Electrolyte and lithium sulfur cell

Abstract

A lithium sulfur cell comprising a working electrode, a counter electrode, and an electrolyte, wherein the working electrode comprises a transition metal dichalcogenide (TMD) of formula (I): LiaMX2 (I); wherein a is from 0 to 2.0; X is selected from S, Se, and Te; and M is a transition metal. The electrolyte is a quasi-solid-state electrolyte (QSSE), the electrolyte comprising a monomer, a polymer formed or formable from the monomer, and a lithium salt. The invention also relates to a method of preparing an electrolyte and a method of preparing a lithium sulfur cell. The lithium sulfur cell may be used for high density energy storage.

Description

[0001] Electrolyte and Lithium Sulfur Cell

[0002] Related Applications

[0003] The present case claims priority to, and the benefit of, GB 2401246.0 filed 31 January 2024 (31.01.2024), the contents of which are incorporated by reference in their entirety.

[0004] Field of the Invention

[0005] The invention relates to an electrolyte, a lithium sulfur cell comprising the electrolyte, a method of preparing an electrolyte and a method of preparing a lithium sulfur cell, as well as a battery comprising the lithium sulfur cell.

[0006] Background

[0007] The transition to net zero carbon emission will require a step change in performance of electronic devices. Batteries based on new chemistries that overcome the fundamental limitations of lithium ion batteries will play an important role in enabling a transition to higher performance electronic devices.

[0008] Lithium sulfur (Li-S) batteries have the potential to offer high energy density, low material cost and excellent safety. However, fundamental challenges such as low sulfur utilization in the cathode, slow reaction rate, self-discharge, poor capacity retention and short lifetimes have hampered Li-S batteries development.

[0009] Many conventional Li-S batteries use a sulfur cathode and a lithium metal anode, but the poor cycle life of Li-S batteries stems from their commonly adopted catholyte-intermediated operating mechanism - that is, the sulfur cathode dissolves into the electrolyte to form soluble lithium polysulfides, which then undergo liquid phase conversion and finally precipitate into solid lithium sulfide. Such dissolution-precipitation chemistry can promote reaction kinetics but on the other hand, also lead to the shuttling effect. This shuttling of dissolved polysulfides results in active material loss on the sulfur cathode and surface corrosion on the lithium anode.

[0010] Numerous strategies have been developed to suppress polysulfide shuttling and improve the cycling stability of Li-S batteries. Porous carbons have been widely used as a sulfur host material that can provide physical confinement to dissolved polysulfides within the cathode. However, their high porosity needs to be filled with abundant electrolyte, which adds considerable weight to the entire cell but without contributing to capacity, hindering the pursuit of high overall energy density.

[0011] The incorporation of catalytically active polar sites or redox mediators is to date the most common approach for facilitating the chemical adsorption and electrocatalysis of polysulfides on the cathode or at the cathode- electrolyte interface. Whilst great progress has been made in kinetic promoters, it is only focused on the cathode side.

[0012] An equally crucial factor that also limits the lifespan of Li-S batteries is the instability of the anode. The lithium metal anode in Li-S batteries, besides the additional corrosion issue introduced by polysulfides, remains similar problems as in lithium metal batteries, including: dendrites formation, dead lithium, unstable SEI, and structural pulverization. In particular, uncontrolled growth of lithium dendrites may induce internal short circuits, leading to safety concerns that impede practical implementation of the batteries with metallic lithium electrode.

[0013] Research has also looked at the composition of the cathode. The present inventors have previously provided a lithium sulfur cell comprising a cathode including stacked layers of metallic phase transition metal dichalcogenide (see WO 2023 / 041799 and Zhuangnan et al ), which was tested with a liquid electrolyte cell with a lithium anode. The TMD electrode was shown to provide good physical and electrochemical properties.

[0014] However, there remains a need for Li-S batteries having improved energy density, reaction rate, resistance to self-discharge, capacity retention and lifetime.

[0015] Summary of the Invention

[0016] At its most general, the invention provides a lithium sulfur cell having a working electrode, wherein the working electrode comprises a transition metal dichalcogenide (TMD) and an electrolyte, wherein the electrolyte comprises a monomer and a polymer formed or formable from the monomer. The electrolyte typically comprises a monomer, a polymer formed from the monomer, and a lithium salt. The electrolyte is typically a quasi-solid-state electrolyte (QSSE).

[0017] The inventors have surprisingly found that a QSSE may be formed using a TMD (such as the TMD present in a working electrode) as an initiator in a polymerisation of the monomer to form the polymer that is a component of the QSSE. This provides unexpected advantages for the production of the lithium-sulfur cell, by avoiding the need for separate reactants and allowing for formation of the QSSE during cell assembly. The QSSE resulting from the preparation with the TMD also provides excellent electrochemical properties to the resulting lithium-sulfur cell.

[0018] In this way, the electrolyte (e.g., QSSE) comprises a monomer and a polymer. The polymer and monomer are structurally related, in that the polymer is formed or formable from the monomer. The polymer may be formed by polymerisation of the monomer. The monomer may be referred to as a precursor to the polymer, or may be referred to as a repeating unit of the polymer. The monomer and polymer may be selected because of the ability of the monomer to be polymerised using the TMD (such as the TMD present in a working electrode) as an initiator in the polymerisation. In other words, the polymer may be a polymer formed or formable from a monomer by a polymerisation reaction, such as a polymerisation reaction using a TMD initiator. The TMD is preferably one which can serve as an electrode in a lithium sulfur cell, or more preferably is an electrode in a lithium sulfur cell.

[0019] In a general aspect of the invention there is provided a lithium sulfur cell comprising a working electrode, a counter electrode, and an electrolyte, wherein the working electrode comprises a transition metal dichalcogenide (TMD) of formula (I):

[0020] LiaMX2(I) wherein a is from 0 to 2.0;

[0021] X is selected from S, Se, and Te; and

[0022] M is a transition metal; and wherein the electrolyte comprises a monomer, a polymer formed or formable from the monomer, and a lithium salt.

[0023] The electrolyte may be referred to as a quasi-solid-state electrolyte (QSSE).

[0024] In a first aspect of the invention there is provided a lithium sulfur cell comprising a working electrode, a counter electrode, and an electrolyte, wherein the working electrode comprises a transition metal dichalcogenide (TMD) of formula (I):

[0025] LiaMX2(I) wherein a is from 0 to 2.0;

[0026] X is selected from S, Se, and Te; and

[0027] M is a transition metal; and wherein the electrolyte is a quasi-solid-state electrolyte (QSSE), the electrolyte comprising a monomer, a polymer formed or formable from the monomer, and a lithium salt.

[0028] The present inventors have found that the Li-S battery of the present invention has excellent electrochemical properties. The QSSE facilitates the favourable catholyte-intermediated dissolution-precipitation reaction kinetics (typically only achieved with a liquid electrolyte - LE), whilst effectively inhibiting the unfavourable polysulfide shuttling phenomenon associated with polysulfide dissolution in a LE. Thus, whilst still ensuring high sulfur utilisation, in contrast to conventional LE-based Li-S cells the parasitic reactions associated with polysulfide shuttling are supressed, and consequently self-discharging is reduced, capacity retention, battery stability and cycling lifetime are improved.

[0029] Without wishing to be bound by theory, it is believed that the LE-associated catholyte- intermediated dissolution-precipitation kinetics are observed because, unlike SEs, the QSSE retains a portion of unpolymerized monomer (see Fig. 1A). Moreover, it is believed that polysulfide shuttling is inhibited by a combination of steric hindrance posed by the polymer framework and the repulsive electrostatic interactions between the negatively charged polysulfide anions and the large number of heteroatoms comprising the polymer framework.

[0030] Additionally, the Li-S battery of the present invention also has advantageously high gravimetric and volumetric energy density. Existing approaches to supress polysulfide shuttling use porous electrode materials that require abundant electrolyte to trap the polysulfides. These are volumetrically and gravimetrically inefficient. In contrast, the QSSE of the present invention inhibits polysulfide shutting independently of the electrode material. Therefore, Li-S cells using a QSSE of the present invention can use more volumetrically and gravimetrically efficient electrode materials, such as transition metal dichalcogenides.

[0031] Moreover, the QSSE of the Li-S battery of the present invention can be used with limited excess anode (described by negative-to-positive, N / P, ratio) and limited excess electrolyte (described by electrolyte-to-sulfur, E / S, ratio). Conventional Li-S cells suffer from parasitic side reactions due to polysulfide shuttling, which corrodes the lithium metal electrode and the solid electrolyte interphase (SEI). In these cells, electrolyte is consumed to repair the damaged SEI. Therefore, Li-S batteries typically require an excess of anode and electrolyte to compensate for the parasitic reactions arising from polysulfide shuttling. In the present invention, inhibition of polysulfide shuttling allows an extended battery lifespan to be observed even in the case where there is limited excess anode and electrolyte. Therefore, the Li-S battery of the present invention can work with a smaller volume of electrolyte and a smaller lithium metal anode. This reduces the mass of the battery, thus improving energy density.

[0032] The Li-S cell of the present invention can be operated at a wider temperature range than conventional LE based Li-S batteries. The Li-S battery comprising QSSE is observed to have excellent capacity retention compared to a conventional LE-based Li-S battery when operating as low as 5 °C and as high as 45 °C. The improved thermal stability advantageously enables the Li-S battery of the first aspect to be used in a wider range of applications than conventional LE-based Li-S batteries.

[0033] The QSSE of the Li-S battery of the invention is also found to be safer than existing Li-S batteries. Conventional LE-based cells do not prevent polysulfide shuttling. Polysulfide shuttling can result in thermal runaway as polysulfides react with the lithium metal anode. Therefore, the Li-S battery of the first aspect is safer with respect to thermal runaways caused by polysulfide shuttling. Additionally, the prevention of polysulfide shuttling by batteries of the first aspect prevents short-circuiting, which may otherwise damage equipment and pose a danger. The Li-S battery of the invention has even been found to be capable of safe operation following mechanical damage, reducing the hazards posed by battery failure resulting from mechanical damage.

[0034] In a second aspect of the invention there is provided a method for forming a polymer mixture, the method comprising: contacting a monomer and a lithium salt with a transition metal dichalcogenide (TMD), wherein the TMD is of formula (I):

[0035] LiaMX2(I) wherein a is from 0 to 2.0;

[0036] X is selected from S, Se, and Te; and M is a transition metal; and polymerising a portion of the monomer to give a polymer mixture comprising the monomer, the polymer formed from the monomer, and the lithium salt.

[0037] The polymer mixture may be used in an electrolyte of a lithium sulfur cell, such as a QSSE of a lithium sulfur cell.

[0038] In a third aspect of the invention there is provided, a method of preparing a lithium sulfur cell, the method comprising: forming a polymer mixture according to the second aspect; and assembling the lithium sulfur cell from a working electrode, a counter electrode, and a QSSE comprising the polymer mixture, wherein the working electrode comprises a transition metal dichalcogenide (TMD) of formula (I).

[0039] The inventors have surprisingly found that a QSSE may be formed using TMD as an initiator for the polymerisation of a monomer to give the polymer that is a component of the QSSE. The partial polymerisation of a mixture containing the monomer, polymer and lithium salt, also forms the QSSE itself. Here, partial refers to the retention of monomer within the product composition, and also within the QSSE. Thus, not all monomer is consumed in the polymerisation.

[0040] Alternatively, the electrolyte may be formed by carrying out a full polymerisation of the monomer using TMD as an initiator, to give the polymer. Further monomer can then be added to the polymer (optionally along with other components) to give the QSSE.

[0041] The TMD may be contacted with the monomer ex-situ (e.g., outside of the Li-S cell). In this case, the TMD can be retained in the resulting electrolyte, as it is compatible with the TMD electrode present in the resulting Li-S cell, thus avoiding the need for a separation step in the QSSE preparation. Preferably, the TMD may be contacted with the monomer in-situ (e.g., using the TMD electrode inside the Li-S cell). By using the TMD present in the cell, the QSSE may be formed during cell formation, without any residual Lewis acidic initiator remaining in the QSSE. This in-situ induced polymerization is facile, high in purity, and produces an excellent interface between the electrode and electrolyte. This results in low interfacial resistance. Many known SE systems have poor electrode-electrolyte contact resulting in high interfacial resistance.

[0042] The polymerization pathway promoted by the TMD may proceed by cation-mediated ringopening polymerization under mild conditions. The properties of the obtained QSSE, such as the degree of polymerization and molecular weight, can be tuned by tuning the Lewis acidity of the TMD initiator or by the addition of lithium salts which promote polymerisation (e.g., LiTFSI) or inhibit polymerisation (e.g., LiNO3). For example, certain lithium salts (such as LiTFSI) may act as catalysts which accelerate polymerisation. This advantageously allows electrolyte properties to be customised.

[0043] The preparation method of the present invention avoids unnecessary manipulation and purification steps typically involved with preparing a QSSE. In particular, the in-situ formation of the QSSE ensures good surface-contact of the QSSE with the working electrode, and more effective polysulfide shuttling inhibition, as the protective polymer framework is formed directly at the electrode-electrolyte interface. This also results in low electrolyte-electrode interfacial resistance. This contrasts to known SE based cells, where poor electrode-electrolyte contact results in undesirable high interfacial resistance.

[0044] Without wishing to be bound by theory, it is thought that the polymerization process is initiated by the Lewis acidic transition metal dichalcogenide (TMD) electrode. It is believed the monomer coordinates to the Lewis acidic transition metal of the TMD electrode, which activates the monomer and initiates polymerization. This reaction mechanism advantageously avoids free-radical polymerization pathways used in other in-situ polymerizations, which require extra non-electrolyte monomers, initiators and special conditions such as high temperature.

[0045] In a fourth aspect of the invention there is provided an electrolyte for a lithium sulfur cell, wherein the electrolyte is a quasi-solid-state electrolyte (QSSE), the electrolyte comprising a monomer, a polymer formed or formable from the monomer, and a lithium salt; and wherein the electrolyte comprises from 5 wt.% to 30 wt.% monomer, based on the total mass of the monomer and the polymer.

[0046] The proportion of monomer to polymer components has been found to provide a Li-S battery having excellent electrochemical properties. The QSSE facilitates the favourable catholyte- intermediated dissolution-precipitation reaction kinetics (typically only achieved with a liquid electrolyte - LE), whilst effectively inhibiting the unfavourable polysulfide shuttling phenomenon. Consequently, self-discharging is reduced, while capacity retention, battery stability and cycling lifetime are improved.

[0047] In fifth aspect of the invention there is provided a polymer mixture for a lithium sulfur cell obtained or obtainable by the method of the second aspect.

[0048] In some embodiments, the polymer mixture is used as an electrolyte, such as QSSE.

[0049] In sixth aspect of the invention there is provided a lithium sulfur cell obtained or obtainable by the method of the third aspect. ln another aspect of the invention there is provided a lithium sulfur battery comprising two or more lithium sulfur cells of the first or sixth aspects.

[0050] In another aspect of the invention there is provided a method of charging and / or discharging the lithium sulfur cell of the first or sixth aspects.

[0051] These and other aspects and embodiments of the invention are described in further detail below.

[0052] Summary of the Figures

[0053] The present invention is described with reference to the figures listed below.

[0054] Figure 1 A shows a schematic of Li-S batteries with different electrolytes, illustrating that the in-situ formed QSSE in Li-S batteries ensures good contact with electrodes, mitigates polysulfides shuttling, and promotes reaction kinetics.

[0055] Figure 1B shows the Raman spectra of a sample of 1T M0S2 and 2H M0S2. The spectra show the Aigand E2g1peaks of the metallic 1T phase, and the J-series peaks of the semiconducting 2H phase. 1T phase is the top line and 2H phase is the bottom line.

[0056] Figure 2 shows photographs of the QSSE formation process in vials with different precursors. The photos show the vials containing DOL solution standing upright, and then standing upside down 1 minute, 10 minutes and 10 hours after addition of the precursors.

[0057] Figure 3 shows the preparation of QSSE. (a) Reaction mechanism illustrating the ringopening polymerization initiated by MoS2-based cathodes, (b) Photographs of original LE and QSSE induced by metallic 1T MoS2.

[0058] Figure 4A shows three stacked1H NMR spectra for an LE, QSSE and SE. The SE spectrum shows the proton peaks associated with the DOL polymer, and the LE spectrum shows the proton peaks associated with the DOL monomer. The QSSE spectrum shows peaks associated with both the DOL polymer and monomer.

[0059] Figure 4B shows three stacked Raman spectra for an LE, QSSE and SE. The SE and QSSE spectra show peaks associated with DOL polymer vibrational modes. The LE spectrum shows a peak associated with the DOL monomer’s ring vibrational mode and all three spectra show a peak associated with CF3 deformation of the TFSI salt.

[0060] Figure 4C shows three stacked FTIR spectra with the characteristic absorption peaks associated with the vibrational modes of the DOL polymer of the SE and QSSE, the DOL monomer of the LE and the N-SO2 of the TFSI salt. Figure 4D shows a graph of molecular weights of polymer and degree of polymerization of DOL in an LE, SE and QSSE. The bar chart portion of the graph shows the molecular weight of the polymer (or monomer in the case of the LE), and the scatter plot portion of the graph shows the degree of polymerization.

[0061] Figure 4E shows a graph of weight (% of initial weight) as a function of time (h) for three different electrolytes (an LE, SE and QSSE) left in uncapped vials shown in the inset photograph and measured over a period of 24 hours.

