Method for preparing an electrode for a sodium-ion secondary cell

Abstract

There is disclosed herein a method for preparing an electrode for a sodium-ion secondary cell, comprising the steps of: applying an electrode coating (204) along a coating axis (c) on an electrode substrate sheet (202) to form a coated substrate sheet, the electrode coating (204) comprising an active electrode material wherein the active electrode material comprises one of hard carbon or a Prussian Blue analogue; cutting the coated substrate sheet across the coating axis (c) into a plurality of portions (210); and calendering a portion of the plurality of portions, wherein the calendering comprises applying pressure with at least one roller (212) having a pressure- applying axis (p) aligned with the coating axis (c). Among other benefits, the risk minimization of damage to the electrode substrate is achieved. Also disclosed herein is an electrode prepared by the aforementioned method, a sodium-ion cell comprising such an electrode, a battery system comprising such a cell, and a vehicle comprising such a battery system.

Description

METHOD FOR PREPARING AN ELECTRODE FOR A SODIUM-ION SECONDARY CELLTechnical Field

[0001] The present disclosure relates to a method for preparing an electrode for a sodium-ion secondary cell, an electrode prepared by said method, a secondary sodium-ion cell comprising said electrode, a battery system comprising said secondary sodium-ion cell, and a vehicle comprising said battery system.Background

[0002] As the world’s resources are finite, the use of them is important. Recycling of materials and improving performance and lifetime of batteries is key for improving the pace of electrification and use of renewable energy, in order to more rapidly decrease the industrialized world’s dependency on petroleum and natural gas. There is an increasing demand for electric vehicles, and battery storage. Current technologies use lithium-ion batteries, which comprise (nowadays) expensive lithium only sourced from few places in the world such as China and South America. Lithium is not abundant and with increasing demand in batteries, the lithium inventory will be depleted.

[0003] In order to provide a sufficient supply to battery demand, batteries consisting of abundant and easy to source materials are needed. Thus, there is a need for new batteries and components thereof to overcome the problems with conventional products and to provide new batteries with improved performance.

[0004] Lithium-ion batteries may currently have a major market share of rechargeable batteries, but alternatives such as sodium ion batteries represent an attractive alternative to lithium-ion batteries. Sodium-ion batteries contain sodium, which is an abundant material and everywhere accessible. Sodium ion batteries are shown an increasing interest in order diversify the battery industry in view of raw materials but also show good storing capacity of energy.Summary

[0005] The present disclosure is therefore directed to a method of manufacture of sodium-ion cells, and more particularly the electrodes thereof. Electrodes for sodium-ion cells differ from those of lithium-ion cells in respect of, among other things, their density and fragility, and thus novel manufacturing techniques are required to manufacture electrodes for sodium- ion cells reliably and at pace, in order to produce sodium-ion cells with enhanced energy density.

[0006] Aspects of the present disclosure are directed to the calendering of electrodes for sodium cells. Calendering is a process of applying high pressure to coated electrodes between two rollers to reduce its thickness and porosity. By calendering, the particle contact within the electrode is enhanced, which increases the electrical conductivity and reduces the internal resistance. Calendering also reduces the amount of inactive material in the electrode, such as binder and solvent, which improves the gravimetric and volumetric energy density of the battery.

[0007] In particular, according to an aspect of the present disclosure, there is provided a method for preparing an electrode for a sodium-ion secondary cell, comprising the steps of: a. applying an electrode coating along a coating axis on an electrode substrate sheet to form a coated substrate sheet, the electrode coating comprising an active electrode material wherein the active electrode material comprises one of hard carbon or a Prussian Blue analogue; b. cutting the coated substrate sheet across the coating axis into a plurality of portions; and c. calendering a portion of the plurality of portions, wherein the calendering comprises applying pressure with at least one roller having a pressure-applying axis aligned with the coating axis.

[0008] Conventional techniques for manufacturing electrodes (e.g., for lithium- ion cells) comprise a step of first calendering the coated electrode substrate, and then cutting the calendered electrode into portions for subsequent processing. However, the presently disclosed techniques comprise a reverse approach of first cutting and then calendering the electrode, which allows forthe calendering of the electrode in such a way that the calenders (i.e., the rollers) apply pressure aligned with the axis along which the coating was applied.

