Sodium metal cell

Abstract

The invention relates to a sodium metal cell comprising a cathode electrode and an anode electrode disposed in an opposite face-to-face arrangement with a separator therebetween, and in which the anode electrode and the separator are located in contact with opposite sides of a free-standing polymeric substrate. A method of manufacturing said sodium metal cell is also disclosed.

Description

[0001] SODIUM METAL CELL

[0002] FIELD OF THE INVENTION

[0003] The present invention relates to a sodium metal cell comprising a free-standing polymeric substrate. A process for preparing such a sodium metal cell is also disclosed.

[0004] BACKGROUND OF THE INVENTION

[0005] Sodium-ion batteries are analogous in many ways to the lithium-ion batteries that are in common use today; they are both reusable secondary batteries that comprise an anode (negative electrode), a cathode (positive electrode) and an electrolyte material, both are capable of storing energy, and they both charge and discharge via a similar reaction mechanism. When a sodium-ion (or lithium-ion) battery is charging, Na+(or Li+) ions deintercalate from the cathode and insert into the anode. Meanwhile charge balancing electrons pass from the cathode through the external circuit containing the charger and into the anode of the battery. During discharge the same process occurs but in the opposite direction.

[0006] A sodium-ion cell comprises an active material layer coated on a current collector foil to form a cathode, and a similar arrangement exists for a corresponding anode. The cathode and anode are physically separated by a separator which allows for a flow of ions within a liquid electrolyte medium, which is present uniformly within the cell and wets the entire cathode, anode and separator. During charging, Na+ions shuttle from the cathode active material and are inserted in the anode active material (electrons flow through the external circuit) and the reverse process occurs during discharging (sodium ions are extracted from the anode active material and are inserted into the cathode active material with the electrons flowing through the external circuit, doing the useful work).

[0007] In a sodium metal cell, the anode is composed of sodium metal (either as standalone Na metal foil / sheet or as sodium metal foil / sheet laminated upon another current collector foil / sheet), with the cathode, separator and electrolyte being the same as that used in the above sodium- ion cell. Therefore, in sodium metal cells during each charge / discharge cycle, Na plating / stripping occurs on the Na metal foil / sheet (the cathode behaves in the same manner as that in sodium-ion cells). Sodium metal cells have generated considerable interest and are considered as promising candidates for next-generation battery technology. However, one area that needs more attention is the development of novel sodium metal cells which can cycle (i.e., charge / discharge) in a more stable way than existing sodium metal cells. This is particularly true when such sodium metal cells have capacities of greater than 3 mAh.

[0008] Prior art techniques to enable more stable cycling in sodium metal cells are briefly summarised below.

[0009] Lu et al., “Regulating Interfacial Na-Ion Flux via Artificial Layers with Fast Ionic Conductivity for Stable and High-Rate Na Metal”, ACS Materials Lett. 2019, 1 , 3, 303-309, investigated the use of a NaxMOeSs coating on top of a metallic Na electrode for use in a sodium metal cell.

[0010] Although the hybrid anode in this prior art exhibited stable cycling, the preparation of the MOeSs / carbon composite was cumbersome and preparing the hybrid anode by direct contact with sodium metal required multiple air-sensitive steps, which would not be suitable in a commercial environment.

[0011] Sun et al., “Dendrite-Free Sodium-Metal Anodes for High-Energy Sodium-Metal Batteries”, Adv. Mater. 2018, 30, 1801334, investigated the use of a nitrogen and sulfur doped carbon nanotube (NSCNT) paper as an ‘interlayer’ in a sodium metal battery. Whilst excellent stability was demonstrated by the use of the NSCNT paper, the nitrogen and sulfur functionalities were said to be essential to achieving such stability. However, this required the use of a complex thermal pyrolysis synthesis method which would be unattractive and costly in a commercial environment.

[0012] Zhen et al., “Realizing high-power and high-capacity zinc-sodium metal anodes through interfacial chemistry regulation”, Nat Commun 12, 3083 (2021), investigated the use of a modified separator having a zinc coating and concluded this as a promising route to superior cycling stabilities in a zinc or sodium metal cell. The zinc coated separator was produced either via a) magnetron sputtering, or b) via a slurry coating by a doctor blading technique. However, the use of a modified separator would be unattractive and costly in a commercial environment. This is because it would be difficult to integrate into an existing process and if off-site, it could crack or be damaged when rolled. The use of magnetron sputtering is also very expensive. Zhao et al., “Inorganic-Organic Coating via Molecular Layer Deposition Enables Long Life Sodium Metal Anode”, Nano Lett. 2017, 17, 9, 5653-5659, investigated the use of a 60 nm ‘aluncone’ layer on sodium metal via molecular layer deposition. Trimethylaluminum (TMA) and ethylene (EG) were used as the inorganic-organic ‘aluncone’ coating precursors. However, molecular layer deposition is an expensive technique to use in a commercial environment.

[0013] Finally, US 10,658,669 B2 discloses the use of a ‘dendrite penetration-layer’ which is ‘integral’ with a sodium metal layer. The dendrite penetration-layer is produced via a series of extremely complex steps which include providing a sodium-containing species from an electrochemical deposition reaction in which a layer of the ‘dendrite penetration-layer’ is used as a working electrode (see Example 14). The assembly of further cells to produce such a layer is clearly unattractive and costly in a commercial environment.

[0014] The present invention therefore aims to mitigate or eliminate one or more of the aforesaid disadvantages of the known art.

[0015] In particular, the aim of the present invention is therefore to provide a sodium metal cell which can cycle (i.e. , charge / discharge) in a more stable way than an existing sodium metal cell.

[0016] The sodium metal cell of the present invention will also be commercially attractive because it will be produced without complex manufacturing steps, such as initial assembly of ‘sacrificial’ cells to generate the final product as is required for US 10,658,669 B2.

[0017] The present invention achieves these aims by providing a sodium metal cell comprising a cathode electrode and an anode electrode disposed in an opposite face-to-face arrangement with a separator therebetween, and in which the anode electrode and the separator are located in contact with opposite sides of a free-standing polymeric substrate.

[0018] The term “sodium metal cell” as used herein refers to an electrochemical cell in which the anode electrode uses sodium metal. Suitable examples include (but the invention is not limited to these examples) electrochemical cells which use sodium metal in the form of foil, sheets, wires, ribbons (either standalone or disposed on a current collector); a mesh-type current collector infused with metallic sodium metal; or any suitable current collector supporting deposited sodium metal (for example by techniques such as magnetron sputtering, physical / plasma vapour deposition or chemical vapour deposition). As used herein the term “free-standing” means not attached to, or fixed to anything else. That is, independent. Thus, for the purposes of this disclosure, the free-standing substate of the present invention is not bonded to, nor fixed to a cathode electrode, an anode electrode, nor a separator, of a sodium metal cell. Whilst the free-standing substate of the present invention may contact and weakly adhere to such components during assembly of a sodium metal cell, or whilst the free-standing substate is in use in a sodium metal cell, it remains independent.

[0019] As used herein the term “substate” refers to any solid substance to which one or more second substrates are applied so that the one or more second substrates can contact the substrate.

[0020] Thus, for the purposes of this disclosure, an anode electrode and a separator (i.e., the one or more second substrates) make contact with at least a portion of opposite sides of the freestanding “substate” in a sodium metal cell. As such, the term “substrate” as defined herein includes within its meaning, for example, any of a film, sheet, plate, panel, or membrane.

[0021] Preferably, the free-standing polymeric substrate is a film, which is ideally flexible. Highly preferably, the free-standing polymeric substrate is an interfacial film. This means that it separates the anode from the cathode via a distinct porous layer of certain properties, as revealed in this disclosure.

[0022] Typically, the substrate (ideally as a film) lies essentially in one plane when installed or in use in a sodium metal cell (i.e., it is substantially flat, or substantially planar). This configuration enables optimum contact in use with the anode electrode and the separator of a sodium metal cell. In one embodiment, the shape of the free-standing polymeric substrate is a cuboid (e.g., rectangular). Of course other alternative shapes (e.g. circular) could also be envisaged to enable optimum contact with the shape of the anode electrode and / or separator of a sodium metal cell.

[0023] Ideally, the free-standing polymeric substrate comprises an interconnected network of pores which are preferably an interconnected network of open pores.

[0024] By providing the free-standing polymeric substrate with an interconnected network of pores it means that the free-standing polymeric substrate is provided with one or more substantially free paths, which provide one or more ‘highways’ to allow homogenous Na+ flux into / out of the anode of the sodium metal cell during charging / discharging. Indeed, this is a key reason why a sodium metal cell according to the present invention results in greater cycling stability when compared to a prior art sodium metal cell.

[0025] Preferably, at least some of these pores are present in a bulk region which extends between opposite sides of the free-standing polymeric substrate. Experimental data disclosed herein confirms that a sodium metal cell is provided with improved electrochemical performance when at least some pores have their openings at a position that is substantially perpendicular to a surface which contacts an anode and / or separator, or additionally or alternatively an opening which is internally present within the free-standing polymeric substrate through which it is connected to other pores. For the avoidance of any doubt, these are examples of pores present within a ‘bulk region’.

[0026] Furthermore, experimental data disclosed herein also confirms that a sodium metal cell is provided with improved electrochemical performance when at least some of the pores are present in the bulk region because this provides a highway to allow homogenous Na+ flux into / out of the anode of the sodium metal cell during charging / discharging.

[0027] A ‘bulk region’ as defined herein is a region that substantially extends across the thickness (in the height direction, when lying flat) of the free-standing polymeric substrate between opposite sides. The ‘bulk region’ as herein does not encompass the ‘surface region’ as defined below. Therefore, unlike the ‘surface region’, the ‘bulk region’ is a region which does not ideally make contact with the anode electrode and / or the separator of the sodium metal cell whilst it is in use.

[0028] Preferably, at least some of these pores are present in a surface region included in at least one of the opposite sides of the free-standing polymeric substrate.

[0029] A ‘surface region’ as defined herein is a region that is a surface of one or more sides (in the height direction, when lying flat) of the free-standing polymeric substrate. Typically, this is a region that is a surface of one or more opposite sides (in the height direction, when lying flat) of the free-standing polymeric substrate which are in contact with the anode electrode and / or the separator of the sodium metal cell whilst it is in use. Ideally, this is a region that is a surface of each of the two opposite sides (i.e. , both sides in the height direction, when lying flat) of the free-standing polymeric substrate which are in contact with the anode electrode and / or the separator of the sodium metal cell whilst it is in use. The ‘surface region’ does not encompass the ‘bulk region’ because these, in the present invention, are distinct regions from each other as discussed above.

[0030] Highly preferably, at least some of these pores are present in a bulk region and a surface region of the free-standing polymeric substrate.

[0031] Indeed, experimental data disclosed herein confirms that a sodium metal cell is provided with improved electrochemical performance in this highly preferred configuration because this optimum structure of the free-standing polymeric substrate provides highways between opposite and perpendicular surface regions of the films to allow homogenous Na+ flux into / out of the anode of the sodium metal cell during charging / discharging, and also openings to said highways, to further allow homogenous Na+ flux into / out of the anode of the sodium metal cell during charging / discharging.

[0032] As the free-standing polymeric substrate ideally comprises an interconnected network of pores the free-standing polymeric substrate therefore comprises porosity. As such, the free-standing polymeric substrate maybe defined as a free-standing porous polymeric substrate.

[0033] Preferably, pores are present in the ‘bulk region’ as discussed above. Therefore, the freestanding porous substrate may comprise ‘bulk region’ porosity.

[0034] Preferably, pores are present in the ‘surface region’ as discussed above. Therefore, the freestanding porous substrate may comprise ‘surface region’ porosity.

[0035] Most preferably, the free-standing porous substrate will comprise ‘bulk region’ porosity as well as ‘surface region’ porosity to provide improved electrochemical performance. When both bulk and surface region porosity are provided, it is possible to provide greater cycling stability to a sodium metal cell.

[0036] The free-standing polymeric substrate may have a porosity of from about 5 to about 95%, or about 10 to about 95%, or about 20 to about 90%, or about 30 to about 80%.

[0037] The porosity corresponds to a value obtained by subtracting the volume expressed from the weight and density of each ingredient in the free-standing polymeric substrate, from the volume calculated from the thickness (height), width, and length of the free-standing polymeric substrate. The porosity maybe determined from scanning electron microscopy images (SEM) or by using a mercury porosimeter.

[0038] According to an embodiment of the present invention, at least some of the pores present in the free-standing polymeric substrate have pore widths of from 200 nm to 50 pm, more preferably from 500 nm to 45 pm and ideally from about 1 pm to 40 pm. Ideally, all of the pores present in the free-standing polymeric substrate have pore widths of from 200 nm to 50 pm, more preferably from 500 nm to 45 pm and ideally from about 1 pm to 40 pm.

[0039] The pore widths may be determined by using field emission scanning electron microscopy (FESEM). In particular, the reader is directed to the experimental section for a more detailed method of using FESEM to determine pore width.

[0040] Preferably, at least some of the pores present in the bulk region will have a pore width of from greater than 0 to about 10 pm. Ideally, all of the pores present in the bulk region will have a pore width of from greater than 0 to about 10 pm.

[0041] Preferably, at least some of the pores present in the surface region will have a pore width of from greater than 0 to about 50 pm. Ideally, all of the pores present in the surface region will have a pore width of from greater than 0 to about 50 pm.

[0042] According to an embodiment of the present invention, the free-standing polymeric substrate has a cross-sectional thickness (in the height direction, when lying flat) from about 1 pm to about 40 pm.

