High-performance sodium-ion cells and method for their preparation

Abstract

The present disclosure relates to a sodium-ion cell and a method for preparing said sodium-ion cell, the sodium-ion cell comprises a cathode comprising sodium-containing active material and a first binder; an anode comprising a carbonaceous active material and a second binder; a separator; and an electrolyte, wherein each of the first binder, the second binder and the separator comprise a cellulosic material. A battery, a vehicle and a stationary energy storage system comprising said sodium-ion cell are also disclosed.

Description

[0001] HIGH-PERFORMANCE SODIUM-ION CELLS AND METHOD FOR THEIR PREPARATION

[0002] TECHNICAL FIELD OF THE INVENTION

[0003] The present disclosure relates to a sodium-ion cell and a method of preparing a sodium- ion cell. In particular, the sodium-ion cell comprises binders and a separator that are each based on a cellulosic material. Further, the present disclosure relates to a battery, a vehicle and a stationary energy storage system comprising said sodium-ion cell.

[0004] BACKGROUND OF THE INVENTION

[0005] Rechargeable batteries or secondary lithium-ion cells find widespread use as electrical power supplies and energy storage systems. For example, in automobiles, battery packs formed of a plurality of lithium-ion battery cells are provided as a means of effective storage and utilization of electric power. However, increasing concerns regarding the sustainability of lithium sources, due to their limited availability and consequent expected price increase, have raised awareness of the importance of developing alternative energystorage candidates that can sustain the ever-growing energy demand. Furthermore, limitations on the availability of certain transition metals used in the manufacturing of cathode materials typically used in lithium-ion cells, together with questionable mining practices, are driving development towards more sustainable elements.

[0006] Sodium-ion batteries (Na-ion batteries) promise to revolutionise the area of low-cost, safe, and rapidly scalable energy-storage technologies. The use of raw elements, obtained ethically and sustainably from inexpensive and widely abundant sources, makes this technology extremely attractive, especially in applications where weight / volume are not of concern.

[0007] The main challenge of sodium-ion batteries is its poor electrochemical performance compared to competing lithium-ion batteries. The performance of sodium-ion batteries is greatly dependent on the properties of the electrode materials. Efforts has been put into development of cathode active materials with high loading capacity of sodium ions. However, it remains that sodium-ion batteries are less energy dense and have less storage capacity compared to lithium-ion batteries, and therefore it is necessary to improve electrochemical performance of sodium-ion batteries for them to be commercially viable. Further, there is an ever-increasing demand for improved batteries providing, for instance, increased charging density and reduced charging time. Hence, there is a need to provide sodium-ion cells with improved performance, such as high charge capacity and extended cycle life. In particular, it would be advantageous to develop a cost-efficient method for preparing sodium-ion cells of high performance suitable for uses in batteries, vehicles, and energy storage systems.

[0008] SUMMARY OF THE INVENTION

[0009] In view of the above, it is an object of the present technology to provide improved sodium- ion cells that may be used in rechargeable batteries, vehicles and energy storage systems, which altogether alleviate at least one of the above-mentioned drawbacks.

[0010] Another object is to provide a method of preparing a sodium-ion cell which is cost-efficient by using raw elements, obtained ethically and sustainably from inexpensive and widely abundant sources.

[0011] Another object is to provide a method of preparing a sodium-ion cell which may endure high drying temperatures during manufacturing of the sodium-ion cell.

[0012] Another object is to provide a method of preparing a sodium-ion cell that is easy to produce and highly scalable at different sizes.

[0013] Another object is to provide a sodium-ion cell that may be used in stationary energystorage technologies.

[0014] Another object is to provide a binder, which has the purpose to adhesively connect active electrode materials in the form of powders together and to also fix the active material on a current collector, e.g., a foil.

[0015] Another object is to provide a water-soluble binder that is able to provide adhesion and cohesion during drying and simultaneously endures high drying temperatures.

[0016] Another object is to provide a sodium-ion cell which contains electrodes comprising binder and a separator, wherein both the binders and the separator are comprising a cellulosic material that may withstand high processing temperatures during preparation of the sodium-ion cell.

[0017] To achieve at least one of the above objects and also other objects that will be evident from the following description, a sodium-ion cell, a method of preparing a sodium-cell, a battery, a vehicle and a stationary energy storage system are as defined in the independent claims. Preferred variations to the inventive concept will be evident from the dependent claims.

[0018] According to a first aspect there is provided a sodium-ion cell comprising:

[0019] - a cathode comprising sodium-containing active material and a first binder;

[0020] - an anode comprising a carbonaceous active material and a second binder;

[0021] - a separator; and

[0022] - an electrolyte, wherein each of the first binder, the second binder and the separator comprise a cellulosic material.

[0023] According to a second aspect there is provided a method of preparing a sodium-ion cell comprising the steps of:

[0024] - providing a cathode, an anode and a separator, wherein the cathode comprises a sodium-containing active material and a first binder; the anode comprises a carbonaceous active material and a second binder; wherein each of the first binder, the second binder and the separator comprise a cellulosic material;

[0025] - assembling the cathode, the anode and the separator to form an electrode assembly;

[0026] - drying the electrode assembly under conditions of a temperature above 100°C, and at reduced pressure, preferably at a pressure from 0.1 Pa to 100 Pa, or under inert atmosphere, such as under nitrogen or argon; and

[0027] - arranging the electrode assembly in a cell casing, wherein the step of arranging the electrode assembly in the cell casing is performed before or after the step of drying.

[0028] According to a third aspect there is provided a battery comprising the sodium-ion cell according to the first aspect.

[0029] According to a fourth aspect there is provided a vehicle comprising the sodium-ion cell according to the first aspect or the battery according to the third aspect.

[0030] According to a fifth aspect there is provided a stationary energy storage system comprising the sodium-ion cell according to the first aspect or the battery according to the third aspect.

[0031] According to a sixth aspect there is provided a sodium-ion cell obtainable by the method of the second aspect and embodiments thereof. One advantage with the proposed inventive concept is that the specific need for drying during production of high-performance sodium-ion cells, has surprisingly been made possible when using a cellulosic based binder material, for each of the anode and the cathode, and as well as for the separator. Other advantages will be appreciated when reading the detailed description.

[0032] BRIEF DESCRIPTION OF THE FIGURES

[0033] The above and other aspects of the present inventive concept will now be described in more detail, with reference to appended drawings showing variants of the present inventive concept. The figures should not be considered limiting the inventive concept, instead they are used for explaining and understanding the inventive concept.

[0034] As illustrated in the figures, the sizes of layers and regions are exaggerated for illustrative purposes and are thus provided to illustrate the general structures of variants of the present inventive concept.

