The invention relates to an electrochemical cell, methods of manufacture of the electrochemical cell, and an electrochemical cell assembly comprising at least one of the electrochemical cells. In particular, the invention relates to electrochemical cells including a cathode comprising a composite, wherein the composite comprises hollow core-shell particles.

In recent years there has been an increased demand for "green energy" due to the detrimental impact that fossil fuels have on the environment. One energy source that has received a great deal of interest is battery technology, in particular rechargeable batteries.

Of the rechargeable battery technologies, lithium-ion (Li-ion) battery technology dominates the commercial market, because of its high energy density compared to competing technologies, such as nickel-cadmium batteries (Ni-Cd). However, Li-ion batteries are expensive to produce, highly flammable, and typically require the use of cobalt and/or nickel in the production of the cathodes. Both cobalt and nickel are costly materials, and there are concerns over the security of the supply chain. In addition, cobalt can be toxic if not handled correctly, increasing the operational complexity of both the manufacture and end of life recycling processes.

In recent years, lithium-sulfur (Li-S) cells have received widespread attention because of their advantages over Li-ion batteries. For instance, they have a higher gravimetric energy (i.e., the measure of how much energy a battery contains in proportion to its weight, which is typically measured in 'watt-hours per kilogram (Wh/kg)', wherein a watt-hour is a measure of electrical energy that is equivalent to the consumption of one watt for one hour), a lower raw material cost, and are more environmentally friendly. Moreover, they do not require the use of nickel or cobalt in their manufacture. Furthermore, there are safety benefits associated with use of Li-S batteries over Li-ion batteries, as there is no longer a need for free metal ions in the materials. Instead, Li-S batteries proceed via a "conversion mechanism", whereby sulfur and lithium react to form polysulfides.

However, there is an important disadvantage surrounding the generation of polysulfides in conventional Li-S batteries. Polysulfides generated at the electrodes dissolve in a liquid electrolyte and undergo a "shuttling effect" between the anode and cathode, which results in an irreversible loss of sulfur. This can result in capacity loss and be detrimental to cyclability of the battery (i.e., the measure of times they can be recharged before they start to break down). One way that the effect of polysulfide shuffle has been limited is through the addition of certain additives in the electrolyte, such as additives that include N-0 bonds (e.g., lithium nitrate (LiNOs)). Without being bound by theory, the presence of additives like LiNOs in the electrolyte results in formation of a passivation layer (commonly known as a solid electrolyte interphase (SEI) layer) on the anode that mitigates the polysulfide shuffle effect. However, there are numerous disadvantages associated with the use of these additives. For instance, LiNOs forms nitrate gases above 40 degrees Celsius, which results in a narrowing of the operating and storage temperature window. Moreover, the presence of these additives results in cell swelling, due to formation of gases during cycling, not to mention safety implications. Therefore, there is a need to eradicate the need for such additives.

However, conventional Li-S batteries often use flammable liquids as the electrolyte, which has resulted in concern over their safety. As a result, there has been a great deal of interest in Li-S solid state batteries (SSBs), which use an inorganic solid-state electrolyte that do not dissolve polysulfides during battery cycling.

Whilst safety would be improved using a solid-state battery, the manufacture of solid- state batteries on a large scale is difficult, and there remains the issue of low sulfur utilisation and poor interfacial contact between the electrolyte and electrode in a solid- state battery, which may result in high impedance within the cell. Moreover, to retain as good an interfacial contact as possible between the solid-state electrolyte and cathode, a high amount of pressure is required.

An alternative way of tackling the polysulfide shuffle effect is shown in WO 2020/053604. Instead of using electrolyte salts such as LiNOs or solid-state electrolytes, there is described the combination of the use of a low porosity cathode having an electrochemically active sulfur component with a liquid electrolyte having no or a low polysulfide solubility (which ultimately reduces polysulfide shuffle).

WO 2021/250380 utilises a cathode comprising a carbon-sulfur composite having greater than 65 wt% sulfur based on the total weight of the composite material and an electroconductive carbon material having an average pore volume of 1.5 cm ³ g ^-1 and an average pore diameter of less than 3 nm. However, although the cell has a high gravimetric energy, it still suffers from low cyclability. Building upon this, WO 2021/074634 demonstrates that a combination of a highly concentrated electrolyte with a low porosity cathode comprising both an electrochemically active sulfur and an electronically conductive carbon material can result in high gravimetric energy and volumetric energy (i.e., the measure of the energy content of a battery in relation to its volume, which is typically measured in 'watt-hours per litre (Wh/L)') densities. Again, whilst this technology was shown to have high gravimetric energy, it still suffers from low cyclability. Without being bound by theory, this could be a result of the poor mechanical strength of the carbon-sulfur composite, which is not able to withstand the large volume of expansion of sulfur during delithiation and lithiation. As such, the interfacial contact between carbon and sulfur would be lost during cycling.

Notwithstanding the above-mentioned advances in the field, there remains a need for an electrochemical cell having not only a high gravimetric energy, volumetric energy density, and broad operating/storage temperature window, but also having improved cyclability.

The invention is intended to overcome or ameliorate at least some aspects of the above- mentioned problems.

Accordingly, in a first aspect of the invention there is provided an electrochemical cell comprising: an anode comprising an alkali metal, an alkali metal alloy, silicon, carbon, or a silicon-carbon composite material; a cathode comprising a composite, wherein the composite comprises (a) sulfur and (b) hollow core-shell particles comprising a carbonaceous material, a metallic material, a metalloid material, a polymer, or a combination thereof; and a liquid electrolyte, wherein the polysulfide solubility of the liquid electrolyte is less than 500 mM.

The electrochemical cell has a high gravimetric energy, a high volumetric energy, a broad operating and storage temperature range, a good interfacial stability between the cathode and the electrolyte (resulting in high sulfur utilisation), and a long cycle life. Moreover, unlike traditional solid-state batteries, use of pressure is not required to obtain good interfacial contact between the electrolyte and cathode, as a liquid electrolyte is used to fabricate the cell. Without being bound by theory, the electrochemical cell according to the first aspect of the invention is believed to operate via a solid-state mechanism, i.e., via the formation of solid (unsolvated) polysulfide species. In such solid-state mechanisms, cathodes according to conventional Li-S batteries and Li-S solid state batteries may have insufficient transport of lithium ions to the active sulfur species present in the cathode, and/or an insufficient sulfur/hollow core-shell particle interface to enable high sulfur utilisations via a solid-state mechanism. However, the combination of a cathode comprising a composite having (a) sulfur and (b) hollow core-shell particles comprising a carbonaceous material, a metallic material, a metalloid material, a polymer, or a combination thereof, with a liquid electrolyte with poor polysulfide solubility may mitigate this issue via the formation of solid polysulfide species that remain in the cathode.

The term "anode" takes its usual meaning in the art and relates to the negative electrode of a cell. The anode releases electrons to the circuit (and effectively oxidises) during the electrochemical reaction.

The term "cathode" takes its usual meaning in the art and relates to the positive electrode of a cell. The cathode acquires electrons from the circuit (and is effectively reduced) during the electrochemical reaction.

The term "electrolyte" takes its usual meaning in the art and relates to the medium that allows for ion transport between the anode and cathode. As used herein, with respect to all aspects of the invention, the term "electrolyte" and "liquid electrolyte" are interchangeable. The term "liquid electrolyte" is intended to include liquid electrolytes and electrolytes where the liquid is in a gel matrix (also referred to as a "gel electrolyte").

The term "composite" takes its usual meaning in the art and relates to materials made from two or more constituent materials having different chemical or physical properties from one another. When the two or more constituent materials are combined, the resultant composite material has properties different to the individual components present. The composite can be manufactured by known techniques, such as melt diffusion, a solutionbased process, or mechanical grinding. Thermal annealing could be performed as an optional step following the melt diffusion step.

Optionally, the composite comprises at least 70 wt% sulfur based on the total weight of the cathode, at least 80 wt%, at least 90 wt% or at least 90 wt%, such that the upper limit would be 100 wt%, 99.9 wt% or possibly 99 wt%.

Optionally, the hollow core-shell particles comprising a carbonaceous material, a metallic material, a metalloid material, a polymer, or a combination thereof, may be present in the composite material at a level of at least 10 wt%, at least 20 wt%, or at least 30 wt%.

