Sodium ion cells with high performance

Abstract

The present disclosure relates to high performance sodium-ion cells. In particular, the sodium-ion cells comprises a cathode with a cathode active layer having a very low water content.

Description

[0001] SODIUM ION CELLS WITH HIGH PERFORMANCE

[0002] TECHNICAL FIELD OF THE INVENTION

[0003] The present disclosure relates to high performance sodium-ion cells. In particular, the sodium-ion cells comprise a cathode with a cathode active layer having a very low water content.

[0004] BACKGROUND OF THE INVENTION

[0005] 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 energy-storage candidates that can sustain the evergrowing 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 Na-ion batteries is its poor electrochemical performance compared to competing Li-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 Na-ion batteries are less energy dense and have less storage capacity compared to Li-ion batteries, and therefore it is necessary to improve electrochemical performance of Na-ion batteries for them to be commercially viable.

[0008] Hence, there is a need to provide Na-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 process for preparing cathode active layers of high performance suitable for use in Na-ion cells. SUMMARY OF THE INVENTION

[0009] The present disclosure relates to a method for preparing a high performing cathode that may be used with Na-ion cells. The cathode comprises a cathode active layer with a very low content of water to reduce corrosion of the cathode active layer and mitigate swelling of the cell during charge-discharge cycles. Reduction of the water content is controlled during the manufacturing process and surprisingly results in a significant improvement in cell performance.

[0010] Thus, an object of the present disclosure relates to provision of a method for preparing an improved cathode for a sodium-ion cell. In particular, the cathode active layer has a very low water content.

[0011] Thus, an aspect of the present disclosure relates to a process of preparing a cathode, said process comprising the following steps:

[0012] (i) providing a slurry comprising a Prussian blue analogue (PBA) material and a solvent;

[0013] (ii) preparing a coated foil by applying the slurry onto at least one side of a conductive foil to form a cathode active layer; and

[0014] (iii) removing water from the cathode active layer to provide a coated foil wherein the water content of the cathode active layer is less than 99 ppm; wherein the PBA material is of a chemical formula represented by 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.

[0015] Another aspect of the present disclosure relates to a cathode obtained by the process as described herein.

[0016] Yet another aspect of the present disclosure relates to a cathode comprising: a conductive foil; and a cathode active layer comprising a Prussian blue analogue (PBA) material; wherein the water content of the cathode active layer is less than 99 ppm, and wherein the PBA material is of a chemical formula represented by 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.

[0017] A further aspect of the present disclosure relates to a sodium-ion cell comprising: an anode; a separator; an electrolyte; and a cathode as described herein.

[0018] A still further aspect of the present disclosure relates to a battery comprising the cathode or the sodium-ion cell as described herein.

[0019] An even further aspect of the present disclosure relates to a vehicle comprising the sodium- ion cell or the battery as described herein.

[0020] Another aspect of the present disclosure relates to a stationary energy storage system comprising the sodium-ion cell or the battery as described herein.

[0021] BRIEF DESCRIPTION OF THE FIGURES

[0022] Figure 1 shows the coulombic efficiency and normalized discharge capacity of a cell according to the disclosure over 100 cycles.

[0023] Figure 2 shows the galvanostatic cycling plot of a cell according to the disclosure on cycle 1, 50 and 100.

[0024] The present disclosure will in the following be described in more detail.

[0025] DETAILED DESCRIPTION OF THE INVENTION

[0026] Definitions

[0027] Prior to outlining the present disclosure in more details, a set of terms and conventions is first defined:

[0028] Cathode active layer

[0029] In the present context, the term "cathode active layer" refers to a layer comprising cathode active material. Preferably, the cathode active layer also comprise a binder, and optionally a conductive agent.

[0030] To produce a cathode active layer, cathode active material, and optionally binder and / or conductive agent are typically dispersed in a dispersant to form a slurry. The slurry is then deposited on a conductive foil to form the cathode active layer.

[0031] Cathode active material In the present context, the term "cathode active material" refers to a material that can reversibly intercalate ions, such as sodium ions, during cell charge and discharge cycles. Preferably, the intercalation metal of the cathode active material is sodium.

[0032] Prussian blue analogue

[0033] In the present context, the term "Prussian blue analogue" (PBA) refers to a cathode active material capable of intercalation and de-intercalation of sodium-ions, and comprising a sodium transition metal cyanide having six cyanide groups per formula unit.