[0035] Figures la-lc is a schematic illustration of a sodium-ion cell according to the first aspect, fig. la shows the sodium-ion cell, fig. lb shows the sodium-ion cell with a casing and fig. 1c shows the sodium-ion cell with a sealed casing.

[0036] Figure 2 schematically illustrates a block scheme of a method of preparing a sodium-ion cell according to one embodiment of the second aspect.

[0037] Figures 3a-3b illustrate the effect of high temperature drying at 117 °C for 2h and 1 Pa on a conventional PE separator.

[0038] Figures 4a-4b illustrate the effect of high temperature drying at 117 °C for 2h and 1 Pa on a conventional PP separator.

[0039] Figures 5a-5b illustrate the effect of high temperature drying at 140 °C for 4h and 1 Pa on a conventional PE separator.

[0040] Figures 6a-6b illustrate the effect of high temperature drying at 140 °C for 4h and 1 Pa on a conventional PP separator. Figures 7a-7b illustrate the effect of high temperature drying at 153 °C for 15h and 1 Pa on a conventional PE separator.

[0041] Figures 8a-8b illustrate the effect of high temperature drying at 140 °C for 20h and 1 Pa on a conventional PP separator.

[0042] Figure 9 illustrates the discharge capacity plotted vs number of cycles for sodium-ion cells according to embodiments herein, that have been dried at different temperatures, i.e., drying at 117 °C, at 140 °C and at 165 °C, respectively.

[0043] The present disclosure will in the following be described in more detail.

[0044] DETAILED DESCRIPTION OF THE INVENTION

[0045] The present invention focuses on a sodium-ion cell, where drying before electrolyte filling and during assembly is important. The essence of the invention is to use a separator and binder materials for the electrodes of the same class of materials wherein said class of materials is relatively insensitive to high temperatures, so as to give as much process space for the drying process during preparation of the sodium-ion cell as possible. Primarily the focus is on using cellulose-based materials or cellulosic materials.

[0046] Definitions

[0047] Prior to outlining the present disclosure in more details, a set of terms and conventions is first defined:

[0048] Electrode active layer

[0049] In the context of this disclosure, an "electrode active layer" is a layer comprising binder, electrode active material, and optionally conductive additives. With "conductive" means electrically conductive, i.e., a material that is able to conduct electricity.

[0050] To produce an electrode active layer, electrode active material, a binder and optionally a conductive additive are typically dispersed in a dispersant to form a slurry. The slurry is then deposited on a current collector to form the electrode active layer.

[0051] The term "electrode active material" is to be understood as an electrochemical species which can be oxidized and reduced in a system which enables a cell to produce electric energy during discharge. The electrode active material may be either cathode active material or anode active material.

[0052] An "anode active layer" is a layer comprising binder, anode active material, and optionally conductive additives.

[0053] A "cathode active layer" is a layer comprising binder, cathode active material, and optionally conductive additives.

[0054] Each of the components within an electrode active layer will be described below:

[0055] Cathode and Sodium-containing active material

[0056] In the present context, the term "Sodium-containing active material" refers to a material that can reversibly intercalate ions, such as sodium ions, during cell charge and discharge cycles. Thus, the intercalation metal of the active material of the cathode is sodium.

[0057] In the present context, the cathode may comprise a sodium-containing active material and a first binder, and optionally a conductive agent. A conductive additive may e.g., be acetylene black, carbon black, graphene, graphite, mesocarbon microbead (MCMB), pitchbased carbon, coke powders, carbon nanotubes or metallic powders, or any combination thereof. The role of the conductive additive is to improve the electronic properties of the cathode and to provide an electrical connection between the particles of cathode active material in the cathode.

[0058] Anode and carbonaceous active material

[0059] The term "carbonaceous" is to be understood as describing a substance or material rich in carbon, such as containing or comprising a relatively high degree of carbon. With the term "carbonaceous active material" according to the present inventive concept may e.g., comprise carbon black, hard carbon and graphite, or any combination thereof. The carbonaceous active material is the active material in the anode of the sodium-ion cell and preferably comprises hard carbon. The role of the anode carbonaceous active material is to reversibly bind sodium-ions during cell charge and discharge cycles.

[0060] Binder

[0061] Binders adhesively connect the cathode active materials and the anode active materials, respectively, for long-term charge / discharge cycling. The binders of the present inventive concept are preferably cellulosic materials, which are materials that may sustain harsh conditions, i.e., drying temperatures above 100°C, during preparation of the anode, cathode and the sodium-ion cell, respectively. Separator

[0062] A separator is typically a permeable membrane, whose main function is to keep the two electrodes apart to prevent electrical short circuits while also allowing the transport of sodium ions that are needed to close the circuit during the passage of current in an electrochemical cell.

[0063] Cellulosic materials typically used in sodium-ion cells are environmentally friendly and have good thermal stability. Typically, when a cellulosic material such as cellulose or derivatives thereof is used as a separator, a sheet of a cellulosic material is disposed between the anode and the cathode during assembly of the cell.

[0064] Typically, a separator should be thin, and ideally as thin as practical while fulfilling its core function of electrically isolating the anode from the cathode. For example, the separator may have a thickness of from 1 pm to 20 pm, such as from 2 pm to 15 pm, for example from 5 pm to 12 pm.

[0065] Cellulosic material

[0066] Cellulose is an organic compound with the formula (CeHioOs , a polysaccharide consisting of a linear chain of several hundred to many thousands of 0(1— >4) linked D-glucose units, i.e., wherein n is the degree of polymerisation and represents the number of glucose groups, which thus may be from several hundred to many thousands D-glucose units. Cellulose is a straight chain polymer. Further, cellulose is crystalline, strong, and resistant to hydrolysis. The multiple hydroxyl groups on the glucose from one chain form hydrogen bonds with oxygen atoms on the same or on a neighbouring chain, holding the chains firmly together side-by-side and forming microfibrils with high tensile strength. The structural form of the cellulosic material may be nano-structured cellulose or nanocellulose. This may be either cellulose nanocrystal (CNC or NCC) or cellulose nanofibers (CNF) also called nanofibrillated cellulose (NFC). CNFs may typically have fibril widths in the range of 5-20 nanometers. Further, CNFs may have a wide range of lengths, typically several micrometers.

