Electrolytes for lithium-sulfur cells

Abstract

Provided are cells, such as lithium-sulfur cells, comprising a quasi-solid state liquid electrolyte, wherein the electrolyte comprises a room temperature ionic liquid and an Li salt, and certain concentration subsets of FSI- anions, wherein the electrolyte has ultralow or negligible solvating power for one or more intermediate polysulfide species (Li2Sn, 4 ≤ n ≤ 8).

Description

[0001] Electrolytes for Lithium-Sulfur Cells

[0002] The present application claims priority to Australian Provisional Application AU 2024900719, filed on 18 March 2024, the entire contents of which are incorporated herein by cross-reference.

[0003] Technical field

[0004] The invention relates to an electrolyte, and lithium sulfur cells with the electrolyte with ultralow or negligible polysulfide solubility, which support efficient and high utilisation of active S material due to limitation of the polysulfide shuttle effect.

[0005] Background

[0006] LI Bs exhibit limited storage capacities (~250 Wh kg1) and require further improvement for large- scale applications including passenger EVs, commercial transport EVs, and stationary grid storage. Li-S batteries (LSBs) involving a sulfur (Ss) cathode and a Li metal anode are a promising next-generation high- energy storage systems that can potentially deliver high energy density of up to 2500 Wh kg1, which is up to 10 times greater than current LI Bs.

[0007] Low-cost sulfur has high theoretical specific capacity of 1672 mA h g1. Sulfur cathodes are in an initially charged state, where during a first discharge cycle step, elemental Ss is reduced to LizS at the cathode via a two-stage mechanism involving formation of intermediate polysulfides (LizSs, LizSg, LizSzt, and U2S2). During charging, the two-stage mechanism operates in reverse and insoluble / solid Li2S is oxidised back to Ss, again via the formation of intermediate polysulfides.

[0008] Commercialisation of lithium sulfur cells has not yet been achieved. Challenges include low electronic conductivity of S, significant volume expansion changes (around 76%) and accompanying loss of conductivities and loss of mechanical strength of cathode that occurs on discharging as LixS species are formed. Two particular problems that have prevented commercial use of LSBs are the polysulfide shuttle (PS) and Li dendrite formation. The PS results from dissolution of long-chain polysulfides (U2S8, Li2Sg) and intermediate-chain polysulfides (Li2S4) which are highly soluble in conventional organic liquid electrolytes. The PS results in the S active material moving back and forth from cathode to anode during cycling, a phenomenon known as the shuttle effect. PS shuttling and the associated uncontrolled deposition of insoluble Li2S throughout the cell is a major reason for rapid capacity fade and low Columbic efficiency in LSBs. Much effort has been made to address the shuttle effect of polysulfide, where proposed mitigating strategies include modification of the cathode, pre-treatment of Li foil, electrolyte modifications including passivating additives, and separator modification.

[0009] On the anode side, the lithium dendrite issue arises from damage to the anode solid electrolyte interphase (SEI) as a result of volume expansion during cycling which causes inhomogeneous deposition of Li metal on the anode in the form of lithium dendrites, continued growth of which during cycling eventually leads to internal short circuit posing a significant safety risk. Constant exposure of electrolyte to fresh lithium metal also quickly uses up the electrolyte in the cell and these processes overall contribute to low Coulombic efficiency and capacity loss. Currently used electrolytes for Li-S cells are typically flammable and potentially explosive, as well as volatile, which make them impractical for commercial manufacture.

[0010] Indeed, there is complexity around the choice of electrolyte for LSBs. Typical Li-ion battery organic carbonate electrolytes (e.g., PC, EC, DEC and mixtures) are not compatible with the Li-S chemistry as long chain PS react with carbonates, resulting in byproduct formation and loss of electrolyte over time. Therefore, most Li-S cells use liquid electrolytes composed of ether solvents (e.g., DOL or DME) or glycol ethers (e.g., DEGDME or TEGDME) which avoid these reactions. A commonly used electrolyte is 1 M LiTFSI in DOL:DME 1:1 vol, with 1% w / w LiNOa additive for lithium passivation. However, once the LiNOa additive is used up, protection against the shuttle effect is lost and the above problems occur. Furthermore, volatility and potentially explosive (LiNOa) also makes manufacturing with these challenging / impractical. Improved solutions are desirable.

[0011] All solid-state Li-S batteries involving solid-state electrolytes (SSEs) (e.g., solid or gel polymer or ceramic electrolytes) in which liquid electrolyte is eliminated from the cell is a reasonably effective strategy to alleviate the PS due to barrier behaviour. Further, mechanically, the SSE can decrease issues arising from Li dendrite growth, thereby producing improved cycling performance and safety for practical applications. However, SSEs in LSBs often suffer from low room temperature ionic conductivity and, most importantly, poor interfacial contact which leads to high interfacial resistance, poor utilisation of active material as well as chemical / electrochemical instability of the SSE component, particularly at the reactive Li metal electrode side of the cell. Lithium dendrites are still an issue with SSEs. While conductivity issues have mostly been solved and indeed can exceed liquids in some cases, problems remain with interface quality issues which means excessive stack pressure is typically required. Certain Li-S cells involve using novel electrolyte systems, including those comprising ionic liquids, in which solubility of the polysulfide intermediates is limited. For example, Watanabe reported a low polysulfide solubility binary IL electrolyte of 0.64 M LiTFSI / [DE M E]-[TFSI] (single TFSI anion) with the structure:

[0012] Watanabe reports the 0.64 M LiTFSI salt concentration (approx. 14 mole% salt) was specifically chosen because the Li(l) diffusion flux in the electrolyte reaches a maximum level at around 0.64 M (approx. 14 mole% salt), while a binary system was chosen as the highest limiting current density can be achieved in a binary ionic liquid. Higher salt concentrations were not therefore considered. Galvanostatic chargedischarge curves of Li-S cells with this electrolyte exhibit discharge curves with two voltage plateau regions signifying a mechanism involving dissolved intermediate polysulfide species in the electrolyte solution. The initial discharge capacity of the cell falls within the range of 600 - 1000 mA h g1, corresponding to 50-60% of the theoretical capacity of elemental sulfur (1672 mA h g1) which signifies occurrence of only relatively low sulfur utilisation. The Coulombic efficiency was >97% over 100 cycles, leading the authors to hypothesize that the solubility of Li2Smin the electrolyte is very low such that the PS redox shuttle mechanism is efficiently inhibited. Solubility analysis revealed a solubility limit of L2S8 in 0.64 M Li [TFSI] / [DEM E]-[TFSI] of 6.96 mM (atomic concentration of S). While the PS solubility overall is considered low, UV-vis analysis of solutions extracted from the separator of cycled cells confirm dissolved polysulfides are present in the single TFSI anion electrolyte (Figure 6(a)). These results confirm that Watanabe's 0.64 M Li [TFSI] / [DEM E]-[TFSI] electrolyte dissolves PS species at a level which is well above the limit of detection for the UV-vis technique.

[0013] Dissolution of PS intermediates in the electrolyte needs to be further controlled to achieve long cycle life for Li-S cells operating with high active material utilisation and high energy density. Improved electrolytes with lower solubilising ability for Li2Smspecies are therefore desirable for decreased polysulfide dissolution and shuttle suppression. The task of zero or ultralow PS solubility is not trivial given the active species (e.g., Ss, Li2Sm) exhibit a wide range of hydrophilic-hydrophobic character. For example, the most hydrophobic Ss species is only soluble in non-polar solvents such as benzene, while in contrast, the final reduction product U2S is only soluble in highly polar solvents such as water. The intermediate Li2Sm species exhibit variable intermediate character depending on the chain length. Notwithstanding these challenges, the electrolyte must also solubilise the lithium metal salt. The electrolyte ideally must also be capable of forming a suitable passivating interphase film on both anode and cathode, particularly importantly on the S cathode, in a way that assists in avoidance of PS dissolution and / or generation of a low impedance film for optimal cell performance.

[0014] Watanabe's group later reported saturation concentrations (in units of total atomic S concentration) of Ss and mixtures of Li2Sn(4 < n < 8) in ILs with different anions in the presence of 0.5 mol. kg1Li [TFSI], noting that the results showed that not all IL electrolytes are effective for suppressing PS dissolution in Li-S cells. The solubility studies showed that TFSI anion ILs have very low solubility values of around 10 mM in atomic S concentration, suggesting that predominantly TFSI anion ILs are preferred for IL based electrolytes in Li-S cells. However, in cell cycling tests, reversible charge / discharge of the Li-S cell was only achieved with one specific electrolyte, namely 0.5 mol.kg-1 Li [TFSI] in [C3mpyr][TFSI] (single TFSI anion system; approx. 17.5 mole% salt) which gave a best discharge capacity of around 600 mAhg1at the 50thcycle and low overvoltage. The poor performance of all the other electrolytes was attributed to polysulfide dissolution, unfavourable side reactions and slower mass transport through the electrolyte. The discharge curves showed a two-voltage plateau discharge signifying formation of at least some higher order PS intermediates from Ss, which are then reduced to Li2S and the same redox behaviour was observed for the earlier [DEM E] [TFSI] system. As reported, the charge / discharge behaviour of the FSI based electrolyte 0.5 mol. kg1LifTFSI] / [C3mpyr][FSI] (mixed FSI / TFSI anion system; approx. 13.4 mol%) was particularly disappointing with a drastic decrease in capacity within 10 cycles, significant overpotential, and the Coulombic efficiency of >110%, which supported occurrence of irreversible side reactions during the discharge process. Indeed, the authors proposed that the FSI anion (with -SO2F groups) is less stable in the presence of Li2Sn(4 < n < 8), and / or that the FSI anion decomposes in the presence of nucleophilic polysulfide anions thereby resulting in poor cycling. SEM and XRD analysis showed solid byproducts were formed on the surface of the KB / Ss cathode after just 3 cycles in the FSI system, while spherical particles of byproducts were observed on the cathode surface after 30 cycles. FSI anion decomposition in this electrolyte was reported as forming a poorly performing insulating interphase layer on the S cathode that dramatically increased the charge transfer resistance during charge / discharge. As a result of the insulating layer formation on the cathode due to FSI anion decomposition, Watanabe's group proposed that FSI anion IL is not at all a suitable electrolyte for a Li-S battery. This was a disappointing conclusion for them, as the dissolution of Li2Smwas suppressed to a low level in the [C3mpyr] [FSI] electrolyte (saturation concentration of individual intermediate polysulfide species is between 1 and 10 mM atomic concentration of S), and this electrolyte exhibited the highest Li+conduction in the series of IL electrolyte tested. On the basis of the issues around the passivating layer, overall, the group proposed that low viscosity TFSI anion based ILs, and in particular [C3mpyr][TFSI] ILs (TFSI single anion IL systems) are the optimal IL electrolytes for Li-S batteries.

[0015] Given the above issues, there is a need for new Li-S cells that address one or more of the above problems would be desirable. For example, new electrolytes that exhibit ultralow or even negligible PS solubility would be highly desirable, as would such electrolytes that are capable of producing a good quality, low impedance passivating film / interphase at the cathode, that may also limit PS dissolution while allowing for optimised cell performance.

[0016] Statements of the Invention

[0017] Surprisingly, particularly in view of the prior art discussed above teaching away from FSI anion electrolyte systems, the inventors have found that certain Li salt concentration subsets of FSI- anion ionic liquids exhibit ultralow or negligible solvating power for one or more intermediate polysulfide species (LizSn, 4 < n < 8). Preferred Li salt concentration subsets of FSI- anion ionic liquids solubilise these intermediate polysulfide species (LiaSn, 4 < n < 8) to such a low degree that on saturation with these polysulfides, they are present in the electrolyte solution in amounts of atomic concentration of S that are at least lower than ImM, and indeed typically are lower than the limit of detection of UV-vis spectroscopy.

[0018] It will be understood that the Sg2-species is detectable by UV spectroscopic analysis at 490-500 nm, the Sg2“ species is detectable by UV spectroscopic analysis at 450-470 nm, and the S42-species is detectable by UV spectroscopic analysis at 420 nm. In the case of the Li salt concentration subsets of FSI- anion ionic liquids electrolytes as defined by the invention, no detectable UV bands are found for at least one or more of, and most typically, all of these high and intermediate order polysulfide species during solution UV studies at room temperature of 25 °C, for example, where the saturated solutions are prepared at 50 °C or under other conditions which would ensure formation of saturated PS solutions. Therefore, for preferred electrolytes, at room temperature of 25 °C, the Li+conductive ionic liquid electrolytes of the invention comprise a concentration of S42-(detectable at 420 nm), Sg2“ (detectable at 450-470 nm), Sg2-at (detectable at 490-500 nm) that at least <lmM, more typically is zero, or at least is negligible in the sense it is not detectable by UV analysis in terms of the concentration present. This means the concentration of dissolved PS species (to the extent they are present at all) is lower than the limit of detection of the UV-vis spectroscopy technique used. That is, each high and intermediate polysulfide species, to the extent it is present in the electrolyte solution at all, is present in an amount that is below the limit of detection of the UV technique for the particular polysulfide under consideration, particularly at analysis at room temperature of 25 °C, at least for saturated solutions prepared at 50 °C, e.g. via methods described elsewhere herein.

[0019] The ultralow or negligible solvating power for one or more intermediate polysulfide species ( LizSn, 4 < n < 8) make these electrolytes suitable for use as potential polysulfide shuttle inhibiting electrolytes for use in Li-S cells. For preferred electrolytes, the ultralow or negligible solvating power supports apparent quasi solid-state redox behaviour in a Li-S cell which includes the electrolytes of the invention. Apparent quasi solid-state redox behaviour is demonstrated by the cell, on cycling, exhibiting a single redox plateau during discharge, for example, under the cell composition and operating parameters described in Examples 1 or 2 below. In addition to the desirably PS solubility, further preferred electrolytes of the invention form a desirably low impedance passivating surface film / interface at the cathode, which assists in cell performance, and which may further limit PS dissolution into the electrolyte. A film is also desirably formed on the anode. Together the surprising ultralow or negligible solvating power for PS and the ability of the electrolytes of the invention to form stable, low impedance interface films on one or more of the cathode and anode, but particularly the cathode, enables Li-S cells in which cell operating parameters are optimised, e.g. with respect to one or more of: high S active material utilisation, low overpotential, chargedischarge efficiency, favourable Li electrode morphology, lean electrolyte conditions and safety. It is remarkable that the electrolytes of the invention simultaneously favourably support each of these distinct parameters. All of the properties of the electrolytes of the invention together represent a significant and notable progression in the art of Li-S cell technology in this regard.

[0020] The desirably lack of PS solubility is confined to two general types of Li+conductive ionic liquid electrolyte, each of which have at least one room temperature ionic liquid (RTIL) and at least one Li salt solubilised in the RTIL, as follows: wherein the Li+conductive ionic liquid electrolyte comprises, consists of or consists essentially of: (A) from about 20 mole% to about 60 mole% of Li+cations, where anions of the electrolyte are predominantly [FSI]- anions, regardless if sourced from Li salt or RTIL (designated herein as a 'single FSI system'); or (B) from about 20 mole% to about 60 mole% of Li+cations and [TFSI]- anions, where anions in the electrolyte are predominantly [FSI]- anions, regardless if sourced from Li salt or RTIL (designated herein as a 'mixed TFSI / FSI system'). About in this context means ±1% of the stated value. In either case, the Li+conductive ionic liquid electrolyte preferably solubilises one or more polysulfide species, LiaSn, 4 < n < 8, to a saturation concentration at 25 °C of less than 1 mM atomic S, more preferably to a saturation concentration at 25 °C that is zero or at least is below the limit of detection of UV spectroscopy.

[0021] It will be understood that sources of the required concentration of Li+in (A) can be from any Li salt in the electrolyte, however, with the condition that the total anions in the electrolyte composition must be predominantly FSI" anions. It will be further understood that sources of the required concentration of Li+cation in (B) can be from any Li salt in the electrolyte, and likewise the source of the required concentration of [TFSI]- anion can be from LiTFSI salt or from a portion of one or more included [ILcatlon] [TFSI] ionic liquids, however, overall, outside the stated concentration range for Li+cation and [TFSI]- anion, the total anions in the electrolyte composition must be predominantly [FSI]" anions.

[0022] In general examples, the Li+conductive ionic liquid electrolyte having at least one room temperature ionic liquid (RTIL) and at least one Li salt solubilised in the RTIL is one, wherein the Li+conductive ionic liquid electrolyte comprises: (A) a single FSI anion system, where an anion of the Li+salt is bis(fluorosulfonyl)imide ([FSI]-), and the Li salt is present in an amount of from about 20 mole% to about 60 mole% of the electrolyte; or (B) a mixed FSI / TFSI anion system, where an anion of the Li+conductive salt is bis[(trifluoromethyl)sulfonyl]imide ([TFSI]-), and the Li salt is present in an amount of from about 20 mole% to about 60 mol% of the electrolyte. About in this context means ±1% of the stated value.

[0023] The anode is described herein as a negative Li metal or Li metal alloy electrode. In one embodiment, the Li metal or Li metal alloy electrode may comprise one or more other non-Li elements. In some embodiments, the anode may be an alloying anode, such as may comprise silicon. Accordingly, in some embodiments, the anode may be an alloying anode comprising silicon and form an alloy when lithiated. In one embodiment, the anode comprising silicon may comprise amorphous silicon, or silicon dioxide, silicon monoxide.

[0024] In a first aspect, the invention provides a liquid electrolyte lithium-sulfur cell, preferably a quasisolid state (QSS) liquid electrolyte lithium-sulfur cell comprising: o a negative Li metal or Li metal alloy electrode (anode); o a positive S electrode (cathode); and o a Li+conductive ionic liquid electrolyte having at least one room temperature ionic liquid (RTIL) and at least one Li salt solubilised in the RTIL, wherein the Li+conductive ionic liquid electrolyte comprises, consists of, or consists essentially of:

[0025] (A) from about 20 mole% to about 60 mole% of Li+cations, where anions of the electrolyte are predominantly [FSI]- anions, regardless if sourced from Li salt or RTIL (single FSI system); or

[0026] (B) from about 20 mole% to about 60 mole% of Li+cations and [TFSI]- anions, where anions in the electrolyte are predominantly [FSI]- anions, regardless if sourced from the Li salt or the RTIL ('mixed TFSI / FSI system'); wherein the Li+conductive ionic liquid electrolyte solubilises one or more polysulfide species, LizSn, 4 < n < 8, to a saturation concentration at 25 °C of less than 1 mM atomic S, more preferably to a saturation concentration at 25 °C that is zero, or at least is below the limit of detection of UV spectroscopy for the one or more polysulfide species.

[0027] Preferably, the Li+conductive ionic liquid electrolyte comprises, consists of, or consists essentially of:

[0028] (A) from about 35 mole% to about 55 mole% of Li+cations, where anions of the electrolyte are predominantly [FSI]- anions, regardless if sourced from Li salt or RTIL (single FSI system); or (B) from about 25 mole% to about 40 mole% of Li+cations and [TFSI]- anions, where anions in the electrolyte are predominantly [FSI]- anions, regardless if sourced from Li salt or RTIL (mixed TFSI / FSI system).

[0029] Preferably, the Li+conductive ionic liquid electrolyte comprises, consists of, or consists essentially of:

[0030] (A) from about 47 mole% to about 53 mole%, preferably about 50 mole% of Li+cations, where anions of the electrolyte are predominantly [FSI]- anions, regardless if sourced from Li salt or RTIL (single FSI system); or

[0031] (B) from about 27 mole% to about 35 mole%, preferably about 30 mole% of Li+cations, and [TFSI]- anions, where anions in the electrolyte are predominantly [FSI]- anions, regardless if sourced from Li salt or RTIL (mixed TFSI / FSI system).

[0032] Suitably, the Li+cations are sourced in (A) from a LiFSI salt and the ionic liquid anions are predominantly FSI anions sourced from the LiFSI salt and the IL anions which are [FSI]- anions; and where in the Li+cations and [TFSI]- anions are sourced in (B) from a LiTFSI salt and the ionic liquid anions are predominantly FSI anions sourced from the IL anions which are [FSI]- anions.

[0033] Preferably, in the single FSI anion system, the Li+cation is present in an amount of from about 30 mole% to about 55 mole% of the electrolyte. Most preferably the Li+cation is present in an amount of from about 45 mole% to about 55 mole% of the electrolyte, most preferably around the 50 mole% concentration, that is, about 45 mole%, about 46 mole%, about 47 mole%, about 48 mole%, about 49 mole%, about 50 mole%, about 51 mole%, about 52 mole%, about 53 mole%, about 54 mole%, or about 55 mole%. In preferred embodiments, the Li+cation is present in an amount of from about 47 mole% to about 53 mole% of the electrolyte, most preferably about 50 mole%. About in this context means ±1% of the stated value.

[0034] In preferred example, in the mixed TFSI / FSI anion system, the Li+cation and the [TFSI]- anion are present in an amount of from about 20 mole% to 50 mole% of the electrolyte. Most preferably the Li+cation and the [TFSI]- anion are present in an amount of from about 25 mole% to about 45 mole% of the electrolyte, most preferably around the 30 mole% concentration, that is, about 25 mole%, about 26 mole%, about 27 mole%, about 28 mole%, about 29 mole%, about 30 mole%, about 31 mole%, about 32 mole%, about 33 mole%, about 34 mole%, or 35 mole%. In preferred embodiments, the Li+cation and the [TFSI]- anion are present in an amount of from about 27 mole% to about 33 mole% of the electrolyte, most preferably about 30 mole%. About in this context means ±1% of the stated value.

[0035] These concentrations ranges are preferred for any IL cation used, however they are particularly preferred for phosphonium and ammonium cations, especially quaternary phosphonium and quaternary ammonium cations For ILs with a pyrrolidinium cation, in the single or mixed anion systems, the Li+cation is preferably present in an amount from about 45 mole% to about 55 mole% of Li+cations, preferably about 50 mole% Li+cations. About in this context means ±1% of the stated value. Desirably, on cycling, for example at least at a current density below 0.5 mA / cm2, preferably below 0.3 mA / cm2, more preferably below 0.1 mA / cm2, the cell exhibits quasi solid-state redox behaviour as evidenced by a single voltage plateau discharge curve feature for the cell which indicates an apparent, direct, one step, Ss to low order lithium sulfides (LiaSa / LiaS) solid to solid conversion reaction which is confined to a S cathode of the cell. Preferred cells also exhibit this behaviour at even higher current densities. As described herein, a quasi -solid-state Li-S cell is one that exhibits this behaviour in its discharge curve. Typical cycle composition and operating conditions where this behaviour is observed with the cells of the invention are described in Examples.

[0036] Desirably, a preferred cell is one wherein, on cycling at a current density of 0.1 mA cm'2or less, more preferably at a current density of 0.500 mA cm'2or less more preferably at a current density of 0.334 mA cm'2or less, which utilises at least 70% of S active during cycling for at least the first discharge cycle, that is the cell exhibits at least 70% conversion of the S active material in the positive electrode into low order lithium sulfides (LizSz / LizS).

[0037] Preferably, the cell on cycling exhibits an overpotential of 0.5 V or less, preferably 0.3 V or less, more preferably 0.275 V or less, more preferably still 0.25 V or less, between the discharge and charge steps of any given cycle, at least when cycled using the electrode preparations of Example 1 or Example 2, e.g., with or without calendaring and employing the same current density, temperature, separator, electrolyte volume, compression, formation and wetting procedure, as described in the Examples.

[0038] Preferably, the cell is substantially self-discharge free, as demonstrated for example, via an open circuit voltage (OCV) of the cell which varies by no more than ± 0.1 V / hour, for example, for at least 1 hour. This is as a result of the ultralow, zero or at least undetectable PS solubility in the electrolytes of the invention used in the cells of the invention.

[0039] For further enhanced performance, preferably the S cathode of the cells of the invention comprises a polyionic liquid (PIL) : Li Salt binder mixture of

[0040] (i) at least one PIL selected from PIL cationic polymers with counter anions; PIL anionic polymers with Li+counter cations; and PIL cationic and / or anion block copolymers with corresponding counter anions or cations, preferably wherein the at least one PIL comprises:

[0041] • a poly ionic liquid cationic polymer comprising a plurality of one or more ionic liquid cations included in, or tethered as pendant groups, to an organic polymer backbone; and a plurality of one or more tethered or untethered counter anions;

[0042] • a poly ionic liquid anionic polymer comprising a plurality of one or more ionic liquid anions included in, or tethered as pendant groups, to an organic polymer backbone; and a plurality of Li+cations;

[0043] • a poly ionic liquid cationic and / or anion block copolymer comprising: a non-ionic block that does not contain ionic charge and comprises polymerised residues of hydrophobic monomers, and an ionic block comprising polymerised monomer residues having covalently coupled thereto one or more of : (a) a pendant organic ionic liquid cation, the pendant organic ionic liquid cation having a counter anion, and

[0044] (b) a pendant anionic moiety, the pendant anionic moiety having a countercation; and

[0045] (ii) one or more Li salts.

