Electrode structure

Abstract

The use of carboxymethyl cellulose within an interlayer of an electrode structure to improve the corrosion resistance of the electrode structure is described. The electrode structure is for use in a battery cell and comprises a current collector layer having a current collector surface, an electrode layer in electrical contact with the current collector, and the interlayer arranged between the current collector surface and the electrode surface, a tab having a tab surface that faces the current collector surface, wherein the tab is electrically conductive and connectable to an external circuit, and an interlayer arranged between the current collector surface and the tab surface. The interlayer comprises an electrically conducting material and a binder, wherein the binder comprises carboxymethyl cellulose. A battery cell incorporating the electrode structure, a method of making the electrode structure, and uses of the interlayer are also described.

Description

[0001] ELECTRODE STRUCTURE

[0002] BACKGROUND

[0003] An electrode structure for a battery typically comprises an electrode and a current collector foil that minimises the path length for conduction of electrical current away from the electrode. In an assembled battery cell, two such electrode structures (one anode and one cathode) are arranged with an electrolyte between them.

[0004] Electrode structures of this type are typically made by forming the electrode directly onto the current collector, for example by slurry casting. In this case, the electrode is typically an oxide material. An electrode can also be formed on a current collector layer using a physical or chemical vapour deposition techniques (PVD and CVD), though such techniques are generally costly and are not compatible with all materials.

[0005] It is also important, especially for high-power applications such as EV batteries, that cells and electrode structures have high resistance to corrosion caused by electrolytes. Corrosion of the current collector of an electrode structure typically results in higher resistance and disconnection of the layers of a battery. This can be detrimental to cell performance and safety.

[0006] SUMMARY

[0007] The invention resides in an electrode structure having an interlayer which is arranged between a surface of a current collector and a surface of a tab to improve the corrosion resistance.

[0008] In a first aspect, provided herein is an electrode structure for use in a battery cell, the electrode structure comprising:

[0009] a current collector layer having a current collector surface;

[0010] an electrode layer in electrical contact with the current collector layer;

[0011] a tab having a tab surface that faces the current collector surface, wherein the tab is electrically conductive and connectable to an external circuit; and an interlayer arranged between the current collector surface and the tab surface, the interlayer comprising an electrically conducting material and a binder, wherein the binder comprises carboxymethyl cellulose.

[0012] The inventors have found that an electrode structure can exhibit improved corrosion resistance when an electrically conducting interlayer, comprising carboxymethyl cellulose (CMC) as a binder, is arranged in the electrode structure between the current collector and the tab. By virtue of the electrically conducting interlayer, electrical contact between the electrode layer and the current collector layer may also be maintained.

[0013] In particular, the interlayer may provide a protective corrosion-resistant barrier between the current collector and the tab to preserve battery cell performance, while maintaining good electrical conductivity between the electrode layer and the current collector layer. The interlayer may also provide no detrimental effect of the welding efficacy of the tab to the current collector surface.

[0014] By “corrosion resistance”, it is meant the ability of the electrode structure or battery cell containing the electrode structure to maintain its microstructure when exposed to an electrolyte. In a typical battery cell, corrosion of the current collector can occur under cell environments using electrolytes. For example, certain electrolytes can react with an aluminium foil current collector to produce salts which are soluble in the electrolyte, thereby causing the underlying aluminium metal to corrode. The effects of corrosion caused by electrolytes can be observed using imaging methods, such as scanning electron microscopy (SEM), as non-uniform, rough and pitted regions on the current collector surface. Corrosion can negatively affect a battery cell’s performance and safety.

[0015] The electrode structure is typically provided as a layered structure. At its broadest, the electrode layer, the current collector layer, the tab and the interlayer are provided in a stacked arrangement in the electrode structure.

[0016] In the electrode structure, the function of the current collector layer is to minimise the path length for conduction of electrical current away from the electrode. Accordingly, the current collector layer may be any material that is suitable for conducting current. The current collector layer may be any suitable thickness. Suitable current collector layers are known.

[0017] In some embodiments, the current collector layer may be a metal, such as a metal foil. In some embodiments, the current collector surface comprises a transition metal. In some embodiments, the metal may be, or comprise, e.g. as part of a laminate, at least one of aluminium, copper and nickel. In some embodiments, the metal may be, or comprise, e.g. as part of a laminate, at least one of aluminium and copper. In some embodiments, the metal may be copper. In some embodiments, the current collector layer comprises or consists essentially of aluminium.

[0018] In some embodiments, the current collector layer may have a maximum thickness of about 20 μm, such as about 18 μm, such as about 15 μm, such as about 12 μm. In some embodiments, the current collector layer may have a minimum thickness of about 1 μm, such as about 3 μm, such as about 5 μm, such as about 7 μm. Any of the foregoing may be combined to form a suitable range, such as in the range of from 1 to 20 μm, such as from 5 to 30 μm, such as from 3 to 18 μm, such as from 3 to 12 μm, such as from 7 to 12 μm. In some embodiments, the thickness here may refer to the average (mean) thickness. The average (mean) thickness may be measured as described herein for the average (mean) interlayer thickness.

[0019] In the electrode structure, the function of the electrode layer is to provide electrical contact with an electrolyte for transporting electrons to another electrode of a battery cell. In some embodiments, the electrode layer is a cathode layer when used in a battery cell. In some such embodiments, the other electrode is an anode.

[0020] In some embodiments, the electrode layer may be a polymer electrode layer. In some embodiments, the polymer electrode layer may be a polymer gel electrode layer. In some embodiments, the polymer electrode layer may be or may comprise the components as set out elsewhere herein. In some embodiments the polymer gel may be compressible, such as for good contact with the interlayer. In some embodiments, a polymer gel electrode may be formed from a polymer-electrolyte gel matrix phase, which in turn may be formed from one or more electrolyte components and at least one gelling polymer.