[0067] A cellulosic material is a cellulose-based material or derivative, that comprises the polymer backbone of cellulose wherein the hydroxyl groups of cellulose may be partially or fully reacted with various reagents to afford cellulosic derivatives or cellulosic materials, with useful properties, e.g., cellulose esters or cellulose ethers. In principle, although not always in current industrial practice, cellulosic polymers are renewable resources. Examples of cellulosic ester derivatives according to the present invention are cellulose acetate, cellulose triacetate, cellulose propionate, cellulose acetate propionate, cellulose acetate butyrate, cellulose nitrate or cellulose sulphate.

[0068] Examples of cellulosic ether derivatives according to the present invention are methyl cellulose (MC), ethyl cellulose (EC), ethyl methyl cellulose (EMC), hydroxyethyl cellulose (HEC), hydroxypropyl cellulose (HPC), hydroxyethyl methyl cellulose (HEMC), hydroxypropyl methyl cellulose (HPMC), ethyl hydroxyethyl cellulose or carboxymethyl cellulose (CMC).

[0069] Several other types of cellulosic materials are known, for example regenerated cellulose. Regenerated cellulose is a class of material manufactured by the conversion of natural cellulose to a soluble cellulosic derivative and subsequent regeneration to form a fibre. Regenerated cellulose includes many types of fibres, such as lyocell and rayon.

[0070] Preferably, the cellulosic fibres comprise regenerated cellulose fibres such as lyocell and rayon. In contrast to cellulose, hemicelluloses are derived from several sugars in addition to glucose, especially xylose but also including mannose, galactose, rhamnose and arabinose. Hemicelluloses are branched, shorter in chain length than cellulose, i.e., between 500 to 3000 sugar units, and have also shown a tendency to crystallize. Cellulose is stable during heating but at temperatures above 350°C cellulose undergoes thermolysis or pyrolysis, thus decomposing into solid char, vapours, aerosols and gases such as carbon dioxide.

[0071] Thus, the cellulosic material according to the present inventive concept is highly advantageous both as separator and binder material in sodium-ion cells as it resists from breaking down during the high-temperature drying required during preparation.

[0072] Electrode formation

[0073] To produce an electrode of a sodium-ion cell, a slurry with all the necessary components is formed. Typically, the slurry comprises electrode active material, binder, dispersing medium, and optionally conductive additive. Preferably, the dispersing medium is water. The addition of a water allows for easy preparation of the electrode.

[0074] The slurry composition is formulated to a suitable viscosity to allow it to be processed into an electrode, for example by slot-die coating. The slurry composition typically has a dynamic viscosity of from about 2 to about 50 Pa-s, for instance, from about 5 to about 40 Pa-s or from about 5 to about 25 Pa-s, as measured at 25°C.

[0075] The slurry composition typically contains from about 50 to 90 wt% solids, such as from about 60 to 80 wt% solids, preferably from about 68 to 73 wt% solids, with the remainder being dispersing medium, i.e., water.

[0076] In an embodiment, a slurry for forming an electrode, such as an anode or a cathode, comprises dispersing medium, such as water, electrode active material, a binder, and optionally conductive additive.

[0077] The slurry is deposited on a foil. Preferably, the foil is an aluminium foil.

[0078] The slurry may be deposited on a foil on one side or on two sides. Any of the well-known deposition processes may be utilised. The slurry is preferably evenly deposited on at least one side of the foil. The thickness of the coating on the foil may vary depending on the specific application and purpose.

[0079] The electrode active material may be pre-dried before the electrode assembly is formed. Upon this pre-drying, an electrode comprising a foil and an electrode active layer is formed. This drying step is typically done in an oven at elevated temperature, for example at a temperature between about 40°C and 120°C, such as between about 60°C and 100°C, for example between 70°C and 90°C. This pre-drying step is typically carried out over a shorter time period and at a lower temperature than the drying stage.

[0080] Typically, the electrode then undergoes calendering to compact the electrode active layer. Calendering typically involves passing the electrode through heated rollers. In doing so, an electrode active layer having more uniform thickness and density is formed, for example the thickness may be determined by the calendering process. Calendering improves electrode adhesion and density and increases the electrical conductivity between particles.

[0081] For example, an electrode may be formed using a process comprising the following steps: providing a foil and a dispersion comprising water, electrode active material, binder, and optionally conductive additives; disposing said dispersion onto said foil to provide a coated foil; calendering the coated foil to provide an electrode comprising a foil and an electrode active layer. Optionally, the electrode forming process may include a heating step, either before, after or during calendering, or any combination of these possibilities.

[0082] The electrode may then be cut into the desired shape by techniques well known in the field. For example, any type of slitting machine may be utilized.

[0083] Electrolyte

[0084] An electrolyte is introduced into the casing to soak the electrode assembly. The electrodes soak the electrolyte into its pores. The electrolyte facilitates the transport of sodium ions between the cathode and anode during the operation of the sodium-ion cell.

[0085] A sodium-ion cell and a method of preparing a sodium-ion cell will now be described with reference to the appended figures, illustrating preferred embodiments of the present inventive concept, as well as figures illustrating the technical effect of the invention in comparison to the state of the art.

[0086] Figure 1 illustrates a sodium-ion cell 10 comprising a cathode 20 comprising sodium- containing active material 21 and a first binder 22. The Na-ion cell 10 comprises an anode 30 comprising a second binder 32 and a carbonaceous active material 31. The sodium-ion cell 10 also comprises a separator 40 and an electrolyte 50. In the sodium-ion cell 10 each of the first binder 22, the second binder 32 and the separator 40 comprise a cellulosic material.

[0087] It is understood that the electrolyte is arranged in contact with the cathode, anode and the separator. The electrolyte may be present in pores of the cathode, anode, and the separator.

[0088] The sodium-ion cell may also comprise current collectors 61, 62, e.g., an electrically conductive foil. Preferably the conductive foil is an aluminium foil.

[0089] Advantages of the sodium-ion cell is that it has enhanced electrochemical performance. Providing the first binder, the second binder, and the separator comprising a cellulosic material allow for a relatively environmentally sodium-ion cell.

[0090] Figure 2 illustrates a method 100 of preparing a sodium-ion cell 10. In step 110 a cathode 20, an anode 30 and a separator 40 are provided. The cathode 20 comprises a sodium- containing active material 21 and a first binder 22. The anode 30 comprises a second binder 32 and a carbonaceous active material 31. Each of the first binder 22, the second binder 32 and the separator 40 comprise a cellulosic material.

[0091] In step 120 the cathode 20, the anode 30 and the separator 40 are assembled to form an electrode assembly 70.

[0092] In step 130 the electrode assembly is dried under conditions of a temperature above 100°C, and at reduced pressure, preferably at a pressure from 0.1 Pa to 100 Pa, or under inert atmosphere, such as under nitrogen or argon. The specific temperature and reduced pressure, or inert atmosphere, has been found to advantageously allow for a relatively time-efficient manufacturing of a well-functioning sodium-ion cell.