[0034] In the present context, the PBA material is of a chemical formula represented by AxPy[Rz(CN)e]w, wherein A is Na, P and R are Fe, l<x<2, 0<y<2, l<z<2, and l<w<2.

[0035] Water content

[0036] In the present context, the term "water content" relates to an amount of water held by (or contained within) the cathode active layer. The water content can be determined by solid state Karl Fischer titration, for instance using a solid state Karl Fischer titration at a heating temperature of 170°C.

[0037] Karl Fischer titration is a widely used analytical technique for determining the moisture content in a substance. The basic principle of Karl Fischer titration involves the reaction between water (moisture) in the sample and a titrant solution containing iodine, sulphur dioxide, and a base. In this reaction, iodine (I2) reacts with sulphur dioxide (SO2) and water (H2O) to form hydriodic acid (2HI) and sulphur trioxide (SOs). The titration is monitored by detection method, which can involve various techniques such as potentiometry, coulometry, or amperometry. The reaction continues until all the water in the sample has reacted with the Karl Fischer reagent. The point at which the titration is complete is determined by a sudden change in the indicator signal. The amount of titrant used in the reaction is directly proportional to the amount of water in the sample. From the volume or amount of titrant required, the moisture content of the sample can be calculated.

[0038] This Karl-Fischer measurement used an evaporation oven instead of direct submersion of the sample electrodes. The purpose of the evaporation oven is to remove water from the sample where the water vapor is transferred to the titration solution, allowing for accurate and precise determination of the moisture content using the Karl Fischer titration method without interference from other substances.

[0039] The instrument used for these measurements consisted of a Metrohm 874 Oven Sample Processor. For this instrument the electrode sample is sealed in a glass vial with septum under inert atmosphere and placed in the sample loader. For the measurement the sample vial is pierced with a needle connecting it to the titration solution whereupon the sample vial is heated to a set temperature and all moisture is measured by the instrument.

[0040] The positive electrode sheets were retrieved by opening the cycled pouch cells in an Ar filled glovebox. Individual electrode sheets were then cut into smaller pieces and transferred to vials which were subsequently sealed inside the glovebox. The mass of the electrode sheets was noted down. A typical sample mass in these measurements was 0.3- 1.0 g.

[0041] The water content of the electrodes was then measured via coulometric Karl Fischer titration using the so-called oven method. In the oven method, the sealed vial is inserted into an oven compartment which heats the sample. As the sample is heated the water is released and transferred via a carrier gas (Ar or N2) into the titration solution. Here, an oven temperature of 170°C was used together with Ar as the carrier gas.

[0042] Inherent to Karl Fischer measurements is the so-called drift which is a continuous drift in the recorded water signal as a result of e.g. water adsorbed on the walls of the vessel containing the titration solution. Before a measurement is started, the system will do a pre-conditioning step and the target drift value for this step should be set to 10 pg / min. The conditions that mark the end of titration is based on an extraction time and the drift value decreasing to a value close to the value at start of determination (driftstart). A minimum extraction time of 300 seconds is recommended, together with a relative drift value (drift - driftstart) of 5-10 pg / min. This means that the titration will end when the drift reaches a value of typically < 15-20 pg / min when 300 seconds has passed. The water content is then calculated using Equation 1. water contentsampie= water contentmeasured— (drift ■ time') — water contentbiank

[0043] ( Eq. 1 )

[0044] The water content of an empty vial (water contentblank) was measured (in triplicate) prior to sample measurements in order to correct for possible water adsorbed to the walls of the vial. All vials were pre-heated at 60°C before introduction to the glovebox, which typically rendered a blank vial value of 0 pg. The water content of a cathode layer in a cell is determined by opening the cell and carefully removing the cathode, and testing a portion thereof. Cell disassembly and measurements are typically performed in a glove box having a dry and inert atmosphere, to avoid moisture depositing on the hygroscopic Prussian blue analogue material.

[0045] The water content is herein reported in part per million (ppm), which is expressed as a gravimetric value so may be used interchangeably with pg / g.

[0046] Mean particle size

[0047] In the present context, the term "mean particle size" refers to the d50 value of a population of particles, e.g. Prussian blue analogue particles. The d50 value sets the value at which 50% of the particles within the population have a diameter below this value.

[0048] The particle diameter reported herein is preferably a d [4,3] diameter as measured by laser diffraction. Suitable apparatus for measuring the mean particle size include Malvern Mastersizer 3000 from Malvern Panalytical.

