Method of preparing a sodium ion cell

Abstract

The disclosure relates to a method of preparing an electrode assembly for a sodium ion cell, comprising a cellulose separator The properties of the cell may be improved by optimising the drying process of the electrode assembly by measuring and analysing the colour of the cellulose separator. In particular, there is provided a method of preparing an electrode assembly comprising a cellulose separator and a Prussian Blue Analogue as a cathode.

Description

[0001] METHOD OF PREPARING A SODIUM ION CELL

[0002] FIELD OF THE INVENTION

[0003] The disclosure relates to a method of preparing an electrode assembly for a sodium ion cell comprising a drying step which is monitored by observing the colour of the separator as it is dried, and a cell comprising said electrode assembly prepared by the method of the present disclosure.

[0004] BACKGROUND

[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 cells (Na-ion cells) 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] A Na-ion cell comprises two electrodes (the cathode and the anode) which are separated by a separator.

[0008] A separator is typically a permeable membrane, which has a main function of keeping 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.

[0009] Aside from being chemically stable, a separator for a Na-ion cell also needs good thermal stability given that elevated temperatures are required to remove liquids (particularly water) prior to refilling with electrolyte and sealing of the cell. This has led to an increased interest in cellulose-based separators as these possess very good thermal stability. However, cellulose separators, if not wet from manufacturing, may pick up moisture during handling and storage. This can be problematic, as cathode active materials, and in particular sodium active materials, are highly sensitive to moisture. It is therefore necessary to thoroughly dry the cellulose separators during the assembly of a cell.

[0010] However, if the cellulose is dried too much (overdried) it may become brittle. The brittle sheets of cellulose may be fragile and prone to tearing, making them difficult handle during manufacture. This issue has led to cell production which involves formation of an electrode assembly, and drying this electrode assembly before incorporation into the cell. The formation of the electrode assembly prior to drying reduces the amount of handling of the separator after drying, reducing the risk of damage.

[0011] Accordingly, it is important to find drying conditions for the electrode assembly comprising a cellulose separator that result in optimised battery / cell performance.

[0012] Prussian blue analogue (PBA) cathode materials stand out as promising cathode materials for use in sodium ion batteries. Prussian blue analogues have a unique crystal structure with open three-dimensional frameworks and large interstitial voids, and are capable of storing sodium ions.

[0013] PBAs have a strong affinity for water, due to the potential for hydrogen bonding between the water molecules and the crystal structure. Any water present in the PBA structure should be removed, or at least the amount should be reduced, for the material to fully utilize its capacity as a cathode in a battery cell. Therefore, when PBA is used as the active cathode material, drying is particularly important.

[0014] A challenge that arises is that monitoring of the water content of the PBA material is often not possible when it is contained within the electrode assembly during drying. Likewise, the structural integrity of the cellulose separator cannot be monitored during drying. There is therefore a need for overcoming the problems of monitoring and controlling the water content of the electrode assembly, particularly in the manufacture of sodium ion batteries comprising Prussian blue analogues as cathode materials. SUMMARY

[0015] An object of the present disclosure is generally to provide a method of preparing a sodium ion electrode assembly for a cell. The electrode assembly comprises a cellulose separator, a cathode and an anode. The drying step of the method is optimised by measuring a colour property of the cellulose separator, and the colour property measurement is compared to a reference range. In particular, there is provided a method of preparing an electrode assembly comprising Prussian Blue Analogues as cathode active materials, comprising measuring a colour property of the cellulose separator during the drying step.

[0016] The present disclosure particularly relates to a method of preparing a sodium ion electrode assembly, comprising:

[0017] (a) providing an electrode assembly comprising an anode, a cathode, and a cellulose separator;

[0018] (b) drying the electrode assembly under drying conditions comprising a temperature above 100°C optionally under vacuum; wherein a colour property of the cellulose separator is measured during step (b), and step (b) is terminated when the colour property of the cellulose separator is in a reference range.

[0019] Also provided in the present disclosure is a process for evaluating a heating cycle, wherein the process comprises:

[0020] (i) subjecting a cellulose separator to the heating cycle comprising a temperature above 100°C optionally under vacuum;

[0021] (ii) measuring a colour property of the cellulose separator after it has been subjected to the heating cycle; and

[0022] (iii) determining if the colour property of the cellulose separator is within a reference range.

[0023] In the process of the disclosure, the heating cycle is preferably part of a manufacturing process for making an electrochemical cell, particularly in the drying cycle used to dry the electrode assembly. Preferably, the colour property is CIELab-b, CIEL*a*b*-b, CIEL*C*h*-C, CIEL*u*v*-v, or an equivalent thereof.

[0024] The present disclosure also relates to a sodium ion electrode assembly comprising an anode, a cathode, and a cellulose separator, as formed by a method of the present disclosure.

[0025] The present disclosure also relates to an electrochemical cell comprising an electrode assembly formed by a method of the present disclosure.

[0026] The present disclosure also relates to use of a cellulose separator to improve cycle life of an electrochemical cell, wherein a colour property of the cellulose separator is measured during the manufacture of the electrochemical cell comprising said cellulose separator.

[0027] The present disclosure also relates to use of a colour measurement device to improve the cycle life of an electrochemical cell, particularly wherein the colour measurement device is capable of detecting a colour change in a cellulose separator during manufacture (and particularly the drying step) of the electrochemical cell.

