Stack for electrochemical cell

Abstract

A stack for an electrochemical cell is described. The stack has a laminate structure comprising a first gel separator layer and a second gel separator layer, and a first electrode layer between the first and second gel separator layers. The first gel separator layer and second gel separator layer each extend beyond a peripheral edge of the first electrode layer. The first gel separator layer and second gel separator layer are adhered to one another by a heat-seal adjacent to opposing edges of the first electrode layer. Electrochemical secondary cells and energy storage devices containing the stack are also described, as are methods of manufacturing the stack.

Description

[0001] STACK FOR ELECTROCHEMICAL CELL

[0002] BACKGROUND

[0003] To ensure high performance of electrochemical cells containing gel polymer components such as gel polymer electrodes and / or gel polymer separators, good contact between the components of the cell is necessary to minimise internal resistance and reduce the risk of a short circuit.

[0004] Without adhesion between the components and / or a constant high pressure applied to the stack, there would be a risk of movement of the components after assembly of the stack. This could cause inhomogeneous surface contact between cell components which would lead to increased cell resistance or an inhomogeneous current density at the electrode.

[0005] Existing stack lamination methods rely on assembling the stack of electrodes and separators and applying heat and / or pressure to adhere the layers together. However, such methods can be detrimental to the performance of the cell due to increased cell resistance and a reduced ability for liquid electrolyte to properly wet into the stack after assembly without the need for elevated temperature.

[0006] SUMMARY

[0007] A first aspect of the invention is a stack for an electrochemical cell, having a laminate structure comprising: a first gel separator layer and a second gel separator layer; and a first electrode layer between the first and second gel separator layers; wherein the first gel separator layer and second gel separator layer each extend beyond a peripheral edge of the first electrode layer such that they contact one another beyond the peripheral edge of the first electrode layer; and the first gel separator layer and second gel separator layer are adhered to one another by a heat-seal adjacent to opposing edges of the first electrode layer.

[0008] The stack of the first aspect comprises an electrode layer positioned between two gel separator layers, in a “sandwich” arrangement. Since the gel separator layers each extend beyond a peripheral edge of the first electrode layer, they lie in contact with one another beyond the peripheral edge. The first and second gel separator layers are also adhered to one another where they are in contact, at locations beyond two respective opposing peripheral edges of the first electrode layer. In this way, the first electrode layer is more effectively held in position between the two separator layers than if the layers were completely free and not adhered together. The electrode layer is prevented from sliding laterally in at least one direction due to the heat-seal between the gel separator layers. There is therefore a reduced risk of movement of the layers within the cell during use, and a concomitant reduction in the risk of short circuit or internal resistance.

[0009] The first and second gel separator layers and the first electrode layer are “film-like” structures, having a third dimension which is significantly shorter than the other two dimensions (the third dimension tends to have a length in the micrometre range, compared with lengths in the centimetre range for the first and second dimensions). Herein, “first dimension” and “second dimension” are used to refer to axes or directions lying parallel with the plane of the film like electrode or separator, and “third dimension” is used to refer to the axis or direction lying perpendicular to such plane, i.e. directed through the laminate structure of the stack when assembled.

[0010] Herein, the term “peripheral edge of the first electrode layer” refers to an edge of the electrode layer which lies parallel with the plane containing the two longer dimensions, i.e. first and second dimensions. The term “opposing edges” refers to edges which lie substantially opposite one another across the centre of the first electrode. So, for example, for a square or rectangular first electrode layer, opposite edges will be either the first pair of parallel edges, or the second pair of parallel edges.

[0011] The term “stack” is understood by the skilled person to refer to the layered or laminate assembly of the components which make up an electrochemical cell, in particular the electrodes and separator(s) which lies between them. The first gel separator layer and second gel separator layer may be adhered to one another by a heat-seal adjacent to opposing edges of the first electrode layer to form an adhered portion (i.e. a heat-sealed portion).

[0012] The first gel separator layer and second gel separator layer may be adhered to one another by one or more heat-seals adjacent to opposing edges of the first electrode layer to form one or more adhered portions (i.e. heat-sealed portions).

[0013] The first gel separator layer and second gel separator layer each extend beyond a peripheral edge of the first electrode layer. In other words, the first gel separator layer is larger than the first electrode layer in at least one of the first and second dimensions; and the second gel separator layer is larger than the first electrode layer in at least one of the first and second dimensions.