[0062] Figure 5A shows three galvanostatic charge-discharge (GCD) curves of Li-S coin cells with different electrolytes (an LE, SE and QSSE) at a current density of 0.1 C. At the top right of the graph, the SE is the left most line, QSSE the middle line and LE the right most line.

[0063] Figure 5B shows three cyclic voltammetry curves of Li-S coin cells with different electrolytes (an LE, SE and QSSE) at a scan rate of 0.1 mV s-1. At the top right side of the graph, the LE is the left most peak, the QSSE is the middle peak and the SE the right most peak.

[0064] Figure 5C shows a photograph demonstrating the permeation behaviour of LizSe in two vials containing an LE and QSSE.

[0065] Figure 5D shows a graph plotting open-circuit voltage of assembled Li-S coin cells with different electrolytes (an LE, SE and QSSE) measured over a period of 180 days. At the vertical axis (left side) the LE line is at the top, the QSSE in the middle and the SE at the bottom.

[0066] Figure 5E shows a graph plotting specific capacity (mAh g-1) against cycle number over a period of 24 cycles, with a 10 day rest between the 10thand 11thcycle.

[0067] Figure 6A shows a plot of coulombic efficiency (%) of a Li stripping and plating process in asymmetric Li||Cu coin cells containing three different electrolytes (an LE, QSSE and SE) against cycle number for a period of over 100 cycles.

[0068] Figure 6B shows a Nyquist plot of symmetric Li cells with different electrolytes (an LE, SE and QSSE) after 200 cycles.

[0069] Figure 6C shows photographs of the lithium metal anodes after adding Li2Se to the asymmetric Li||Cu coin cells of Figure 4A.

[0070] Figure 6D shows a graph of long-term cycling performance of symmetric Li-S cells with different electrolytes (an LE, SE and QSSE). The graph shows a continuous plot of voltage against Time (h) as the cells are continuously cycled over 400 hours. Figure 7A shows galvanostatic charge-discharge (GCD) curves of three Li-S pouch cells with an LE at three different temperatures (5, 25 and 45 °C) and at a current density of 0.05 C. At the top of the graph, the LE_5°C is the left most line, the LE_45°C the middle line and the LE_25°C the right most line.

[0071] Figure 7B shows galvanostatic charge-discharge (GCD) curves of three Li-S pouch cells with a QSSE at three different temperatures (5, 25 and 45 °C) and at a current density of 0.05 C. At the top of the graph, the QSSE_5°C is the left most line, the QSSE_25°C the middle line and the QSSE_45°C the right most line.

[0072] Figure 7C shows a bar chart plotting the specific capacities (mAh g-1) of Li-S pouch cells with LE (at 25 °C) and QSSE (at 25 and 45 °C) at different current densities. The QSSE_45°C is the left column, the QSSE_25°C the middle column and the LE_25°C the right column.

[0073] Figure 7D shows a graph of long-term cycling stability of Li-S pouch cells. The graph shows a plot of specific capacity (mAh g-1) against cycle number (over 200 cycles) for three Li-S pouch cells: with LE (under 25 °C) and QSSE (under 25 and 45 °C). The inset photograph shows light-emitting diodes powered by QSSE-based Li-S pouch which has undergone mechanical damage.

[0074] Figure 8 shows a graph of molecular weights of polymer and degree of polymerization for polymers formed using three different Lewis acids; 1T MoS2 / graphene, 1T M0S2, and 1T MoS2 / NbSe2. The bar chart portion of the graph shows the molecular weight of the polymer, and the scatter plot portion of the graph shows the degree of polymerization.

[0075] Figure 9 is an SEM images showing the morphology of 1T MoS2-based cathodes before (a) and after (b) QSSE integration. The scale bar is 1 pm.

[0076] Figure 10 is a graph of the self-discharge current of Li-S coin cells with a SE, QSSE and LE. The solid lines indicate the evolution trend of self-discharge current over OCVs.

[0077] Figure 11 is a graph of the self-discharge current of Li-S coin cells with a QSSE and LE at different temperatures. The solid lines indicate the evolution trend of self-discharge current over OCVs.

[0078] Detailed Description of the Invention

[0079] The present invention provides a lithium sulfur cell comprising a working electrode, wherein the working electrode comprises a transition metal dichalcogenide (TMD) and an electrolyte, wherein the electrolyte is a quasi-solid-state electrolyte (QSSE). The electrolyte comprises a monomer, a polymer formed from the monomer, and a lithium salt. US 2019 / 372148 describes lithium cells with a cathode-protecting layer comprising a lithium ion-conducting polymer matrix. The document does not describe the combination of a TMD and QSSE for use in a lithium sulfur cell. Example 4 describes a M0S2 cathode with a quasisolid electrolyte including LiPF6salt, and a propylene carbonate (PC) and ethylene carbonate (EC) solvent mixture. Example 4 is not a lithium sulfur cell. The polymer used for the QSSE is not a polymer formed or formable from the polymerisation of a monomer which is present in the electrolyte. Instead, the QSSE is an unspecified polymer matrix with an unrelated PC / EC liquid component.

[0080] US 2014 / 363746 also describes a lithium cell including a quasi-solid state electrolyte. The document does not describe the combination of a TMD and QSSE in a lithium sulfur cell.

[0081] Lithium Sulfur Cell

[0082] In a first aspect of the invention there is provided a lithium sulfur cell comprising a working electrode, a counter electrode, and an electrolyte, wherein the working electrode comprises a transition metal dichalcogenide (TMD) of formula (I):

[0083] LiaMX2(I) wherein a is from 0 to 2.0;

[0084] X is selected from S, Se, and Te; and

[0085] M is a transition metal; and wherein the electrolyte is a quasi-solid-state electrolyte (QSSE), the electrolyte comprising a monomer, a polymer formed or formable from the monomer, and a lithium salt.

[0086] The invention also provides a lithium sulfur cell obtained or obtainable by the preparation methods described herein.

[0087] The Li-S cell of the invention typically comprises a working electrode, a counter electrode and an electrolyte. The Li-S cell may also include a separator. The Li-S cell typically comprises terminals for connection to an external device or an external power supply.

[0088] The working electrode and electrolyte are described in more detail below.

[0089] Typically, the counter electrode is the negative electrode. A negative electrode comprising any suitable anode active material may be used. Examples of suitable anode active materials include lithium metal or lithium alloys. The lithium alloys may include alloys of lithium with Mg, Zn, Sn, Sb, Si or Al, such as Li-Sn2O3 and Li-SnO2. Preferably, the counter electrode comprises lithium metal.

[0090] The counter electrode may comprise a current collecting substrate. Any suitable current collecting substrate may be used. Examples of suitable current collecting substrates include a copper plate or foil. The anode active material may be disposed on the surface of the current collecting substrate.

[0091] The counter electrode may comprise a binder to improve adhesion of the active material to a current collecting surface. Examples of typical binders are PVDF, PTFE, CMC, PAA, PM MA, PEO, SBR and co-polymers thereof.

[0092] The electrochemical cell may also include a solid porous membrane positioned between the negative and positive electrodes. The solid-porous membrane may be known as a separator. The solid porous membrane may partially or completely replace the liquid electrolyte. The solid porous membrane may comprise a polymer (e.g., polyethylene, polypropylene, or copolymer thereof) or an inorganic material, such as a transition metal oxide (e.g., titania, zirconia, yttria, hafnia, or niobia) or main group metal oxide, such as silicon oxide, which can be in the form of glass fibre.

[0093] Preferably, the solid porous membrane comprises polypropylene.

[0094] The solid non-porous membrane may comprise a lithium-ion conductor. For example, LLZO (garnet family), LSPO (LISICON family), LGPS (thio-LISICON family), LATP / LAGP (NASICON family), LLTO (perovskite family) and phosphide / sulfide glass ceramics.

[0095] The lithium sulfur cell set out above, and the lithium sulfur cell obtained or obtainable by the method of manufacture set out herein, have excellent electrochemical properties.

[0096] The lithium sulfur cells of the invention have excellent capacity retention. In some embodiments, the cell has a capacity retention of 70% or more over 200 cycles at 25 °C and a C-rate of 0.1C, preferably 75% or more, more preferably 80% or more.

[0097] The lithium sulfur cells of the invention have excellent capacity retention at elevated temperatures. In some embodiments, the cell has a capacity retention of 65% or more over 200 cycles at 45 °C and a C-rate of 0.1C, preferably 70% or more, more preferably 75% or more.

[0098] The lithium sulfur cells of the invention have excellent capacity at reduced temperatures. In some embodiments, the cell has a capacity retention of 60% or more at 5 °C compared to the capacity at 25 °C and a C-rate of 0.1C, preferably 65% or more, more preferably 70% or more.

[0099] The lithium sulfur cells of the invention have excellent Coulombic efficiency. In some embodiments, the cell has a coulombic efficiency of 80% or more over 100 cycles, preferably 85% or more, more preferably 90% or more. The lithium sulfur cells of the invention have excellent high-rate capacity. In some embodiments, the capacity at 0.5C is 50% or more compared to the capacity at 0.05C, when the capacity is measured at 25 °C.

[0100] The lithium sulfur cells of the invention have excellent resistance to self-discharge. In some embodiments, the cell retains an open circuit voltage of 2.3V or more, following a 180-day period of inactivity. This typically represents a reduction of less than about 0.1 over a 180-day period.

[0101] The lithium sulfur cells of the invention also have excellent sulfur utilisation. Sulfur utilisation is calculated relative to the complete usage of sulfur in the working electrode. Such an electrode will produce 1,675 mAh of charge per gram of sulfur. That is, 100% utilization corresponds to a gravimetric capacity of 1 ,675 mAh g-1of the sulfur in the cell.

[0102] Typically, the lithium sulfur cells of the invention have a sulfur utilisation rate or 80% or more. Preferably, the lithium sulfur cells of the invention have a sulfur utilisation rate of 82% or more, more preferably 83% or more.

[0103] The lithium sulfur cells of the invention have excellent gravimetric energy density. Typically, the lithium sulfur cells of the invention have a gravimetric energy density of 350 Wh kg-1or more. Preferably, the lithium sulfur cells of the invention have a gravimetric energy density of 400 Wh kg-1or more, more preferably 450 Wh kg-1or more.

[0104] The lithium sulfur cells of the invention have excellent volumetric energy density. Typically, the lithium sulfur cells of the invention have a volumetric energy density of 600 Wh L'1or more. Preferably, the lithium sulfur cells of the invention have a volumetric energy density of 650 Wh L'1or more, more preferably 700 Wh L'1or more.

[0105] Working Electrode

[0106] The Li-S cell of the invention comprises a working electrode. The working electrode may be a positive (cathode) or negative (anode) electrode, for example during a discharge step. Typically, the working electrode is the positive electrode (cathode).

[0107] The working electrode comprises a (lithiated) transition metal dichalcogenide of formula (I): LiaMX2(I)

[0108] The group M in formula (I) is the transition metal. The transition metal M may be selected from Ti, Hf, Zr, V, Nb, Ta, Mo, W, Tc, Re, Pd and Pt. Preferably M is V, Nb, Mo or W. More preferably M is Mo or Nb.

[0109] In some embodiments, X is S. In some embodiments, the TMD is a metallic phase transition metal dichalcogenide, preferably comprising stacked layers of a metallic phase transition metal dichalcogenide.

[0110] In some embodiments, the TMD is M0S2, such as 1T-phase M0S2. The M0S2 may be lithiated, and so may in some embodiments the TMD is LiaMoS2, such as 1T-phase LiaMoS2.

[0111] Preferably, the transition metal M is selected from V, Nb, Ta, Mo and W. More preferably, the transition metal M is selected from V, Nb, Mo and W. Even more preferably, the transition metal is selected from Nb or Mo. Most preferably, the transition metal M is Mo.

[0112] The worked examples demonstrate that a QSSE based Li-S cell in which the working electrode comprises lithiated molybdenum disulfide (LixMoS2) has excellent sulfur utilization, excellent capacity retention and long cycling lifetime. The inventors expect additional lithiated transition metal dichalcogenide to perform well in an QSSE based Li-S cell based on similar the electrochemical properties that have demonstrated in related systems such as electrocatalytic hydrogen evolution, lithium ion batteries and supercapacitors (Chhowalla et al., 2013).

[0113] The transition metal dichalcogenide is preferably in the metallic phase.

[0114] The metallic phase is a phase having its Fermi level (Ef) within an electron orbital band. This may occur when the highest energy occupied electron orbital is partially filled. This contrasts with a semiconducting phase, where the Ef is in an energy gap between two orbital bands. This may occur when the highest energy occupied electron orbital is fully filled, and there is an energy gap between the highest energy occupied orbital and lowest energy unoccupied orbital. Metallic phase materials are conductors at absolute zero (0 K), whereas semiconductors are insulators at absolute zero.

[0115] In the metallic phase, Li+ions possess strong binding with the metallic phases of transition metal dichalcogenides such as M0S2, which is beneficial in minimizing the dissolution of LiPS in electrolyte.

[0116] The metallic phase is also highly conducting, mitigating the need to use additional conductive additives, such as conductive carbon, in the working electrode.

[0117] The transition metal dichalcogenide may have any structure (polymorph) having a metallic phase. Transition metal dichalcogenide may be in the metallic phase in the 1T, 2H or 3R polymorph. Here, the letters stand for trigonal, hexagonal and rhombohedral, respectively, while the number indicates the number of X-M-X units in the unit cell. For example, the 1T polymorphs of M0S2 and WS2 are metallic, while both the 2H and 3R polymorphs of NbS2 are metallic. In a particularly preferred embodiment, the transition metal dichalcogenide is in the metallic 1T phase. Li+ions possess strong binding with the 1T phase of M0S2, which is beneficial in minimizing the dissolution of Li PS in electrolyte (Bediako et al., 2018).

[0118] In an additional preferred embodiment, the transition metal dichalcogenide is in the metallic 3R phase. For example, the transition metal dichalcogenide may be a NbS? in the metallic 3R phase.

[0119] The working electrode may comprise lithiated transition metal dichalcogenide in additional phases. Preferably, the proportion of lithiated transition metal dichalcogenide in the metallic phase is from 60% to 100%. More preferably, the proportion of lithiated transition metal dichalcogenide in the metallic phase is from 70% to 95%, even more preferably from 80% to 90%.

[0120] The working electrode may comprise lithiated transition metal dichalcogenide in additional phases. Preferably, the proportion of lithiated transition metal dichalcogenide in the 1T phase is from 60% to 100%. More preferably, the proportion of lithiated transition metal dichalcogenide in the 1T is phase is from 70% to 95%, even more preferably from 80% to 90%.

[0121] The phase of the transition metal dichalcogenide may be known or it may be determined using standard techniques, such as X-ray photoelectron spectroscopy (XPS).

[0122] The working electrode of the invention preferably comprises stacked layers of the transition metal dichalcogenide.

[0123] Accordingly, the transition metal dichalcogenide is preferably a layered material. In the layered material, the transition metal is sandwiched between layers of the chalcogen to form an X-M-X stack or sheet. In layered materials, the bonding in-plane (i.e. within the layer or sheet) typically comprises strong chemical bonds, whereas the layers themselves are held together by weaker forces, such as van der Waals forces.

[0124] Preferably, the transition metal dichalcogenide is a two-dimensional (2D) material. Thus, the working electrode comprises nanosheets or monolayers of the transition metal dichalcogenide.

[0125] Individual nanosheets can have a (geometric) surface area of the order of pm2, but typically have a thickness of about a nanometre. In some embodiments the thickness of a nanosheet is about 0.7 nm.

[0126] Individual nanosheets or flakes of the transition metal dichalcogenide may be restacked to form a film. Thus, the transition metal dichalcogenide preferably comprises restacked transition metal dichalcogenide, such as restacked flakes of exfoliated, transition metal dichalcogenide.

[0127] The restacked film of nanosheets may have a surface area of the order of mm2, and typically have a thickness of from 1 to 100 pm. Typically, the restacked film comprises 10 or more layers, preferably 20 or more layers, more preferably 50 or more layers, even more preferably 100 or more layers.

[0128] The 2D transition metal disulfide may be prepared by exfoliation. Thus, the transition metal dichalcogenide is preferably an exfoliated transition metal dichalcogenide, such as exfoliated flakes, nanosheets or monolayers of the transition metal dichalcogenide.

[0129] The working electrode typically comprises a film comprising stacked layers of the transition metal dichalcogenide. Thus, the film has a layered (or laminar) structure. The layered working electrode has improved cycle stability in comparison to the bulk metallic TMD crystals as it able to withstand volumetric expansion, such as that arising from the conversion between Ss and l_i2S, and so is less susceptible to fracturing.

[0130] The orientation of each layer (e.g. each nanosheet, monolayer or flake) is typically parallel to the orientation of the film. (That is, the normal vector of each individual layer is aligned with the normal vector of the film to within a few degrees, such as within 10° or 5°).

[0131] As discussed in more detail below, the film may be prepared by restacking exfoliated, metallic phase, transition metal dichalcogenide, such as individual transition metal disulfide nanosheets, monolayers or flakes.

[0132] The thickness of the film may depend on the sulfur loading (see below). Typically, the film has a thickness of from 1 to 100 pm. Preferably, the film has a thickness of from 2 to 10 pm, more preferably 2 to 5 pm, even more preferably 2 to 3 pm.