[0028] The colour measurement device may be any device that it set up to take measurements of a colour property of a cellulose separator, preferably to take quantitative measurements that can be used to measure a colour change. It may for example be a commercially available colorimeter or spectrophotometer. The colour measurement device may be set up to measure a colour property at intervals, or continuously, for example throughout a drying process.

[0029] Surprisingly, the inventors have found that monitoring a colour property, in particular a colour change, of the cellulose separator during the drying step of cell formation allows the performance properties, particularly the charge capacity and the cycle life, of the cell to be optimised. Monitoring of the colour property can therefore be used as an indicator or quality check for when the drying process of the electrode assembly is complete.

[0030] Likewise, in the process of the disclosure, the colour change of the cellulose separator may be used to evaluate the suitability of a heating cycle in the production of an electrochemical cell comprising said cellulose separator. This finds particular use in quality control, for instance in a manufacturing process where a continuous production line may be configured in a number of different ways, depending on what particular cell type it is being used to produce, the process of the disclosure can be used to test that the production line is configured correctly before production is started.

[0031] Both the method and process rely on the observation that, during drying, the off-white cellulose separator becomes yellow and eventually if overdried a brown colour develops. The extent of the colour change during heating is in part linked to drying of the cellulose, but surprisingly has also been found to be linked to the performance properties of the cell, particularly the charge capacity and cycle life. If the electrode assembly is underdried, the initial discharge capacity may not be optimised. The discharge capacity can be increased by suitably drying at a higher temperature. However, if it becomes a dark brown colour, this indicates that the separator may have been overdried and potentially damaged, resulting in the properties of the cell no longer being optimal. For example, the loss of capacity over the number of charge-discharge cycles may be increased (i.e. reduced cycle life) if the cellulose separator becomes dark brown (i.e. it is overdried).

[0032] The poor cell performance can be attributed to the cellulose separator rather than the cathode and anode, as it has been found that subjecting these electrodes to similar conditions that lead to overdrying and forming an electrode assembly with a non-cellulose separator has no impact on cell performance. Without wishing to be bound by theory, it is believed that the loss in cycle life is not directly correlated to the water content of the cellulose separator, which does not significantly change as the brown colour develops. Rather, it is thought that the dark brown colour that develops in cellulose separators may arise due to unwanted chemical species that interfere with the battery performance. Additionally, it is indicative of a cellulose separator that has poor structural integrity.

[0033] Accordingly, by observing the colour of the cellulose separator during drying, for example with a colour measurement device such as a colorimeter or spectrophotometer, the drying process can be stopped at the point at which the desired colour indicates that the optimum amount of drying has taken place.

[0034] FIGURES SUMMARY

[0035] Figure 1 contains a series of graphs showing the decrease in the discharge capacity (mAh / g) over the number of charge cycles of the electrochemical cell of the disclosure, when it is dried at different temperatures. Figure 2 shows electrode assemblies comprising cellulose Separator A, which have been dried at different temperatures in the oven.

[0036] Figure 3 shows electrode assemblies comprising cellulose separator B, which have been dried at different temperatures in the oven.

[0037] Figure 4 shows electrode assemblies comprising a cellulose separator C, which have been dried at different temperatures in the oven.

[0038] DETAILED DESCRIPTION

[0039] The disclosure relates to a method of preparing an electrode assembly comprising a cellulose separator. The properties of the cell, particularly the charge capacity and cycle life, are improved by optimising the drying process of the electrode assembly by measuring and analysing the colour of the cellulose separator. In particular, there is provided a method of preparing an electrode assembly comprising a cellulose separator and a Prussian Blue Analogue as a cathode.

[0040] ELECTRODE ACTIVE LAYER

[0041] In the context of this disclosure, an "electrode active layer" is a layer comprising binder, electrode active material, and optionally conductive additives.

[0042] 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.

[0043] 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.

[0044] The electrode active material may be either cathode active material or anode active material.

[0045] An "anode active layer" is a layer comprising binder, anode active material, and optionally conductive additives. A "cathode active layer" is a layer comprising binder, cathode active material, and optionally conductive additives.

[0046] Each of the components within an electrode active layer will be described below:

[0047] Binder

[0048] The binder adhesively connects all the electrode materials for long-term charge / discharge cycling. The role of the optional 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.

[0049] Suitable binders are well known in the art and may be water-insoluble or water-soluble.

[0050] Examples of binders for a sodium ion cell include 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), hexafluoro propylene (HFP), or any combination thereof.

[0051] Preferably, the binder comprises carboxymethyl cellulose (CMC), styrene butadiene rubber (SBR), and / or a combination thereof. Even more preferred, the binder comprises both carboxymethyl cellulose (CMC) and styrene butadiene rubber (SBR).

[0052] The electrode active layer may comprise, by weight, from 0.01-10 wt% binder, for example from about 0.02-8 wt%, from about 0.05-6 wt%, or from about 0.06-4 wt% binder. Preferably the cathode active layer comprises from about 0.1 to 3 wt% binder, even more preferably 0.2 wt% to 2 wt% binder, such as from 0.5 wt% to 1.5 wt% binder. Most preferably the electrode active layer comprises about 1 wt% binder.