As used herein, the term "hollow core-shell particle" takes its usual meaning in the art and refers to particles comprising a shell surrounding a hollow core, which is effectively a single pore. The pore may be a void, or may contain an active or inactive material, such as sulfur, which may pass into and out of the pore during cycling or manufacture. Following composite formation, sulfur is generally housed within the hollow core-shell particles. As such, following composite formation, the hollow core-shell particles may be at least partially filled with sulfur, on occasion largely or completely filled; however, it is advantageous if the particles are only partially filled (often in the range 5 - 80 vol%, or 10 - 50 vol%), as this reduces the mechanical stress undergone by the shell structure as sulfur expands or contracts during cycling, and enables extended cycle life. It may be the case that the sulfur is at least partly covering the shell of the particle, optionally covering substantially all of the shell of the hollow core-shell particle, such that the sulfur permeates from the shell of the core-shell particle into the hollow core. As used herein, covering "substantially" all of the shell of the hollow core-shell particle is intended to mean that at least 70% of the shell is covered, often at least 80%, often at least 90%, such that the upper limit would be 100%, 99.9% or possibly 99%.

It may be the case that the hollow core-shell particles are in the form of one or more agglomerates. As used herein, the term "agglomerate" refers to a collection of hollow coreshell particles that form a mass or cluster. Where the hollow core-shell particles are in the form of one or more agglomerates, following composite formation, sulfur is generally housed within the hollow core-shell particles within the agglomerate. The hollow core-shell particles may be at least partially filled with sulfur, on occasion largely or completely filled. It may be the case that the sulfur is at least partly covering the shell of one or more particles present within the agglomerate, optionally covering substantially all of the shell of one or more of the hollow core-shell particles within the agglomerate, such that the sulfur permeates from the shell of the one or more hollow core-shell particles into the hollow core of said one or more hollow core-shell particles. As used herein, covering "substantially" all of the shell of the hollow core-shell particle is intended to mean that at least 70% of the shell is covered, often at least 80%, often at least 90%, such that the upper limit would be 100%, 99.9% or possibly 99%.

Incorporation of hollow core-shell particles comprising a carbonaceous material, a metallic material, a metalloid material, a polymer, or a combination thereof, allows for a mechanically stable host structure for the delithiation and lithiation of sulfur (and subsequent volume expansion). Moreover, without being bound by theory, particles having a hollow core-shell structure are generally electronically conductive, and when combined with sulfur (for instance using melt diffusion techniques), sulfur can be housed within the core of the particles and remain there during cycling. As such, sulfur utilisation is high, leading to high charge/discharge capacities. Optionally, the hollow core-shell particles have an average particle diameter in the range of from 0.1 nm to 40 nm, often in the range of from 0.5 nm to 35 nm, often in the range of from 1 nm to 30 nm, often in the range of from 4 nm to 20 nm, often in the range of from 3 nm to 15 nm. Average particle diameters in this range have high interfacial contact between sulfur and the core-shell particles, which allows for electrochemical cells having high sulfur utilisation. Particle size analysis can be determined using any known technique, such as dynamic image analysis (DIA), static laser light scattering, dynamic light scattering (DLS), sieve analysis, or by visual analysis of Transmission Electron Microscopy (TEM) or Scanning Electron Microscopy (SEM) images. In instances where the hollow core-shell particles are in the form of one or more agglomerates, it may be the case that the one or more agglomerates have an average diameter in the range of from 10 nm to 100 nm, often in the range of from 40 nm to 80 nm. Without being bound by theory, an agglomerate may arise from the fusion of two or more hollow core-shell particles via their external shells to create a cluster or mass of particulate matter comprising multiple voids (i.e., the hollow cores of each hollow core-shell particle within the agglomerate). Size can be determined using any known technique, such as dynamic image analysis (DIA), static laser light scattering, dynamic light scattering (DLS), sieve analysis, or by visual analysis of Transmission Electron Microscopy (TEM) or Scanning Electron Microscopy (SEM) images.

The hollow core-shell particles may have a wide range of geometries including, but not limited to, spherical, cubic, prismatic, pentagonal, hexagonal, heptagonal, or octagonal. The geometries will often be mathematically imperfect, such that they may be "substantially" of a given geometry. As used herein, with regard to these geometries, the term "substantially" can be taken to mean clearly recognisable as a given geometry (for instance spherical), but not mathematically of that geometry (e.g., not a perfect sphere). This could include elongation along one axis by, perhaps, ±20% or ±10%, maybe in the range 20% or 10% to 1%; the presence of surface roughness be that protrusions or recesses in the surface of, perhaps, ±10% or less (maybe 10% - 1%) of the thickness of the shell.

Often, the hollow core-shell particles are substantially spherical. Spherical hollow coreshell particles have a high surface area, which results in a high sulfur loading and therefore high sulfur utilisation.

The hollow core-shell particles will generally form a complete shell surface. However, in some instances, it may be the case that at least a proportion of the particles are partially formed, such that there are holes present in the form of a missing face or faces (or part faces) of the surface of the shell. Often, at least 75% of the hollow core-shell particles will form a complete shell surface, often at least 80%, often at least 85%, often at least 90%, such that the upper limit would be 100%, 99.9% or possibly 99%. When at least 75% of the hollow core-shell particles form a complete shell surface, the particles are more robust to cycling and more resistant to degradation during battery operation. It may be the case that, when the hollow core-shell particles are in the form of agglomerates, that at least 75% of the hollow core-shell particles within the agglomerate will form a complete shell surface, often at least 80%, often at least 85%, often at least 90%, such that the upper limit would be 100%, 99.9% or possibly 99%.

Optionally, the hollow core-shell particles have a unimodal, bimodal, or multimodal particle size distribution. Often, the hollow core-shell particles have a unimodal particle size distribution.

Optionally, the hollow core-shell particles have a shell thickness in the range of from 0.01 and 50 nm, often from 0.1 nm to 40 nm, often from 0.5 to 30 nm. This range allows for optimum structural rigidity and elasticity of the particles, such that they can better withstand cell swelling.

The shell of the hollow core-shell particles may be porous, or partially porous, in addition to or instead of holes being present as described above. As used herein the term "porous" is to be given it's common meaning in the art and refers to the presence of one or more pores, often many pores, in the shell. Each pore will typically be of cross-section in the range of from 0.1 to 3.0 nm, often in the range of from 0.3 to 2.0 nm.

It may be the case that the shell the hollow core-shell particles comprises two or more layers. Provision of hollow core-shell particles wherein the shell comprises two or more layers will improve cyclability, as a layered shell can better withstand expansion of sulfur during cycling. In instances where the shell of the hollow core-shell particles comprises two or more layers, following composite formation, sulfur may be housed within the hollow core-shell particles. The hollow core-shell particles may be at least partially filled with sulfur, on occasion largely or completely filled. It may be the case that the sulfur is at least partly covering the shell of the particle, optionally covering substantially all of the shell of the hollow core-shell particle, such that the sulfur permeates from the shell of the coreshell particle into the hollow core. As used herein, covering "substantially" all of the shell of the hollow core-shell particle is intended to mean that at least 70% of the shell is covered, often at least 80%, often at least 90%, such that the upper limit would be 100%, 99.9% or possibly 99%. It may be the case that sulfur is present between the two or more layers present in the shell of the hollow core-shell particle. It may be the case that the hollow core shell particles are in the form of one or more agglomerates, and the shell of one or more hollow core-shell particles within the agglomerate comprises two or more layers. It may be the case that the shell of each hollow core-shell particle within the agglomerate comprises two or more layers.

The hollow core-shell particles comprise a carbonaceous material, a metallic material, a metalloid material, a polymer, or a combination thereof. As such, all four components may be present, or three, or two, or just any one of the components.

Optionally, the polymer comprises polyacrylonitrile; cellulose; a polyether (an example includes, but is not limited to, polyethylene glycol (PEG)); polyvinylpyrrolidone (PVP); poly(3,4-ethylenedioxythiophene) (PEDOT); poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT: PSS); polythiophene (PTh); polydopamine (PDA); polyaniline (PANI); triallyl isocyanurate polymer; polypyrrole (PPY); an ionomer (examples include, but are not limited to, a sulfonated tetrafluoroethylene-based fluoropolymercopolymer (Nation®), and copolymers of ethylene and acrylic and/or methacrylic acid); an ethylene oxide (EO) based polymer (an example includes, but is not limited to, PEO); an acrylate based polymer (an example includes, but is not limited to, PMMA); a polyamine (an example includes, but is not limited to, polyethyleneimine); a siloxane (an example includes, but is not limited to, poly(dimethylsiloxane)); a polyheteroaromatic compound (an example includes, but is not limited to, polybenzimidazole); a polyamide (examples include, but are not limited to, Nylons); a polyimide (an example includes, but is not limited to, Kapton®); a polyvinyl polymer (examples include, but are not limited to, poly(2- vinyl pyridine), poly(vinyl acetate), poly (vinyl alcohol), poly(vinyl chloride), and poly(vinyl fluoride)); a polycyanoacrylate (an example includes, but is not limited to, poly(methylcyanoacrylate)); an inorganic polymer (examples include, but are not limited to, polysilane, polysilazane, polyphosphazene, polyphosphonate); a polyurethane; a polyolefin (examples include, but are not limited to, polyacrylamide, polypropylene, polytetrafluoroethylene); a polyester (examples include, but are not limited to, polycarbonate and polybutylene terephthalate); or a combination thereof.