[0133] The TMD may be applied to a current collector with a loading of 0.5 to 15 mg / cm2, preferably 1 mg cm-2to 10 mg cm-2. Preferably, the loading is from 2 mg cm-2to 9 mg cm-2, more preferably 3 mg cm-2to 8 mg cm-2, even more preferably 6 mg cm-2to 8 mg cm-2, and most preferably 7 mg cm-2to 8 mg cm-2. Typically the TMD loading is about 8 mg / cm2.

[0134] The value ‘a’ in formula (I) may be referred to as the lithiation rate. The lithiation rate a is 0 or more. The upper limit for the lithiation rate is not particularly limited. Typically, the upper limit for the lithiation rate is around 2.0.

[0135] In some embodiments, a is from 0.1 to 2.0, preferably 0.5 to 1.0, more preferably from 0.6 to 0.8. In some embodiments, the lithiation rate is 0.05 or less, such as 0. In such cases, the metallic phase transition metal dichalcogenide is unlithiated.

[0136] Preferably, the transition metal dichalcogenide is lithiated (comprises lithium). Lithiation of the transition metal dichalcogenide enhances the Li+ion diffusion kinetics within the electrode material.

[0137] As discussed in more detail below, the lithiated transition metal dichalcogenide can retain the layered structure of the material. Thus, the lithiated transition metal dichalcogenide is a two- dimensional (2D) material, and the working electrode comprises nanosheets or monolayers of the lithiated transition metal dichalcogenide. Typically, the lithiated material exists as individual stacks or sheets of transition metal disulfide (X-M-X) containing lithium ions (Li+) in between the sheets.

[0138] The lithiated transition metal dichalcogenide typically includes lithium ions which are not from a lithium (poly)sulfide. In such cases, the lithiation is distinct from the inclusion of lithium poly(sulfide) in the working electrode. In other words, the lithium ions are not from the lithium poly(sulfide).

[0139] Accordingly, the lithiation rate a is typically from 0.1 to 2.0. Preferably, the lithiation rate is from 0.1 to 1.5, more preferably 0.3 to 1 .2, even more preferably 0.5 to 0.9, and most preferably from 0.6 to 0.8. In the worked examples, the lithiation rate is about 0.7.

[0140] Preferably, the lithiation occurs prior to formation of the Li-S cell. Thus, the working electrode may be described as a “pre-lithiated” electrode. In such cases, the working electrode comprises stacked layers of the pre-lithiated, metallic phase transition metal dichalcogenide of formula (I). In such cases, the lithiation occurs before sulfur or lithium (poly)sulfide is added to the working electrode (e.g. before operation of the Li-S cell). In other words, the lithium ions are not from the lithium poly(sulfide). The lithium poly(sulfide) may only be introduced during the first cycle of the Li-S cell.

[0141] The lithiated transition metal dichalcogenide may be prepared by chemical or electrochemical lithiation. Preferably, the lithiated transition metal disulfide is prepared by chemical lithiation. Thus, the transition metal disulfide is preferably a “chemically lithiated” transition metal dichalcogenide.

[0142] The lithiated transition metal dichalcogenide is preferably a layered material, as discussed above. The lithium ions are inserted (intercalated) between the layers of the transition metal dichalcogenide. Thus, the material is a transition metal dichalcogenide that is intercalated with lithium.

[0143] The working electrode of the invention preferably comprises sulfur (Ss) or a lithium (poly)sulfide. Examples of lithium (poly)sulfides include Li2S, U2S2, Li2S4, Li2Se, and Li2Ss. In some embodiments, the mass ratio of LiaMX2 to sulfur is from 1 :2 to 1 :3.

[0144] The quantity of sulfur in the working electrode may be specified using the mass ratio of (lithiated) transition metal dichalcogenide to sulfur. Typically, the total mass of sulfur, including the sulfur component of lithium polysulfides, is used to calculate the sulfur mass ratio.

[0145] Preferably, the mass ratio of LiaMX2to sulfur is from 1 :2 to 1:3. More preferably, the mass ratio of LiaMX2to sulfur is about 1 :2.5.

[0146] The quantity of sulfur in the working electrode may alternatively be specified using the percentage weight of sulfur against the total weight of sulfur and active material (lithiated transition metal dichalcogenide) in the working electrode. This may be known as the sulfur fraction. Typically, the sulfur fraction is from 20 wt.% to 90 wt.%. Preferably, the sulfur fraction is from 40 wt.% to 85 wt.%, more preferably 50 wt.% to 80 wt.%, even more preferably 60 wt.% to 75 wt.%, and most preferably from 70 to 75 wt.%.

[0147] The quantity of sulfur in the working electrode may also be specified using the areal sulfur loading. That is, the mass of sulfur per unit area of the working electrode. Typically, the areal sulfur loading is 1 mg cm-2to 10 mg cm-2. Preferably, the areal sulfur loading is from 2 mg cm-2to 9 mg cm-2, more preferably 3 mg cm-2to 8 mg cm-2, even more preferably 6 mg cm-2to 8 mg cm-2, and most preferably 7 mg cm-2to 8 mg cm-2.

[0148] The form of the sulfur components, whether elemental sulfur or lithium (poly)sulfide, will change during charging and discharging of the electrochemical cell. In the charged state (prior to discharging) the sulfur component is in the form of element sulfur (Ss). During discharging, the sulfur components will change to lithium (poly)sulfides.

[0149] The sulfur components, whether elemental sulfur or lithium (poly)sulfide, are typically located between the restacked nanosheets of transition metal dichalcogenide.

[0150] Typically, the Li-S cell is manufactured in the charged state. Thus, the working electrode comprises elemental sulfur (Ss). Typically, the working electrode comprises a composite of the metallic phase transition metal dichalcogenide and elemental sulfur. Preferably, the sulfur component is added prior to formation of the Li-S cell. Thus, the working electrode may be described as a “pre-sulfinated” electrode. In such cases, the working electrode comprises stacked layers of the pre- sulfinated”, metallic phase transition metal dichalcogenide of formula (I). The lithium poly(sulfide) may only be introduced during the first cycle of the Li-S cell.

[0151] The metallic phase transition metal dichalcogenide is typically an electrical conductor. Accordingly, the working electrode does not need to contain additional conductive components, such as conductive carbon components. Typical conductive carbon components include carbon black, graphite, graphene, nanoparticulate carbon powder, carbon fibre, carbon nanotubes. Specific examples include Ketjen black or Super P carbon. Additional examples include exfoliated graphene or reduced graphene oxide. Preferably the additional conductive component is graphene.

[0152] Known working electrodes for Li-S cells (such as those using a semiconducting phase transition metal dichalcogenide), include a significant amount of conductive additives (such as conductive carbon). These known electrodes may include conductive carbon at an amount of 10% or more, 20% or more, 30% or more, such as from 30 to 40 %. Electrodes using TMD in a semiconducting phase will not have sufficient conductivity when the amount of conductive additive is 10% or less, 5% or less, or 1% or less. Electrodes using TMD in a semiconducting phase will not have sufficient conductivity when the amount of carbon conductive additive, such as graphene, is 10% or less, 5% or less, or 1% or less.

[0153] The working electrode may comprise conductive carbon in an amount of 10 wt.% or less, such as 5 wt.% or less. Typically, the working electrode comprises conductive carbon in an amount of 4 wt.% or less, such as 3 wt.% or less. Preferably, the working electrode comprises conductive carbon in an amount of 2 wt.% or less, more preferably 1 wt.% or less, and even more preferably 0.5 wt.% or less. Most preferably, the working electrode is substantially free from conductive carbon. In some embodiments, the working electrode may comprise conductive carbon in an amount of from 0.5 to 5 wt.%, such as 1 to 4 wt.%.

[0154] The lithiated transition metal dichalcogenide is typically a free-standing material, such as a free-standing film. That is, the lithiated transition metal dichalcogenide does not need to rely on, or be bound to, a support material. Accordingly, the working electrode does not need to contain additional binder components. Typical binder components include PVDF, PTFE, CMC, PAA, PM MA, PEO, SBR and co-polymers thereof.

[0155] Typically, the working electrode comprises binder in an amount of 3 wt.% or less, preferably, the working electrode comprises binder in an amount of 2 wt.% or less, more preferably 1 wt.% or less, and even more preferably 0.5 wt.% or less. Most preferably, the working electrode is substantially free from binder.

[0156] The working electrode in the Li-S cell of the invention comprises the metallic phase transition metal dichalcogenide of formula (I). The working electrode may consist essentially of the metallic phase transition metal dichalcogenide of formula (I).

[0157] The working electrode may comprise a current collecting substrate. Any suitable current collecting substrate may be used. Examples of suitable current collecting substrates include an aluminium plate or foil. The lithiated transition metal dichalcogenide of formula (I) may be disposed on the surface of the current collecting substrate. In a particularly preferred embodiment, the working electrode comprises: stacked layers of a metallic phase niobium disulfide of formula (IA); and sulfur or a lithium (poly)sulfide,

[0158] NbS2(IA).

[0159] In some such embodiments, the NbS2is in a metallic 3R phase.

[0160] In another particularly preferred embodiment, the working electrode comprises: stacked layers of a lithiated, metallic phase molybdenum disulfide of formula (IB); and sulfur or a lithium (poly)sulfide,

[0161] LiaMoS2(IB) where: a is from 0.5 to 0.9, such as 0.6 to 0.8.

[0162] In some embodiments, the working electrode further comprises a Lewis acidity modifier, such as graphene or NbSe2. The Lewis acidity modifier acts to change the Lewis acidity of the TMD. The Lewis acidity modifier preferably has a 2D-structure, such as layered structure. Graphene and NbSe2have a 2D structure. The 2D structure of the Lewis acidity modifier allows the TMD and Lewis acidity modifier to form a stacked, layered structure. The stacked structure is as described herein.

[0163] The working electrode may comprise a mixture of a Lewis acidity modifier and the T D. For example, the working electrode may comprise a mixture of graphene and TMD, or a mixture of NbSe2and TMD. In the mixture, the Lewis acidity modifier and TMD are not chemically bonded, and so the identifies of the Lewis acidity modifier and TMD are separate.

[0164] In some embodiments, the working electrode further comprises graphene. In such embodiments, the TMD is coupled with graphene, to form a TMD / graphene composite. For example, the working electrode may include a MoS2 / graphene composite, such as a 1T-MoS2 / graphene composite. The graphene acts to reduce the Lewis acidity of the MoS2, and as such reduces the propensity of the MoS2to assist in the polymerisation of the monomer in the electrolyte.

[0165] In some embodiments, the working electrode further comprises NbSe2. In such embodiments, the TMD is coupled with NbSe2, to form a TMD / NbSe2composite. For example, the working electrode may include a MoS2 / NbSe2composite, such as a 1T-MoS2 / NbSe2composite. The NbSe2acts to increase the Lewis acidity of the MoS2, and as such increases the propensity of the MoS2to assist in the polymerisation of the monomer in the electrolyte.

[0166] The Lewis acidity modifier may be incorporated into the working electrode using any suitable method for mixing the TMD and Lewis acidity modifier. The mixture may be formed using solution processing. For example, the TMD, Lewis acidity modifier and optionally other working electrode components, may be dispersed in a solvent (e.g., by sonication) and dried to give a mixture of the Lewis acidity modifier and TMD.

[0167] The mixture may be formed using powder mixing. For example, the TMD and Lewis acidity modifier may be milled (e.g., by ball-milling) and the resulting powders combined. In addition or alternatively, the TMD and Lewis acidity modifier may be milled together (e.g., by ballmilling).

[0168] Preparation of Working Electrode

[0169] The transition metal dichalcogenide may be prepared by any suitable method, such as those described in WO 2023 / 041799, the contents of which are incorporated herein by reference. In some embodiments the transition metal dichalcogenide is prepared by the preparation method described herein.

[0170] In some embodiments the transition metal dichalcogenide is prepared by known methods, such as a chemical vapour transport method, an electrochemical treatment method, an e-beam irradiation method, or by pressure application method (Chhowalla et al., 2015).

[0171] Transition metal dichalcogenide from any source may be used. The transition metal dichalcogenide may be prepared by lithiating a non-metallic transition metal dichalcogenide. A phase transition to metallic typically accompanies the lithiation. The lithium ions may then be removed (see below) while the material remains in the metallic phase. Chemical or electrochemical lithiation methods may be used.

[0172] The transition metal dichalcogenide may be a two-dimensional transition metal dichalcogenide, such as nanosheets, monolayers or flakes of transition metal dichalcogenide. The 2D transition metal disulfide may be prepared by exfoliation.

[0173] Typically, the method comprises exfoliating a transition metal dichalcogenide to provide a metallic phase transition metal dichalcogenide. This may be referred to as the exfoliation step, step (a).

[0174] The transition metal dichalcogenide typically has formula (II): M2X22(II) where X2is selected from S, Se and Te, and M2is a transition metal.

[0175] The transition metal M2may be selected from Ti, Zr, Hf, V, Nb, Ta, Mo, W, Tc, Re, Pd and Pt. Preferably, the transition metal M2is selected from V, Nb, Ta, Mo and W. More preferably, the transition metal M2is Mo.

[0176] The chalcogen X2is selected from S, Se and Te. Preferably, the chalcogen X2is S. As noted above, transition metal dichalcogenides are layered materials. In layered materials, the bonding in-plane (i.e. within the layer or sheet) typically comprises strong chemical bonds, whereas the layers themselves are held together by weaker forces, such as van der Waals forces. Thus, exfoliation provides a quick and efficient route to prepare individual nanosheets or monolayers of the material.

[0177] Accordingly, the exfoliation step provides flakes, nanosheets or monolayers of the transition metal dichalcogenide. The flakes, nanosheets or monolayers are two-dimensional materials. The flakes, nanosheets or monolayers may also be referred to as “exfoliated” material (i.e., exfoliated transition metal dichalcogenide).

[0178] Preferably, the exfoliation step comprises chemically exfoliating the transition metal dichalcogenide, such as exfoliating the transition metal dichalcogenide with lithium ions.

[0179] Additionally, the exfoliation step may comprise chemically exfoliating with any suitable metal ion. Suitable metal ions include group 1 metal ions, such as lithium ions, sodium ions or potassium ions. Preferably, the metal ion is a lithium ion.

[0180] The chemical exfoliation step provides lithiated transition metal dichalcogenide in the metallic phase. This avoids the need for a later phase transition step. The exfoliation step may provide the transition metal dichalcogenide in the 1T, 2H or 3R polymorph. In a preferred embodiment, the exfoliation step provides the transition metal dichalcogenide is in the metallic 1T phase.

[0181] Preferably, the exfoliation step comprises treating the transition metal dichalcogenide with an organolithium compound. Suitable organolithium compounds include alkyl- and aryllithium compounds. Specific examples of suitable organolithium compounds include butyllithium (such as n-butyllithium, sec-butyllithium, iso-butyllithium and tert-butyllithium) and phenyllithium. Preferably, n-butyllithium is used.

[0182] Additionally, the exfoliation step may comprise treating the transition metal dichalcogenide with any suitable metal ion source, such as a group 1 metal ion source. Suitable metal ion sources include organometallic compounds, metal borane compounds, Grignard reagents or metal-metal alloys. Suitable organometallic compounds include alkyl- and aryl-metallic compounds. Specific examples of suitable organometallic compounds include butyllithium (such as n-butyllithium, sec-butyllithium, iso-butyllithium and tert-butyllithium) and phenyllithium. Preferably, n-butyllithium is used. Suitable metal borane compounds include NaBH4and LiBH4, preferably LiBH4. Suitable Grignard reagents include alkyl- or arylmagnesium chloride compounds and alkyl- or aryl-magnesium bromide compounds. Suitable metal-metal alloys include sodium-potassium alloy.

[0183] Typically, the mole ratio of metal ion source to transition metal dichalcogenide in the exfoliation step is from 1 :1 to 5:1. Preferably, the mole ratio of metal ion source to transition metal dichalcogenide in the exfoliation step is from 1 :1 to 4:1 , more preferably from 1 :2 to 1 :3. Typically, the mole ratio of metal ion source to transition metal dichalcogenide in the exfoliation step is 1:1 or more. Preferably, the mole ratio of metal ion source to transition metal dichalcogenide in the exfoliation step is 1:2 or more, more preferably 1:3 or more.

[0184] Typically, the mole ratio of organolithium reagent to transition metal dichalcogenide in the exfoliation step is from 1 :1 to 5:1. Preferably, the mole ratio of organolithium reagent to transition metal dichalcogenide in the exfoliation step is from 1 :1 to 4:1 , more preferably from 1:2 to 1 :3. Typically, the mole ratio of organolithium reagent to transition metal dichalcogenide in the exfoliation step is 1:1 or more. Preferably, the mole ratio of organolithium reagent to transition metal dichalcogenide in the exfoliation step is 1 :2 or more, more preferably 1 :3 or more.

[0185] Typically, the concentration of the metal ion source (such as organolithium reagent) is such that it is present in excess compared to the transition metal dichalcogenide. In other words, the transition metal dichalcogenide is saturated with the metal ion source (such as organolithium reagent). Preferably, the concentration of the metal ion source (such as organolithium reagent) is 1 M or more, more preferably 1.5 M or more, even more preferably 2 M or more. In some embodiments, the concentration may be from 1 M to 3 M, preferably 1.5 M to 2.5 M.