[0053] Conductive additives

[0054] Suitable conductive additives include acetylene black, carbon black, graphene, graphite, mesocarbon microbead (MCMB), pitch-based carbon, coke powders, carbon nanotubes or metallic powders. These conductive additives may be used alone or in combination. Preferred conductive additives are selected from carbon black, graphite, carbon nanotubes, or mixtures thereof.

[0055] In an embodiment, the cathode active layer may comprise from 0-10 wt% conductive additive, for example from about 0.01-8 wt%, from about 0.05-6 wt%, or from about 0.06- 4 wt% conductive additive. Preferably the cathode active layer comprises from about 0.1 to 3 wt% conductive additive, even more preferably 0.2 wt% to 2 wt% conductive additive, such as from 0.5 wt% to 1.5 wt% conductive additive. Most preferably the cathode active layer comprises about 1 wt% conductive additive.

[0056] In an embodiment, the anode active layer may comprise from 0-10 wt% conductive additive, for example from about 0.01-8 wt%, from about 0.05-6 wt%, or from about 0.06- 4 wt% conductive additive. Preferably the anode active layer comprises from about 0.5 to 3 wt% conductive additive.

[0057] Typically, when the anode active material comprises graphite, no conductive additive is needed. Other carbon based anode active materials such as hard carbon often require a conductive additive to be included.

[0058] Cathode active material

[0059] The role of the cathode active material is to reversibly intercalate ions (such as sodium ions) during cell charge and discharge cycles.

[0060] The cathode active material of the disclosure is an intercalation material, wherein the intercalation metal is sodium.

[0061] In the context of the disclosure "cathode active material" refers to any material that is suitable for use as the positive electrochemically active material in a cathode, and suitable for use in a cell.

[0062] The cathode active material may comprise any one or a mixture of two or more sodium metal, sodium containing alloys, sodium containing oxides, sodium containing cyanides, or any combination thereof.

[0063] Preferably, the cathode active material comprises sodium transition metal cyanides having six cyanide groups per formula unit. Each of these cyanide groups connect transition metals within the cathode active material to form a framework with large voids that allow intercalation and de-intercalation of sodium-ions.

[0064] Exemplary sodium transition metal cyanides include Prussian Blue (PB) and its derivatives, i.e. Prussian Blue Analogues (PBA). Even more preferably, the cathode active material comprises Prussian Blue Analogues (PBA).

[0065] Exemplary Prussian blue analogues include Prussian White, Turnbull's blue, potassium ferricyanide, and potassium ferrocyanide.

[0066] In some embodiments, the cathode active material comprises Prussian Blue Analogues (PBAs) having a formula of AxPy[Rz(CN)6]w, where A is a sodium ion, and P and R are transition metals. Prussian Blue Analogues (PBA) offer many opportunities for structural variation and hence the properties are highly tunable. For example, the stoichiometry may vary: l<x<2, 0<y<2, l<z<2, and l<w<2.

[0067] The transition metals P and R may each be selected from manganese (Mn), Iron (Fe), Aluminium (Al), Titanium (Ti), Nickel (Ni), Vanadium (V) and Cobalt (Co).

[0068] Preferably, R is iron (Fe). More preferably, P and R are both iron (Fe).

[0069] Preferred are Prussian Blue Analogues (PBA) selected from Fe-Fe-PBA, Mn-Fe-PBA, Fe-Ni- PBA, Ni-PBA, Co-PBA, or any combination thereof, especially Fe-Fe-PBA.

[0070] Prussian Blue Analogues (PBA) may be complexed to water.

[0071] In the cathode active layer, the cathode active material is typically present in an amount of, by weight, from about 60-99.9 wt% active material, for example from about 70-99.9 wt%, from about 80-99.8 wt%, from about 90-99.6 wt%, or from about 95-99.5 wt% active material. Preferably the cathode active layer comprises from about 96-99.5 wt% active material, even more preferably the cathode active layer comprises about 98 wt% active material.

[0072] Anode active material

[0073] The role of the anode active material is to reversibly bind ions (such as sodium ions) during cell charge and discharge cycles. In the context of the disclosure "anode active material" refers to any material that is suitable for use as the negative electrochemically active material in a cathode, and suitable for use in a cell.

[0074] The anode active material may comprise any one or a mixture of two or more of carbon black, hard carbon or graphite.

[0075] Preferably, the anode active material comprises hard carbon.

[0076] In the anode active layer, the anode active material is typically present in an amount of, by weight, from about 60-99.9 wt% active material, for example from about 70-99.9 wt%, from about 80-99.8 wt%, from about 90-99.6 wt%, or from about 95-99.5 wt% active material. Preferably the anode active layer comprises from about 96-99.5 wt% active material, even more preferably the anode active layer comprises about 98 wt% active material.

[0077] SEPARATOR 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.

[0078] Cellulose based materials typically used in sodium ion cells are environmentally friendly and have good thermal stability. Typically, when cellulose or derivatives thereof is used as a separator, a sheet of cellulose or a derivative thereof is disposed between the anode and the cathode during formation of the cell.

[0079] The separator of the disclosure comprises fibres comprising cellulose. The separator of the disclosure may be commercially available.

[0080] In the context of this disclosure, "fibres comprising cellulose" and "cellulosic fibres" are used interchangeably and are meant to mean fibres made from cellulose or a derivative thereof. For example, the cellulosic fibres may comprise hemicellulose or carboxy methyl cellulose (CMC), or a combination thereof.

[0081] Alternatively or additionally, the cellulosic fibres may comprise regenerated cellulose.