[0021] The one or more electrolyte components may include a solvent suitable for use as an electrolyte solvent in a gel electrode, for example an organic solvent. The one or more electrolyte components may include a salt. In some embodiments, the one or more electrolyte components may constitute an electrolyte salt solution or liquid electrolyte.

[0022] In some embodiments, the one or more electrolyte components may comprise a solvent comprising one or more cyclic or linear carbonate compounds. In some embodiments, the solvent may comprise one or more cyclic carbonate compounds. In some embodiments, the solvent may comprise one or more of ethylene carbonate, propylene carbonate, butylene carbonate, vinylene carbonate, fluoroethylene carbonate, fluoropropylene carbonate and y-butyrolactone.

[0023] In some embodiments, the solvent may comprise a blend of at least two different compounds, for example at least three or at least four different compounds. In some embodiments the solvent may comprise a blend of at least two different organic carbonate compounds, for example at least three or at least four different organic carbonate compounds.

[0024] In some embodiments, the one or more electrolyte components may comprise an alkali metal salt. The alkali metal of the alkali metal salt may be any suitable alkali metal (Group I of the periodic table). The alkali metal salt may be a lithium, sodium, or potassium salt.

[0025] The anion of the alkali metal salt may be any suitable anion. Typical anions are known to the skilled person and may be chosen based on the kind of alkali metal. In some embodiments, when the alkali metal is lithium, the anion of the salt may comprise a halogen such as fluorine. Examples of the anion include BFr, PFe’, TFST, FST, OTf, DFOB" and TDT.

[0026] In some embodiments, the one or more electrolyte components may comprise a lithium salt. In some embodiments, the electrolyte may comprise a mixture of at least two different lithium salts. Examples of suitable lithium salts include LiPFe, LiCICU, LiBF4, LiTFSI, LiFSI, LiOTf, LiDFOB and LiTDI.

[0027] One or more kinds of alkali metal salt may be used in accordance with the present invention. Typically, but not exclusively, when more than one kind of alkali metal salt is used, they may share a common alkali metal.

[0028] The polymer-electrolyte gel matrix phase may comprise a gel matrix formed by the gelling of one or more gelling polymers when the polymer(s) absorb a liquid electrolyte. The polymer-electrolyte gel matrix phase may therefore comprise a gel comprising the polymer(s) and absorbed liquid electrolyte.

[0029] The gelling polymer may comprise one or more gelling polymers independently selected from poly(ethyleneglycol dimethacrylate), poly(ethyleneglycol diacrylate), poly(propyleneglycol dimethacrylate), poly(propyleneglycol diacrylate), poly(methyl methacrylate) (PMMA), poly(acrylonitrile) (PAN), polyurethane (PU), poly(vinylidene difluoride) (PVDF), poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP), poly(ethylene oxide) (PEO), poly-L-lactic acid (PLA), polystyrene (PS), poly(ethyleneglycol dimethylether), poly(ethyleneglycol diethylether), poly[bis(methoxy ethoxyethoxide)-phosphazene], poly(dimethylsiloxane) (PDMS), polyacene, polydisulfide, polystyrene, polystyrene sulfonate, polypyrrole, polyaniline, polythiophene, polythione, polyvinyl pyridine (PVP), polyvinyl chloride (PVC), polyaniline, poly(3,4-ethylenedi oxy thiophene) (PEDOT), poly(p-phenylene), poly(triphenylene), polyazulene, polyfluorene, polynaphthalene, polyanthracene, polyfuran, polycarbazole, tetrathiafulvalene- substituted polystyrene, ferrocene-substituted polyethylene, carbazolesubstituted polyethylene, polyoxyphenazine, poly(heteroacene), poly[(4-styrenesulfonyl)(trifluoromethanesulfonyl)imide-co-methoxy -polyethyleneglycolacrylate] (Li[PSTFSI-co-MPEGA]), sulfonated poly(phenylene oxide) (PPO), N, N-dimethylacryl amide (DMAAm), lithium 2-acrylamido-2-methyl-l -propane sulfonate (LiAMPS), Poly(lithium 2-Acrylamido-2-Methylpropanesulfonic Acid-Co- Vinyl Triethoxysilane), polyethyleneoxide(PEO) / poly(lithium sorbate), PEO / poly(lithium muconate), PEO / [poly(lithium sorbate)+BF3], PEO copolymer, PEO terpolymer, and NIPPON SHOKUBAI® polymer, or mixtures or co-polymers thereof.

[0030] In some embodiments, the gelling polymer may comprise one or more gelling polymers independently selected from poly(vinylidene difluoride) (PVDF), poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP), poly(m ethyl methacrylate) (PMMA), poly(ethylene oxide) (PEO), poly-L-lactic acid (PLA) and polystyrene (PS).

[0031] In some embodiments, the electrode layer may have a maximum thickness of about 150 pm, such as about 145 pm, such as about 140 pm, such as about 130 pm, such as about 120 pm, such as about 110 pm, such as about 100 pm. In some embodiments, the electrode layer may have a minimum thickness of about 5 pm, such as about 10 pm, such as about 15 pm. Any of the foregoing may be combined to form a suitable range of the thickness of the electrode layer, such as from 5 to 150 pm, such as from 5 to 145 pm, such as from 5 to 140 pm, such as from 5 to 120 pm, such as from 5 to 100 pm, such as from 10 to 150 pm, such as from 10 to 145 pm, such as from 10 to 140 pm, such as from 10 to 130 pm, such as from 10 to 120 pm, such as from 10 to 100 pm, such as from 15 to 150 pm, such as from 15 to 145 pm, such as from 15 to 140 pm, such as from 15 to 130 pm, such as from 15 to 120 pm, such as from 15 to 110 pm, such as from 15 to 100 pm.