[0043] Preferably, the free-standing polymeric substrate has a cross-sectional thickness (in the height direction when lying flat) from about 2 pm to about 15 pm. More preferably, the free-standing polymeric substrate has a cross-sectional thickness from about 3 pm to about 12 pm, and most preferably the free-standing polymeric substrate has a cross-section thickness from about 6 pm to about 12 pm.

[0044] When the cross-sectional thickness falls within the above-mentioned range, it is possible to provide greater electrochemical performance to a sodium metal cell.

[0045] Exemplary cross-sectional thicknesses include 6 pm, 7 pm, 8 pm or 9 pm. The cross-sectional thickness maybe determined by using a digital micrometer, or by using an optical microscope.

[0046] According to an embodiment of the present invention, the free-standing polymeric substrate further includes one or more stabilising additives. As used herein the term “stabilising additives” means an agent that affects interfacial interactions which occur in the method of preparing a free-standing polymeric substate as disclosed herein, and which stabilise the formation of such substrate or its precursor. Therefore, when one or more stabilising additives are provided to the free-standing polymeric substrate, it is possible to provide greater electrochemical performance to a sodium metal cell.

[0047] Preferably, the one or more stabilising additives are provided in solid form, typically at room temperature (i.e. in contrast to liquid or gaseous form). Ideally, the one or more stabilising additives of the present invention are selected from one or more of carbon black, carbon nanotubes, carbon nanofibers, graphite / graphene, hard carbon, glassy carbon, soft carbon, activated carbon, silica, silicates, alumina, aluminosilicates, titania, talc, zeolites, nanozeolites, or a combination thereof.

[0048] In one embodiment, the one or more stabilising additives of the present invention does not include graphite and / or graphene.

[0049] Preferably, the one or more stabilising additives are selected from one or more of carbon black, nanozeolites, or a combination thereof. That is, the one or more stabilising additives preferable include carbon black and / or nanozeolites.

[0050] Examples of carbon black include a C65 powder. Examples of nano zeolite include a 5 A nanozeolite powder, the ‘Nano H-ZSM-5 P-9T material provided by ACSMaterial (which is a class of nanosized crystalline aluminosilicates with the molar ratio of SiC^AhOs being 91 :1) - here 5 A is the pore size of the nanozeolite.

[0051] Preferably, the free-standing polymeric substrate of the present invention is prepared using one or more stabilising additives in their “as made” state. That is, the one or more stabilising additives have not previously undergone one or more chemical or heating operations prior to preparing the free-standing polymeric substrate of the present invention. Preferably, the free-standing polymeric substrate comprises one or more polymers. That is, one or more polymeric components.

[0052] The one or more polymers may be selected from one or more of the group consisting of polyvinylidene fluoride (PVDF), polyethylene oxide (PEO), and combinations thereof. That is, the one or more polymers may include PVDF and / or PEO. As such, the one or more polymers may include one or more fluoropolymers and / or one or more polyether polymers.

[0053] Alternative one or more polymers may include polytetrafluoroethylene (PTFE), polyacrylic acid (PAA), sodium salt of polyacrylic acid (PAANa), polyvinyl alcohol (PVA), ethylene-vinyl acetate copolymer (EVA), styrene butadiene rubber (SBR), carboxymethyl cellulose (CMC) (especially the sodium salt of CMC), sodium alginate (SA), polymethacrylic acid (PMA), carboxymethyl chitosan (CMCS), and combinations thereof.

[0054] Preferably, the free-standing polymeric substrate comprises one or more hydrophobic polymers and optionally one or more hydrophilic polymers. Examples of one or more hydrophobic polymers include polyvinylidene fluoride (PVDF). Examples of one or more hydrophilic polymers include polyethylene oxide (PEO).

[0055] Alternatively, the free-standing polymers may comprise two or more polymers. That is, two or more polymeric components. In particular, the free-standing polymer may comprise a first polymer and a second polymer, in which the first polymer is different from the second polymer.

[0056] Preferably, the free-standing polymeric substrate may comprise one or more hydrophobic polymers and one or more hydrophilic polymers. An ideal mixture of one or more hydrophobic polymers and one or more hydrophilic polymers includes polyvinylidene fluoride (PVDF) and polyethylene oxide (PEO).

[0057] When free-standing polymeric substrate comprises a mixture of hydrophobic and hydrophilic polymers, the weight ratio of the hydrophobic polymer to the hydrophilic polymer is in the range 1 : 1 to 8:1 , further preferably 2:1 to 6:1 , and particularly preferably 3:1 to 5:1. A ratio in the range 3:1 is especially preferred, particularly when using a mixture of polyvinylidene fluoride (PVDF) and polyethylene oxide (PEO).

[0058] According to an embodiment of the present invention, the free-standing polymeric substrate may comprise one or more polymers and one or more stabilising additives. When a stabilising additive is incorporated in the free-standing polymeric substrate, it is convenient to express the total amount of the polymeric component (that is, the amount of the one or more hydrophobic polymers together with any optional one or more hydrophilic polymers) relative to the amount of one or more stabilising additives, as a weight ratio. Preferably, the weight ratio of the one or more polymers to the one or more stabilising additives is in the range 1 :1 to 10:1 , further preferably 2:1 to 9:1 , and particularly preferably 3:1 to 5:1. A ratio in the range 4:1 is especially preferred.

[0059] According to a preferred embodiment of the present invention, the free-standing polymeric substrate of the present invention is derived from solvent casting into an aqueous phase. The aqueous phase will comprise (i) water, or (ii) optionally water in combination with a surface tension lowering additive such as an alcohol, for example isopropyl alcohol (I PA), (ii) is typically used in the case that the polymeric component of the free-standing polymeric substrate comprises one or more optional hydrophilic polymers (including mixtures with one or more hydrophobic polymers).

[0060] A “surface tension lowering additive” in this context means a substance which, when added to the aqueous phase above, results in a reduction of the overall surface tension of the aqueous phase. Therefore, it will be appreciated that the surface tension lowering additive itself has a surface tension less than that of water (i.e. , less than 0.072 N / m at 25 °C).

[0061] In a preferred embodiment, the surface tension lowering additive is a liquid compound, such as a solvent. Preferably, the surface tension lowering additive is a liquid compound with a surface tension of at least 0 N / m and less than 0.072 N / m at 25 °C. Preferably, the surface tension lowering additive has a surface tension of at least 0 N / m and less than 0.07 N / m at 25 °C, optionally less than 0.06 N / m, or further optionally less than 0.05 N / m at 25 °C. In some embodiments, the surface tension lowering additive may have a surface tension of less than 0.05 N / m at 25 °C, less than 0.04 N / m at 25 °C, or less than 0.03 N / m at 25 °C.

[0062] Very preferably, the surface tension lowering additive is an alcohol. That is, an organic molecule which carries at least one hydroxyl (-OH) functional group bound to a saturated carbon atom. Examples of alcohols might be isopropyl alcohol (I PA), ethanol, methanol, butanol etc. Particularly useful are alcohols which have a surface tension of between around 0 to around 0.07 N / m at 25 °C. Ideally, the alcohol includes isopropyl alcohol (I PA). Preferably, the alcohol (ideally IPA) is mixed with water in a weight ratio of 1 :2. Therefore, preferably IPA:water = 1 :2 wt / wt. In one embodiment, the alcohol (ideally IPA) is mixed with water in a weight ratio from 3:1 to 1 :5, more preferably 1 :1 to 1 :3.

[0063] According to one embodiment of the present invention, when the free-standing polymeric substrate is made using a polymeric component which consists essentially of one or more hydrophobic polymers such as polyvinylidene fluoride (PVDF) the free-standing polymeric substrate of the present invention is derived from solvent casting into an aqueous phase that consists essentially of water and at least one surface tension lowering additive such as IPA However, when the aqueous phase consists essentially of water it is a requirement that the free-standing polymeric substrate further includes one or more stabilising additives.

[0064] According to an alternative embodiment of the present invention, when the free-standing substrate is made using a polymeric component which comprises one or more optional hydrophilic polymers (including mixtures with one or more hydrophobic polymers), the freestanding polymeric substrate of the present invention is derived from solvent casting into an aqueous phase that comprises water and one or more surface tension lowering additives.

[0065] As explained above, ideally, the surface tension lowering additive is an alcohol, preferably having a surface tension of from around 0 to around 0.07 N / m at 25 °C. Ideally, the alcohol includes isopropyl alcohol (IPA). Preferably, IPA is mixed with water in a ratio of 1 :2 (IPA:water = 1 :2 wt / wt).

[0066] When the free-standing polymeric substrate is derived from solvent casting using the abovementioned solvent system, it is possible to provide at least some pores in a bulk and surface region of the free-standing polymeric substrate without the need for one or more stabilising agents. When one or more stabilising agents are avoided improvements in toxicity levels and manufacturing costs may be observed.

[0067] Preferably, the anode electrode of the sodium metal cell comprises metallic sodium. Ideally, the anode electrode comprises a current collecting element (i.e. , a current collector) which comprises a layer of metallic sodium.

[0068] Preferably, the cathode electrode of the sodium metal cell comprises one or more active materials which are adapted to insert and extract sodium ions when the sodium metal cell undergoes charging and discharging operations. Ideally, the cathode electrode comprises a current collecting element (i.e., a current collector) which comprises one or more active materials which are adapted to insert and extract sodium ions when the sodium metal cell undergoes charging and discharging operations.

[0069] The cathode electrode preferably further includes a polymeric binder. Typically, the polymeric binder is selected from polyvinylidene fluoride (PVDF), styrene-butadiene rubber (SBR), and carboxymethylcellulose (CMC). Preferably, the polymer binder comprises polyvinylidene fluoride (PVDF).

[0070] The cathode electrode ideally comprises one or more active materials (A), one or more polymeric binders (B), and one or more conductive additives (C). Such a mixture is typically mixed with an aqueous or non-aqueous solvent (such as water or N-methyl Pyrrolidone (NMP)) and then disposed as a layer or film on one or more surfaces of an anode current collector via techniques such as doctor blade or slot die methods. Typically, A:B:C are present in a weight ratio of from (80 to 98) : (1 to 19) : (1 to 19).

[0071] The one or more conductive additives (C) may include one or more of carbon black, carbon nanotubes, graphene, acetylene black, graphite and carbon nanofiber. Preferably, the one or more conductive additives includes carbon black, such as TIMCAL Super C65 and / or Ketjen Black (KB), and carbon nanotubes.

[0072] When the one or more active materials include a sodium transition metal oxide (e.g., of the general formula as set out below) A:B:C are preferably in a weight ratio of about 92:3:5. Additionally, the one or more polymeric binders (B) typically include PVDF and the one or more conductive additives (C) typically include carbon black, such as TIMCAL Super C65.

[0073] When the one or more active materials include a vanadium-based phosphate (e.g., sodium vanadium phosphate (NVP) - or NVP / C as discussed below) A:B:C are preferably in a weight ratio of about 90:5:5. Additionally, the one or more polymeric binders (B) typically include PVDF and the one or more conductive additives (C) typically include carbon black, such as TIMCAL Super C65.

[0074] When the one or more active materials include an iron-based phosphate (e.g., Na4Fe3(PO4)2(P2O?) (NFPP) or NFPP / C as discussed below) A:B:C are preferably in a weight ratio of about 90:4:6. Additionally, the one or more polymeric binders (B) typically include PVDF and the one or more conductive additives (C) typically include carbon black. Ideally, in one embodiment, the one or more conductive additives include two or more conductive additives. Examples include either a mixture of TIMCAL Super C65 and Ketjen Black (KB). Or, alternatively, a mixture of TIMCAL Super C65 and carbon nanotubes.

[0075] When a mixture of TIMCAL Super C65 (C1) and KB (C2) is used, then A:B:C1 :C2 are preferably in a weight ratio of about 90:4:5.8:0.2. Similarly, when a mixture of TIMCAL Super C65 (C1) and carbon nanotubes (C2) is used, then A:B:C1 :C2 are preferably in a weight ratio of about 90:4:5.8:0.2. Two or more conductive additives may be used preferably in the case that the one or more active materials include an iron-based phosphate.

[0076] The one or more active materials may include sodium transition metal oxides, polyanionic compounds (including fluorinated polyanionic compounds), Prussian blue analogue (PBA) compounds (such as Prussian White or Berlin Green), materials storing sodium via a conversion reaction, sodium transition metal fluorides, oxyfluorides, phosphates, pyrophosphates, mixed phosphates-pyrophosphates, sulfates, and silicates (and their fluorinated versions).

[0077] Preferably, the one or more active materials may include sodium transition metal oxides and polyanionic compounds. Preferred polyanionic compounds may include phosphates, such as vanadium-based phosphates and iron-based phosphates. Highly preferred polyanionic compounds include iron-based phosphates.

[0078] Examples of phosphates are those such as sodium vanadium phosphate (NVP) - i.e., a vanadium-based phosphate, or sodium iron mixed phosphates such as Na4Fe3(PO4)2(P2O?) - i.e., an iron-based phosphate. Na4Fe3(PO4)2(P2O?) is abbreviated herein as ‘NFPP’.

[0079] Iron-based phosphates such as NFPP are particularly favourable cathode active materials because sodium metal cells according to the present invention demonstrate particularly high performance when using NFPP.