[0046] Preferably, each Li salt in the binder mixture has a Li+cation and a salt counter anion, wherein counter anions and the Li salt anions in the electrolyte are the same or different. Preferred binder mixture Li salt counter anions in the PIL : Li Salt mixture are [FSI]- anions and / or [TFSI]- anions, preferably [TFSI]- anions. Preferably, the one or more Li salts in the PIL binder mixture are identical to the one or more Li metal salts used in the Li+conductive ionic liquid electrolyte. For example, if the electrolyte comprises LiTFSI, the binder Li salt is preferably LiTFSI. Likewise, if the electrolyte comprises Li FSI salts only, the binder Li salt may be Li FSI . In some embodiments, the binder salt may be Li FSI or LiTFSI, regardless of the Li salt used in the electrolyte composition. Overall the most preferred binder mixture Li salts are LiTFSI.

[0047] Preferably, wherein the at least one ionically conductive organic polymer ionic liquid (PIL) binder comprises a polymeric backbone [PDADMA]+and tethered ionic liquid anion component [TFSI]- such that the PIL corresponds to [PDADMA][TFSI]. Other PI L:Li Salt binder mixtures include [PMTFSI.TFSI] :LiTFSI and [PILblocCo-polymer(plmTFSI)]:LiTFSI. Preferably, the PIL:Li Salt binder mixture is 50:50 mole% [PDADMA.TFSI]:UTFSI, 50:50 mole% [PMTFSI.TFSI]:UTFSI, or 50:50 mole% [PILblocCo- polymer(plmTFSI)]:LiTFSI.

[0048] Suitably, at least one ionically conductive PI L:Li salt binder mixture is present in the S cathode in an amount up to about 20 wt% (about means ±5%), preferably up to about about 15 wt% (about means ±5%), preferably up to about 10 wt%, most preferably up to about 5 wt% (about means ±5%) of the total positive S electrode (cathode) composition.

[0049] Preferably, the positive S electrode (cathode) further comprises 2D and / or nanomaterials such as boron nitride or an allotrope of boron nitride, such as BNNTs, BNNSs, or BNNWs, or a ceramic nanomaterial such as graphene, functionalised graphene, MXenes and other 2D nanomaterials, preferably in an amount of up to about 3 wt% (about means ±5%), of the positive S electrode (cathode), preferably up to 2 wt% (about means ±5%), and preferably still up to 1 wt% (about means ±5%), most preferably up to 0.5 wt% (about means ±5%). Boron nitride or an allotrope of boron nitride, such as BNNTs, BNNSs, or BNNWs are particularly preferred. Excellent results have been achieved with BNNTs. Preferably, the positive S electrode (cathode) further comprises up to about 1 wt% (about means ±5%), most preferably up to about 0.5 wt% (about means ±5%) of BNNT Preferred cells, wherein on cycling at a current density of 0.5 mA cm- 2, more preferablyO.334 mA cm'2(C / 10) utilises at least 50% of S active during cycling on the 40thdischarge cycle. These cells typically comprise an S cathode with P I L: Li Salt binder mixture and the 2D or nanomaterials described herein, particularly a boron nitride, e.g., BNNT or an allotrope thereof. In a second aspect, the invention provides a use a Li+conductive ionic liquid electrolyte having at least one room temperature ionic liquid (RTIL) and at least one Li salt solubilised in the RTIL, wherein the Li+conductive ionic liquid electrolyte comprises, consists of, or consists essentially of:

[0050] (A) from about 20 mole% to about 60 mole% of Li+cations, where anions of the electrolyte are predominantly [FSI ]’ anions, regardless if sourced from Li salt or RTIL (single FSI system); or

[0051] (B) from about 20 mole% to about 60 mole% of Li+cations and [TFSI ]- anions, where anions in the electrolyte are predominantly [FSI]- anions, regardless if sourced from Li salt or RTIL (mixed TFSI / FSI system); wherein the Li+conductive ionic liquid electrolyte solubilises one or more polysulfide species, LizSn, 4 < n < 8, to a saturation concentration at 25 °C of less than 1 mM atomic S, more preferably to a saturation concentration at 25 °C that is zero, or at least is below the limit of detection of UV spectroscopy for the one or more polysulfide species, in an energy storage application.

[0052] In a third aspect, the invention provides a use of a Li+conductive ionic liquid electrolyte having at least one room temperature ionic liquid (RTIL) and at least one Li salt solubilised in the RTIL, wherein the Li+conductive ionic liquid electrolyte comprises an ammonium cation, and the electrolyte comprises:

[0053] (A) from about 20 mole% to about 60 mole% of Li+cations, where anions of the electrolyte are predominantly [FSI]- anions, regardless if sourced from Li salt orRTIL ('single FSI system'); or

[0054] ( B) from about 20 mole% to about 60 mole% of Li+cations and [TFSI]- anions, where anions in the electrolyte are predominantly [FSI]- anions, regardless if sourced from Li salt or RTIL ('mixed TFSI / FSI system'), wherein the Li+conductive ionic liquid electrolyte solubilises one or more polysulfide species, Li 2Sn, 4 < n < 8, to a saturation concentration at 25 °C of less than 1 mM atomic S, more preferably to a saturation concentration at 25 °C that is zero, or at least is below the limit of detection of UV spectroscopy for the one or more polysulfide species in a Li-S cell.

[0055] In a related aspect the invention provides a use of a Li+conductive ionic liquid electrolyte having at least one room temperature ionic liquid (RTIL) and at least one Li salt solubilised in the RTIL, wherein the Li+conductive ionic liquid electrolyte comprises, consists of, or consists essentially of:

[0056] (A) about 50 mole% of Li+cations, where anions of the electrolyte are predominantly [FSI]" anions, regardless if sourced from the Li salt or the RTIL (single FSI system); or

[0057] (B) about 30 mole% of Li+cations and [TFSI]- anions, where anions in the electrolyte are predominantly bis(fluorosulfonyl)imide [FSI]- anions, regardless if sourced from Li salt or RTIL (mixed TFSI / FSI system); wherein the Li+conductive ionic liquid electrolyte solubilises one or more polysulfide species, LizSn, 4 < n < 8, to a saturation concentration at 25 °C of less than 1 mM atomic S, more preferably to a saturation concentration at 25 °C that is zero, or at least is below the limit of detection of UV spectroscopy for the one or more polysulfide species, in a Li-S cell. In a fourth aspect, the invention provides a use of a Li+conductive ionic liquid electrolyte having at least one room temperature ionic liquid (RTIL) and at least one Li salt solubilised in the RTIL, wherein the Li+conductive ionic liquid electrolyte comprises, consists of, or consists essentially of:

[0058] (A) from about 20 mole% to about 60 mole% of Li+cations, where anions of the electrolyte are predominantly [FSI ]’ anions, regardless if sourced from Li salt or RTIL (single FSI system); or

[0059] (B) from about 20 mole% to about 60 mole% of Li+cations and [TFSI]- anions, where anions in the electrolyte are predominantly [FSI]- anions, regardless if sourced from Li salt or RTIL (mixed TFSI / FSI system); wherein the Li+conductive ionic liquid electrolyte solubilises one or more polysulfide species, LizSn, 4 < n < 8, to a saturation concentration at 25 °C of less than 1 mM atomic S, more preferably to a saturation concentration at 25 °C that is zero, or at least is below the limit of detection of UV spectroscopy for the one or more polysulfide species, to manufacture a Li-S cell, preferably a quasi-solid-state Li-S cell.

[0060] In a fifth aspect, the invention provides a use of a Li+conductive ionic liquid electrolyte having at least one room temperature ionic liquid (RTIL) and at least one Li salt solubilised in the RTIL, wherein the Li+conductive ionic liquid electrolyte comprises, consists of, or consists essentially of:

[0061] (A) from about 20 mole% to about 60 mole% of Li+cations, where anions of the electrolyte are predominantly [FSI]- anions, regardless if sourced from Li salt or RTIL (single FSI system); or

[0062] (B) about 20 mole% to about 60 mole% of Li+cations and [TFSI]- anions, where anions in the electrolyte are predominantly [FSI]- anions, regardless if sourced from Li salt or RTIL (mixed TFSI / FSI system); wherein the Li+conductive ionic liquid electrolyte solubilises one or more polysulfide species, LizSn, 4 < n < 8, to a saturation concentration at 25 °C of less than 1 mM atomic S, more preferably to a saturation concentration at 25 °C that is zero, or at least is below the limit of detection of UV spectroscopy for the one or more polysulfide species, to reduce or eliminate the polysulfide shuttle effect in a Li-S cell.

[0063] In a sixth aspect, the invention provides a method operating a Li-S cell with a S active material utilisation of >60% on a 20thdischarge cycle of the cell, by cycling the cell with a Li+conductive ionic liquid electrolyte having at least one room temperature ionic liquid (RTIL) and at least one Li salt solubilised in the RTIL, wherein the Li+conductive ionic liquid electrolyte comprises, consists of, or consists essentially of:

[0064] (A) from about 25 mole% to about 60 mole% of Li+cations, where anions of the electrolyte are predominantly [FSI]- anions, regardless if sourced from Li salt or RTIL (single FSI system); or

[0065] ( B) from about 20 mole% to about 60 mole% of Li+cations and [TFSI]- anions, where anions in the electrolyte are predominantly [FSI]- anions, regardless if sourced from Li salt or RTIL (mixed TFSI / FSI system); wherein the Li+conductive ionic liquid electrolyte solubilises one or more polysulfide species, LizSn, 4 < n < 8, to a saturation concentration at 25 °C of less than 1 mM atomic S, more preferably to a saturation concentration at 25 °C that is zero, or at least is below the limit of detection of UV spectroscopy for the one or more polysulfide species. In a seventh aspect, the invention provides a use of a Li+conductive ionic liquid electrolyte having at least one room temperature ionic liquid (RTIL) and at least one Li salt solubilised in the RTIL, wherein the Li+conductive ionic liquid electrolyte comprises, consists of, or consists essentially of:

[0066] (A) from about 20 mole% to about 60 mole% of Li+cations, where anions of the electrolyte are predominantly [FSI ]’ anions, regardless if sourced from Li salt or RTIL (single FSI system); or

[0067] (B) from about 20 mole% to about 60 mole% of Li+cations and [TFSI ]- anions, where anions in the electrolyte are predominantly [FSI]- anions, regardless if sourced from Li salt or RTIL (mixed TFSI / FSI system); wherein the Li+conductive ionic liquid electrolyte solubilises one or more polysulfide species, LizSn, 4 < n < 8, to a saturation concentration at 25 °C of less than 1 mM atomic S, more preferably to a saturation concentration at 25 °C that is zero, or at least is below the limit of detection of UV spectroscopy for the one or more polysulfide species, to manufacture a Li-S cell, preferably a quasi solid-state Li-S cell.

[0068] In an eighth aspect, the invention provides Li-S cell, preferably a quasi solid-state Li-S cell, comprising: o a negative Li metal or Li metal alloy electrode (anode); o a positive S electrode (cathode); and o a Li+conductive ionic liquid electrolyte having at least one room temperature ionic liquid (RTIL) and at least one Li salt solubilised in the RTIL, wherein the Li+conductive ionic liquid electrolyte comprises, consists of, or consists essentially of:

[0069] (A) from about 20 mole% to about 60 mole% of Li+cations, where anions of the electrolyte are predominantly [FSI]- anions, regardless if sourced from the Li salt orthe RTIL (single FSI system); or

[0070] ( B) from about 20 mole% to about 60 mole% of Li+cations and [TFSI]- anions, where anions in the electrolyte are predominantly [FSI]- anions, regardless if sourced from the Li salt or the RTIL (mixed TFSI / FSI system); wherein the Li+conductive ionic liquid electrolyte solubilises one or more polysulfide species, LizSn, 4 < n < 8, to a saturation concentration at 25 °C of less than 1 mM atomic S, more preferably to a saturation concentration at 25 °C that is below the limit of detection of UV spectroscopy for the one or more polysulfide species, wherein the positive S electrode (cathode) comprises at least one ionically conductive organic polymeric ionic liquid (PIL) binder : Li salt composite; and wherein the positive S electrode (cathode) further comprises a 2D and / or a nanomaterial such as boron nitride or an allotrope of boron nitride, such as BNNT, BNNS, BNNW, or a ceramic nanomaterial such as graphene, functionalised graphene, MXenes and other 2D nanomaterials, preferably in an amount of up to 3 wt% of the positive S electrode (cathode), preferably up to 1 wt%, most preferably in an amount of up to 0.5% wt %.

[0071] The PI L:Li salt binder mixture is described elsewhere herein. Desirably, the RTIL comprises an RTIL cation component which comprises a phosphonium cation, an ammonium cation, a sulfonium cation, a pyrrolidinium cation, an imidazolium cation, pyridinium cation, or combinations thereof. Preferably, the RTIL cation component is an alkyl phosphonium cation, an alkyl ammonium cation, an alkyl sulfonium cation, an alkyl pyrrolidinium cation, an alkyl imidazolium based cation, an alkyl pyridinium cation or combinations thereof.

[0072] For the above first to eighth aspects, in some preferred examples, the Li+conductive ionic liquid electrolyte comprises a phosphonium cation, and the electrolyte comprises, consists of, or consists essentially of:

[0073] (A) from about 25 mole% to about 60 mole% of Li+cations, preferably about 35 mole% to about 55 mole% Li+cations, most preferably 47 mole% to about 53 mole% Li+cations, most preferably still about 50 mole% Li+cations, where anions of the electrolyte are predominantly [ FSI ] anions, regardless if sourced from the Li salt or the RTIL (single FSI system); or

[0074] (B) from about 25mole% mole% to about 60 mole% of Li+cations, preferably about 27 mole% to about 40 mole% Li+cations, most preferably 27 mole% to about 35 mole% Li+cations, most preferably still about 30 mole% Li+cations, and [TFSI]- anions, where anions in the electrolyte are predominantly [FSI]- anions, regardless if sourced from the Li salt or the RTIL (mixed TFSI / FSI system).

[0075] For the above first to eighth aspects, in some preferred examples, the Li+conductive ionic liquid electrolyte comprises an ammonium cation, and the electrolyte comprises, consists of, or consists essentially of:

[0076] (A) about 20 mole% to about 60 mole% of Li+cations, preferably about 35 mole% to about 55 mole% Li+cations, most preferably 47 mole% to about 53 mole% Li+cations, most preferably still about 50 mole% Li+cations, where anions of the electrolyte are predominantly [FSI]- anions, regardless if sourced from the Li salt or the RTIL (single FSI system); or

[0077] ( B) about 20 mole% to about 60 mole% of Li+cations, preferably about 25 mole% to about 55 mole% Li+cations, most preferably 27 mole% to about 35 mole% Li+cations, most preferably still about 30 mole% Li+cations, and [TFSI]- anions, where anions in the electrolyte are predominantly [FSI]- anions, regardless if sourced from the Li salt or the RTI L (mixed TFSI / FSI system).

[0078] For the above first to eighth aspects, in some preferred examples, the Li+conductive ionic liquid electrolyte comprises a pyrrolidinium cation, and the electrolyte comprises, consists of, or consists essentially of:

[0079] (A) from about 45 mole% to about 55 mole% of Li+cations, preferably about 50 mole% Li+cations, where anions of the electrolyte are predominantly [FSI]- anions, regardless if sourced from the Li salt or the RTIL (single FSI system).

[0080] Suitably, the RTIL cation component is selected from the group consisting of: [Pmi4]+, [NUB] / [Campyr]+, or combinations thereof. Preferably, the RTIL of the ionic liquid electrolyte is selected from the group consisting of: [Pmi4] [FSI], [NUB] [FSI], [C3mpyr][FSI], and combinations thereof. Desirably, phosphonium cation is a quaternary phosphonium cation, preferably a quaternary alkyl phosphonium cation such as [Piiii4]+, [P1222F, [Pi22a]+, [P1224 , [P2222]+, [P2223]+, [P2224]+, [Pi444]+[Pii4i4i4]+and related ether oxygen containing analogues. In the notation [PXxxx], each x is an integer representing an alkyl group, wherein the integer corresponds to a number of carbon atoms in the alkyl group, e.g., 1 = methyl, 2 = ethyl, 3= propyl, 4 = butyl, i4 = iso-butyl, etc.

[0081] Desirably, ammonium cation is a quaternary ammonium cation, X preferably a quaternary alkyl ammonium cation such as [Nm3]+, [Nuu]+, [NIU2]+, [Nm4]+, [Ni222]+, [Ni222]+, [Ni22a]+, [NI224]+, [N2222F, [N2223]+, [N2224]+, NI333]+, [NH33]+[N3333]+, [ N i444]+[ N ij4j4i4]+and related ether oxygen containing analogues. In the notation [NXxxx], each x is an integer representing an alkyl group, wherein the integer corresponds to a number of carbon atoms in the alkyl group, e.g., 1 = methyl, 2 = ethyl, 3= propyl, 4 = butyl, i4 = iso-butyl, etc.

[0082] Desirably, the pyrrolidinium cation is an alkyl pyrrolidinium such as [C3mpyr]+, [Clmpyr]+, [C2mpyr]+, [C4mpyr]+, [Clepyr]+, [C2epyr]+, [C3epyr]+, [C4epyr]+and related ether oxygen containing analogues.

[0083] In some embodiments, the IL cation is not an alkyl imidazolium cation, such as is not [EMIm]+.

[0084] Brief Description of Drawings

[0085] Embodiments of the invention will herein be illustrated by way of example only with reference to the accompanying drawings in which:

[0086] Figure 1 illustrates (A) the effect of a single FSI- anion ionic liquid based electrolyte system (IL1 is [Piiij4] [ESI] / Saltl is LiFSI) compared to conventional solvent-based 1:1 DME:DOL analogue: (I) voltage vs capacity profile (a) and capacity vs cycle number (b) of a LiS cell with a typical S cathode [15 wt % CMC, 21 wt % HSAC, 14 wt % C65 and 50 wt % S] in the single FSI anion phosphonium cation based electrolyte 30 mole% LiFSI / [Pmj4] [FSI] IL electrolyte, cell cycled at C / 10 at 50 °C; (II) voltage vs capacity profile (c) and capacity vs cycle number (d) of a LiS cell with a typical S cathode [15 wt % CMC, 21 wt % HSAC, 14 wt % C65 and 50 wt % S] in conventional electrolyte 1 M LiTFSI & 0.2 M LiNO3in 1:1 DME:DOL electrolyte at C / 10 at 50 °C; (B) the effect of a single FSI anion ionic liquid-based electrolyte compared to conventional solvent-based analogue (IL 2 [Nm3][FSI] / salt 1: LiFSI): (I) voltage vs capacity profile (a) and capacity vs cycle number (b) of a LiS cell with a typical S cathode [15 wt % CMC, 21 wt % HSAC, 14 wt % C65 and 50 wt % S] in the single FSI anion ammonium cation based electrolyte 30 mol% LiFSI / [Nm3][FSI] FSI IL electrolyte, with the cell cycled at C / 10 at 50 °C; voltage vs capacity profile (c) and capacity vs cycle number (d) of LiS cell with a typical S cathode ([15 wt % CMC, 21 wt % HSAC, 14 wt % C65 and 50 wt % S] in 1 M LiTFSI and 0.2 M LiNO3in 1:1 DME:DOL electrolyte at C / 10 at 50 °C;

[0087] Figure 2 illustrates (A) the effect of salt medium and low concentration (30 mole% v 10 mole%) on the performance of the sulfur cathode in the single FSI anion ionic liquid based electrolyte system (IL1 is [Pnii4] [FSI] / Saltl is LiFSI): (I) voltage vs capacity profile (a) and capacity vs cycle number (b) of LiS cell in 30 mole% LiFSI / [Pmj4] [FSI] IL electrolyte at C / 10 at 50 °C; (III) voltage vs capacity profile (c) and capacity vs cycle number (d) of LiS cell in a typical S cathode [15 wt % CMC, 21 wt % HSAC, 14 wt % C65 and 50 wt % S] in 10 mole% LiFSI / [Pmj4] [FSI] IL electrolyte at C / 10 at 50 °C; (B) the effect of salt concentration (30 mole% v 20 mole%) on the performance of the sulfur cathode in the single FSI anion ammonium system (IL2: [Nma] [FSI] / Salt 1: LiFSI): (I) voltage vs capacity profile (a) and capacity vs cycle number (b) of LiS cell with a typical S cathode (15 wt % CMC, 21 wt % HSAC, 14 wt % C65 and 50 wt % S) in 30 mol% LiFSI / [Nu ] [FSI] IL electrolyte at C / 10 at 50 °C. (Ill) voltage vs capacity profile (c) and capacity vs cycle number (d) of LiS cell with a typical S cathode [15 wt % CMC, 21 wt % HSAC, 14 wt % C65 and 50 wt % S) in 20 mol% LiFSI / [N1113] [FSI] IL electrolyte at C / 10 at 50 °C.

[0088] Figure 3 illustrates (A) the effect of a mixed FSI / TFSI anion ionic liquid-based electrolyte (IL1 is [PHM] [FSI] / Salt2 is LiTFSI) compared to conventional solvent-based 1:1 DME:DOL analogue: (IV) voltage vs capacity profile (a) and capacity vs cycle number (b) of LiS cell with typical S cathode [15 wt % CMC, 21 wt % HSAC, 14 wt % C65 and 50 wt % S] in 30 mole% LiTFSI / [Pmi4] [FSI] electrolyte at C / 10 at 50 °C; (II) voltage vs capacity profile (c) and capacity vs cycle number (d) of LiS cell in conventional electrolyte 1 M LiTFSI & 0.2 M LiNOa in 1:1 DME:DOL electrolyte at C / 10 at 50 °C; (B) the effect of a mixed FSI / TFSI ionic liquid-based electrolyte compared to conventional solvent-based analogue (IL 2 [Nma] [FSI] / salt 2: LiTFSI): (IV) voltage vs capacity profile (a) and capacity vs cycle number (b) of LiS cell with typical S cathode (15 wt % CMC, 21 wt % HSAC, 14 wt % C65 and 50 wt % S) in 30 mol% LiTFSI / [Nm3][FSI] IL electrolyte at C / 10 at 50 °C. (II) voltage vs capacity profile (c) and capacity vs cycle number (d) of LiS cell in 1 M LiTFSI and 0.2 M Li NO3 in 1:1 DME:DOL electrolyte at C / 10 at 50 °C.

[0089] Figure 4 illustrates (A) the effect of a low (10 mole%) and medium (30 mole%) salt concentration on the performance of the sulfur cathode in a mixed FSI / TFSI anion phosphonium ionic liquid-based electrolyte (IL1: [Pini4] [FSI] / Salt 2: LiTFSI): (IV) voltage vs capacity profile (a) and capacity vs cycle number (b) of LiS cell in 30 mole% LiTFSI / [Pmi4] [FSI] IL electrolyte at C / 10 at 50 °C; (V) voltage vs capacity profile (c) and capacity vs cycle number (d) of LiS cell with typical S cathode [15 wt % CMC, 21 wt % HSAC, 14 wt % C65 and 50 wt % S] in 10 mole% LiTFSI / [PUM] [FSI] IL electrolyte at C / 10 at 50 °C; (B) the effect of a low (20 mole%) and medium (30 mole%) salt concentration on the performance of the sulfur cathode in a mixed FSI / TFSI anion ammonium ionic liquid-based electrolyte (IL2: [N1113] [FSI ] / Salt 2: LiTFSI): (IV) voltage vs capacity profile (a) and capacity vs cycle number (b) of LiS cell in 30 mol% LiTFSI / [N1113] [FSI] IL electrolyte at C / 10 at 50 °C. (V) voltage vs capacity profile (c) and capacity vs cycle number (d) of LiS cell with a typical S cathode (15 wt % CMC, 21 wt % HSAC, 14 wt % C65 and 50 wt % S) in 20 mol% LiTFSI / [Nm3][FSI] IL electrolyte at C / 10 at 50 °C.