As used herein, the term "ionomer" is intended to take its usual meaning in the art, and refers to synthetic polyelectrolytes, which consist of both electrically neutral and ionized groups along the polymer backbone. The electrically neutral and ionized groups can be regularly distributed or randomly distributed. The term "inorganic polymer" is intended to take its usual meaning in the art, and refers to polymers with an inorganic backbone, which is composed of atoms other than carbon. As used herein, the term "polyvinyl polymer" is intended to take its usual meaning in the art and refers to polymers derived by polymerization from compounds containing the vinyl group.

It may be the case that the polymer comprises polyacrylonitrile, cellulose, polyethylene glycol (PEG), polyvinylpyrrolidone (PVP), poly(3,4-ethylenedioxythiophene) (PEDOT), poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT: PSS), polythiophene (PTh), polydopamine (PDA), nylon, polyaniline, triallyl isocyanurate, polypyrrole, or a combination thereof.

Optionally, the metallic material comprises a metal, a metal oxide, a metal hydroxide, a metal sulfide, or a combination thereof.

As used herein, the term "metal" is intended to include all metal elements, such as transition and post-transition metals, alkali metals, alkaline earth metals, lanthanides, and actinides. Examples of metals include, but are not limited to nickel, manganese, platinum, silver, gallium, copper, palladium, or a combination thereof. Examples of metal oxides include, but are not limited to, zinc oxide, aluminium oxide, titanium oxide, vanadium oxide, ruthenium oxide, nickel oxide, manganese oxide, or a combination thereof. Often, the metal oxide comprises manganese oxide. Examples of metal hydroxides include, but are not limited to, cobalt (II) hydroxide, nickel (II) hydroxide, or a combination thereof. An example of a metal sulfide includes, but is not limited to, molybdenum sulfide. Metallic materials allow for hollow-core shell particles that strike a balance between high structural rigidity and electron conductivity.

Optionally, the metalloid material comprises a metalloid, a metalloid oxide, a metalloid sulfide, or a combination thereof. Examples of metalloids include, but are not limited to silicon, boron, germanium, antimony, tellurium, or a combination thereof. Examples of metalloid oxides include silicon oxide, germanium oxide, boron oxide or a combination thereof. Examples of metalloid sulfides include, but are not limited to, boron sulfide, germanium sulfide, antinomy sulfide, tellurium sulfide, or a combination thereof.

Optionally, the cathode comprises hollow core-shell particles comprising a carbonaceous material. Often, the carbonaceous material comprises carbon black (e.g., PBX® 51 Sterling® V (SV), Vulcan® 3 (V3), Vulcan® 6 (V6), Black Pearls® 880 (BP880), Black Pearls® 1300 (BP1300), and Black Pearls® 2000 (BP2000)); and activated carbon (e.g., UNICARB® activated carbons), activated carbon (e.g., UNICARB® activated carbons), graphene, or combinations thereof. As used herein, the term "graphene" is intended to include two- dimensional graphene and three-dimensional graphene. Often, the graphene will be three- dimensional. Carbonaceous hollow core-shell particles have a very high electron conductivity, which results in high charge/discharge capacities. Optionally, the carbon material comprises carbon black in the form of hollow core-shell particles. It may be the case that the carbon material comprises Black Pearls® 2000, PBX® 51, or combinations thereof. It may be the case that the cathode comprises hollow core-shell particles comprising a carbonaceous material, wherein the shell of the hollow core-shell particle comprises two or more layers.

It may be the case that the carbonaceous material is doped with a heteroatom. Examples of dopants include, but are not limited to, nitrogen, boron, oxygen, sulfur, or phosphorous. Often, the carbonaceous material is doped with boron, often the carbonaceous material is boron-doped graphene. Doping the carbon network results in an increase in the electronic conductivity of the carbonaceous material and improves the mechanical robustness of the resultant hollow core-shell particles.

Optionally, the hollow core-shell particles comprise a carbonaceous material, a metallic material, a metalloid material, or a combination thereof. Optionally, the carbonaceous material, metallic material, metalloid material, or the combination thereof, is functionalised. Examples of functional groups include, but are not limited to, hydroxyl groups (-OH), carboxylic acid groups (-COOH), amine groups, amide groups, nitrogen. Functionalisation of carbonaceous hollow core-shell particles, metallic hollow core-shell particles and metalloid hollow core-shell particles can be achieved using any known technique. Functionalisation of the carbonaceous hollow core shell particles improves adhesion and structural stability, increases the lithium/sodium conductivity, and improves retention of polysulfides. In addition, functionalisation of carbonaceous hollow core-shell particles can help with dispersion of said particles within a cathode slurry, improving homogeneity of said slurry. Moreover, functionalisation increases the hydrophilicity of the composite material, which facilitates the manufacture of the cathode, as it results in preparation of highly stable water-based slurries. Hydroxyl group and carboxylic acid group functionalisation are particularly effective in improving dispersion of the hollow coreshell particles in water.

Optionally, the cathode further comprises an electronically conductive current collector. The current collector may comprise aluminium, copper, titanium, platinum, zinc, or stainless steel. Often, the current collector is aluminium foil. Aluminium foil can form a passive film, which results in a stable electrolyte/aluminium interface. Aluminium foil is also light weight, low cost, and provides strong adhesion to the cathode. Moreover, aluminium foil has good electronic conductivity. It may be the case that the current collector is coated with a protective layer. Typically, the protective layer is a carbon-based layer.

Optionally, the cathode further comprises a binder. Without being bound by theory, the binder may act to bind the cathode components together. Additionally, or alternatively, the binder may also help bind the cathode components to the current collector. In doing so, the binder can provide a cathode with enhanced mechanical robustness and can improve the processability of the cathode.

The binder may be guar gum, xanthan gum, gum arabic, a polymeric binder, or a combination thereof. Optionally, the binder may be guar gum, xanthan gum, or a combination thereof.

The binder may be a polymeric binder, for example, a polyether such as poly(ethylene oxide)s, polyethylene glycols, polypropylene glycols, polytetramethylene glycols (PTMGs), polytetramethylene ether glycols (PTMEGs), or mixtures thereof.

The binder may be selected from halogenated polymers, for instance, the binder may be selected from a fluorinated polymer. Examples of suitable binders include, but are not limited to, poly(vinylidene fluoride) (PVDF), often in the a form poly(trifluoroethylene) (PVF3); polytetrafluoroethylene (PTFE); copolymers of vinylidene fluoride with either hexafluoropropylene (HFP) or trifluoroethylene (VF3) or tetrafluoroethylene (TFE) or chlorotrifluoroethylene (CTFE); fluoroethylene/propylene (FEP) copolymers; copolymers of ethylene with either fluoroethylene/propylene (FEP) or tetrafluoroethylene (TFE) or chlorotrifluoroethylene (CTFE); perfluoropropyl vinyl ether (PPVE); perfluoroethyl vinyl ether (PEVE); and copolymers of ethylene with perfluoromethyl vinyl ether (PMVE); or blends or mixtures thereof.

Other examples of suitable binders include polyacrylonitrile, polyurethane, PVDF-acrylic co-polymer; polyacrylic acid, polyimides and polyvinyl alcohol. Further suitable binders include rubber (e.g., styrene butadiene rubber), cellulose-based binders (e.g., carboxymethyl cellulose), or gelatine.

Optionally, the binder is selected from polyethylene oxide (PEO), polyvinylidene fluoride (PVDF), poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP), a sulfonated tetrafluoroethylene-based fluoropolymer-copolymer (Nation®), polypyrole, polythiophene, polyaniline, polyvinyl alcohol, poly(ethylene) imine, polyacetylene, polyphenylene vinylene, poly(3,4-ethylenedioxythiophene), polyphenylene sulfide, gelatine, or a combination thereof.