[0186] Typically, the exfoliation step takes place in a solvent. Typically, an organic solvent is used. Most commonly, a non-polar organic solvent is used. Preferably, a hydrocarbon solvent is used.

[0187] The hydrocarbon solvent may be an aliphatic or aromatic hydrocarbon solvent.

[0188] Examples of suitable aliphatic hydrocarbon solvents include linear alkanes such as pentane, hexane, heptane and octane; cycloalkanes such as cyclopentane, cyclohexane, cycloheptane and cyclooctane; and petroleum fractions such as kerosene and petroleum ether. Mixtures of these solvents may be used.

[0189] Examples of suitable aromatic hydrocarbon solvents include benzene, toluene and xylene.

[0190] Preferably, the organic solvent is an aliphatic hydrocarbon solvent, more preferably the organic solvent is hexane.

[0191] The exfoliation step may be performed at ambient temperature (approximately 20 °C). Alternatively, the exfoliation step may be performed at elevated temperature (above ambient temperature; above approximately 20 °C). Methods for providing heat during the exfoliation step are known and include, for example, using a reaction vessel having an external heating jacket. Typically, the exfoliation step comprises heating the transition metal dichalcogenide at reflux. That is, at the boiling point of the solvent.

[0192] The exfoliation step may be performed for sufficient time to allow a desired quantity of the lithiated transition metal dichalcogenide to form. Typically, the exfoliation step is performed until substantially all the transition metal dichalcogenide is consumed.

[0193] Typically, the exfoliation step comprises treating the transition metal dichalcogenide with an organolithium compound for 12 hours to 72 hours. Preferably, the exfoliation step comprises exfoliating the transition metal dichalcogenide for 24 hours to 72 hours, more preferably 48 hours to 72 hours.

[0194] Typically, the exfoliation step comprises treating the transition metal dichalcogenide with an organolithium compound for 6 hours or more, 8 hours or more, 10 hours or more, 12 hours or more. Preferably, the exfoliation step comprises exfoliating the transition metal dichalcogenide for 24 hours or more, more preferably 36 hours or more, even more preferably 48 hours or more.

[0195] Preferably, exfoliation is carried out under conditions that provide a metallic phase TMD. For example, exfoliation is carried by treating the TMD with organolithium compound for a sufficient time and / or at a sufficient mole ratio to provide a metallic phase T D. The presence of a metallic phase TMD may be confirmed by any suitable analysis method, such as X-ray diffraction, Raman spectroscopy or XPS. For example, an XPS for metallic 1T phase LixMoS2 has peaks at approx. 232 eV and 228 eV compared to the semiconducting 2H phase which has peaks at approx. 233 eV and 229 eV.

[0196] The lithiated transition metal dichalcogenide may be collected, for example, by filtration. Methods of filtration are known.

[0197] The lithiated transition metal disulfide may be washed to ensure removal of remaining organolithium regents and organic residues. For example, the lithiated transition metal dichalcogenide may be washed with an organic solvent, typically a non-polar solvent. Preferably, the lithiated transition metal dichalcogenide is washed with a hydrocarbon solvent, such as hexane.

[0198] As noted above, the lithium ions may be removed from the lithiated, metallic phase transition metal dichalcogenide while retaining the material in the metallic phase. Removal of the lithium ions may be achieved my washing the lithiated transition metal dichalcogenide with an aqueous solvent. Accordingly, the exfoliation step may optionally comprise washing the lithiated transition metal dichalcogenide with water or an aqueous solvent. Washing the lithiated transition metal dichalcogenide with water can reduce the lithiation rate of the material to 0.05 or less, such as 0. Sonication improves the removal of the lithium ions. Accordingly, the aqueous washing step may comprise sonicating the lithiated transition metal dichalcogenide in water or an aqueous solvent. The sonication step is performed until substantially all the lithium ions are removed from the transition metal dichalcogenide. Typically, the sonication step is performed for from 15 minutes to 4 hours, such as from 30 minutes to 2 hours, such as about 1 hour.

[0199] The (non-lithiated) metallic phase transition metal dichalcogenide may be collected, for example, by filtration or by centrifugation.

[0200] The method comprises assembling a working electrode comprising a film comprising stacked layers of the metallic phase transition metal dichalcogenide and sulfur or a lithium (poly)sulfide. This may be referred to as the restacking step, step (b).

[0201] The restacking step may comprise filtering a suspension comprising flakes, monolayers, or nanosheets of the metallic phase transition metal dichalcogenide.

[0202] The restacking step also comprises loading the working electrode with sulfide or a lithium polysulfide salt.

[0203] Any suitable method may be used to load the sulfur component onto the metallic phase transition metal dichalcogenide. Preferably, however, high-temperature annealing or molten diffusion processes are avoided. High-temperature annealing or molten diffusion process processes may cause a partial phase transition from the 1T phase to the less-desired 2H phase.

[0204] The loading may take place by coprecipitation or by solvent-based loading.

[0205] In the coprecipitation method, a suspension of transition metal dichalcogenide and elemental sulfur (Ss), such as powdered sulfur, is prepared and mixed. The suspension may be prepared by suspending the material obtained in the exfoliation step in a suitable solvent.

[0206] Preferably, the mass ratio of LiaMX2 to sulfur in the suspension is from 1:2 to 1:3. More preferably, the mass ratio of LiaMX2 to sulfur is about 1:2.5.

[0207] Typically, the suspension is prepared in an organic solvent. Suitable solvents include carbon disulfide.

[0208] Typically, the suspension is mixed to ensure a uniform distribution of sulfur and metallic phase transition metal dichalcogenide throughout the suspension. Suitable dispersion methods include sonication.

[0209] The suspension is filtered to prepare a composite of the metallic phase transition metal dichalcogenide and elemental sulfur. The composite may be removed from the filter medium and used in the working electrode. Alternatively, the suspension may be filtered over a porous conductive material. The porous, conductive material may then be used as the current collector in the working electrode.

[0210] Alternatively, a solvent-based loading approach may be used. In such cases, a solution of a lithium (poly)sulfide may be applied to the stacked, layered metallic phase transition metal dichalcogenide, such as a film of the metallic phase transition metal dichalcogenide.

[0211] Suitable lithium (poly)sulfides include U2S4, LisSsand LisSs. The lithium (poly)sulfides may be purchased commercially. Alternatively, they may be prepared by reacting lithium sulfide (l_i2S) with sulfur in the appropriate molar quantities.

[0212] Suitable solvents include the electrolyte solvents set out above.

[0213] Preferably, the loading comprises coprecipitating the metallic phase transition metal dichalcogenide with sulfur.

[0214] Electrolyte

[0215] In general, the electrolyte is a quasi-solid-state electrolyte (QSSE), and the electrolyte comprises a monomer, a polymer formable from the monomer, and a lithium salt.

[0216] The polymer may be formed or formable from the monomer. Preferably, the polymer is formed from the monomer components present in the QSSE.

[0217] The polymer and monomer are structurally related, in that the polymer is formed or formable from the monomer. The monomer may be referred to as a precursor to the polymer, or may be referred to as a repeating unit of the polymer.

[0218] The polymer may be formable from the monomer together with co-polymerizable monomers. The polymer may be formed from the monomer together with co-polymerizable monomers. In this way, the polymer may be a copolymer,

[0219] Preferably ,the polymer is formed or formable from the monomer. Preferably, the polymer is a homopolymer.

[0220] The electrolyte is typically positioned between the working and counter electrodes (e.g., negative and positive electrodes). The QSSE occupies the interstitial space between the electrodes of the Li-S cell. The electrolyte facilitates ion transport between the electrodes. Typically, the electrolyte in the electrochemical cell is suitable for solubilising lithium ions. Typically, the electrolyte in a charged and discharged cell contains lithium ions. In a fourth aspect of the invention there is provided an electrolyte for a lithium sulfur cell, wherein the electrolyte is a quasi-solid-state electrolyte (QSSE), the electrolyte comprising a monomer, a polymer formed or formable from the monomer, and a lithium salt; and wherein the electrolyte comprises from 5 wt.% to 30 wt.% monomer, based on the total mass of the monomer and the polymer.

[0221] The invention also provides an electrolyte obtained or obtainable by the preparation method described herein.

[0222] The electrolyte is a quasi solid-state electrolyte (QSSE).

[0223] QSSEs are a specific type of solid-state electrolyte. A QSSE typically comprises a polymer matrix containing a liquid component. The QSSE may be a freestanding membrane that contains a liquid component. The liquid component is typically located inside the polymer matrix. QSSE may also be known as gel polymer electrolytes (GPE).

[0224] The QSSE typically has a gel-like character, and may be known as a gel electrolyte. The gel is typically a mixture of solid and liquid components, such as solid polymer components and liquid monomer components. The solid polymer components may be formed or formable from the same monomers present in the liquid monomer component.

[0225] The QSSE may have a viscosity of from 5,000 to 20,000 mPa s, measured at 25°C according to ISO 1628-1:2021. Typically, the QSSE has a viscosity of from 10,000 to 15,000 mPa s.

[0226] The QSSE may be homogeneous. The QSSE may be uniform and / or homogeneous across the inter-electrode space. The QSSE may have long range order. The QSSE differs from, for example, a polymer separator and liquid electrolyte, because the separator and electrolyte mixture is not homogeneous and uniform across the inter-electrode space.

[0227] QSSEs differ from a liquid electrolyte (LE) which is predominately liquid, such as 90 wt.% or more, or 95 wt.% or more liquid. Liquid electrolytes do not typically include a polymer matrix containing the liquid. Some liquid electrolytes may be provided in a separator. However, the separator is typically a different polymer material to the liquid electrolyte. For example, the separator may be polypropylene, while the liquid electrolyte is 1 ,3-dioxolane. In a QSSE of the present invention, the polymer matrix is formed from a monomer present in the liquid contained within the polymer matrix.

[0228] QSSEs differ from all-solid-electrolytes (SE) which are predominately free of liquid. That is, SEs are predominantly polymer, such as 90 wt.% or more, or 95 wt.% or more polymer, based on the total mass of the electrolyte. Typically, SEs conduct ions through interaction with the polymer chains. In contrast, QSSEs may conduct ions through the liquid contained within the polymer matrix. The electrolyte comprises a monomer, a polymer formed or formable from the monomer, and a lithium salt.

[0229] The monomer and polymer may be selected because of the ability of the monomer to be polymerised using the TMD as an initiator in the polymerisation. The polymer may be a polymer formed or formable from a monomer by a polymerisation reaction, such as a polymerisation reaction using a TMD.

[0230] The monomer is a polymerizable monomer. The monomer is typically a monomer which is polymerizable in the presence of a Lewis acidic initiator, and preferably a monomer which is polymerizable in the presence of a TMD initiator, such as M0S2. Preferably, the monomer is a monomer which is polymerizable in the presence of a TMD initiator, such as M0S2, and lithium salt, such as LiTFSI.

[0231] It is thought that the Lewis acidity of the T D allows it to initiate the polymerisation of the monomer. It is thought that the lithium salt, such as LiTFSI, catalyses the polymerisation of the monomer.

[0232] In this context, monomer refers to free monomer. That is, monomer which is not incorporated into a polymer.

[0233] The polymer is a polymer formed or formable from the polymerizable monomer. The polymer is typically a polymer which was polymerized in the presence of a Lewis acidic initiator, and preferably a polymer which was polymerized in the presence of a TMD initiator, such as M0S2. Preferably, the polymer is a polymer which was polymerized in the presence of a TMD initiator, such as M0S2, and lithium salt, such as LiTFSI.

[0234] In some embodiments, the monomer is a cyclic acetal. A cyclic acetal is cyclic molecule comprising a -O-CH2-O- moiety, or substituted equivalent, as part of a heterocycloalkyl ring. A cyclic acetal may be a cyclic molecule comprising an unsubstituted -O-CH2-O- moiety as part of a heterocycloalkyl ring.

[0235] The polymer may be a polyether. In some such embodiments, the polymer formed or formable from the cyclic acetal monomer is a polyether. The polyether typically comprises a -O-CH2-O- moiety, or substituted equivalent, as part of a polymer chain. In some embodiments, the polyether polymer may comprise an unsubstituted -O-CH2-O- moiety as part of a polymer chain.

[0236] In some embodiments the electrolyte comprises a monomer of Formula (A). wherein -L- is -(CR2)ni- or -(CR2)n2-O-(CR2)n3-; n1 is from 2 to 6; n2 and n3, are each independently from 1 to 5; each R is independently H, or C1-6 alkyl, or two R groups, either connected to the same carbon atom or connected to different carbon atoms, taken together with the carbon atom(s) to which they are attached, form a C5-10 cycloalkyl or C5-10 heterocycloalkyl ring; and wherein -X1and -X2are independently H or C1-6 alkyl, or -X1and -X2taken together with the carbon atom to which they are attached form a C5-10 cycloalkyl or a C5-10 heterocycloalkyl ring.

[0237] In some embodiments the electrolyte comprises a polymer of Formula (B). wherein -L- is -(CR2)ni-, or -(CR2)n2-O-(CR2)n3-; n1 is from 2 to 6; n2 and n3, are each independently from 1 to 5; each R is independently H, or C1-6 alkyl, or two R groups, either connected to the same carbon atom or connected to different carbon atoms, taken together with the carbon atom(s) to which they are attached, form a C5-10 cycloalkyl or C5-10 heterocycloalkyl ring; wherein -X1and -X2are independently H or C1-6 alkyl, or -X1and -X2taken together with the carbon atom to which they are attached form a C5-10 cycloalkyl or a C5-10 heterocycloalkyl ring; and wherein n is 10 or more.

[0238] In the present invention, the polymer is a polymer formed or formable from the polymerizable monomer. Accordingly, the polymer may be a polymer of Formula (B) and the monomer a monomer of Formula (A), wherein the definitions of -L-, -X1, -X2, n1, n2, n3, and R is the same for Formula (A) and Formula (B).

[0239] In some embodiments, -X1and -X2are each independently H or C1-6 alkyl. C1-6 alkyl is a monovalent alkyl group having from 1 to 6 carbon atoms forming the alkyl chain. The alkyl chain may be linear or branched. For example, the alkyl group may be selected from methyl, ethyl, and propyl, including n-propyl and / -propyl, butyl, pentyl or hexyl. Preferably the C1-5 alkyl is C1-4 alkyl, such as C1-3 alkyl, such as C3 alkyl.

[0240] In some embodiments, -X1and -X2are Ci-s alkyl, preferably C1.3 alkyl, more preferably C1-2 alkyl.

[0241] In some embodiments, -X1and -X2taken together with the carbon atom to which they are attached form a spiro C5-10 cycloalkyl or C5-10 heterocycloalkyl ring. Preferably, -X1and -X2taken together with the carbon atom to which they are attached form a spiro C5-10 cycloalkyl, such as a spiro C5-6 cycloalkyl.

[0242] C5-10 cycloalkyl is a saturated cyclic group having from 5 to 14 carbon atoms forming the cycloalkyl ring. For example, the cycloalkyl group may be a 5 membered cycloalkyl group, or a 6 membered cycloalkyl group, or a 10 membered cycloalkyl group. The cycloalkyl ring may be unsubstituted or substituted with one or more additional C1-6 alkyl groups or C1-6 heteroalkyl groups.

[0243] C5-10 heterocycloalkyl is a saturated cyclic group having from 5 to 14 atoms forming the cycloalkyl ring, including at least one heteroatom selected from -O-, -N(H)- or -S-. For example, the cycloalkyl group may be a 5 membered heterocycloalkyl group, or a 6 membered heterocycloalkyl group, or a 10 membered heterocycloalkyl group. The heterocycloalkyl ring may be unsubstituted or substituted with one or more additional Ci-6alkyl groups or C1-6 heteroalkyl groups.

[0244] Preferably, -X1and -X2are H.

[0245] In some embodiments the electrolyte comprises a monomer of Formula (A1). wherein -L-, n1, n2, n3 and R are as defined for Formula (A). In some embodiments the electrolyte comprises a polymer of Formula (B1). wherein -L-, n, n1 , n2, n3 and R are as defined for Formula (A).

[0246] In some embodiments, n is 700 or less, preferably 500 or less, more preferably 300 or less, yet more preferably 100 or less. In some embodiments, n is 10 or more, more preferably 30 or more, yet more preferably 45 or more.

[0247] In some embodiments, n is from 20 and 340, preferably from 30 to 150, more preferably from 35 to 100, yet more preferably from 40 to 50. For example, n may be about 45.

[0248] In the present invention, the polymer is a polymer formed or formable from the polymerizable monomer. Accordingly, the polymer may be a polymer of Formula (B1) and the monomer a monomer of Formula (A1), wherein the definitions of -L-, n1, n2, n3, and R is the same for Formula (A1) and Formula (B1).

[0249] In some embodiments, the electrolyte comprises a monomer of the Formula (A1) and a polymer of the Formula (B1), wherein -L- is -(CR2)ni-, or -(CR2)n2-O-(CR2)n3-; n1 is from 2 to 6; n2 and n3, are each independently from 1 to 5; each R is independently H or Ci-6 alkyl, or two R groups taken together with the carbon atom(s) to which they are attached form a C5-10 cycloalkyl or C5-10 heterocycloalkyl ring; wherein n is 10 or more.