[0049] About

[0050] Wherever the term "about" is employed herein in the context of amounts, for example absolute amounts, such as numbers, purities, concentrations, weights, sizes, etc., or relative amounts (e.g. percentages, equivalents or ratios), timeframes, and parameters such as temperatures, pressure, etc., it will be appreciated that such variables are approximate and as such may vary by ±10%, for example ± 5% and preferably ± 2% (e.g. ± 1%) from the actual numbers specified. This is the case even if such numbers are presented as percentages in the first place (for example 'about 10%' may mean ±10% about the number 10, which is anything between 9% and 11%).

[0051] Cathode for sodium-ion cell

[0052] Cathodes are conventionally manufactured by mixing of a cathode active material in powder form with a solvent and further ingredients of the cathode active layer, such as a binder and conductive agent, to form a slurry. The slurry is then coated onto a conductive foil. The conductive foil is typically a metal foil, such as an aluminium foil. Following coating, the solvent is removed by drying to form the cathode active layer. Subsequently, the cathode may be assembled with an anode, a separator and an electrolyte to form a working electrochemical cell.

[0053] Prussian blue analogue (PBA) materials are promising cathode active materials for use in sodium ion batteries. PBA materials have a unique crystal structure with open three- dimensional frameworks and large interstitial voids, and are capable of storing sodium ions. The PBA material is largely hygroscopic and will capture water in its structure if moisture is available in the environment. However, water in electrochemical cells are known to pose at least two problems; (i) electrolysis of water during charge-discharge cycles produce by H2 and O2 gas which leads to swelling of the cell, and (ii) interaction of water with fluorine-containing electrolytes leads to generation of HF resulting in leaching of the cathode active material. The two problems are known to reduce cell performance, and therefore it is desired to remove residual water in the PBA material.

[0054] When water is removed from the PBA material it transitions from monoclinic crystal structure to a rhombohedral crystal structure which is favourable for maximizing the energy density and cycling stability of the electrochemical cell. However, the water captured in the hygroscopic PBA material plays a central role in supporting the crystal structure of the material. Removal of water from the PBA material may lead to collapse of the crystal structure, and therefore less interstitial voids into which sodium ions can intercalate and de-intercalate during charge-discharge cycles, ultimately leading reduced charge capacity and cycle life.

[0055] Herein it has surprisingly been found that water can be removed from a particular class of PBA materials to produce cathodes which perform extremely well in sodium-ion cells. The identified class of PBA materials comprises, as the transition metal, two Fe atoms per formula unit (Fe-Fe-PBA). Sodium-ion cells based on cathodes comprising this PBA material at low content resulted in increased charge capacity and extended cycle life.

[0056] Without being bound by theory, it is contemplated that inclusion of Fe atoms in the PBA material allows the three-dimensional framework and large interstitial voids to be sustained in the rhombohedral crystal structure after removal of water. The ability to reduce the water content mitigates the performance-reducing effects of leaching of the cathode active material and swelling of the cell.

[0057] Thus, an aspect of the present disclosure relates to a process of preparing a cathode, said process comprising the following steps:

[0058] (i) providing a slurry comprising a Prussian blue analogue (PBA) material and a solvent;

[0059] (ii) preparing a coated foil by applying the slurry onto at least one side of a conductive foil to form a cathode active layer; and

[0060] (iii) removing water from the cathode active layer to provide a coated foil wherein the water content of the cathode active layer is less than 99 ppm; wherein the PBA material is of a chemical formula represented by 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.

[0061] Water may be removed from the cathode active layer at several stages of the manufacturing process. Thus, water may be removed from the manufactured cathode before assembling of the cell, after assembly of an electrode stack, and / or during initial cycling of the battery cell when the solid electrolyte interphase (SEI) is formed.

[0062] 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.

[0063] Therefore, an embodiment of the present disclosure relates to the process as described herein, wherein removal of water comprises drying conducted under vacuum.

[0064] Another embodiment of the present disclosure relates to the process as described herein, wherein the removal of water comprises drying with a drying time in the range of 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.

[0065] 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.

[0066] However, an embodiment of the present disclosure relates to the process as described herein, wherein the pressure of the vacuum is less than or equal to about 0.1 MPa.

[0067] Since applying a vacuum accelerates the drying process, it is helpful to apply the vacuum gradually to avoid risk of damage to the cathode layer 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.

[0068] 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. Another embodiment of the present disclosure relates to the process as described herein, wherein water is removed by drying at temperature in the range of from about 130°C to about 200°C, such as from about 140°C to about 175°C, preferably from to about 150°C to about 170°C.