[0009] When the electrode substrate is coated along a coating axis to form a coated electrode substrate, an uneven profile is formed across the coating axis, whereby the coated electrode substrate may be thicker in a central longitudinal coated section, and thinner in the surrounding uncoated parts. Moreover, the coated part may itself have irregularities along its width as a result of the coating mechanism or due to additives in the electrode coating.

[0010] If the coated electrode substrate is calendered perpendicular to the coating axis, there will thus be an irregular thickness of material passing between the rollers. This irregularity of thickness may result in a corresponding irregularity in the stress applied to the material and hence the material may deform accordingly. For example, the electrode substrate, which is typically a thin metal foil, may crinkle, crack, or tear if the calenders apply too much pressure.

[0011] Accordingly, a limit is placed on the amount of pressure that the calenders can apply, and an associated limit on the improvements of porosity and / or density of the resulting calendered electrodes.

[0012] By calendering the coated electrode substrate along the same axis that it was coated, a same thickness of material can be entered into the calendars at any given time (i.e., the irregularities are substantially perpendicular to the coating axis, and the thickness is substantially even along the coating axis). Thus, the calenders do not apply an uneven pressure to the coated electrode substrate and the risk of damage thereto is thus substantially reduced.

[0013] Accordingly, the amount of pressure applied by the calenders can be increased, resulting in a greater reduction in porosity / increase in density. Indeed, in preferred embodiments of the present disclosure, the calendering comprises applying a pressure of between 20 to 60 MPa to the portion of the coated substrate sheet. Electrodes formed according to the presently disclosed approach may preferably have a coating density of 1 .0 to 1 .8 g / cm3after the calendering (which may depend at least in part on the composition of the coating).

[0014] Therefore, aspects of the present disclosure advantageously provide electrodes with greater energy density. Hence, cells containing such electrodes will have a corresponding increase in energy density.

[0015] The method described above may be embodied as part of a manufacturing process line employing conveyor belts, robotic manipulators, or the like. In some examples, the method may further comprise a step of conveying the portion of the coated substrate sheet to the roller, wherein said conveying comprises rotating and / or translating the portion to align the coating axis with the pressure-applying axis.

[0016] Rotating the portion may comprise a substantially 90-degree rotation and may be performed through a computerized control of a manipulator, or may be a manual step. In some examples, the rotation may be performed under the guidance of computer vision or the like. In this way, the process line may remain linear without complicated constructions of conveyor belts, etc.

[0017] Translating the portion may comprise a substantially 90-degree translation relative to a previous conveyance direction of the portion (e.g., the direction along which coating was applied). A purely translational movement of the portion may be less complicated to implement, and thus more reliable, than a rotational movement, and could be enacted by, e.g., a conveyer belt having a right-angled junction before the roller(s). In some examples, a partial rotation and a partial translation may be performed to align the coating axis of the portion with the pressure-applying axis of the roller(s).

[0018] The portion of the coated substrate sheet may correspond to an individual electrode sheet for the sodium-ion secondary cell. That is, an electrode assembly for a cell may comprise a plurality of electrode sheets having a desired shape, this being the same shape for all, or different shapes for different sheets. The cutting step may either form the electrode sheets into this desired shape, or may be approximate to the desired shape, e.g., a general outline or dimension of the desired shape, with a subsequent cutting step being performed after the calendering to bring the electrode sheets into the desired shape.

[0019] That is, the method may further comprise a step of shaping the calendered portion into a desired electrode shape. The cutting and / or theshaping may comprise applying a blade or a laser beam to the coated substrate sheet. In a preferred example, the shaping may comprise directing a laser beam onto the electrode coating of the calendered portion, so as to enhance the energy absorption of the laser beam.

[0020] The active electrode material may comprise hard carbon, in which case the electrode coating is preferably applied such that the mass loading of the electrode coating is at least 10 mg / cm2. An electrode manufactured accordingly may be intended as an anode for a cell.

[0021] Alternatively, the active electrode material may comprise a Prussian Blue analogue, in which case the electrode coating is preferably applied such that the mass loading of the electrode coating is at least 15 mg / cm2. An electrode manufactured accordingly may be intended as a cathode for a cell.