Optionally, the cathode may comprise 0.05 to 20 wt% binder based on the total weight of the cathode, often 0.5 to 10 wt%, for example 1 to 5 wt% based on the total weight of the cathode, for example 2 to 3 wt%.

Optionally, the cathode further comprises an ionically conductive material. The ionically conductive material may have a bulk ionic conductivity of greater than 10' ⁷ S/cm at 25 °C, for example greater than 10' ⁶ S/cm. Where the cathode contains an electroactive, ionically conductive material such as IJ3PS4, or LixPySz, a further ionically conductive material may be absent.

Optionally, the ionically conductive material is selected from a conducting ceramic material, an ionically conducting polymer, or a combination thereof.

The ceramic material may have a crystalline, polycrystalline, partially crystalline, or amorphous structure. Suitable ceramic materials include, but are not limited to, oxides, carbonates, nitrides, carbides, sulfides, oxysulfides, and/or oxynitrides of metals and/or metalloids. Where the anode comprises lithium or a lithium alloy, the ceramic material generally comprises lithium; similarly, where the anode comprises sodium or a sodium alloy, the ceramic material generally comprises sodium. Non-limiting examples of suitable solid-state electrolytes of sufficient ionic conductivity for use in lithium-based systems may be produced by a combination of various lithium compounds, such as ceramic materials including lithium include lithium oxides (LizO, KiO, UO2, LiRC , where R is scandium, yttrium, lanthanum, cerium, praseodymium, neodymium, promethium, samarium, europium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, ytterbium, and/or lutetium), lithium carbonate (IJ2CO3), lithium nitrides (e.g., LisN), lithium oxysulfide, lithium oxynitride, lithium garnet-type oxides (e.g., Li?La3Zr20i2)), LiwGeP2Si2, lithium phosphorus oxynitride, lithium silicosulfide, lithium germanosulfide, lithium lanthanum oxides, lithium titanium oxides, lithium borosulfide, lithium aluminosulfide, lithium phosphosulfide, lithium silicate, lithium borate, lithium aluminate, lithium phosphate, lithium halides, and combinations of the above. In certain cases, the ceramic material comprises a lithium oxide, a lithium nitride, or a lithium oxysulfide. In some embodiments, the ceramic includes a carbonate and/or a carbide.

Examples of ceramic materials that can be used as a lithium-ion containing conductive material include: Li-containing oxides such as Lis.sLao.seTiCh; Nasicon structure such as LiTi(PO4)3; LiSICON (Lii4Zn(GeO4)4); LiwGePzSiz; Garnet: Li LasZrzOiz; U2O; other oxides such as AI2O3, TiO2, ZrC , SiC , ZnO; sulfides such as US-P2S5; antiperovskites such as IJ3OCI; hydrides such as LiBH4, LiBH4-LiX (where X = Cl, Br, or I), LiNH, UNH2, LiAIHe, IJ2NH; borates or phosphates such as IJ2B4O7, IJ3PO4, LiPON; carbonates or hydroxides such as U2CO3, LiOH; fluorides such as LiF; nitrides such as U3N; sulfides such as lithium borosulfides; lithium phosphosulfides, lithium aluminosulfides, oxysulfides, praseodymium oxide. At least one of said ceramic materials may be used, or a combination thereof. As noted above, where the anode comprises sodium metal or a sodium alloy, the sodium ion equivalent of any of these conductive materials may be utilised.

In some examples, the ionically conductive material may be formed of a polymeric material which is inherently ionically conductive, such as a sulfonated tetrafluoroethylene-based fluoropolymer-copolymer (Nation®). Alternatively, polymers blended with lithium (or sodium) salts, which can achieve bulk conductivities of greater than 10' ⁷ S/cm, may also be used. Examples of suitable polymers include, but are not limited to, ethylene oxide (EO) based polymers (for example PEO); acrylate based polymer (for example PMMA); polyamines (polyethyleneimine); siloxanes (poly(dimethylsiloxane)); polyheteroaromatic compounds (e.g., polybenzimidazole); polyamides (e.g. Nylons); polyimides (e.g. Kapton®); polyvinyls (e.g. polyacrylamide, poly(2-vinyl pyridine), poly(N- vinylpyrrolidone), poly(methylcyanoacrylate), poly(vinyl acetate), poly(vinyl alcohol), poly(vinyl chloride), poly(vinyl fluoride)); inorganic polymers (e.g. polysilane, polysilazane. polyphosphazene, polyphosphonate); polyurethanes; polyolefins (e.g., polypropylene, polytetrafluoroethylene); polyesters (e.g., polycarbonate, polybutylene terephthalate); and combinations thereof. Optionally, co-block polymers such as a sulfonated tetrafluoroethylene-based fluoropolymer-copolymer (Nation®) may be used. At least one of said polymeric materials may be used, or a combination thereof. It may be the case that the cathode contains ceramic particles in combination with one or more ionically conductive polymers.

Optionally, the conducting ceramic material is selected from at least one of lithium lanthanum zirconium oxide (Li?La3Zr20i2) (LLZO), lithium aluminium titanium phosphate (Lii. ₃ Alo.3Tii.7(P04)3) (LATP), lithium germanium phosphorus sulfide (LiioGeP2Si2) (LGPS), or lithium sulfide-phosphorous pentasulfide (U2S-P2S5); and the ionically conductive polymer is selected from at least one of polypyrole, polythiophene, polyaniline, polyacetylene, polyphenylene vinylene and poly(3,4-ethylenedioxythiophene), or a combination thereof. Optionally, the cathode contains from 1 to 60 % by weight ionically conductive material based on the total weight of the cathode.

Optionally, the cathode further comprises an electronically conductive carbon material. Often, the electronically conductive carbon material is selected from carbon nanotubes, carbon nanofibers, graphene, graphene oxide, graphite, carbon black, activated carbon (e.g., Maxorb III®), or a combination thereof. The electronically conductive carbon material may be present in the range of from 0.1 to 30 wt%, optionally 1 to 20 wt%, optionally 5 to 15 wt% of the total weight of the cathode. Inclusion of an electronically conductive carbon material results in an increase of the electronic conductivity within the cathode. In addition, inclusion of an electronically conductive carbon material adds a degree of porosity to the cathode, allowing for a better electrolyte penetration, which contributes to shortening the lithium-ion migration path within the cathode. As a result, power performance of the cell is enhanced.

Optionally, the cathode is pre-lithiated or pre-sodiated. The terms "pre-l ith iated" and "pre- sodiated" take their usual meaning and relate to a pretreatment step where lithium ions or sodium ions are added to the cell before operation. Pre-lithiation or pre-sodiation of the cathode results in an increase in energy density of the cell.

Optionally, the cathode further comprises selenium, thereby resulting in the formation of sulfoselenide within the composite material. Addition of selenium improves the electronic conductivity of the cathode, resulting in higher volumetric capacity of the cell.

Optionally, the hollow core-shell particles comprise an ionically conducting coating. The ionically conducting coating may comprise a ceramic material, a polymeric material, or a combination thereof. Use of an ionically conducting coating helps contain the polysulfides within the cathode. The provision of an ionically conducting coating on the hollow coreshell particles is particularly effective at inhibition of polysulfide dissolution, prolonging cyclability.

Optionally, the coating has a low area specific lithium and/or sodium ionic resistance. Typically, the area specific lithium and/or sodium ionic resistance is less than 20 Q/cm ² , more typically less than 5 Q/cm ² . As used herein, the unit "Q/cm ² " relates to the area of the coating in contact with the electrolyte.

The coating may be applied to the hollow core-shell particles using conventional coating techniques. For instance, the coating can be applied via chemical vapour deposition (CVD), plasma-enhanced CVD, sol-gel techniques, hydrothermal or solvothermal precipitation, molecular layer deposition (MLD), or atomic layer deposition (ALD). Where the coating comprises a ceramic material, it is typically the case that the coating is applied via ALD. Where the coating comprises a ceramic-polymer composite, it is typically the case that the coating is applied via MLD.