[0069] In some variations of the process, it is preferred to delay water removal from the cathode active layer until a late stage of the manufacturing process. By doing so, the initial steps of the manufacturing process can be performed at ambient temperature, which reduce 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.

[0070] Accordingly, an embodiment of the present disclosure relates to the process as described herein further comprising the steps of:

[0071] (a) assembling the cathode with an anode and a separator to form an electrode stack; and

[0072] (b) arranging the electrode stack in a cell (or battery) casing.

[0073] It is to be understood that additional steps (a) and (b) may be performed before or after removal of water from the cathode active layer.

[0074] Thus, an embodiment of the present disclosure relates to the process as described herein, wherein removal of water from the cathode active layer is performed after the electrode stack has been arranged in the cell (or battery) casing.

[0075] The removal of water, such as by drying, may be performed in a transfer chamber (or antechamber) which is connected to or is part of a glove box. Vacuum conditions may be introduced in the transfer chamber.

[0076] Assembling of the electrode stack may be carried out by stacking or winding the electrodes (cathode and anode) with the separator. Preferably, the electrodes are stacked.

[0077] The cell (or battery) casing is not particularly limited and include both pouch cells, prismatic cells and cylindrical cells. Preferably, the cell (or battery) casing is in the form of a pouch cell. The pouch cell can be made of flexible material and can therefore accomodate different sizes or shapes. Water may also be consumed as the solid electrolyte interphase (SEI) is formed during initial cycling of the cell comprising the sodium-ion electrode stack. Thus, the process may also include steps of injecting electrolyte, sealing the cell (or battery) casing, and performing initial cycling of the cell (or battery). This does not exclude the possibility to also include an earlier step of water removal.

[0078] Therefore, an embodiment of the present disclosure relates to the process as described herein, wherein the process further comprises the steps of:

[0079] (c) adding an electrolyte to the cell (or battery) casing; and

[0080] (d) sealing the cell (or battery) casing to form a cell (or battery).

[0081] Another embodiment of the present disclosure relates to the process as described herein, wherein water is removed from the cathode active layer during initial cycling of the cell (or battery).

[0082] It is to be understood that additional steps (c) and (d) may be performed before or after removal of water from the cathode active layer.

[0083] Another embodiment of the present disclosure relates to the process as described herein, wherein water is removed from the cathode active layer during initial cycling of the cell the cycling. The cycling comprises a step of charging the cell with a rate of at most C / 5, optionally at most C / 10. An advantage of selecting such a rate is that the water removal is more efficient.

[0084] A preferred embodiment of the present disclosure relates to the process as described herein, wherein the step of charging the cell is the first charging step of the cell. By removing water from the cathode active layer during the first charging step, further sidereactions with water are reduced in following charging and discharging steps.

[0085] Another embodiment of the present disclosure relates to the process as described herein, wherein water is removed from the cathode active layer during initial cycling of the cell. The water content of the cathode active layer is at least 99 ppm, optionally at least 140 ppm, optionally at least 200 ppm, prior to the cycling of the cell. Such a process limits or completely negates the energy and time consumption in the optional drying steps prior to assembling the cell.

[0086] Another embodiment of the present disclosure relates to the process as described herein, wherein water is removed from the cathode active layer during initial cycling of the cell. The water content of the cathode active layer is at most 500 ppm, optionally at most 300 ppm, optionally at most 200 ppm, prior to the cycling of the cell. Such a process limits the amount of water content of the active cathode material, corresponding to an increased cell performance.

[0087] Addition of electrolyte and sealing of the cell (or battery) casing is preferably performed without exposure to moisture and air. Accordingly, steps (c) and (d) may be performed in a closed chamber under inert conditions, i.e. in an atmosphere devoid of oxygen and water. The closed chamber may be a glove box, e.g. with sealed-in gloves and with Argon gas.

[0088] To produce a cathode of a sodium-ion cell, a slurry with all the necessary components is formed. Typically, the slurry comprises cathode active material, binder, solvent, and optionally conductive agents. The addition of a solvent allows for easy preparation of the cathode. Suitable solvents are capable of dispersing the various components in the slurry without leading to undesirable side reactions or degradation of the components.

[0089] Accordingly, an embodiment of the present disclosure relates to the process as described herein, wherein the solvent is selected from the group consisting of NMP (N-methyl pyrrolidone), DMSO (dimethyl sulphoxide), DMF (dimethylformamide), and water.