[0022] In preferred examples, the Prussian Blue analogue has the formula AaMb[M’c(CN)6]d, wherein A is sodium, and 1 < a < 2, wherein M and M’ are transition metals, preferably selected from iron and / or manganese, wherein 0 < b < 2, 1 < c < 2, and 1 < d < 2.

[0023] In a further preferred subset of such examples, the Prussian Blue analogue is Prussian white having the formula AaFe[Fe(CN)e], wherein A is sodium, and wherein 1 .8 < a < 2, preferably wherein 1.9 < a < 2.

[0024] The Prussian blue analogue may be in a hydrated state when the electrode coating is applied. More generally speaking, the electrode coating is preferably applied as an aqueous slurry, e.g., via aqueous slurry coating, spraying, etc.

[0025] The electrode coating may further comprise a binder of carboxymethyl cellulose (CMC) and / or styrene-butadiene rubber (SBR), wherein the binder is preferably up to 10% by weight of the electrode coating.

[0026] According to conventional techniques, electrode coatings comprising PVDF (polyvinylidene fluoride) and NMP (N-Methylpyrrolidone) may be applied to at least the sides of the coated areas. However, PVDF is a PFAS (per- / polyfluoroalkyl substance), which has a substantial environmental impact.

[0027] Therefore, CMC and SBR binders are preferred, but there is a prejudice against such binders by those skilled in the art due to the creation ofhigher side edges on the coated region of the coated electrode substrate, which can exacerbate the risk of damage to the electrode substrate during calendering.

[0028] Thus, according to the presently disclosed techniques, more environmentally friendly materials can be used to manufacture the electrodes for sodium-ion cells, without the associated drawbacks.

[0029] The electrode substrate may be an aluminum sheet or foil. As discussed above, the presently disclosed techniques allow for a substantially reduced risk of crinkles, rips, or other damage. Therefore, when employing the presently disclosed techniques, a thinner aluminum sheet can be used. The aluminum sheet may thus have a thickness less than 20pm, preferably less than 15pm, and optionally a thickness of at least 4pm.

[0030] By reducing the thickness of the aluminum sheet (or other metal foil electrode substrate), the amount of material required to manufacture the electrode (and thus a sodium-ion cell) can be substantially reduced.Moreover, the reduction in thickness allows for a greater energy density of the electrode, and thus a greater energy density of a cell in which the electrode is installed.

[0031] According to further aspects of the present disclosure, there is provided an electrode prepared by the above-described method, a secondary sodium-ion cell comprising said electrode, a battery system comprising said secondary sodium-ion cell, and a vehicle comprising said battery system.

[0032] Benefits of the presently disclosed method include increased energy density, and thus it will be appreciated that a cell, battery system, and / or vehicle incorporating an electrode manufactured according to such a method will also benefit from, among others, increased energy density.Brief Description of the Drawings

[0033] 0ne or more example implementations of the present disclosure will be described, by way of example only, and with reference to the following figures, in which:

[0034] Figure 1 schematically shows a cross-sectional view of an example coated electrode substrate, with an irregular thickness across the coating axis;

[0035] Figures 2A and 2B schematically show a side and top view of a process line for the manufacture of an electrode for a sodium-ion cell, according to an example implementation of the presently disclosed techniques;

[0036] Figure 3 illustrates a method for preparing an electrode for a sodium- ion cell, according to aspects of the present disclosure;

[0037] Figure 4 shows a battery pack comprising a plurality of cells; and

[0038] Figure 5 shows a vehicle comprising the battery pack shown in figure 4.Detailed Description

[0039] The present disclosure is described in the following by way of a number of illustrative examples. It will be appreciated that these examples are provided for illustration and explanation only and are not intended to be limiting on the scope of the present disclosure. Instead, the scope of the present disclosure is defined by the appended claims.

[0040] Furthermore, although examples may be presented individually for the sake of focused discussion of particular features, it will be recognized that the present disclosure also encompasses combinations of the examples described herein.

[0041] Figure 1 schematically shows a cross-sectional view of an example coated electrode substrate 100, with an irregular thickness T across the coating axis, i.e., along the width wof the coated electrode substrate 100.

[0042] The coated electrode substrate 100 comprises an electrode substrate 102, such as an aluminum sheet, with a substantially uniform thickness t. The electrode substrate 102 is partially coated (in a central width region) with a coating 104, where the coating comprises hard carbon or Prussian blue analog, depending on, e.g., whether the resulting electrode is to be an anode or a cathode.