The ceramic material may have a crystalline, polycrystalline, partially crystalline, or amorphous structure. Suitable ceramic materials include, but are not limited to, oxides, carbonates, nitrides, carbides, silicides, sulfides, oxysulfides, and/or oxynitrides of metals and/or metalloids. Examples of ceramic materials that can be used include, but are not limited to, oxides such as titanium oxide, aluminium oxide, zinc oxide, silicon oxide, boron oxide, vanadium oxide, zirconium oxide, magnesium oxide, or a combination thereof; nitrides such as aluminium nitride, boron nitride, silicon nitride, or a combination thereof; carbides such as tungsten carbide (WC), chromium carbide (O3C2), titanium carbide (TiC), tantalum carbide (TaC), silicon carbide (e.g. sintered silicon carbide (SSiC), liquid phase sintered silicon carbide (LPS-SiC), reaction bonded Silicon Carbide (RBSiC), nitride bonded silicon carbide (NSiC), or recrystalised silicon carbide), or a combination thereof; hydrides such as LiBH ₄ , LiBH ₄ -LiX (where X = Cl, Br, or I), LiNH, LiNH ₂ , LiAIHe, Li ₂ NH, or a combination thereof; or any combination thereof. Often, the coating comprises a ceramic oxide selected from titanium oxide, aluminium oxide, zinc oxide, silicon oxide, boron oxide, vanadium oxide, zirconium oxide, magnesium oxide, or a combination thereof, more often the coating comprises aluminium oxide.

Examples of the polymeric material include, but are not limited to, a poly(p-phenylene vinylene), a poly(acetylene), a polyphenylene, a polyphenylene sulfide, a polyaniline, a polythiophene, a polycarbazole, a polyfluorene, a polyazulene, a polypyrene, a poly(3,4- ethylenedioxythiophene), polystyrene sulfonate (PEDOT:PSS), a polyindole, a polypyrene, a polynaphthalene, polyethylene oxide; or a combination thereof. Often, the polymeric material comprises polyethylene oxide.

It may be the case that the coating comprises a ceramic-polymer composite material. Examples of ceramic-polymer composite materials include, but are not limited to, metalcones (e.g., alucone, zincone, zircone, titacone, or a combination thereof).

Optionally, the coating has a thickness in the range of from 1 nm to 20 nm, often less than 15 nm, more often less than 10 nm (such as 1 nm to 10 nm or 15 nm). A coating in this range allows for fast lithium or sodium ion diffusion, as well as allowing conduction of electrons.

Optionally, the cathode has a thickness in the range of from 20 pm to 300 pm. Often in the range of 50 pm to 200 pm, often in the range of from 75 pm to 150 pm. The cathode may be a single or double-sided cathode, although dimensions are quoted without the current collector.

As noted above, the anode comprises an alkali metal, alkali metal alloy, silicon, carbon, or a silicon-carbon composite material.

Optionally, the alkali metal or alkali metal alloy comprises lithium and/or sodium. It may be the case that the anode comprises a foil formed of lithium metal or lithium metal alloy. Examples of lithium alloys include, but are not limited to, lithium indium alloy, lithium aluminium alloy, lithium magnesium alloy and lithium boron alloy. It may be the case that the anode comprises a foil formed of sodium metal or sodium metal alloy. Examples of sodium alloys include, but are not limited to, sodium indium alloy, sodium aluminium alloy, sodium magnesium alloy and sodium boron alloy. Often the anode is a lithium metal foil or a sodium metal foil because of their high specific capacity.

Alternatively, the anode may comprise silicon. Where the anode comprises silicon, this may be lithiated or sodiated. As used herein, the term "lithiated" takes its usual meaning in the art and refers to the combination or impregnation with lithium or a lithium compound. Similarly, the term "sodiated" takes its usual meaning in the art and refers to the combination or impregnation with sodium or a sodium compound.

Alternatively, the anode may comprise carbon, for instance as carbon nanotubes, carbon nanofibers, graphene, graphene oxide, graphite, carbon black, or a combination thereof.

It may be the case that the anode comprises a silicon-carbon composite material. Examples of silicon-carbon composites include, but are not limited to, Silicon-doped graphite.

The electrolyte according to the first aspect of the invention has a polysulfide solubility at room temperature (approximately 20 °C) of less than 500 mM. For example, the liquid electrolyte may have a polysulfide solubility less than 400 mM, optionally less than 200 mM, optionally less than 150 mM, optionally less than 100 mM, optionally less than 10 mM, or optionally less than 1 mM. In some cases, the electrolyte may not dissolve polysulfides. For example, the electrolyte may have a polysulfide solubility in the range of from 0.001 mM to 500 mM, often 0.01 to 400 mM, often 0.1 mM to 200 mM, more often 1 mM to 10 mM. Correspondingly, the electrolyte may have a low solubility for sulfur- containing species (such as polysulfides and sulfur) in general.

The use of an electrolyte having poor or no solubility of polysulfides can prevent polysulfide shuttle within an electrolyte and is therefore beneficial in cells such as lithium-sulfur cells. As noted above, the polysulfide shuttle effect is an undesirable reaction, as it results in loss of coulombic efficiency and can impact cyclability.

It is typically the case that the electrolyte comprises a suitable solvent system, liquid or gel, or mixture of liquids and/or gels; and an alkali metal salt.

Suitable organic solvents for use in the electrolyte are ethers (e.g. linear ethers, diethyl ether (DEE), diglyme (2-methoxyethyl ether), tetraglyme, diethylene glycol diethyl ether, tetra hydrofuran, 2-methyltetrahydrofuran, di methoxyethane (DME), dioxolane (DIOX)); carbonates (e.g. dimethylcarbonate, diethylcarbonate, ethylmethylcarbonate, methylpropylcarbonate, ethylene carbonate (EC), propylene carbonate (PC)); sulfones (e.g. dimethyl sulfone (DMS), ethyl methyl sulfone (EMS), tetramethyl sulfone (TMS)); esters (e.g. methyl formate, ethyl formate, methyl propionate, methylpropylpropionate, ethylpropylpropionate, ethyl acetate and methyl butyrate); ketones (e.g. methyl ethyl ketone); nitriles (e.g. acetonitrile, proprionitrile, isobutyronitrile); amides (e.g. dimethylformamide, dimethylacetamide, hexamethyl phosphoamide, N, N, N, N-tetraethyl sulfamide); lactams/lactones (e.g. N- methyl-2-pyrrolidone, butyrolactone); ureas (e.g. tetra methyl urea); sulfoxides (e.g. dimethyl sulfoxide); phosphates (e.g. trimethyl phosphate, triethyl phosphate, tributyl phosphate); phosphoramides (e.g. hexamethylphosphoramide); or a combination thereof. Further suitable solvents include toluene, benzene, heptane, xylene, dichloromethane, and pyridine. Often the organic solvent will comprise an ether, for instance DEE, diglyme, tetraglyme, diethylene glycol diethyl ether, tetra hydrofuran, 2-methyltetrahydrofuran, DME, DIOX or combinations thereof. Often the organic solvent will comprise diethylene glycol diethyl ether alone or in combination with another ether, or another solvent from the list above.

It may be the case that the ethers, carbonates, sulfones, esters, ketones, nitriles, amides, lactams, ureas, phosphates, phosphoramides are fluorinated.

Optionally, the liquid electrolyte comprises a mixture of fluorinated and non-fluorinated solvents. For example, the organic solvent may comprise a mixture of one or more ethers and fluorinated ethers, or one or more carbonates and fluorinated carbonates. Examples of fluorinated ethers include l,l,2,2,-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether (TTE), hydrofluorinated ether (HFE) and 2,2,2-trifluoroethyl methyl ether ethylene glycol (TFEG). An example of a fluorinated carbonate is monofluoroethylene carbonate. Often the organic solvent will comprise a mixture of one or more ethers and fluorinated ethers, for instance, a combination of TTE and diethylene glycol diethyl ether. Where present in combination, the fluorinated and non-fluorinated solvents may be present in the range 6: 1 - 1:6 non-fluorinated :fluorinated solvent v/v. It may be the case that the fluorinated and non-fluorinated solvents are present in equal amounts. It may be the case that there is more fluorinated solvent present than non-fluorinated solvent, such that the ratio may be in the range 1: 1 - 1 :6 non-fluorinated :fluorinated solvent v/v.