[0090] Figure 5 illustrates analysis of electrochemical performance of sulfur-electrode [15 wt % CMC, 21 wt % HSAC, 14 wt % C65 and 50 wt % S] in [Pmj4] [FSI] electrolyte system in terms of capacity and polarization. Figure 6 illustrates (A) the effect of inclusion of a polyionic liquid (PIL) binder in the cathode composition on the performance of a cell with a mixed FSI / TFSI salt phosphonium IL system (IL1 is [Pmj4] [FSI] / Salt 2 is LiTFSI): (VI) voltage vs capacity profile (a) and capacity vs cycle number (b) of LiS cell in mixed FSI / TFSI system, 10 mole% LiTFSI / [Pmi4] [FSI] IL electrolyte cell with a cathode with PIL [5 wt % [PDADMATFSI] :[LiTFSI], lOwt % CMC, 21 wt % HSAC, 14 wt % C65 and 50 wt % S] at C / 10 at 50 °C; (V) voltage vs capacity profile (c) and capacity vs cycle number (d) of LiS in mixed FSI / TFSI phosphonium system 10 mole% LiTFSI / [PunJ [FSI] IL electrolyte cell with a typical cathode without PIL:Li salt binder mixture [15wt % CMC, 21 wt % HSAC, 14 wt % C65 and 50 wt % S] at C / 10 at 50 °C; (B) the effect of inclusion of a poly ionic liquid (PIL) binder addition on the performance of the sulfur cathode in a mixed FSI / TFSI ammonium anion system (IL2: [NUB] [FSI] / Salt 2: LiTFSI): (V) voltage vs capacity profile (a) and capacity vs cycle number (b) of LiS cell in 20 mol% LiTFSI / [Nuu] [FSI] IL electrolyte cell with S cathode with PIL [5 wt % [PDADMATFSI] :[LiTFSI], lOwt % CMC, 21 wt % HSAC, 14 wt % C65 and 50 wt % S] at C / 10 at 50 °C. (VI) voltage vs capacity profile (c) and capacity vs cycle number (d) of LiS in 20 mol% LiTFSI / [NmaJfFSI] IL electrolyte cell with a typical S cathode without PI L:Li salt binder mixture [15wt % CMC, 21 wt % HSAC, 14 wt % C65 and 50 wt % S] at C / 10 at 50 °C.

[0091] Figure 7 illustrates (A) the effect of poly ionic liquid (PIL) : Li Salt binder mixture addition and different electrolyte salt contents (low 10 mole% v medium 30 mole%) on the performance of the sulfur cathode mixed TFSI / FSI anion phosphonium IL system (IL1 is [Pmi4] [FSI] / Salt 2 is LiTFSI): (VII) voltage vs capacity profile (a) and capacity vs cycle number (b) of LiS cell in 30 mole% LiTFSI / fPuii [FSI] IL electrolyte cell with S cathode with PILI binder [5 wt % [PDADMATFSI] :[LiTFSI], lOwt % CMC, 21 wt % HSAC, 14 wt % C65 and 50 wt % S] at C / 10 at 50 °C; (VI) voltage vs capacity profile (c) and capacity vs cycle number (d) of LiS in 10 mole% LiTFSI / [Pmi4] [FSI] IL electrolyte cell with a cathode with PILI [5 wt % [PDADMA.TFSI]:[LiTFSI], lOwt % CMC, 21 wt % HSAC, 14 wt% C65 and 50 wt% S] at C / 10 at 50 °C; (B) the effect of poly ionic liquid (PIL) binder addition and different electrolyte salt contents (low 20 mole% v medium 30 mole%) on the performance of the sulfur cathode in a mixed FSI / TFSI anion ammonium IL system (IL2: [Nma] [FSI] / Salt 2: LiTFSI): (VII) voltage vs capacity profile (a) and capacity vs cycle number (b) of LiS cell in 30 mol% LiTFSI / [Nu ] [FSI] IL electrolyte cell with a cathode with PILI [5 wt % [PDADMATFSI] :[UTFSI], lOwt % CMC, 21 wt % HSAC, 14 wt % C65 and 50 wt % S] at C / 10 at 50 °C. (VI) voltage vs capacity profile (c) and capacity vs cycle number (d) of LiS in 20 mol% LiTFSI / [NmaHFSI] IL electrolyte cell on a cathode with PILI [5 wt % [PDADMATFSI] :[LiTFSI], lOwt % CMC, 21 wt % HSAC, 14 wt % C65 and 50 wt % S] at C / 10 at 50 °C.

[0092] Figure 8 illustrates (A) the effect of BNNT addition on the performance of the sulfur cathode in a medium concentration (30 mole%) mixed FSI / TFSI anion phosphonium IL system (IL1 is [Pmu] [FSI] / Salt 2 is LiTFSI / binder is PDADMA / CMC): (VIII) voltage vs capacity profile (a) and capacity vs cycle number (b) of LiS cell in 30 mole% LiTFSI / [Puii [FSI] IL electrolyte cell with a S cathode with PIL : Li salt binder mixture and BNNT [5 wt % [PDADMATFSI] :[LiTFSI], 9.5wt % CMC, 0.5 wt % BNNT, 21 wt % HSAC, 14 wt % C65 and 50 wt % S] cycled at C / 10 at 50 °C; (VII) voltage vs capacity profile (c) and capacity vs cycle number (d) of LiS in 30 mole% LiTFSI / [PUM] [FSI] IL electrolyte cell with S cathode with PIL: Li Salt binder mixture but no BNNT [5 wt % [PDADMA.TFSI] :[UTFSI], lOwt % CMC, 21 wt % HSAC, 14 wt % C65 and 50 wt % S] at C / 10 at 50 °C; (B) the effect of BNNT addition on the performance of the sulfur cathode in a medium concentration (30 mole%) mixed FSI / TFSI anion ammonium IL mixed FSI / TFSI salt IL system (IL2: [Nma] [FSI] / Salt 2: LiTFSI / binder: PDADMA / CMC): (VIII) voltage vs capacity profile (a) and capacity vs cycle number (b) of LiS cell in 30 mol% LiTFSI / [NUB] [FSI] IL electrolyte cell with PIL but with BNNT [5 wt % [PDADMATFSI] : [LiTFSI], 9.5wt % CMC, 0.5 wt % BNNT, 21 wt % HSAC, 14 wt % C65 and 50 wt % S] at C / 10 at 50 °C. (VIII voltage vs capacity profile (c) and capacity vs cycle number (d) of LiS in 30 mol% LiTFSI in [NUB] [FSI] IL electrolyte cell with PIL but without BNNT [5 wt % [PDADM ATFSI] :[LiTFSI], lOwt % CMC, 21 wt % HSAC, 14 wt % C65 and 50 wt % S] at C / 10 at 50 °C.

[0093] Figure 9 illustrates (A) the effect of a low (10 mole%) and a high concentration (50 mole%) electrolyte salt content on the performance of a PIL binder-containing sulfur cathode in a single FSI anion phosphonium IL system (IL1 is [Pmi4] [FSI] / Saltl is LiFSI / binder is PDADMA / CMC): (X) voltage vs capacity profile (a) and capacity vs cycle number (b) of LiS cell with an S cathode with PILI [5 wt % [PDADMA.TFSI] :[LiTFSI], lOwt % CMC, 21 wt % HSAC, 14 wt % C65 and 50 wt % S] in 50 mole% LiFSI / [Pnii4] [FSI] IL electrolyte at C / 10 at 50 °C; (IX) voltage vs capacity profile (c) and capacity vs cycle number (d) of LiS cell with S cathode with PIL : Li salt binder mixture [5 wt % [PDADMA.TFSI] :[UTFSI], lOwt % CMC, 21 wt % HSAC, 14 wt % C65 and 50 wt % S] in 10 mole% LiFSI / [Pmi4] [FSI] IL electrolyte at C / 10 at 50 °C; (B) the effect of a low (20 mole%) and a high concentration (50 mole%) electrolyte salt content on the performance of the PIL binder-containing sulfur cathode in a single FSI anion ammonium IL system (IL2: [Nma] [FSI] / Salt 1: LiFSI / binder: PDADMA / CMC): (X) voltage vs capacity profile (a) and capacity vs cycle number (b) of LiS cell in 50 mol% LiFSI / [NIH3][FSI] IL electrolyte at C / 10 at 50 °C. (IX) voltage vs capacity profile (c) and capacity vs cycle number (d) of LiS cell with S cathode including PILI (5 wt % [PDADMA.TFSI] : [LiTFSI], lOwt % CMC, 21 wt % HSAC, 14 wt % C65 and 50 wt % S) in 20 mol% LiFSI / [Nm3][FSI] IL electrolyte at C / 10 at 50 °C.

[0094] Figure 10 illustrates (A) the effect of a low (10 mole%) and a high concentration (50 mole%) (LiTFSI) electrolyte salt content on the performance of the PIL binder-containing sulfur cathode in a mixed FSI / TFSI anion phosphonium IL system with a S cathode with PILI (IL1 is [Pmu] [FSI] / Salt2 is LiTFSI / binder is PDADMA / CMC); (VI) voltage vs capacity profile (a) and capacity vs cycle number (b) of LiS cell with cathode with PIL: Li salt binder mixture [5 wt % [PDADMA.TFSI] :[UTFSI], lOwt % CMC, 21 wt % HSAC, 14 wt % C65 and 50 wt % S] in 10 mole% LiTFSI / [Pmi4] [FSI] at C / 10 at 50 °C; (XI) voltage vs capacity profile (c) and capacity vs cycle number (d) of LiS cell with and the same S cathode with PIL [5 wt % [PDADMA.TFSI]:[LiTFSI], lOwt % CMC, 21 wt % HSAC, 14 wt % C65 and 50 wt % S] in 50 mole% LiTFSI / [Pnii [FSI] electrolyte at C / 10 at 50 °C; (B) the effect of a low (20 mole%) and a high concentration (50 mole%) LiTFSI electrolyte salt content on the performance of the PILI binder-containing sulfur cathode in a mixed FSI / TFSI ammonium anion IL system (IL2: [Nma] [FSI ] / Salt 2: LiTFSI / binder: PDADMA / CMC): (XI) voltage vs capacity profile (a) and capacity vs cycle number (b) of LiS cell in 50 mol% LiTFSI / [Nma] [FSI] FSI IL electrolyte at C / 10 at 50 °C. (VI) voltage vs capacity profile (c) and capacity vs cycle number (d) of LiS cell with a S cathode with PILI [5 wt % [PDADMA.TFSI] :[UTFSI], lOwt % CMC, 21 wt % HSAC, 14 wt % C65 and 50 wt % S] in 20 mol% LiTFSI / Puii4FSI IL electrolyte at C / 10 at 50 °C.

[0095] Figure 11 illustrates analysis of electrochemical performance (discharge capacity and cell polarisation) of a S cathode with PIL:Li Salt binder mixture [5 wt % [PDADMA.TFSI]:[UTFSI], 10wt% CMC, 21 wt% HSAC, 14 wt% C65 and 50 wt% S] in various salt concentration [Pmi4] [FSI] electrolytes for a range of up to 40 discharge cycles. Figure 12 illustrates an analysis of PIL vs non-PIL binder wrt electrochemical performance (discharge capacity and cell polarisation) of cells with a sulfur-electrode with PIL : Li Salt binder mixture [5 wt % [PDADMA.TFSI] :[LiTFSI], lOwt % CMC, 21 wt % HSAC, 14 wt % C65 and 50 wt % S] vs S cathode without PIL binder [15 wt % CMC, 21 wt % HSAC, 14 wt % C65 and 50 wt % S] in [Pmi4] [FSI] electrolyte system. Figure 13 illustrates optical images of solution of neat-IL with 0, 30, 40, 45, and 50 mole% LiTFSI / [Pmi4] [FSI] electrolyte before (clear colour) and after addition of Ss and LizS, taken 24h after addition and after 3- months rest, where for solutions with at least 30 mole% of Li salt, the solutions remain colourless indicating ultralow polysulfide formation / dissolution in solution, even after 3 months rest. The dark red colour is in the neat IL without salt is due to formation of higher order polysulfides, LizSn, LiaSn(4 < n < 8).

[0096] Figure 14 illustrates visual depiction of solutions used in Raman analysis, including studies in neat and 30 mole% LiTFSI in [Pnii4] [FSI] and 30 mole% LiTFSI in [Nm3][FSI].

[0097] Figure 15 illustrates Raman Spectroscopy of system involved in polysulfide dissolution test: (a) Li3S+S in [Pnii4] [FSI] showing strong PS peaks; (b) LiaS+S in 30 mole% LiTFSI / [Pmi4] [FSI] with undetectable PS peaks; (c) 30 mole% [Li] [TFSI] in [Nu ] [FSI]; (d, e and f) Zoomed view of (a, b, and c) respectively; (g) LiaS+S in 30 mole% [Li] [TFSI] in [N1113] [FSI]; (h) 30 mole% [Li] [TFSI] in [N1113] [FSI]; (I and j) are the zoomed view of (g and h). P-X represents the percentage of LAZER power utilized during Raman Spectroscopy experiment. The Raman shift due to polysulfide is at positions 437 and 474 cm1. Due to the ultralow / negligible PS solubility in the electrolytes of the invention, it is evident that any PS present are ultralow concentrations that are lower than the limit of quantification, if not lower than the limit of detection of the high power (P- 100) Raman technique.

[0098] Figure 16 illustrates optical images of solutions of [Pmi4][FSI] with 5, 10, or 20 mole% LiTFSI, before and after Ss+LijS addition. It is clear that PS immediately dissolve and remain dissolved for at least 24 - 48 hours in the neat and 5 mole% to 20 mole% LiTSI / [PUM] [FSI] electrolyte. This is in contrast to the lack of PS dissolution visibly observable for the 30 mole% LiTFSI / [Pmi4] [FSI] system of Figure 15.

[0099] Figure 17 illustrates UV spectroscopy analysis of polysulfides solubility in [Pmi4] [FSI] and in 30 mole% LiTFSI / [Pnii4] [FSI], showing the saturated solubility at 25 °C is much less than 1.25 mm atomic S for the 30 mole% LiTFSI / [Pmi4][FSI] electrolyte.

[0100] Figure 18 illustrates cell impedance analysis via comparison of Nyquist plots which were obtained for different Li-S cells containing [Pmi4] [FSI] electrolyte system with different Li-salt concentrations, (a) 25 mole% LiTFSI and 25 mole% Li FSI, (b) 50 mole% LiFSI, (c) 15 mole% LiTFSI and 15 mole% LiFSI, and (d-e) 30 mole% LiTFSI.

[0101] Figure 19 which illustrates a cell overpotential analysis carried out by galvanostatic cycling stability study from the 5thto 15thcycle of a Li / S cell with microporous carbon-sulfur cathode with PIL:Li salt composite binder and CMC (5PD10CMC) with a sulfur loading of 2 mg cm'2at 1 / 10 C (1C = 1672 mA / g), where the cell uses the 30 mole% LiTFSI / [Pmi4] [FSI] electrolyte of the invention.

[0102] Figure 20 illustrates Raman Spectroscopy analysis to compare the solubility limit of polysulfides of 30 mole% LiTFSI / PUMFSI electrolyte with (a) 2.5 mM S in PH^FSI (b) 1.25 mM S in PHMFSI, and (c) 30 mole% LiTFSI / PHMFSI when saturated with LizSs (d) 30 mole% LiTFSI / PH FSI blank electrolyte (e, f, g and h) Zoomed view of (a, b, c and d) respectively. The tests were carried out at power of P (100%) which is a very high sensitivity. The Raman shift due to dissolved Polysulfide positions is 437 and 474 cm1. As can be seen from the studies the bands corresponding to dissolved polysulfides are smaller than those for 1.25 mM S in [Pmi4] [FSI] indicating a lower saturated solubility for PS than 1.25 mM. Figure 21 illustrates evidence of favourably controlled and stable interface formed in the Li-S cell in the electrolyte systems of the invention: (IV) EIS data (a) and (b) of LiS cell in S cathode without PIL: Li Salt binder mixture (15 wt % CMC, 21 wt % HSAC, 14 wt % C65 and 50 wt % S) in preferred 30 mole% LiTFSI / PHMFSI mixed FSI / TFSI anion IL electrolyte at OCV and after lithiation; (VI) EIS data (c) and (d) of LiS cell with S cathode without PIL (15 wt % CMC, 21 wt % HSAC, 14 wt % C65 and 50 wt % S) in preferred 50 mole% Li FSI / PH FSI single FSI anion IL electrolyte at OCV and after lithiation; (B) Electrochemical Impedance spectroscopy data (a) and (b) of LiS cell with S cathode without PIL (15 wt % CMC, 21 wt % HSAC, 14 wt % C65 and 50 wt % S) in preferred 30 mol% LiTFSI / PHMFSI mixed FSI / TFSI anion IL electrolyte at OCV and after lithiation. Electrochemical Impedance spectroscopy data (c) and (d) of LiS cell with S cathode with PIL [5 wt % [PDADMATFSI] : [LiTFSI], lOwt % CMC, 21 wt % HSAC, 14 wt % C65 and 50 wt % S) in 50 mol% Li FSI / PHMFSI single FSI anion IL electrolyte at OCV and after lithiation. The results show that regardless so of the electrolyte composition used, after the 1st lithiation of S-electrode, the cell resistance decreases significantly, which might be attributed to the favourable modification of the sulfur-electrode and electrolyte interface. It is believed that the interface modification is due to the good SEI formation capability of the electrolytes through the electrochemical reduction of electrolyte components during the initial cycle. After the lithiation of the sulfur electrodes, the cell resistance is stable. In Figure 21(e), the cell resistance after cycling to the 30th cycle shows no significant change in cell resistance, which is believed to be due to the high quality and stable interface of the Li-S cells formed and maintained in the electrolyte systems of the invention. In particular, the sulfur electrode and its electrolyte interface is very stable. The stable Interface depicts that there is an almost negligible parasitic reaction occurs at the Sulfur-electrode electrolyte interface.

[0103] Figure 22 illustrates that cells using the electrolyte systems of the invention are substantially self-discharge free: Voltage vs time profile of LiS cell [5 wt % [PDADMA.TFSI] :[LiTFSI], 10 wt % CMC, 21 wt % HSAC, 14 wt % C65 and 50 wt % S] resting at OCV (open circuit voltage) at 50 °C, (a) in 30 mol% LiTFSI / PHMFSI IL electrolyte (mixed TFSI / FSI anion system), (b) in 50 mol% LiFSI in PH FSI IL electrolyte (singe FSI anion system), (c) in 30 mol% LiTFSI in NmsFSI IL electrolyte (mixed TFSI / FSI anion system), and (d) in 50 mol% LiFSI in NmaFSI IL electrolyte (single FSI anion system). As further evidence for the ultralow / negligible polysulfide dissolution, preferred cells of the invention are substantially self-discharge free, as demonstrated, via an open circuit voltage (OCV) of the cell which varies by no more than ± 0.1 V / hour.

[0104] Figure 23 illustrates the effect of another polyionic liquid : Li salt binder mixture (PIL-2: [PMTFSI-Li] : [LiTFSI] ) addition on the performance of the sulfur cathode (IL1: Pii FSI / Salt 2: LiTFSI) in the mixed anion electrolyte. Voltage vs capacity profile (a) and capacity vs cycle number (b) of LiS cell in 30 mole% LiTFSI / PH FSI IL electrolyte cell using S cathode with PIL-2 [15 wt % [PMTFSI-LI] :[LiTFSI], 21 wt % HSAC, 14 wt % C65 and 50 wt % S] at C / 10 at 50 °C; voltage vs capacity profile (c) and capacity vs cycle number (d) of LiS in 30 mol% LiTFSI / PHMFSI IL electrolyte cell using S cathode without PIL 2 (15wt % CMC, 21 wt % HSAC, 14 wt % C65 and 50 wt % S) at C / 10 at 50 °C.

[0105] Figure 24 illustrates a further example of a PIL which is a PBC (PILBLOCK co-polymer, PLmTFSI) in a further PIL : Li Salt binder mixture (PIL-3).

[0106] Figure 25 illustrates the effect of PIL-3 (PIL-3 [PILBIockCo-polymer (plm.TFSI )]: [LiTFSI ] ) binder addition on the performance of the sulfur cathode in a mixed anion electrolyte system (IL1: Puii FSI / Salt 2: LiTFSI): voltage vs capacity profile (a) and capacity vs cycle number (b) of LiS cell in 30 mol% LiTFSI / PHMFSI IL electrolyte cell [15 wt % [PILBIock Co-polymer(plmTFSI)]:[LiTFSI], 21 wt % HSAC, 14 wt % C65 and 50 wt % S] at C / 10 at 50 °C. Voltage vs capacity profile (c) and Capacity vs cycle number (d) of LiS in 30 mol% LiTFSI / Pnii4FSI IL electrolyte cell with S cathode without PIL-3 [15wt % CMC, 21 wt % HSAC, 14 wt % C65 and 50 wt % S] at C / 10 at 50 °C.

[0107] Figure 26 illustrates the effect of different cations (IL1: [Pmi4 FSI] / IL3: [C3mpyr][FSI]) on the performance of the sulfur cathode (Saltl: Li FSI ) : voltage vs capacity profile (a) and capacity vs cycle number (b) of LiS cell in single anion system 30 mole% LiFSI / [Pmi ] [FSI] IL electrolyte at C / 10 at 50 °C. Voltage vs capacity profile (c) and capacity vs cycle number (d) of LiS cell [15 wt % CMC, 21 wt % HSAC, 14 wt % C65 and 50 wt % S] in 30 mol% LiFSI / [C3mpyr][FSI] IL electrolyte at C / 10 at 50 °C.

[0108] Figure 27 illustrates the effect of different cations (ILl[Pmi4 FSI] / [C3mpyr] [FSI]) on the performance of the sulfur cathode (salt2 LiTFSI): Voltage vs capacity profile (a) and Capacity vs cycle number (b) of LiS cell in mixed anion system 30 mol% LiTFSI / PUM FSI IL electrolyte at C / 10 at 50 °C. Voltage vs capacity profile (c) and capacity vs cycle number (d) of LiS cell (15 wt % CMC, 21 wt % HSAC, 14 wt % C65 and 50 wt % S) in 30 mol% LiTFSI / [C3mpyr][FSI] IL electrolyte at C / 10 at 50 °C.

[0109] Figure 28 illustrates the effect of different salts (Li FSI / LiTFSI) on the performance of the sulfur cathode (IL3: [C3mpyr] [FSI]) : voltage vs capacity profile (a) and capacity vs cycle number (b) of LiS cell in single anion system 30 mol% LiFSI / C3mpyrFSI IL electrolyte at C / 10 at 50 °C; Voltage vs capacity profile (c) and capacity vs cycle number (d) of LiS cell [15 wt % CMC, 21 wt % HSAC, 14 wt % C65 and 50 wt % S] in mixed anion system 30 mol% LiTFSI / [C3mpyr][FSI] IL electrolyte at C / 10 at 50 °C. The results show that the [C3mpyr][FSI] based electrolyte utilising 30 mole% LiFSI or LiTFSI salts are not suitable for cycling stability of Li-S cells. Higher salt contents are required for cycling stability.

[0110] Figure 29 illustrates the effect of salt concentration (50 mole% v 30 mole%) on the performance of the sulfur cathode (IL3: [C3mpyr][FSI] / Salt 1: LiFSI): Voltage vs capacity profile (a) and capacity vs cycle number (b) of LiS cell in 50 mol% LiFSI / C3mpyrFSI IL electrolyte at C / 10 at 50 °C. Voltage vs capacity profile (c) and capacity vs cycle number (d) of LiS cell [15 wt % CMC, 21 wt % HSAC, 14 wt % C65 and 50 wt % S] in 30 mol% LiFSI / [C3mpyr][FSI] IL electrolyte at C / 10 at 50 °C. Results show that the 50 mole% system works better than the 30 mole% system for this IL cation, particular for the single FSI anion system.