[0043] It can be seen that the thickness T of the coated electrode substrate 100 is a combination of the thickness t of the electrode substrate 102 and the height h of the coating 104. The height h of the coating 104 is shown as varying along the width wof the electrode substrate 102. Firstly, it can be seen that a substantial height difference exists between the coated and noncoated parts of the electrode substrate 102, but there is also a varying height h of the coating, which may be due to additives, or uneven distribution during the coating process.

[0044] It can be seen from figure 1 that, were the coated electrode substrate 100 to be calendered along the width w direction - that is, such that the pressure-applying axis of the rollers was along the width w direction - then, at any given time, a varying thickness T of material would be fed through the rollers. As discussed above, the varying thickness T of material may result in an uneven stress applied by the rollers onto the coated electrode substrate 100, which could risk tearing or otherwise damaging the electrode substrate 102.

[0045] Therefore, figures 2A and 2B schematically show a side and top view of an example process line 200 for the manufacture of an electrode for a sodium-ion cell, according to an example implementation of the presently disclosed techniques. The process line 200 progresses from left to right as illustrated. These figures do not show any conveying means, but it will be appreciated that the illustrated techniques could be readily implemented with conveyor belts, robotic manipulators and / or the like.

[0046] In the illustrated example, a coating dispenser 208 dispenses a coating 204 onto an electrode substrate 202 (also referred to herein as simply ‘substrate 202’). In this example, the electrode substrate 202 is an aluminum sheet and the coating 204 is Prussian white having the formula AaFe[Fe(CN)e], wherein A is sodium, and wherein 1 .8 < a < 2, in hydrated form. The coating 202 is aqueous when it is applied, and the dispenser 208 may be configured to apply the coating 204 onto the substrate 202.

[0047] As shown in figure 2B, the coating 204 is applied onto the substrate 202 along a coating axis c which thus forms a coating axis c. In this illustrated example, the coating axis c is along the direction of conveyance, and along alongitudinal direction of the pre-cut substrate 202. Once the substrate 202 has been coated, the coating axis c along which it was coated may be readily identifiable via the directional irregularities in thickness, as shown in figure 1 . In this example, these irregularities are substantially perpendicular to the coating axis c.

[0048] The coated substrate is then cut into a plurality of portions 210a, 210b, 210c, 21 Od (collectively referred to in the following as ‘portions 210’), by a cutter (not shown) such as a mechanical blade or a laser cutter. Although the portions 210 are shown as being the same size, it will be appreciated that, in some examples, they could be different sizes. The coated substrate 202 is cut across the coating axis c so as to form portions 210 that have a central coated region and an uncoated region at either side of the coated region. Specially, in the illustrated example, the coated substrate 202 is cut perpendicular to the coating axis c.

[0049] After cutting, as shown in figures 2A and 2B, the portions 210 are rotated. Specifically, it can be seen that the portions 210b, 210c, and 21 Od are rotated 90 degrees in the plane of the substrate 202 relative to the portion 210a. The rotation may be performed by any suitable means, including manually, but preferably by some automated conveyance or robotic means.

[0050] After being rotated, the portions 210 are then calendered by calenders 212, also referred to as Tollers 212’. In some examples, only one calender 212 may be used, pressing against a static surface, for example. The calenders apply a pressure, in a manner understood by those skilled in the art, along a pressure-applying axis p indicated in figure 2B.

[0051] It can be seen in figure 2B that, as a result of the rotation, the coating axis c becomes aligned with the pressure applying axis p. It will be appreciated that neither of these axes are direction-dependent. The portions 210 may have a thickness varying perpendicular to the coating axis, such as the example shown in figure 1 (which could be understood as being a cross- sectional view of a portion 210 looking along the coating axis c).

[0052] Thus, as the portion 210c is fed through the calenders, it will be understood that, at any given time, a substantially uniform thickness is presented to the pressure-applying axis p of the calenders 212. Hence, asdiscussed above, the calenders 212 apply a substantially uniform stress along the pressure-applying axis p and the risk of the substrate 202 tearing, or any other damage being inflicted, is substantially reduced.

[0053] It can be seen that the portion 21 Od, having passed through the rollers 212, has mostly been compressed along its thickness. Specifically, the coating 202 has been reduced in thickness as a result of the porosity thereof being reduced, and hence the density of the coating 202 has been increased.