[0093] Providing the first binder and second binder comprising a cellulosic material allow for the use of readily available solvent during manufacturing. Said readily available solvent may be an environmentally friendly and safe solvent such as water. Furthermore, by providing a first and second binder, and a separator comprising cellulosic material allows for similar properties thus a more effective manufacturing method.

[0094] Water may be removed from the sodium-containing active material of the cathode at several stages of the manufacturing process. Thus, water may be removed from the produced cathode before assembling of the cell, after assembly of an electrode stack, and / or during initial cycling of the battery cell when a solid electrolyte interphase (SEI) is formed. A SEI layer is a component of alkali-ion batteries, and it is formed by the decomposition of materials in the electrolyte of the battery. The SEI layer is a vital factor for the function of a secondary battery. The reactivity of the electrode active material and the oxidation / reduction reaction that occur on the surface of the electrodes. The SEI layer plays an important role for the performance of sodium-ion batteries in terms of cycle life limitations, the capacity for reversibility and safety. The role of the SEI layer involves the prevention of further electrolyte decomposition in order to maintain cycling of the battery cell.

[0095] Water removal by drying may be performed immediately following formation of the coated foil and / or after the cathode has been assembled into an electrode stack. For water removal by drying, vacuum conditions are desirable since it allows water removal at lower temperatures and reduced time consumption.

[0096] Therefore, an embodiment of the present disclosure relates to the method 100 as described herein, wherein removal of water comprises drying 130 conducted under vacuum. Another embodiment of the present disclosure relates to the method 100 as described herein, wherein the removal of water comprises drying 130 with a drying time in the range of about 10 minutes to about 50 hours, such as about 10 hours to about 50 hours, such as about 12 hours to about 40 hours, such as about 15 hours to about 35 hours, such as about 20 hours.

[0097] Suitable pressure of the vacuum is dependent on the temperature utilized for drying and vice versa. Therefore, the pressure may be adjusted depending on the temperature used for drying.

[0098] However, one embodiment of the present disclosure relates to the method 100 as described herein, wherein the pressure of the vacuum is less than or equal to about 100 Pa. The specific pressure allows for decreased drying temperature, drying time and thus a more cost-effective method.

[0099] Since applying a vacuum accelerates the drying process, it is helpful to apply the vacuum gradually to avoid risk of damage to the cathode active material during drying. Initial stages of drying may therefore occur at standard pressure. Often, dry air or an inert atmosphere such as nitrogen or argon is used.

[0100] Typical pressures used for the initial drying include less than or equal to 1000 Pa, for instance less than or equal to 100 Pa. Typical final pressures used during the drying include less than or equal to 10 Pa, for instance less than or equal to 1 Pa. The typical final pressures have been found particularly suitable for the method described herein.

[0101] The drying may be at a temperature of above 100°C, such as above 120 °C, such as in a range of 120-200°C, preferably in a range of 130-200°C, preferably in a range of 130- 175°C, preferably in a range of 140-170°C. The specific drying conditions that may be chosen according to the present inventive concept provides an end-product, e.g., sodium- ion cell, battery or energy storage device, etc., with enhanced electrochemical properties.

[0102] In some variations of the method 100, it is preferred to delay water removal from the sodium-containing active material of the cathode 20 until a late stage of the preparation process. By doing so, the initial steps of the method can be performed at ambient temperature, i.e., room temperature at around 20-25°C, which reduces complexity and costs of production as dry rooms with controlled environments are avoided. Thus, removal of water can be performed after the cathode has been assembled in an electrode stack. This does not exclude the possibility to also include an earlier and / or later step of water removal.

[0103] In step 140 the electrode assembly 70 is arranged in a cell casing 80. The step 140 of arranging the electrode assembly 70 in the cell casing 80 may be performed before or after the step 130 of drying for example, in a method of preparing a prismatic cell. In one embodiment the step 140 of arranging the electrode assembly 70 in the cell casing 80 may be performed after the step 130 of drying. In another embodiment the step 140 of arranging the electrode assembly 70 in the cell casing 80 may be performed before the step 130 of drying.

[0104] In some embodiments, the cell casing comprises aluminium, aluminium alloys, nickel- plated iron, and / or steel. For example, the cell casing may comprise an aluminium laminated polymeric film, such as an aluminium laminated polyethylene film or an aluminium laminated polypropylene film. The casing should preferably be capable of withstanding the step of drying without any substantial mechanical or physical changes.

[0105] Figure 1 shows a schematic figure of a sodium-ion cell; however, the cell is not limited to the planar stacking of the components in the cell as pictured in figures la-lc. The assembling of the electrode assembly may be selected from other arrangements e.g., cylindrical winding, prismatic winding, Z-stacking and single sheet stacking, preferably the assembling electrode assembly is by Z-stacking.

[0106] The method 100 of preparing a sodium-ion cell 10 may comprise further steps. In step 150 an electrolyte may be added to the cell casing 80 of the sodium-ion cell 10. Preferably, the electrolyte 50 may be a non-aqueous electrolyte. Examples of electrolytes may be sodium hexafluorophosphate, sodium tetrafluoroborate, sodium bis(oxalato)borate salt organic carbonates, propylene carbonate, ethylene carbonate, vinylene carbonate, ethyl methyl carbonate, dimethyl carbonate, or diethyl carbonate, or any combination thereof. Using non-aqueous electrolyte allows for having a relatively high cell voltage and thus an improved energy.

[0107] In an even further step 160 the cell casing 80 may be sealed to form a closed sodium-ion cell, such as a cylindrical cell, a prismatic cell or a pouch cell. An electrolyte is introduced into the casing to soak the electrode assembly. The electrolyte facilitates the transport of sodium ions between the cathode and anode. The casing is typically sealed to ensure the electrolyte is retained within the casing. Said casing usually includes terminals, such as current collectors in electrical contact with the anode and cathode. The cellulosic material of the first binder, the second binder and the separator may be independently selected from a polymer of formula I, or a salt thereof:

[0108] Formula I wherein

[0109] R is independently selected from H, CH3, CH2CH3, CH2CH(OH)CH3, CH2CH2OH, C(O)CH3, CH2CO2H and D-ribofuranose, preferably R is independently selected from H, CH2CO2H and D-ribofuranose;

[0110] R1 is independently selected from H, CO2H and CH2OR; and n is degree of polymerization and represents the number of repeating groups, preferably n is at least 300, preferably n is at least 500, preferably n is at least 1000, preferably n is at least 5000, preferably n is at least 10000.