[0111] Figure 30 illustrates the effect of high salt concentration (50 mole%) and different cations (IL1 [PHM FSI] / IL: 3 [C3mpyrFSI]) on the performance of the sulfur cathode in a single anion system (Saltl: LiFSI): Voltage vs capacity profile (a) and capacity vs cycle number (b) of LiS cell in single anion system 50 mole% LiFSI / Pnii4 FSI IL electrolyte at C / 10 at 50 °C. Voltage vs capacity profile (c) and capacity vs cycle number (d) of LiS cell [15 wt % CMC, 21 wt % HSAC, 14 wt % C65 and 50 wt % S] in single anion system 50 mole% LiFSI / C3mpyrFSI IL electrolyte at C / 10 at 50 °C. The results show that the phosphonium cation system gives the best S utilisation, and capacity retention in the longer cycling tests. Figure 31 illustrates the effect of salt concentration on the performance of the sulfur cathode (IL3: C3mpyrFSI / Salt 1: LiTFSI) : voltage vs capacity profile (a) and capacity vs cycle number (b) of LiS cell in mixed anion system 50 mole% LiTFSI / C3mpyrFSI IL electrolyte at C / 10 at 50 °C. Voltage vs capacity profile (c) and capacity vs cycle number (d) of LiS cell [15 wt % CMC, 21 wt % HSAC, 14 wt % C65 and 50 wt % S] in mixed anion system 30 mole% LiTFSI / [C3mpyr][FSI] IL electrolyte at C / 10 at 50 °C.

[0112] Figure 32 illustrates the effect of high concentration different salts (Li FSI / LiTFSI ) on the performance of the sulfur cathode (IL3: C3mpyrFSI): Voltage vs capacity profile (a) and capacity vs cycle number (b) of LiS cell in single anion system 50 mole% LiFSI / [C3mpyr][FSI] IL electrolyte at C / 10 at 50 °C. Voltage vs capacity profile (c) and capacity vs cycle number (d) of LiS cell [15 wt % CMC, 21 wt % HSAC, 14 wt % C65 and 50 wt % S] in mixed anion system 50 mole% LiTFSI / [C3mpyr][FSI] IL electrolyte at C / 10 at 50 °C. Results show that the 50 mole% system works slightly better in the single FSI anion system than the mixed TFSI / FSI anion system.

[0113] Figure 33 illustrates the effect of high salt concentration (50 mole%) and different cations (phosphonium v imidazolium) in a (ILl[Pmi4 FSI] / [EMImFSI]) on the performance of a sulfur cathode (saltl LiFSI) in a single FSI anion system: (VI) voltage vs capacity profile (a) and capacity vs cycle number (b) of LiS cell in 50 mol% LiFSI / Pnii4 FSI IL electrolyte at C / 10 at 50 °C. Voltage vs capacity profile (c) and capacity vs cycle number (d) of LiS cell with typical S cathode [15 wt % CMC, 21 wt % HSAC, 14 wt % C65 and 50 wt % S] in 50 mol% LiFSI in [EMIm] [FSI] IL electrolyte at C / 10 at 50 °C. The results show that the imidazolium cation ionic liquid system even at 50 mole% LiFSI is not suitable for long term cycling.

[0114] Figure 34 illustrates the effect of different salts (Li FSI / LiTFSI ) on the performance of the PIL bindercontaining sulfur cathode (IL2: [NUB] [FSI] / binder: PDADMA / CMC): (VI) voltage vs capacity profile (a) and capacity vs cycle number (b) of LiS cell with S cathode with PILI [5 wt % [PDADMA.TFSI]:[LiTFSI], lOwt % CMC, 21 wt % HSAC, 14 wt % C65 and 50 wt % S] in 50 mol% LiFSI / [Nm3][FSI] IL electrolyte at C / 10 at 50 °C. (VI) voltage vs capacity profile (c) and capacity vs cycle number (d) of LiS cell with the same S cathode with PILI [5 wt % [PDADMA.TFSI] :[LiTFSI], lOwt % CMC, 21 wt % HSAC, 14 wt % C65 and 50 wt % S] in 50 mol% LiTFSI / [NUB] [FSI] FSI IL electrolyte at C / 10 at 50 °C. The results show similar performance at the 1stand 2ndcycles, with the single FSI anion system showing a slight benefit at the 20thcycle.

[0115] Figure 35 illustrates physiochemical properties of the mixed anion based ionic liquid electrolytes, and density and viscosity measurements showing that for the same concentration of salt, NimFSI electrolytes are denser and more viscous than their PHMFSI counterparts and that density and viscosity increases with increasing salt concentration.

[0116] Detailed Description of the Invention The invention provides a liquid electrolyte which is suitable for an lithium-sulfur (Li-S) cell, which means, on using the electrolyte in a Li-S cell, the cell preferably discharges in a way that exhibits apparent quasi solid-state redox behaviour which is identified by a discharge curve of the cell that involves an apparent direct, one step, Ss to low order lithium sulfides (LiaSa / LiaS) solid to solid conversion reaction which is confined to a S cathode of the cell. The prior art Li-S cells that use electrolytes with low solubilising power for polysulfides, when operated under similar conditions, discharge in a way where their discharge curve exhibit two plateaus, indicating some PS dissolution occurs in the electrolyte. The inventors believe that in the case of the ultralow, zero or negligible PS solubility electrolytes of the invention, exhibit a discharge curve, at least under the considered cell operating parameters and conditions, that appears to suggest, that mechanistically, the redox process for discharge predominantly / substantially involves a direct solid-state conversion of Ss to low order lithium sulfides ( LizSa / LizS) . The inventors believe that it may be possible that this conversion might still involve at least an initial very fast polysulfide dissolution step at the start of the first discharge cycle, but that the ultralow or negligible PS solubilising power and desirable film forming property of the electrolytes, means that such initial step (to the degree it may be occur) is very fast, and is confined by a surface film of the cathode formed early on during initial discharge cycle. In other words, at the start of the discharge reaction, it may be possible that initially only very trace amounts of high order polysulfides are formed within the cathode, but which are not solubilised in the solution phase, but which are rapidly converted to low order polysulfides within the cathode, whereas after sufficient film formation on the cathode (usually early in the first cycle), only lower order polysulfides (LizS / U2S) are formed in a direct Ss to low order lithium sulfides ( LiaSa / LiaS) conversion reaction. This means that for the electrolytes of the invention, PS do not dissolve into the bulk electrolyte. Further detailed and demanding operando mechanistic studies of the S cathode are required to fully understand the presence or absence of a rapid conversion step as described. However, regardless, the net effect is not differentiated from the direct solid-solid conversion reaction in the presented herein by the electrochemical testing data. Regardless, cells exhibiting such one step quasi solid-state redox behaviour are preferred for a Li-S cell as the one voltage plateau feature essentially signifies the complete avoidance or at least almost complete avoidance of formation of intermediate polysulfide species that are associated with the polysulfide redox shuttle experienced in typical Li-S cells that use conventional organic solvent-based electrolytes, as well as the prior art IL systems 0.64 M LiTFSI / [DEME]-[TFSI] and 0.5 mol. kg1Li[TFSI] in [C3mpyr] [TFSI] . Another advantage of the ultralow or negligible polysulfide species solubility in the electrolytes of the invention is that the FSI decomposition observed by Watanabe's 0.5 mol / kg1FSI IL electrolyte to lead to a poor quality film and catastrophic cycling is much better controlled by use of the electrolytes of the invention, such that the process is faster and / or cleaner and results in a much better, higher quality functioning surface film (i.e., more protective and more Li conducting than Watanbe's film) As a result the problems encountered by the prior art system do not appear to be an issue when using the electrolytes of the invention. This is a potential further advantage of the much more limited amount of polysulfides solubilised in the electrolyte of the invention and is likely to avoid the vigorous FSI reduction by PS observed in the prior art system. Typical cells and electrolyte compositions are described above in the first to eight aspects. Predominantly [FSI]- anions respect of the ionic liquid anion component means of the total electrolyte composition, the majority of anions present are [FSI]- anions. This means however, small quantities of other ionic liquid anions can also be present as long as they are (on their own or in total in the case of a mixture of other IL anions) in the minority to the FSI anions.

[0117] Predominantly [FSI]- anions respect of the Li salt component means of the total Li salt component present, the majority of anions present / contributed from the IL are [FSI]- anions. This means however, small quantities of other Li salt anions can also be present as long as they are (on their own or in total in the case of a mixture of other Li salt anions) in the minority to the FSI anions from the Li salt.

[0118] Predominantly [TFSI]- anions respect of the Li salt component means of the total Li salt component present, the majority of anions present / contributed from the salt are [TFSI]- anions. This means however, small quantities of other Li salt anions can also be present as long as they are (on their own or in total in the case of a mixture of other Li salt anions) in the minority to the TFSI anions from the Li salt. Predominately means more than 50%, more than 55%, more than 60%, more than 65%, more than 70%, more than 75%, more than 80%, more than 85%, more than 90%, or in some cases more than 95%, of the anions of the respective component.

[0119] The other anions, if present, can be selected from one or more of [BF ]’, [OTf]-, lithium perchlorate (LiCIO i), lithium dicyanamide (LiDCA), lithium cyanate (LiOCN), lithium thiocyanate (LiSCN), lithium N- cyanoamide (TFSAM), lithium 2,2,2-trifluoro- / V-(trifluoromethylsulfonyl) acetamide (TSAC), lithium nonafluorobutanesulfonate (NF), lithium carborane, lithium borate salts, lithium difluoro(oxolato)borate and combinations thereof. Preferred lithium borate salts are disclosed in Physical Chemistry Chemical Physics 25.40 (2023): 27718-27730, and Advanced Energy Materials 11.36 (2021): 2101422, the contents of which are hereby incorporated by reference. In some examples, predominately means 100% of the anions are the stated anion for the component in question. In other words, no additional anions to FSI are present, and no additional anions to FSI or TSFI from the Li salt are present. Desirably, the Li+conductive ionic liquid electrolyte, at room temperature of 25 °C, the Li+conductive ionic liquid electrolyte comprises a concentration of one or more, and indeed preferably all of the individual species: S42-(detectable at 420 nm with UV spectroscopic analysis), Sg2“ (detectable at 450-470 nm with UV spectroscopic analysis), Sg2-at (detectable at 490-500 nm with UV spectroscopic analysis) that is less than 1 mM in atomic S, or zero / negligible (not detectable), that is, in that the concentration present (if present at all) is lower than the limit of quantification and more preferably still lower than the limit of detection of the UV-vis spectroscopy technique used. That is, each polysulfide species, to the event it is at all present under saturated conditions in electrolyte solution at 25°C, is present in an amount that is less than 1 mM atomic S, or is at a concentration of atomic S that is below the limit of quantification (LOQ) or preferably below the limit of detection (LOD) of the UV spectroscopic technique for the particular individual or more preferable the total polysulfide species under consideration. Suitably, this is with respect to UV analysis at room temperature, and at least for saturated solutions in the electrolyte as prepared at 50 °C or under any other conditions that would ensure formation of a saturated solution as described elsewhere herein or under any other conditions that would ensure formation of a saturated solution. In preferred examples, the total polysulfide (Li2Sn, 4 < n < 8) species solubility measured at room temperature of 25 °C is lower than 1 mM atomic S, or is zero or is at least below the limit of detection of UV-vis spectroscopy for an individual or total LizSn (4 < n < 8) in terms of atomic S concentration.

[0120] The electrolytes of the invention are convenient to prepare. The electrolytes solutions are prepared by direct addition of the Li salt ( Li FSI / LiTFSI) to the dried ionic liquid and dissolved with stirring and heat (24h at 50 °C). Preparation is performed in an Argon glovebox (O2 < 0.1 ppm and H2CX 0.1 ppm), the ionic liquid and Li salts are dried on a Schlenk line at 50 °C, 0.6 kPa for 72 h. The obtained clear solutions are stored under argon at RT. The mole % of the Li+salt in the Li+conductive ionic liquid electrolyte may be determined by considering the mole fraction of the Li+salt component in the Li+conductive ionic liquid electrolyte relative to the total moles of all components in the electrolyte and converting that mole fraction to a percentage. For example, a 30 mole% LiTFSI / [PUM] [FSI] electrolyte has 30 moles of LiTFSI and 70 moles of [Pnii4] [FSI] present in the composition to a total of 100 moles. Therefore, 30 moles of LiTFSI (Mwt is 287.08 g / mole) and 70 moles of [Pmi4] [FSI] (Mwt is 310.173 g / mol) is equivalent to 1.38 mol / kg1LiTFSI in [Pnii4] [FSI] .

[0121] A room temperature ionic liquid (RTIL) is a salt consisting of an organic cation and an organic or inorganic anion, whose melting temperature falls below a conventional limit of 100°C.

[0122] In preferred examples, the electrolyte involves predominantly an Li FSI salt, that is, for a single [FSI]- anion IL electrolyte where the anion of the Li+salt is [FSI]-, the LiFSI salt is preferably present in an amount of from about 20 mole% to about 60 mole% of the electrolyte. More preferably, the LiFSI salt is present in an amount of from about 30 mole% to about 55 mole% of the electrolyte. Most preferably the LiFSI salt is present in an amount of from about 45 mole% to about 55 mole% of the electrolyte, most preferably around the 50 mole% concentration, that is, about 45 mole%, about 46 mole%, about 47 mole%, about 48 mole%, about 49 mole%, about 50 mole%, about 51 mole%, about 52 mole%, about 53 mole%, about 54 mole%, or about 55 mole%. In preferred embodiments, the LiFSI salt is present in an amount of from about 47 mole% to about 53 mole% of the electrolyte, most preferably about 50 mole%. About in this context means ±1% of the stated value.

[0123] In alternative preferred electrolyte involves predominantly a LiTFSI salt, that is, for a mixed [FSI] / [TFSI ]" anion electrolyte, wherein the anion of the Li+salt is [FSI]-, the LiTFSI salt is present in an amount of from about 20 mole% to about 60 mole% of the electrolyte. In preferred examples, the LiTFSI salt is present in an amount of from about 20 mole% to 40 mole% of the electrolyte. Most preferably the LiTFSI salt is present in an amount of from about 25 mole% to about 35 mole% of the electrolyte, most preferably around 30 mole% concentration, that is, about 25 mole%, about 26 mole%, about 27 mole%, about 28 mole%, about 29 mole%, about 30 mole%, about 31 mole%, about 32 mole%, about 33 mole%, about 34 mole%, or 35 mole%. In preferred embodiments, the LiTFSI salt is present in an amount of from about 27 mole% to about 33 mole% of the electrolyte, most preferably about 30 mole%. About in this context means ±1% of the stated value.

[0124] Preferred cells of the invention using the electrolytes of the invention are those in which conductivity, Li transport etc. are optimised through use of the electrolytes of the invention. For example, the Li metal and S electrode surface film conductivity is sensitive to Li salt concentration and composition and for any electrolyte system should be optimised for low polarisation and stability of the polarisation magnitude with respect to cycling, as has been demonstrated herein for the preferred cells of the examples. That is, the person skilled in the art will recognise that the specific concentration within the stated range must be tuned to the particular S cathode compositions used, to obtain the optimum interfacial properties. This means that the electrolyte concentration may vary slightly within the stated subset of concentration ranges for any specific cathode design and composition and must be tuned to obtain the minimum polarisation and optimal stability. For example, for the S cathodes exemplified herein, the particularly useful LiFSI and LiTFSI concentrations for the S cathodes described herein are about 50 mol% (single anion system) and 30 mol% (mixed anion system) respectively. For the particular S cathode described herein, cells comprising these S cathodes and the preferred salt concentrations have been found to achieve an optimal balance of Li transport, Li metal cycling, low PS solubility and cathode and anode SEI resistance and stability where the respective about 50 mol% (single anion system) and 30 mol% (mixed anion system) electrolytes are used. The skilled person can readily screen the electrolytes of the invention to determine good and the very best salt concentration ranges and / or IL cations for any given S cathode used. For example, the screening may involve analysing a series of salt concentrations to determine the best S utilisation after a predetermined number of cycles. Also, as shown in the examples described herein, cell polarisation studies can be carried out by determining the different potentials at the 50% SoC point during discharge and the corresponding charge potential at C / 10 using the cells described here) and preferred electrolytes will maintain cell polarisation below 400 mV and with less than 20 % change or less between the 1stand 20thcycles, and 3 % or less between 20thand 40thCycle, for example.

[0125] If required, the electrolyte of the invention may comprise one or more cosolvents as diluent, preferably DME. Such diluents can be used to reduce the viscosity of the electrolyte. If desired. This may be useful for cells that involve low temperature operating conditions or electrolytes having Li salt concentrations of above around 50 / 55 mole% salt content. Depending on the target cell operating conditions, e.g., temperature, no diluent may be required. Different IL cations and salt anions result in different viscosity of solution. Need for diluent may also depend on the IL cation and / or the salt anion used. Preferred electrolyte viscosities are in the range of 50 mPa.s to 65 mPa.s as measured by micro viscometer at a temperature of about 50°C. (See Figure 35). Diluents may be used to adjust the viscosity to within this range. Suitably, diluents include DME, sulfolane, DOL, TEGDME, acetonitrile, BTFE, DMSO, 1, 1,2,2- Tetrafluoroethyl 2,2,2-trifluoroethyl ether (TFEFE), 1,1,2,2-Tetrafluoroethyl 2,2,2-trifluoroethylether, Bis(2,2,2-trifluoroethyl) ether [BTFE], 1,2-Difluorobenzene (DFB), Fluorobenzene and hydrofluoroether (HFE). The amount of diluent can range from 5 to 90 wt% of the total electrolyte composition, preferably from 5 to 35 wt% of the total electrolyte composition, most preferably from 20 - 30 wt%.

[0126] Preferred electrolytes have an ionic conductivity of between 1 and 10 mS / cm at 50 °C. (See Figure 35).

[0127] Desirably, the IL is an aprotic IL. Preferably, the RTIL cation component of the RTIL ionic liquid of the Li+conductive ionic liquid electrolyte of the invention comprises one or more of a phosphonium cation, an ammonium cation, a sulfonium cation, a pyrrolidinium cation, an imidazolium cation, pyridinium cation, or combinations thereof. In some examples, a phosphonium cation or an ammonium cation is preferred. In examples, the RTIL cation component may be an alkyl phosphonium cation, an alkyl ammonium cation, an alkyl sulfonium cation, an alkyl pyrrolidinium cation, an alkyl imidazolium based cation, an alkyl pyridinium cation or combinations thereof. In some examples, a quaternary phosphonium cation or a quaternary ammonium cation is preferred. In some examples, it is preferred that the RTIL cation component is free from oxygen atoms, and in particular is free from ether and / or alkoxyl functionalities, in particular it is preferred that the IL cation is not [DEME]+cation (N, / -diethyl-A / -methyl-A / -(2- methoxyethyl)ammonium). In some examples, it is preferred that the IL cation is not [C3mpyr]+. In some examples, the IL is not LiTFSI / [DEME]-[TFSI] and or LifTFSI] in [C3mpyr] [TFSI]. In some examples, the RTIL cation component is selected from the group consisting of: [Puii4]+, [NUB] / [Campyr]+, or combinations thereof. Desirably, the RTIL of the ionic liquid electrolyte is selected from the group consisting of: [Pnii [FSI], [Nina] [FSI], [Campyr] [FSI], and combinations thereof.

[0128] In the subranges of salt concentration mentioned above, the electrolytes of the invention have been found to have an ultralow or even negligible solubilising power for intermediate polysulfides (LizSn, 4 < n < 8). There is no colour change in the tested electrolytes of >30 mole% of LiTFSI or in [Pmi4] [FSI] ionic liquid electrolyte at 50 °C (on formation) or indeed on cooling to room temperature (between 25 and 30 °C). This was a surprising finding as there was nothing in the art that suggested that such a particular subset of Li salt containing single [FSI]- anion electrolytes or mixed [FSI] ’ / [TFSI]- anion electrolytes would have such low or even negligible polysulfide solubilising ability. Indeed, preferred electrolytes of the invention solubilise polysulfides to saturated concentrations (even at 50 °C) which are below the limit of quantification (LOQ), and more preferably below the limit of detection (LOD) of the individual or more preferably total high and intermediate polysulfide species [S42-(detectable at 420 nm with UV spectroscopic analysis), Sg2“ (detectable at 450-470 nm with UV spectroscopic analysis), Sg2-at (detectable at 490-500 nm)] of UV spectroscopic techniques. In some examples, the preferred electrolytes of the invention solubilise polysulfides to saturated concentrations (even when formed at 50 °C) which are below the limit of quantification (LOQ), and more preferably below the limit of detection (LOD) of the individual and more preferably total intermediate polysulfide species (S2-, Sg2”, Sg2-) of ultrasensitive Raman spectroscopic techniques, even under high Laser powers of at least 17 mW (100 %). For example, at around 437 cm1and 1 around 474 cm1peaks corresponding to S42, Sg2, Ss2are not detectable or are detectable only in negligible quantities in solution.

[0129] Advantageously, due to the ultralow or even negligible intermediate PS solubility exhibited by the electrolytes of the invention, the electrolytes can be used in Li-S cells, and as a result of the desirably low PS solubility, the Li-S cell discharge cycle predominantly exhibits quasi solid-state redox behaviour, which as intended herein is identified by a discharge curve of the cell that involves an apparent direct, one step, Sg to low order lithium sulfides (LizSa / LiaS) solid to solid conversion reaction which is confined to a S cathode. The electrolytes therefore are thought to support Li-S cell chemistry that does not involve, or involves only to a negligible degree, dissolution of intermediate PS species in the electrolyte. Advantageously, such electrolytes avoid the deleterious effects of polysulfide shuttle, particularly poor utilisation of active material and rapid loss of electrolyte in the cell on cycling.

[0130] The skilled person will understand that such proposed quasi solid state discharge behaviour will be particularly evident when the cell is cycled at a low C-rate (i.e., a current density below 0.1 mA / cm2) as this would ensure the discharge curve would reflect primarily the thermodynamic potentials. In the cells of the invention this behaviour is observed on cycling at C / 10 (0.334 mA / cm2). Conversely, the benefit / effects of the desirable low impedance cathode surface film formed from the electrolytes of the invention would be more obvious at higher rates (i.e., above 0.5 to 1 mA / cm2) where such testing would differentiate cell behaviour and allow optimisation of the electrolyte of the invention against similar candidate IL electrolytes having low PS solubility that may support poor S cathode impedance, despite exhibiting low PS solubilising power.

[0131] Desirably, the Li+conductive ionic liquid electrolyte is free of one or more solid matrix materials such as a ceramic material or a gel polymer matrix. Such materials rely on physical / mechanical strength barrier effects to prevent the polysulfide shuttle effect. Due to the ultralow or negligible polysulfide solubility in the electrolytes of the invention, such materials are not necessary in the cell. Similarly, a preferred Li+conductive ionic liquid electrolyte is free of film forming additive such as nitrate salts, for example, LiPS, P2S5, glutamate, LiBR, MEP+, LiNOa for anode and / or cathode passivation. Because polysulfides do not dissolve into the electrolyte in any significant amounts, and no such protective / passivating additive is needed. Desirably, the electrolyte is free of conventional solvents (non-IL solvents) of one or more of: ether solvents, such as DEE, THF, DME, DIOX, DMC, EMC, EC, PC, MF, EF, EA, etc.

[0132] As a result of the ultralow, or indeed even negligible, dissolution of high and intermediate order polysulfide species ( LiaSn, 4 < n < 8) into the electrolytes, and the preferred accompanying quasi solid state redox behaviour, in some examples, the cell discharge utilises 60% or more of the S active material during cycling for at least the first discharge cycle (e.g., produces a discharge capacity of at least 60% of the theoretical S capacity of 1672 mA g1sulfur), that is the cell exhibits at least 60% direct conversion of the S active material in the positive electrode into low order lithium sulfides ( LizSa / LiaS) . For example, as evident from Table 1, such performance in terms of S active utilisation occurs for 30 mole% LiFSI / [Pnii4] [FSI], 30 mole% LiTFSI / [Pmw] [FSI], 30 mole% LiFSI / [N1113] [FSI], 30 mole% LiTFSI / [N1113] [FSI], 30 mole% LiFSI / [C3mpyr] [FSI], 30 mole% LiTFSI / [C3mpyr] [FSI], and 30 mole% LiFSI / [EIX / I I m] [FSI ] . This is certainly the case for the basic cathode described in the examples.