[0054] Prussian blue analogues (such as Prussian white) have particularly low densities, and thus may require a greater pressure applied by the rollers 212 than lower density coatings. Hence, it will be appreciated that the risk of tearing of electrodes is especially high for such low-density coatings, and hence the presently disclosed techniques are especially beneficial for such coatings.

[0055] It will further be appreciated that the illustrated output 21 Od of the process line 200 may not be a finished electrode as there may be further steps performed, such as shaping, drying, etc.

[0056] Figure 3 illustrates a method 3000 for preparing an electrode for a sodium-ion secondary cell, according to aspects of the present disclosure comprising the steps of applying 3010 an electrode coating along a coating axis on an electrode substrate sheet to form a coated substrate sheet, cutting 3020 the coated substrate sheet across the coating axis into a plurality of portions, and calendering 3030 a portion of the plurality of portions, wherein the calendering comprises applying pressure with at least one roller having a pressure-applying axis aligned with the coating axis.

[0057] The method is performed in an order that goes against conventional techniques, in that conventional techniques typically involve calendering before cutting, while according to the presently disclosed techniques, the calendering 3030 is performed after the cutting 3020, so as to enable an alignment of a coating axis of a cut portion with a pressure-applying axis of the calenders, as discussed above.

[0058] Figure 4 shows a perspective view of an example battery pack 400 with a portion of its casing (being illustrated in a purely schematic way) cut away to schematically show a plurality of sodium-ion cells 410 housedtherein. In this example, the cells 410 comprise electrodes formed according to the techniques described above. As such, the benefits of increased energy density, etc., afforded to the electrodes formed according to such techniques, are also afforded to the cells 410, and by extension the battery pack 400.

[0059] The secondary cells 410 are connected together in series and / or parallel, and in an optionally modular fashion, so as to form a combined electrical storage capacity. In some examples, the cells have a common orientation such that failure vents of the cells are oriented in a same direction, and such that the terminals of the cells can be accessed at the same side.

[0060] Figure 5 schematically shows an example vehicle 500 comprising the battery pack 400 shown in figure 4. In this example, the battery pack is arranged at a lower portion of the vehicle 500, which may be an electric or hybrid vehicle. Other uses for the battery pack 400 may comprise a standalone battery pack for powering devices or installations or the like.

[0061] While the present disclosure is susceptible to various modifications and alternative forms, specific embodiments are shown and described above by way of example in relation to the drawings, with a view to clearly explaining the various advantageous aspects of the present disclosure. It should be understood, however, that the detailed description herein and the drawings attached hereto are not intended to limit the disclosure to the particular form disclosed. Rather, the intention is to cover all modifications, equivalents, and alternatives falling within the scope of the following claims.