[0080] Highly preferably, the one or more active materials may include polyanionic compounds in which such compounds are embedded in a carbon matrix or comprise a carbon-coating. A carbon-coating is understood to mean that they adopt a core-shell like structure in which a core of polyanionic compound is surrounded in a shell of carbon. Indeed, the purpose of a carbon-coating or a carbon matrix is to enhance electronic conductivity within the cathode coating. In particular, carbon-coated polyanionic compounds may include carbon-coated phosphates, such as carbon-coated vanadium-based phosphates and carbon-coated iron-based phosphates. Highly preferred carbon-coated polyanionic compounds include carbon-coated iron-based phosphates.

[0081] Examples of carbon-coated phosphates are those such as carbon-coated sodium vanadium phosphate (NVP / C) - i.e., a carbon-coated vanadium-based phosphate, or carbon-coated sodium iron mixed phosphates such as carbon-coated Na4Fe3(PO4)2(P2O?) - i.e., a carbon- coated iron-based phosphate. Carbon coated Na4Fe3(PO4)2(P2O?) is abbreviated herein as ‘NFPP / C’.

[0082] The carbon-coating may be present in an amount from 0.5% to about 5 wt% of the total weight of the active material. Indeed, the carbon-coating may account for 3 wt% of NVP, or the carbon-coating may account for 1 wt% of NFPP.

[0083] Alternative examples are sodium transition metal oxides.

[0084] Preferred sodium transition metal oxides are of the general formula:

[0085] AI±6M1VM2WM3X M4YM5ZO2-C wherein

[0086] A is one or more alkali metals selected from sodium, potassium and lithium;

[0087] M1comprises one or more redox active metals in oxidation state +2, preferably selected from the group consisting of nickel, copper, cobalt and manganese;

[0088] M2comprises a metal in oxidation state greater than 0 to less than or equal to +4;

[0089] M3comprises a metal in oxidation state +2;

[0090] M4comprises a metal in oxidation state greater than 0 to less than or equal to +4;

[0091] M5comprises a metal in oxidation state +3; wherein 0 < 5 < 1 ;

[0092] V is > 0;

[0093] W is Ss 0;

[0094] X is Ss 0;

[0095] Y is Ss 0; at least one of W and Y is > 0

[0096] Z is Ss 0; C is in the range 0 < c < 2 wherein V, W, X, Y, Z and C are chosen to maintain electrochemical neutrality.

[0097] Ideally, metal M2comprises one or more transition metals, and is preferably selected from manganese, titanium and zirconium; M3is preferably one or more selected from magnesium, calcium, copper, tin, zinc and cobalt; M4comprises one or more transition metals, preferably selected from manganese, titanium and zirconium; and M5is preferably one or more selected from aluminium, iron, cobalt, tin, molybdenum, chromium, vanadium, scandium and yttrium.

[0098] A particularly preferred cathode electrode active material for use in a sodium metal cell will be a nickelate-based material. A sodium-containing active material with any crystalline structure may be used, however, preferably the structure will be 03 or P2 or a derivative thereof, but, specifically, it is also possible that the cathode electrode active material will comprise a mixture of phases, i.e. it will have a non-uniform structure composed of several different crystalline forms. For example, the cathode active material will comprise a compound with the general formula detailed above in a mixture of 03 and P2 phases. The ratio of 03: P2 phases is preferably 1 to 99: 99 to 1.

[0099] The sodium metal cell according to the present invention ideally additionally comprises an electrolyte. Advantageously, the sodium metal cell according to the present invention may use an electrolyte in any form, i.e., solid, liquid or gel composition. Ideally, the sodium metal cell according to the present invention uses a liquid electrolyte.

[0100] Suitable examples include liquid electrolytes such as >0 to 10 molar alkali metal salt such as NaPFe, NaBF4, sodium bis(oxalate) (NaBOB), sodium triflate (NaOTf), NaFSI, NaTFSI, NaDFOB, and mixtures thereof, preferably in one or more solvents selected from carbonate- ester-based solvents, gamma butyrolactone (GBL) sulfolane, monoglyme (also known as 1 ,2- Dimethoxyethane), diglyme, triglyme, tetraglyme, dimethyl sulfoxide (DMSO), dioxolane, and mixtures thereof.

[0101] Optional electrolyte additives such as one or more of 1 ,3-propanediolcyclic sulfate (PCS), P123 surfactant and tris(trimethylsilyl) borate (TMSB) may also be included. Preferably, the electrolyte includes a sodium metal salt selected from NaPFe, NaBF4, highly preferably NaBF4. According to an embodiment of the present invention, a preferred electrolyte may include one or more glymes together with one or more surfactants; one or more boron-containing compounds; and one or more sulfur-containing compounds.

[0102] Preferably, the one or more glymes may include a mixture of two or more glymes, such as tetraglyme and diglyme, ideally in a 1 :1 ratio (wt / wt).

[0103] Preferably, the one or more surfactants may include a non-ionic block copolymer such as poloxamer P123, typically in an amount from 0 > to 5 wt%, ideally about 1 wt%.

[0104] Preferably, the one or more boron-containing compounds may include a borate such as tris(trimethylsilyl) borate (TMSB), typically in an amount from 0 > to 5 wt%, ideally about 5 wt%.

[0105] Preferably, the one or more sulfur-containing compounds may include a sulfate such as 1 ,3- propanediolcyclic sulfate (PCS), typically in an amount from 0 > to 5 wt%, preferably in an amount of about 2 wt%, and most preferably in an amount of about 4 wt%.

[0106] Preferably, the present invention further includes an electrolyte which comprises one or more sodium-containing salts, and a solvent system which comprises a first component which comprises one or more glyme-based solvents; and a second component which comprises one or more additives selected from the group consisting of sulfur-containing compounds, boron- containing compounds, and surfactants.

[0107] Highly preferably, the one or more sodium-containing salts are selected from sodium tetrafluoroborate (NaBF4), sodium hexafluorophosphate (NaPFe), sodium trifluoromethanesulfonate (CFsNaSOs), sodium bis(fluorosulfonyl)imide (Na(SC>2F)2N), sodium bis(trifluoromethanesulfonyl)imide (C2FeNNaO4S2), sodium bis(oxalate) (NaB(C2O4)2), and sodium-difluoro(oxalato)borate. Most preferably, the one or more sodium-containing salt is sodium tetrafluoroborate (NaBF4).

[0108] It is convenient to express the amount of the one or more sodium-containing salts in terms of its molality of the components in the solvent system; that is, the total number of moles of the one or more sodium-containing salts (i.e. solute), per kilogram of the solvent system (the combined weight of the first component and the second component) - i.e. mol / kg. The total molality of all of the sodium-containing salts is preferably in the range 0.1 mol / kg to 10 mol / kg, and more preferably in the range 0.1 mol / kg to 6 mol / kg, and most preferably in the range 0.1 mol / kg to < 4 mol / kg. Sodium tetrafluoroborate (NaBF4) is typically used at a molality of 1.5 mol / kg.

[0109] Ideally, the first component comprises tetraglyme and / or diglyme. Futher ideally, the first component comprises a mixture of tetraglyme and diglyme. When a mixture of tetraglyme and diglyme is used this would ideally be in the weight ratio 1 to 20: 1 to 20 wt / wt, further preferably in the weight ratio 1 to 10 : 1 to 10 wt / wt, also preferably 1 to 5 : 1 to 5 wt / wt, and most preferably in the weight ratio 1 :1 wt / wt.

[0110] The second compound may comprise one or more additives selected from the group consisting of sulfur-containing compounds in an amount of >0 to <10% by weight of the solvent system; boron-containing compounds in an amount of >0 to <10% by weight of the solvent system; and surfactants in an amount of >0 to < 10% by weight of the solvent system.

[0111] The phase “weight of the solvent system” means the weight of the first component of the solvent system combined with the weight of the second component of the solvent system.

[0112] The second compound may comprise two or more additives selected from the group consisting of sulfur-containing compounds in an amount of >0 to <10% by weight of the solvent system; boron-containing compounds in an amount of >0 to <10% by weight of the solvent system; and surfactants in an amount of >0 to <10% by weight of the solvent system.

[0113] The second compound may comprise three or more additives selected from the group consisting of sulfur-containing compounds in an amount of >0 to <10% by weight of the solvent system; boron-containing compounds in an amount of >0 to <10% by weight of the solvent system; and surfactants in an amount of >0 to <10% by weight of the solvent system.

[0114] Preferably, the second component may comprise three or more additives comprising sulfur- containing compounds in an amount of >0 to <10% by weight of the solvent system, boron- containing compounds in an amount of >0 to <10% by weight of the solvent system, and surfactants in an amount of >0 to < 10% by weight of the solvent system.

[0115] As set out above, the one or more sulfur-containing compounds may include a sulfate such as 1 ,3-propanediolcyclic sulfate (PCS), typically in an amount from 0 > to 5 wt%, preferably in an amount of about 2 wt% or about 4 wt%. The one or more boron-containing compounds may include a borate such as tris(trimethylsilyl) borate (TMSB), typically in an amount from 0 > to 5 wt%, ideally about 5 wt%. And, the one or more surfactants may include a non-ionic block copolymer such as a poloxamer, e.g., P123, typically in an amount from 0 > to 5 wt%, ideally about 1 wt%.

[0116] Most preferably, the three or more additives as a second component of the solvent system may comprise boron-containing compounds (e.g., TMSB) in an amount of about 5% by weight of the solvent system, sulfur-containing compounds (e.g., PCS) in an amount of about 2 to 4% by weight of the solvent system, and surfactants (e.g., poloxamers) in an amount of about 1 % by weight of the solvent system.

[0117] One extremely preferred example of three or more additives as a second component of the solvent system may comprise boron-containing compounds (e.g., TMSB) in an amount of about 5% by weight of the solvent system, sulfur-containing compounds (e.g., PCS) in an amount of about 2% by weight of the solvent system, and surfactants (e.g., poloxamers) in an amount of about 1% by weight of the solvent system. Indeed, this example is particularly preferred because sodium metal cells according to the present invention demonstrate particularly high performance when using this combination of additives. This is particularly the case when these additives are also used in conjunction with cathode active materials including iron-based phosphates such as NFPP.

[0118] A sodium metal cell disclosed herein may also include an anode (negative) electrode that includes an anode current collector and / or a cathode (positive) electrode that includes a cathode current collector. The cathode current collector, the anode current collector, or both the cathode current collector and the anode current collector can be formed from aluminium or aluminium alloy (e.g., an alloy of aluminium and one or more of Mg, Mn, Cr, Zn, Si, Fe, and Ni). Preferably, the anode current collector comprises an aluminium current collector. Highly preferably, both the cathode current collector and the anode current collector comprise an aluminium current collector. Alternatively, copper, magnesium, carbon and tin might also be used as current collector materials. Furthermore, both the cathode or anode current collector could be in the form of thin foils (and if foils, also with one surface rougher than the other), or in the form of porous mesh-type foils.

[0119] Preferably, the anode current collector also includes one or more carbon-containing layers formed on one or more surfaces of the anode current collector prior to an initial first charge cycle of the sodium-ion cell. Such layers can comprise amorphous carbon (e.g., a carbon black, such as TIMCAL Super C65), ideally having a thickness from about 10 Angstrom to about 1000 pm.

[0120] A sodium metal cell disclosed herein may also include a separator located between the cathode and the anode current collector. A polyolefin separator is preferable. Ideally, the separator has a thickness of 5 to 25 pm.

[0121] A sodium metal cell disclosed herein may also have a capacity of greater than 3 mAh.

[0122] An ideal upper cut-off voltage of the sodium metal cell disclosed herein is greater than 3V vs Na / Na+.

[0123] Preferably, the free-standing polymeric substrate is made porous by a “phase inversion” or “phase separation” process. This is discussed in detail below. Ideally, the free-standing polymeric substrate is made without using a liquid pore-forming agent prior to casting.

[0124] The present invention also provides in another aspect an apparatus comprising a sodium metal cell as defined herein. A typical apparatus may include a device such as a battery pack that may have use in either a stationary or a mobile application.

[0125] The present invention also provides in another aspect a method for manufacturing a sodium metal cell as described herein. Therefore, the preferred features which are disclosed with respect to the sodium metal cell as described herein apply equally to a method of manufacturing such sodium metal cell.

[0126] In particular, a preferred aspect of the present invention provides a method for manufacturing a sodium metal cell comprising a cathode electrode and an anode electrode disposed in an opposite face-to-face arrangement with a separator therebetween, in which the anode electrode and the separator are located in contact with opposite sides of a free-standing polymeric substrate, optionally in which the free-standing polymeric substrate comprises an interconnected network of pores, and optionally in which at least some of the pores are present in a bulk region extending between the opposite sides of the free-standing polymeric substrate, and in which the preferred method for manufacturing the sodium metal cell comprises: a. providing a first liquid phase including (a) a polymeric component comprising one or more hydrophobic polymers and optionally one or more hydrophilic polymers; and (b) one or more organic solvents which are miscible with water; b. providing a second liquid phase either (i) comprising water; or in the case that the polymeric component of the first liquid phase comprises one or more optional hydrophilic polymers, (ii) comprising water and one or more surface tension lowering additives; c. casting said first liquid phase onto a surface; d. contacting at least part of said first liquid phase on said surface with said second liquid phase; e. removing said contacted product of step d) from said surface; f. drying said removed product of step e) to provide a free-standing polymeric substrate; and g. assembling a cathode electrode and an anode electrode disposed in an opposite face-to-face arrangement with a separator therebetween, and in which the anode electrode and the separator are located to be in contact with opposite sides of the free-standing polymeric substrate provided at the end of step f), to form the sodium metal cell; and optionally further in which the first liquid phase provided in step a) further includes one or more stabilising additives in the case when the second liquid phase provided in (i) of step b) consists essentially of water.