[0133] In some examples, the cell discharge utilises 70% or more of the S active material during cycling for at least the first discharge cycle (e.g., produces a discharge capacity of at least 70% of the theoretical S capacity of 1672 mA g1sulfur). For example, as evident from Table 1, such performance in terms of S active utilisation occurs for 50 mole% LiFSI / [PunJ [FSI], 50 mole% LiFSI / [C3myr][FSI], and 50 mole% LiTFSI / [C3myr][FSI], and 30 mole% LiFSI / [E IX / llm] [FSI] . This is certainly the case for the basic cathode described in the Example 1. Such excellent S utilisation of 60% or more, carries over to the 2nddischarge cycle as demonstrated by the data in Table 1. Further enhanced S utilisation is achieved with a PI L binder is included in the cathode used in the cell with the electrolyte. In examples, such a cell discharge utilises 80% or more of the S active material during cycling for at least the first discharge cycle (e.g., produces a discharge capacity of at least 80% of the theoretical S capacity of 1672 mA g1sulfur). For example, as evident from Table 1, such performance in terms of S active utilisation occurs for 50 mole% LiFSI / [Pmj4] [FSI], 30 mole% LiTFSI / [Pini4][FSI], 50 mole% LiTFSI / [Pmw] [FSI], 50 mole% LiFSI / [Nm3][FSI] and 30 mole% LiTFSI / [N1113] [FSI]. In some cases, where PIL is included in the cathode, the S utilisation on the first discharge can be 100%, e.g., see Table 1, 50 mole% LiFSI / [Pmw] [FSI], 30 mole% LiTFSI / [Pnii4] [FSI], 30 mole% LiTFSI / [NIH3] [FSI] and 50 mole% LiTFSI / [NUB] [FSI] . In cases where PIL is included in the S in the cell with the electrolytes of the invention, the S utilisation is boosted to a degree where S utilisation on the 2nddischarge cycle is 75% or more, preferably 80% or more, and in some cases as shown in Table 1, can be 95% or more. The benefits of including PIL in the S cathode carry over to the 20thcycle where S utilisation may be as high as 45% or greater. In some preferred cases, the S utilisation on the 20th cycle may be 60% or higher, particularly for 50 mole% single salt systems and 30 mole% mixed salt systems (see Table 1). Particularly enhanced S utilisation is achieved with a PIL binder in combination with a nanomaterial or 2D material such as boron nitride or an allotrope of boron nitride, such as BNNT, BNNS, BNNW, preferably in an amount of up to about 3 wt% of the positive S electrode (cathode), preferably up to about 1 wt%, most preferably up to about 0.75 wt% and most preferably up to about 0.5 wt% of the cathode composition, included in the cathode used in the cell with the electrolyte. About means ±5% of the stated value. In examples, such a cell discharge advantageously involves S utilisation of 50% and higher on the 40thdischarge cycle. The excellent results, particularly at 40thdischarge, is due to combined effects of the S cathode and also the excellent low impedance SEIs formed on the cathode and the anode via the electrolytes of the invention. Thus, in some examples, the electrolytes of the invention result in the formation of a desirable electrodeelectrolyte interface, which is, e.g., of low resistance, that is an insulating layer with good transmission of Li+. As a result, preferred Li-S cells including the electrolytes of the invention, on cycling, may exhibit a midpoint (or 50% SOC) overpotential of 0.5 V or less, preferably 0.3 V or less, more preferably 0.275 V or less, more preferably still 0.25 V or less, between the discharge and charge steps of any given cycle, when cycled using the electrode preparations as described here, with or without calendaring but employing the same current density, temperature, separator, electrolyte volume, compression, formation and wetting procedure, etc. In other words, an equivalent cell operated under the condition described herein to allow a direct comparison of electrolyte and / or S cathode variation.

[0134] The anode is described herein as a negative Li metal or Li metal alloy electrode. In one embodiment, the anode is a Li metal anode. In one embodiment, the anode is a Li metal alloy electrode. In one embodiment, the Li metal or Li metal alloy electrode may comprise one or more other non-Li elements. In one embodiment, the anode may be an alloying anode, such as may comprise silicon and form an alloy when lithiated. In one embodiment, the anode comprising silicon may comprise amorphous silicon, or silicon dioxide, silicon monoxide. In one embodiment, the anode comprising silicon may comprise amorphous silicon, silicon dioxide, silicon monoxide, or any combination of these. In one embodiment, the anode comprising silicon may comprise amorphous silicon. In one embodiment, the Li metal or Li metal alloy electrode may comprise a non-Li current collector.

[0135] Limited self-discharge

[0136] As evidence for the ultralow / negligible polysulfide solubility behaviour in the electrolytes of the invention, preferred cells of the invention may be substantially self-discharge free, as demonstrated for example, via an open circuit voltage (OCV) of the cell which varies by no more than ± 0.1 V / hour, for example, for 30 mins, 60 mins, 3 hours, 10 hours, and in some cases at least 48 hours. Such results are not possible for cells where PS are dissolved in the bulk of the electrolyte.

[0137] S electrode composition

[0138] The S electrode includes electrochemically active sulfur, e.g. elemental sulfur or LizS. Preferably, the electrochemically active sulfur is provided in the form an electrochemically active sulfur-carbon composite, particularly a high surface area S-carbon composite (HSAC-S) where the electrochemically active sulfur, is associated with high surface area carbon.

[0139] Desirably, in addition to active S, the positive S electrode (cathode) further comprises one or more conductivity enhancing carbonaceous materials, such as, carbon nanotubes, carbon nanofibres, graphene, reduced graphene oxide, carbon black or other conductive carbons, such as C65.

[0140] Suitably, the positive S electrode (cathode) may further comprise one or more non-ionic binders in an amount up to about 15 wt%, such as carboxy methyl cellulose (CMC), poly(vinylidene fluoride) (PVDF- HFP), polyvinylpyrrolidone (PVP), mix of PVP with polyethylene imine (PEI), polyaniline (PANI) and lithium polyacrylate (LiPAA), PEI, polyethylene glycol) diglycidyl ether (PEGDGE), crosslinked PEI, cationized PEI, PEI-ER(diglycidyl ether) and PEI-gelatin.

[0141] The skilled person will be aware of the range of materials and typical ranges of such conventional components of S cathodes. For example, typical ranges of each component may be from 50 wt% to about 80 wt% S active material, 40 wt% to about 10 wt% conductive carbon, 5 wt% to about 15 wt% non-ionic binder. In some examples the mass ratio of S to conductive carbon to non-ionic binder is about 70:15:15. In preferred examples the inventors have tested, the mass ratios are 70% wt%, HSAC-S; 15 wt% C65, and 15 wt% CMC. The S electrode may be provided on aluminium current collector, particularly a carbon coated aluminium current collector. In some embodiments, a preferred S cathode may comprise 50 wt % S, 21 wt % HSAC, 14 wt % C65 and 15 wt % CMC.

[0142] PIL:Li Salt binder mixture in S electrode

[0143] Generally speaking, the inventors believe that the benefits of the electrolytes system of the invention (primarily ultralow / negligible PS solubilising power, and in particular, for preferred electrolytes capable of desirable film formation) should be derivable from any kinds of S cathode, including the preferred S cathodes described herein that include a PIL : Li salt binder mixture with or without 2D or nanostructured materials as described above. Indeed, the benefits provided by the electrolytes of the invention means that the S cathodes themselves used do not necessarily need sophisticated, expensive or complex nanostructures / morphologies, and / or other complex additive components to enhance activity and / or suppress PS shuttle. An advantage therefore is that practical S cathodes from commercial manufacturing perspective can readily be used and expected to provide good performance when used with the electrolytes of the invention. Addition of the simple 2D or nanostructured materials such as BNNTs, etc., is convenient to achieve and merely involves addition of the material via dispersion into a solution of water and binder such as CMC prior to inclusion in the electrode slurry in the desired quantity. In some embodiments where longer cycle life is important, inclusion of the simple 2D or nanostructured materials such as BNNTs, etc., is desirable in addition to the PIL : Li Salt binder mixture of the invention, as in particular, they prolong higher S utilisation and discharge capacity at up to at least 40 cycles as described elsewhere herein.

[0144] In a preferred cell, the S electrode used in the cell, preferably with the electrolyte of the invention, comprises at least one ionically conductive organic polymeric ionic liquid (poly ionic liquid, PIL) binder which is included in the positive electrode composition as a PIL : Li salt binder mixture. Inclusion of one or more PILs in the S cathodes in this manner have been shown to support longer cycling performance as can be seen from Table 1. PILS are a type of polyelectrolyte that have a typical ionic liquid cationic or anionic species in some if not all of the polymerised monomer repeating residues or blocks, which are connected through a polymeric backbone to form an ionically conducting macromolecular architecture. Cationic PILs have cationic IL based polymeric backbone or a neutral backbone with pendant cationic groups, and have a tethered or untethered IL anion, e.g., [FSI]-, [TFSI]-, [CIO4]-, [OTf]-, [BOB]-, [DFOB]-, [BETI]-, [TDI]- [PFg]', [BF4]", organic phosphates (such as DEP, DBP), borates, [OTf]-, and existing [TFSI] analogues, which are well known to the skilled person.

[0145] The PIL may be a homopolymer with the same IL cationic or anionic repeating unit or can be a copolymer with different IL cationic or anionic repeating units. Block polymers which include a mixture of non-ionic blocks (e.g., hydrophobic blocks) and ionic blocks are also possible. Suitably, the PIL binder comprises a plurality of one or more ionic liquid cations, or one or more ionic liquid anions, tethered to an organic polymer backbone or forming part of the polymeric back bone, and a plurality of one or more tethered or untethered counter anions. In some examples, the ionically conductive organic polymer ionic liquid (PI L) binder is a polyelectrolyte, that is a macromolecule in which repeating units of the polymer bear one or more electrolyte groups which are typically derived from ionic liquid cations and / or ionic liquid anions. Depending on the electrolyte groups present, the PIL binder can be polycation (positively charged), a polyanion (negatively charged) or even a polyzwitterionic polymer (bear cationic and anionic repeating groups). Such polysalts in solution are conductive. The PIL may comprises a polymeric backbone comprising one repeating unit (homopolymer) or more repeating units (copolymer or block / block copolymer).

[0146] In some examples, the PIL includes one or more onium cation groups that are derived from typical ionic liquid cations. The onium cations may comprise aromatic groups or heteroaryl rings which comprise one or more heteroatoms. Suitably, the onium cations may be selected from ammonium cation groups, pyridinium cation groups, imidazolium cation groups, pyrazolium cation groups, sulfonium, piperidinium, pyrrolidinium cation groups, morpholinium cation groups, and phosphonium cation groups. These groups may be alkylated with one or more functional groups, often Ci.g alkyl groups. Preferably, the onium cation groups in the PIL are quaternary onium cations. Typical examples include one or more onium cations derived from the following typical IL cations: wherein R1, R2, R3, and R4, when present are each independently selected from hydrogen, alkyl, alkenyl, alkyl ether, heteroalkyl, and heteroalkenyl, and wherein at least one of R1, R2, R3, and R4when present represents a linkage to the polymer backbone, which can be a direct linkage or a linking through one or more additional functional groups located between R1, R2, R3, and R4and the polymer backbone. In another embodiment, the onium cations are optionally quaternary ammonium cations or quaternary phosphonium cations. Preferred such cations are of the general formula as follows:

[0147] Ammonium Phosphonium wherein R1, R2, R3, and R4, are each independently selected from hydrogen, alkyl, alkenyl, alkyl ether, heteroalkyl, and heteroalkenyl, and wherein at least one of R1, R2, R3, and R4, represents a linkage to the polymer backbone, which can be a direct linkage or a linking through one or more additional functional groups located between R1, R2, R3, and R4, and the polymer backbone.

[0148] In some examples, the PIL polymer back bone itself may directly comprises the onium cation, preferably any ammonium, pyrrolidinium, sulfonium, zwitterion IL cation etc. Preferably, a suitable PIL backbone comprises a pyrrolidinium cation. For example, a suitable PIL is poly(diallyldimethylammonium) polymer [PDADMA]+which provides a cationic polymer backbone as follows:

[0149] The anion of a salt form of the PIL may be TFSI or FSI.

[0150] In preferred embodiments, the PIL comprises a homo polymer cationic backbone of poly(diallyldimethylammonium) [PDADMA]+cations together with an untethered ionic liquid anion component [TFSI]- such that the PIL corresponds to the salt form [PDADMA][TFSI] having the structure below.

[0151] In some examples, the PIL comprises a polymeric back bone that comprises one or more polymerised monomer residues that provide pendant anionic groups that are derived from typical ionic liquid anions. For example, the PIL may comprise one or more anionic groups derived from:

[0152] The cation in such polyanionic PILS is Li+cation.

[0153] The polymer may be a homopolymer or a copolymer where another polymerised monomer residue forms a different pendant group which may be ionic or non-ionic, or an AB di-block polymer comprising two distinct blocks of pendant groups where at least one group is a pendant anionic group, or a triblock polymer with three distinct blocks of pendant groups where at least one group is a pendant anionic group.

[0154] In some embodiments, the polymer comprises a repeating unit having the following structure (I): where R5, R6, and R7are each independently H or optionally substituted C 1.12a I ky I, and n has a value in a range from 0 to about 20, or from 0 to about 10, or from 0 to about 5. For example, R5and R6may be both H, R7and R8may be both Ci.galkyl, and / or R5and R6may be both H, and R7may be methyl, and the anionic group may be one or more anionic groups (*) described above.

[0155] The polymer may be provided in a salt form, where the cation is Li+.

[0156] The anionic groups may be linked to the polymer backbone via direct linkage or a via a linkage through one or more additional functional groups located between the polymer back bone and the anionic component. For example, the anionic group may be linked to the polymer backbone via an attachment through an optionally substituted polyacrylic acid or an optionally substituted polymethacrylic acid group. The backbone may be any of PMA, PMMA, PEMA, polyHEMA etc. The optional substituents are described in the below next section. Preferably, the anionic groups are derived from TFSI, FSI, borate, phosphate or sulfonate, most preferably TFSI or FSI, more preferably still TFSI. In one example, the PIL may be poly((trifluoromethane)sulfonimide lithium methacrylate (PMTFSI-Li), having the following structure:

[0157] FSI, borate, phosphate or sulfonate versions of PMTFSI-Li are also desirable.

[0158] In another example, the PIL is a PILBIoc co-polymer comprising, wherein the block copolymer comprises a non-ionic block (i.e. that does not contain ionic charge / neutral polymer block) and an ionic block, the non-ionic block comprising polymerised residues of hydrophobic monomers (i.e. a hydrophobic non-ionic block), and the ionic block comprising polymerised monomer residues having covalently coupled thereto one or more of (a) a pendant organic ionic liquid cation, the pendant organic ionic liquid cation having a counter anion, and (b) a pendant anionic moiety, the pendant anionic moiety having a counter cation. By "hydrophobic monomers" it is meant monomers that when homo-polymerised or co- polymerised with each other form polymer that is substantially insoluble in water. Provided the required hydrophobic character is derived, there is no limitation as to the type of the residues of hydrophobic monomers that can be used for the purpose of the present invention. For example, the residues of hydrophobic monomers may be derived from acrylate monomer, vinyl monomer, styrenic monomer, or combinations thereof. In one embodiment, the non-ionic block may be described as a hydrophobic non-ionic block. In some embodiments the residues of hydrophobic monomers are derived from styrene or styrene derivatives, indene or indene derivatives, vinylpyridine or vinylpyridine derivatives, methyl methacrylate or methacrylate derivatives, or a combination thereof. For example, residues of hydrophobic monomers may be derived from a-methylstyrene, methylstyrene, chlorostyrene, hydroxystyrene, vinylbenzyl chloride, methylindene, ethylindene, trimethylindene, vinylmethylpyridine, vinylbutylpyridine, vinylquinioline, vinylacrydine, hydroxyethyl methacrylate, dimethylamino-ethyl methacrylate, vinylcarbazole, or a combination thereof. In some preferred embodiments, the residues of hydrophobic monomers are derived from styrene.

[0159] In some embodiments, the non-ionic block comprises a repeating unit having either of the following structures (I) and (II): where R1, R2, R3, and R4are each independently H or Ci-galkyl. For example, R1and R2may both be H, both R3and R4may be Ci-aalkyl, or R1and R2may be both H and R3and R4may be both methyl. The block copolymer of the electrolyte composition of present invention also comprises an ionic block. By the expression "ionic block" is meant a polymer block that contains an overall ionic charge. The monomer residues of the ionic block may derive from styrene or styrene derivatives, indene or indene derivatives, vinylpyridine or vinylpyridine derivatives, methyl methacrylate or methacrylate derivatives, methyl acrylate or acrylate derivatives, methacrylamide or acrylamide derivatives, or a combination thereof. In some embodiments, the monomer residues of the ionic block derive from a-methylstyrene, methylstyrene, chlorostyrene, hydroxystyrene, vinylbenzyl chloride, methylindene, ethylindene, trimethylindene, vinylmethylpyridine, vinylbutylpyridine, vinylquinioline, vinylacrydine, hydroxyethyl methacrylate, dimethylamino-ethyl methacrylate, vinylcarbazole, or a combination thereof. The type of the pendant organic ionic liquid cation is not particularly limited provided it presents as a pendant moiety to the monomer residues forming the backbone of the ionic block.

[0160] In some embodiments, the pendant organic ionic liquid cation comprises any known ionic liquid cation type. The cation may be mono-, di-, or tri-substituted, typically alkyl substituted, where each alkyl independently defined to include Ci-8 linear, branched, or cyclic carbon moieties. In some embodiments, the pendant organic ionic liquid cation comprises the onium ions described above at (**). Preferred onium ions include imidazolium, ammonium, pyrrolidinium, sulfonium and zwitterion cations.

[0161] In some embodiments, the pendant organic ionic liquid cation comprises a carboalkoxy, carboxylato, carboxyamino, alkylene, alkenylene, or ether group linking the cation to the polymerised monomer residues of the ionic block. In some embodiments, the ionic block comprises a repeating unit having the following structure (III): where R5, R6, R7, and R8are each independently H or optionally substituted C 1.12a I ky I, and n has a value in a range from 0 to about 20, or from 0 to about 10, or from 0 to about 5. For example, R5and R6may be both H, R7and R8may be both Ci-galkyl, and / or R5and R6may be both H, R7may be methyl, and R8may be n-butyl, with n between 1 or about 10.

[0162] In some embodiments, polymerised monomer residues of the ionic block derive from monomers that comprise a pendant organic ionic liquid cation or the ionic liquid anion of the kind described herein. There is no particular limitation as to the type of such monomers, provided they comprise a polymerizable moiety and a pendant organic ionic liquid cation. Examples of such monomers include acryloyl-imidazolium, acryloyl- pyrrolidinium, acryloyl-pyridinium, vinyl-imidazolium, vinyl-pyrrolidinium, vinyl-pyridinium, styrene- imidazolium, styrene- pyrrolidinium, styrene-pyridinium, and a combination thereof.

[0163] The pendant organic ionic liquid cation has a counter anion. Provided the counter anion neutralizes the charge of the pendant organic ionic liquid cation, there is no limitation as to the nature of that counter anion. In some embodiments, the counter anion of the pendant organic ionic liquid cation is selected from bis(trifluoromethanesulfonyl)imide (TFSI), Triflate (OTf), Borate, sulfonate, Tetrafluoroborate (BF4), hexafluorophosphate (PFg), and bis(fluorosulfonyl)imide (FSI), fluorosulfonyl- (trifluoromethanesulfonyl) imide (FTFSI), and a combination thereof. TFSI and FSI are preferred. TFSI is particular preferred.

[0164] In addition to or instead of the pendant organic ionic liquid cation, the ionic block may comprise a pendant anionic moiety. The nature of the pendant anionic moiety is not particularly limited provided it presents as a pendant moiety to the monomer residues forming the backbone of the ionic block. In some embodiments, the pendant anionic moiety comprises derivatives of TFSI, OTf, BF4, PFg, and FSI, FTFSI, and a combination thereof. The pendant anionic moiety has a counter cation. In one embodiment, the polymerised monomer residues of the ionic block do not have covalently coupled thereto a pendant anionic moiety.

[0165] In one preferred example, the PILbloc co-polymer has the following structure: In a preferred example, the PlmTFSI has a molecular weight of about 55,000 g / mol, and preferably comprises 41 wt% of polystyrene and 59 wt% of the polyionic liquid cation.

[0166] Suitably, the PIL binder is provided in the form of a composite of the PIL binder and one or more Li metal salts, each Li metal salt having a Li+cation and a salt anion, wherein the counter anions of the PIL and the Li metal salt anions are the same or different. Suitably, the counter anions and Li metal salt anions are predominantly [FSI ]’ anions and / or [TFSI]" anions, most preferably [TFSI]- anions. Preferably, the one or more Li metal salts in the PIL binder are identical to the Li metal salts used in the Li+conductive ionic liquid electrolyte. Desirably, the at least one ionically conductive organic polymer ionic liquid (PIL) binder composite is present in an amount up to about 15 wt% (±5%), more preferably up to about 10 wt% (±5%), more preferably up to about 7.5 wt% (±5%), most preferably up to about 5 % (±5%) of the total positive S electrode (cathode) composition.

[0167] The PIL:LI salt composite preferably comprises a ratio of Li salt and PIL in the binder mixture of from 25 to 75 mole% PIL to Li salt or from 75 to 25 mole% PIL, more preferably 50 to 50 mole% PIL to Li salt. For example the composite may comprise / consist of: 50:50 mole% [PDADMA.TFSI]:LiTFSI, e.g., in an amount of 5 wt% of the cathode; 50:50 mole% [PMTFSI .TFSI] : LiTFSI, e.g., in an amount of 15 wt% of the cathode; or 50:50 mole% [PILblocCo-polymer plmTFSI )] :LiTFSI, e.g., 15 wt% of the cathode. In the latter two examples at 15wt%, no non-ionic binder is included, that is, no CMC is included in the cathode.

[0168] Typical ranges of each cathode component having PI L: Li salt composite may be from 50 wt% to about 80 wt% S active material, 40 wt% to about 10 wt% conductive carbon, 5 wt% to about 15 wt% nonionic binder. In preferred examples, the mass ratios are 5 wt % [PDADMATFSIj:[l.iTFSI], lOwt % CMC, 21 wt % HSAC, 14 wt % C65 and 50 wt % 5. The S electrode may be provided on aluminium current collector, particularly a carbon coated aluminium current collector. In some embodiments, the S cathode may comprise 50 wt % S, 21 wt % HSAC, 14 wt % C65, 10 wt % CMC and 5 wt % [PDADMATFSI] :[LiTFSI] . In this case, the S is regular sulfur impregnated / melt diffuse in an HSA-carbon. In another example, the cathode with PIL:Li Salt binder mixture may comprise or consist of: 50 wt % S, 21 wt % HSAC, 15 wt % [PMTFSI .TFSI] : LiTFSI (50:50 mole%) and 14 wt% C65. In another example, the cathode with PIL may comprise or consist of: 50 wt % S, 21 wt % HSAC, 15 wt % [PilblocCopolymer(plmTFSI)]:LiTFSI (50:50 mole%) and 14 wt% C65.

[0169] 2D & Nanomaterials in S electrode

[0170] In some examples, the positive S electrode (cathode) further comprises 2D materials and / or nanomaterials such as boron nitride or an allotrope of boron nitride, such as BNNT, BNNS, or BNNW, or a ceramic nanomaterial such as graphene, functionalised graphene, MXenes and other 2D nanomaterials, preferably in an amount of up to 3 wt% (±5%) of the positive S electrode (cathode), preferably up to 2 wt% (±5) and more preferably up to 1 wt% (±5%), more preferably up to 0.75 wt% (±5%), most preferably up to 0.5 wt% (±5%) are particularly preferred. BNNTs at up to 0.5wt% (±5%) of the cathode are particularly preferred. The 2D materials and / or nanomaterials may be in addition to the PIL component for the S cathode as described above.

[0171] A preferred cell comprises a separator such as a polyolefin separator such as a polypropylene separator. In some embodiments the separator may comprise a S polymer, such as a polysulfone.

[0172] In another aspect, the invention provides for a Li+conductive ionic liquid electrolyte composition having at least one room temperature ionic liquid (RTIL) comprising a RTIL cation component and an RTIL anion component which is predominantly bis(fluorosulfonyl)imide ([FSI]-), and at least one Li+salt solubilised in the RTIL, wherein the Li+salt is either: (i) present in an amount of from about 20 mole% to about 60 mole% of the electrolyte when an anion of the Li+sa It is [FSI]-; or (ii) present in an amount of from about 20 mole% to about 60 mole% of the electrolyte when an anion of the Li+conductive salt is bis[(trifluoromethyl)sulfonyl]imide ( [TFSI] ); wherein the Li+conductive ionic liquid electrolyte has an total polysulfide (LizSm) solubility that is lower than 1 mM atomic S, or zero, or at least lower than the limit of detection of UV-vis spectroscopy.