[0111] As an alternative and to illustrate that the cellulosic material of the first binder 22, the second binder 32 and the separator 40 may be independently selected from a polymer, or salt thereof, wherein the R at the same position may be different in different unit groups or monomers of the polymer formula I, i.e., partial functionalization, the cellulosic material of the first binder 22, the second binder 32 and the separator 40 may be independently selected from a polymer of formula II, or a salt thereof:

[0112] Formula II wherein n is degree of polymerization and represents the number of repeating groups, preferably n is at least 300, preferably n is at least 500, preferably n is at least 1000, preferably n is at least 5000, preferably n is at least 10000; for each repeating group

[0113] R1 is independently selected from H, CO2H and CH2OR;

[0114] R is independently selected from H, CH3, CH2CH3, CH2CH(OH)CH3, CH2CH2OH, C(O)CH3, CH2CO2H and D-ribofuranose, preferably R is independently selected from H, CH2CO2H and D-ribofuranose;

[0115] R2 is independently selected from H, CH3, CH2CH3, CH2CH(OH)CH3and CH2CH2OH; and R3 is independently selected from H, CH3, CH2CH3, CH2CH(OH)CH3and CH2CH2OH. Thus, it is to be understood that the polymer of formula I or formula II may be a mixed polymer, meaning that the repeating monomer may differ within the polymer by independently varying the R-, R1-, R2- and R3-groups of formula I or formula II.

[0116] The cellulosic material may be selected from the group consisting of: carboxymethyl cellulose (CMC), methyl cellulose (MC), ethyl cellulose (EC), ethyl methyl cellulose (EMC), hydroxypropyl cellulose (HPC), hydroxypropyl methyl cellulose (HPMC), hydroxyethyl cellulose (HEC), hydroxyethyl methyl cellulose (HEMC), hemicellulose (HC), cross-linked hemicellulose and regenerated cellulose (RC), or a salt thereof, or any combination thereof. In one embodiment R is H or CH2CO2H, i.e., the cellulosic material may be carboxymethyl cellulose (CMC).

[0117] The cellulosic material of the separator, the first binder and the second binder may be the same or different cellulosic materials. There may also be a mixture of at least two different cellulosic materials within each of the separator, the first binder and the second binder.

[0118] In one embodiment R is H or CH3, i.e., the cellulosic material may be methyl cellulose (MC).

[0119] In one embodiment R is H or CH2CH3, i.e., the cellulosic material may be ethyl cellulose (EC).

[0120] In one embodiment wherein R is H, CH3 or CH2CH3, i.e., the cellulosic material may be ethyl methyl cellulose (EMC)

[0121] In one embodiment R is H or CH2CH(OH)CH3, i.e., the cellulosic material may be hydroxypropyl cellulose (HPC).

[0122] In one embodiment R is H, CH3 or CH2CH(OH)CH3, i.e., the cellulosic material may be hydroxypropyl methyl cellulose (HPMC).

[0123] In one embodiment R is H or CH2CH2OH, i.e., the cellulosic material may be hydroxyethyl cellulose (HEC).

[0124] In one embodiment R is H, CH3 or CH2CH2OH, i.e., the cellulosic material may be hydroxyethyl methyl cellulose (HEMC)

[0125] In one embodiment the cellulosic material may be cross-linked hemicellulose. In one embodiment the cellulosic material may be regenerated cellulose (RC).

[0126] In one embodiment the cellulosic material may be the sodium salt of carboxymethyl cellulose, i.e., sodium carboxymethyl cellulose) (Na-CMC). Na-CMC has proven particularly suitable as first and second binder. Na-CMC has proven thermally stable over a relatively large temperature interval, and in particular at temperatures needed for drying the sodium-ion cell described herein. The Na-CMC may be considered especially suitable for sodium-ion cells as it does not introduce other cations which may negatively affect the electrochemical properties of the cell.

[0127] The cellulosic material may be a cellulose derivative as disclosed herein, or a salt thereof, or any combination thereof.

[0128] The first binder and the second binder may comprise a same or distinct cellulosic materials. Preferably the first binder and the second binder may be selected from the group consisting of: carboxymethyl cellulose (CMC), methyl cellulose (MC), ethyl cellulose (EC), ethyl methyl cellulose (EMC), hydroxypropyl cellulose (HPC), hydroxypropyl methyl cellulose (HPMC), hydroxyethyl cellulose (HEC), and hydroxyethyl methyl cellulose (HEMC), or a salt thereof, or any combination thereof, more preferably carboxymethyl cellulose (CMC), or a salt thereof. Most preferably the first binder and the second binder may be sodium carboxymethyl cellulose (Na-CMC). In one embodiment the first binder may be sodium carboxymethyl cellulose (Na-CMC). In another embodiment the second binder may be sodium carboxymethyl cellulose (Na-CMC). An advantage of the cellulosic binder is that it facilitates ionic and electrical conductivity. The cellulosic binders adhesively connect all the cathode materials and anode materials, respectively, for long-term charge / discharge cycling. Preferably, the cellulosic binders are thermally stable at a temperature above 140°C or even higher. Thus, the sodium-ion cell withstands the high drying temperatures of above 100°C which is necessary for the preparation of a working sodium-ion cell.

[0129] The cathode 20 may comprise the first binder at a maximum of 3 wt%, preferably at a maximum of 1 wt%, preferably the cathode comprises the first binder in a range of 0.5-3 wt%, more preferably in a range of 0.7-1.5 wt%, most preferably about 1 wt%. The specified amounts of first binder advantageously allows for a satisfactory combination of electrochemical and mechanical properties of the cathode.

[0130] The anode 30 may comprise the second binder at a maximum of 3 wt%, preferably at a maximum of 1 wt%, preferably the anode comprises the second binder in a range of 0.5- 3 wt%, more preferably in a range of 0.7-1.5 wt%, most preferably about 1 wt%. The specified amounts of second binder advantageously allows for a satisfactory combination of electrochemical and mechanical properties of the anode.

[0131] At least one of the cathode or the anode may comprise a further binder selected from styrene butadiene rubber (SBR) and polyacrylic acid (PAA). In one embodiment the cathode 20 may comprise a further binder selected from styrene butadiene rubber (SBR) and polyacrylic acid (PAA). In another embodiment the anode 30 may comprise a further binder selected from styrene butadiene rubber (SBR) and polyacrylic acid (PAA). The further binder may advantageously contribute to adhesively connecting active electrode materials in the form of powders together and fixing the active material on a current collector 61, 62, e.g., a foil.