[0090] Another embodiment of the present disclosure relates to the process as described herein, wherein the solvent is NMP (N-methyl pyrrolidone).

[0091] A preferred embodiment of the present disclosure relates to the process as described herein, wherein the solvent is water, or is water-based.

[0092] A preferred embodiment of the present disclosure relates to the process as described herein, wherein water is removed from the cathode active layer during initial cycling of the cell. The solvent is water, or is water-based. Such a process, in contrast to a process including an organic solvent-based solvent, enables removal of water emanating from other parts of the cathode, which may be the foil, and / or the binder, and / or the conductive agent. A cathode made using an organic solvent-based solvent in the slurry, typically has much lower water content than cathode made using water, or a water-based solvent, as solvent in the slurry. The slurry composition is formulated to a suitable viscosity to allow it to be processed into a cathode, for example by slot-die coating.

[0093] Therefore, an embodiment of the present disclosure relates to the process as described herein, wherein the slurry has a dynamic viscosity of from about 2 Pa-s to about 50 Pa-s, for instance from about 5 Pa-s to about 40 Pa • s or from about 5 Pa-s to about 25 Pa-s, as measured at 25°C.

[0094] Another embodiment of the present disclosure relates to the process as described herein, wherein the slurry is applied to the conductive foil by slot-die coating.

[0095] The slurry may be disposed on the conductive foil to provide coatings of different thicknesses and densities. The amount of solids in the slurry will influence the final weight of the coating.

[0096] Therefore, an embodiment of the present disclosure relates to the process as described herein, wherein the thickness of the cathode active layer is in range of about 50 pm to about 500 pm, preferably about 100 pm to about 250 pm.

[0097] Another embodiment of the present disclosure relates to the process as described herein, wherein the slurry comprises from about 50 to about 90 wt% solids, such as from about 60 to about 80 wt% solids, preferably from about 68 to about 73 wt% solids, with the remainder being solvent.

[0098] A further embodiment of the present disclosure relates to the process as described herein, wherein the weight of the cathode active layer with respect to the area of the conductive foil is in the range of about 5 mg / cm2to about 30 mg / cm2, preferably about 10 mg / cm2to about 20 mg / cm2.

[0099] The PBA material may particularly enriched in sodium to increase the charge capacity of the sodium-ion cell. Cathode active layers comprising high contents of the cathode active material can be beneficial, and in particular PBA material with large particle size produce sodium-ion cells with high performance.

[0100] Therefore, an embodiment of the present disclosure relates to the process as described herein, wherein the PBA material has a sodium content x of above 1.8, preferably above 1.9 or even more preferably at 1.92. Another embodiment of the present disclosure relates to the process as described herein, wherein the mean particle size (d50) of the PBA material is in the range of about 8 pm to about 15 pm.

[0101] A further embodiment of the present disclosure relates to the process as described herein, wherein in the cathode active layer, the cathode active material comprises, by weight, from about 60-99.9 wt% active material, such as about 70-99.9 wt%, such as about 80-99.8 wt%, such as about 90-99.6 wt%, such as about 95-99.5 wt%, such as about 96-99.5 wt%, such as about 98 wt%.

[0102] Binders adhesively connect all the cathode materials for long-term charge / discharge cycling. It is preferred that the slurry (and therefore the cathode active layer) comprises a binder. Suitable binders are well known in the art and may be water-insoluble or water- soluble. The role of the conductive agent 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.

[0103] Thus, an embodiment of the present disclosure relates to the process as described herein, wherein the slurry further comprises a binder and / or a conductive agent.

[0104] Another embodiment of the present disclosure relates to the process as described herein, wherein the slurry further comprises a binder.

[0105] Yet another embodiment of the present disclosure relates to the process as described herein, wherein the binder is selected from the group consisting of styrene butadiene rubber (SBR), carboxymethyl cellulose (CMC), sodium lignosulfonate (NaLS or LgSA), sodium alginate, polyvinylidene fluoride (PVDF), polyacetylene, sodium carboxymethyl cellulose (Na-CMC), polyacrylic acid (PAA), sodium polyacrylate (PANa), polytetrafluoro ethylene (PTFE), and hexafluoro propylene (HFP), or any combination thereof.

[0106] Preferably, the binder is selected from carboxymethyl cellulose (CMC), styrene-butadiene rubber (SBR), or mixtures thereof, typically a mixture of these binders.