The liquid electrolyte may comprise one or more ionic liquids as a solvent. Said ionic liquids may comprise salts comprising organic cations such as imidazolium, ammonium, pyrrolidinium, and/or organic anions such as bis(trifluoromethanesulfonyl)imide (TFST), bis(fluorosulfonyl)imide (FST), triflate, tetrafluoroborate (BF4'), dicyanamide (DCA ^_ ), and/or chloride (Ch)- The ionic liquid is liquid at room temperature (20 °C). Examples of suitable ionic liquids include (N,N-diethyl-N-methyl-N(2-methoxyethyl)ammonium bis(trifluoromethanesulfonyl), N,N-diethyl-N-methyl-N-propylammonium bis(fluorosulfonyl)imide, N,N-diethyl-N-methyl-N-propylammonium bis(fluorosulfonyl)imide, N,N-dimethyl-N-ethyl-N-(3-methoxypropyl)ammonium bis(fluorosulfonyl)imide, N,N-dimethyl-N-ethyl-N-(3-methoxypropyl)ammonium bis(trifluoromethanesulfonyl)imide, N,N-dimethyl-N-ethyl-N-benzylammonium bis(trifluoromethanesulfonyl)imide, N,N-dimethyl-N-Ethyl-N-phenylethylammonium bis(trifluoromethanesulfonyl)imide, N-Ethyl-N,N-dimethyl-N-(2-methoxyethyl)ammonium bis(fluorosulfonyl)imide, N-Ethyl-N,N-dimethyl-N-(2-methoxyethyl)ammonium bis(trifluoromethanesulfonyl)imide, N-tributyl-N-methylammonium bis(trifluoromethanesulfonyl)imide, N-tributyl-N-methylammonium dicyanamide, N- tributyl-N-methylammonium iodide, N-trimethyl-N-butylammonium bis(trifluoromethanesulfonyl)imide, N-trimethyl-N-butylammonium bromide, N-trimethyl- N-hexylammonium bis(trifluoromethanesulfonyl)imide, N-trimethyl-N-propylammonium bis(fluorosulfonyl)imide, N-trimethyl-N-propylammonium bis(trifluoromethanesulfonyl)imide, (N,N-diethyl-N-methyl-N(2methoxyethyl)ammonium bis(fluorosulfonyl)imide, 1 -Butyl-l-methylpyrrolidinium bis(fluorosulfonyl)imide, 1-ethyl- 3-methylimidazolium bis(fluorosulfonyl)imide, l-methyl-l-(2-methoxyethyl)pyrrolidinium bis(fluorosulfonyl)imide, N,N-diethyl-N-methyl-N-propylammonium bis(fluorosulfonyl)imide, N-Ethyl-N,N-dimethyl-N-(2-methoxyethyl)ammonium bis(fluorosulfonyl)imide, N-propyl-N-methylpiperidinium bis(fluorosulfonyl)imide, N- trimethyl-N-butyl ammonium bis(fluorosulfonyl)imide, N-methyl-N- butyl-piperidinium bis(trifluoromethanesulfonyl) imide, N-methyl-N-propylpyrrolidinium bis(trifluoromethanesulfonyl)imide, or a combination thereof.

Alternatively, or additionally, the liquid electrolyte may be a gel electrolyte. As noted above, as used herein the term "liquid electrolyte" is intended to include liquid electrolytes and electrolytes where the liquid is in a gel matrix (also referred to as a "gel electrolyte"). The gel electrolyte may comprise polyethylene oxide with a gelling liquid electrolyte, for example an ether such as dimethyl ether. In one example, the electrolyte may comprise polyethylene oxide in combination with LiTFSI in dimethylether.

Any combination of one or more of the above solvents may be included in the liquid electrolyte. For example, the electrolyte may comprise the combination of an ionic liquid with a fluorinated ether, or the combination of an ionic liquid within a gel, or the combination of a fluorinated ether within a gel. The electrolyte may comprise a combination of two or more of any of the liquids and/or gels detailed above.

Optionally, the liquid electrolyte comprises a solvent selected from linear ethers, diethyl ether (DEE), tetra hydrofuran (THF), Dimethoxyethane (DME), Dioxolane (DIOX), Diglyme, dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), ethylene carbonate (EC), propylene carbonate (PC), methyl formate (MF), ethyl formate (EF), methyl propionate (MP), ethyl acetate (EA) and methyl butyrate (MB), methyl ethyl ketone, acetonitrile (ACN), propionitrile (PN), isobutyronitrile (iBN), Dimethylformamide (DMF), Dimethylacetamide (DMAc), N-Methyl-2-pyrrolidone (NMP), Tetram ethyl urea (TMU), Dimethyl sulfoxide (DMSO), Trimethyl phosphate, Triethyl phosphate, Hexamethylphosphoramide, toluene, benzene, heptane, xylene, dichloromethane, ionic liquids, fluorinated ethers, fluorinated carbonates, fluorinated sulfones, fluorinated esters, fluorinated ketones, fluorinated nitriles, fluorinated amides, fluorinated lactams, fluorinated ureas, fluorinated phosphates, fluorinated phosphoramides, gels, or a combination thereof; and at least one alkali metal salt.

Optionally, the alkali metal salt comprises lithium when the anode comprises lithium or a lithium alloy; and wherein the alkali metal salt comprises sodium when the anode comprises sodium or a sodium alloy.

Optionally, the alkali metal salt is at least one lithium salt selected from lithium hexafluoroarsenate, lithium hexafluorophosphate, lithium perchlorate, lithium sulfate, lithium nitrate, lithium trifluoromethanesulfonate, lithium bis(trifluoromethane)sulfonimide, lithium bis(fluorosufonyl)imide, lithium bis(oxalate) borate, lithium difluoro(oxalate)borate, lithium bis(pentafluoroethanesulfonyl)imide, lithium 2-trifluoromethyl-4,5-dicyanoimidazole, or a combination thereof; or wherein the alkali metal salt is at least one sodium salt selected from sodium hexafluoroarsenate, sodium hexafluorophosphate, sodium perchlorate, sodium sulfate, sodium nitrate, sodium trifluoromethanesulfonate, sodium bis(trifluoromethane)sulfonimide, sodium bis(fluorosufonyl)imide, sodium bis(oxalate) borate, sodium difluoro(oxalate)borate, sodium bis(pentafluoroethanesulfonyl)imide, sodium 2-trifluoromethyl-4,5- dicyanoimidazole, or a combination thereof.

Often, the alkali metal salt is lithium trifluoromethanesulfonate (also known as lithium triflate or LiOTf), lithium bis-trifluoromethanesulfonimide (LiTFSI), and/or lithium bis(fluorosufonyl)imide (LiFSI). In conventional Li-S cells, LiFSI lacks stability in the presence of polysulfides, such that this salt would usually be considered unsuitable. However, with the cathode of the invention, which operates without the formation of polysulfides, the electrolyte can include salts and solvents, such as LiFSI, that otherwise would not be stable, resulting in a broader range of materials that can be used in the fabrication of the claimed cells.

Optionally, the concentration of the at least one alkali metal salt is at least 75% of the saturation concentration of the electrolyte. As used herein, the term "saturation concentration" relates to the extent of solubility of a particular solute in a particular solvent. The point of saturation is where the addition of solute does not result in an increase in concentration. Often the concentration of the at least one alkali metal salt is at least 80% of the saturation concentration of the solvent system, often at least 85% of the saturation concentration of the solvent system, often at least 90% of the saturation concentration of the solvent system. It may be the case that the concentration of the at least one alkali metal salt is about 100% of the saturation concentration, i.e., the electrolyte is fully saturated by the alkali metal salt. Often the concentration of the alkali metal salt will be in the range 75% to 90%, or 80% to 100%, of the saturation concentration of the solvent system in the electrolyte. For example, the concentration of lithium or sodium salt in the electrolyte may be within the range of 0.05 M to 10 M, often 1 M to 5 M, for example, 3 M. It may be the case that the lithium salt is present in the electrolyte at a concentration of 0.1 M to 6 M, often 0.5 M to 4 M, often, 1 to 3M for example, 2 M.

The saturation concentration is determined at room temperature, for example at 20 °C. The saturation concentration of polysulfides within a particular solvent may be determined by known methods, for example by determining the point at which just enough electrolyte is added to dissolve all solid residues.

The electrolyte is liquid across the range of operating temperatures of the cell, which may be from -30 to 120 °C, often from -10 to 90 °C, often from 0 to 60 °C. Operating pressures of the cell may be from 5 mbar to 100 bar, often from 10 mbar to 50 bar, for example 100 mbar to 20 bar. It may be the case that the cell according to the invention is operated at room temperature and pressure. The high concentration of the electrolyte in accordance with the invention means that the electrolyte has a lower vapour pressure than a standard electrolyte used in conventional lithium-sulfur cells. Thus, the cell in accordance with the invention may perform better than a standard lithium-sulfur cell at a low pressure.