[0032] In some embodiments, the electrode layer may comprise an electrochemically active material which is a positive active material so that the electrode layer is a cathode layer. In some embodiments, the positive active material may be a lithium transition metal oxide material. In some embodiments, the positive active material may be a lithium transition metal oxide material comprising a mixed metal oxide of lithium and one or more transition metals, optionally further comprising one or more additional non-transition metals. In some embodiments, the positive active material may be a lithium transition metal oxide material comprising lithium and one or more transition metals selected from nickel, cobalt and manganese. In some embodiments, the positive active material may be selected from one or more of lithium cobalt oxide (LCO), lithium manganese oxide (LMO), lithium nickel cobalt oxide (NCO), aluminium-doped lithium nickel cobalt oxide (NCA), lithium nickel manganese cobalt oxide (NMC), lithium nickel oxide (LNO), lithium nickel manganese oxide (LNMO), lithium iron phosphate (LFP), lithium manganese iron phosphate (LFP) and lithium nickel vanadate (LNV). In some embodiments, the positive active material may be lithium nickel manganese cobalt oxide (NMC), optionally doped with another metal such as aluminium. Such positive active materials may be commercially available or may be manufactured by methods known to the skilled person, for example through the precipitation of mixed metal hydroxide intermediates from a reaction mixture containing different precursor metal salts, followed by calcination to form a mixed metal oxide and optionally lithiation to incorporate lithium into the oxide.

[0033] The electrochemically active material may be undoped or uncoated, or may contain one or more dopants and / or a coating. For example, the electrochemically active material may be doped with small amounts of one or more metal elements. The electrochemically active material may comprise a carbon coating on the surface of the particles of the material.

[0034] The electrochemically active material may be a particulate material, i.e. materials made up of a plurality of discrete particles. The particles may comprise primary particles and / or secondary particles formed from the agglomeration of a plurality of primary particles.

[0035] In some embodiments, the electrochemically active material may make up at least 50 vol% of the electrode layer, based on the total volume of the electrode layer, for example at least 55 vol%, such as at least 60 vol%, such as at least 62 vol%, such as at least 64 vol%, such as at least 65 vol%. Suitably, in some embodiments, the electrochemically active material may make up about 64 vol% of the electrode layer.

[0036] The electrode layer may be separable from the interlayer. In this way, there is no need to adhere the electrode to the interlayer. The electrode layer may be a free-standing electrode layer. In this way, the electrode layer can be made separately from the current collector, and applied to the current collector in a subsequent process. The electrode layer may for example be an extruded electrode, made by extrusion of a polymer gel. The polymer gel may be a compressible material.

[0037] In some embodiments, the electrode layer is a cathode layer and the current collector surface comprises aluminium. In some embodiments, the electrode layer is a cathode layer and the current collector layer comprises or consists of aluminium.

[0038] In the electrode structure, the function of the tab is to provide an electrically conductive and connectable contact point between the battery cell and an external circuit which is outside the battery cell. Accordingly, the tab may be made of any material that is suitable for conducting current. The tab may be any suitable thickness. Suitable tabs are known.

[0039] In some embodiments, the tab is arranged between the electrode and the current collector. In some embodiments, the tab is in electrical contact and indirect physical contact with the current collector via the interlayer, wherein the interlayer is arranged therebetween. In some embodiments, the tab is in direct physical contact with the electrode.

[0040] The tab provides an electrical connection to an external circuit, and the tab is typically characterised by its structural location relative to the other components within the electrode structure. In some embodiments, the tab comprises an end surface which extends beyond the current collector, wherein the end surface is connectable to the external circuit. The end surface of the tab is electrically conducting, and its extension is for electrical connection to an external circuit.

[0041] In some embodiments, the tab comprises a metal. In some embodiments, the tab comprises aluminium, nickel or copper. In some embodiments, the tab comprises aluminium.

[0042] In some embodiments, the tab surface comprises a metal. In some embodiments, the tab surface comprises aluminium, nickel or copper. In some embodiments, the tab surface comprises aluminium. The tab typically has a rectangular profile, and so the tab may be characterised in three dimensions: thickness, length and width. The length is typically longer than the width. The width is typically wider than the thickness.

[0043] When arranged in the electrode structure, the tab may have a first tab surface and a second tab surface opposite the first tab surface. The distance between the first tab surface and the second tab surface is the thickness of the tab, and is measured in the same direction as the thickness of the current collector layer, electrode layer and interlayer.

[0044] In some embodiments, the tab may have an average thickness of about 0.50 mm or less, such as about 0.40 mm or less, such as about 0.30 mm or less. In some embodiments, the tab may have an average thickness of about 0.05 mm or more, such as about 0.10 mm or more, such as about 0.15 mm or more. Any of the foregoing can be combined to form an appropriate range of thickness for the tab. Suitable ranges may include, for example, from 0.05 to 0.50 mm, such as from 0.10 to 0.40 mm, such as from 0.15 to 0.30 mm.

[0045] In some embodiments, the tab may have a length of about 30 mm or more, such as about 40 mm or more, such as about 45 mm or more, such as about 50 mm or more. In some embodiments, the tab may have a length of about 80 mm or less, such as about 70 mm or less, such as about 60 mm or less. Any of the foregoing can be combined to form an appropriate range of length for the tab. Suitable ranges may include, for example, from 40 to 80 mm, such as from 45 to 70 mm, such as from 50 to 60 mm.

[0046] In some embodiments, the tab may have a width which is less than the width of the current collector and / or the width of the electrode.