[0132] The sodium-containing active material may further comprise at least one transition metal, and wherein the sodium-containing active material is configured to enable reversible intercalation of sodium ions. For example, the sodium-containing active material may be a Prussian blue analogue (PBA) material. Preferably the PBA material is of a chemical formula represented by AxPy[Rz(CN)e]w, wherein A is Na, P and R are individually selected from one or more transition metals, preferably selected from Fe and / or Mn, more preferably P and R are Fe, l<x<2, 0<y<2, l<z<2, and l<w<2. Preferably, the sodium-containing active material comprises sodium transition metal cyanides having six cyanide groups per formula unit. Each of these cyanide groups connect transition metals within the sodium-containing active material of the cathode to form a framework with large voids that allow intercalation and de-intercalation of sodium-ions. The specified sodium-containing active material has shown particularly desirable electrochemical properties, and is especially suitable for the drying procedures described herein.

[0133] In preferred embodiments, the sodium-containing cathode active material comprises a PBA material represented by the formula AxPy[Rz(CN)6]w, wherein A is Na, P and R are Fe, l<x<2, 0<y<2, l<z<2, and l<w<2; and the first and the second binder comprises Na- CMC. The inventors have found that this particular combination of sodium-containing cathode active material and first and second binders provide a cell with relatively high cycle life and specific capacity. This may further be considered relatively cost-effective sodium- ion cell due to the ease of manufacturing as separate drying steps of anode, cathode, and separator can be avoided.

[0134] The carbonaceous active material may be selected from carbon black, hard carbon and graphite, or any combination thereof. Preferably the anode comprises hard carbon.

[0135] The hard carbon is especially suitable for intercalating sodium ions during, i.e., charging of the cell. The hard carbon provides particularly suitable ion pathways for intercalation of sodium ions.

[0136] In one embodiment the anode 30 may comprise:

[0137] - a conductive foil or current collector 62, preferably an aluminium foil;

[0138] - a carbonaceous active material 31 selected from carbon black, hard carbon and graphite, or any combination thereof, preferably the anode comprises hard carbon; - optionally at least one conductive additive selected from the group consisting of: acetylene black, carbon black, graphene, graphite, mesocarbon microbead (MCMB), pitchbased carbon, coke powders, carbon nanotubes or metallic powders, or any combination thereof;

[0139] - a second binder 32 selected from the group consisting of: carboxymethyl cellulose (CMC), methyl cellulose (MC), ethyl cellulose (EC), ethyl methyl cellulose (EMC), hydroxypropyl cellulose (HPC), hydroxypropyl methyl cellulose (HPMC), hydroxyethyl cellulose (HEC), and hydroxyethyl methyl cellulose (HEMC), or a salt thereof, or any combination thereof, preferably the binder is selected from carboxymethyl cellulose (CMC), or a salt thereof, preferably sodium carboxymethyl cellulose (Na-CMC); and

[0140] - optionally a further binder selected from styrene butadiene rubber (SBR) and polyacrylic acid (PAA). In another embodiment, the anode comprises a further binder selected from styrene butadiene rubber (SBR) and polyacrylic acid (PAA). The anode may comprise the second binder and the further binder at a maximum of 5 wt%, preferably at a maximum of 3 wt%, of the total weight of the anode. In one embodiment the anode may comprise the second binder at a maximum of 1 wt% and the further binder at a maximum of 4 wt%, preferably the further binder at a maximum of 2 wt%.

[0141] In one embodiment the cathode 20 may comprise:

[0142] - a conductive foil or current collector 61, preferably an aluminium foil;

[0143] - a sodium-containing active material 21 comprising a transition metal material, such as a Prussian blue analogue (PBA) material, wherein the sodium-containing active material in configured to enable reversible intercalation of sodium ions

[0144] - optionally at least one conductive additive selected from the group consisting of: acetylene black, carbon black, graphene, graphite, mesocarbon microbead (MCMB), pitchbased carbon, coke powders, carbon nanotubes or metallic powders, or any combination thereof;

[0145] - a first binder 22 selected from the group consisting of: carboxymethyl cellulose (CMC), methyl cellulose (MC), ethyl cellulose (EC), ethyl methyl cellulose (EMC), hydroxypropyl cellulose (HPC), hydroxypropyl methyl cellulose (HPMC), hydroxyethyl cellulose (HEC), and hydroxyethyl methyl cellulose (HEMC), or a salt thereof, or any combination thereof, preferably the binder is selected from carboxymethyl cellulose (CMC), or a salt thereof, preferably sodium carboxymethyl cellulose (Na-CMC); and

[0146] - optionally a further binder selected from styrene butadiene rubber (SBR) and polyacrylic acid (PAA). In another embodiment, the cathode comprises a further binder selected from styrene butadiene rubber (SBR) and polyacrylic acid (PAA). The cathode may comprise the first binder and the further binder at a maximum of 5 wt%, preferably at a maximum of 3 wt%, of the total weight of the cathode. In one embodiment the cathode may comprise the first binder at a maximum of 1 wt% and the further binder at a maximum of 4 wt%, preferably the further binder at a maximum of 2 wt%.

[0147] After drying, the cathode may comprise a water content of less than 99 ppm, preferably the cathode may comprise a water content in a range of 10-95 ppm, preferably in a range of 12-90 ppm, preferably in a range of 14-80 ppm, preferably in a range of 16-70 ppm, preferably in a range of 18-60 ppm, preferably in a range of 20-50 ppm. This water content advantageously allows for a decreased risk of parasitic cell reactions and deterioration of desirable cell properties of the sodium-ion cell.

[0148] The separator may comprise cellulose, cross-linked hemicellulose, carboxy methyl cellulose (CMC), regenerated cellulose (RC), or any combination thereof.

[0149] The separator 40 may have one or more properties individually selected from: i) a thickness, according to test standard ISO 534, of less than 25 pm, preferably less than 20 pm, preferably in a range of 1-25 pm, more preferably in a range of 5-25 pm, most preferably in a range of 15-20 pm; ii) an air permeability, according to test standard ISO5636-51 JIS P8117, in a range of 35-65 s / 100 mL; iii) a porosity, according test standard ISO5341 ISO536, in a range of 40-50 %, calculated on dry matter; iv) a shrinkage, according to test standard ASTM D2732, in machine direction (MD) and cross direction (CD) of less than 2 %, preferably less than 1 %, calculated on dry matter; v) a tensile strength, according to test standard ISO 1924-2, TAPPI T494, in the machine direction (MD) of at least or more than 0.8 kN / m, preferably at least or more than 0.9 kN / m, preferably a tensile strength in a range of 0.8-2 kN / m, more preferably in a range of 0.9-1.5 kN / m, most preferably 1.0 kN / m; vi) a tensile strength, according to test standard ISO 1924-2, TAPPI T494, in the cross direction (CD) of at least or more than 0.6 kN / m; vii) an elongation, according to test standard ISO 1924-2, TAPPI T494, in the machine direction (MD) of at least or more than 1.55 %, preferably at least or more than 1.8 %, most preferably at least or more than 2.5 %, preferably an elongation in a range of 1.55-3.0 %, more preferably in a range of 1.8-2.7 %; and viii) an elongation, according to test standard ISO 1924-2, TAPPI T494, in the cross direction (CD) of at least or more than 6 %.