[0082] 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.

[0083] Preferably, the cellulosic fibres comprise regenerated cellulose fibres such as lyocell and rayon.

[0084] In an embodiment of the disclosure, the separator comprises cellulosic fibres, wherein the cellulose fibres comprise hemicellulose, carboxy methyl cellulose (CMC), regenerated cellulose or a combination thereof, and wherein the regenerated cellulose may include fibres such as lyocell and / or rayon.

[0085] By "cellulosic material" is meant any kind of material that comprises cellulose or derivatives thereof. For example hemicellulose, carboxy methyl cellulose (CMC), regenerated cellulose or a combination thereof, and wherein the regenerated cellulose may include fibres such as lyocell and / or rayon.

[0086] The cellulose separator may contain other components to act as a stabiliser within the layer. Suitable materials include polyaramid fibres.

[0087] 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.

[0088] ELECTRODE FORMATION

[0089] 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. The addition of a dispersing medium allows for easy preparation of the electrode. Suitable dispersing media are capable of dispersing the various components in the slurry without leading to undesirable side reactions or degradation of the components. Suitable dispersing medium include NMP (N-methyl pyrrolidone), DMSO (dimethyl sulphoxide), DMF (dimethylformamide), and water.

[0090] Preferably, the dispersing medium is water.

[0091] The slurry composition is formulated to a suitable viscosity to allow it to be processed into an electrode, for example by slot-die coating.

[0092] 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.

[0093] 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.

[0094] In an embodiment, a slurry for forming an electrode comprises dispersing medium, electrode active material, a binder, and optionally conductive additive.

[0095] The slurry is deposited on a foil. Preferably, the foil is an aluminium foil.

[0096] 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.

[0097] 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. 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.

[0098] For example, an electrode may be formed using a process comprising the following steps:

[0099] 1. providing a foil and a dispersion comprising a solvent, electrode active material, binder, and optionally conductive additives;

[0100] 2. disposing said dispersion onto said foil to provide a coated foil;

[0101] 3. calendering the coated foil to provide an electrode comprising a foil and an electrode active layer.

[0102] Optionally, the electrode forming process may include a heating step, either before, after or during calendering, or any combination of these possibilities.

[0103] 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.

[0104] ELECTRODE ASSEMBLY

[0105] After an electrode is formed, an electrode stack comprising an anode, a cathode and the separator may be assembled to form an "electrode assembly".

[0106] The anode and cathode may be stacked or wound with the separator to from an electrode assembly. For example, the electrode assembly may be assembled by any of the techniques well known in the art, such as cylindrical winding, prismatic winding, Z-folding, and single sheet stacking. Preferably, the electrode assembly is formed by Z-folding.

[0107] The cellulose separator may protrude from the electrode assembly, preferably so that it is visible during the drying process. The protruding cellulose separator may enable easier and / or more efficient measurement of the colour property of the cellulose separator.

[0108] In particular, the cellulose separator may be wrapped or folded around the remainder of the electrode assembly. For example, a portion of the cellulose separator may protrude from the electrode assembly and be wrapped around the electrode assembly to encase the assembly. Preferably, the cellulose separator may form an enclosure around the electrode assembly.

[0109] A colour property of the cellulose separator may be measured by observing the actual cellulose separator of the electrode assembly. This is possible when the cellulose separator protrudes out of the cell assembly, and is particularly when the cellulose separator forms an enclosure around the electrode assembly.

[0110] Additionally or alternatively, a colour property of a reference cellulose separator may be measured, wherein the reference cellulose separator is formed from identical material as the cellulose separator in the electrode assembly, but is separate from the electrode assembly. The separate sheet of cellulose may be placed in the same oven, under the same drying conditions (time and temperature). The colour property of this separate cellulose sheet may be used as a proxy for determining the optimum drying conditions for the electrode assembly. Using this proxy may be particularly useful when a colour property of the cellulose separator is not visible or able to be measured during the drying process, for example if it is not protruding from the electrode assembly.

[0111] DRYING

[0112] Prior to introduction of the electrolyte, the electrode assembly is dried under drying conditions comprising a temperature above 100°C, for instance from 100°C to 200°C, preferably from 120°C to 180°C, more preferably from 140°C to 175°C.

[0113] The drying may occur in an oven, for example for up to 48 hours, typically from 10 hours to 36 hours, often from 10 hours to 24 hours, preferably from 12 hours to 20 hours.

[0114] The drying stage preferably occurs in a vacuum.

[0115] Thus, 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.

[0116] 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. 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.

[0117] Drying may therefore take place at a pressure of from 0.1 to 100 Pa.

[0118] 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.

[0119] The drying process removes water from the cathode and anode, as well as residual water in the cellulose separator. Typically, drying will remove water adhered to the surfaces as well as interstitial water (i.e. water in the intercalation sites within the cathode and anode), which is beneficial for the capacity of the cell.

[0120] In relation to cathode materials, drying at an increased temperature typically leads to improved cell properties, especially as the extremely hydrophilic nature of a PBA cathode means that a high temperature is required to break the hydrogen bonds and release the water molecules from the structure. Indeed, it is generally found that cathode active materials have optimum performance under a range of drying conditions, provided sufficient amounts of water is removed from the cathode layer.

[0121] However, the cellulose separator can become damaged and lose structural integrity if exposed to too harsh conditions when drying, impacting the cycle life of the cell.