[0047] The length of the end surface of the tab may be the length of the portion of the tab which extends beyond the current collector in the electrode structure. The length of the end surface may be adapted to provide and receive electrical connection to an external circuit. In some embodiments, the end surface of the tab may have a length of about 10 mm or more, such as about 15 mm or more, such as about 20 mm or more. In some embodiments, the end surface of the tab may have a length of about 40 mm or less, such as about 35 mm or less, such as about 30 mm or less. Any of the foregoing can be combined to form an appropriate range of length for the end surface of the tab. Suitable ranges may include, for example, from 10 to 40 mm, such as from 15 to 35 mm, such as from 20 to 30 mm.

[0048] The electrode layer may comprise an electrode surface that faces the current collector surface, and the interlayer may be arranged between the electrode surface and the current collector surface.

[0049] The current collector may comprise a first current collector surface, and the current collector may further comprise a second current collector surface opposite the first current collector surface. In some embodiments, the electrode structure may comprise a second electrode layer. The second electrode layer may have a second electrode surface that faces the second current collector surface. A second interlayer may be arranged between the second current collector surface and the second electrode surface. The second interlayer may comprise an electrically conducting material and a binder. The binder may comprise carboxymethyl cellulose. In this way, a single current collector layer can act as a current collector for two electrodes, maximising efficiency of the cell.

[0050] The current collector surface may be a first current collector surface, and the current collector may further comprise a second current collector surface opposite the first current collector surface. In some embodiments, a second interlayer may be arranged on the second collector surface. The second interlayer may comprise an electrically conducting material and a binder The binder may comprise carboxymethyl cellulose. In this way, corrosion resistance may be imparted while maintaining electrical conductivity between the first current collector layer and a second current collector layer.

[0051] The tab surface may be a first tab surface, and the tab may further comprise a second tab surface opposite the first tab surface. In some embodiments, a second interlayer may be arranged on the second tab surface. The second interlayer may comprise an electrically conducting material and a binder. The binder may comprises carboxymethyl cellulose. In this way, corrosion resistance may be imparted while maintaining electrical conductivity between the second current collector layer and the tab.

[0052] Thus, the present invention may also provide an electrode structure comprising a current collector layer having a plurality of interlayers and a plurality of current collector surfaces, wherein at least one interlayer, such as each interlayer, may be arranged on at least one current collector surface, such as each current collector surface.

[0053] In some embodiments, the first and / or second interlayers may have some or all of the optional features described elsewhere herein for the interlayer of the electrode structure of the first aspect.

[0054] In some embodiments, the first and second interlayers may be the same. In some embodiments, the first and second interlayers may have the same composition. In some embodiments, the first and second interlayers may have the same electrically conducting material. In some embodiments, the first and second interlayers may have the same conductive material content. In some embodiments, the first and second interlayers may have the same CMC content. In some embodiments, the first and second interlayers may have the same thickness.

[0055] In some embodiments, the first and second interlayers may be different. In some embodiments, the first and second interlayers may have a different composition. In some embodiments, the first and second interlayers may have a different electrically conducting material. In some embodiments, the first and second interlayers may have a different conductive material content. In some embodiments, the first and second interlayers may have a different CMC content. In some embodiments, the first and second interlayers may have a different thickness.

[0056] In the electrode structure, the interlayer is arranged between the between the current collector surface and the tab surface. The interlayer is electrically conducting, and maintains electrical contact between the electrode layer and the current collector layer. In some embodiments, the interlayer may have an average thickness of about 150 nm or less. In some embodiments, the interlayer may have an average thickness of about 140 nm or less, such as about 130 nm or less, such as about 120 nm or less, such as about 110 nm or less, such as about 100 nm or less. In some embodiments, the interlayer may have an average thickness of about 50 nm or more, such as about 60 nm or more, such as about 70 nm or more, such as 80 nm or more, such as about 90 nm or more, such as about 100 nm or more. Any of the foregoing can be combined to form an appropriate range of average thickness for the interlayer. Suitable ranges may include, for example, from 50 to 150 nm, such as from 60 to 140 nm, such as from 70 to 130 nm, such as from 80 to 120 nm, such as from 90 to 110 nm, or 100 nm.

[0057] The average thickness may be determined using SEM. The average thickness as referred to herein means the average, i.e. mean, thickness value of at least 3 thickness measurements taken from at least 1 SEM image, such as at least 3 SEM images, such as 4 SEM images, wherein the SEM images are taken from different parts of the interlayer cross-section across its width. In some embodiments, the average thickness may be the average of at least 4 thickness measurements taken from at least 1 SEM image, such as at least 3 SEM images, such as 4 SEM images, wherein the SEM images are taken from different parts of the interlayer cross-section across its width. In some embodiments, the average thickness may be the average of at least 5 thickness measurements taken from at least 1 SEM image, such as at least 3 SEM images, such as 4 SEM images, wherein the SEM images are taken from different parts of the interlayer cross-section across its width. A thickness measurement may be obtained using an SEM image of a cross-section of the interlayer, which may be formed on a current collector surface and / or on an electrode surface, from which a thickness of the interlayer is determined using the cross-sectional image with appropriate software.

[0058] In some embodiments, the interlayer may have a maximum thickness of about 200 nm and / or a minimum thickness of about 100 nm. The interlayer may have a maximum thickness of about 150 nm or less. In some embodiments, the interlayer may have a maximum thickness of about 140 nm or less, such as about 130 nm or less, about 120 nm or less, such as about 110 nm or less, or about 100 nm or less.

[0059] The interlayer may have a minimum thickness of about 10 nm or more. In some embodiments, the interlayer may have a minimum thickness of about 15 nm or more, such as about 20 nm or more, such as about 25 nm or more, or about 30 nm or more, or about 40 nm or more.

[0060] The maximum thickness may be determined using SEM. The maximum thickness as referred to herein means the largest thickness value measured during the measurement of average thickness of the interlayer, as described elsewhere herein.