[0107] A further embodiment of the present disclosure relates to the process as described herein, wherein the cathode active layer comprises, by weight, from about 0.01-10 wt% binder, such as about 0.02-8 wt%, such as about 0.05-6 wt%, such as about 0.06-4 wt% binder, such as 1 to 3 wt% binder, such as 0.2 wt% to 2 wt% binder, such as 0.5 wt% to 1.5 wt% binder. A still further embodiment of the present disclosure relates to the process as described herein, wherein the slurry further comprises a conductive agent.

[0108] An even further embodiment of the present disclosure relates to the process as described herein, wherein the conductive agent is selected from the group consisting of acetylene black, carbon black, graphene, graphite, mesocarbon microbead (MCMB), pitch-based carbon, coke powders, carbon nanotubes and metallic powders, or combinations thereof.

[0109] Preferably, the conductive agent is carbon black.

[0110] Another embodiment of the present disclosure relates to the process as described herein, wherein the cathode active layer comprises, by weight, from 0-10 wt% conductive agent, such as from about 0.01-8 wt%, such as about 0.05-6 wt%, such as about 0.06-4 wt%, such as from about 0.1 to 3 wt% conductive agent, such as about 0.2 wt% to 2 wt%, such as about 0.5 wt% to 1.5 wt%.

[0111] For the purpose of sodium-ion cells it is possible to replace the typical copper conductive foil with an aluminium conductive foil because sodium does not react with aluminium. The aluminium conductive foil is advantageous because it is cheaper.

[0112] Accordingly, an embodiment of the present disclosure relates to the process as described herein, wherein the conductive foil is an aluminium foil.

[0113] Typically, the cathode undergoes calendering to press and compact the cathode active layer onto the conductive foil. Calendering typically involves passing the cathode through heated rollers. In doing so, a cathode active layer having more uniform thickness and density is formed, for example the thickness may be altered by the calendering process. Calendering improves electrode adhesion and density and increases the electrical conductivity between particles.

[0114] Thus, an embodiment of the present disclosure relates to the process as described herein, wherein the process further comprises a calendering step either before, after or during the removal of water.

[0115] The low water content of the cathode active layer ensures that performance of the cathode is optimal, and in particular that challenges with leaching of the PBA material and swelling of the sodium-ion cell is reduced. Determination of the water content may be performed by conventional methods, such as Karl Fischer titration.

[0116] Therefore, an embodiment of the present disclosure relates to the process as described herein, wherein the water content of the cathode active layer is in the range of about 5 ppm to about 99 ppm, such as about 10 ppm to about 95 ppm, such as about 12 ppm to about 90 ppm, such as about 14 ppm to about 80 ppm, such as about 16 ppm to about 70 ppm, such as about 18 ppm to about 60 ppm, such as about 20 ppm to about 50 ppm.

[0117] Another embodiment of the present disclosure relates to the process as described herein, wherein the water content of the cathode active layer is in less than about 99 ppm, such as less than about 95 ppm, such as less than about 90 ppm, such as less than about 80 ppm, such as less than about 70 ppm, such as less than about 60 ppm, such as less than about 50 ppm.

[0118] A further embodiment of the present disclosure relates to the process as described herein, wherein the water content is measured by solid state Karl Fischer titration.

[0119] The ASTM D6304-20 test provides a general guide to determine free water and water of hydration in most liquid or solid organic and inorganic compounds using volumetric Karl Fischer titration. This test uses automatic titration for determining the Karl Fischer titration end-point potentiometrically.

[0120] Accordingly, an embodiment of the present disclosure relates to the process as described herein, wherein the water content is measured according to ASTM D6304-20 using a heating temperature of 170°C.

[0121] The process described herein allows manufacture of cathodes that may be utilised in sodium-ion cell to improve their performance, such as increased charge capacity or extended cycle life. The process exploit the finding that Prussian blue analogue materials with two Fe atoms in unit formula (Fe-Fe-PBA) yield advantageous properties when applied at very low water content.

[0122] Thus, an aspect of the present disclosure relates to a cathode obtained by the process as described herein.

[0123] Another aspect of the present disclosure relates to a cathode comprising: a conductive foil; and a cathode active layer comprising a Prussian blue analogue (PBA) material; wherein the water content of the cathode active layer is less than 99 ppm, and wherein the PBA material is of a chemical formula represented by 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.

[0124] The cathodes described herein can be implemented in sodium-ion cell through conventional cell production. A sodium-ion cell comprises a cathode, an anode, a separator disposed between the anode and cathode, said cathode, anode and separator forming an electrode assembly, the cell further comprising a casing for the electrode assembly. An electrolyte is introduced into the casing to surround 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 in electrical contact with the anode and composite cathode.