[0122] Therefore, it is challenging to determine the optimal conditions required to dry the electrode assembly comprising a cellulose separator.

[0123] The inventors have surprisingly found that monitoring the colour of the cellulose separator during the drying process can overcome this challenge, as the colour acts as an indicator of the drying extent, and the colour of the cellulose is correlated with improved properties of the resulting cell, particularly improved charge capacity and improved cycle life.

[0124] Without wishing to be bound by theory, it is hypothesized that the heating of the electrode assembly during the drying stage and the resulting colour change may be causing a chemical reaction in the cellulose to occur, in addition to reducing the water content of the electrode assembly. For example, a Maillard reaction (non-enzymatic browning) or a caramelisation of sugars released from the cellulose may be occurring, which may alter the chemical structure and / or properties of the cellulose separator impacting the properties of the resulting cell. At least initially, there is improved charge capacity and improved cycle life. However, if the browning progresses too far, cell performance and particularly cell cycle life is significantly reduced. Without wishing to be bound by theory, it may be possible that the yellowing and ultimate browning of the cellulose during drying is indicative of a redox active components being formed in the cellulose, which may be interfering with the cell properties.

[0125] The yellowness of the cellulose separator may correspond to the effectiveness of an electrochemical cell formed from the electrode assembly prepared by the method of the present disclosure. The yellowness of the separator (i.e. the colour of the separator) is preferably monitored by the CIELab-b value, CIEL*a*b*-b value, CIEL*C*h*-C value, CiEL*u*v*-v value, or an equivalent thereof.

[0126] It has been observed for a number of cellulose separators that the CIELab-b value increases during the initial stages of drying. The increase in CIELab-b value correlates with an increase in cell charge capacity and improvement in cycle life. However, prolonged exposure to the wrong drying conditions and particularly prolonged exposure to high temperatures leads to the CIELab-b value decreasing and a resultant drop in cycle life.

[0127] During this progression, the CIELab-a value of the cellulose separator will typically increase, while the CIELab-L value of the cellulose separator will typically decrease. These colour values do not therefore directly correlate with the performance of the dried electrode assembly.

[0128] Surprisingly therefore the CIELab-b value can be used to determine the optimised drying conditions of the electrode assembly. Furthermore, the drop in CIELab-b value can be indicative that overdrying of the electrode assembly may be occurring and the drying process can be terminated.

[0129] As the CIELab-b value correlates within the came colour space as the CIEL*a*b*-b value, CIEL*C*h*-C value, CIEL*u*v*-v value, any of these values (or any equivalent thereof) may be used in a similar way to the CIELab-b value to monitor the progress of the drying process.

[0130] In the method of the disclosure, the reference range may be an optimum range, i.e. the range within which the colour property (preferably the CIELab-b value, CIEL*a*b*-b value, CIEL*C*h*-C value, CIEL*u*v*-v value, or an equivalent thereof) should be maintained during drying.

[0131] The reference range may also be a termination range, or in other words if the colour develops to provide a value in the termination range, the drying cycle should be stopped. This allows the colour property of the cellulose separator to be used as a means to monitor the drying cycle, and to stop the method if colours indicative of poor cell performance start to develop. This is particularly useful in large scale manufacturing where there may be temperature gradients within the drying oven. Monitoring of any cellulose separators which develop a colour in a termination range can be indicative of a 'local hot spot' within the oven, for instance.

[0132] The reference range (for instance the optimum range or termination range) is defined by an upper and lower limit. The colour property value can enter this range by decreasing from a value above the upper limit to a value at or below the upper limit, or by increasing from a value below the lower limit to a value at or below the lower limit.

[0133] In the context of the reference range, "within" means that a value can be the above, or at, the lower limit of the range, and below, or at, the upper limit of the range.

[0134] For example, when the colour property is CIELab-b, the lower limit of the optimum range may be 11, typically 12, such as 12.5, such as 13, such as 13.5, or even 14.

[0135] For the CIELab-b colour property, the value decreases as the cellulose separator becomes overdried and cycle life begins to drop. The optimum range does not therefore have an upper value which, if passed, correlates with a decrease in cell performance. The optimum range may therefore be described as 11 or above, typically 12 or above, such as 12.5 or above, such as 13 or above, such as 13.5 or above, or even 14 or above. CIELab CIELabln other cases, particularly if it is favourable to have colour property below a certain value, the optimum range may be solely defined by an upper limit and the colour property may be maintained below this upper limit.

[0136] Some cellulose separators may be intrinsically yellow, and may have a CIELab-b value above 11 even before exposure to the drying conditions. In such cases, the lower limit of the optimum range is a CIELab-b value of 1 unit above the CIELab-b value of the cellulose separator prior to exposure to the drying conditions.

[0137] In some embodiments, the reference range is a termination range. The termination range can be used as a safety means to check that negative cell properties (such as loss of cycle life) do not develop as a result of poor drying conditions. The drying process can therefore be terminated once the colour property (preferably a CIELab-b value) is in a termination range.

[0138] As the CIELab-b value increases upon exposure to the drying conditions, overdrying can be detected and prevented by monitoring when the CIELab-b value begins to decrease. When the colour property is CIELab-b, the upper limit of the termination range may be 13, preferably 13.5. When the CIELab-b value decreases and goes below this upper limit (i.e. it is in the termination range) then the drying process may be terminated to prevent overdrying.