[0127] Preferably, as described above, the cathode electrode comprises one or more active materials which are selected from one or more of sodium transition metal oxides and polyanionic compounds.

[0128] According to a preferred embodiment of the present invention, by providing the free-standing polymeric substrate with an interconnected network of pores it means that the free-standing polymeric substrate is provided with one or more substantially free paths, which provides a highway to allow homogenous Na+ flux into / out of the anode of the sodium metal cell during charging / discharging. Indeed, this is a key reason why a sodium metal cell according to the present invention result in greater cycling stability when compared to a prior art sodium metal cell.

[0129] Preferably, at least some of these pores are present in a bulk region which extends between opposite sides of the free-standing polymeric substrate. Preferably, at least some of these pores are present in a surface region included in at least one of the opposite sides of the freestanding polymeric substrate. Highly preferably, at least some of these pores are present in a bulk region and a surface region of the free-standing polymeric substrate.

[0130] Step a) of the method of the present invention includes providing a first liquid phase including (a) a polymeric component comprising one or more hydrophobic polymers and optionally one or more hydrophilic polymers; and (b) one or more organic solvents which are miscible with water.

[0131] According to an embodiment of the present invention, the polymeric component of the first liquid phase comprises one or more hydrophobic polymers, such as polyvinylidene fluoride (PVDF). In one embodiment, this may be a fluoropolymer.

[0132] According to an alternative embodiment of the present invention, the polymeric component of the first liquid phase comprises a mixture of one or more hydrophobic polymers and one or more hydrophilic polymers. An ideal mixture includes polyvinylidene fluoride (PVDF) and polyethylene oxide (PEO). Thus, the one or more hydrophilic polymers may include polyethylene oxide (PEO). In one embodiment, this may be a fluoropolymer and a polyether polymer.

[0133] When the first liquid phase comprises a mixture of hydrophobic and hydrophilic polymers, the weight ratio of the hydrophobic polymer to the hydrophilic polymer is in the range 1 :1 to 8:1 , further preferably 2:1 to 6:1 , and particularly preferably 3:1 to 5:1. A ratio in the range 3:1 is especially preferred, particularly when using a mixture of polyvinylidene fluoride (PVDF) and polyethylene oxide (PEO).

[0134] According to an embodiment of the present invention, the first liquid phase may also comprise, a third component, (c), in the form of one or more stabilising additives. Ideally, these are in solid form. As discussed above, preferably, the one or more stabilising additives are selected from one or more of the group consisting of carbon black, nanozeolites, or a combination thereof. That is, the one or more stabilising additives may be carbon black and / or nanozeolites.

[0135] One or more stabilising additives may be added to the first liquid phase. The first liquid phase may include one or more stabilising additives when the second liquid phase comprises water and one or more surface tension lowering additives. Alternatively, the first liquid phase may include one or more stabilising additives when the second liquid phase consists essentially of water.

[0136] When a stabilising additive is incorporated in the first liquid phase, it is convenient to express the total amount of the polymeric component (that is, the amount of the one or more hydrophobic polymers together with any optional one or more hydrophilic polymers) relative to the amount of one or more stabilising additives, as a weight ratio. Preferably, the weight ratio of the polymeric component to the one or more stabilising additives is in the range 1 : 1 to 10: 1 , further preferably 2:1 to 9:1 , and particularly preferably 3:1 to 5:1. A ratio in the range 4:1 is especially preferred.

[0137] As discussed above, the first liquid phase comprises one or more organic solvents which are miscible with water. The purpose of the one or more organic solvents is to solubilise the polymeric component, and disperse any optional one or more stabilising additives. Ideally, the organic solvents include N-Methyl-2-pyrrolidone (NMP) and / or acetonitrile. Ideally, the organic solvent is NMP.

[0138] According to an embodiment of the invention, the one or more organic solvents are miscible with water at room temperature.

[0139] In a preferred embodiment, the polymeric component is provided only with one or more organic solvents such as NMP and / or acetonitrile, ideally NMP. That is, the first liquid phase preferably consists essentially of polymeric component and organic solvent. Indeed, as the first liquid phase is ideally provided as an organic phase, it is preferred that water is not included within the first liquid phase. Therefore, ideally, the first liquid phase is substantially free of water. Indeed, water which is present in the first liquid phase may act as a liquid pore-forming agent prior to casting, which is undesirable in the present invention. Instead, water is introduced later in step b) to provide an aqueous phase as discussed below.

[0140] Preferably, step a) comprises mixing one or more polymeric components with one or more organic solvents such as NMP and / or acetonitrile to provide the first liquid phase. Indeed, such mixing is preferably performed at 25 °C, and ideally not at an elevated temperature such as above 30 °C or 35 °C. Therefore, the first liquid phase is preferably provided at a temperature of no greater than 30 °C or 35 °C. Ideally, it is provided at a temperature from 10 °C to 35 °C. Step b) of the method of the present invention includes providing a second liquid phase either (i) comprising water; or in the case that the first liquid phase comprises one or more optional hydrophilic polymers, (ii) comprising water and one or more surface tension lowering additives.

[0141] As will be appreciated, the second liquid phase is therefore an aqueous phase because it includes water.

[0142] The method of the present invention enables the formation of a free-standing polymeric substrate which comprises an interconnected network of pores in which at least some of the pores are present in a bulk region of the free-standing polymeric substrate. Without wishing to bound by any particular theory, this is achieved by the formation of micelles or a process of phase separation, which after evaporation, leave pores behind in the bulk region of the substrate. Thus, a critical part of the method of the present invention is achieving such successful micelle formation.

[0143] According to an embodiment of the present invention the second liquid phase comprises water.

[0144] Thus, this embodiment may include embodiments that use water alone, or embodiments that use water in combination with one or more surface tension lowering additives, in the case when the polymeric component of the first liquid phase consists essentially of a hydrophobic polymer such as polyvinylidene fluoride (PVDF). That is, where a hydrophilic polymer is not present in the polymeric component of the first liquid phase. Thus, in one embodiment, the polymeric component of the first liquid phase consists essentially of polyvinylidene fluoride (PVDF) and the second liquid phase comprises water and one or more surface tension lowering additives.

[0145] The embodiment in which the second liquid phase comprises water is conveniently used to form micelles in the case that the polymeric component of the first liquid phase comprises one or more hydrophobic polymers. Preferably, this embodiment is used to form micelles in the case that polymeric component of the first liquid phase consists essentially of one or more hydrophobic polymers. For instance, when the polymeric component is only polyvinylidene fluoride (PVDF). Without wishing to bound by any particular theory, this embodiment is best avoided if the first liquid phase comprises one or more hydrophilic polymers, or mixtures of hydrophobic and hydrophilic polymers because micelles cannot be readily formed in these cases without the addition of a surface tension lowering additive to the second liquid phase as discussed below. According to an embodiment of the present invention the second liquid phase consists essentially of water. This is embodiment is typically used in the case that the polymeric component of the first liquid phase consists essentially of one or more hydrophobic polymers. For instance, when the polymeric component is only polyvinylidene fluoride (PVDF). However, without wishing to be bound by any particular theory, when the second liquid phase consists essentially of water and the polymeric component of the first liquid phase consists essentially of one or more hydrophobic polymers (in particular, PVDF) it is a requirement to enable the successful formation of micelles that the first liquid phase provided in step (a) further includes one or more stabilising additives.

[0146] According to an alternative embodiment of the present invention the second liquid phase comprises water and one or more surface tension lowering additives. This embodiment is preferably used in the case that the polymeric component of the first liquid phase comprises one or more optional hydrophilic polymers (including mixtures with one or more hydrophobic polymers). That said, if this embodiment is used when the first liquid phase consists essentially of one or more hydrophobic polymers (in particular, PVDF), then it is possible to avoid using one or more stabilising additives unlike the embodiment described immediately above.

[0147] Ideally, the surface tension lowering additive is an alcohol, preferably with a surface tension of between around 0 to around 0.07 N / m at 25 °C. Ideally, the alcohol includes isopropyl alcohol (I PA). Preferably, alcohol (ideally I PA) is mixed with water in a weight ratio of 1 :2. Preferably, IPA:water = 1 :2 wt / wt. In one embodiment, the alcohol (ideally IPA) is mixed with water in a weight ratio from 3:1 to 1 :5, more preferably 1 :1 to 1 :3.

[0148] To enable the successful formation of pores in the interfacial film when the polymeric component of the first liquid phase comprises one or more optional hydrophilic polymers, a surface tension lowering additive (such as IPA) is added to water. Without wishing to be bound by any particular theory, the addition of the surface tension lowering additive to the water of the second liquid phase lowers the surface tension of the second liquid which helps to avoid the one or more hydrophilic polymers from swelling upon contact in step d). The surface tension lowering additive is also able to take part in the micelle formation process. Thus, micelle formation is achieved.

[0149] Preferably, step b) comprise providing a second liquid phase either (i) comprising water; or in the case that the first liquid phase comprises one or more optional hydrophilic polymers, (ii) mixing water with one or more surface tension lowering additives. In either case, such second liquid phase is preferably provided at 25 °C, and ideally not at an elevated temperature such as above 30 °C, or above 35 °C. Ideally, it is provided at a temperature from 10 °C to 35 °C.

[0150] Step c) of the method of the present invention includes casting said first liquid phase onto a surface.

[0151] Ideally, the surface is a substantially flat surface, which is preferably made of glass. However, surfaces made from plastic, metal, wood, fluorinated polymers such as Teflon etc are equally useful.

[0152] The casting step preferably includes casting (which also encompasses coating) the first liquid phase using known techniques such as doctor blade, slot die, tape casting etc, on a suitable substrate such as glass, Teflon etc. Preferably, the first liquid is cast as a film on the surface.

[0153] Preferably, the first liquid phase is provided at room temperature, and ideally not at an elevated temperature such as above 30 °C, or above 35 °C. Therefore, the first liquid phase is cast when it is at a temperature of no greater than 30 °C (such as 25 °C), or no greater than 35 °C. Ideally, it is cast when it is at a temperature from 10 °C to 35 °C.

[0154] Step d) of the method of the present invention comprises contacting at least part of said first liquid phase on said surface with said second liquid phase. Preferably, the method includes contacting substantially all of said first liquid phase on said surface with said second liquid phase. Preferably, the second liquid phase is provided at 25 °C, and ideally not at an elevated temperature such as above 30 °C, or above 35 °C. Ideally, it is provided at a temperature from 10 °C to 35 °C.

[0155] Preferably, step d) comprises immersing the product of step c) in the second liquid phase (which is typically retained in a container such as a water bath) at room or ambient temperature such as 25 °C for a period of time typically from 30 seconds to 30 minutes. When this step occurs, it is often observed that the product of step d) will mostly delaminate from the substrate.

[0156] In one embodiment, the first liquid phase is contacted with a single solution (i.e. , only one solution) after being cast onto the surface. The single solution in this case is the second liquid phase. That is, the first liquid phase may not be contacted with two or more different solutions, such as an initial immersion in ethanol followed by an immersion in water. Indeed, by using only a single solution the invention is more efficient.

[0157] Optionally, prior to removal, step d) may also include rinsing the contacted product of step d) with one or more rinsing fluids. Such fluids may include, for example water, I PA, ethanol or acetone. Thus, in one embodiment, the first liquid phase is contacted firstly with said second liquid phase, and after immersion, secondly, with one or more fluids for rinsing.

[0158] Step e) of the method of the present invention includes removing said product of step d) from said surface. As explained above, this step may occur in conjunction with step d) as the first liquid phase is contacted with the second liquid phase.

[0159] Alternatively, or additionally, step d) comprises removing the product of step d) by lifting the product of step d) away from said casting surface. Appropriate tools such as glass rods or a spatula may be used to aid the process. That said, the skilled person will understand that it is important to be careful in this step and to avoid excessive force, which might tear the substrate (film). Thus, use of sharp objects is not advisable in this step.

[0160] Step f) of the method of the present invention includes drying said removed product of step e) to provide a free-standing polymeric substrate in accordance with the present invention.

[0161] This step encourages the formation of empty pores from the micelles by the use of evaporation (i.e., drying). Thus, step f) includes evaporation to remove liquids and / or vapours (e.g. liquids such as the second liquid phase). Optional heating and / or a vacuum may be used to aid the process.

[0162] In one preferred embodiment, drying in step f) is achieved by contacting the product of step d) with an environment (ideally, a dry environment) at room or ambient temperature for a period of time, typically between 1 h to 24 h. Evaporation of liquids and / or vapours then occurs.

[0163] Step g) of the method of the present invention includes assembling a cathode electrode and an anode electrode disposed in an opposite face-to-face arrangement with a separator therebetween, in which the anode electrode and the separator are located to be in contact with opposite sides of the free-standing polymeric substrate provided at the end of step f), to form the sodium metal cell. This step typically can either be performed in ambient air, or in moisture controlled- environments (such as those inside an argon-filled or dry air-filled glove box). It will be understood by the skilled person that if moisture-insensitive cathode and anode active materials are used in the full cell, then this step can be performed in ambient air; if moisture sensitive materials are used (such as sodium metal anode), then this step needs to be performed in a moisture-controlled environment. This step involves taking the cathode electrode, separator, free-standing polymeric film and anode electrode and combining these together to form an electrode stack, using any method known in the prior art - these could be via traditional stacking of different layers and securing them via tape or electrode stacking / winding using semi / fully automated lines. This step can be performed in ambient air, or it can be performed in a moisture-controlled air, as mentioned above.