In conventional Li-S cells, certain additives may be included in the electrolyte to prevent or limit the effect of polysulfide shuttle. For instance, additives including N-0 bonds, such as LiNCh. However, there are some disadvantages to the inclusion of additives of this nature, such as depletion during cell operation and resultant cell swelling due to formation of gases during cycling, particularly at higher temperatures. This could not only have safety implications but can also have an adverse effect on cycle life of the cell. Moreover, these additives may also limit the voltage range of the cell. The electrolyte of the invention removes the need for these additives. It may be the case that the electrolyte according to the invention does not comprise additives comprising N-0 bonds.

Optionally, the electrolyte loading is in the range of from 0.5 pL/mAh to 3 pL/mAh. Often in the range of from 0.75 pL/mAh to 2 pL/mAh, often in the range of 1 to 2 pL/mAh. As used herein, "pL/mAh" relates to the electrolyte/sulfur ratio in the cell (i.e., pL of electrolyte per milliampere-hour of sulfur). Typically, conventional Li-S requires a high amount of electrolyte (i.e., high loading) to dissolve polysulfides contained in the cathode. With cells according to the invention this is not necessary. A low electrolyte loading is beneficial as it makes the cell lighter, resulting in higher gravimetric energy.

According to a second aspect of the invention, there is described a method of producing a cell according to the first aspect of the invention comprising the following steps:

(i) forming a cathode by (a) optionally mixing sulfur and hollow core-shell particles comprising a carbonaceous material, a metallic material, a metalloid material, a polymer, or a combination thereof, with a solvent to produce a slurry, (b) optionally depositing the slurry onto a current collector, (c) optionally removing the solvent to produce a cathode, and (d) optionally cutting the cathode into the desired shape;

(i) placing a separator on the cathode;

(ii) placing an anode on the separator; and

(iii) adding an electrolyte, wherein the polysulfide solubility of the electrolyte is less than 500mM. According to a third aspect of the invention, there is described a method of producing a cell according to the first aspect of the invention comprising the following steps:

(i) forming a cathode by (a) optionally mixing sulfur and hollow core-shell particles comprising a carbonaceous material, a metallic material, a metalloid material, a polymer, or a combination thereof, with a solvent to produce a slurry, (b) optionally depositing the slurry onto a current collector, (c) optionally removing the solvent to produce a cathode, and (d) optionally cutting the cathode into the desired shape;

(ii) placing a separator on an anode;

(iii) placing the cathode on the separator; and

(iv) adding an electrolyte, wherein the polysulfide solubility of the electrolyte is less than 500mM.

It may be the case that at least 70 wt% sulfur based on the total weight of the cathode is mixed with the hollow core-shell particles in step (i) of the methods according to the second or third aspect of the invention.

It may be the case that the cell is a pouch cell, a prismatic cell, or a cylindrical cell.

Where the cell is a pouch cell, the method according to the second aspect or the third aspect of the invention may further include the step of placing the cathode, separator, and anode in a pouch prior to addition of the electrolyte.

A plurality of cells according to the first aspect of the invention may be combined to form a cell stack.

The solvent according to the second or third aspect of the invention may be selected from water or a suitable organic solvent, such as N-Methyl-2-pyrrolidone (NMP).

The current collector according to the second or third aspect of the invention may comprise aluminium, copper, titanium, or stainless steel. Often, the current collector is aluminium foil.

The separator according to the second or third aspect of the invention may be formed from a wide variety of materials. Examples of material used for the separator include, but are not limited to, polyolefin-based materials such as polyethylene, polypropylene, or a combination thereof. The composite according to the second or third aspect of the invention may be mixed with, for example, an electronically conductive carbon material and/or further optional components, such as binders, prior to forming the slurry. Examples of the electronically conductive carbon material, and binder(s) are detailed above in relation to the first aspect of the invention.

Optionally, the method according to the second or third aspect of the invention further comprises the step of calendaring or pressing of the cathode prior to cutting. As used herein, the term "calendaring" refers to the compaction process for the cathode. Calendaring can be carried out via conventional methods, such as through the use of calendar rollers.

Where calendaring takes place via calendar rollers, the cathode may be passed through rollers up to five times, often one or two times. The rollers may be made of any suitable material, for example steel, glass, or ceramics. A force may be applied on the rollers of 0 kN to 100 kN, often 0 to 80 kN, often, for example, 20 to 80 kN. Calendaring may take place at room temperature (i.e., in the range of from 15 to 25 °C). Heating may optionally be applied to the rollers. The temperature of the rollers may be in the range of from 15 to 80 °C. During calendaring or pressing, the thickness of the cathode decreases. The thickness of the cathode following calendaring or pressing may be from 1 to 50 pm, often 10 to 40 pm, often 15 to 30 pm. Calendaring may result in the reduction of the porosity of the cathode, which allows for a lower electrolyte loading and results in an increase in volumetric energy (Wh/L). As used herein, the term "volumetric energy" relates to the amount of energy stored in the cell per volume in litres. Moreover, calendaring may also bring about smoothing and levelling of the cathode, which can help extend the life cycle of the cell without affecting the cell utilisation.

In a fourth aspect of the invention there is provided an electrochemical cell assembly comprising at least one electrochemical cell according to the first aspect of the invention; and a means of applying pressure to the at least one electrochemical cell.

In a fifth aspect of the invention there is provided a method of cycling the cell according to the first aspect of the invention, the method comprising cycling between a charge voltage and a discharge voltage for at least one cycle (charge voltage X to discharge voltage Y and return to charge voltage X), generally at least two cycles, often multiple cycles. Often the cell will be cycled in the voltage range (charge to discharge) IV to 4V, enabling higher charge capacity relative to known systems, enhanced coulombic efficiency, and cyclability. The cell of the invention has a long cycle life. During cycling of the cell, there is an expansion of sulfur, which results in swelling of the cathode. Application of pressure helps to retain the integrity of the cathode structure, which can be changed due to volume expansion of the sulfur-hollow core-shell particles. As a result, application of pressure as discussed in the fourth aspect of the invention, helps extend the life cycle of the cell.

Optionally, the means of applying pressure comprises at least one of a band, wrap or tubing positioned on the outside of the cell assembly. A band, wrap or tubing positioned on the outside of the cell assembly allows for a stable constricting force to be applied during cycling. As used herein, the outside of the cell refers to the surface of the anode. Pressure may be applied across the entire surface of the anode. Alternatively, the force may be applied over a portion of the surface of the anode, such as over at least 20% of the surface of the anode. Often, pressure may be applied over at least 40% of the surface of the anode, often over at least 60%, often over at least 80%.

The cell assembly may comprise one or more plates located outside the cell. Where one or more plates are present, pressure may be applied to the one or more plates.

The cell assembly may be located within a housing. When the cell is located within a housing, pressure may be applied to the housing.

The band wrap or tubing can be made of any suitable material, such as elastic materials or shrink-wrap materials. Examples of suitable elastic materials include, but are not limited to, natural or synthetic rubber materials. Examples of shrink-wrap materials include, but are not limited to, polyvinyl chloride (PVC), polyethylene (PE), and polyolefin (POF).

Pressure may be applied to the cell or plurality of cells present in the cell assembly continuously. Alternatively, the pressure may vary over time. Other means of applying pressure may include use of screws or weights.

Unless otherwise stated, each of the integers described may be used in combination with any other integer as would be understood by the person skilled in the art. Further, although all aspects of the invention preferably "comprise" the features described in relation to that aspect, it is specifically envisaged that they may "consist" or "consist essentially" of those features outlined in the claims. In addition, all terms, unless specifically defined herein, are intended to be given their commonly understood meaning in the art.

Further, in the discussion of the invention, unless stated to the contrary, the disclosure of alternative values for the upper or lower limit of the permitted range of a parameter, is to be construed as an implied statement that each intermediate value of said parameter, lying between the smaller and greater of the alternatives, is itself also disclosed as a possible value for the parameter.

In addition, unless otherwise stated, all numerical values appearing in this application are to be understood as being modified by the term "about".

In order that the invention may be more readily understood, it will be described further with reference to the figures and to the specific examples hereinafter.