[0125] Accordingly, an aspect of the present disclosure relates to a sodium-ion cell comprising: an anode; a separator; an electrolyte; and a cathode as described herein.

[0126] Examples of suitable anode include, but is not limited to, metal composite oxides, sodium metal, sodium alloys, silicon, silicon-based alloys, tin-based alloys, bismuth-based alloys, metal oxides, conductive polymers, Na-Co-Ni-based materials, and hard carbon.

[0127] An embodiment of the present disclosure relates to the sodium-ion cell as described herein, wherein the anode comprise any one or a mixture of two or more of carbon black, hard carbon or graphite.

[0128] An embodiment of the present disclosure relates to the sodium-ion cell as described herein, wherein in the anode active layer, the anode active material comprises, by weight, from about 60-99.9 wt% active material, such as about 70-99.9 wt%, such as about 80-99.8 wt%, such as about 90-99.6 wt%, such as about 95-99.5 wt%, such as about 96-99.5 wt%, such as about 98 wt%.

[0129] A separator is included in the sodium-ion cell to prevent electrical short circuit between the cathode and anode. Moreover, the separator also provide stability to the cell. The material of the separator is not particularly limited and can be any material which is chemically stable and electrically insulating. An embodiment of the present disclosure relates to the sodium-ion cell as described herein, wherein the separator is selected from the group consisting of polypropylene film, polyethylene film, polyethylene / polypropylene / polyethylene composite film, cellulose film, non-woven fabrics film and glass fibre film, preferably cellulose film.

[0130] The electrolyte is preferably a non-aqueous electrolyte which prevents water from interfering with the PBA material of the cathode. Non-aqueous electrolytes are also advantageous in that they offer a large voltage window compared to aqueous electrolytes.

[0131] An embodiment of the present disclosure relates to the sodium-ion cell as described herein, wherein the electrolyte is selected from the group consisting of sodium hexafluorophosphate, sodium tetrafluoroborate, sodium bis(oxalato)borate salt organic carbonates, propylene carbonate, ethylene carbonate, vinylene carbonate, ethyl methyl carbonate, dimethyl carbonate, and diethyl carbonate, or any combination thereof.

[0132] The sodium-ion cells may be combined to form a battery system (i.e. an array of cells), such as a stationary energy storage system, for instance a back-up generator, reserve power source, portable power bank (e.g. suitable for temporary use such as music events), or the like.

[0133] Accordingly, an aspect of the present disclosure relates to a battery comprising the cathode or the sodium-ion cell as described herein.

[0134] Another aspect of the present disclosure relates to a stationary energy storage system comprising the sodium-ion cell or the battery as described herein.

[0135] The disclosure also relates to an electrical device comprising a cell of the disclosure. For instance, the disclosure relates to a vehicle comprising a cell (or battery system) of the disclosure. The vehicle is preferably an electric vehicle, such as a car, truck, bus, scooter, motorbike, bicycle or the like, preferably a car, truck or bus.

[0136] Therefore, an aspect of the present disclosure relates to a vehicle comprising the sodium- ion cell or the battery as described herein.

[0137] The listing or discussion of an apparently prior published document in this specification should not necessarily be taken as an acknowledgement that the document is part of the state of the art or is common general knowledge. Unless otherwise indicated, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this disclosure pertains.

[0138] Preferences, options and embodiments for a given aspect, feature or parameter of the disclosure should, unless the context indicates otherwise, be regarded as having been disclosed in combination with any and all preferences, options and embodiments for all other aspects, features and parameters of the disclosure. This is especially true for the description of the process of preparing cathode, and all its features, which may readily be part of the cathode as such, as well as the sodium-ion cell and batteries comprising the cathode as described herein.

[0139] EXAMPLES

[0140] EXAMPLE 1: EVALUATION OF WATER CONTENT IN ASSEMBLED SODIUM-ION CELLS

[0141] Sodium-ion cells were prepared according to the process described herein. Briefly, a slurry comprising a Fe-Fe-PBA material was disposed on an aluminium foil to provide a coated foil. The coated foil was further processed using calendering to a final cathode. Cathodes were assembled with anodes and separators to form an electrode stack and arranged in a battery casing. The electrode stack was then dried at 165°C for 24 hours initially under a nitrogen atmosphere, then under reduced pressure ultimately at 1 Pa. Electrolyte was injected into the battery casing to form a sodium-ion cell.