[0164] In an alternative embodiment, one can choose to make ‘half-stacks’ first, especially considering cases where extremely moisture-sensitive sodium metal anodes will be used as the anode electrode. In this case, this step involves taking the cathode electrode, separator and free-standing polymeric film and combining these together to form an electrode half-stack, using any method known in the prior art - these could be via traditional stacking of different layers and securing them via tape or electrode stacking / winding using semi / fully automated lines. The advantage of the half-stack approach is that this step can be performed in ambient air, if the cathode used is sufficiently moisture (and air)-insensitive.

[0165] The present invention also provides in another aspect a sodium metal cell obtained by the process as defined herein.

[0166] BRIEF DESCRIPTION OF THE DRAWINGS

[0167] The present invention will now be described with reference to the following figures in which:

[0168] FIGURE 1A shows a plot of discharge capacity (mAh / gactive cathode) vs cycle number for sodium metal cells that are not according to the present invention;

[0169] FIGURE 1 B shows a plot of discharge capacity (mAh / gactive cathode) vs cycle number for sodium metal cells according to the present invention compared against sodium metal cells that are not according to the present invention; FIGURE 2A shows a photograph of the front side of the sodium metal anode which is facing the cathode (and separator) of cell NB1C2FP230818, after disassembly;

[0170] FIGURE 2B shows a photograph of the front side of the sodium metal anode which is facing the cathode (and separator) of cell NB1C2FP230823, after disassembly;

[0171] FIGURE 2C shows a photograph of the back side of the sodium metal anode which is facing its current collector (thus, the electrochemically inactive side of the sodium metal anode) of cell NB1C2FP230823, after disassembly;

[0172] FIGURE 2D shows a photograph of the interfacial film of cell NB1C2FP230812 (the film between the front side of the sodium metal anode and the separator), after disassembly;

[0173] FIGURE 2E shows a photograph of the front side of the sodium metal anode (facing the interfacial film) of cell NB1C2FP230812, after disassembly.

[0174] FIGURE 3 shows a plot of discharge capacity (mAh / gactive cathode) vs cycle number for two sodium metal cells according to the present invention compared against a sodium metal cell that is not according to the present invention;

[0175] FIGURE 4 shows a plot of discharge capacity (mAh / gactive cathode) vs cycle number for sodium metal cells according to the present invention;

[0176] FIGURE 5 shows a plot of discharge capacity (mAh / gactive cathode) vs cycle number for a sodium metal cell according to the present invention compared against a sodium metal cell that is not according to the present invention;

[0177] FIGURE 6 shows a plot of discharge capacity (mAh / gactive cathode) vs cycle number for sodium metal cells according to the present invention compared against three sodium metal cells that are not according to the present invention;

[0178] FIGURE 7 shows normal (i.e. taken from above the plane) and cross-sectional FESEM images of one free-standing substrate not according to the present invention, and two free-standing substrates according to the present invention; FIGURE 8 shows a plot of discharge capacity (mAh / gactive cathode) vs cycle number for two sodium metal cells according to the present invention compared against a sodium metal cell that is not according to the present invention;

[0179] FIGURE 9 shows a schematic representation of a sodium metal cell including a free-standing polymeric substrate according to the present invention;

[0180] FIGURE 10 shows a schematic representation of a free-standing polymeric substrate according to the present invention to show the ‘bulk region’ and the ‘surface region’;

[0181] FIGURE 11 shows a plot of specific discharge capacity (mAh / g) vs cycle number for two sodium metal cells according to the present invention;

[0182] FIGURE 12 shows a plot of specific discharge capacity (mAh / g) vs cycle number for a sodium metal cell according to the present invention compared against a sodium metal cell that is not according to the present invention;

[0183] FIGURE 13 shows a plot of coulombic efficiency (%) vs cycle number for a sodium metal cell according to the present invention compared against a sodium metal cell that is not according to the present invention;

[0184] FIGURE 14 shows normal (i.e. taken from above the plane) and cross-sectional FESEM images of two free-standing substrates not according to the present invention; and

[0185] FIGURE 15 shows normal (i.e. taken from above the plane) and cross-sectional FESEM images of three free-standing substrates according to the present invention.

[0186] DETAILED DESCRIPTION

[0187] Figure 9 illustrates a free-standing polymeric substrate (e.g., a film) 12 according to the present invention. More particularly, Figure 9 illustrates a sodium metal cell 10 comprising a free-standing polymeric film 12 according to the present invention.

[0188] The sodium metal cell 10 is made up of a positive (cathode) electrode 14 and a negative (anode) electrode 16 which are disposed in an opposite face-to-face arrangement with a separator 18 therebetween. The anode electrode 16 and the separator 18 are positioned to be in contact with opposite sides 20, 22 of the free-standing polymeric film 12.

[0189] The anode electrode 16 is made up of an aluminium current collecting element 24 in contact with a layer of sodium metal 26. The layer of sodium metal 26 is in contact with one 20 of the opposite sides 20, 22 of the free-standing polymeric film 12. The separator 18 is in contact with the other 22 of the opposite sides 20, 22 of the free-standing polymeric film 12.

[0190] The cathode electrode is made up of a coating of the cathode active material, conductive additives and binder upon the aluminium current collector.

[0191] In the sodium metal cell 10, liquid electrolyte is introduced to facilitate transfer of Na+ ions between the cathode and the anode (the liquid electrolyte can achieve this also due to the fact that it can fill the pores between such electrodes).

[0192] Figure 10 schematically illustrates the locations of pores 34, 36, and 38 (i.e. , openings in the film) in the free-standing substrate 12. In particular, pores 34 are shown in a surface region 19 which is a region that is limited to the planar surfaces 20 and 22 of the film 12 that contact the separator 18 and the layer of sodium metal 26 when the film 12 is installed in a sodium metal cell 10. Pores 36, 38 are also shown in a bulk region 40 which is a region of the film 12 that does not contact the separator 18 and / or the layer of sodium metal 26 when the film 12 is installed in a sodium metal cell 10. Thus, this bulk region 40 encompasses pores 36 which are present along the cross-section of the film 12 and / or pores 38 which are internally present in the bulk of the film 12. In particular, pores 36 are examples of pores which may be present in the bulk region 40 because they have an opening in one or more sides which are substantially perpendicular to opposite sides 20, 22. Pores 38 are also examples of pores that may be present in the bulk region 40 because they are internal pores. In this example, the free-standing polymeric film 12 is a cuboid and the pores 38 do not contact any of the six sides of the cuboid.

[0193] Figure 10 also shows the presence of an interconnected network pores 34 and 38. That, is pores 34 and 38 (i.e., openings in the film 12) which are connected together by one or more through-film ‘highways’ 30, 32 which allow for the passage of Na+ flux into / out of the anode electrode 16 of the sodium metal cell 10 during charging / discharging. Without wishing to be bound by any particular theory, it is thought that such highways 30, 32 are wet with a liquid electrolyte when the film is installed in a sodium metal cell 10, facilitating and regulating good Na+ flux through the film 12 and into / out of the Na metal anode 16.

[0194] It is envisaged that several of such through-film highways 30, 32 exist throughout the bulk region 40 of the film 12. These highways 30, 32 could be oriented in several directions and between different starting and end points, such as from the planar surfaces 20 to 22 and vice versa (highways marked as 30), or highways from planar surfaces 20 and / or 22 to the crosssections 28 of the film 12 and vice-versa (highways marked as 32). Furthermore, it is also thought that there will be several regions where highways 30 and 32 intersect inside the bulk 40 of the film, in internal pore cavities marked as 38 - such pore cavities are expected to be sufficiently numerous inside the bulk of the film 12, and are expected to help facilitate Na+ transport through the film.

[0195] EXAMPLES

[0196] The sodium metal cells under investigation were prepared using the following general procedure:

[0197] General procedure(s) to make an interfacial film

[0198] Method 0a Free-standing film (hydrophobic) made using no additives not according to the present invention.

[0199] The interfacial film was prepared via a slurry casting and delamination technique. It involved first making a mixture of polymer (PVDF only) with solvent (such as NMP), slurry casting this on a flat glass plate, soaking this casted polymer film (in the wet state) into a water bath (containing only water), followed by delamination (peeling off the layer), and then finally placing the film for drying at room temperature in ambient air.

[0200] Free-standing film (hydrophilic and hydrophobic) made not according to the present invention.

[0201] The interfacial film was prepared via a slurry casting and delamination technique. It involved first making a mixture of polymer (PVDF and PEO, as indicated) with solvent (such as NMP), slurry casting this on a flat glass plate, soaking this cast polymer film (in the wet state) into a water bath (containing only water), followed by delamination (peeling off the layer), and then finally placing the film for drying at room temperature in ambient air.

[0202] Method 0c - Free-standing film (hydrophilic and hydrophobic) made not according to the present invention.

[0203] The interfacial film was prepared via a slurry casting and delamination technique. It involved first making a mixture of polymer (PVDF and PEO, as indicated) with solvent, followed by adding a stabilising additive to the mixture (such as C65 powder and / or 5 A nanozeolite powder), slurry casting this on a flat glass plate, soaking this casted polymer film (in the wet state) into a water bath (containing water only) followed by delamination (peeling off the layer), and then finally placing the film for drying at room temperature in ambient air.

[0204] Method 1 - Free-standing film (hydrophobic) made according to the present invention.

[0205] The interfacial film was prepared via a slurry casting and delamination technique. It involved first making a mixture of a hydrophobic polymer (such as PVDF) with solvent (such as NMP, although other suitable solvents can be used here as well, such as acetonitrile), followed by adding a stabilizing additive to the mixture (such as C65 powder and / or 5 A nanozeolite powder), slurry casting this on a flat glass plate, soaking this casted polymer film (in the wet state) into a water bath, followed by delamination (peeling off the layer), and then finally placing the film for drying at room temperature in ambient air.

[0206] Method 2 - Free-standing film (hydrophobic) made according to the present invention.

[0207] The interfacial film was prepared via a slurry casting and delamination technique. It involved first making a mixture of hydrophobic polymer (such as PVDF) with solvent (such as NMP), followed by adding a stabilizing additive to the mixture (C65 powder and / or nanozeolite powder) slurry casting this on a flat glass plate, soaking this casted polymer film (in the wet state) into a water bath (containing water and at least one lower surface tension solvent - i.e. a ‘surface tension lowering additive’ (such as I PA)) serving as a transitory stabilizing additive (for example, with a ratio such as water: I PA = 2:1 wt / wt), followed by delamination (peeling off the layer), and then finally placing the film for drying at room temperature in ambient air.

[0208] Method 3 - Free-standing film (hydrophilic and hydrophobic) made according to the present invention. The interfacial film was prepared via a slurry casting and delamination technique. It involved first making a mixture containing hydrophobic polymers (such as PVDF) and hydrophilic polymers (such as PEO) with solvent (such as NMP, although other suitable solvents can be used here as well, such as acetonitrile), followed by adding a stabilizing additive to the mixture (such as C65 powder and / or 5 A nanozeolite powder), slurry casting this on a flat glass plate, soaking this casted polymer film (in the wet state) into a water bath (containing water and at least one lower surface tension solvent - i.e. a ‘surface tension lowering additive’ (such as I PA) serving as a transitory stabilizing additive (for example, with a ratio such as water: I PA = 2: 1 wt / wt), followed by delamination (peeling off the layer), and then finally placing the film for drying at room temperature in ambient air.

[0209] Method 4 - Free-standing film (hydrophobic) made according to the present invention:

[0210] The interfacial film was prepared via a slurry casting and delamination technique. It involved first making a mixture of hydrophobic polymers (such as PVDF) with solvent (such as NMP, although other suitable solvents can be used here as well, such as acetonitrile), slurry casting this on a flat glass plate, soaking this casted polymer film (in the wet state) into a water bath (containing water and at least one lower surface tension solvent - i.e. a ‘surface tension lowering additive’ (such as I PA) serving as a transitory stabilizing additive (for example, with a ratio such as water: I PA = 2:1 wt / wt), followed by delamination (peeling off the layer), and then finally placing the film for drying at room temperature in ambient air.

[0211] The precise composition of each of the films as investigated, is detailed in Table 1 below:

[0212] TABLE 1

[0213] The film thickness of FC Oa (not according to the present invention) was approximately 13 pm; FC Ob (not according to the present invention) was approximately typically 1 pm; FC Oc (not according to the present invention) was approximately typically 9 pm; FC 1 was approximately typically 9 pm; the film thickness of FC 2 was approximately typically 6 - 7 pm; the film thickness of FC 3 was approximately typically 9 pm; the film thickness of FC 4 was approximately typically 9 pm; the film thickness of FC 5 was approximately typically 8 pm; and the film thickness of FC 6 was approximately typically 9 pm. All of the films (FC 1 to FC 6) used NMP as the solvent.

[0214] Abbreviations used:

[0215] PVDF = Polyvinylidene fluoride, PEO = polyethylene glycol, C65 = Timcal Super C65 Conductive Carbon Black Powder, NZ 5 A = Nanozeolite 5 A, Tetraglyme = Tetraethylene glycol dimethyl ether, Diglyme = diethylene glycol dimethyl ether, NaBF4 = sodium tetrafluoroborate, PCS = 1,3-propanediolcyclic sulfate, P123 = Poloxamer (Pluronic) P123, TMSB = Tris(trimethylsilyl) borate.

[0216] General procedure for electrolyte preparation The electrolyte was prepared as follows.