[0061] In some embodiments, the interlayer may have a uniform thickness. In some such embodiments, the interlayer may have a uniform thickness across its width.

[0062] By ‘uniform thickness’, it is meant that the interlayer has a low variation in thickness, and the interlayer may vary across its width by no more than 40%, such as no more than 35%, such as no more than 33% of the average thickness. For example, an interlayer having an average thickness of 80 nm with a uniform thickness as defined herein may have a thickness of no more than 112 nm and no less than 48 nm across its width, such as a thickness of no more than 108 nm and no less than 52 nm, or no more than 106.4 nm and no less than 53.6 nm.

[0063] In some embodiments, the uniformity of thickness can be determined by SEM measurements as described herein. Laser profilometry may alternatively be applied. In some embodiments, a surface roughness can be determines using atomic force microscopy (AFM).

[0064] In embodiments, an interlayer with a uniform thickness may have a thickness that varies across its width by no more than 30%. Such interlayers may show no visible edge defects and therefore improved performance. By visible edge defects of an interlayer, the present application refers to unevenness of the edge of the interlayer as observed by eye. The edge as referred to herein is an edge bounding the width of the interlayer. In some embodiments, a visible edge defect may have the form of a waviness or curve along the edge of the interlayer. In some embodiments, a visible edge defect may have a different appearance, such as a chip or other non-linearity. In some embodiments, a visible edge defect may be identified by comparing the straightness of the edge of the interlayer with a ruler or similar.

[0065] The interlayer contains a binder comprising CMC, and an electrically conducting material. The binder can adhere the interlayer to the electrode and to the current collector layer, while the electrically conducting material provides electrical conductivity. The binder may be a thermoplastic material: a thermoplastic material is particularly easy to handle and easily applied as a layer to the current collector. The binder comprises CMC. In some embodiments, the binder consists of CMC. Adhering the electrode to the current collector, via the interlayer, secures the electrode structure together.

[0066] The arrangement of the interlayer may suitably provide a particularly effective protective barrier against electrolyte corrosion, resulting in a particularly corrosion-resistant interface between the current collector and the tab, and without any detriment to the welding efficacy of the tab to the current collector.

[0067] In some embodiments, the interlayer may consist of the electrically conducting material and the binder, the binder comprising or consisting of CMC. In some embodiments, the interlayer may comprise at least 12 wt% CMC, such as at least 13 wt%, such as at least 14 wt%, such as at least 15 wt%. In some embodiments, the interlayer may comprise at most 80 wt% CMC, such as at most 75 wt%, such as at most 74 wt%, such as at most 73 wt%, such as at most to 72 wt%. In some embodiments, the interlayer may comprise from 12 to 80 wt% CMC, such as from 13 to 75 wt%, such as from 14 to 74 wt%, such as from 15 to 72 wt%. In some embodiments, the interlayer may comprise from 30 to 80 wt% CMC, such as from 30 to 75 wt%, such as from 35 to 75 wt%, such as from 37 to 72 wt%. In some embodiments, alongside this amount of CMC, the balance of the weight of the interlayer may be the electrically conducting material (with the exception of possible trace amounts of solvent remaining in the interlayer after drying steps).

[0068] Amounts of CMC within these ranges may help to provide optimal improvements in the corrosion resistance of the electrode structure.

[0069] In some embodiments, the interlayer may comprise at least 20 wt% electrically conducting material, such as at least 25 wt%, such as at least 26 wt%, such as at least 28 wt%. In some embodiments, the interlayer may comprise up to 90 wt% electrically conducting material, such as at most 88 wt%, such as at most 87 wt%, such as at most 86 wt%, such as at most 85 wt%. In some embodiments, the interlayer may comprise from 20 to 90 wt% electrically conducting material, such as from 25 to 90 wt%, such as from 26 to 88 wt%, such as from 27 to 85 wt%. In some embodiments, the interlayer may comprise from 20 to 70 wt% electrically conducting material, such as from 25 to 65 wt%, such as from 26 to 64 wt%, such as from 28 to 63 wt%. In some embodiments, alongside this amount of electrically conducting material, the balance of the weight of the interlayer may be CMC (with the exception of possible trace amounts of solvent remaining in the interlayer after drying steps).

[0070] In some embodiments, the electrically conducting material may comprise a tubular carbon material as a majority component by weight.

[0071] The term ‘majority component’ is used herein to define that the component constitutes at least 50 wt% of the total mass of the electrically conductive material. In some embodiments, the electrically conductive material may be 60 wt% or more, such as 70 wt% or more, such as 80 wt% or more, such as 90 wt% or more, such as 95 wt% or more, such as 99 wt% or more tubular carbon. The component may also constitute at least 50 vol% of the electrically conductive material, such as 60 vol% or more, such as 70 vol% or more, such as 80 vol% or more, such as 90 vol% or more, such as 95 vol% or more, such as 99 vol% or more tubular carbon. The term ‘tubular carbon’ is used herein to define carbonaceous materials having a generally tubular form. Tubular carbon materials may include, but are not limited to, carbon nanotubes (CNTs) - including single-walled carbon nanotubes (SWCNTs) and multi-walled carbon nanotubes (MWCNTs) - as well as carbon fibres such as carbon nanofibers (CNFs) and vapor-grown carbon fibres (VGCFs).

[0072] In some embodiments, the electrically conductive material may comprise 90 wt% or more of tubular carbon material. In some embodiments, the electrically conductive material may consist essentially of the tubular carbon material. In such embodiments, the interlayer may not comprise alternative non-tubular electrically conductive material components. That is, the interlayer may contain substantially no conductive carbonaceous material other than the tubular carbon materials - e.g. the interlayer may not comprise graphite, graphene and / or amorphous carbon (such as carbon black). In some embodiments, the interlayer may contain less than 0.5 wt% of conductive carbonaceous materials other than the tubular carbon material, such as 0.1 wt% or less, such as 0.05 wt% or less, such as 0.01 wt% or less.