[0250] -L- is -(C 2)n1-, or -(C 2)n2-O-(C 2)n3-. In some embodiments, n1 , n2 and n3 are each independently from 2, 4 or 5. Preferably n2 and n3 are each independently 2 or 4, such as 2. Preferably n1 is 2, 4 or 5, such as 2 or 4.

[0251] In some embodiments, -L- is -(CR2)ni- and n1 is 2, 4 or 5, such as 2 or 4.

[0252] In some embodiments, -L- is -(CR2)n2-O-(CR2)n3- and n2 and n3 are 2 or 4, such as 2.

[0253] Preferably -L- is -(CR2)2-.

[0254] In some embodiments, each R is independently H or Ci-6 alkyl. The Ci-s alkyl may be C1-3 alkyl, preferably C1-2 alkyl. Preferably, each R is independently H or C1-2 alkyl.

[0255] In some embodiments, at least one R is C1.6 alkyl, preferably C1.3 alkyl, more preferably C1-2 alkyl. In some embodiments, one R is C1-6 alkyl, preferably C1.3 alkyl, more preferably C1-2 alkyl.

[0256] In some embodiments, each R is independently H or two R groups taken together with the carbon atom(s) to which they are attached form a C5-10 cycloalkyl or C5-10 heterocycloalkyl ring. In some embodiments, two R groups taken together with the carbon atom(s) to which they are attached form a C5-10 cycloalkyl or C5-10 heterocycloalkyl ring and the remaining R groups are H.

[0257] In some embodiments, each R is independently H or two R groups taken together with the carbon atom(s) to which they are attached form a C5-10 cycloalkyl ring. In some embodiments, two R groups taken together with the carbon atom(s) to which they are attached form a C5-10 cycloalkyl ring and the remaining R groups are H.

[0258] In some embodiments, each R is independently H or two R groups taken together with the carbon atom(s) to which they are attached form a C5-6 cycloalkyl or C5-6 heterocycloalkyl ring. In some embodiments, two R groups taken together with the carbon atom(s) to which they are attached form a C5-6 cycloalkyl or C5-6 heterocycloalkyl ring and the remaining R groups are H.

[0259] In some embodiments, each R is independently H or two R groups taken together with the carbon atom(s) to which they are attached form a Cs-s cycloalkyl ring. In some embodiments, two R groups taken together with the carbon atom(s) to which they are attached form a C5-6 cycloalkyl ring and the remaining R groups are H.

[0260] The cycloalkyl or heterocycloalkyl ring may be unsubstituted or substituted with one or more C1-5 alkyl groups or C1-6 heteroalkyl groups . Preferably the cycloalkyl or heterocycloalkyl ring is unsubstituted.

[0261] Preferably, each R is H. In some embodiments, -L- is -(CH2)ni-, or -(CH2)n2-O-(CH2)n3-.

[0262] In some embodiments, n1 , n2 and n3 are each independently from 2, 4 or 5. Preferably n2 and n3 are each independently 2 or 4, such as 2. Preferably n1 is 2, 4 or 5, such as 2 or 4.

[0263] In some embodiments, -L- is -(CH2)ni- and n1 is 2, 4 or 5, such as 2 or 4.

[0264] In some embodiments, -L- is -(CH2)n2-O-(CH2)n3- and n2 and n3 are 2 or 4, such as 2.

[0265] Preferably

[0266] Preferably -L- is -(CH2)2-, -(CH2)4-, -(CH2)5-, -(CH2)4- or -(CH2)2-O-(CH2)2-.

[0267] Particularly preferably -L- is -(CH2)2-.

[0268] In the examples of the invention, the monomer is 1 ,3 dioxolane, and the polymer is a polymer formed or formable from 1 ,3 dioxolane.

[0269] QSSEs may be formed or formable from the partial polymerization of the monomers. Accordingly, QSSEs may be characterized according to their degree of polymerization, their number-average molecular weight, and the unpolymerized original monomer fraction remaining in the formed QSSE.

[0270] The QSSE typically has a lower degree of polymerization and lower Mnthan a SE. The QSSE typically has a higher degree of polymerization and higher Mnthan a LE.

[0271] The unpolymerized monomer fraction remaining in the electrolyte can be provided as a wt.% based on the total mass of the polymerizable monomer and the polymer formed from the polymerizable monomer. The ratio may be measured, for example, using1H NMR spectroscopy by integrating peaks attributable to the monomers and polymers, as described in the example section. The ratio may be measured by preparing a representative sample of the QSSE, for example by an ex-situ preparation using equivalent ratios of polymerizable components, initiators and catalysts, which is equivalent to the QSSE formed in-situ in the lithium-sulfur cell. Alternatively, the ratio may be measured by disassembling the lithiumsulfur cell and extracting the QSSE formed in-situ.

[0272] In contrast to a SE, the QSSE retains some unpolymerized monomer within the electrolyte.

[0273] In some embodiments, the electrolyte comprises 3 wt.% or more monomer, based on the total mass of the monomer and the polymer. Preferably, the electrolyte comprises 5 wt.% or more monomer, more preferably 10 wt.% or more, yet more preferably 12 wt.% or more.

[0274] In some embodiments, the electrolyte comprises 25 wt.% or less monomer, based on the total mass of the monomer and the polymer. Preferably, the electrolyte comprises 20 wt.% or less monomer, more preferably 15 wt.% or less, yet more preferably about 14 wt.% or less.

[0275] In some embodiments, the electrolyte comprises from 3 to 40 wt.% monomer, based on the total mass of the monomer and the polymer. Preferably, the electrolyte comprises from 5 to 30 wt.% monomer, more preferably 8 to 25 wt.%, yet more preferably 10 to 20 wt.%, yet more preferably 11 to 15 wt.%. The electrolyte may comprise about 13 wt.% monomer, based on the total mass of the monomer and the polymer.

[0276] The number-average molecular weight (Mn) of the polymer formed from the polymerizable monomer may be determined by gel permeation chromatography, using an Agilent 1260 Infinity II using tetrahydrofuran as the mobile phase. The measurement method is as described in the examples section. The number-average molecular weight may be measured by preparing a representative sample of the QSSE, for example by an ex-situ preparation using equivalent ratios of polymerizable components, initiators and catalysts, which is equivalent to the QSSE formed in-situ in the lithium-sulfur cell. Alternatively, the ratio may be measured by disassembling the lithium-sulfur cell and extracting the QSSE formed in-situ for measurement as described herein.

[0277] In some embodiments, the polymer has a number-average molecular weight (Mn) of 1,500 g mol-1or more, preferably 2,000 g mol'1or more, more preferably 3,000 g mol'1or more.

[0278] In some embodiments, the polymer has a number-average molecular weight (Mn) of 50,000 g mol'1or less, preferably 25,000 g mol'1or less, more preferably 10,000 g mol'1or less, yet more preferably 5,000 g mol'1or less.

[0279] In some embodiments, the polymer has a number-average molecular weight (Mn) of from 1,500 to 25,000 g mol'1, preferably from 2,000 to 10,000 g mol'1, more preferably from 3,000 to 5,000 g mol'1, yet more preferably from 3,000 to 4,000 g mol'1. The number-average molecular weight of the polymer may be about 3,300 g mol'1.

[0280] The degree of polymerisation is the ratio of Mn / Mo, whereinnis the number-average molecular weight and Mois the monomer unit molecular weight. The degree of polymerisation is indicative of the number of monomers in each polymer chain (as a number average). The Mnis measured as described above and in the examples section. The Mois taken as the monomer weight when the monomer is present in the polymer (e.g., excluding any leaving groups which may be lost during polymerisation). For example, for 1,3 dioxolane Mo is taken as 74 g / mol. In some embodiments, the polymer has a degree of polymerization of 700 or less, preferably 500 or less, more preferably 300 or less, yet more preferably 100 or less. In some embodiments, the polymer has a degree of polymerization of 5 or more, preferably 10 or more, more preferably 30 or more, yet more preferably 45 or more.

[0281] In some embodiments, the polymer has a degree of polymerization of from 20 and 340, preferably from 30 to 150, more preferably from 35 to 100, yet more preferably from 40 to 50. The degree of polymerisation may be about 45.

[0282] The electrolyte comprises a lithium salt. The lithium salt may be a lithium salt having good solubility in the monomer and / or ether solvent. The lithium salt may be a lithium salt which is able to assist, such as catalyse, the polymerisation of the monomer.

[0283] The lithium salt may be LiTFSI, (bis(trifluoromethane)sulfonimide lithium salt, LiPF6, LiBF4, UCIO4, LiTF (lithium triflate), lithium difl uoro(oxalato) borate (LiDFOB) and lithium bis(oxalato) borate (LiBOB). Preferably, the electrolyte comprises a lithium salt that has good solubility in ether solvents, such as LiTFSI, LiCICU, LiTF and LiBOB. Preferably, the electrolyte comprises a lithium salt that may catalyse the polymerisation reaction, such as UCIO4, LiTFSI, LiPF6, and LiDFOB, more preferably LiTFSI or UCIO4.

[0284] The lithium salt is preferably LiTFSI.

[0285] In some embodiments, the electrolyte comprises the lithium salt, such as LiTFSI, at a concentration of from 0.1 to 2.0 M, preferably 0.5 to 1.5 M, more preferably 0.8 to 1.2 M, yet more preferably about 1.0 M.

[0286] In some embodiments, the electrolyte comprises 10 to 40 wt.% lithium salt, such as LiTFSI, based on the total mass of the electrolyte, preferably 20 to 30 wt.%, more preferably 22 to 25 wt.%.

[0287] Without wishing to be bound by theory, it is thought that the coordination of the TFSL anion to the TMD electrode results in the formation of an electron deficient cationic sulfonyl species. The monomer coordinates more strongly to the sulfonyl cation than the TMD electrode and is therefore more activated. Thus, the rate of polymerization of the monomer is increased by the inclusion of LiTFSI.

[0288] The electrolyte may be a liquid electrolyte, such as a liquid at ambient temperature, for example at 25 °C.

[0289] In some embodiments, the electrolyte may further comprise an organic solvent. This solvent is provided in addition to the monomer described above, and so may be known as an additional organic solvent. Preferably, the electrolyte comprises a non-aqueous electrolyte. The electrolyte may comprise a polar aprotic solvent. The electrolyte may comprise an organic solvent. Solvents for dissolving lithium ions are well known in the art.

[0290] Preferably, the solvent is a solvent having a poor solubility for sulfur and lithium polysulfide.

[0291] Preferably, the solvent is an ether solvent. Lithium polysulfide is poorly soluble in ether solvents. Suitable ether solvents include acyclic ethers, cyclic ethers and polyethers.

[0292] Examples of suitable acyclic ethers include diethyl ether, dipropyl ether, dibutyl ether, dimethoxymethane, trimethoxymethane, dimethoxyethane, diethoxyethane, 1,2-dimethoxypropane, and 1,3-dimethoxypropane.

[0293] Examples of suitable cyclic ethers include tetrahydrofuran, tetrahydropyran, 2-methyltetrahydrofuran, 1,4-dioxane, 1 ,3-dioxolane, and trioxane.

[0294] Examples of suitable polyethers include, diethylene glycol dimethyl ether (diglyme), triethylene glycol dimethyl ether (triglyme), tetraethylene glycol dimethyl ether (tetraglyme), higher glymes, ethylene glycol divinylether, diethylene glycol divinylether, triethylene glycol divinylether, dipropylene glycol dimethyl ether, and butylene glycol ethers.

[0295] In some embodiments, the electrolyte further comprises an organic solvent, preferably an acyclic ether solvent or a cyclic ether solvent, such as 1 ,2-Dimethoxyethane (DME).

[0296] In some embodiments, the electrolyte further comprises 20 to 70 wt.% organic solvent, based on the total mass of the monomer, the polymer and the organic solvent, preferably from 30 to 60 wt.%, more preferably from 40 to 50 wt.%, yet more preferably about 45 wt.%.

[0297] In some embodiments, the electrolyte further comprises 25 to 75 wt.% organic solvent and monomer, based on the total mass of the monomer, the polymer and the organic solvent, preferably from 35 to 65 wt.%, more preferably from 45 to 55 wt.%, yet more preferably about 52 wt.%.

[0298] In some embodiments, the electrolyte is free of additional organic solvent. That is, the electrolyte consists of a monomer and polymer formed or formable from the monomer as the solvent component. In some embodiments, the electrolyte may consist of, such as essentially consist of, a monomer, a polymer formed or formable from the monomer, a lithium salt and a TMD of Formula (I). In some embodiments, the electrolyte may consist of, such as essentially consist of, a monomer, a polymer formed or formable from the monomer and a lithium salt.

[0299] In some embodiments, the electrolyte comprises a TMD powder of Formula (I). The definition and preferred embodiments of Formula (I) is as described in the working electrode section. In some embodiments, the electrolyte comprises 0.01 wt.% or more TMD powder of Formula (I), based on the total mass of the electrolyte, preferably 0.02 wt.% or more. In some embodiments, the electrolyte comprises 1 wt.% or less TMD powder of Formula (I), based on the total mass of the electrolyte, preferably 0.1 wt.% or less. In some embodiments, the electrolyte comprises from 0.01 to 0.1 wt.% TMD powder of Formula (I), based on the total mass of the electrolyte, preferably 0.02 to 0.05 wt.%.

[0300] Preferably the TMD powder of Formula (I) has the same composition as the working electrode of the Li-S cell. The TMD powder is typically used to catalyse the formation of the QSSE, where the QSSE is formed outside of the Li-S cell.

[0301] In some embodiments, the electrolyte is substantially free of UNO3. The UNO3 is thought to inhibit the polymerisation of the monomer, and thus prevents formation of a QSSE. In the examples, LiNOs is used in the comparative LE and substantially no polymerisation is observed.

[0302] In some embodiments, the electrolyte is substantially free of initiators, other than the TMD.

[0303] In some embodiments, the electrolyte is substantially free of catalysts, other than lithium salt. For example, the electrolyte may be substantially free of AIF3, Al l3, and AI(OTf)3. In some embodiments, the electrolyte is substantially free of catalysts, other than LiTFSI. For example, the electrolyte may be substantially free of Li PF6, LiDFOB, AIF3, Alls, and AI(OTf)3.

[0304] Substantially free typically means less than 0.02 wt.%, such as less than 0.01 wt.%.

[0305] In some embodiments, the electrolyte consists essentially of a monomer, a polymer formed or formable from the monomer, an additional solvent, a lithium salt and optionally a TMD of Formula (I).

[0306] In preferred embodiments, the electrolyte consists essentially of a 1,3 dioxolane, a polymer formed or formable from 1 ,3 dioxolane, an additional solvent, a lithium salt and optionally a TMD of Formula (I).

[0307] In particular embodiments, the electrolyte consists essentially of a 1,3 dioxolane, a polymer formed or formable from 1 ,3 dioxolane, 1 ,2-dimethoxyethane, LiTFSI and optionally a TMD of Formula (I).

[0308] In particularly preferred embodiments, the electrolyte consists essentially of a 1 ,3 dioxolane, a polymer formed or formable from 1 ,3 dioxolane, 1 ,2-dimethoxyethane, LiTFSI and optionally M0S2.

[0309] The amounts of the components contained in the electrolyte are as described above. The QSSE of the invention facilitates rapid lithium ion diffusion through the liquid monomer portion of the QSSE, whilst inhibiting polysulfide shuttling by a combination of steric hindrance and electrostatic repulsion between the polysulfide anions and the heteroatoms of the polymer framework.

[0310] The worked examples demonstrate that a Li-S cell in which the electrolyte comprises a DOL- based QSSE has excellent electrochemical characteristics not limited to; high sulfur utilization, low self-discharge rates, polysulfide shuttling inhibition, high energy density, high capacity, excellent thermal stability, excellent capacity retention, good resistance to mechanical stress, excellent dynamic (cycling) and static (storage) stability and extended cycling lifetime.

[0311] The inventors expect the advantageous electrochemical characteristics imparted by the DOL-based QSSE to be observed for other polymerizable monomers and in the presence of transition metal dichalcogenides other than 1T M0S2. In particular, when the electrolyte has the characterises described above, such as monomer fraction, degree of polymerisation, Mn, it has been shown that there is a balance of sufficient polymer framework to inhibit polysulfide shuttling and sufficient unpolymerized monomer to facilitate LE-like catholyte-intermediated dissolution-precipitation reaction kinetics.

[0312] The characteristic properties of the QSSE can be modified by tuning the Lewis acidity of the TMD initiator and components of the electrolyte. Therefore, a large range of QSSEs with different properties can be made, and the properties of the QSSE can be optimized.

[0313] Preparation of Polymer Mixture and Electrolyte

[0314] In a second aspect of the invention there is provided a method for forming a polymer mixture, the method comprising: contacting a monomer and a lithium salt with a transition metal dichalcogenide (TMD), wherein the TMD is of formula (I):

[0315] LiaMX2(I) wherein a is from 0 to 2.0;

[0316] X is selected from S, Se, and Te; and M is a transition metal; and polymerising a portion of the monomer to give a polymer mixture comprising the monomer, the polymer formed from the monomer, and the lithium salt.

[0317] The polymer mixture may be used in an electrolyte of an electrochemical cell. The polymer mixture may be used in an electrolyte of a lithium sulfur cell, such as a QSSE of a lithium sulfur cell.