[0150] By using one or two electrodes that are coated using e.g., a CMC type binder together with a cellulosic separator, the drying time and profiles can be improved. In state-of-the-art batteries CMC is used together with a PP / PE separator, thereby limiting the drying time and profile. For example, figures 3-8 show illustrations of what happens with a conventional separator containing polyethylene (PE) and polypropylene (PP), respectively. For example, figure 7a shows a PE separator before drying and figure 7b shows the same PE separator after drying at a temperature of 153 °C at IPa for 15h. From figure 7b it may be seen that, e.g., the dimensions of the PE separator have been altered. Similarly, Figure 8a shows a PP separator before drying and figure 8b shows the same PP separator after drying at a temperature of 140 °C at IPa for 20h. As may be seen when comparing figures 8a and 8b, the dimensions of the PP separator has been altered due to, e.g., shrinkage upon the drying conditions of the present inventive concept.

[0151] The Gurley second or Gurley unit is a unit that describes air permeability as a function of the time required for a specified amount of air to pass through a specified area of a separator under a specified pressure.

[0152] A separator having a porosity of from 40-50% has been found particularly suitable.

[0153] During the production of a sodium-ion cell, i.e., the stacking step, the separator experiences relatively high tensile forces in the machine direction (MD) direction as compared to the cross direction (CD), thus the property in this direction may be considered a relatively important factor for the quality of the separator. The tensile strength in the machine direction (MD) of the separator may be at least 0.8 kN / m, such as about 1.0 kN / m. A preferred range of the tensile strength in the machine direction (MD) may be 0.95-1.4 kN / m.

[0154] EXAMPLES

[0155] EXAMPLE 1 : PREPARATION OF SODIUM-ION CELLS

[0156] Sodium-ion cells were prepared according to the process described herein. Briefly, an aqueous slurry comprising a Fe-Fe-PBA material and CMC was disposed on an aluminium foil to provide a coated cathode foil. The coated cathode foil was further processed using calendering to a final cathode. A slurry comprising a hard carbon and CMC was disposed on an aluminium foil to provide a coated anode foil. The coated anode foil was further processed using calendering to a final cathode.

[0157] Cathodes were assembled with anodes and cellulosic separators to form a standard z- stacked stack arranged in a casing. The z-stacked stacks were dried for 20h at different temperatures (117 °C, 140 °C and 165 °C) under 1 Pa pressure. After drying casing was injected with electrolyte and formation was carried out by conducting one C / 10 and two C / 5 cycles.

[0158] Thereafter cycling was conducted at C / 3 in a voltage range from 1.8 V to 3.8 V and the discharge capacity vs cycle number can be observed in figure 9. From figure 9, it can be concluded that the inventive cells exhibit a relatively stable discharge capacity over 40 cycles.

[0159] EXAMPLE 2: THERMAL STABILITY OF REFERENCE SEPARATORS COMPRISING PE OR PP

[0160] The thermal stability of conventional polyethylene (PE) and polypropylene (PP) separators were investigated at various temperatures and dryings times. The pressure during drying was 1 Pa. Figures 3a, 5a, and 7a depict a PE separator before drying and figures 3b, 5b, and 7b show the PE separators after drying at 117 °C, 140 °C and 153 °C, respectively.

[0161] As can be seen when comparing figure 3b, 5b, and 7b, the drying temperature have a prominent impact on the degree of structural malformation. All three PE separators shrink during the drying process. The shrinkage of the separator may lead to, e.g., short-circuiting when arranged in a cell. Furthermore, the PE separator dried at 153 °C obtained after drying showed a clear colour change. The colour change indicates a thermal shutdown of the separator. The thermal shutdown mechanism leads to a decreased ionic conductivity when arranged in a cell. In summary, conventional PE separators dried according to the process described herein, would not function properly in a sodium-ion cell.

[0162] The same applies for the PP separator. Figures 4a, 6a, and 8a depict PP separators before drying and figure 4b, 6b, and 8b show the PP separator after drying at 117 °C, 140 °C (drying time 4h) and 140 °C (drying time 20h) respectively.

[0163] It can be concluded that a conventional PE or PP separator when exposed to drying conditions of the present inventive concept would not provide a satisfactory functioning cell, since PE and PP separators dried already above 117 °C lose their shape, integrity and functionality.

[0164] The embodiments described above are to be understood as a few illustrative examples of the present invention. It will be understood by those skilled in the art that various modifications, combinations and changes may be made to the embodiments without departing from the scope of the present invention. In particular, different part solutions in the different embodiments can be combined in other configurations, where technically possible. The scope of the present invention is, however, defined by the appended claims.

Claims

CLAIMS1. A sodium-ion cell comprising:- a cathode comprising a sodium-containing active material and a first binder;- an anode comprising a carbonaceous active material and a second binder;- a separator; and- an electrolyte,- wherein each of the first binder, the second binder and the separator comprise a cellulosic material.

2. A method of preparing a sodium-ion cell comprising the steps of:- providing a cathode, an anode and a separator, wherein the cathode comprises a sodium-containing active material and a first binder; the anode comprises a carbonaceous active material and a second binder; wherein each of the first binder, the second binder and the separator comprise a cellulosic material;- assembling the cathode, the anode and the separator to form an electrode assembly;- drying the electrode assembly under conditions of a temperature above 100°C, and at reduced pressure, preferably at a pressure from 0.1 Pa to 100 Pa, or under inert atmosphere, such as under nitrogen or argon; and- arranging the electrode assembly in a cell casing, wherein the step of arranging the electrode assembly in the cell casing is performed before or after the step of drying.

3. The sodium-ion cell according to claim 1, or the method of preparing a sodium-ion cell according to claim 2, wherein the cellulosic material of the first binder, the second binder and the separator is independently selected from a polymer of formula I, or a salt thereof:Formula I whereinR is independently selected from H, CH3, CH2CH3, CH2CH(OH)CH3, CH2CH2OH, C(O)CH3, CH2CO2H and D-ribofuranose, preferably R is independently selected from H, CH2CO2H and D-ribofuranose;R1 is independently selected from H, CO2H and CH2OR; and n is degree of polymerization and represents the number of repeating groups, preferably n is at least 300, preferably n is at least 500, preferably n is at least 1000, preferably n is at least 5000, preferably n is at least 10000.