[0073] In some embodiments, the electrically conducting material may comprise carbon nanotubes. This kind of carbon material can show particularly good performance and stability, and high electrical conductivity.

[0074] In some embodiments, the carbon nanotubes may comprise or consist of single-walled carbon nanotubes (SWCNTs).

[0075] In some embodiments, the carbon nanotubes may comprise or consist of multi-walled carbon nanotubes (MWCNTs).

[0076] In some embodiments, the electrically conducting material may comprise graphene.

[0077] In a second aspect, provided here is a battery cell incorporating the electrode structure of the first aspect as a cathode, further comprising at least one electrolyte and an anode. In some embodiments, the battery cell may be a lithium-ion battery.

[0078] The at least one electrolyte may take the form of a liquid or solid. In some embodiments, the at least one electrolyte comprises a halogenated species. In some embodiments, the at least one electrolyte may comprise one or more of LiPF6, LiClO4and LiBF4.

[0079] In some embodiments, the anode may comprise carbon.

[0080] In a third aspect, there is a method for making an electrode structure for use in a battery cell, the method comprising:

[0081] providing a current collector layer having a current collector surface; providing an electrode having an electrode surface, and arranging the electrode in electrical contact with the current collector;

[0082] providing a tab having a tab surface;

[0083] arranging an interlayer between the current collector surface and the tab surface, the interlayer comprising an electrically conducting material and a binder, wherein the binder comprises carboxymethyl cellulose.

[0084] In some embodiments, the method may comprise providing the current collector layer having a current collector surface, providing the tab having a tab surface, and then arranging the interlayer between the current collector surface and the tab surface.

[0085] In some embodiments, the current collector layer may be first provided. The tab may then be arranged on a current collector surface of the current collector layer. The interlayer may then be arranged between the current collector surface and the tab surface, such that the tab surface is in indirect contact with the current collector surface via the interlayer. The electrode layer may then be arranged on the interlayer.

[0086] In some embodiments, the current collector layer may be first provided. The interlayer may then be arranged on a current collector surface of the current collector layer. For example, the interlayer may be coated on the current collector layer. The tab having a tab surface may then be arranged on the interlayer, such that the interlayer may be arranged between the current collector surface and the tab surface. The electrode layer may be then arranged on the interlayer.

[0087] In a fourth aspect, provided herein is a use of an interlayer arranged between a current collector surface and a tab surface within an electrode structure to improve the corrosion resistance of the electrode structure;

[0088] the electrode structure comprising:

[0089] a current collector layer having a current collector surface;

[0090] an electrode layer having an electrode surface that faces the current collector surface;

[0091] a tab having a tab surface that faces the current collector surface, wherein the tab is electrically conductive and connectable to an external circuit; and

[0092] the interlayer arranged between the current collector surface and the tab surface, the interlayer comprising an electrically conducting material and a binder, wherein the binder comprises carboxymethyl cellulose.

[0093] In a fifth aspect, provided herein is a use of an interlayer arranged on a current collector surface and / or tab surface within an electrode structure to improve the corrosion resistance of the electrode structure, the interlayer comprising an electrically conducting material and a binder, wherein the binder comprises carboxymethyl cellulose.

[0094] Surprisingly, the present inventors have found that the use of the interlayer to improve the corrosion resistance of an electrode structure may be applicable throughout the electrode structure. The interlayer of the fifth aspect is identical to the interlayer of the first aspect. The interlayer of the fifth aspect may have some or all of the optional features described elsewhere herein for the interlayer of the first aspect.

[0095] In some embodiments, the interlayer may improve corrosion resistance of the electrode structure to at least one electrolyte. In some embodiments, the at least one electrolyte may have some or all of the optional features described elsewhere herein for the electrolyte of the battery cell of the second aspect.

[0096] These, and other aspects and embodiments of the invention, are described in further detail herein.

[0097] BRIEF DESCRIPTION OF THE DRAWINGS

[0098] Figure 1 is a perspective view of an electrode structure according to an embodiment of the invention comprising a current collector, an electrode, a tab, and an electrically conducting interlayer arranged between the tab and the current collector.

[0099] Figure 2 to 4 are steps in the process of assembling the electrode structure of Figure 1.

[0100] Figure 5 is a further embodiment of an electrode structure, comprising a further interlayer and a further electrode.

[0101] Figure 6 shows a Keyence image of the reverse side of a single-layer pouch (SLP) electrode according to an embodiment of the invention, with two darker corroded regions highlighted in boxes.

[0102] Figure 7 shows a SEM micrograph of (left) one of the edges of the darker corroded regions of the SLP electrode of Figure 6, and (right) a non-corroded bare aluminium surface for comparison. Scale bars are 10 pm.

[0103] Figure 8 shows SEM micrographs of a multi-layer pouch (MLP) cathode electrode according to an embodiment of the invention, with an exposed Al region and a primed Al region at different degrees of magnification, (a) scale bar of 100 μm; (b) scale bar of 1 μm.

[0104] DETAILED DESCRIPTION

[0105] Figure 1 illustrates an example of an electrode structure 10. This electrode structure includes a current collector layer 12 having a current collector surface 13, an electrode layer 16 having an electrode surface 17 that faces the current collector surface 13, a tab 15 having a tab surface 15’ that faces the current collector surface 13, and an electrically conducting interlayer 14 provided between the current collector surface 13 and the tab surface 15’. The interlayer 14 is electrically conducting, so as to conduct current between the electrode layer 16 and the current collector layer 12.