[0318] In a related aspect, there is also provided a polymer mixture obtained or obtainable by the method of the second aspect. Accordingly, in some embodiments, there is provided a method of preparing an electrolyte for a lithium sulfur cell, the method comprising: contacting a monomer, a lithium salt and a transition metal dichalcogenide (TMD), wherein the TMD is of formula (I):

[0319] LiaMX2(I) wherein a is from 0 to 2.0;

[0320] X is selected from S, Se, and Te; and M is a transition metal polymerising a portion of the monomer into a polymer, to provide the electrolyte, wherein the electrolyte is a quasi-solid-state electrolyte (QSSE); and the electrolyte comprises the monomer, the polymer formed from the monomer, and the lithium salt.

[0321] In a related aspect, there is provided a polymerizable composition comprising a monomer, a lithium salt and a transition metal dichalcogenide (TMD), wherein the TMD is of formula (I).

[0322] The components of the electrolyte are as described herein, such as in the Electrolyte section. The TMD is as described herein, such as in the Working Electrode section.

[0323] The step of contacting a monomer, a lithium salt and a transition metal dichalcogenide (TMD), may be known as the contacting step.

[0324] Contacting refers to bringing the components together such that they are able to react. In other words, the components are in reactive contact.

[0325] The components may be contacted using any suitable means. The component may be contacted by mixing in solution or mixing in suspension, or by applying the components to a surface.

[0326] The step of polymerising a portion of the monomer into a polymer to provide the electrolyte may be known as the polymerising step.

[0327] The polymerisation may occur by any suitable mechanism which results in a polymer of the invention. The polymerisation may be referred to as a partial polymerisation. That is, where only a portion of the monomer is reacted to form the polymer. Thus, a portion of the monomer is retained in the product mixture, which product mixture may be, and preferably is, the QSSE.

[0328] The polymerisation may be a co-polymerisation, with a further monomer component. Preferably, the polymerisation is a homopolymerisation. In some embodiments, 40 wt.% or less of the monomer is present, based on the total mass of the monomer and the polymer. Preferably, 30 wt.% or less of the monomer is present, more preferably 20 wt.% or less, yet more preferably 15 wt.% or less.

[0329] In some embodiments, from 3 to 40 wt.% of the monomer is present, based on the total mass of the monomer and the polymer. Preferably, from 5 to 30 wt.% of the monomer is present, more preferably 8 to 25 wt.%, yet more preferably 10 to 20 wt.%, even more preferably 11 to 15 wt.%. In some embodiments, about 13 wt.% of the monomer is present, based on the total mass of the monomer and the polymer

[0330] In some embodiments, 60 wt.% or more of the polymer is present, based on the total mass of the monomer and the polymer. Preferably, 70 wt.% or more of the polymer is present, more preferably 80 wt.% or more, yet more preferably about 85 wt.% or more.

[0331] In some embodiments, from 60 to 98 wt.% of the polymer is present, based on the total mass of the monomer and the polymer. Preferably, from 70 to 95 wt.% of the polymer is present, more preferably from 80 to 92 wt.%, even more preferably from 85 to 90 wt.%, yet more preferably from 86 to 89 wt.%. In some embodiments, about 87 wt.% of the polymer is present, based on the total mass of the monomer and the polymer.

[0332] Preferably, the amount of monomer present corresponds to the amount of monomer which has not been polymerised. Preferably, the amount of polymer present corresponds to the amount of monomer which has been polymerised. Accordingly, the amounts described above may apply to the amount of monomer which has not been polymerised and / or the amount of monomer which has not been polymerised, respectively.

[0333] The amount of monomer and polymer present typically refers to the amount present at the completion of the polymerisation reaction. The amount of monomer and polymer present may refer to where the polymerisation composition is isolatable. The amount typically refers to the where the composition is present in a QSSE.

[0334] The polymerisation typically uses an initiator, such as a Lewis acidic initiator. The initiator is preferably the TMD as described herein.

[0335] The polymerisation typically uses a catalyst, such as lithium salt. The catalyst is preferably a lithium salt, such as LiTFSI or UCIO4, preferably LiTFSI.

[0336] The polymer mixture, such as the electrolyte, may be prepared using an ex-situ preparation. That is, where the polymer mixture is prepared outside of the lithium sulfur cell. Specifically, this may refer to where the contacting step and / or the polymerisation step occurs outside of the lithium sulfur cell. Preferably, in an ex-situ preparation the contacting step and the polymerisation step occur outside of the lithium sulfur cell. In some such embodiments, the TMD is a TMD powder. In some such embodiments, in the step of contacting the monomer, the lithium salt and the TMD comprises mixing the monomer and the lithium salt with the T D powder.

[0337] The TMD powder may be added to the monomer to polymerise the monomer. The TMD powder may be contacted with the monomer outside of a lithium sulfur cell, to result in polymerisation outside of the lithium sulfuric cell. The TMD powder may be retained in the electrolyte composition when assembled into a Li-S cell.

[0338] Alternatively, the polymer mixture, such as the electrolyte, may be prepared using an in-situ preparation. That is, where the polymer mixture is prepared in a lithium sulfur cell assembly. Specifically, this may refer to where the contacting step and / or the polymerisation step occurs inside of the lithium sulfur cell assembly. Preferably, in an in-situ preparation the contacting step and the polymerisation step occurs inside of the lithium sulfur cell assembly.

[0339] In some such embodiments, the TMD is the working electrode in the lithium sulfur cell assembly.

[0340] In some embodiments, the step of contacting the monomer, the lithium salt and the TMD comprises applying the monomer and the lithium salt to a surface of the TMD working electrode in the lithium sulfur cell assembly.

[0341] The lithium sulfur cell assembly typically refers to a precursor of a lithium sulfur cell. That is, a lithium sulfur cell where the electrolyte is not yet present or formed. The lithium sulfur cell assembly may comprise a working electrode, a counter electrode and optionally a separator. The lithium sulfur cell may also comprise a polymerisation mixture, which is used to form the QSSE.

[0342] In some embodiments, the monomer and the lithium salt are applied to the surface of the TMD working electrode during preparation of a lithium sulfur cell.

[0343] In this way, the TMD electrode may be used to polymerise the monomer during electrolyte preparation. The TMD electrode may be contacted with the monomer inside of a lithium sulfur cell assembly, to result in polymerisation inside of the lithium sulfuric cell assembly and form a working lithium sulfur cell. In addition, or alternatively, the TMD electrode may be contacted with the monomer before or during cell assembly.

[0344] The present invention also provides an electrolyte obtained or obtainable by the method of manufacture described above. Preparation of Li- S Cell

[0345] In a third aspect of the invention there is provided, a method of preparing a lithium sulfur cell, the method comprising: forming a polymer mixture according to the second aspect; and assembling the lithium sulfur cell from a working electrode, a counter electrode, and a QSSE comprising the polymer mixture, wherein the working electrode comprises a transition metal dichalcogenide (TMD) of formula (I).

[0346] In some embodiments the polymer mixture is an electrolyte, such as an QSSE.

[0347] In some embodiments, there is provided a method of preparing a lithium sulfur cell, the method comprising: contacting a monomer, a lithium salt and a transition metal dichalcogenide (TMD), wherein the TMD is of formula (I):

[0348] LiaMX2(I) wherein a is from 0 to 2.0;

[0349] X is selected from S, Se, and Te; and M is a transition metal polymerising a portion of the monomer into a polymer, to provide an electrolyte, wherein the electrolyte is a quasi-solid-state electrolyte (QSSE), and the electrolyte comprises the monomer, the polymer formed from the monomer, and the lithium salt; and assembling the lithium sulfur cell comprising a working electrode, a counter electrode, and the electrolyte, wherein the working electrode comprises a transition metal dichalcogenide (TMD) of formula (I).

[0350] The components of the electrolyte are as described herein, such as in the Electrolyte section. The TMD is as described herein, such as in the Working Electrode section. Other components of the Li-S cell are as described herein. The description of the method of preparing the electrolyte provided above also applies to the method of preparing a lithium sulfur cell.

[0351] In this method, the working electrode comprises the transition metal dichalcogenide (TMD) of formula (I). The TMD in the working electrode may be the same or a different type of TMD to the TMD used in the contacting step. Preferably, the working electrode comprises the same type of TMD of Formula (I) as used in the contacting step. Preferably the working electrode comprises 1T phase M0S2 and the contacting step uses 1T phase M0S2.

[0352] Preferably, the working electrode TMD is the TMD used in the contacting step. The method comprises a step of contacting a monomer, a lithium salt and a transition metal dichalcogenide (TMD). This step is as described herein, for the method of preparing the electrolyte.

[0353] The TMD may be a powder. In some embodiments, the step of contacting the monomer, the lithium salt and the TMD comprises mixing the monomer and the lithium salt with a TMD powder.

[0354] In alternative embodiments, the step of contacting the monomer, the lithium salt and the TMD comprises applying the monomer and the lithium salt to a surface of the working electrode. For example, the step of contacting a monomer, a lithium salt and a TMD comprises applying the monomer and the lithium salt to a working electrode for a lithium-sulfur cell, wherein in the working electrode comprises a TMD of formula (I).

[0355] The method comprises a step of polymerising a portion of the monomer into a polymer, to provide an electrolyte. This step is as described herein, for the method of preparing the electrolyte.

[0356] In some embodiments, the polymerisation step occurs on the surface of the working electrode, to provide an electrolyte on the surface on the working electrode.

[0357] The method comprises a step of assembling the lithium sulfur cell comprising a working electrode, a counter electrode, and the electrolyte. This may be referred to as the assembly step.

[0358] In some embodiments, the QSSE electrolyte has formed before the cell is assembled. Accordingly in some embodiments, assembling the lithium sulfur cell comprises applying the electrolyte to the working electrode and / or counter electrode. Preferably, the electrolyte applied to the working electrode and / or counter electrode comprises the TMD powder.

[0359] In some embodiments, the polymerising step occurs on the surface of the working electrode, to provide an electrolyte on the surface on the working electrode, preferably wherein the polymerising step occurs in a lithium-sulfur cell assembly.

[0360] In some such embodiments, the method of preparing a lithium-sulfur cell comprises in order: contacting a monomer, a lithium salt and a transition metal dichalcogenide (TMD), wherein the TMD is of formula (I):

[0361] LiaMX2(I) wherein a is from 0 to 2.0;

[0362] X is selected from S, Se, and Te; and

[0363] M is a transition metal polymerising a portion of the monomer into a polymer, to provide an electrolyte, wherein the electrolyte is a quasi-solid-state electrolyte (QSSE), and the electrolyte comprises the monomer, the polymer formed from the monomer, and the lithium salt; and assembling a working electrode, a counter electrode, and the electrolyte into a lithium sulfur cell, wherein the working electrode comprises a transition metal dichalcogenide (TI ID) of formula (I).

[0364] In alternative embodiments, the QSSE electrolyte forms during and / or after the assembly of the working electrode and the counter electrode. This may be due to the monomer being contacted with the TMD working electrode, which acts as a polymerisation initiator. Accordingly, in some embodiments, the step of assembling the lithium sulfur cell occurs simultaneously with the polymerising step.

[0365] In some such embodiments, the method of preparing a lithium-sulfur cell comprises in order: contacting a monomer, a lithium salt and a transition metal dichalcogenide (TMD) of a working electrode, wherein the TMD is of formula (I):

[0366] LiaMX2(I) wherein a is from 0 to 2.0;

[0367] X is selected from S, Se, and Te; and

[0368] M is a transition metal polymerising a portion of the monomer into a polymer, to provide an electrolyte on the working electrode, wherein the electrolyte is a quasi-solid-state electrolyte (QSSE), and the electrolyte comprises the monomer, the polymer formed from the monomer, and the lithium salt; and assembling the working electrode, the electrolyte, and a counter electrode into a lithium sulfur cell.

[0369] In alternative embodiments, the QSSE electrolyte forms after the electrodes are arranged into a lithium sulfur cell assembly. In a related embodiment, the working electrode, counter electrode, and where present a separator, are assembled prior to the contacting step. Accordingly, in some embodiments, the method comprises a step of assembling the working electrode and counter electrode into a cell assembly, followed by applying the monomer and the lithium salt to a space between the working electrode and the counter electrode. In such embodiments, the polymerisation step occurs after the assembly step and the contacting step.

[0370] In some such embodiments, the method of preparing a lithium-sulfur cell comprises in order: assembling a working electrode and a counter electrode into a lithium sulfur cell, wherein the working electrode comprises a transition metal dichalcogenide (TMD) of formula (I):

[0371] LiaMX2(I) wherein a is from 0 to 2.0; X is selected from S, Se, and Te; and

[0372] M is a transition metal contacting a monomer and a lithium salt with the transition metal dichalcogenide (TMD) of the working electrode, polymerising a portion of the monomer into a polymer, to provide an electrolyte, wherein the electrolyte is a quasi-solid-state electrolyte (QSSE), and the electrolyte comprises the monomer, the polymer formed from the monomer, and the lithium salt.

[0373] The assembly step may also include a step of assembling a separator. The separator is as described herein, such as in the lithium sulfur section. Thus, in some embodiments, the assembly step comprises assembling a working electrode, electrolyte, separator and counter electrode into a lithium sulfur cell.

[0374] The assembly step typically takes place in the absence of oxygen (for example, in an atmosphere containing less than 10 ppm oxygen). The assembly step typically takes place in the absence of water (for example, in an atmosphere containing less than 10 ppm water vapor).

[0375] Preferably, the assembly step typically takes place under inert atmosphere, such as an atmosphere or nitrogen or argon.

[0376] Any suitable lithium sulfur cell geometry may be used. Suitable geometries include coin cells and pouch cells.

[0377] The present invention also provides a lithium sulfur cell obtained or obtainable by the method of manufacture set out above.

[0378] The lithium sulfur cell set out above, and the lithium sulfur cell obtained or obtainable by the method of manufacture set out above, have excellent electrochemical properties.

[0379] Battery, Uses and Applications

[0380] The invention also provides a battery comprising one or more lithium sulfur cells of the invention.

[0381] Where there is a plurality of cells, these cells may be provided in series or parallel.

[0382] A battery of the invention may be provided in a road vehicle, such as an automobile, moped or truck. Alternatively, a battery of the invention may be provided in a rail vehicle, such as a train or a tram. A battery of the invention may also be provided in an electric bicycle (e-bike), a drone, an electric aircraft, and an electric or hybrid boat. Similarly, batteries of the invention may be provided in power tools such as powered drills or saws, garden tools such as lawnmowers or grass trimmers, or home appliances such as toothbrushes or hair dryers. The batteries of the invention may be provided in a drone or a satellite, such as a low orbit satellite.

[0383] A battery of the invention may be provided in a regenerative braking system.

[0384] A battery of the invention may be provided in a portable electronic device, such as a mobile phone, laptop or tablet.

[0385] A battery of the invention may be provided in a power grid management system.

[0386] The invention also provides a use of an electrolyte in a lithium sulfur cell, wherein the electrolyte is a quasi-solid-state electrolyte (QSSE), the electrolyte comprising a monomer, a polymer formed or formable from the monomer, and a lithium salt; and wherein the electrolyte comprises from 5 to 30 wt.% monomer, based on the total mass of the monomer and the polymer components.

[0387] In particular, the invention provides a use of an electrolyte in a lithium sulfur cell, wherein the electrolyte is a quasi-solid-state electrolyte (QSSE), the electrolyte comprising a monomer, a polymer formed or formable from the monomer, and a lithium salt; wherein the lithium sulfur cell comprises a working electrode comprising a TMD. Preferably the TMD is of Formula (I):

[0388] LiaMX2(I) where: a is from 0.1 to 1.0;

[0389] X is selected from S, Se and Te; and M is a transition metal.

[0390] Preferences for the working electrode and electrolyte, and other components of the lithiumsulfur cell, are as set out above.

[0391] Other Preferences

[0392] Each and every compatible combination of the embodiments described above is explicitly disclosed herein, as if each and every combination was individually and explicitly recited.

[0393] Various further aspects and embodiments of the present invention will be apparent to those skilled in the art in view of the present disclosure.

[0394] “and / or” where used herein is to be taken as specific disclosure of each of the two specified features or components with or without the other. For example “A and / or B” is to be taken as specific disclosure of each of (i) A, (ii) B and (iii) A and B, just as if each is set out individually herein. Unless context dictates otherwise, the descriptions and definitions of the features set out above are not limited to any particular aspect or embodiment of the invention and apply equally to all aspects and embodiments which are described.

[0395] Certain aspects and embodiments of the invention will now be illustrated by way of example and with reference to the figures described above.

[0396] Examples

[0397] The following examples are provided to further illustrate the present invention and are not intended to limit the scope of the invention.

[0398] Experimental Methods and Materials

[0399] Preparation of MoSz-based sulfur hosts and sulfur composites for cathodes

[0400] Preparation of M0S2

[0401] Bulk 2H M0S2 powder (0.3 g; Alfa Aesar) was used as purchased without further modification.

[0402] 1T M0S2 was synthesized by chemical exfoliation of bulk 2H M0S2 with organolithium. The 2H M0S2 was first immersed in hexane (15 mL; Sigma-Aldrich), followed by adding n-butyllithium solution (1.6 M in hexane, 3 ml; Sigma-Aldrich) and refluxing for 2 days under argon protection. The product was then washed with hexane (50 mL) 3 times and dispersed in deionized water (1 mg mL-1) with the aid of ultrasonication (20 minutes). After centrifuging to remove the unreacted parts and residues, the resultant powder was freeze-dried to yield 1T M0S2.