[0142] For measurement of the water content in the cathode active layer, cathodes were removed from the sodium-ion cell and analysed by solid state Karl Fischer titration. Cathodes were removed from three positions within the sodium-ion cell; (i) top, (ii) top-middle, and (iii) middle of the electrode stack.

[0143] Titration measurements showed that the position of the cathode in the electrode stack made a small difference, but that all cathodes comprised a low amount of water, i.e. below 50 ppm (see Table 1). Table 1. Cathode position in electrode stack Water content (ppm)

[0144] The cell underwent Galvanostatic cycling using a Neware BTS4000 galvanostat under controlled conditions. Formation was carried out by conducting one C / 10 and two C / 5 cycles. Voltage cut-offs were adjusted to 3.8 V and 1.8 V, respectively. Thereafter, cycling was conducted with a C / 3 current between 3.8 and 1.8V. Additionally, a constant voltage hold was employed during charge until the current drops to a C / 10 current.

[0145] Figure 1 shows the coulombic efficiency(top) and normalised discharge capacity through 100 cycles. The cell shows excellent performance characteristics retaining above 95% efficiency through 100 cycles. Figure 2 shows the cycling plot on cycle 1, 50 and 100.

Claims

CLAIMS1. A process of preparing a cathode, said process comprising the following steps:(i) providing a slurry comprising a Prussian blue analogue (PBA) material and a solvent;(ii) preparing a coated foil by applying the slurry onto at least one side of a conductive foil to form a cathode active layer; and(iii) removing water from the cathode active layer to provide a coated foil wherein the water content of the cathode active layer is less than 99 ppm; wherein the PBA material is of a chemical formula represented by 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.

2. The process according to claim 1, wherein removing water comprises drying under vacuum.

3. The process according to any one of claims 1 or 2, wherein removing water comprises drying at temperature in the range of about 130°C to about 200°C, preferably about 140°C to about 175°C.

4. The process according to any preceding claim, wherein removing water comprises cycling the cell thereby consuming water during formation of the solid electrolyte interface.

5. The process according to claim 4, wherein the cycling comprises a step of charging the cell with a rate of at most C / 5, optionally at most C / 10.

6. The process according to claim 5, wherein the step of charging the cell is the first charging step of the cell.

7. The process according to any one of claims 4-6, wherein the water content of the cathode active layer is at least 99 ppm, optionally at least 140 ppm, optionally at least 200 ppm, prior to the cycling of the cell.

8. The process according to any one of claims 4-7, wherein the water content of the cathode active layer is at most 500 ppm, optionally at most 300 ppm, optionally at most 200 ppm, prior to the cycling of the cell.

9. The process according to any one of the preceding claims, wherein the solvent is water.

10. The process according to any one of the preceding claims, wherein the slurry further comprises a binder and / or a conductive agent.

11. The process according to any one of the preceding claims, wherein the slurry is applied to the conductive foil by slot-die coating.

12. The process according to any one of the preceding claims, wherein the process further comprises a calendering step either before, after or during the removal of water.

13. The process according to any one of the preceding claims, wherein the water content of the cathode active layer is in the range of about 5 ppm to about 99 ppm, such as about 10 ppm to about 95 ppm, such as about 12 ppm to about 90 ppm, such as about 14 ppm to about 80 ppm, such as about 16 ppm to about 70 ppm, such as about 18 ppm to about 60 ppm, such as about 20 ppm to about 50 ppm.

14. The process according to any one of the preceding claims, wherein the water content is measured by solid state Karl Fischer titration.

15. The process according to any one of the preceding claims, wherein the water content is measured using a solid state Karl Fischer titration at a heating temperature of 170°C, for instance using ASTM D6304 using a heating temperature of 170°C.

16. A cathode obtained by the process according to any one of the preceding claims.

17. A cathode comprising: a conductive foil; and a cathode active layer comprising a Prussian blue analogue (PBA) material; wherein the water content of the cathode active layer is less than 99 ppm, and wherein the PBA material is of a chemical formula represented by 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.

18. A sodium-ion cell comprising: an anode; a separator; an electrolyte; anda cathode according to any one of claims 16 or 17.

19. A battery comprising the cathode according to any one of claims 16 or 17 or the sodium-ion cell according to claim 18.

20. A vehicle comprising the sodium-ion cell according to claim 18 or the battery according to claim 19.

21. A stationary energy storage system comprising the sodium-ion cell according to claim 18 or the battery according to claim 19.

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