[0062] Moreover, the present disclosure may be better understood through the following numbered clauses:1 . A method for preparing an electrode for a sodium-ion secondary cell, comprising the steps of: applying an electrode coating along a coating axis on an electrode substrate sheet to form a coated substrate sheet, the electrode coating comprising an active electrode material wherein the active electrode material comprises one of hard carbon or a Prussian Blue analogue;cutting the coated substrate sheet across the coating axis into a plurality of portions; calendering a portion of the plurality of portions, wherein the calendering comprises applying pressure with at least one roller having a pressure-applying axis aligned with the coating axis.2. The method according to clause 1 , further comprising a step of conveying the portion of the coated substrate sheet to the roller, wherein said conveying comprises rotating and / or translating the portion to align the coating axis with the pressure-applying axis.3. The method according to clause 1 or clause 2, wherein the portion of the coated substrate sheet corresponds to an individual electrode sheet for the sodium-ion secondary cell.4. The method according to any preceding clause, wherein the cutting comprises applying a blade or a laser beam to the coated substrate sheet.5. The method according to any preceding clause, wherein the calendering comprises applying a pressure of between 20 to 60 MPa to the portion of the coated substrate sheet.6. The method according to any preceding clause, wherein the electrode has a coating density of 1 .0 to 1 .8 g / cm3after the calendering.7. The method according to any preceding clause, wherein the active electrode material comprises hard carbon, and wherein the electrode coating is applied such that the mass loading of the electrode coating is at least 10 mg / cm2.8. The method according to any of clauses 1 to 6, wherein the active electrode material comprises a Prussian Blue analogue, and wherein the electrode coating is applied such that the mass loading of the electrode coating is at least 15 mg / cm2.9. The method according to clause 8, wherein the Prussian Blue analogue has the formula AaMb[M’c(CN)6]d, wherein A is sodium, and 1 < a <2, wherein M and M’ are transition metals, preferably selected from iron and / or manganese, wherein 0 < b < 2, 1 < c < 2, and 1 < d < 2.10. The method according to clause 9, wherein the Prussian Blue analogue is Prussian white having the formula AaFe[Fe(CN)e], wherein A is sodium, and wherein 1 .8 < a < 2, preferably wherein 1 .9 < a < 2.1 1 . The method according to clause 9 or claim 10, wherein the Prussian blue analogue is in a hydrated state when the electrode coating is applied.12. The method according to any preceding clause, wherein the electrode coating further comprises a binder of carboxymethyl cellulose, CMC, and / or styrene-butadiene rubber, SBR.13. The method according to clause 12, wherein the binder is up to 10% by weight of the electrode coating.14. The method according to any preceding clause, wherein the electrode coating is applied as an aqueous slurry.15. The method according to any preceding clause, wherein the electrode substrate is an aluminum sheet.16. The method according to clause 15, wherein the aluminum sheet has a thickness less than 20pm, preferably less than 15pm.17. The method according to claim 16, wherein the aluminum sheet has a thickness greater than or equal to 4pm.18. The method according to any preceding clause, further comprising a step of shaping the calendered portion into a desired electrode shape.19. The method according to clause 18, wherein the shaping comprises directing a laser beam onto the electrode coating of the calendered portion.20. An electrode prepared by the method defined in any preceding clause.20. A secondary sodium-ion cell comprising an electrode as defined in clause 19 as anode or cathode.21 . A battery system comprising the secondary sodium-ion cell as defined in clause 20.22. A vehicle comprising the battery system as defined in clause 21 .

Claims

C L A I M S1 . A method for preparing an electrode for a sodium-ion secondary cell, comprising the steps of: applying an electrode coating along a coating axis on an electrode substrate sheet to form a coated substrate sheet, the electrode coating comprising an active electrode material wherein the active electrode material comprises one of hard carbon or a Prussian Blue analogue; cutting the coated substrate sheet across the coating axis into a plurality of portions; calendering a portion of the plurality of portions, wherein the calendering comprises applying pressure with at least one roller having a pressure-applying axis aligned with the coating axis.

2. The method according to claim 1 , further comprising a step of conveying the portion of the coated substrate sheet to the roller, wherein said conveying comprises rotating and / or translating the portion to align the coating axis with the pressure-applying axis.

3. The method according to claim 1 or claim 2, wherein the portion of the coated substrate sheet corresponds to an individual electrode sheet for the sodium-ion secondary cell.

4. The method according to any preceding claim, wherein the calendering comprises applying a pressure of between 20 to 60 MPa to the portion of the coated substrate sheet.

5. The method according to any preceding claim, wherein the electrode has a coating density of 1 .0 to 1 .8 g / cm3after the calendering.

6. The method according to any preceding claim, wherein the active electrode material comprises hard carbon, and wherein the electrode coating is applied such that the mass loading of the electrode coating is at least 10 mg / cm2.

7. The method according to any of claims 1 to 6, wherein the active electrode material comprises a Prussian Blue analogue, and wherein the electrode coating is applied such that the mass loading of the electrode coating is at least 15 mg / cm2.

8. The method according to any preceding claim, wherein the electrode coating further comprises a binder of carboxymethyl cellulose, CMC, and / or styrene-butadiene rubber, SBR.

9. The method according to any preceding claim, wherein the electrode coating is applied as an aqueous slurry.

10. The method according to any preceding claim, wherein the electrode substrate is an aluminum sheet.11 . The method according to any preceding claim, further comprising a step of shaping the calendered portion into a desired electrode shape.

12. An electrode prepared by the method defined in any preceding claim.

13. A secondary sodium-ion cell comprising an electrode as defined in claim 12 as anode or cathode.

14. A battery system comprising the secondary sodium-ion cell as defined in claim 13.

15. A vehicle comprising the battery system as defined in claim 14.

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