[0403] The 1T and 2H M0S2 was characterised using Raman spectroscopy. The identity of 1T and 2H M0S2 was confirmed by the Aigand E12gpeaks in the Raman spectra for the semiconducting 2H MoS2 and the J-series peaks of the metallic 1T M0S2 phase (see Fig. 1B).

[0404] Preparation of sulfur composites

[0405] Sulfur composites were prepared by mixing M0S2 host material (40 mg) and sulfur powder (100 mg; Alfa Aesar) through a ball milling process. The mixture was next ground with poly(vinylidene fluoride) binder (MTI Corporation) at a mass ratio of 9:1 in A / -Methyl-2- pyrrolidone (Sigma-Aldrich) to form a homogenous slurry.

[0406] For the slurry of 2H M0S2, an additional 10 wt.% Super P carbon (MTI Corporation) was added to ensure sufficient conductivity (that is, sulfur composite, carbon, binder at a 8:1 :1 mass ratio). For the 1T M0S2 no conductive carbon was used (giving sulfur composite and binder at a 9:1 mass ratio). The slurry was then coated onto Al foils (MTI Corporation) using a doctor blade and dried at 60 °C for 24 hours. The loading was 8 mg / cm2. The coated aluminium foils were used as the MoS2-sulfur-based cathodes in the electrochemical cells described below.

[0407] Preparation of electrolytes and polysulfide solutions

[0408] All electrolytes and polysulfide solutions were prepared in an argon-filled glovebox. LiTFSI salt (1.0 M; Sigma-Aldrich) was first dissolved in mixed 1 ,3 dioxolane (DOL) and 1,2-dimethoxyethane (DME) (1 :1 by volume; Sigma-Aldrich) solvents to produce a liquid precursor.

[0409] Comparative liquid electrolyte (LE) examples were prepared by dissolving LiNOs (0.2 M; Sigma-Aldrich) in the precursor.

[0410] Quasi solid state electrolyte (QSSE) examples were formed in situ by dropping the liquid precursor onto 1T M0S2 -based cathodes, during cell assembly. Alternatively, the QSSE without integration into cells was obtained by adding 1T M0S2 powders (0.5 mg) to the liquid precursor (2 mL).

[0411] Comparative solid electrolyte (SE) examples were formed in situ by dropping the liquid precursor onto 2H MoS2-based cathodes, during cell assembly. Alternatively, the SE without integration into cells was obtained by adding 2H M0S2 powders (0.5 mg) to the liquid precursor (2 mL).

[0412] Polysulfide solutions were prepared by mixing sulfur and lithium sulfide (Alfa Aesar) powders in stoichiometric proportion in the liquid precursor, followed by stirring overnight at 50 °C.

[0413] Materials characterisation

[0414] Structural information of materials was detected by NMR spectroscopy (Bruker 400 MHz Avance III HD using chloroform-d as a deuterated solvent), Raman spectroscopy (Renishaw InVia using a 514 nm laser beam), FTIR spectroscopy (Bruker Tensor 27 using an ATR mode) and gel permeation chromatography (Agilent 1260 Infinity II using tetrahydrofuran as the mobile phase).

[0415] Electrochemical characterisation

[0416] Electrochemical performance of electrolytes was characterized in coin cells (CR2032) and pouch cells (6 cm 4.5 cm in dimension). All coin cells were fabricated in an argon-filled glovebox.

[0417] Li-S coin cells were assembled with the MoS2-sulfur-based cathode, the lithium foil anode, Celgard separator and the electrolyte. Asymmetric Li||Cu and symmetric Li coin cells were assembled with similar components but by changing the electrodes into lithium foil versus Cu foil and two pieces of lithium foil, respectively. Pouch cells were fabricated in a dry room (relative humidity <0.1%) with the MoSz-based cathode (Al current collector), the lithium foil anode (Cu current collector), Celgard separator, and the electrolyte. The Al and Ni tabs were welded together with the cathodes and anodes, respectively. The entire cell core was encapsulated into the Al-laminated films.

[0418] Galvanostatic charge-discharge (GCD) tests were carried out on a battery cycler (LANHE CT3002A) in the voltage range of 1.7 V to 2.8 V at various C rates (1 C = 1,672 mAh g-1). An oven and a fridge were coupled with the battery cycler to control the temperature during GCD measurements.

[0419] Cyclic voltammetry (CV) tests were conducted with an electrochemical workstation (BioLogic VSP-300) from 1.5 V to 3.0V at various scan rates.

[0420] Open Circuit Voltage (OCV) of freshly assembled cells before measurements was recorded for 180 days to monitor the self-discharge behaviour. The cycling stability was evaluated by performing continuous GCD cycles. The Li stripping and plating tests were investigated in both asymmetric Li||Cu (stripping cutoff voltage = 1 V) and symmetric Li cells at various current densities.

[0421] Electrochemical impedance spectroscopy was measured at open circuit under a sinusoidal signal over the frequency range from 100 kHz to 100 mHz with an amplitude of 10 mV.

[0422] Unless specified otherwise, all electrochemical characterisation was carried out at 25 °C.

[0423] Example 1 - Investigating the Formation of QSSE

[0424] Three partially filled vials of DOL and 1T MoS2were made up to give a precursor composition. A first vial was left as the precursor composition. LiNOs was added to a second vial. LiTFSI was added to a third vial.

[0425] The vials were capped and left to rest for 1 minute before upturning them to mix the contents. The solutions in all three vials immediately flowed to the capped end of the vial (see Fig. 2).

[0426] The vials were re-set and left to stand for 10 minutes, before upturning them again. The solutions in the vials containing 1T M0S2 and LiNOs (second vial) and 1T M0S2 (first vial) both immediately flowed to the capped end of the vial, but the vial containing M0S2 and TFSI' (third vial) remained in the base of the vial. The increased viscosity of the third vial is thought to indicate that some polymerisation of the DOL occurred, to form a QSSE composition (see Fig. 2).

[0427] The remaining two vials (first and second vials) were re-set and left to stand for 10 hours before upturning them again. The vial containing LiNOs (second vial) remained a solution and immediately flowed to the capped end of the vial. The vial containing M0S2 (first vial) flowed down slowly, behaving as a viscous liquid. The increased viscosity is thought to indicate that some polymerisation of the DOL occurred, to form a QSSE composition (see Fig. 2).

[0428] It is thought that the polymerisation and thus formation of QSSE was accelerated by the addition of TFSI' and inhibited by the presence of UNO3. Without wishing to be bound by theory, it is thought that the IJNO3 inhibits polymerization due to stabilising coordination interactions with the DOL monomer. On the other hand, the TFSI' anion accelerates the rate of reaction by generating an electropositive sulfonyl leaving group with an electron deficient S centre by interaction with the M0S2.

[0429] A proposed mechanism for the polymerisation reaction of DOL with TFSI' anion is shown in Fig. 3a. The mechanism shown in Figure 2a illustrates the chemical process of cationic ringopening polymerization initiated by MoS2-based cathodes.

[0430] It is thought that the Lewis acidic Mo site in M0S2 interacts with the TFSI' anion in the electrolyte, creating an electron-deficient N centre. Subsequently, the electron of the adjacent S atom transfers to the electropositive N site due to relatively high electronegativity of the N atom, which thus generates a sulfonyl leaving group and a residual electron-deficient S centre. This electropositive S site is then attacked by the lone-pair electron of the O atom in DOL solvents, forming an oxonium ion that initiates the ring-open polymerization of DOL monomers. As the polymer chains grow, the electrolyte transforms from its original liquid state into a transparent solid (see Fig. 3b).

[0431] It is believed that the ring-opening reaction is primarily associated with ring strain, and therefore will be greatly affected by the composition of the electrolyte. For example, in the most commonly used Li-S electrolyte that contains lithium nitrate (UNO3) as an additive, the ring-opening polymerization of DOL is inhibited due to the strong coordination between DOL and UNO3. In addition, it understood that the activation of polymerization to the Lewis acidic site produced by the interaction between M0S2 and TFSI', rather than to the intuitively believed Mo atom in M0S2. This is because, although sole MoS2 is capable of inducing the ring-opening polymerization, the presence of TFSI-anions in the electrolyte renders higher Lewis acidity and consequently accelerates such reaction.

[0432] This test shows that the gelation process of DOL can be manipulated by tuning the intrinsic properties of M0S2, leading to a QSSE that substantially enhances the cycling stability of Li-S batteries.

[0433] Example 2 - Material Characterisation of SE, LE and QSSE Compositions

[0434] A comparative LE composition was prepared as described above, by preparing a liquid precursor of 1,3 dioxolane (DOL) and 1 ,2-dimethoxyethane (DME) (1 :1 by volume) with LiTFSI salt (1.0 M) and LiNO3(0.2 M). A QSSE composition was prepared as described above, by preparing a liquid precursor of 1,3-dioxolane (DOL) and 1 ,2-dimethoxyethane (DME) (1 :1 by volume) solvent with LiTFSI salt (1.0 M). The liquid precursor (2 mL) was combined with 1T phase M0S2 powder (0.5 mg).

[0435] The weakly Lewis acidic 1T M0S2 cathode results in partial polymerisation, and formation of a QSSE.

[0436] A comparative SE composition was prepared as described above, by preparing a liquid precursor of 1,3 dioxolane (DOL) and 1,2-dimethoxyethane (DME) (1 :1 by volume) solvent with LiTFSI salt (1.0 M). The liquid precursor (2 mL) was combined with 2H phase M0S2 powder (0.5 mg). The more strongly Lewis acidic 2H phase M0S2 cathode results in a greater degree of polymerisation, and formation of a SE.

[0437] Without wishing to be bound by theory, it is thought that the extra electron present in the metallic 1T phase M0S2 partially neutralizes the Lewis acidity compared to 2H M0S2, and thus weakens its interaction with the TFIS-anions. As a consequence, the polymerization process initiated by 1T M0S2 terminates at a lower reaction degree than that with 2H M0S2, producing a QSSE - a polymer framework incorporating a portion of unpolymerized liquid.

[0438] NMR Characterisation

[0439] The different electrolytes were analysed using1H N R.

[0440] Analysis by1H NMR (see Fig. 4A) showed the LE containing a polymerisation inhibitor (UNO3) retained peaks corresponding to the DOL monomer, with1H peaks at 3.87 ppm and 4.90 ppm. No additional peaks were visible in the NMR spectra, suggesting the LE included substantially all DOL monomer with no significant polymerisation.

[0441] The SE was found to have1H peaks at 3.73 and 4.76 ppm, which are attributable to the DOL polymer. No additional peaks were visible in the NMR spectra, suggesting the SE included substantially all DOL polymer with no significant amount of remaining DOL monomer.

[0442] The QSSE was found to have a mix of1H peaks at 3.87 and 4.90 ppm (corresponding to the DOL monomer) and 3.73 and 4.76 ppm (corresponding to the DOL polymer). This suggests that the QSSE is predominantly DOL polymer, but with significant amount of unreacted DOL monomer. Integration of the area of the peaks indicates the unpolymerized original DOL fraction in the QSSE is approximately 13%.

[0443] Raman Characterisation

[0444] The structural changes were further investigated by Raman spectroscopy. Analysis by Raman spectroscopy (see Fig. 4B) shows a high intensity peak at approximately 950 cm-1for the LE, which is thought to correspond to the C-O-C ring vibration in the DOL monomer.

[0445] In contrast both the SE and QSSE show high intensity peaks at approximately 850 cm-1, which is thought to correspond to the C-0 chain stretching vibration in the DOL polymer.

[0446] FTIR Characterisation

[0447] The structural changes were further investigated by fourier-transform infrared spectroscopy (FTIR).

[0448] Analysis by FTIR spectroscopy (see Fig. 4C) shows a strong and narrow absorption for LE at approximately 1080 cm-1, corresponding to the C-O-C ring vibration, and a peak at approximately 900 cm-1for the out-of-plane C-H vibration. These are thought to be characteristic of the DOL monomer.

[0449] In contrast both SE and QSSE show a broader absorption with a maximum absorption at approximately 1,000 cm-1from the C-0 chain vibration and a sharp peak at approximately 850 cm-1for the long chain. These are thought to be characteristic of the DOL polymer.

[0450] Gel Permeation Chromatography Characterization

[0451] The QSSE and SE, prepared as described above, were purified by washing to remove the M0S2 powder.

[0452] Analysis by gel permeation chromatography was performed on the purified products.

[0453] The number-average molecular weight (Mn) of QSSE was determined to be 3,300 g mol’1. The Mnof the purified SE was found to be 53,700 g mol'1.

[0454] This corresponds to a degree of polymerization (Mn / M0where Mois the weight of the monomer unit [Mo is taken to be 74 g / mol]) of approximately 45 for the QSSE and of approximately 725 for the SE.

[0455] The results are shown graphically in Fig. 4D.

[0456] Resistance to Volatilization

[0457] The resistance to volatilization of the LE, QSSE and SE was investigated.

[0458] The LE, QSSE and SE were left in an uncapped vial (see Fig. 2E), and their mass measured periodically over a 24 hour period. After 24 hours, the LE weighed only 55.3 % of its original weight. After 24 hours the QSSE weighed 88.9 % of its original weight, and the SE weighed 94.2 % of its original weight. The QSSE demonstrates superior resistance to volatilization loss compared to the LE, despite the lower Mnand the presence of DOL monomer. It is thought that this is due to the formation of a polymer framework within the QSSE, which traps the more volatile DOL monomer.

[0459] SEM Characterisation

[0460] In addition, SEM images of the 1T MoS2-based cathodes were obtained. SEM images were obtained using an FEI Magellan 400.

[0461] An SEM image was obtained before the QSSE was formed on the electrode surface. This is shown in Figure 9a. A second SEM image was obtained after the QSSE was formed on the electrode surface. This is shown in Figure 9b.

[0462] The SEM images show the excellent integration of the QSSE into the varied morphology of the electrode surface. This is thought to provide a good electrode-electrolyte interface.

[0463] Example 3 - Electrochemical characterisation of QSSE, SE and LE

[0464] A comparative LE composition was prepared as described above, by preparing a liquid precursor of 1,3 dioxolane (DOL) and 1 ,2-dimethoxyethane (DME) (1 :1 by volume) with LiTFSI salt (1.0 M) and LiNO3(0.2 M).

[0465] A QSSE composition was prepared as described above, by preparing a liquid precursor of 1,3-dioxolane (DOL) and 1 ,2-dimethoxyethane (DME) (1 :1 by volume) solvent with LiTFSI salt (1.0 M). The liquid precursor was added to a 1T phase M0S2 based cathode, during cell assembly. The weakly Lewis acidic 1T M0S2 cathode results in partial polymerisation, and formation of a QSSE.

[0466] A comparative SE composition was prepared as described above, by preparing a liquid precursor of 1,3-dioxolane (DOL) and 1,2-dimethoxyethane (DME) (1:1 by volume) solvent with LiTFSI salt (1.0 M). The liquid precursor was added to a 2H phase M0S2 based cathode, during cell assembly. The more strongly Lewis acidic 2H phase M0S2 cathode results in a greater degree of polymerisation, and formation of a SE.

[0467] The electrolytes were formed into coin cells, as described above.

[0468] A series of Li-S coin cells were assembled with the MoS2-sulfur-based cathode, the lithium foil anode, Celgard separator and either the LE, QSSE or SE electrolyte. The QSSE and comparative SE electrolytes were formed in situ with the 1T M0S2 and 2H M0S2 cathodes, respectively. The comparative LE was tested using a 1T M0S2 cathode. The electrolyte-to- sulfur (E / S) and negative-to-positive (N / P) ratios were kept at ideal conditions for all coin cells. Galvanostatic charge-discharge (GCD)

[0469] Galvanostatic charge-discharge (GCD) curves were generated for each of the coin cells, as described above. The discharge-charge curves are shown in Fig. 5A.

[0470] The GCD curves for the QSSE and comparative LE coin cells both exhibit two discharge plateaus at 2.4 V and 2.1 V, suggesting a typical catholyte-intermediated redox mechanism. Considering that the dissolution of polysulfides in the electrolyte is primarily ascribed to the 1,2-dimethoxyethane (DME) solvent, the presence of two characteristic plateaus indicates the cathode surface is still accessible to DME in a QSSE based coin cell.

[0471] The GCD curve of the comparative SE coin cell shows only slightly plateaus at around 2.4 V and 2.1 V. This indicates a deviation from a catholyte-intermediated redox mechanism, suggesting that DME is less able to access the cathode and that an alternative solid-solid sulfur conversion route occurs.

[0472] Without wishing to be bound by theory, it is thought that this difference is due to the retention of unpolymerized DOL within QSSE, which is highly compatible with DME.

[0473] As a consequence, sulfur cathodes in QSSE through the dissolution-based stepwise reaction pathway delivered a specific capacity of 1 ,146 mAh g-1at 0.1C, higher than the cathodes in SE (723 mAh g-1) and even comparable to those in LE (1, 180 mAh g-1).

[0474] Cyclic Volta metry (CV)

[0475] Cyclic Voltametry (CV) was carried out on each of the coin cells, as described above. The discharge-charge curves are shown in Fig. 5B.

[0476] The CV trace is consistent with that observed in the GCD. The comparative SE coin cells show only one representative anodic and cathodic peak, but two representative cathodic peaks (2.4 V and 2.1 V) and one anodic peak (2.5V) are observed for the LE and QSSE coin cells.