[0139] This termination range may be particularly useful as a safety means for situations where there are unexpected issued in the drying process, for example a malfunction with the drying apparatus causing the oven to overheat, or local hot spots within a large scale oven.

[0140] The termination range may also be relative to the maximum CIELab-b value, for instance the upper limit of the termination range may be 1 unit below the maximum CIELab-b value obtained during drying, preferably 0.5 units below the maximum CIELab-b value obtained during drying.

[0141] While the above has been described in relation to the CIELab-b value, it will be recognised that similar ranges can be applied to the , CIEL*a*b*-b value, CIEL*C*h*-C value, CiEL*u*v*-v value, or any equivalent thereof. For instance, for a CIEL*a*b*-b value the optimum range may be described as 11 or above, typically 14 or above, such as 15 or above, such as 16 or above, such as 16.5 or above, or even 17 or above, or alternatively if the CIEL*a*b*-b value is above 11 prior to drying, the lower limit of the optimum range is a CIEL*a*b*-b value of 1 unit above the CIEL*a*b*-b value of the cellulose separator prior to exposure to the drying conditions

[0142] Likewise, for a CIEL*a*b*-b value, the upper limit of the termination range may be 17, preferably 17.5. When the CIEL*a*b*-b value decreases and goes below this upper limit (i.e. it is in the termination range) then the drying process may be terminated to prevent overdrying.

[0143] For a CIEL*C*h*-C value the optimum range may be described as 11 or above, typically 16 or above, such as 17 or above, such as 18 or above, such as 18.5 or above, or alternatively if the CIEL*C*h*-C value is above 11 prior to drying, the lower limit of the optimum range is a CIEL*C*h*-C value of 2 units above the CIEL*C*h*-C value of the cellulose separator prior to exposure to the drying conditions

[0144] Likewise, for a CIEL*C*h*-C value, the upper limit of the termination range may be 17.5, preferably 18 and more preferably 18.5. When the CIEL*C*h*-C value decreases and goes below this upper limit (i.e. it is in the termination range) then the drying process may be terminated to prevent overdrying.

[0145] For a CIEL*u*v*-v value the optimum range may be described as 17 or above, typically 22 or above, such as 23 or above, such as 23.5 or above, such as 24 or above, or alternatively if the CIEL*u*v*-v value is above 17 prior to drying, the lower limit of the optimum range is a CIEL*u*v*-v value of 2.5 units above the CIEL*u*v*-v value of the cellulose separator prior to exposure to the drying conditions

[0146] Likewise, for a CIEL*u*v*-v value, the upper limit of the termination range may be 21.5, preferably 22 and more preferably 22.5. When the CIEL*u*v*-v value decreases and goes below this upper limit (i.e. it is in the termination range) then the drying process may be terminated to prevent overdrying. Preferably, the colour property of the separator is CIELab-b, CIEL*a*b*-b, CIEL*C*h*-C, CiEL*u*v*-v, or equivalent thereof measured in accordance with ASTM E313-20.

[0147] Preferably, an X-rite CI60 device is used to obtain the CIELab, CIEL*a*b*-b, CIEL*C*h*- C, CIEL*u*v*-v, or equivalent thereof measurement

[0148] Preferably, the CIELab, CIEL*a*b*-b, CIEL*C*h*-C, CIEL*u*v*-v, or equivalent thereof measurement is carried out using a D65 illumination source, 8mm aperture, spin mode, taking an average of five measurements.

[0149] Without wishing to be bound by theory, it is hypothesized that as the drying process is carried out, the water content of the cathode is reduced as the water molecules escape via complex pathways in the electrode assembly, typically via the cellulose separator. As cellulose is an organic material, there may be unpredictable variations in the drying rate. Therefore, the monitoring of a colour property (particularly the CIELab-b value, CIEL*a*b*-b value, CIEL*C*h*-C value, CIEL*u*v*-v value, or an equivalent thereof) of the cellulose separator can also act as an in situ 'safety check' to ensure that the drying process is being carried out under conditions that lead to an improvement in cell properties. Therefore, it is important that the colour property of the cellulose is actively monitored throughout the drying step (step (b)).

[0150] The colour property of the cellulose separator may be measured continuously during drying. Alternatively, the colour property of the cellulose separator may be measured intermittently, for instance at intervals of from 1 to 60 minutes.

[0151] EVALUATION PROCESS

[0152] The disclosure also relates to a process for evaluating a heating cycle for suitability in the manufacture of electrochemical cells, particularly sodium ion cells having a cellulose separator.

[0153] The process of the disclosure finds particular use in quality control procedures, as well as in design of process lines on scale up. Thus, large scale manufacturing processes may utilise a manufacturing line in multiple different production processes. When the manufacturing line is in use, it can effectively run continuously. However, there is some inevitable downtime when the manufacturing line is reconfigured to make a difference cell type. After reconfiguring the line, the process of the disclosure can be carried out to check that the heating and drying portions of the manufacturing line are at the correct settings and suitable for use in making cells having cellulose separators. This can easily be done by simply running a cellulose separator through the drying cycle without the remainder of the cell assembly. This provides a low cost testing process to determine the suitability of the line.