4. The sodium-ion cell according to claim 1 or 3, or the method of preparing a sodium- ion cell according to claim 2 or 3, wherein the cellulosic material is selected from the group consisting of: carboxymethyl cellulose (CMC), methyl cellulose (MC), ethyl cellulose (EC), ethyl methyl cellulose (EMC), hydroxypropyl cellulose (HPC), hydroxypropyl methyl cellulose (HPMC), hydroxyethyl cellulose (HEC), hydroxyethyl methyl cellulose (HEMC), hemicellulose (HC), cross-linked hemicellulose and regenerated cellulose (RC), or a salt thereof, or any combination thereof.

5. The sodium-ion cell according to any one of claims 1 or 3 to 4, or the method of preparing a sodium-ion cell according to any one of claims 2 to 4, wherein the first binder and the second binder are comprising a same or distinct cellulosic materials, preferably the first binder and the second binder are selected from the group consisting of: carboxymethyl cellulose (CMC), methyl cellulose (MC), ethyl cellulose (EC), ethyl methyl cellulose (EMC), hydroxypropyl cellulose (HPC), hydroxypropyl methyl cellulose (HPMC), hydroxyethyl cellulose (HEC), and hydroxyethyl methyl cellulose (HEMC), or a salt thereof, or any combination thereof, more preferably carboxymethyl cellulose (CMC), or a salt thereof, most preferably sodium carboxymethyl cellulose (Na-CMC).

6. The sodium-ion cell according to any one of claims 1 or 3 to 5, or the method of preparing a sodium-ion cell according to any one of claims 2 to 5, wherein the sodium-containing active material further comprises at least one transition metal, and wherein the sodium-containing active material is configured to enable reversible intercalation of sodium ions.

7. The sodium-ion cell according to any one of claims 1 or 3 to 6, or the method of preparing a sodium-ion cell according to any one of claims 2 to 6, wherein the sodium-containing active material is a Prussian blue analogue (PBA) material, preferably the PBA material is of a chemical formula represented by AxPy[Rz(CN)6]w, wherein A is Na, P and R are individually selected from one or more transition metals,preferably selected from Fe and / or Mn, more preferably P and R are Fe, l<x<2, 0<y<2, l<z<2, and l<w<2.

8. The sodium-ion cell according to any one of claims 1 or 3 to 7, or the method of preparing a sodium-ion cell according to any one of claims 2 to 7, wherein the cathode comprises the first binder at a maximum of 3 wt%, preferably at a maximum of 1 wt%, preferably the cathode comprises the first binder in a range of 0.5-3 wt%, more preferably in a range of 0.7-1.5 wt%, most preferably about 1 wt%.

9. The sodium-ion cell according to any one of claims 1 or 3 to 8, or the method of preparing a sodium-ion cell according to any one of claims 2 to 8, wherein the carbonaceous active material is selected from carbon black, hard carbon and graphite, or any combination thereof, preferably the anode comprises hard carbon.

10. The sodium-ion cell according to any one of claims 1 or 3 to 9, or the method of preparing a sodium-ion cell according to any one of claims 2 to 9, wherein the anode comprises the second binder at a maximum of 3 wt%, preferably at a maximum of 1 wt%, preferably the cathode comprises the first binder in a range of 0.5-3 wt%, more preferably in a range of 0.7-1.5 wt%, most preferably about 1 wt%.

11. The sodium-ion cell according to any one of claims 1 or 3 to 10, or the method of preparing a sodium-ion cell according to any one of claims 2 to 10, wherein at least one of the cathode or the anode comprises a further binder selected from styrene butadiene rubber (SBR) and polyacrylic acid (PAA).

12. The sodium-ion cell according to any one of claims 1 or 3 to 11, or the method of preparing a sodium-ion cell according to any one of claims 2 to 11, wherein the separator comprises cellulose, cross-linked hemicellulose, carboxy methyl cellulose (CMC), regenerated cellulose (RC), or any combination thereof.

13. The sodium-ion cell according to any one of claims 1 or 3 to 12, or the method of preparing a sodium-ion cell according to any one of claims 2 to 12, wherein the separator has one or more properties individually selected from: i) a thickness, according to test standard ISO 534, of less than 25 pm, preferably less than 20 pm, preferably in a range of 1-25 pm, more preferably in a range of 5- 25 pm, most preferably in a range of 15-20 pm; ii) an air permeability, according to test standard ISO5636-5 I JIS P8117, in a range of 35-65 s / 100 mL;iii) a porosity, according test standard ISO534 I ISO536, in a range of 40-50 %, calculated on dry matter; iv) a shrinkage, according to test standard ASTM D2732, in machine direction (MD) and cross direction (CD) of less than 2 %, preferably less than 1 %, calculated on dry matter; v) a tensile strength, according to test standard ISO 1924-2, TAPPI T494, in the machine direction (MD) of at least or more than 0.8 kN / m, preferably at least or more than 0.9 kN / m, preferably a tensile strength in a range of 0.8-2 kN / m, more preferably in a range of 0.9-1.5 kN / m, most preferably 1.0 kN / m; vi) a tensile strength, according to test standard ISO 1924-2, TAPPI T494, in the cross direction (CD) of at least or more than 0.6 kN / m; vii) an elongation, according to test standard ISO 1924-2, TAPPI T494, in the machine direction (MD) of at least or more than 1.55 %, preferably at least or more than 1.8 %, most preferably at least or more than 2.5 %, preferably an elongation in a range of 1.55-3.0 %, more preferably in a range of 1.8-2.7 %; and viii) an elongation, according to test standard ISO 1924-2, TAPPI T494, in the cross direction (CD) of at least or more than 6 %.

14. The method of preparing a sodium-ion cell according to any one of claims 2 to 13, wherein the drying is at a temperature of above 100°C, such as in a range of 120- 200°C, preferably 130-200°C, preferably 130-175°C, preferably 140-170°C.

15. A battery comprising the sodium-ion cell according to any one of claims 1 or 3 to 13.

16. A vehicle comprising the sodium-ion cell according to any one of claims 1 or 3 to 13 or the battery according to claim 15.

17. A stationary energy storage system comprising the sodium-ion cell according to any one of claims 1 or 3 to 13 or the battery according to claim 15.

18. A sodium-ion cell obtainable by the method according to any one of claims 2-14.

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