[0477] Assessment of Polysulfide Shuttling

[0478] Dissolution of polysulfides is thought to be responsible for the high specific capacity of cathodes, but it also thought to be the root of the shuttling effect that results in poor cycling stability and self-discharge.

[0479] The polysulfide shuttling effect was tested by adding a strongly coloured solution of U2S6, a lithium polysulfide, into a LE and QSSE composition (see Fig. 5C). It was observed that the LE quickly became dark brown in colour, indicating facile permeation of the LE by U2S6. In contrast, the QSSE remained colourless, indicating poor permeation of the QSSE by Li2Ss. This qualitative test suggests that in the QSSE, while allowing the catholyte-intermediated Li-S chemistry, the polymer framework prevents polysulfides from penetrating through the electrolyte and thereby adequately suppresses their shuttling, as compared to a LE.

[0480] Self-discharge Behaviour

[0481] Freshly assembled coin cells include QSSE and comparative SE and LE, were prepared as described above.

[0482] The Open-circuit voltage (OCV) of each cell was measured over a period of 180 days (see Fig. 5D), as described above.

[0483] The OCV of freshly assembled cells with LE dropped by 0.08 V (from 2.43 V to 2.35 V) after resting for 30 days, and further decreased to below 2.3 V over 100 days.

[0484] The OCV of freshly assembled cells with SE is 2.38 V and remains at 2.38 V for the duration of the 180 days of measuring.

[0485] The OCV of freshly assembled cells with QSSE showed a small OCV drop of only 0.01 V (from 2.41 V to 2.4 V) after the initial 30 days, and still maintained above 2.38 V after 180 days.

[0486] The greatly improved retention of capacity indicates the QSSE-containing cells suffer a much lower degree of self-discharge and represents a vital improvement for the real-world feasibility of Li-S cells.

[0487] Further tests were carried out on a fresh set of coin cells. The specific capacity of the cells was measured every day over 24 days. The cells underwent one galvanostatic chargedischarge cycles at 0.1C per day, and the capacity was measured. The results are shown in Fig. 3E.

[0488] The cells were paused mid-discharge at 2.1 V of their 11thdischarge cycle, where polysulfides are most concentrated, and rested for 10 days before resuming discharging. After 10 days of resting at this half-way discharged state, the cell discharge was resumed for a further 14 cycles, and the specific capacity measured.

[0489] The LE cells showed a significant capacity loss of 17% after the 10 day rest, whilst the QSSE cells showed a capacity loss of less than 4%. This % change is comparable to the SE, although the overall capacity of the SE cell is much lower.

[0490] The QSSE cell demonstrates a capacity retention of over 95% after 10 days of inactivity in a partially discharged state. In addition, self-discharge current at various OCVs was examined after the cells reached voltage equilibrium during the relaxation period. This was carried out using the method described in Knap et al..

[0491] A first test was carried out using a LE, QSSE and SE in a coin cell (prepared as described above). The results are shown in Figure 10. This demonstrates that the self-discharge current for the QSSE is consistently lower than for LE based cells.

[0492] A further test was carried out using a LE and QSSE coin cell stored at 25°C and 45°C. The results are shown in Figure 11. This demonstrates that the self-discharge current for the QSSE is consistently lower than for LE based cells, and that the QSSE is relatively unaffected by increases in temperature compared to the LE cells.

[0493] This is indicative of relatively low levels of self-discharge for QSSE base cells, especially at high temperatures.

[0494] Anode Stability and SEI Formation

[0495] Asymmetric Li||Cu coin cells were assembled using the QSSE and comparative LE and SE, as described above.

[0496] The coulombic efficiency of the cells was measured over 100 charge-discharge cycles at a current density of 1 mA cm-2and a capacity density of 1 mAh cm-2. The results are shown in Fig. 6 A

[0497] The coulombic efficiency of the cells with LE fades rapidly after 50 cycles. The efficiency of the LE cell falls from between approximately 80-95% after 20 cycles, to between about 70- 85% after 50 cycles, and then to about 40% after 80 cycles. The poor coulombic efficiency of cells containing LE is thought to be due to the instability of the solid electrolyte interphase (SEI) derived from LE, which fractures in each cycle and is repaired by continuously consuming the electrolyte.

[0498] In contrast, the coulombic efficiency of cells containing a QSSE or SE were stable for over 100 cycles. The coulombic efficiency of the QSSE-containing cells rose to >90% after fewer than 10 cycles and remained stable at >90% for over 100 cycles. The coulombic efficiency of the SE-containing cells was lower and fluctuated between approximately 85-95%, which is thought to be due to relatively poor ionic conductivity of the SE.

[0499] Moreover, the SEIs formed in polymer-based electrolytes (QSSE and comparative SE) also provided better protection to the lithium metal anode from parasitic reactions — for example, the polysulfide shuttling effect — than that in comparative LE.

[0500] The SEI was qualitatively compared between each of the cells. Li2Se was added to each of the coin cells, and anode corrosion visually compared (see Fig. 6C). The cells comprising QSSE and SE visibly showed minimal corrosion compared to the cells comprising LE, where the anode showed significant corrosion. This suggests that the SEI formed in the QSSE and SE cells provides better protection for the electrode, than the LE cells.

[0501] The long-term reversibility of Li stripping and plating processes in different electrolytes was further evaluated in symmetric Li cells. Symmetric lithium coin cells were assembled using LE, QSSE and SE. Their galvanostatic polarization behaviours were measured at a current density of 1 mA cm-2and a capacity density of 1 mAh cm-2over 400 cycles (see Fig. 6D). This is equivalent to 1 hour per cycle.

[0502] As shown in Fig. 6D, the polarization in SE is consistently higher than that in LE and QSSE during the initial 150 cycles. SE cells polarize between approximately +0.07 and -0.09 V, gradually increasing to and fluctuating between approximately +0.11 and -0.12 V after 150 cycles. This is thought to be associated with the lower ionic conductivity, in good agreement with the results from asymmetric Li||Cu cells.

[0503] The polarization of LE and QSSE cells is observed to be similar for the first 50 cycles and both polarize between approximately +0.03 and -0.04 V. As cycling continues, however, a gradual increase in overpotential is observed for LE cells. By 190 cycles the LE cell polarizes between approximately + / - 0.3 V.

[0504] The higher overpotential in LE cells means that a larger driving force is required to strip and plate Li in each cycle, which is unfavourable for Li deposition and is believed to result in uncontrolled lithium growth, accumulation of dead lithium, and ultimately internal short circuits.

[0505] In comparison QSSE cells operated stably for more than 400 hours with no deviation from polarization values between approximately +0.03 and -0.04 V.

[0506] A Nyquist plot for the symmetric lithium cells with different electrolytes was plotted after 200 cycles, which showed the QSSE cells to have the smallest resistance (see Fig. 6B). The greater operating stability and low resistance of QSSE suggests superior compatibility with lithium metal anodes.

[0507] Example 4 - Variable Temperature Testing

[0508] Pouch-type Li-S cells with LE and QSSE were assembled, as described above.

[0509] Electrochemical performance was evaluated at different temperatures. GCD curves were obtained at 5 °C, 25 °C and 45 °C (see Fig. 7A and 7B). At 25 °C, both of the QSSE and comparative LE Li-S cells exhibit a similar specific capacity of 1,050 mAh g-1.

[0510] However, as the temperature increased to 45 °C, although LE-based pouch cells show enhanced reaction kinetics (smaller polarization voltage gap between charge and discharge curves), their specific capacity (716 mAh g-1) drops greatly. In contrast, at 45 °C QSSE-based pouch cells deliver a higher specific capacity of 1,213 mAh g-1as the result of promoted reaction kinetics at high temperature. These results are thought to be due to the exacerbated shuttling of polysulfides in LE at high temperature, as also revealed by the poorly maintained second discharge plateau in the GCD curve.

[0511] At 5 °C, the LE-based cells can barely operate with a specific capacity of less than 300 mAh g-1, while the QSSE pouch-type Li-S cell retains a considerable specific capacity of 753 mAh g-1, suggesting good low temperature adaptability.

[0512] The rate capability for QSSE and comparative LE pouch cells was also tested at different temperatures, and the results are shown in Fig. 7C.

[0513] High Rate Testing

[0514] Rate capability for pouch-type cells containing LE and QSSE was measured at 25 °C, and for QSSE batteries at 45 °C. Capacity (mAh g-1) was measured at C-rates of 0.05, 0.1 , 0.2, 0.3, 0.4 and 0.5 C.

[0515] At the lowest current density of 0.05 C, the specific capacity of the QSSE batteries at 45 °C was measured as 1,200 mAh g-1, whilst the QSSE and LE batteries at 25 °C were very similar at approximately 1 ,050 mAh g-1.

[0516] As C-rate increased, specific capacity decreased across all three battery types, however the capacity retention for the QSSE batteries at 45 °C was 55% at 0.5 C, which is approximately 20% higher capacity retention than that of the LE batteries at 25 °C and approximately 30% higher than that of QSSE batteries at 25 °C.

[0517] Long Term Testing

[0518] The long term capacity retention for QSSE and comparative LE pouch cells was also tested at different temperatures, and the results are shown in Fig. 7D.

[0519] Pouch-type Li-S cells with LE and QSSE were tested at 25 °C, and QSSE was also tested at 45 °C. The specific capacity (mAh g-1) was measured at a C-rate of 0.1 C over chargedischarge cycles.

[0520] After 200 cycles, the QSSE cells (at 45 °C) retained approximately 75% capacity, whilst the QSSE cells (at 25 °C) retained 80.7% of their initial capacity. In contrast, the LE cells (at 25 °C) only retained approximately 60% of their initial capacity after 60 cycles and 30% of their initial capacity after 70 cycles.

[0521] Using a QSSE over a LE therefore provides a projected threefold increase in the lifetime of the Li-S battery. This is thought to be the highest reported capacity retention for pouch-type Li-S batteries.

[0522] Example 5 - Safety of QSSE batteries

[0523] A pouch-type Li-S cell comprising QSSE was submitted to mechanical damage by cutting the outer casing of the pouch cell. The damaged cell was then assembled into a circuit comprising LEDs (see Fig. 7D). The LEDs lit up, indicating that QSSE-based cells are safely operable even after mechanical damage to the battery.

[0524] The above exemplified improvements in working temperature range, cycle life, and safety demonstrate great promise of implementing QSSE in next-generation Li-S batteries.

[0525] References

[0526] A number of publications are cited above in order to more fully describe and disclose the invention and the state of the art to which the invention pertains. Full citations for these references are provided below. The entirety of each of these references is incorporated herein.

[0527] Bediako et al. Nature, 2018, Vol. 558, pp. 425^429.

[0528] Chhowalla et al., Nature Chemistry, 2013, Vol. 5, pp. 263-275

[0529] Chhowalla et al., Chem. Soc. Rev., 2015, Vol. 44, pp. 2702-2712

[0530] Knap et al., J. Electrochem. Soc. 2018, Vol. 165, A1601- A1609

[0531] Zhuangnan et al., Nature Energy, 2023, Vol. 8, pp. 84-93

[0532] WO 2023 / 041799

Claims

Claims1. A lithium sulfur cell comprising a working electrode, a counter electrode, and an electrolyte, wherein the working electrode comprises a transition metal dichalcogenide (TMD) of formula (I):LiaMX2(I) wherein a is from 0 to 2.0;X is selected from S, Se, and Te; andM is a transition metal; and wherein the electrolyte is a quasi-solid-state electrolyte (QSSE), the electrolyte comprising a monomer, a polymer formed or formable from the monomer, and a lithium salt.

2. The lithium sulfur cell of claim 1 , wherein the electrolyte comprises from 5 to 30 wt.% of the monomer, based on the total mass of the monomer and the polymer, preferably from 8 to 25 wt.%, more preferably from 10 to 20 wt.%, yet more preferably from 11 to 15 wt.%.

3. The lithium sulfur cell of claim 1 or claim 2, wherein the polymer has a number-average molecular weight (Mn) of from 1 ,500 to 25,000 g mol’1, preferably from 2,000 to10,000 g mol-1, more preferably from 3,000 to 5,000 g mol’1, yet more preferably 3,000 and 4,000 g mol’1.

4. The lithium sulfur cell of any one of claims 1 to 3, wherein the polymer has a degree of polymerization of from 20 and 340, wherein the degree of polymerization is the ratio of Mn / Mo, wherein Mnis the number-average molecular weight and Mo is the monomer unit molecular weight, preferably wherein the degree of polymerization is from 30 to 150, more preferably from 35 to 100, yet more preferably from 40 to 50.

5. The lithium sulfur cell of any one of claims 1 to 4, wherein the polymer is a polyether.

6. The lithium sulfur cell of any one of claims 1 to 5, wherein the electrolyte comprises a monomer of the Formula (A1):wherein -L- is -(CRzJni-, or -(CR2)n2-O-(CR2)n2-; n1 is from 2 to 6;n2 and n3 are each independently from 1 to 5; each R is independently H, or C1-6 alkyl, or two R groups, either connected to the same carbon atom or connected to different carbon atoms, taken together with the carbon atom(s) to which they are attached, form a C5-10 cycloalkyl or C5-10 heterocycloalkyl ring.

7. The lithium sulfur cell of claim 6, wherein:-L- is -(CR2)ni- and n1 is 2, 4 or 5, such as 2 or 4; or-L- is -(CR2)n2-O-(CR2)n3- and each of n2 and n3 is independently 2 or 4, such as 2.

8. The lithium sulfur cell of claim 6, wherein -L- is -(CH2)2-, -(CH2)4-, -(CH2)s-,preferably wherein -L- is -(CH2)2-.

9. The lithium sulfur cell of any one of claims 1 to 8, wherein the electrolyte further comprises an organic solvent, preferably an acyclic ether solvent or a cyclic ether solvent, such as 1 ,2-dimethoxyethane (DME).

10. The lithium sulfur cell of any one of claims 1 to 9, wherein the lithium salt is LiTFSI , (bis(trifluoromethane)sulfonimide lithium salt, LiPFe, LiBF4, LiCIO4, LiTF (lithium triflate), lithium difluoro(oxalato) borate (LiDFOB), lithium bis(oxalato) borate (LiBOB), or a combination thereof, preferably LiTFSI or LiCIO4, more preferably LiTFSI.

11. The lithium sulfur cell of any one of claims 1 to 10, wherein the TMD is a metallic phase transition metal dichalcogenide, preferably comprising stacked layers of a metallic phase transition metal dichalcogenide.

12. The lithium sulfur cell of any one of claims 1 to 11 , wherein a is from 0.1 to 2.0, preferably 0.5 to 1.0, more preferably from 0.6 to 0.8.

13. The lithium sulfur cell of any one of claims 1 to 12, wherein X is S.

14. The lithium sulfur cell of any one of claims 1 to 13, wherein the transition metal is selected from Ti, Zr, Hf, V, Nb, Ta, Mo, W, Tc, Re, Pd and Pt, preferably selected from V, Nb, Mo and W; more preferably selected from Mo and Nb.

15. The lithium sulfur cell of any one or claims 1 to 14, wherein the TMD is LiaMoS2, such as 1T-phase LiaMoS2.

16. The lithium sulfur cell of any one of claims 1 to 15, wherein the working electrode further comprises sulfur or a lithium (poly)sulfide.

17. The lithium sulfur cell of any one of claims 1 to 16, wherein the working electrode further comprises a Lewis acidity modifier, such as graphene or NbSe2.

18. A method for forming a polymer mixture, the method comprising: contacting a monomer and a lithium salt with a transition metal dichalcogenide (TMD), wherein the TMD is of formula (I):LiaMX2(I) wherein a is from 0 to 2.0;X is selected from S, Se, and Te; and M is a transition metal; and polymerising a portion of the monomer to give a polymer mixture comprising the monomer, the polymer formed from the monomer, and the lithium salt.

19. A method of preparing a lithium sulfur cell, the method comprising: forming a polymer mixture according to claim 18; and assembling the lithium sulfur cell from a working electrode, a counter electrode, and a QSSE comprising the polymer mixture, wherein the working electrode comprises a transition metal dichalcogenide (TMD) of formula (I).

20. The method of claim 18 or claim 19, wherein the TMD is a powder.

21. The method of claim 18 or claim 19, wherein the step of contacting a monomer, a lithium salt and a TMD comprises applying the monomer and the lithium salt to a working electrode for a lithium-sulfur cell, wherein in the working electrode comprises a TMD of formula (I).

22. The method of claim 21, wherein the polymerising step occurs on the surface of the working electrode, to provide an electrolyte on the surface on the working electrode, preferably wherein the polymerising step occurs in a lithium-sulfur cell assembly.

23. An electrolyte for a lithium sulfur cell, wherein the electrolyte is a quasi-solid-state electrolyte (QSSE), the electrolyte comprising a monomer, a polymer formed or formable from the monomer, and a lithium salt; and wherein the electrolyte comprises from 5 to 30 wt.% monomer, based on the total mass of the monomer and the polymer.

24. The electrolyte of claim 23, wherein the electrolyte further comprises a transition metal dichalcogenide (TMD) of Formula (I):LiaMX2(I) wherein a is from 0 to 2.0;X is selected from S, Se, and Te; and M is a transition metal; and / orwherein the electrolyte is substantially free of IJNO3.

25. A lithium sulfur battery comprising one or more lithium sulfur cells of any one of claims 1 to 17.

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