[0173] Description of Preferred Embodiments

[0174] The inventors have discovered unexpectedly that a subset of high concentration Li salt single anion, or mixed TFSI / FSI anion, ionic liquid electrolytes where the electrolyte comprises predominantly [FSI]- anions, have particularly low, such as ultralow or indeed negligible polysulfide solubility, making them particularly useful as electrolytes for Li-S cells, where they limit or avoid undesirable effects associated with polysulfide shuttle effect, including low S active material utilisation, loss of electrolyte and rapid capacity fade in the absence of sacrificial additives. In some examples, the electrolytes of the invention result in the formation of a desirable electrode-electrolyte interface on the S cathode and / or the anode which is, e.g., of low resistance, that is an effective insulating layer that allows good Li+transmission, which in addition to the favourable PS solubility assist in the observed excellent performance of the cells of the invention comprising the electrolytes of the invention.

[0175] Embodiments

[0176] Embodiment 1. A quasi-solid state (QSS) liquid electrolyte lithium-sulfur cell comprising: o a negative Li metal or Li metal alloy electrode (anode); o a positive S electrode (cathode); and o a Li+conductive ionic liquid electrolyte having at least one room temperature ionic liquid (RTIL) and at least one Li salt solubilised in the RTIL, wherein the Li+conductive ionic liquid electrolyte comprises an ammonium cation, and the electrolyte comprises:

[0177] (A) from about 20 mole% to about 60 mole% of Li+cations, where anions of the electrolyte are predominantly [FSI]- anions, regardless if sourced from the Li salt or the RTIL (single FSI system); or

[0178] ( B) from about 20 mole% to about 60 mole% of Li+cations and [TFSI]- anions, where anions in the electrolyte are predominantly [FSI]- anions, regardless if sourced from Li salt or RTIL (mixed TFSI / FSI system), wherein the Li+conductive ionic liquid electrolyte solubilises one or more polysulfide species, LizSn, 4 < n < 8, to a saturation concentration at 25 °C of less than 1 mM atomic S, more preferably to a saturation concentration at 25 °C that is zero, or at least is below the limit of detection of UV spectroscopy for the one or more polysulfide species.

[0179] Embodiment !. The cell of Embodiment 1, wherein the Li+conductive ionic liquid electrolyte comprises in

[0180] (A) from about 35 mole% to about 55 mole% of Li+cations, where anions of the electrolyte are predominantly [FSI]- anions, regardless if sourced from Li salt or RTIL (single FSI system); or

[0181] (B) from about 25 mole% to about 40 mole% of Li+cations and [TFSI]- anions, where anions in the electrolyte are predominantly [FSI]- anions, regardless if sourced from Li salt or RTIL (mixed TFSI / FSI system).

[0182] Embodiment s. The cell of Embodiment 1 or Embodiment 2, wherein the Li+conductive ionic liquid electrolyte comprises in

[0183] (A) from about 47 mole% to about 53 mole%, preferably about 50 mole% of Li+cations, where anions of the electrolyte are predominantly [FSI]- anions, regardless if sourced from Li salt or RTIL (single FSI system); or

[0184] (B) from about 27 mole% to about 35 mole%, preferably about 30 mole% of Li+cations and [TFSI]- anions, where anions in the electrolyte are predominantly [FSI]- anions, regardless if sourced from Li salt or RTIL (mixed TFSI / FSI system).

[0185] Embodiment 4. The cell of any one of Embodiments 1 to 3, wherein the Li+cations are sourced in (A) from Li FSI salt and the ionic liquid anions are predominately [FSI]- anions; and where in the Li+cations and [TFSI]" anions are sourced in (B) from LiTFSI salt and the ionic liquid anions are predominately FSI anions.

[0186] Embodiment 5. The cell of any one of the preceding Embodiments, wherein on wherein on cycling at a current density of C / 10 (0.334 mA cm'2) or below, the cell exhibits quasi solid-state redox behaviour as evidenced by a single voltage plateau discharge curve feature for the cell which indicates a direct, one step, Sg to low order lithium sulfides ( Li 2S2 / U2S) solid to solid conversion reaction which is confined to a S cathode of the cell.

[0187] Embodiment 6. The cell of any one of the preceding Embodiments, wherein on cycling at a current density of C / 10 (0.334 mA cm'2) or below, which utilises at least 70% of S active during cycling for at least the first discharge cycle, that is the cell exhibits at least 70% conversion of the S active material in the positive electrode into low order lithium sulfides (Li2S2 / Li2S).

[0188] Embodiment 7. The cell of any one of the preceding Embodiments, wherein the Li+salt is present in an amount of from about 35 mole% to about 55 mole%, preferably about 50 mole%, of the electrolyte and the anion of the Li+salt is [FSI]- (single FSI anion system); or wherein the Li+salt is present in an amount of from about 25 mole% to about 40 mole%, preferably about 30 mole%, of the electrolyte and the anion of the Li+conductive salt is [TFSI]- (mixed TFSI / FSI anion system). Embodiment s. The cell of any one of the preceding Embodiments, which on cycling exhibits an overpotential of 0.5 V or less, preferably 0.3 V or less, more preferably 0.275 V or less, more preferably still 0.25 V or less, between the discharge and charge steps of any given cycle, when cycled using the electrode preparations and conditions of Example 1 or Example 2.

[0189] Embodiment 9. The cell of any one of the preceding Embodiments, wherein the RTIL cation component is predominantly an ammonium cation, and optionally, lesser amount of a phosphonium cation, a sulfonium cation, a pyrrolidinium cation.

[0190] Embodiment 10. The cell of any one of the preceding Embodiments, wherein the RTIL cation component is predominantly an alkyl ammonium cation, and optionally, lesser amounts of an alkyl phosphonium cation, an alkyl sulfonium cation, an alkyl pyrrolidinium cation, an alkyl imidazolium based cation, an alkyl pyridinium cation or combinations thereof.

[0191] Embodiment 11. The cell of any one of the preceding Embodiments, wherein the RTIL cation component is selected from the group consisting of: [Nni2]+, [Nm4]+, [Ni222]+, [Ni222]+, [Ni22a]+, [Ni224]+, [N2222 , [N2223 , [N2224 , Ni333]+, [Ni444]+[ N ii4i4i4]+and related ether oxygen containing analogues.

[0192] Embodiment 12. The cell of any one of the preceding Embodiments, wherein the RTIL of the ionic liquid electrolyte is predominantly [N1113] [ESI] or exclusively [N1113] [FSI] .

[0193] Embodiment 13. The cell of any one of the preceding Embodiments, wherein the cell is substantially self-discharge free, as demonstrated for example, via an open circuit voltage (OCV) of the cell which varies by no more than ± 0.1 V / hour, for example, for 30 mins, 60 mins, 3 hours, 10 hours, and in some cases at least 48 hours.

[0194] Embodiment 14. The cell of any one of the preceding Embodiments, wherein the S cathode comprises at least one ionically conductive organic polymeric ionic liquid (PI L):Li salt binder mixture, comprising at least one PIL selected from PIL cationic polymers with counter anions; PIL anionic polymers with Li+counter cations; and PIL cationic and / or anion block copolymers with corresponding counter anions or cations, preferably wherein the at least one PIL comprises:

[0195] • a poly ionic liquid cationic polymer comprising a plurality of one or more ionic liquid cations included in, or tethered as pendant groups, to an organic polymer backbone; and a plurality of one or more tethered or untethered counter anions;

[0196] • a poly ionic liquid anionic polymer comprising a plurality of one or more ionic liquid anions included in, or tethered as pendant groups, to an organic polymer backbone; and a plurality of Li+cations;

[0197] • a poly ionic liquid cationic and / or anion block copolymer comprising: a non-ionic block that does not contain ionic charge and comprises polymerised residues of hydrophobic monomers, and an ionic block comprising polymerised monomer residues having covalently coupled thereto one or more of o (a) a pendant organic ionic liquid cation, the pendant organic ionic liquid cation having a counter anion, and o (b) a pendant anionic moiety, the pendant anionic moiety having a counter cation. Embodiment 15. The cell of Embodiment 14, wherein the counter anions and Li salt anions are [ESI]- anions and / or [TFSI]- anions, preferably [TFSI]- anions.

[0198] Embodiment 16. The cell of Embodiment 14 or Embodiment 15, wherein the one or more Li salts in the PI L:Li salt binder mixture are identical to the Li metal salts used in the Li+conductive ionic liquid electrolyte.

[0199] Embodiment 17. The cell of any one of Embodiments 14 to 16, wherein the PI L:Li salt binder mixture is 50:50 mole% [PDADMA.TFSI]:LiTFSI, 50:50 mole% [PMTFSI .TFSI] :LiTFSI, or 50:50 mole% [PILblocCo-polymer (plmTFSI)]:UTFSI.

[0200] Embodiment 18. The cell of any one of Embodiments 14 to 17, wherein the PIL:Li salt binder mixture is present in an amount up to about 20 wt%, preferably up to about 15 wt%, preferably up to about 10 wt%, most preferably up to about 5 wt% (about means ±5%) of the total positive S electrode (cathode) composition.

[0201] Embodiment 19. The cell of any one of Embodiments 14 to 18, wherein the positive S electrode (cathode) further comprises 2D and / or nanomaterials such as boron nitride or an allotrope of boron nitride, such as BNNTs, BNNSs, or BNNWs, or a ceramic nanomaterial such as graphene, functionalised graphene, MXenes and other 2D nanomaterials, preferably in an amount of up to 3 wt% of the positive S electrode (cathode), preferably up to 2 wt% and most preferably up to 1 wt%, most preferably up to 0.5 wt%..

[0202] Embodiment 20. The cell of Embodiment 19, wherein on cycling at a current density C / 10 (0.334 mA cm'2) or below, the cell utilises at least 50% of S active during cycling on the 40thdischarge cycle.

[0203] Embodiment 21. The cell of any one of the preceding Embodiments, wherein the Li+conductive ionic liquid electrolyte is free of one or more solid matrix materials such as a ceramic material or a gel polymer matrix.

[0204] Embodiment 22. The cell of any one of the preceding Embodiments, wherein the Li+conductive ionic liquid electrolyte is free of film forming additive such as nitrate salts, for example, LiNOa.

[0205] Embodiment 23. The cell of any of the preceding Embodiments, wherein the electrolyte comprises a cosolvent as diluent, preferably DME.

[0206] Embodiment 24. Use a Li+conductive ionic liquid electrolyte having at least one room temperature ionic liquid (RTIL) and at least one Li salt solubilised in the RTIL, wherein the Li+conductive ionic liquid electrolyte comprises an ammonium cation, and the electrolyte comprises:

[0207] (A) from about 20 mole% to about 60 mole% of Li+cations, where anions of the electrolyte are predominantly [FSI]- anions, regardless if sourced from the Li salt orthe RTIL (single FSI system); or ( B) from about 20 mole% to about 60 mole% of Li+cations and [TFSI ]- anions, where anions in the electrolyte are predominantly [FSI]- anions, regardless if sourced from Li salt or RTIL (mixed

[0208] TFSI / FSI system), wherein the Li+conductive ionic liquid electrolyte solubilises one or more polysulfide species, LizSn, 4 < n < 8, to a saturation concentration at 25 °C of less than 1 mM atomic S, more preferably to a saturation concentration at 25 °C that is zero, or at least is below the limit of detection of UV spectroscopy, in an energy storage application.

[0209] Embodiment 25. Use of a Li+conductive ionic liquid electrolyte having at least one room temperature ionic liquid (RTIL) and at least one Li salt solubilised in the RTIL, wherein the Li+conductive ionic liquid electrolyte comprises an ammonium cation, and the electrolyte comprises:

[0210] (A) from about 20 mole% to about 60 mole% of Li+cations, where anions of the electrolyte are predominantly [FSI]- anions, regardless if sourced from the Li salt orthe RTIL (single FSI system); or

[0211] ( B) from about 20 mole% to about 60 mole% of Li+cations and [TFSI]- anions, where anions in the electrolyte are predominantly [FSI]- anions, regardless if sourced from Li salt or RTIL (mixed TFSI / FSI system), wherein the Li+conductive ionic liquid electrolyte solubilises one or more polysulfide species, LizSn, 4 < n < 8, to a saturation concentration at 25 °C of less than 1 mM atomic S, more preferably to a saturation concentration at 25 °C that is zero, or at least is below the limit of detection of UV spectroscopy in a Li-S cell.

[0212] Embodiment 26. Use of a Li+conductive ionic liquid electrolyte having at least one room temperature ionic liquid (RTIL) and at least one Li salt solubilised in the RTIL, wherein the Li+conductive ionic liquid electrolyte comprises an ammonium cation, and the electrolyte comprises:

[0213] (A) from about 20 mole% to about 60 mole% of Li+cations, where anions of the electrolyte are predominantly [FSI]- anions, regardless if sourced from the Li salt or the RTIL (single FSI system); or

[0214] ( B) from about 20 mole% to about 60 mole% of Li+cations and [TFSI]- anions, where anions in the electrolyte are predominantly [FSI]- anions, regardless if sourced from Li salt or RTIL (mixed TFSI / FSI system), wherein the Li+conductive ionic liquid electrolyte solubilises one or more polysulfide species, LizSn, 4 < n < 8, to a saturation concentration at 25 °C of less than 1 mM atomic S, more preferably to a saturation concentration at 25 °C that is below the limit of detection of UV spectroscopy, to reduce or eliminate the polysulfide shuttle effect in a Li-S cell.

[0215] Embodiment 27. Use of a Li+conductive ionic liquid electrolyte having at least one room temperature ionic liquid (RTIL) and at least one Li salt solubilised in the RTIL, wherein the Li+conductive ionic liquid electrolyte comprises an ammonium cation, and the electrolyte comprises:

[0216] (A) from about 20 mole% to about 60 mole% of Li+cations, where anions of the electrolyte are predominantly [FSI]- anions, regardless if sourced from the Li salt orthe RTIL (single FSI system); or

[0217] (B) from about 20 mole% to about 60 mole% of Li+cations and [TFSI ]- anions, where anions in the electrolyte are predominantly [FSI]- anions, regardless if sourced from Li salt or RTIL (mixed TFSI / FSI system), wherein the Li+conductive ionic liquid electrolyte solubilises one or more polysulfide species, LizSn, 4 < n < 8, to a saturation concentration at 25 °C of less than 1 mM atomic S, more preferably to a saturation concentration at 25 °C that is zero, or at least is below the limit of detection of UV spectroscopy, to manufacture a quasi-solid state Li-S cell.

[0218] Embodiment 28. A method operating a Li-S cell with a S active material utilisation of >60% on a 20thdischarge cycle of the cell, by cycling the cell with a Li+conductive ionic liquid electrolyte having at least one room temperature ionic liquid (RTIL) and at least one Li salt solubilised in the RTIL, wherein the Li+conductive ionic liquid electrolyte comprises an ammonium cation, and the electrolyte comprises:

[0219] (A) from about 20 mole% to about 60 mole% of Li+cations, where anions of the electrolyte are predominantly [FSI]- anions, regardless if sourced from Li salt or RTIL (single FSI system); or

[0220] ( B) from about 20 mole% to about 60 mole% of Li+cations and [TFSI]- anions, where anions in the electrolyte are predominantly [FSI]- anions, regardless if sourced from Li salt or RTIL (mixed TFSI / FSI system), wherein the Li+conductive ionic liquid electrolyte solubilises one or more polysulfide species, LizSn, 4 < n < 8, to a saturation concentration at 25 °C of less than 1 mM atomic S, more preferably to a saturation concentration at 25 °C that is zero, or at least is below the limit of detection of UV spectroscopy.

[0221] Embodiment 29. A quasi-solid state (QSS) liquid electrolyte lithium-sulfur cell, preferably that operate with a S active material utilisation of >50% on a 40thdischarge cycle of the cell, the cell comprising: o a negative Li metal or Li metal alloy electrode (anode); o a positive S electrode (cathode); and o a Li+conductive ionic liquid electrolyte having at least one room temperature ionic liquid (RTIL) and at least one Li salt solubilised in the RTIL, wherein the Li+conductive ionic liquid electrolyte comprises an ammonium cation, and the electrolyte comprises:

[0222] (A) from about 20 mole% to about 60 mole% of Li+cations, where anions of the electrolyte are predominantly [FSI]- anions, regardless if sourced from Li salt or RTIL (single FSI system); or ( B) from about 20 mole% to about 60 mole% of Li+cations and [TFSI ]- anions, where anions in the electrolyte are predominantly [FSI]- anions, regardless if sourced from Li salt or RTIL (mixed TFSI / FSI system), wherein the Li+conductive ionic liquid electrolyte solubilises one or more polysulfide species, LizSn, 4 < n < 8, to a saturation concentration at 25 °C of less than 1 mM atomic S, more preferably to a saturation concentration at 25 °C that is zero, or at least is below the limit of detection of UV spectroscopy, wherein the positive S electrode (cathode) comprises at least one ionically conductive organic polymeric ionic liquid (PI L):Li salt binder mixture; and wherein the positive S electrode (cathode) further comprises boron nitride or an allotrope of boron nitride.

[0223] Embodiment 30. The cell of Embodiment 29, wherein the at least one ionically conductive organic polymeric ionic liquid (PI L):Li salt binder mixture comprising a composite of:

[0224] (i) at least one PIL selected from PIL cationic polymers with counter anions; PIL anionic polymers with Li+counter cations; and PIL cationic and / or anion block copolymers with corresponding counter anions or cations, preferably wherein the at least one PIL comprises:

[0225] • a poly ionic liquid cationic polymer comprising a plurality of one or more ionic liquid cations included in, or tethered as pendant groups, to an organic polymer backbone; and a plurality of one or more tethered or untethered counter anions;

[0226] • a poly ionic liquid anionic polymer comprising a plurality of one or more ionic liquid anions included in, or tethered as pendant groups, to an organic polymer backbone; and a plurality of Li+cations;

[0227] • a poly ionic liquid cationic and / or anion block copolymer comprising: a non-ionic block that does not contain ionic charge and comprises polymerised residues of hydrophobic monomers, and an ionic block comprising polymerised monomer residues having covalently coupled thereto one or more of

[0228] (a) a pendant organic ionic liquid cation, the pendant organic ionic liquid cation having a counter anion, and

[0229] (b) a pendant anionic moiety, the pendant anionic moiety having a counter cation; and

[0230] (ii) one or more Li salts.

[0231] Embodiment 31. The cell of Embodiment 30, wherein the PI L:Li Salt binder mixture is 50:50 mole% [PDADMA.TFSI]:UTFSI, 50:50 mole% [PMTFSI.TFSI]:UTFSI, or 50:50 mole% [PILblocCo- polymer(plmTFSI)]:LiTFSI.

[0232] EXAMPLES

[0233] The present invention is described with reference to the following examples. It is to be understood that the examples are illustrative of and not limiting to the invention described herein.

[0234] Example 1 - Basic S-electrode and cell preparation In a first example, the inventors have prepared a S electrode for use in the Li-S cells containing the electrolyte of the invention.

[0235] Electrode prep procedure: 15 wt % CMC, 21 wt % HSAC, 14 wt % C65 and 50 wt % S

[0236] Basic electrodes were prepared by making a slurry of high surface area carbon-sulfur composite (30-70 wt% HSAC-S), conductive carbon-C65, and carboxymethyl cellulose (CMC) binder keeping the mass ratio as 70:15:15, respectively. The solvent used with the CMC binder was deionized water (1 mL DI water / 20 mg of CMC-binder). The slurry was first cast by using the doctor blade gap of 400 pm on the carbon- coated aluminium current collector and then dried at 80°C in the oven for 24 h. After drying, the electrode thickness was 110 to 120 pm which reduced to 70 to 80 pm after calendering using a roll drum calendar (Media-tech). The calendared electrode was utilized for cell making. Further, the electrode is referred to as "15CMC" S-electrode.

[0237] Electrolyte composition preparation

[0238] The electrolytes solutions are prepared by direct addition of the Li salt ( Li FS 1 / LiTFSI ) in the desired concentration to the dried ionic liquid (Schlenk line at 50 °C, 0.6 kPa for 72 h) and dissolved with stirring and heat (24h at 50 °C). Preparation is performed in an Argon glovebox (Oa< 0.1 ppm and H2CX 0.1 ppm), the ionic liquid and Li salts are further dried on a Schlenk line at 50 °C, 0.6 kPa for 72 h. The obtained clear solutions are stored under argon at RT.

[0239] Cell assembly and electrochemical measurements:

[0240] The 15CMC S-electrode was cut into the disks of 8 mm diameter, area 0.5 cm-2Disks were then used as cathodes to fabricate Li-S cells. To assemble a CR 2032 coin cell, a 15CMC cathode was placed against a 100 pm thin lithium metal foil anode. A polypropylene (Celgard 3501) membrane was incorporated as a separator between the cathode and anode. 45 pL of electrolyte was utilized for each cell. The electrochemical measurements of Li-S cells were carried out at a constant temperature of 50 ± 2°C in the potential window of 1.5 -2.8 V versus Li / Li+at a current rate of 0.1 C (167 mA g1Sulfur).

[0241] Example 2 - PIL S-electrode and cell preparation

[0242] In a second example, the inventors have prepared a S electrode including a PIL for use in the Li-S cells containing the electrolyte of the invention.

[0243] Electrode prep procedure: 5 wt % [PDADMA.TFSI]:[LiTFSI], lOwt % CMC, 21 wt % HSAC, 14 wt % C65 and 50 wt % S

[0244] Here 50 : 50 mol% [PDADMA.TFSI] : [LiTFSI] is used as the PI L:Li salt binder mixture.

[0245] Electrodes were prepared by making a slurry of high surface area carbon-sulfur composite (30-70 wt% HSAC-S), conductive carbon-C65, 50:50 mole% [PDADMA.LiTFSI] : LiTFSI solution, and carboxymethyl cellulose (CMC) binder keeping the mass ratio as 70:15:5:10, respectively. Firstly, HSAC-S, conductive carbon-C65 and the Li-conducting PIL binder were mixed in acetone and dried, the fine powder of the dried mixture was further utilized to make the slurry with 10 wt. % of CMC binder pre-dissolved in deionized water (1 mL DI water / 20 mg CMC-binder). The slurry was first cast by using the doctor blade gap of 400 pm on the carbon coated aluminium current collector and then dried at 80°C in oven for 24 h. After drying, the electrode thickness was 110 to 120 pm which reduced to 70 to 80 pm after calendaring. The calendared electrode was utilized for cell making. Further the electrode referred as 5PD10CMC S-electrode.

[0246] Cell assembly and electrochemical measurements

[0247] The 5PD10CMC S-electrode was cut into the disks of 8 mm diameter, area 0.5 cm'2. Disks were then used as cathodes to fabricate Li-S cells. To assemble a CR2032 coin-cell, 15CMC cathode was placed with respect to 100 pm thin lithium metal foil anode. Polypropylene membrane was incorporated as a separator in between cathode and anode. The amount of electrolyte used was 45 pL for each cell. The electrochemical measurements of Li-S cells were carried out at a constant temperature of 50 ± 2°C in the potential window of 1.5 -2.8 V versus Li / Li+at a current rate of 0.1C (167 mA g1Sulfur).

[0248] Example 3 - Cell cycling performance

[0249] The results of cell cycling tests for the S cathodes of Examples 1 and 2 as used in cells with various electrolytes of the invention are provided below.

[0250] [Pnii4][FSI] System

[0251] Figure 1A illustrates the effect of a single [FSI]' anion ionic liquid based electrolyte system (IL1 is [Pnii [FSI] / Saltl is LiFSI) on cycling and S active material utilisation compared to an otherwise equivalent conventional solvent-based 1:1 DME:DOL cell. Figure 1 shows that the single [FSI]' anion ionic liquid based electrolyte system, 30 mole% LiFSI / [Pmi4] [FSI], dramatically improves the utilisation of the cathode active S material which leads to much higher discharge capacity (discharge capacity of approx. 1270 mAhg'1on 1stcycle, approx. 430 mAhg1on 20thcycle), compared to the cell with the conventional solvent-based electrolyte (discharge capacity of about 410 mAhg1on 1stcycle, approx. 240 mAhg1on 20thcycle). The initial cycles discharge capacities (lower line) are higher than the charge capacities (upper line) due to the amount of electrolyte decomposition and irreversible lithiation of sulfur which occurs during the initial cycle(s) on formation of the protective electrode-electrolyte interface, which appears to be fully formed after around 12 / 14 cycles for the cell of the invention.