[0217] Appropriate amounts of solvents for a desired solvent system were mixed together in a glass vial in the desired weight ratio. Where appropriate, one or more performance additives (e.g., one or more surfactants, one or more boron-containing compounds, one or more sulfur- containing compounds, and combinations thereof) were also added to prepare the solvent system. To dry the solvent-mix, activated 4 A molecular sieves were added and the solvent mix was allowed to dry at last for 18 h.

[0218] In a separate glass bottle with a magnetic pellet, required weight of NaBF4 salt was added, followed by the required quantity of the required solvent mix. The salts and solvent mix were magnetically stirred at least for 18 h, whereupon the resultant electrolyte was ready for use.

[0219] The electrolyte used throughout this disclosure was ‘TEL 85k’: 1.5 m NaBF4 in (Tetragylme: Diglyme = 1 :1 wt / wt with 2 wt% PCS, 1 wt% P123 and 5 wt% TMSB); or ‘TEL 85L’: 1.5 m NaBF4 in (Tetragylme: Diglyme = 1 :1 wt / wt with 4 wt% PCS, 1 wt% P123 and 5 wt% TMSB). In this regard, the weight % is with respect to the total solvent system weight (solvents and additives) and does not include the salt weight.

[0220] General procedure for cathode preparation

[0221] The cathode electrode was prepared by solvent-casting a slurry of an active material, conductive carbon, binder and solvent.

[0222] In some examples (as indicated), the cathode comprised O3 / P2 Nickelate oxide material as cathode active material, PVDF as binder and C65 as conductive additives, in 92:3:5 wt ratio, coated on C-coated current collectors. Hereinafter referred to as ‘cathode formulation A’.

[0223] In other examples (as indicated), the cathode comprised NasX^PCUh / C (NVP / C) as the active material (NVP with 3 wt% in-situ carbon - i.e., carbon-coated NVP): the weight ratio NVP / C to PVDF binder to C65 carbon additives were 90:5:5 wt / wt. Herein after referred to as ‘cathode formulation B’. NVP / C was purchased from MTI.

[0224] In other examples (as indicated), the cathode comprised carbon-coated Na4Fe3(PO4)2(P2O?) (referred as ‘NFPP / C’ henceforth) available commercially from TOB New Energy. The wt% of the carbon-coating in NFPP / C was 1 wt%. Two different formulations using NFPP / C were adopted.

[0225] NFPP / C Electrode Formulation 1 (hereinafter referred to as ‘cathode formulation CT):

[0226] The weight ratio of NFPP / C: PVDF: C65 Carbon Black: KB (Ketjen Black) was 90:4:5.8:0.2 wt / wt. Note that KB is a form of carbon black.

[0227] NFPP / C Electrode Formulation 2 (hereinafter referred to as ‘cathode formulation C2’):

[0228] The weight ratio of NFPP / C: PVDF: C65 Carbon Black: CNTs (Carbon Nanotubes) was 90:4:5.8:0.2 wt / wt.

[0229] General procedure for ‘half electrode stack'

[0230] Outside a glove box, ‘half’ electrode stacks were fabricated with the following configuration:

[0231] Cathode (as prepared above) | separator (25 pm polypropylene separator used in all examples) | film (as prepared above)

[0232] This ‘half electrode stack’ was placed for drying overnight in dynamic vacuum at a suitable temperature (such as 60 °C), before being brought into the glove box.

[0233] General procedure for anode preparation and cell construction

[0234] The anode comprised Na metal film on a current collector polymer film: to achieve this, a lump of sodium metal was rolled flat inside an Argon filled glove box, cut in size to the desired dimensions, and then placed on top of an Al-based current collector. This Na metal was then combined with the ‘half electrode stack’, to result in the following stack configuration:

[0235] Al-based current collector | Na metal layer | film | separator | cathode

[0236] The above stack configuration was then placed inside a pouch cell and then, the appropriate amount of liquid electrolyte was added, before then sealing the pouch cell inside the glove box. The sealed pouch cell was brought out of the glove box, placed between clamps to achieve a desired pressure and internal resistance, incubated for 9-24 h at 30 °C, then placed for cycling. General procedure for measuring pore width using FESEM

[0237] A free-standing film was mounted on a suitable FESEM sample holder on top of carbon double-sided tape. The sample was then sputter coated by an electronically conducting material, such as carbon, platinum or gold - to ensure that the sample does not get ‘charged’ once it's exposed to the incoming high-energy electrons from the FESEM (this is because accumulation of electrons on non-electronically-conducting samples results in very poor image guality in FESEM).

[0238] The sputtered sample was then mounted inside the FESEM’s transfer chamber before being inserted into the main chamber where it was pumped to a low pressure. The FESEM's electron gun was then fired up to a high voltage in secondary electron mode ( the FESEM images used in the present disclosure were acguired at 20 kV).

[0239] After achieving good focus, images were taken which were then analysed via software capable of annotating and calculating the length scales of different features on the image (the software package “Imaged” was used for the analyses, in the present work, which is available free of charge here:

[0240] During the pore-size analyses, after calibrating the software’s length scale to the Scale Bar mentioned at the bottom right of each image, one can use the measuring tool to click at the start of a pore, drag the measuring tool till the end of the pore and then click again - this will then determines the width of each pore seen in the image (as the measuring tool was already previously calibrated via the Scale Bar). In any image, pores of varying widths / diameters were analysed, to get a good estimation of the ranges of pore widths / diameters observed and these are shown in the figures.

[0241] General procedure for measuring film thickness

[0242] The film thickness was measured via a digital micrometer, obtained from Mitutoyo, with a resolution of 1 pm. Each film’s thickness was measured various times from different regions in the film, and the film thickness was found to be homogenous. To complement the slight variations in thickness that might arise due to differences in slight pressure application whilst measuring the film thickness via the micrometer, cross-sectional images of the films were also obtained, via an optical microscope (obtained from Leica). To prepare such cross-sectional images, the samples were cut by hand with sharp scissors, and then made to stick on a firm double-sided tape. Since hand-cutting with scissors might induce warping of edges (since these polymer films are not hard), twelve measurements were obtained on each film, from different locations of the film, to result in a statistically-reliable dataset with low standard deviations. Table 2 shows thickness values obtained correlations seen (shown below in Table 2). As can be seen from Table 2, the thickness values obtained from the digital micrometer were largely consistent with those obtained from the optical microscope (the value obtained from the digital micrometer were generally 1-3 pm lower than those measured from the optical microscope - this is not surprising as the former method required a slight pressure on the sample during measurement, whilst the latter method induced no externally applied pressure, during measurement).

[0243] It should also be stated that the thickness values obtained from the micrometer and the optical microscope were consistent with the thickness values gleaned from cross-sectional FESEM images of the films.

[0244] TABLE 2

[0245] Cell Testing

[0246] The cells were tested using Constant Current (Galvanostatic) Cycling techniques. Generally speaking, the cells were first charged via the constant current (CC) mode to a pre-defined maximum voltage limit. Afterwards, the cell was made to undergo a constant voltage (CV) step at that maximum voltage limit, to either a pre-defined time or this CV step was made to last until the current dropped to a pre-defined value, as indicated in the examples. The discharge process was conducted at CC-mode to the lower cut-off voltage.

[0247] A commercial battery cycler from MTI Inc. (Richmond, CA, USA) or Maccor (Tulsa, OK, USA) was used. On charge, Na ions are extracted from the cathode active material and are plated onto the Na metal anode. During discharge, Na ions are stripped from the Na metal anode and re-inserted into the cathode active material.

[0248] All cells were subject to cycling experiments at charge rates such as ±1C (this can also be stated as ±0 / 1), ±0 / 5, ±0 / 10 or +C / 3,-1C (or other rates, as mentioned for each experiment).

[0249] Alternate Embodiment

[0250] In alternate embodiments (dubbed the ‘Cell 1 , Cell 1a’ approach), pouch cells were fabricated by electroplating sodium onto what were otherwise “anode-free cells”. These Na metal anode -based pouch cells were fabricated as follows:

[0251] First, a pouch cell was fabricated in the glove box with just the carbon coated current collector foil (such as En Safe 92) as the anode - the cathode, electrolyte and separator were the same as mentioned above. Thus, this ‘Cell T can be regarded as an ‘anode-free cell’. After sealing the cell inside the glove box, this Cell 1 was brought out of the glove box, clamped, incubated and then made to charge to a pre-defined voltage via CC method. By controlling the mass loading of the cathode and the voltage, an exact amount of Na could be plated on the current collector (for example 0.3 mAh / cm2 or 1.5 mAh / cm2, which equates to ~2.7 pm or 13 pm of plated sodium, respectively).

[0252] Cell 1 was brought back into the glove box. The cathode from Cell 1 was then removed and replaced by a fresh cathode and electrolyte. This Cell 1a was then sealed inside the glove box - the subsequent steps were similar to the ones outlined above.

[0253] The Cell 1 , Cell 1a approach enabled the inventors to fabricate Na metal anode pouch cells with a much thinner layer of plated sodium, than the previous embodiment. This approach can also be used with interfacial films, as detailed in Experiment 4. Experiment 1 : Na Metal Anode Pouch Cells with Interfacial Films

[0254] TABLE 3

[0255] * = control cell.

[0256] Figure 1a shows long-term cycling of Na metal anode pouch cells containing no interfacial films, whilst Figure 1b shows corresponding long-term cycling of Na metal anode pouch cells containing interfacial films FC1 or FC2. A summary of the cells studied is shown in Table 3 as set out above. The cells in Figure 1a and Figure 1b and shown in Table 3 were prepared by using ‘cathode formulation A’.

[0257] For ease of comparison, Figure 1b also includes two control cells (with no interfacial films) from Figure 1a (NB1C2FP230823 - cycled between 3.9-2.5 V; and NB1C2FP230824 - cycled between 3.7 - 2.5 V).

[0258] The cells were cycled either to 3.9 V, or 3.7 V, as indicated (these Na metal anode based pouch cells were cycled for 1 cycle at 3.9 - 2.5 V at ±C / 10, followed by different C rates, as indicated).

[0259] The following conclusions can be drawn from Figure 1a and Figure 1b:

[0260] • Figure 1a firstly shows that, without an interfacial film, it is very difficult to secure repeatable performance from Na metal anode-based pouch cells: four out of the five cells shown in the figure failed early; and

[0261] • Figure 1b secondly shows that under two different voltages and for two different formulations (i.e., FC 1 - NB1C2FP230811, NB1C2FP230812; FC 2 - NB1C2FP230903, NB1C2FP230913), the film containing cells outperform the cells which don’t contain films.

[0262] In particular:

[0263] • The Na metal anode pouch cells of the present invention using TEL 85k (NB1C2FP230811 and NB1C2FP230812) delivered higher capacities and better cycling stabilities, over the respective control cells (2EAFC230612, NB1C2FP230814, NB1C2FP230823 and NB1C2FP230818 for the 3.9 V charged cells, and NB1C2FP230824 for the 3.7 V charged cell). • With particular reference to Figure 1 b, it can be seen that the highest performing control cells (from Figure 1a) could only deliver cycles lives not exceeding 188 cycles (to 20 % capacity fade), irrespective of the voltage window (this is cell NB1C2FP230823). This translates to a capacity fade rate of 0.106 % / cycle (this metric - capacity fade rate / cycle - can be used as a gauge to study cycling stability: the lower the value of this metric, the more stable the cycling of the cell).

[0264] • In contrast, cells made with FC 1 film, cell NB1C2FP230811 , cycled between 3.9 - 2.5 V, demonstrated just 20 % capacity fade only after a much longer 466 cycles translating to a capacity fade rate of 0.043 % / cycle (and 720 cycles to 30 % capacity fade), whilst cell NB1C2FP230812, cycled between 3.7 - 2.5 V, demonstrated cycle life of 297 cycles to 20% capacity fade translating to a capacity fade rate of 0.067 % / cycle, or 448 cycles to 30% capacity fade.

[0265] • With reference to Figure 1b, it can also be seen that a cell made with FC 2 film, cell NB1C2FP230903, demonstrated 20 % capacity fade only after Cycle 612 translating to a capacity fade rate of 0.033 % / cycle, effectively more than tripling the cycle life with respect to the control cell, NB1C2FP230823.

[0266] • Furthermore, looking at cycling data of cell NB1C2FP230913 cycled to 3.7 - 2.5 V and using FC 2 film, but using an alternative electrolyte, TEL 85L, it can be seen that the cell demonstrated very stable cycling, showing a capacity fade of just 15.3 % after 624 cycles translating to a capacity fade rate of just 0.024 % / cycle.

[0267] Further conclusions can also be made with reference to Figures 2A to 2E. In particular, Figures 2A to 2E compares disassembly pictures of some of the cells shown in Figure 1 , to try to explore the role of the interfacial film. The cells were discharged to 2.5 V prior to disassembly inside the Ar-filled glove box.

[0268] It can be seen that the two control cells in Fig 1a and Fig 1b (i.e. NB1C2FP230818 and NB1C2FP230823, that is without an interfacial film), showed an inhomogeneous surface of the Na metal anode on the side facing the cathode (the electrochemically active side) - such inhomogeneity is a classic signature of dendritic sodium plating / stripping. This can be seen from Figure 2A which shows the side facing the cathode of cell NB1C2FP230818, and Figure 2B which similarly shows the side facing the cathode of cell NB1C2FP230823.

[0269] It can also be seen from Figure 2C that the backside of the Na metal anode layer used in cell NB1C2FP230823, which was electrochemically inactive (the side facing the current collector and not the cathode), was smooth, proving that the rough surface of the electrochemically active side was not due to some artifact, but because of dendritic plating / stripping.