[0106] The current collector layer 12 may be made of any material that is suitable for conducting current. Preferably, the current collector layer is a metal foil, and the material is selected depending on the electrode. Transition metals including Al, Cu, Pt, Ni, Mo, and W are particularly effective. For example, aluminium may be a preferred material where the electrode is a cathode, and copper may be a preferred material where the electrode is an anode. The current collector layer may be any suitable thickness, for example between approximately 5 pm and 20 pm.

[0107] In this example, the electrode 16 comprises an electrode surface 17 that faces the current collector surface 13, and the interlayer 14 is arranged between the electrode surface 17 and the current collector surface 13.

[0108] In this example, the electrode 16 is a gel polymer electrode. The gel polymer electrode 16 may also be a free-standing electrode, though embodiments are also envisaged in which the gel polymer electrode is not freestanding. The gel polymer electrode 16 may be an extruded electrode.

[0109] Freestanding in this sense means that the electrode layer has initially been made separately from the current collector layer, without a current collector layer to support it. A freestanding electrode layer is therefore of sufficient integrity to be self-supporting without a current collector layer. When initially provided, the electrode layer 16 comprises two electrode surfaces 17 that are free surfaces.

[0110] In this example, the interlayer 14 acts as a binder or an adhesion layer that adheres the electrode layer 16 to the current collector layer 12. To this end, the interlayer 14 comprises a binder comprising CMC, and a conducting material, to perform the functions of adhesion, electrical conduction and corrosion resistance.

[0111] The gel polymer electrode 16 comprises a gel matrix formed from a polymer and a solvent. One or more electrochemically active materials are loaded into the gel matrix, typically in the form of solid particles. The electrochemically active material is capable of releasing or receiving an ion species, preferably an alkali metal ion, and most preferably lithium and / or sodium. The solvent of the gel matrix will typically be an electrolyte material, for example a carbonate electrolyte. In this particular example, the electrochemically active material is a lithium-containing metal oxide material, and preferably a lithium transition metal oxide such as a lithium cobalt oxide.

[0112] Typically, the polymer gel electrode layer 16 has a thickness of approximately 10 μm to approximately 200 μm.

[0113] Considering the interlayer 14 in more detail, as noted above, the interlayer comprises a binder and a conducting material. The binder of the interlayer 14 is CMC.

[0114] Furthermore, it was found that the electrode structure 10 has surprising corrosion resistance when CMC is used as a binder, and when the interlayer 14 is arranged between the current collector surface 13 and the tab surface 15’.

[0115] The conducting material may be any suitable material capable of conducting current, with any suitable physical form. For example, the conducting material may take the form of carbon nanotubes, though it is also envisaged that the conducting material may be particles or flakes of metal, or other carbon allotropes such as graphite or graphene.

[0116] The interlayer 14 may optionally include a plasticiser to increase the adhesive properties of the interlayer even further. Any suitable plasticiser may be used, but in one particular example the plasticiser is propylene carbonate. The interlayer 14 may also optionally include a salt additive, particularly in combination with a plasticiser. The salt additive may be selected so as to act to passivate the material of the current collector layer 12. To this end, the salt additive preferably contains ions of the species that will be exchanged between the anode and the cathode. For example, where the battery is a lithium battery, the salt additive may be a lithium-based salt.

[0117] The interlayer 14 may be any suitable thickness, but a thickness of between 100 to 150 nm is preferred.

[0118] To form the electrode structure 10, the current collector layer 12 having a current collector surface 13 is first provided. The interlayer 14 is then arranged on the current collector layer 12, such that the interlayer faces the current collector surface 13. The electrode layer 16 is arranged on the interlayer 14, leaving a portion of the interlayer 14 exposed. The tab 15 is then arranged on the exposed portion of the interlayer 14.

[0119] To form the interlayer 14 on the current collector layer 12, the binder comprising CMC and the conducting material (and optionally the plasticiser and salt additive) are mixed with a sacrificial solvent. The solvent may be selected for compatibility with the binder and the electrode material. For example, the solvent may be water, which has good compatibility with CMC. Where the plasticiser is used, the plasticiser and sacrificial solvent are selected such that a boiling point and vapour pressure of the solvent is lower than a boiling point and vapour pressure of the plasticiser.

[0120] To form the tab 15 on the interlayer 14 on the current collector layer 12, the tab 15 is welded onto the current collector layer 12 via the tab surface 15’. Typically, the tab 15 is welded onto the notch section of the current collector layer 12. The tab surface 15’ is therefore in contact with the interlayer 14, and is in indirect contact with the current collector layer 12 via the interlayer 14.

[0121] To form the electrode structure 10, the current collector layer 12 is first provided as shown in Figure 2. The interlayer 14 is then arranged on the current collector layer 12, as shown in Figure 3 and the electrode layer 16 is arranged on the interlayer 14 leaving a portion of the interlayer 14 exposed, as shown in Figure 4. The tab 15 is then arranged on the exposed portion of the interlayer 15 to form the electrode structure 10 shown in Figure 1.

[0122] Figure 5 illustrates an alternative electrode structure 210, which may encompass the gel polymer electrode and associated binder-based interlayer.

[0123] The alternative electrode structure 210 is substantially the same as the electrode structure 10 of Figure 1, except that both surfaces 213, 213f of the current collector layer 212 is provided with corresponding interlayers 214, 214f and electrodes 216, 216f, and the tab 215 comprising a tab surface 215’ is arranged such that an interlayer 214 is between the current collector 212 and the tab surface 215’. To this end, the alternative electrode structure 210 comprises a further current collector surface 213f, with a further interlayer 214f arranged thereon. A further electrode 216f is arranged over the further current collector 213f, such that a further electrode surface 217f is in electrical contact with the further interlayer 214f. The alternative electrode structure 210 may be made using the same methods already described above.