[0252] Figure 2A illustrates the effect of salt concentration on the performance of the sulfur cathode in the single anion ionic liquid based electrolyte system (IL1 is [PHM] [FSI] / Saltl is LiFSI). Figure 2 shows that in the [PHM] [FSI] single FSI anion ionic liquid based electrolyte system (30 mole% LiFSI / [PUM] [FSI]), higher LiFSI salt content of 30 mole% salt leads to higher cathode utilization (discharge capacity of approx. 1270 mAhg1on 1stcycle, approx. 430 mAhg1on 20thcycle) compared to the lower salt concentration electrolyte (10 mole% LiFSI / [Pmj4] [FSI] ) in the same anion ionic liquid (discharge capacity of approx. 1040 mAhg1on 1stcycle, approx. 260 mAhg1on 20thcycle). This data supports the concept that for a single anion IL based electrolyte system, a certain minimum salt concentration of greater than 10 mole %salt concentration range and closer to 30 mole% of in the FSI anion IL electrolyte ensure greater reduction in polysulfide dissolution in the electrolyte and to ensure greater utilisation of the S active material provided in the positive electrode of the cells.

[0253] Figure 3A illustrates the effect of a mixed FSI / TFSI anion ionic liquid-based electrolyte (IL1 is [Pnii4] [FSI] / Salt2 is LiTFSI) on cell performance compared to conventional solvent-based 1:1 DME:DOL electrolyte. Figure 3 shows that for the mixed FSI / TFSI anion [PunJ [FSI] ionic liquid-based electrolyte (30 mole% LiTFSI / [Pmu] [FSI] (IV)), utilising LiTFSI instead of Li FSI salt drastically improves the utilisation of the cathode active S material, leading to much higher charge and discharge capacity in the mixed anion electrolyte (discharge capacity of approx. 1050 mAhg1on 1stcycle, approx. 740 mAhg1on 20thcycle), compared to the conventional solvent-based electrolyte (discharge capacity of approx. 410 mAhg1on 1stcycle, approx. 240 mAhg1on 20thcycle). While the single FSI system has a higher discharge capacity on the 1stcycle than the mixed system, at the 20thcycle, the discharge capacity (approx. 740 mAhg1) is much better than that of the single FSI system (approx. 430 mAhg1), suggesting at least for this S electrode, the mixed anion system is better for long term cycling performance.

[0254] Figure 4A illustrates the effect of salt concentration on the performance of the sulfur cathode in a mixed FSI / TFSI anion ionic liquid-based electrolyte (IL1: [Pnn [FSI] / Salt 2: LiTFSI). Figure 4 shows that in the mixed FSI / TFSI anion [Pma] [FSI] electrolyte, a higher LiTFSI salt content (30mol%) leads to improved cathode utilisation (discharge capacity of approx. 1050 mAhg1on 1stcycle, approx. 740 mAhg1on 20thcycle) compared to the lower LiTFSI salt concentration (10mol%) in the same [Pmi4] [FSI] ionic liquid (discharge capacity of approx. 1010 mAhg1on 1stcycle, approx. 370 mAhg1on 20thcycle). This is consistent with the hypothesis that higher concentrated mixed anion IL electrolytes reduce polysulfide dissolution in the electrolyte more than the lower concentration mixed anion ILs. Likewise, the higher concentration contributes to more stable and conductive electrode interphases (i.e., lower cell polarisation), leading to higher utilisation and sustained capacity during cycling.

[0255] Figure 5A illustrates analysis of electrochemical performance of sulfur-electrode [15 wt % CMC, 21 wt % HSAC, 14 wt % C65 and 50 wt % S] in [Pmu] [FSI] electrolyte system in terms of capacity and polarization. Figure 5A compares the 10thand 20thcycle capacity in single and mixed anion IL electrolyte systems with varying concentration of Li salt (10 mole% and 30 mol%). The difference between the capacities depicts the suitability of the electrolyte composition for use in Li-sulfur batteries. The results show that the mixed FSI / TFSI anion IL electrolyte, 30 mole% LiTFSI / [Pmu] [FSI], is the best-performing electrolyte in terms of highest discharge capacity at cycle 20, i.e., approx. 740 mAhg1for the 30 mole% salt mixed FSI / TFSI anion systems vs about approx. 260 mAhg1for the 10 mole% salt single FSI anion system. Given the significantly higher utilisation of S behind these results, it is evident that the higher 30 mole% concentration salt electrolyte are preferred over the lower 10 mole% concentration, and further the mixed anion system is superior to the single anion system in terms of discharge capacity achievable for long term cycling of 20 cycles.

[0256] Further, the polarization can be defined as the difference between charge and discharge voltage plateau at 50 % of depth of charge (DOC) and depth of discharge (DOD) for a given cell assembly. The degree of polarization difference between the 1st and 2nd cycle reflects the decreased resistance of the electrode-electrolyte interphase formed during 1st lithiation whereby lower resistance interphase is much preferred. The data show that all electrolytes show decreased resistance of the electrode-electrolyte interphase after 1st lithiation. However, the electrical properties of the interphase formed during 1st lithiation in both 50 mole% of LiFSI / [PIHM] [FSI] (single FSI anion) electrolyte and 30 mole% of LiTFSI / [Pmu] [FSI] (mixed FSI / TFSI anion) electrolyte system are better than the lower salt concentration equivalent of same single FSI anion or mixed FSI / TFSI anion system (10mol% LiFSI / [Pnii [FSI] or 10 mole% LiTFSI / [Pnii4][FSI]) in the same ionic liquid. This indicates that the concentration of salt in either system should be at minimum greater than 10 mole% and indeed at a minimum closer to at least 30 mole% for the best performance. Further, the polarization difference between the 2ndand 20threflects the chemical and mechanical stability of the interphase. The data show that the 30 mole%TFSI and 50 mole% FSI are very similar in terms of desirably polarisation differences.

[0257] Figure 6A illustrates the effect of inclusion of a (poly)ionic liquid (PIL) binder in the cathode composition on the performance of a cell with mixed FSI / TFSI salt IL system (IL1 is [PUM] [FSI] / Salt 2 is LiTFSI). Figure 6A shows that in the mixed FSI / TFSI system with lower concentration of salt, 10 mole% LiTFSI / [Pnii4] [FSI], the effect of the addition of a polyionic liquid (PIL) binder ([PDADMA][TFSI]) at 5 wt% to the typically used CMC binder results in greater cathode active material utilization and therefore higher capacity retention in the first 20 cycles (discharge capacity of approx. 810 mAhg1on 20th cycle with PIL v discharge capacity of approx. 370 mAhg1on 20thcycle without PIL). The PIL greatly improves performance of the cell with the mixed FSI / TFSI system of 10 mole% LiTFSI / [Pmu] [FSI] compared to the cell with the mixed FSI / TFSI system of 10 mole% LiTFSI / [Pmu] [FSI] but without the PIL.

[0258] Figure 7A illustrates the effect of the (poly)ionic liquid (PIL) binder addition on different electrolyte salt contents on the performance of the sulfur cathode in a high and low concentration mixed FSI / TFSI salt IL system (IL1 is [Pmi4] [FSI] / Salt 2 is LiTFSI). The effect of PIL binder addition is investigated in the mixed FSI / TFSI systems having two different salt contents (10 and 30 mole%). The PIL increased the capacity retention in the first 20 cycles for both systems. The results show good cathode active material utilization in both electrolyte systems. However, the mixed FSI / TFSI salt IL electrolyte containing 30 mole% LITFSI shows the highest capacity retention in the first 20 cycles (discharge capacity of approx. 1020 mAhg1on 20thcycle with PIL in 30 mole% salt v discharge capacity of approx. 730 mAhg1on 20thcycle without PIL). Including PIL in the cathode of a cell with the higher concentration mixed anion electrolyte, 30 mole% LiTFSI / [Pnii4] [FSI] boosts the discharge capacity to more than 1000 mAhg1on the 20thcycle.

[0259] Figure 8A illustrates the effect of BNNT addition on the performance of the sulfur cathode in a mixed FSI / TFSI salt IL system (IL1 is [PUM] [FSI] / Salt 2 is LiTFSI / binder is PDADMA / CMC). Figure 8 shows that addition of both 0.5 wt% BNNT and 5 wt% Li conducting PIL binder mixture into the cathode results in slightly improved capacity retention at 20 cycles (discharge capacity of approx. 1100 mAhg1on 20thcycle with PIL in 30 mole% salt v discharge capacity of approx. 1050 mAhg1on 20thcycle without BNNT). Moreover, the replicate cell cycling is more reliable and less variable than cathodes without BNNTs. Therefore, the data supports the finding that addition of BNNT into the S cathode results in improved longterm cycling performance.

[0260] Figure 9A illustrates the effect of a much-increased electrolyte salt content (50 mole%) on the performance of the PIL binder-containing sulfur cathode in a single FSI anion IL system. Figure 9 shows that in the single FSI anion [PUM] [FSI] electrolyte, a very high LiFSI salt content of 50 mole% LiFSI leads to further improved cathode utilization compared to the lower salt concentrations (10 mole% and 30 mole%) in the same ionic liquid (discharge capacity of approx. 1025 mAhg1on 20thcycle with PIL in 50 mole% salt v discharge capacity of approx. 500 mAhg1on 20thcycle with PIL in 10 mole% salt, and 450 mAhg1on 20thcycle with no PIL in 30 mole% salt). This data shows that very high salt concentrations are preferred for the single FSI anion IL electrolyte systems of the invention.

[0261] Figure 10A illustrates the effect of increased (LiTFSI) electrolyte salt content of 50 mole% on the performance of the PIL binder-containing sulfur cathode in a mixed FSI / TFSI anion IL system. Figure 10 shows that for the mixed FSI / TFSI anion [Pmi4] [FSI] electrolyte, a very high LiTFSI salt content of 50 mole% LiTFSI leads to decreased cathode utilization at the 20thcycle compared to the lower LiTFSI salt concentration of 10 mole% in the same ionic liquid (discharge capacity of approx. 750 mAhg1on 20thcycle with PIL in 50 mole% salt v discharge capacity of approx. 850 mAhg1on 20thcycle with PIL binder mixture in 10 mole% LiTFSI salt). Therefore, for the mixed FSI / TFSI anion IL systems, very high concentration of LiTFSI of 50 mole% or over are not desired for optimised performance in terms of high-capacity retention after many cycles. For optimum cycle performance at 20 cycles, for the mixed anion system, the salt content should be 20 mole% or above but less than 50 mole% concentration.

[0262] Figure 11 illustrates analysis of electrochemical performance of sulfur-electrode [5 wt % [PDADMATFSI] :[LiTFSI], 10wt% CMC, 21 wt% HSAC, 14 wt% C65 and 50 wt% S] in [Pnii4] [FSI] electrolyte system. In Figure 11(a), the 10th, 20thand 40thcycle capacity and differences therebetween depicts the suitability of the electrolyte composition for PIL containing sulfur-electrode in Li-sulfur batteries, where the 30 mole% LiTFSI mixed anion system and the 50 mole% LiFSI single anion system give the highest discharge capacity retention on 20thand 40thcycles, signifying they are the best electrolytes for the Li-S cell performance. The data confirms that the mixed FSI / TFSI anions ILs containing 30 mole% of LiTFSI / [Pnii [FSI], and single anion systems containing 50 mole% of LiFSI / [Pmi4] [FSI] are the best-performing electrolytes. In Figure 11(b), the polarization difference between 1stand 2ndcycle reflects the decreased resistance of the electrode-electrolyte interface formed during 1stlithiation. All electrolyte compositions show decreased electrode-electrolyte interface resistance after 1stlithiation. The electrical properties of the interface formed during 1stlithiation in both 30 mole% of LiTFSI / [PUM] [FSI] and 50 mole% LiFSI / [Pnii [FSI] electrolyte systems are better than the lower salt concentration electrolyte (10 mole% LiFSI or LiTFSI) and the very high concentration (50 mole% TFSI) electrolyte using the same ionic liquid. The polarization difference between the 2ndand 20thcycle reflects the chemical and mechanical stability of the interface where the lower salt concentration electrolyte (10 mole% LiFSI or LiTFSI) give the poorest result, indicating the least optimised interphase.

[0263] Figure 12 illustrates an analysis of PIL vs non-PIL binder S cathodes wrt electrochemical performance of sulfur-electrode [5 wt % [PDADMATFSI]:[LiTFSI], lOwt % CMC, 21 wt % HSAC, 14 wt % C65 and 50 wt % S] vs [15 wt % CMC, 21 wt % HSAC, 14 wt % C65 and 50 wt % S] in [Pnii4] [FSI] electrolyte system. Figure 12 shows that for all of the single FSI anion and mixed FSI / TFSI anion [PUM] [FSI] electrolytes, a PIL containing s-electrode performs better than a non-PIL containing S-electrode in the otherwise same cell system. Furthermore, inclusion of PIL shows cell polarization behaviour of the PIL containing S-electrode is smallerthan non-PIL containing S-electrode in an otherwise same cell system. The S active utilization of the PIL containing S-electrodes is higher than for otherwise equivalent non- PIL containing S-electrodes. Capacity retention of the PIL containing s-electrodes are more than non- PIL containing S-electrodes. The data suggests that for the best results in terms of long term discharge capacity retention, the electrolytes of the invention should be used in combination with S cathode that include a PIL binder.

[0264] [NIH3][FSI] System

[0265] Figure IB illustrates the effect of a single [FSI]- anion ionic liquid based electrolyte system (IL2 is [NUB] [FSI] / Saltl is LiFSI) on cycling and S active material utilisation compared to an otherwise equivalent conventional solvent-based 1:1 DME:DOL cell. Figure IB shows that the single [FSI]- anion ionic liquid based electrolyte system, 30 mole% LiFSI / [NUB] [FSI], dramatically improves the utilisation of the cathode active S material which leads to much higher discharge capacity (discharge capacity of approx. 1291 mAhg1on 1stcycle, approx. 565 mAhg1on 20th cycle), compared to the cell with the conventional solvent-based electrolyte (discharge capacity of about 400 mAhg1on 1stcycle, approx. 300 mAhg1on 20thcycle). Overall, the performance of the 30 mole% / [Pmi4] [FSI] System and the 30 mole% / [NUB] [FSI] System are similar, with a slightly better discharge capacity in the case of the ammonium system at the 20thcycle.

[0266] Figure 2B illustrates the effect of salt concentration in a single FSI electrolyte on the cycling performance (IL2: [Nma] [FSI] / Salt 1: LiFSI) for a cycled LiS cell in 30 mole% LiFSI / [NUB] [FSI] IL electrolyte at C / 10 at 50 °C compared to a LiS cell involving 20 mole% LiFSI / [NUB] [FSI] IL electrolyte at C / 10 at 50 °C. Thus, Figure 2B illustrates the effect of high and low salt concentration on the performance of the sulfur cathode in the single anion ionic liquid based electrolyte system (IL2 is [Nun] [FSI] / Saltl is LiFSI). Figure 2B shows that in the [Nun] [FSI] single FSI anion ionic liquid based electrolyte system (30 mole% LiFSI / [Nina] [FSI] ), a higher LiFSI salt content of 30mol% salt leads to slightly higher cathode utilization (discharge capacity of approx. 1300 mAhg1on 1stcycle, approx. 699 mAhg-1 on 20thcycle) compared to the lower salt concentration electrolyte (20 mole% LiFSI / [Nim][FSI]) in the same anion ionic liquid (discharge capacity of approx. 1300 mAhg1on 1st cycle, approx. 550 mAhg1on 20thcycle). In this case, a lower LiFSI concentration of 10 mole% was not possible, so 20 mole% LiFSI in [Nim][FSI] (IL-2) as the lowest viable concentration. Note, [NUB] [FSI] is solid at RT, and after mixing 20 mol% Ll-salt the composition is in liquid form. The inventors have found that if less than 20 mole% Li-salt is used with [NUB] [FSI], the viscosity of electrolyte is unsuitably high.

[0267] Figure 3B illustrates the effect of a mixed FSI / TFSI anion ionic liquid-based electrolyte (IL2 is [NIH3] [FSI] / Salt2 is LiTFSI) on cell performance compared to conventional solvent-based 1:1 DME:DOL electrolyte. Figure 3B shows that for the mixed FSI / TFSI anion [NmaHFSI] ionic liquid-based electrolyte (30 mole% LiTFSI / [NUB] [FSI], utilising LiTFSI instead of LiFSI salt drastically improves the utilisation of the cathode active S material, leading to much higher charge and discharge capacity in the mixed anion electrolyte (discharge capacity of approx. 1300 mAhg1on 1stcycle, approx. 700 mAhg1on 20thcycle), compared to the conventional solvent-based electrolyte (discharge capacity of approx. 400 mAhg1on 1st cycle, approx. 450 mAhg1on 20thcycle). The results suggest that the ammonium single and mixed anion systems perform quite similarly, in contrast to the phosphonium system where the mixed anion system gave better performance.

[0268] Figure 4B illustrates the effect of low (20 mole%) and medium (30 mole%) salt concentration on the performance of the sulfur cathode in a mixed FSI / TFSI anion ammonium ionic liquid-based electrolyte (IL2: [Nma] [FSI] / Salt 2: LiTFSI). Figure 4B shows that in the mixed FSI / TFSI anion [N1113] [FSI] electrolyte, a higher LiTFSI salt content (30mol%) leads to discharge capacity of approx. 1050 mAhg-1 on 1st cycle, approx. 736 mAhg-1 on 20th cycle) compared to the lower LiTFSI salt concentration (20mol%) in the same [NUB] [FSI] ionic liquid (discharge capacity of approx. 1350 mAhg-1 on 1st cycle, approx. 650 mAhg-1 on 20th cycle). Even though the 20 mole% system gave a slightly higher S utilisation on the 1st discharge cycle, at the 20th cycle, the 30 mole% system performs better. This is consistent with the hypothesis that higher concentrated mixed anion IL electrolytes reduce polysulfide dissolution in the electrolyte more than the lower concentration mixed anion ILs. Likewise, the higher concentration contributes to more stable and conductive electrode interphases (i.e., lower cell polarisation), leading to higher utilisation and sustained capacity during cycling.

[0269] Figure 6B illustrates the effect of inclusion of a (poly)ionic liquid (PIL) binder in the cathode composition on the performance of a cell with mixed FSI / TFSI salt ammonium IL system (IL2 is [NUB] [FSI] / Salt 2 is LiTFSI). Figure 6B shows that in the mixed FSI / TFSI ammonium system with lower concentration of salt, 20 mole% LiTFSI / [NinslfFSI], the effect of the addition of a polyionic liquid (PIL) binder ([PDADMA][TFSI]) at 5 wt% to the typically used CMC binder results in greater cathode active material utilization and therefore higher capacity retention in the first 20 cycles (discharge capacity of approx. 1500 mAhg-1 on 20th cycle with PIL v discharge capacity of approx. 1200 mAhg-1 on 20th cycle without PIL). As was the case for the phosphonium system, the PIL greatly improves performance of the cell with the mixed FSI / TFSI ammonium system of 20 mole% LiTFSI / [Pmi4] [FSI] compared to the same cell without the PIL. Figure 7B illustrates the effect of the (poly)ionic liquid (PILI) binder addition on different electrolyte salt contents on the performance of the sulfur cathode in a medium (30 mole%) and low (20 mole%) concentration mixed FSI / TFSI salt IL system (IL2 is [Nma] [FSI] / Salt 2 is LiTFSI). The effect of PILI binder addition is investigated in the mixed FSI / TFSI anion ammonium IL systems having two different salt contents (20 mole% and 30 mole%). The PILI in the cathode increased the capacity retention in the first 20 cycles for both systems. The results show good cathode active material utilization in both electrolyte systems. However, the mixed FSI / TFSI salt ammonium IL electrolyte containing 30 mole% LITFSI shows the highest capacity retention in the first 20 cycles (discharge capacity of approx. 700 mAhg-1 on 20th cycle with PIL in 30 mole% salt v discharge capacity of approx. 1020 mAhg-1 on 20th cycle without PIL). Including PIL in the cathode of a cell with the higher concentration mixed anion electrolyte, 30 mole% LiTFSI / [N ma] [FSI] boosts the discharge capacity to more than 1000 mAhg-1 on the 20th cycle. Figure 8B illustrates the effect of BNNT addition on the performance of the sulfur cathode in a mixed FSI / TFSI anion ammonium IL system (IL2 is [NUB] [FSI] / Salt 2 is LiTFSI / binder is PDADMA / CMC). Figure 8B shows that addition of both 5 wt% BNNT and 5 wt% PIL binder into the cathode results in slightly improved capacity retention at 20 cycles (discharge capacity of 1033 mAhg-1 on 20th cycle with PIL in 30 mole% salt v discharge capacity of approx. 1020 mAhg-1 on 20th cycle without BNNT). Moreover, the replicate cell cycling is more reliable and less variable than cathodes without BNNTs. Therefore, the data supports the finding that addition of BNNT into the S cathode results in slightly improved long-term cycling performance.

[0270] Figure 9B illustrates the effect of a much-increased electrolyte salt content (50 mole%) on the performance of the PIL binder-containing sulfur cathode in a single FSI anion ammonium IL system.

[0271] Figure 9B shows that in the single FSI anion phosphonium [Pmi4] [FSI] electrolyte, a very high Li FSI salt content of 50 mole% LiFSI leads to further improved cathode utilization compared to the lower salt concentrations (10 mole% and 30 mole%) in the same ionic liquid (discharge capacity of approx. 1016 mAhg-1 on 20th cycle with PIL in 50 mole% salt v discharge capacity of approx. 691 mAhg-1 on 20th cycle with PIL in 10 mole% salt). This data shows that very high salt concentrations are preferred for the single FSI anion ammonium IL electrolyte systems of the invention.

[0272] Figure 10B illustrates the effect of increased (LiTFSI) electrolyte salt content of 50 mole% on the performance of the PIL binder-containing sulfur cathode in a mixed FSI / TFSI anion ammonium IL system. Figure 10B shows that for the mixed FSI / TFSI anion [NUB] [FSI] electrolyte, a very high LiTFSI salt content of 50 mole% LiTFSI leads to similar performance as the 20 mole% system in the same ionic liquid (discharge capacity of approx. 830 mAhg-1 on 20th cycle with PIL in 50 mole% salt v discharge capacity of approx. 823mAhg-l on 20th cycle with PI L in 20 mole% LiTFSI salt). However the capacity on the 1st cycle is higher for the 50mole% TFSI / [NmaHFSI] electrode. Therefore, for the mixed FSI / TFSI anion ammonium IL systems, very high concentration of LiTFSI of 50 mole% or over are not desired for optimised performance in terms of high-capacity retention after many cycles. For optimum cycle performance at 20 cycles, for the mixed anion ammonium system, the salt content should be above 10 mole% but less than 50 mole% concentration.