[0270] The cell according to the present invention, NB1C2FP230812, was disassembled after 448 cycles. The disassembly pictures of Figures 2D and 2E revealed two crucial points:

[0271] 1) There was no sodium deposited on the surface of the black C65+PVDF film facing towards the cathode as shown by Figure 2D, helping to prove that Na+ passes through this interfacial film and deposits on the underlying Na metal anode; and

[0272] 2) The surface of the underlying Na metal layer (of the side facing the cathode of the cell) was found to be homogenous (visually devoid of any significant surface features) as shown by Figure 2E, confirming that the interfacial films supports non-dendritic Na plating / stripping on the underlying Na metal anode.

[0273] Experiment 2: Sodium metal anode based pouch cell fabricated according to the alternate embodiment at 100-150 mAh Pouch Cell Level

[0274] TABLE 4

[0275] = control cell.

[0276] Figure 3 shows three Na metal anode based pouch cells with a capacity greater than an order of magnitude (150 mAh) with respect to the pouch cells shown in Experiment 1 (around 6 mAh). Cell FPC230928 is the control sample without interfacial film. Cell FPC230938 and cell FPC231088 both have interfacial films according to FC 1 , with a film thickness of approximately ~9 pm between the Na metal anode and the separator. The studied calls are set out in Table 4. From Figure 3, it can be seen that both FPC230938 and FPC231088 showed higher capacity after 175 cycles (60.6 mAh / g and 70.8 mAh / g) relative to the control cell, FPC230928 (45.7 mAh / g), further proving the beneficial effects of the interfacial film towards supporting more stable cycling of sodium metal anode based cells.

[0277] Experiment 3: Effect of interfacial film thickness

[0278] In this experiment the effect of the thickness of the interfacial film was investigated with respect to the specific energy of a cell. Indeed, Table 5 compares the modelled specific energy (Wh / kg) of two cells with the following configuration:

[0279] Al foil | 2 mAh / cm2plated Na | FC 1 , with different film thicknesses of approximately ~9 pm or ~50 pm | 16 pm polypropylene separator | O3 / P2 Nickelate Oxide Cathode.

[0280] In the modelling, values are shown for the following cycling conditions: 4 - 1 V at ±C / 10. The modelling has been done from capacity and voltage values derived from an actual 150 mAh Na metal anode pouch cell, but scaled up to the 32 Ah scale. In the modelling, it is also assumed that the final capacity delivered by both cells is the same (in mAh / g for example) - the only difference in resultant Wh / kg of the two cells would then come from the extra electrolyte absorbed by the thicker interfacial film. It can be seen from Table 5, that an interfacial film of 50 pm will lead to 8 % lower Wh / kg.

[0281] TABLE 5

[0282] The electrochemical performance of interfacial films with different thicknesses was also confirmed with reference to Figure 4 and Table 6 below.

[0283] TABLE 6

[0284] Figure 4 shows two different thicknesses of FC 2: a) 6-7 m (cell NB1C2FP230903, whose performance was also shown in Figure 1 b); and b) 24-28 pm (cell NB1C2FP231012).

[0285] Apart from the differences in film thickness, these cells were identical (including using the same electrolyte loading) and experienced the same cycling protocol.

[0286] It can be seen from Figure 4 that the 24-28 pm film cell showed lower ~8 mAh / g lower capacity than the 6-7 pm cell. This indicates that, not only would the resultant Wh / kg of the cell decrease as the film thickness increases, but inventors have found that the effective capacity of the cell can also decrease. Thus, there exists an optimum range of interfacial film thickness, keeping in mind performance and resultant cell weight (which influences a cell’s Wh / kg).

[0287] Experiment 4: Different cathode materials

[0288] This experiment investigates the use of an interfacial film with an alternative cathode material, sodium vanadium phosphate (NVP) as shown in Table 7.

[0289] TABLE 7

[0290] = control cell.

[0291] Figure 5 shows the cycling performance of two cells:

[0292] 1) NB1C2FP230810: Fabricated without an interfacial film.

[0293] 2) NB1C2FP231007: fabricated with a ~9 pm FC 1 interfacial film. It can be seen that the cell fabricated according to the present invention, cell NB1C2FP231007, demonstrates greatly improved cycling stability than the control cell, without any interfacial film (cell NB1C2FP230810). Specifically, the latter cell demonstrated a capacity retention of just 74% (with respect to the 2ndcycle capacity) after 92 cycles, whilst the former cell demonstrated a much higher capacity retention (86.9 % with respect to its 2ndcycle), after a much longer 298 cycles.

[0294] Experiment 5: Different Types of Films

[0295] This experiment investigates different types of interfacial films as set out in Table 8.

[0296] TABLE 8

[0297] = control cell. Figure 6 shows the cycling performance of cells indicated in Table 8:

[0298] 1) NB1C2FP230823: Fabricated without an interfacial film (control cell, also shown in Figure 1).

[0299] 2) NB1C2FP231113: Fabricated with a ~ 13 pm FC 0a interfacial film (not according to the present invention).

[0300] 3) NB1C2FP231112: Fabricated with a ~ 1 pm FC Ob interfacial film (not according to the present invention).

[0301] 4) NB1C2FP231108: Fabricated with a ~ 9 pm FC 0c interfacial film (not according to the present invention).

[0302] 5) NB1C2FP230811 : fabricated with a ~9 pm FC 1 interfacial film (also shown in Figure

[0303] 1).

[0304] 6) NB1C2FP231027: fabricated with a ~9 pm FC 3 interfacial film.

[0305] 7) NB1C2FP231110: fabricated with a ~9 pm FC 4 interfacial film.

[0306] 8) NB1C2FP231202: fabricated with a ~8 pm FC 5 interfacial film.

[0307] 9) NB1C2FP231114: fabricated with a ~9 pm FC 6 interfacial film.

[0308] From Figure 6, it can be seen that all cells made with interfacial films according to the present invention delivered higher capacities and capacity retentions at Cycle 100, and lower capacity fade rates, with respect to the first cycle at +C / 3.-1C rate (Cycle 2 of the respective cells), than the control cell and those cells made with films not according to the present invention. Specifically:

[0309] 1) NB1C2FP230823 (not of the present invention) delivered just 63.9 mAh / g in Cycle 100 with a 14 % capacity fade. This translates to a capacity fade rate of 0.140 % / cycle over 100 cycles.

[0310] 2) NB1C2FP231113 (not of the present invention) reached 20 % capacity fade (55.6 mAh / g) in just 66 cycles. This translates to a capacity fade rate of 0.303 % / cycle over 66 cycles.

[0311] 3) NB1C2FP231112 (not of the present invention) demonstrated very erratic and unstable cycling, and reached 20 % capacity fade (49.7 mAh / g) in just 43 cycles. This translates to a capacity fade rate of 0.465 % / cycle over 43 cycles.

[0312] 4) NB1C2FP231108 (not of the present invention) demonstrated erratic and unstable cycling, essentially failing after 80 cycles, at which point it delivered 55 mAh / g with a capacity fade of 17.66 %. This translates to a capacity fade rate of 0.221 % / cycle over 80 cycles, followed by cell failure. 5) NB1C2FP230811 (of the present invention) delivered 76.4 mAh / g in Cycle 100 with a

[0313] 4.9 % capacity fade. This translates to a capacity fade rate of 0.049 % / cycle over 100 cycles.

[0314] 6) NB1C2FP231027 (of the present invention) delivered 69.5 mAh / g in Cycle 100 with a

[0315] 8.9 % capacity fade. This translates to a capacity fade rate of 0.089 % / cycle over 100 cycles.

[0316] 7) NB1C2FP231110 (of the present invention) delivered 75.2 mAh / g in Cycle 100 with a 4.4 % capacity fade. This translates to a capacity fade rate of 0.044 % / cycle over 100 cycles.

[0317] 8) NB1C2FP231202 (of the present invention) delivered 66 mAh / g in Cycle 100 with a 12.7 % capacity fade. This translates to a capacity fade rate of 0.127 % / cycle over 100 cycles.

[0318] 9) NB1C2FP231114 (of the present invention) delivered 66.9 mAh / g in Cycle 100 with a 3.8 % capacity fade. This translates to a capacity fade rate of 0.038 % / cycle over 100 cycles.

[0319] To provide further understanding of how the free-standing polymeric films work, Figure 7 (top row) shows FESEM images of the surface of the films parallel to the surface on which it was cast. Similarly, this is shown in Figures 14 and 15. Figure 7 (bottom row) also shows FESEM images of cross sections at right angles to this surface (i.e., the thickness of the films). Similarly, this is shown in Figures 14 and 15.

[0320] Whilst cutting the relatively soft films with scissors during sample preparation for cross- sectional FESEM, slight blurring of edges can occur and wherever such blurring of edges occurred, line markers have been placed to identify the upper and lower boundaries of the cross sections. Also, since this film was mounted on carbon tape, in some samples, line markers identifying the upper level of the carbon tape have also been shown in Figure 7.

[0321] The following information can be gleaned from Figure 7 and 14:

[0322] Film FC 0c (not according to the present invention): this film, comprised of C65:PVDF:PEO = 2:6:2 wt / wt in water, showed some surface porosity (pore widths between ~1 to 10 pm), but no through-film porosity at all. With reference to this film’s electrochemical cycling shown in Figure 6, it is also not surprising that cell NB1C2FP231108 showed noticeably worse performance than the no film (control) cell, NB1C2FP230823.

[0323] The following information can be gleaned from Figure 7 and 15:

[0324] Film FC 1 (according to the present invention): this film, comprised of C65:PVDF = 1 :4 wt / wt in water, showed well-ordered surface porosity (pore widths between -5 to 50 pm), and crucially, discernible through-film porosity (pore widths in its thickness being of the order of magnitude around 200-300 nm to 2-4 pm). Due to this interconnected pore network, the electrochemical performance of cell NB1C2FP230811 using this film, was significantly better than that of the control cell without any film, cell NB1C2FP230823.

[0325] The following information can be gleaned from Figure 7 and 15:

[0326] Film FC 3 (according to the present invention): this film, comprised of C65:PVDF = 1 :4 wt / wt in water: I PA = 2:1 wt / wt (I PA serving as the role of the transitory stabilizing additive), showed well-ordered surface porosity with noticeably smaller pore widths than Film FC 1 (pore widths largely between sub-micron to 5 pm), and crucially, discernible through-film porosity (pore widths in its thickness being around sub-micron size to -7 pm). Due to this interconnected pore network, the electrochemical performance of cell NB1C2FP231027 using this film, was significantly better than that of the control cell without any film, cell NB1C2FP230823.

[0327] The following information can be gleaned from Figure 14:

[0328] Film FC 0a (not according to the present invention): this film, comprised of 100% PVDF film cast in water, showed a good degree of surface porosity (large pore widths between -10 to 70 pm), but no through-film porosity at all. With reference to this film’s electrochemical cycling shown in Figure 6, it is also not surprising that cell NB1C2FP231113 showed noticeably worse performance than the no film (control) cell, NB1C2FP230823.

[0329] The following information can be gleaned from Figure 15:

[0330] Film FC 2 (according to the present invention): this film, comprised of NZ 5 A:PVDF = 1 :4 wt / wt in water, showed well-ordered surface porosity (pore widths in the sub-micron range, such as between ~0.2 to 1 pm), and crucially, discernible through-film porosity (pore widths in its thickness spanning from sub-micron range to pore widths as large as -7.4 pm). The electrochemical performance of film FC 2 with respect to the control sample (without any film) has been detailed in the next experiment.

[0331] Experiment 7: Enhanced Performance of Films vs Separators of Similar Total Thickness

[0332] The purpose of this experiment was to prove that, given a fixed total separation between the cathode and the Na metal anode (this separation is due to either a traditional polypropylene - PP - separator or a combination of PP separator and interfacial film), the cells with the interfacial film outperformed those with just the PP separator, with an equivalent separation. This has been clarified in Table 9.

[0333] TABLE 9

[0334] = control cell.

[0335] From Table 9 above, it can be seen that all in three cells, the cathode was separated from the Na metal anode by around ~25 pm distance. The difference in the three cells was the makeup of the materials used to result in this displacement.

[0336] From Figure 8, it can be seen that the two cells made with interfacial films delivered higher capacities and lower capacity fade rates, with respect to the first cycle at +C / 3.-1C rate (Cycle 2 of the respective cells). Specifically:

[0337] 1) NB1C2FP230823 (not of the present invention) utilised just a 25 pm PP separator between the cathode and the Na metal anode. This cell showed 20 % capacity fade after just 188 cycles, translating to a capacity fade rate of 0.106 % / cycle. 2) NB1C2FP231120 utilised a 16 m PP separator in combination with a 9 pm FC 1 interfacial film, to result in an effective distance between the cathode and the Na metal anode of 25 pm, which is exactly the same as that of the control NB1C2FP230823 cell. This NB1C2FP231120 cell showed 14.72 % capacity fade after 301 cycles, translating to a much lower capacity fade rate of 0.049 % / cycle with respect to the control NB1C2FP230823 cell.

[0338] 3) NB1C2FP231121 utilised a 16 pm PP separator in combination with a 7 pm FC 2 interfacial film, to result in an effective distance between the cathode and the Na metal anode of 23 pm, which is similar as that of the control NB1C2FP230823 cell. This NB1C2FP231121 cell showed 26.6 % capacity fade after 408 cycles, translating to a much lower capacity fade rate of 0.065 % / cycle with respect to the control NB1C2FP230823 cell.