[0124] Both interlayers 214, 214f comprise a binder comprising CMC.

[0125] Any of the methods described above may be implemented as continuous methods. For example, a continuous roll of current collector may be supplied to an interlayer station, where the interlayer is formed continuously on the current collector to ‘prime’ the current collector. A continuous roll of free-standing electrode may then be supplied to the primed current collector to arrange the electrode on top. The assembled structure may then be pressurised and / or heated. Pressure may be supplied by rollers, for example at a calendaring station. Where heat is also applied, the rollers may be heated rollers.

[0126] The completed structure may be fed onwards to a battery assembly station, to be assembled with other components into a battery. In a typical single-layer pouch electrode, regions which are corroded due to exposure to electrolyte may be seen as a dark region on the surface. This is shown by the Keyence image in Figure 6, in which two such regions are highlighted in the boxes.

[0127] The microstructure of the electrode surface can be seen with a SEM micrograph. This is shown in Figure 7. The darker corroded region clearly shows a rough, pitted surface caused by electrolyte corrosion (left). A non-corroded bare aluminium surface is provided for comparison to show a smooth surface (right).

[0128] Figure 8 shows SEM micrographs of a multi-layer pouch (MLP) cathode electrode, with an exposed Al region and a primed Al region at different degrees of magnification, (a) scale bar of 100 μm; (b) scale bar of 1 μm. The exposed Al region shows a region of the current collector in which the bare Al foil was exposed to the electrolyte. The region clearly shows a pitted surface caused by electrolyte corrosion. The primed Al region, the interlayer of which was removed from the surface for the purposes of taking the SEM micrograph, clearly shows a non-pitted surface which indicates that electrolyte corrosion was suppressed.

Claims

CLAIMS:

1. An electrode structure for use in a battery cell, the electrode structure comprising:a current collector layer having a current collector surface;an electrode layer in electrical contact with the current collector;a tab having a tab surface that faces the current collector surface, wherein the tab is electrically conductive and connectable to an external circuit; andan interlayer arranged between the current collector surface and the tab surface, the interlayer comprising an electrically conducting material and a binder, wherein the binder comprises carboxymethyl cellulose.

2. The electrode structure of claim 1, wherein the tab is provided between the electrode and the current collector.

3. The electrode structure of claim 1 or 2, wherein the tab comprises an end surface which extends beyond the current collector, wherein the end surface is connectable to the external circuit.

4. The electrode structure of any one of claims 1 to 3, wherein the tab surface comprises aluminium.

5. The electrode structure of any one of claims 1 to 4, wherein the interlayer has an average thickness of 150 nm or less.

6. The electrode structure of any one of claims 1 to 5, wherein the interlayer has a maximum thickness of 200 nm and / or a minimum thickness of 100 nm.

7. The electrode structure of any one of claims 1 to 6, wherein the interlayer has a uniform thickness.

8. The electrode structure of any one of claims 1 to 7, wherein the current collector surface comprises aluminium.

9. The electrode structure of any one of claims 1 to 8, wherein the electrically conducting material comprises a tubular carbon material as a majority component by weight.

10. The electrode structure of any one of claims 1 to 9, wherein the electrically conducting material comprises carbon nanotubes.

11. The electrode structure of claim 10, wherein the carbon nanotubes comprise or consist of single-walled carbon nanotubes (SWCNTs).

12. The electrode structure of claim 10, wherein the carbon nanotubes comprise or consist of multi-walled carbon nanotubes (MWCNTs).

13. The electrode structure of any preceding claim, wherein the current collector surface is a first current collector surface, and the current collector has a second current collector surface opposite the first current collector surface, wherein a second interlayer is arranged on at least a portion of the second current collector surface, the interlayer comprising an electrically conducting material and a binder, wherein the binder comprises carboxymethyl cellulose.

14. The electrode structure of any preceding claim, wherein the tab surface is a first tab surface, and the tab has a second tab surface opposite the first tab surface, wherein a second interlayer is arranged on at least a portion of the second tab surface, the interlayer comprising an electrically conducting material and a binder, wherein the binder comprises carboxymethyl cellulose.

15. A battery cell incorporating the electrode structure of any one of claims 1 to 14 as a cathode, further comprising at least one electrolyte and an anode.

16. The battery cell of claim 15, wherein the at least one electrolyte comprises one or more of LiPF6, LiClO4and LiBF4.

17. A method for making an electrode structure for use in a battery cell, the method comprising:providing a current collector layer having a current collector surface; providing an electrode having an electrode surface, and arranging the electrode in electrical contact with the current collector;providing a tab having a tab surface;arranging an interlayer between the current collector surface and the tab surface, the interlayer comprising an electrically conducting material and a binder, wherein the binder comprises carboxymethyl cellulose.

18. Use of an interlayer arranged between a current collector surface and a tab surface within an electrode structure to improve the corrosion resistance of the electrode structure; the electrode structure comprising:a current collector layer having a current collector surface;an electrode layer having an electrode surface that faces the current collector surface;a tab having a tab surface that faces the current collector surface, wherein the tab is electrically conductive and connectable to an external circuit; andthe interlayer arranged between the current collector surface and the tab surface, the interlayer comprising an electrically conducting material and a binder, wherein the binder comprises carboxymethyl cellulose.

19. Use of an interlayer arranged on a current collector surface and / or tab surface within an electrode structure to improve the corrosion resistance of the electrode structure, the interlayer comprising an electrically conducting material and a binder, wherein the binder comprises carboxymethyl cellulose.

20. Use of the interlayer of claim 19, wherein the interlayer improves corrosion resistance of the electrolyte surface to at least one electrolyte.

21. Use of the interlayer of claim 20, wherein the at least one electrolyte comprises one or more of LiPF6, LiClO4and LiBF4.

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