[0154] Likewise, if a large scale manufacturing line is in use and producing cell assemblies (and finalised cells) with significant differences in performance, the process of the disclosure can be used to determine whether any part of the drying cycle is not suitable. For instance, cellulose separators may be positioned at various points of the manufacturing line and exposed to the drying cycle. Any cellulose separators not resulting in a colour in the optimum range, or alternatively any separators developing colour in a termination range, may be indicative of a poorly configured manufacturing line, such as a line having hot spots or cool spots.

[0155] ELECTROCHEMICAL CELL

[0156] After the drying stage is complete, the electrode assembly may be arranged into a housing to form an electrochemical cell.

[0157] The present disclosure also relates to electrochemical cells comprising an electrode assembly prepared by the method of the disclosure.

[0158] An electrochemical 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 housing for the electrode assembly. To formulate the cathode and / or the anode of the disclosure into an electrochemical cell, typically, the anode and the cathode is cut into a shape and size suitable for a cell.

[0159] In the cell of the disclosure, the cathode, the anode, or both, comprises a composite comprising a foil coated with an electrode active layer (i.e. a cathode active layer or an anode active layer) and a separator, which are formed into an electrode assembly, which is then dried by the method of the present disclosure.

[0160] Following drying, an electrolyte is introduced into the housing to surround the electrode assembly. The non-aqueous electrolyte facilitates the transport of sodium ions between the cathode and anode. Electrolytes used in the sodium iron cell may be comprising sodium compositions, such as sodium salts, e.g. sodium hexafluorophosphate; organic carbonates, e.g. propylene carbonate, ethylene carbonate, vinylene carbonate, ethyl methyl carbonate, dimethylcarbonate, or diethylcarbonate; or any combination thereof.

[0161] The housing is typically sealed to ensure the electrolyte is retained within the housing. Said housing usually includes terminals in electrical contact with the anode and composite cathode.

[0162] These 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.

[0163] 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.

[0164] 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.

[0165] 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.

[0166] Preferences, options and embodiments for a given aspect, feature or parameter of the invention 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 invention. This is especially true for the description of the process of preparing a cathode, and all its features, which may readily be part of the electrode stack, coated foil, sodium-ion cell, and battery as described herein. EXAMPLES

[0167] Example 1

[0168] An electrode assembly was prepared according to a process of the present disclosure.

[0169] The cathode was a Prussian Blue Analogue (PBA), the anode was hard carbon, and the cellulose separator was Separator A. The electrode assembly was assembled by Z-folding the cellulose separator alternately around the anode and cathode to form a stack containing 21 layers (10 cathode, 11 anode). After the Z-folding was complete, the remainder of the cellulose separator was wrapped around the electrode assembly and taped in place.

[0170] The resulting electrode assembly was dried under vacuum in an oven for 20 hours. This drying process was repeated on four electrode assemblies, assembled as described above, at temperatures of 117 °C, 140°C, 165°C, 175°C and 207 °C.

[0171] After 20 hours, the colour of the cellulose separator was measured by CIELab measurements in accordance with standard ASTM E313, using an X-rite CI60 and a D65 illumination source, 8mm aperture, spin mode. The reported results are the average of five measurements. The results of the colour analysis are shown in Table 1. Out of the temperatures tested, the CIELab-b value peaks when the assembly is dried at 175 °C (see Table 1).

[0172] Table la - Results of colour analysis of Separator A after drying Table lb - Results of colour analysis of Separator A after drying

[0173] Table lc - Results of colour analysis of Separator A after drying Table Id - Results of colour analysis of Separator A after drying

[0174] The effect of varying the drying temperature on the cell performance is shown in Figure 1. Thus, Figure la and Figure lb show that as the drying temperature is increased from 117°C to 140°C, the capacity and cycle life increase. As the temperature is increased further, the capacity is seemingly not affected, but above a certain point, the cycle life dramatically decreases (see Figures lc and Id). The colour of the cellulose separator after drying at different temperatures are shown in Figure 2. There is clear correspondence between the colour of the cellulose separator (particularly the CIELab-b value) and the cycle life efficiency and charge capacity of the cell. In particular, a CIELab-b colour above 11 shows good capacity and cycle life, unless the separator is overdried and thus the CIELab-b value decreases from a maximum to below 13.

[0175] Example 2 An electrode assembly was prepared and the colour of the cellulose separator was monitored in accordance with the method described in Example 1. The only difference was that the cellulose separator was Separator B, which was a high tensile strength, carboxy methyl cellulose (CMC) separator and obtained from the same manufacturer as Separator A. The colour of the cellulose separator after drying at different temperatures are shown in Figure 3.

[0176] The results of the colour analysis are shown in Table 2. Out of the temperatures tested, the CIELab-b value peaks when the assembly is dried at 140 °C (see Table 2).

[0177] Table 2a -results of colour analysis of Separator B after drying

[0178] Table 2b - Results of colour analysis of Separator B after drying Table 2c - Results of colour analysis of Separator B after drying

[0179] Table 2d - Results of colour analysis of Separator B after drying

[0180] The cell performance was evaluated and results consistent with Example 1 were obtained.

[0181] Example 3

[0182] An electrode assembly was prepared and the colour of the cellulose separator was monitored in accordance with the method described in Example 1. The only difference was that the cellulose separator was Separator C, obtained from a different manufacturer than Separators A and B. Separator C also contains a small amount of polyaramid as a stabilising agent. As a result, its initial (before drying) colour is more yellow than Separators A and B.