Figure 1 illustrates electrochemical performance data (charge-discharge capacity throughout cycling) of a cell designed in accordance with Example 1. The data series denoted by the square legend relates to discharge capacity and the data series denoted by the star legend relates to charge capacity;

Figure 2 illustrates electrochemical performance data (charge-discharge voltage curves at a C/10 rate and room temperature) of a cell designed in accordance with Example 1. The voltage can be found along the y axis and the specific capacity per unit mass (mAh/g of sulfur) can be found along the x axis;

Figure 3 illustrates electrochemical performance data (charge-discharge capacity throughout cycling) of a cell designed in accordance with Example 2. The data series denoted by the square legend relates to discharge capacity and the data series denoted by the star legend relates to charge capacity;

Figure 4 illustrates electrochemical performance data (charge-discharge voltage curves at a C/10 rate and room temperature) of a cell designed in accordance with Example 2. The voltage can be found along the y axis and the specific capacity of sulfur per unit mass (mAh/g of sulfur) can be found along the x axis;

Figure 5 illustrates electrochemical performance data (charge-discharge capacity throughout cycling) of a cell designed in accordance with Example 3. The data series denoted by the square legend relates to discharge capacity and the data series denoted by the circle legend relates to charge capacity;

Figure 6 illustrates electrochemical performance data (charge-discharge capacity throughout cycling) of a cell designed in accordance with Example 4. The data series denoted by the square legend relates to discharge capacity and the data series denoted by the circle legend relates to charge capacity; Figure 7 displays a Transmission electron microscopy (TEM) image of BP2000/sulfur composite (with 75 wt% sulfur in composite) prepared via melt infusion. The image shows a collection of agglomerates of hollow core-shell particles. The data was imaged at 200kV on a JOEL 2100F FEG-TEM with a Gatan K3 IS camera;

Figure 8 displays a magnified Transmission electron microscopy (TEM) image of the BP2000/sulfur composite as detailed in Figure 7. The image shows that the hollow coreshell particles comprise layers;

Figure 9 illustrates electrochemical performance data (charge-discharge capacity throughout cycling) of a cell designed in accordance with Example 1. The data series denoted by the square legend relates to discharge capacity and the data series denoted by the star legend relates to charge capacity; and

Figure 10 illustrates electrochemical performance data (charge-discharge voltage curves at a C/10 rate and room temperature) of a cell designed in accordance with Example 1. The voltage can be found along the y axis and the specific capacity per unit mass (mAh/g of sulfur) can be found along the x axis.

Example 1

A lithium sulfur cell with a sulfur-hollow core-shell particle composite containing cathode is provided. The cathode comprised 80 wt% composite, 10% Acetylene Black and 10 wt% PVDF as a binder. The composite comprised 75 wt% sulfur and 25 wt% BLACK PEARLS® 2000 and was fabricated via melt diffusion.

The PDVF binder and solvent were added to form an aqueous slurry, which was coated onto an aluminium based current collector to form a cathode.

The liquid electrolyte consisted of Lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) dissolved within Diethylene glycol diethyl ether:TTE, wherein the volume ratio of Diethylene glycol diethyl ether:TTE is in the region of 1:3, v:v. Lithium metal foil 100 micron thick was utilised as the negative electrode (anode). The liquid electrolyte component was held within an inert separator placed between the electrodes.

Electrochemical performance data characteristics of the cell is provided in Figures 1 and 2, based upon cycling of the cell between 1 and 4V under an applied current equivalent to a rate of C/10 based upon the total sulfur content of the cathode, measured by mass. The theoretical capacity of sulfur is c. 1675 mA h g ^-1 . As can be seen from Figures 1 and 2, the lithium sulfur cell of example 1 shows high discharge/charge capacities. Equivalent performance data characteristics of the cell based upon cycling of the cell between 1 and 3V but otherwise identical conditions, is provided in Figures 9 and 10.

As can be seen from a comparison of Figures 1 and 2 with Figures 9 and 10, the lithium sulfur cell of example 1 shows good discharge/charge capacities across both ranges, although, with this particular system, performance is optimal across when cycled in the range 1 to 4V.

Example 2

A lithium sulfur cell with a sulfur-hollow core-shell particle composite containing cathode is provided. The cathode comprised 80 wt% composite, 10% Acetylene Black and 10 wt% PVDF as a binder. The composite comprised 75 wt% sulfur and 25 wt% PBX® 51 and was fabricated via melt diffusion.

The PDVF binder and solvent were added to form an aqueous slurry, which was coated onto an aluminium based current collector to form a cathode.

The liquid electrolyte consisted of Lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) dissolved within Diethylene glycol diethyl ether:TTE, wherein the volume ratio of Diethylene glycol diethyl ether:TTE is in the region of 1:3, v:v. Lithium metal foil 100 micron thick was utilised as the negative electrode (anode). The liquid electrolyte component was held within an inert separator placed between the electrodes.

Electrochemical performance data characteristics of the cell is provided in Figures 3 and 4, based upon cycling of the cell between 1 and 4V under an applied current equivalent to a rate of C/10 based upon the total sulfur content of the cathode, measured by mass. The theoretical capacity of sulfur is c. 1675 mA h g ^-1 . As can be seen from Figures 3 and 4, the lithium sulfur cell of example 2 shows high discharge/charge capacities.

Example 3

A lithium sulfur cell with a sulfur-hollow core-shell particle composite containing cathode is provided. The cathode can comprise 90 wt% composite and 10 wt% PEO as a binder. The composite can comprise 70 wt% sulfur and 30 wt% polypyrrole hollow core-shell particles. The composite can be synthesised as follows: 0.05M of NazSzC -SHzO is completely dispersed into 500 mL of deionized water through vigorous stirring. Following this step, 5% polyvinylpyrrolidone (PVP) is added and the mixture is stirred continuously for 3 hours at a temperature of 70 °C. Then, 10 mL of hydrochloric acid (HCI) is added slowly to the solution, which is then stirred for 24 hours. The product obtained is washed multiple times with deionized water and ethanol, and subsequently dried in a vacuum at a temperature of 60 °C for 24 hours.

The binder can be added in a second step to form an aqueous slurry which can be coated onto an aluminium based current collector to form a cathode.

The liquid electrolyte of the cell can contain a lithium salt, the salt being at a concentration above 75% of its saturation concentration. In this example, the liquid electrolyte may consist of Lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) dissolved within Diethylene glycol diethyl ether:TTE, wherein the volume ratio of Diethylene glycol diethyl ether:TTE is in the region of 1:3, v:v. Lithium metal foil 100 micron thick can be utilised as the negative electrode (anode). The liquid electrolyte component is held within an inert separator placed between the electrodes.

Electrochemical performance data characteristics of the cell is provided in Figure 5, based upon cycling of the cell between 1 and 4V under an applied current equivalent to a rate of C/10 based upon the total sulfur content of the cathode, measured by mass, and assuming the theoretical capacity of sulfur to be 1675 mA h g ^-1 .

Example 4

A lithium sulfur cell with a sulfur-hollow core-shell particle composite containing cathode is provided. The cathode can comprise 90 wt% composite and 10 wt% PEO as a binder. The composite can comprise 70 wt% sulfur and 30 wt% manganese oxide hollow coreshell particles.

The composite can be synthesised as follows: monodispersed solid sulfur nanospheres can be prepared using a solution reaction of sodium thiosulfate (NazSzCh) with hydrochloric acid (HCI) in the presence of 1% (weight ratio) polyvinylpyrrolidone (PVP) (Mw ~ 40 000) at room temperature. Following this, the solid sulfur nanospheres are then washed with water in order to remove any polyvinylpyrrolidone (PVP) on the nanosphere surface. Next, the solid sulfur nanospheres are subsequently dispersed in an aqueous solution of manganese sulfate (MnSC ), followed by addition of potassium permanganate (KMnC ) under continuous and vigorous stirring for 30 minutes. The y-MnC -coated solid sulfur nanospheres can be obtained through use of a redox reaction between potassium permanganate (KMnC ) and manganese sulfate (MnSC ).

The binder can be added in a second step to form an aqueous slurry which can be coated onto an aluminium based current collector to form a cathode.

The liquid electrolyte of the cell can contain a lithium salt, the salt being at a concentration above 75% of its saturation concentration. In this example, the liquid electrolyte may consist of Lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) dissolved within Diethylene glycol diethyl ether:TTE, wherein the volume ratio of Diethylene glycol diethyl ether:TTE is in the region of 1:3, v:v. Lithium metal foil 100 micron thick can be utilised as the negative electrode (anode). The liquid electrolyte component is held within an inert separator placed between the electrodes.

Electrochemical performance data characteristics of the cell is provided in Figure 6, based upon cycling of the cell between 1 and 4V under an applied current equivalent to a rate of C/10 based upon the total sulfur content of the cathode, measured by mass, and assuming the theoretical capacity of sulfur to be 1675 mA h g ^-1 .

It would be appreciated that the process and apparatus of the invention are capable of being implemented in a variety of ways, only a few of which have been illustrated and described above.