[0339] Experiment 8: Investigating the State- of- Matter of the Films

[0340] The purpose of this experiment was to investigate whether any of the solvents used in the first liquid phase (for example, NMP) or the second liquid phase (for example, water or IPA) were actually retained in the final films. For this purpose, Loss of Ignition (LOI) experiments were performed on Film FC 1 , as shown in Table 10 below. Film FC 1 used only PVDF as the polymer. It is well known that PVDF’s decomposition temperature is around 350 °C, NMP’s boiling point is 202 °C and water’s boiling point is 100 °C. The film FC 1 was fired to 300 °C in air for 1.5 h, and its weight before and after firing was recorded. Furthermore, the weight after 10 min of firing was also recorded (during this 10 min time duration, the film was exposed to ambient air). The results are shown below in Table 10.

[0341] TABLE 10

[0342] Table 10 above shows that after firing to 300 °C for 1.5 h in air, the film FC 1 lost ~7.5 % of its weight, but upon measuring the weight of this film just 10 min after firing, it was observed that the film’s weight was essentially the same as before firing. This indicates that the initial weight loss was only due to the surface adsorbed moisture in the films and that there was no presence of NMP or water in the make-up of the films. In other words, it has been proven, beyond any doubt, that the state- of- matter of the films is as shown in Table 1 .

[0343] Experiment 9: Long-term cycling using a cathode comprising NFPP / C.

[0344] This experiment investigates the use of an interfacial film with an alternative cathode material, NFPP / C in a long-term cycling experiment as shown in Table 11.

[0345] TABLE 11

[0346] Figure 11 shows the cycling performance of two cells (F2400361 ; F2400362) fabricated with a 6-7 pm FC 2 interfacial film. Figure 11 shows the post-formation cycling data of these cells at ±C / 3 at 3.75 - 1.5 V (after 4 formation cycles at ±C / 10 at 3.75 - 1 V). Both cells used TEL 85k electrolyte.

[0347] It can be seen that both cells demonstrated excellent cycling over 200+ cycles. In particular:

[0348] Cell F2400361 demonstrated a capacity retention of 99.76% after 207 cycles. This translates to a capacity fade rate of just 0.24% in 207 cycles, or a capacity fade rate of 0.00116% / cycle.

[0349] Cell F2400362 demonstrated a capacity retention of 99.24% after 207 cycles. This translates to a capacity fade rate of just 0.76% in 207 cycles, or a capacity fade rate of 0.00367% / cycle.

[0350] This example therefore demonstrates that a sodium metal cell according to the present invention provides exceptional electrochemical performance when using an interfacial film in combination with a particular cathode active material (i.e., NFPP / C) as well as a particular electrolyte (i.e., TEL 85k). Indeed, excellent long-term cycling with nearly 100% capacity retention after 200 cycles with non-dendritic plating / stripping was provided. This is highly surprising. Experiment 10: Further benefits of using a cathode comprising NFPP / C.

[0351] This experiment further investigates the benefits of the use of an interfacial film with an alternative cathode material, NFPP / C as shown in Table 12.

[0352] TABLE 12

[0353] = control cell.

[0354] Figures 12 and 13 show the cycling performance of two cells:

[0355] 1) F2400782: Fabricated without an interfacial film.

[0356] 2) F2400785: fabricated with a 6-7 pm FC 2 interfacial film.

[0357] Each cell used TEL 85k electrolyte. Both cells were cycled as follows at 30 degrees C.

[0358] Cycles 1 - 4 (Formation): ±C / 10 at 3.75 - 1 V.

[0359] Cycle 5 onwards (Post formation): ±C / 3 at 3.75 - 1 .5 V.

[0360] The long-term cycling of the two cells is shown in Figure 12, which presents the specific discharge capacity of the cathode in units of mAh / g(active NFPP / C weight). The coulombic efficiencies of both cells during the cycling experiments are shown in Figure 13.

[0361] From Figure 12, it can be seen that the cell of the present invention (F2400785) showed higher capacities and more stable cycling than the control cell (F2400782). Furthermore, it can be seen from Figure 13 that the cell of the present invention demonstrated high and stable coulombic efficiencies, whilst the cell without an interfacial film showed highly erratic coulombic efficiencies.

[0362] In particular, the following metrics are relevant: Comparative Example: Cell F2400782: After 37 cycles in total (33 Post formation cycles at ±C / 3), this cell could only deliver 102.65 mAh / g capacity, equating to a capacity retention of 96.97 % (relative to the 5th cycle, or the 1st cycle at ±C / 3 at 3.75 - 1.5 V). This translated to a fade rate of 3.03 % in 33 cycles, or a capacity fade rate of 0.0918% / cycle. Furthermore, the average coulombic efficiency over 33 POST cycles was only 86.95%.

[0363] Example of the present Invention: Cell F2400785: After 37 cycles in total (33 Post formation cycles at ±C / 3), this cell could still deliver a high value of 113.79 mAh / g capacity, equating to a capacity retention of 99.63 % (relative to the 5th cycle, or the 1st cycle at ±C / 3 at 3.75 - 1.5 V). This translated to a fade rate of 0.37 % in 33 cycles, or a capacity fade rate of just 0.0112% / cycle. Furthermore, the average coulombic efficiency over 33 post-formation cycles was very high, being 99.59%.

[0364] This experiment further demonstrates that a sodium metal anode cell made with an interfacial film demonstrated higher capacities and much higher capacity retention than the cell made without an interfacial film. Furthermore, the cell made with an interfacial film demonstrated smooth cycling with high average coulombic efficiencies, which is a hallmark of non-dendritic plating / stripping. In contrast, the cell made without the interfacial film resulted in erratic cycling with much lower coulombic efficiencies, which is a hallmark of dendritic plating / stripping. Thus, it follows that a sodium metal cell according to the present invention provides enhanced performance compared to a sodium metal cell not according to the present invention, and particularly when the present invention uses certain cathode active materials (i.e., NFPP / C) as well as certain electrolytes (i.e., TEL 85k).

[0365] Overall conclusions

[0366] It has been demonstrated in multiple cells, and with different types of materials, that adding a free-standing polymeric substrate according to the present invention enables reliable cycling of Na metal anode based cells with liquid electrolytes, at scale. Furthermore, it was also demonstrated that free-standing polymeric substrates not according to the present invention (without ‘through-film’ porosity) led to worse cycling than that of free-standing polymeric substrates according to the present invention.

Claims

CLAIMS1. A sodium metal cell (10) comprising a cathode electrode (14) and an anode electrode (16) disposed in an opposite face-to-face arrangement with a separator (18) therebetween, and in which the anode electrode (16) and the separator (18) are located in contact with opposite sides (20, 22) of a free-standing polymeric substrate (12), in which the free-standing polymeric substrate (12) comprises an interconnected network of pores (34, 38), in which at least some of the pores (38) are present in a bulk region (40) extending between the opposite sides (20, 22) of the free-standing polymeric substrate (12), and in which the cathode electrode (14) comprises one or more active materials which are selected from one or more of sodium transition metal oxides and polyanionic compounds.

2. The sodium metal cell according to claim 1 , in which at least some of the pores (34) are further present in a surface region (19) included in at least one of the opposite sides (20, 22) of the free-standing polymeric substrate (12).

3. The sodium metal cell according to claim 1 or claim 2, in which at least some of the pores (38) present in the bulk region (40) have a pore width of from greater than 0 to 10 pm as determined using field emission scanning electron microscopy (FESEM).

4. The sodium metal cell according to any one of claims 2 to 3, in which at least some of the pores (34) present in the surface region (19) have a pore width of from greater than 0 to 50 pm as determined using field emission scanning electron microscopy (FESEM).

5. The sodium metal cell according to any one of claims 1 to 4, in which the free-standing polymeric substrate (12) has a thickness from about 1 pm to about 40 pm, preferably from about 3 pm to about 15 pm.

6. The sodium metal cell according to any one of claims 1 to 5, in which the free-standing polymeric substrate (12) further includes one or more stabilising additives such as nanozeolites and / or carbon black.

7. The sodium cell metal cell according to any one of claims 1 to 6, in which the freestanding polymeric substrate (12) is made by solvent casting, preferably into an aqueous phase comprising water and an alcohol such as isopropyl alcohol (I PA).

8. The sodium metal cell according to any one of claims 1 to 7, in which the free-standing polymeric substrate is made porous by a process without using a liquid pore-forming agent prior to casting9. The sodium metal cell according to any one of claims 1 to 8, in which the one or more sodium transition metal oxides are of the general formula:AI±6M1VM2WM3X M4YM5ZO2-C whereinA is one or more alkali metals selected from sodium, potassium and lithium;M1comprises one or more redox active metals in oxidation state +2;M2comprises a metal in oxidation state greater than 0 to less than or equal to +4;M3comprises a metal in oxidation state +2;M4comprises a metal in oxidation state greater than 0 to less than or equal to +4;M5comprises a metal in oxidation state +3; wherein0 < 5 < 1;V is > 0;W is Ss 0;X is Ss 0;Y is Ss 0; at least one of W and Y is > 0;Z is Ss 0;C is in the range 0 < c < 2; and wherein V, W, X, Y, Z and C are chosen to maintain electrochemical neutrality.

10. The sodium-cell according to any one of claims 1 to 8, in which the polyanionic compounds include iron-based phosphates, preferably carbon-coated iron-based phosphates.

11. The sodium metal cell according to any one of claims 1 to 10, in which the free-standing polymeric substrate (12) includes polyvinylidene fluoride (PVDF).

12. The sodium metal cell according to any one of claims 1 to 11 , in which the free-standing polymeric substrate (12) includes polyvinylidene fluoride (PVDF) and polyethylene oxide (PEO).

13. The sodium metal cell according to any one of claims 1 to 12, further including an electrolyte which comprises one or more sodium-containing salts, and a solvent system which comprises a first component which comprises one or more glyme-based solvents; and a second component which comprises one or more additives selected from one or more of sulfur-containing compounds, boron-containing compounds, and surfactants.

14. The sodium metal cell according to claim 13, in which the first component comprises tetraglyme and diglyme, preferably in the weight ratio 1 :1.

15. The sodium metal cell according to any one of claims 13 to 14, in which the second component comprises three or more additives comprising sulfur-containing compounds in an amount of >0 to <10% by weight of the solvent system, boron- containing compounds in an amount of >0 to <10% by weight of the solvent system, and surfactants in an amount of >0 to < 10% by weight of the solvent system.

16. An apparatus comprising a sodium metal cell according to any one of claims 1 to 15.

17. A method for manufacturing a sodium metal cell according to any one of claims 1 to 15, comprising: a. providing a first liquid phase including (a) a polymeric component comprising one or more hydrophobic polymers and optionally one or more hydrophilic polymers; and (b) one or more organic solvents which are miscible with water; b. providing a second liquid phase either (i) comprising water; or in the case that the polymeric component of the first liquid phase comprises one or more optional hydrophilic polymers, (ii) comprising water and one or more surface tension lowering additives; c. casting said first liquid phase onto a surface; d. contacting at least part of said first liquid phase on said surface with said second liquid phase; e. removing said contacted product of step d) from said surface; f. drying said removed product of step e) to provide a free-standing polymeric substrate; and g. assembling a cathode electrode and an anode electrode disposed in an opposite face-to-face arrangement with a separator therebetween, in which the cathodeelectrode comprises one or more active materials which are selected from one or more of sodium transition metal oxides and polyanionic compounds, and in which the anode electrode and the separator are located to be in contact with opposite sides of the free-standing polymeric substrate provided at the end of step f), to form the sodium metal cell; further in which the first liquid phase provided in step a) further includes one or more stabilising additives in the case when the second liquid phase provided in (i) of step b) consists essentially of water.

18. The method according to claim 17, in which the second liquid phase comprises water and one or more surface tension lowering additives.

19. The method according to claim 18, in which the first liquid phase further includes one or more stabilising additives when the second liquid phase comprises water and one or more surface tension lowering additives.

20. The method according to any one of claims 17 to 19, in which the one or more hydrophobic polymers in step a) include polyvinylidene fluoride (PVDF).21 . The method according to any one of claims 17 to 20, in which step a) includes one or more hydrophilic polymers.

22. The method according to claim 21 , in which the one or more hydrophilic polymers include polyethylene oxide (PEO).

23. The method according to any one of claims 21 to 22, in which the weight ratio of the hydrophobic polymer to the hydrophilic polymer is in the range 1 to 1 : 8 to 1 .

24. The method according to any one of claims 17 to 23, in which the one or more organic solvents include N-Methyl-2-pyrrolidone (NMP) and / or acetonitrile.

25. The method according to any one of claims 17 to 24, in which the one or more surface tension lowering additives includes one or more alcohols, preferably isopropyl alcohol (IPA).

26. The method according to any one of claims 17 to 25, in which step e) includes evaporation to remove liquids and / or vapours.

27. The method according to claim 17, in which the polymeric component of the first liquid phase consists essentially of polyvinylidene fluoride (PVDF) and the second liquid phase comprises water and one or more surface tension lowering additives.

28. The method according to any one of claims 17 to 27, in which the first liquid phase is substantially free of water.

29. The method according to any one of claims 17 to 28, in which the first liquid phase is provided at a temperature of no greater than 35 °C.

30. The method according to any one of claims 17 to 29, in which said first liquid phase is contacted with a single solution after being cast onto the surface.

31. A sodium metal cell obtainable by the method according to any one of claims 17 to 30.

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