[0183] The colour of the cellulose separator after drying at different temperatures are shown in Figure 4. The results of the colour analysis are shown in Table 3. Out of the temperatures tested, the CIELab-b value peaks when the assembly is dried at 140 °C (see Table 3). Table 3a -results of colour analysis of Separator C after drying

[0184] Table 3b - Results of colour analysis of Separator C after drying Table 3c - Results of colour analysis of Separator C after drying

[0185] Table 3d - Results of colour analysis of Separator C after drying

[0186] The cell performance was evaluated and results consistent with Example 1 were obtained. Comparative Example 1

[0187] Electrodes (no separator) were dried separately under the various drying conditions. Then, the electrodes were assembled with a typical polymer separator to form an electrode assembly, which was then formed into an electrochemical cell. The electrochemical cell did not show any change in performance between drying temperatures ranging from 140°C to 207°C, showing that the cellulose separator must be present in the drying stage to obtain the optimised properties shown by the present invention.

[0188] It is thus hypothesised that the improvement in performance of the electrode assemblies prepared by the process of the present disclosure may be due to a reaction occurring in the cellulose.

Claims

CLAIMS1. A method of preparing a sodium ion electrode assembly, comprising:(a) providing an electrode assembly comprising an anode, a cathode, and a cellulose separator;(b) drying the electrode assembly under drying conditions comprising a temperature above 100°C optionally under vacuum; wherein a colour property of the cellulose separator is measured during step (b), and step (b) is terminated when the colour property of the cellulose separator is within a reference range.

2. The method of claim 1, wherein the colour property of the cellulose separator prior to step (b) is outside the reference range.

3. The method of any preceding claim, wherein the colour property is CIELab-b, CIEL*a*b*-b, CIEL*C*h*-C, CIEL*u*v*-v, or an equivalent thereof.

4. The method of claim 3, wherein the CIELab-b, CIEL*a*b*-b, CIEL*C*h*-C, CiEL*u*v*-v, or equivalent thereof value is measured in accordance with ASTM E313-20, preferably with a D65 illumination source, 8mm aperture, spin mode, taking an average of five measurements.

5. The method of any preceding claim, wherein the reference range is an optimum range.

6. The method of claim 5, wherein the optimum range is a CIELab-b value which has a lower limit which is the higher of:(A) 11, or(B) 1 unit above the CIELab-b value of the cellulose separator prior to exposure to the drying conditions, preferably wherein the lower limit is a CIELab-b value of 14.

7. The method of any of claims 1 to 4, wherein the reference range is a termination range.

8. The method of claim 7, wherein the step b) is terminated when the CIELab-b value falls below the upper limit of the termination range, said upper limit of the termination range is a CIELab-b value (particularly as measured according to claim 4) of:(A) 13, or(B) 1 unit below the maximum CIELab-b value obtained during step (b).

9. The method of any preceding claim, wherein the temperature T of step (b) or step (i) is from 120°C to 180°C at a pressure of from 0.1 to 100 Pa, preferably from 140°C to 175°C.

10. The method of any preceding claim, wherein in step (b), the electrode assembly is dried for up to 48 hours, preferably from 10 hours to 24 hours.

11. The method of any preceding claim, wherein the colour property in step (b) is measured continuously, or at intervals of from 1 to 60 minutes.

12. The method of any preceding claim, wherein the cathode comprises a Prussian Blue Analogue, preferably Prussian White, Turnbull's blue, potassium ferricyanide, or potassium ferrocyanide.

13. The method of any preceding claim, wherein the anode comprises carbon black, hard carbon or graphite, or a combination thereof, preferably wherein the anode comprises hard carbon.

14. The method of any preceding claim, wherein the separator comprises hemicellulose, carboxy methyl cellulose (CMC), regenerated cellulose or a combination thereof.

15. The method of any preceding claim, wherein the separator comprises polyaramid fibres.

16. The method of any preceding claim, wherein part of the separator protrudes from the electrode assembly, preferably wherein part of the separator is wrapped and / or folded around the electrode assembly.

17. The method of any preceding claim, wherein the colour property in step (b) is measured on a reference cellulose separator, said reference cellulose separator beingformed from identical material to the cellulose separator in the electrode assembly, but is separate from said electrode assembly.

18. A sodium ion electrode assembly comprising an anode, a cathode, and a cellulose separator, as formed by the method of any of the preceding claims.

19. An electrochemical cell comprising the electrode assembly of claim 18.

20. A vehicle comprising the cell of claim 19.

21. A stationary energy storage system comprising the cell of claim 19.

22. Use of a colour measurement device to improve the cycle life of an electrochemical cell, particularly wherein the colour measurement device is capable of detecting a change in a colour property in a cellulose separator during manufacture of the electrochemical cell.

23. Use according to claim 22, wherein the colour property is CIELab-b, CIEL*a*b*-b, CIEL*C*h*-C, CIEL*u*v*-v, or an equivalent thereof.

24. A process for evaluating a heating cycle, wherein the process comprises:(i) subjecting a cellulose separator to the heating cycle comprising a temperature above 100°C;(ii) measuring a colour property of the cellulose separator after it has been subjected to the heating cycle; and(iii) determining if the colour property of the cellulose separator is within a reference range.

25. The process of claim 24, wherein the heating cycle is part of a manufacturing process for making an electrochemical cell, particularly in the drying cycle used to dry the electrode assembly.

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