[0273] Figure 10C illustrates the effect of different salts ( Li FSI / LiTFSI ) on the performance of the PIL binder-containing sulfur cathode (IL2: [NUB] [FSI] / binder: PDADMA / CMC) of a cycled of a LiS cell (5 wt % [PDADMA.TFSI]:[LiTFSI], lOwt % CMC, 21 wt % HSAC, 14 wt % C65 and 50 wt % S) in 50 mol% LiFSI / [NIH3] [FSI] IL electrolyte at C / 10 at 50 °C, compared to a cycled LiS cell (5 wt % [PDADMA.TFSI]:[LiTFSI], lOwt % CMC, 21 wt % HSAC, 14 wt % C65 and 50 wt % S) in 50 mol% LiTFSI / [Nm3][FSI] FSI IL electrolyte, cycled at C / 10 at 50 °C. The data shows that for the [NUB] [FSI] electrolyte, a higher LiFSI salt content of 50 mole% leads to improved cathode utilization compared to the higher 50 mole% LiTFSI salt concentration in the same ionic liquid. This might be due to the high viscosity of 50 mole% LiTFSI-containing electrolytes. Figure 34 shows the physiochemical properties of the mixed anion based ionic liquid electrolytes; namely 1.4 mol / kg LiTFSI / PmwFSI, 2.1 mol / kg LiTFSI / PmwFSI; 3.2 mol / kg LiTFSI / PmwFSI, 3.2 mol / kg LiTFSI / Pmi4FSI+10wt% DME, 3.2 mol / kg LiTFSI / Pmi4FSI+20wt% DME (a) Ionic conductivity test of the as-prepared mixed anion based ionic liquid electrolytes; (b) Diffusion coefficient of 7Li in various electrolytes measured by pulsed field gradient nuclear magnetic resonance spectroscopy (PFG-NMR) as a function of temperature. The data shows that the ionic conductivity decreases with salt increasing concentration owing to increasing viscosity. The addition of DME diluent increases the ionic conductivity from 0.59 mS / cm for 3.2 mol / kg LiTFSI / Pnii4FSI to 3.9 mS / cm for 3.2 mol / kg UTFSI / PHMFSI+20wt% DME at 50°C, which is close to 1.4 mol / kg LiTFSI / Pnii4FSI (5.4 mS / cm). Li+diffusion coefficients are largely affected by salt concentration; 1.4 mol / kg LiTFSI / Pnii4FSI has the highest diffusion coefficient number at 50°C, with 6.35 x 10-13 m2 / s. 3.2 mol / kg LiTFSI / Pnii4FSI+20wt% DME electrolyte has the second highest diffusion coefficient value with 5.45 x 10-13 m2 / s, and 3.2 mol / kg LiTFSI / Pmi4FSI has the lowest value of 1.18 x 10-13 m2 / s The above results clearly demonstrate the superior physiochemical properties for 1.4 mol / kg LiTFSI / Pmi4FSI and 3.2 mol / kg LiTFSI / Pnii4FSI+20wt% DME electrolytes, which corresponds to good electrochemical performance for the Li / Li symmetrical cell in terms of the smallest overpotential and most stable cycling behaviour that will be discussed below.

[0274] Example 4 - Polysulfide dissolution testing

[0275] In a third example, polysulfide solubility in the mixed FSI / TFSI anion electrolytes was examined.

[0276] Mixing of LizS and S-powder in Ionic-liquid electrolytes

[0277] Polysulfide dissolution in electrolyte is one of the major challenges in Li-S batteries, which cause shuttling of polysulfides between lithium anode and sulfur cathode, which results in poor utilisation of the S active material and rapid capacity fade. Stoichiometric amounts of elemental sulfur and U2S powder were mixed together in different compositions of IL-electrolyte (LiTFSI+[Pmi4] [FSI] and LiTFSI+[Nma] [FSI]), whereby the ratio of sulfur and LizS added corresponds to LizSs- In more detail, 50 mg of (LizS+S mixture in LizSs stoichiometry) was used in every sample during the experiment and stirred at with heat for a prolonged period long time so as to provide enough time, temperature, and mount of PS to form and dissolve. Finally, the mixture has been centrifuged to separate out the un-reacted LizS and S from liquid solution. The solutions were stirred for 120 hours at 50°C to ensure saturation of the solutions. The purpose of this experiment was to determine any colour change of the electrolytes+S+LizS solutions that would be attributable to intermediate polysulfide dissolution in the solutions, and to detect / determine any trace amount of dissolved polysulfides by utilizing the optical / visual examination, UV Vis spectroscopy and Raman Spectroscopy which is particularly sensitive towards the detection of sulfur and polysulfides. Electrolytes of different compositions including 0 (blank / neat I L-[Pnii4] [FSI] without salt), 30 mole% of LiTFSI in [Pmi4] [FSI] and 30 mole% of LiTFSI in [NmslfFSI] have also been tested.

[0278] Example 5 - Polysulfide dissolution testing - visual / optical solubility studies

[0279] Electrolytes containing 0, 5, 10, 20, 30, 40, 45, and 50 mole% LiTFSI / [Pmi4] [FSI] solutions were made. Stoichiometric amount of elemental sulfur and U2S powder were mixed into the different electrolytes and the solutions were stirred for 48 hours at 50°C. The objective of these experiments was to examine polysulfide dissolution in the phosphonium IL electrolytes containing low (<20 mole% LiTFSI salt) and high concentration of lithium salt (>20 mole% LiTFSI salt). Observation of a colour change of electrolytes+S+LizS solutions would signify formation and dissolution of any higher order polysulfides.

[0280] Dark red colour in the blank / neat IL is due to the formation of intermediate polysulfides LizSn (4 < n < 8) which readily dissolve in the neat IL and give it a strong colour and readily visually detectable colour to the solution.

[0281] Figure 13 illustrates optical images of neat-IL 0, 30, 40, 45, and 50 mole% LiTFSI / [PUM] [FSI] electrolyte before and after addition of Sgand LizS, even after 24h and 3-months rest.

[0282] Figure 16 illustrates optical images of solutions of [PHM] [FSI] with 5, 10, or 20 mole% LiTFSI, before and after Sg+LizS addition. No visible colour change occurs in the >30 mole% LiTFSI / [Pmi4] [FSI] ionic liquid solution occurred, signifying ultralow or negligible intermediate polysulfide dissolution in the electrolytes. The lower concentrations 5, 10, or 20 mole% LiTFSI / [Pmi4] [FSI] produced an observable colour change in all three electrolytes containing <20 mole% of LiTFSI.

[0283] Interestingly, it was noted that in the first hour after mixing, the intensity of the red colour is in reverse order of the LiTFSI concentration in the electrolyte, that is, the intensity of the red colour decreases with increasing LiTFSI concentration, signifying that the amount of PS dissolution depends on the LiTFSI salt concentration present in the electrolyte.

[0284] Further interestingly, as time progressed, the red colour of the lower concentration solutions disappeared, and the solution became colourless again. The time required for the solutions to become colourless again is in reverse order of LiTFSI concentration in electrolytes. That is low concentration salt electrolyte were slower to revert to colourless than the higher concentration salt electrolytes. Finally, the colour for all three solutions including the low 5 mole% LiTFSI salt concentration disappear within 48 hours. The result from this experiment indicates that soluble polysulfides are formed in the low concentration electrolytes but that the solubility is reversed over time and the dissolved PS precipitates out of solution due to the lower solvation energy for polysulfides in the electrolytes described herein. Notably, any PS formed during cell cycling immediately dissolves in the electrolyte to the degree support by the particular electrolyte used.

[0285] Example 6 - UV spectroscopy study of polysulfide solubility

[0286] Figure 17 illustrates UV spectroscopy (UV-2600, SHIMADZU) used to analysis of polysulfides solubility in [Pmu] [FSI] and in 30 mole% LiTFSI / [Pmi4] [FSI]. While it is challenging to analyze the exact composition of the polysulfide species from the UV-vis spectra only, the assignment of the UV-vis absorption bands in organic solvents has been reported as 490-500 nm for Sg2-, 450-470 nm for Sg2“, ~420 nm for S42-. The present study involved UV analysis of a 30 mole% LiTFSI in [Pmu] [FSI] solution which was saturated with LizSs and different molar concentration of LizSn (n > 4) containing [Pmu] [FSI] solutions. As shown in Figure 17, the black-color graph (1) shows almost zero absorbance above 400 nm signifying that none of the above PS species (Ss2-, Sg2“, S42-) are solubilized in the 30 mole% LiTFSI in [PUM] [FSI] solution, at least to a level that is detectable by UV spectroscopic analytical method. In contrast, the lowest concentration 1.25 mM S containing [Pmu] [FSI] solution without Li salt (green color (4)) shows a significant UV absorption value. This signifies that the concentration of atomic S in the 30 mole% LiTFSI in [Pmj4] [FSI ] trace (1) is significantly lower than 1.25 mM S. It is most likely at a concentration below the limit of quantification, or indeed below the limit of detection via the UV spectroscopic method used, suggesting an ultralow or negligible solubility that is zero or close to zero. This is in stark contrast to the best prior art FSI based electrolyte 0.5 mol.kg-1 Li[TFSI] / [Pi3][FSI] (mixed FSI / TFSI anion system; approx. 13.4 mol%) which had an atomic S concentration between 1 and 10 mM.

[0287] Example 7 - Raman Spectroscopy of Polysulfide dissolution test

[0288] In a next example, polysulfide solubility in the mixed FSI / TFSI anions electrolytes was examined by Raman spectroscopy which is ultrasensitive for dissolved PS species. Renishaw inVia Visible Raman Microscope, 633nm laser, power 17mW, 20x magnification was used to perform the test. The limit of detection of the technique can be increased by using an increasingly higher power laser in the technique. Sample preparation for Raman Spectroscopy:

[0289] As before, the goal of the sample preparations was to saturate the IL electrolyte with polysulfides. After vigorous mixing at 50 °C, each mixture was centrifuged to separate unreacted sulfur and precipitated U2S solid particles from the liquid phase (supernatant) of mixture. The supernatant and dissolved, saturated polysulfide is then transferred to transparent glass narrow capillary tubes (I) to (V) for Raman analysis. Capillary tubes (III) and (V) were loaded with the blank Li salt / IL electrolyte (Figure 14).

[0290] The Raman spectra of the dark red colour solutions formed after mixing of sulfur and U2S powder in the neat [Pmj4] [FSI] ionic liquid (0 mole% Li salt) readily shows Raman peaks of dissolved lithium polysulfides at 438 and 474 cm1at a low laser power of just 5 % of LAZER power (0.85 mW). The Raman shift due to polysulfide is at positions 437 and 474 cm1which are believed to be attributable to an equilibrium disproportionation of high order polysulfides (Li2Sn, 4 < n < 8). As clear in Figure 15(a) and zoomed view Figure 15(d), the dissolved PS peaks are sharp and intense. Notable, the Raman spectra of this sample required illumination by a low power LAZER of just 5% of LAZER power (0.85 mW) power, signifying that large quantity of dissolved PS is present as they are readily detectable at just 0.85 mW power.

[0291] In contrast, the Raman spectra of the colourless solution resulting from mixing of sulfur and U2S powder in 30 mole% LiTFSI in [Pmi4] [FSI] ionic liquid is shown in Figure 15(b) and its zoomed view in Figure 15(e). The polysulfides peaks at 438 and 474 cm1are not apparent. The Raman spectra of blank electrolyte i.e. 30 mole% LiTFSI in [PUM] [FSI] ionic liquid (no S and Li2S added) is shown in Figure 15(c) and its zoomed view in Figure 15(e). Notably, both samples of Figure 15(b) and 15(c) are illuminated by a higher LAZER 100% (17 mW) power, which is 20 times more than needed to detect the PS peaks in the neat (0 mole% salt) electrolyte (Figure 15(a)). The purpose of illumination with high power LAZER is to further increase the sensitivity of the technique for the detection of lithium polysulfides. Even at higher power, the Raman peaks for polysulfide are essentially absent.

[0292] The Raman spectra of another colourless solution formed after mixing of sulfur and U2S powder in 30 mole% LiTFSI in [NUB] [FSI] ionic liquid is shown in Figure 15(g) and its zoomed view in Figure 15(i). The Raman spectra of the blank electrolyte i.e. 30 mole% LiTFSI in [Nma] [FSI] ionic liquid is shown in Figure 15(h) and its zoomed view in Figure 15(j). There is no significant difference between the RAMAN spectra in Figure 15(g) and Figure 16(h), which is attributed to the practical insolubility of lithium polysulfides in 30 mole% [Li] [TFSI] in [NmsHFSI] ionic liquid electrolyte.

[0293] Example 8 - Cell impedance / EIS analysis of LiS cell

[0294] Interface studies were carried out by EIS technique. Figure 18 illustrates cell impedance analysis via comparison of Nyquist plots which were obtained for different Li-S cells containing [Pnii [FSI] electrolyte system with different Li-salt concentrations, (a) 25 mole% LiTFSI and 25 mole% LiFSI, (b) 50 mole% LiFSI, (c) 15 mole% LiTFSI and 15 mole% LiFSI, and (d-e) 30 mole% LiTFSI.

[0295] Irrespective of the electrolyte composition used, after the 1stlithiation of the S-electrode, the cell resistance decreased significantly which is thought to be attributable to the modification of the Sulfur- electrode and electrolyte interface. The interface modification is thought to be due to the good electrodeelectrolyte interface formation capability of the electrolytes of the invention which are generated through the electrochemical reduction of electrolyte components during the initial cycle.

[0296] After the lithiation of the sulfur electrodes, the cell resistance is stable. Figure 18(e) shows a plot of the cell resistance after cycling for up to the 30thcycle, and there is no observable significant change in cell resistance which is attributed to the stable interface of Li-S Batteries in the electrolyte system of the invention. In particular, the S electrode and electrolyte interface is very stable. This evidence of formation of a stable interface indicates a system where there is almost negligible parasitic reactions occurring at the S-electrode electrolyte interface.

[0297] Example 9 - Cell polarisation / overvoltage analysis of LiS cell

[0298] To further understand the impact of a stable S electrode-electrolyte interface, voltage vs capacity graphs for the Li-S cell were collected to consider cell polarisation / overvoltage. The results are shown in Figure 19 which illustrates a cell overpotential analysis carried out by galvanostatic cycling stability study from the 5thto 15thcycle of a Li / S cell with microporous carbon-sulfur cathode with PIL binder and CMC (5PD10CMC) with a sulfur loading of 2 mg cm'2at 1 / 10 C (1C = 1672 mA h / g), where the cell uses the 30 mole% LiTFSI / [Pmi4] [FSI] electrolyte of the invention. The voltage difference between the charge and discharge of the cell corresponds to polarization which is directly related to the stability of the S electrodeelectrolyte interface. Generally, the polarization increases as resistance of the Sulfur-electrode-electrolyte interface increased. A high resistance interface will be reflected in a highly polarised cell, with large overpotential. The capacity fade in Li-S cell (Figure 19(a) voltage vs real capacity graph) might be attributable to a variety of possible reasons associated with sulfur electrodes, so for a better understanding of polarization during cycling, the charge and discharge capacity was normalized (Figure 19(b) voltage vs normalized capacity graph of the same cell). Overall, there is no significant change in polarization during cycling, which indicates that the S electrode-electrolyte interface formed from the electrolytes of the invention is quite stable and exhibits low resistance during cycling in the electrolytes of the invention. It is believed this effect in combination with the ultralow / zero / negligible PS solubilising power of the electrolytes of the invention means excellent S active utilisation / discharge capacity on cycles of at least up to 40.

[0299] able 1 - Summary of discharge capacities of exemplary Li-S cells

[0300] able 2

Claims

1. Claims1. A quasi-solid state (QSS) liquid electrolyte lithium-sulfur cell comprising: o a negative Li metal or Li metal alloy electrode (anode); o a positive S electrode (cathode); and o a Li+conductive ionic liquid electrolyte having at least one room temperature ionic liquid (RTIL) and at least one Li salt solubilised in the RTI L, wherein the Li+conductive ionic liquid electrolyte comprises an ammonium cation, and the electrolyte comprises:(A) from about 20 mole% to about 60 mole% of Li+cations, where anions of the electrolyte are predominantly [FSI]- anions, regardless if sourced from the Li salt or the RTIL (single FSI system); or(B) from about 20 mole% to about 60 mole% of Li+cations and [TFSI ]- anions, where anions in the electrolyte are predominantly [FSI]- anions, regardless if sourced from Li salt or RTIL (mixed TFSI / FSI system), wherein the Li+conductive ionic liquid electrolyte solubilises one or more polysulfide species, LizSn, 4 < n < 8, to a saturation concentration at 25 °C of less than 1 mM atomic S, more preferably to a saturation concentration at 25 °C that is zero, or at least is below the limit of detection of UV spectroscopy for the one or more polysulfide species.

2. The cell of claim 1, wherein the Li+conductive ionic liquid electrolyte comprises in(A) from about 35 mole% to about 55 mole% of Li+cations, where anions of the electrolyte are predominantly [FSI]- anions, regardless if sourced from Li salt or RTIL (single FSI system); or(B) from about 25 mole% to about 40 mole% of Li+cations and [TFSI]- anions, where anions in the electrolyte are predominantly [FSI]- anions, regardless if sourced from Li salt or RTIL (mixed TFSI / FSI system).

3. The cell of claim 1 or claim 2, wherein the Li+conductive ionic liquid electrolyte comprises in(A) from about 47 mole% to about 53 mole%, preferably about 50 mole% of Li+cations, where anions of the electrolyte are predominantly [FSI]- anions, regardless if sourced from Li salt or RTIL (single FSI system); or(B) from about 27 mole% to about 35 mole%, preferably about 30 mole% of Li+cations and [TFSI]- anions, where anions in the electrolyte are predominantly [FSI]- anions, regardless if sourced from Li salt or RTIL (mixed TFSI / FSI system).

4. The cell of any one of claims 1 to 3, wherein the Li+cations are sourced in (A) from Li FSI salt and the ionic liquid anions are predominately [FSI]- anions; and where in the Li+cations and [TFSI]- anions are sourced in (B) from LiTFSI salt and the ionic liquid anions are predominately FSI anions.

5. The cell of any one of the preceding claims, wherein the Li+salt is present in an amount of from about 35 mole% to about 55 mole%, preferably about 50 mole%, of the electrolyte and the anion of the Li+salt is [FSI]- (single FSI anion system); or wherein the Li+salt is present in an amount of from about 25 mole% to about 40 mole%, preferably about 30 mole%, of the electrolyte and the anion of the Li+conductive salt is [TFSI]- (mixed TFSI / FSI anion system).

6. The cell of any one of the preceding claims, wherein the RTIL cation component is predominantly an ammonium cation, and optionally, lesser amount of a phosphonium cation, a sulfonium cation, a pyrrolidinium cation.

7. The cell of any one of the preceding claims, wherein the RTIL cation component is predominantly an alkyl ammonium cation, and optionally, lesser amounts of an alkyl phosphonium cation, an alkyl sulfonium cation, an alkyl pyrrolidinium cation, an alkyl imidazolium based cation, an alkyl pyridinium cation or combinations thereof.

8. The cell of any one of the preceding claims, wherein the RTIL cation component is selected from the group consisting of: [Nuis]4, [Nun]*, [Nm2]+, [Nm4]+, [Ni222]+, [Ni222]+, [Ni223]+, [Ni224]+, [N2222]+, [N2223]+, [N2224F, N1333F, [Nn33]+[N333S]+, [Ni444]+[Nii4i4i4]+and related ether oxygen containing analogues.

9. The cell of any one of the preceding claims, wherein the RTIL of the ionic liquid electrolyte is predominantly [N1113] [FSI] or exclusively [N1113] [FSI] .

10. The cell of any one of the preceding claims, wherein the S cathode comprises at least one ionically conductive organic polymeric ionic liquid (PI L):Li salt binder mixture, comprising at least one PIL selected from PIL cationic polymers with counter anions; PIL anionic polymers with Li+counter cations; and PIL cationic and / or anion block copolymers with corresponding counter anions or cations, preferably wherein the at least one PIL comprises:• a poly ionic liquid cationic polymer comprising a plurality of one or more ionic liquid cations included in, or tethered as pendant groups, to an organic polymer backbone; and a plurality of one or more tethered or untethered counter anions;• a poly ionic liquid anionic polymer comprising a plurality of one or more ionic liquid anions included in, or tethered as pendant groups, to an organic polymer backbone; and a plurality of Li+cations;• a poly ionic liquid cationic and / or anion block copolymer comprising: a non-ionic block that does not contain ionic charge and comprises polymerised residues of hydrophobic monomers, and an ionic block comprising polymerised monomer residues having covalently coupled thereto one or more ofo (a) a pendant organic ionic liquid cation, the pendant organic ionic liquid cation having a counter anion, and o (b) a pendant anionic moiety, the pendant anionic moiety having a counter cation.

11. The cell of claim 10, wherein the counter anions and Li salt anions are [FSI]- anions and / or [TFSI ]" anions, preferably [TFSI]- anions.

12. The cell of claim 10 or claim 11, wherein the one or more Li salts in the PI L:Li salt binder mixture are identical to the Li metal salts used in the Li+conductive ionic liquid electrolyte.

13. The cell of any one of claims 10 to 12, wherein the PIL:Li salt binder mixture is 50:50 mole% [PDADMA.TFSI]:LiTFSI, 50:50 mole% [PMTFSI.TFSI] :LiTFSI, or 50:50 mole% [PILblocCo-polymer (plmTFSI)]:UTFSI.

14. The cell of any one of claims 10 to 13, wherein the PI L:Li salt binder mixture is present in an amount up to about 20 wt%, preferably up to about 15 wt%, preferably up to about 10 wt%, most preferably up to about 5 wt% (about means ±5%) of the total positive S electrode (cathode) composition.

15. The cell of any one of claims 10 to 14, wherein the positive S electrode (cathode) further comprises 2D and / or nanomaterials such as boron nitride or an allotrope of boron nitride, such as BNNTs, BNNSs, or BNNWs, or a ceramic nanomaterial such as graphene, functionalised graphene, MXenes and other 2D nanomaterials, preferably in an amount of up to 3 wt% of the positive S electrode (cathode), preferably up to 2 wt% and most preferably up to 1 wt%, most preferably up to 0.5 wt%..

16. The cell of any one of the preceding claims, wherein the Li+conductive ionic liquid electrolyte is free of one or more solid matrix materials such as a ceramic material or a gel polymer matrix and / or is free of a film forming additive, such as nitrate salts, for example, LiNOa.

17. The cell of any of the preceding claims, wherein the electrolyte comprises a cosolvent as diluent, preferably DME.

18. A quasi-solid state (QSS) liquid electrolyte lithium-sulfur cell, preferably that operate with a S active material utilisation of >50% on a 40thdischarge cycle of the cell, the cell comprising: o a negative Li metal or Li metal alloy electrode (anode); o a positive S electrode (cathode); and o a Li+conductive ionic liquid electrolyte having at least one room temperature ionic liquid (RTIL) and at least one Li salt solubilised in the RTIL,wherein the Li+conductive ionic liquid electrolyte comprises an ammonium cation, and the electrolyte comprises:(A) from about 20 mole% to about 60 mole% of Li+cations, where anions of the electrolyte are predominantly [FSI]- anions, regardless if sourced from Li salt or RTIL (single FSI system); or(B) from about 20 mole% to about 60 mole% of Li+cations and [TFSI ]- anions, where anions in the electrolyte are predominantly [FSI]- anions, regardless if sourced from Li salt or RTIL (mixed TFSI / FSI system), wherein the Li+conductive ionic liquid electrolyte solubilises one or more polysulfide species, LizSn, 4 < n < 8, to a saturation concentration at 25 °C of less than 1 mM atomic S, more preferably to a saturation concentration at 25 °C that is zero, or at least is below the limit of detection of UV spectroscopy, wherein the positive S electrode (cathode) comprises at least one ionically conductive organic polymeric ionic liquid (PI L):Li salt binder mixture; and wherein the positive S electrode (cathode) further comprises boron nitride or an allotrope of boron nitride.

19. The cell of claim 18, wherein the at least one ionically conductive organic polymeric ionic liquid ( PI L) : Li salt binder mixture comprises a composite of:(i) at least one PIL selected from PIL cationic polymers with counter anions; PIL anionic polymers with Li+counter cations; and PIL cationic and / or anion block copolymers with corresponding counter anions or cations, preferably wherein the at least one PIL comprises:• a poly ionic liquid cationic polymer comprising a plurality of one or more ionic liquid cations included in, or tethered as pendant groups, to an organic polymer backbone; and a plurality of one or more tethered or untethered counter anions;• a poly ionic liquid anionic polymer comprising a plurality of one or more ionic liquid anions included in, or tethered as pendant groups, to an organic polymer backbone; and a plurality of Li+cations;• a poly ionic liquid cationic and / or anion block copolymer comprising: a non-ionic block that does not contain ionic charge and comprises polymerised residues of hydrophobic monomers, and an ionic block comprising polymerised monomer residues having covalently coupled thereto one or more of(a) a pendant organic ionic liquid cation, the pendant organic ionic liquid cation having a counter anion, and(b) a pendant anionic moiety, the pendant anionic moiety having a counter cation; and (ii) one or more Li salts.

20. The cell of claim 19, wherein the PI L:Li Salt binder mixture is 50:50 mole% [PDADMA.TFSI]:LiTFSI, 50:50 mole% [PMTFSI.TFSI] :LiTFSI, or 50:50 mole% [PILblocCo-polymer(plmTFSI)]:LiTFSI.

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