[0014] In some embodiments, the first gel separator layer and second gel separator layer each extend beyond the peripheral edge of the first electrode layer around the entire peripheral edge of the first electrode layer. In other words, in some embodiments the first gel separator layer is larger than the first electrode layer in both the first and second dimensions; and the second gel separator layer is larger than the first electrode layer in both the first and second dimensions. In this way, it is possible to stack the first electrode layer between the first and second gel separator layers in a way which ensures that a border of each gel separator layer protrudes around the entire periphery of the first electrode layer. This allows the two gel separator layers to contact one another around the entire periphery of the first electrode layer, forming an uninterrupted “envelope” around the first electrode layer.

[0015] When the first and second gel separator layers provide such a border around the entire periphery of the first electrode layer, they are adhered to one another by a heat-seal adjacent to opposing edges of the first electrode layer. In some embodiments, they are adhered to one another by first and second heat-seals adjacent to each of a first pair of opposing edges of the first electrode layer; and third and fourth heat-seals adjacent to each of a second pair of opposing edges of the first electrode layer. In this way, lateral sliding movement of the first electrode layer between the gel separator layers is further hindered, since the movement of the first electrode layer is limited in both first and second dimensions by the two pairs of heat-seals which effectively surround the first electrode layer.

[0016] In some embodiments, the first gel separator layer and second gel separator layer are adhered to one another by a first heat-sealed strip along one edge of the stack and a second heat-sealed strip along a second edge of the stack which is opposite the first edge of the stack, wherein the first gel separator layer and second gel separator layer are unsealed between the two first and second heat-sealed strips. The two opposing strips offer a secure seal which effectively secures the first electrode layer. The unsealed portion between the strips offers fluid communication between the first electrode layer and the outside of the stack, such that liquid electrolyte added to the stack after assembly is able to more easily pass between the two gel separator layers into the space between the two gel separator layers surrounding the first electrode layer, without the need to take measures such as heating the liquid electrolyte.

[0017] In some embodiments, the first gel separator layer and second gel separator layer are adhered to one another by a heat-sealed border region which extends around substantially the entire periphery of the first electrode layer. Such a heat-sealed border region may be continuous, providing an uninterrupted seal around substantially the entire periphery of the first electrode layer. By “substantially the entire periphery” it is meant that the heat-sealed border region extends around the entire periphery apart from one or more small breaks in the seal which may be necessary to provide cell functionality, for example to allow an electrical connector to pass between the first and second gel separator layers to provide electrical connection to the first electrode layer.

[0018] In other embodiments the heat-sealed border region is discontinuous, comprising at least one unsealed portion or section.

[0019] In some embodiments, the heat-sealed border region is sealed around substantially the entire periphery of the stack with the exception of an unsealed section making up a portion of the length of a single edge of the stack. In some embodiments the unsealed section lies substantially centrally along the edge of the stack. In some embodiments, the heat-sealed border region is sealed around substantially the entire periphery of the stack with the exception of two unsealed sections, each making up a portion of the length of an edge of the stack. In some embodiments, the two unsealed sections lie within respective opposite edges of the stack. For example, if the stack is square or rectangular in shape, the two unsealed sections may lie within respective parallel edges of the stack, with the remaining pair of parallel edges being completely sealed. In some embodiments, the heat-sealed border region is sealed around substantially the entire periphery of the stack with the exception of three unsealed sections, each making up a portion of the length of a different edge of the stack. For example, if the stack is square or rectangular in shape, first and second unsealed sections may lie within respective parallel edges of the stack, with a third unsealed section lying within one of the remaining two edges, with the final edge being completely sealed.

[0020] In some embodiments, the heat-sealed border region is discontinuous, comprising at least one unsealed portion. Again, the unsealed portion may provide fluid communication between the first electrode layer and the outside of the stack. In this way, liquid electrolyte added to the stack after assembly is able to more easily pass between the two gel separator layers into the space between the two gel separator layers surrounding the first electrode layer.

[0021] In some embodiments, the heat-sealed border region comprises a plurality of unsealed portions, each lying between two sealed portions, for example 2, 3, 4, 5, 6, 7, 8, 9 or 10 unsealed portions. In some embodiments, the plurality of unsealed portions are regularly distributed around the heat-sealed border region. In this way, a plurality of unsealed portions exists distributed around the entire periphery of the first electrode layer, providing a respective plurality of points of fluid communication between points around the entire edge of the first electrode layer and the outside of the stack. This provides a means for liquid electrolyte to pass between the gel separator layers at points around the entire periphery of the first electrode layer, ensuring a more even and complete distribution of liquid electrolyte within the entire space surrounding the first electrode layer between the gel separator layers. This helps to reduce the internal resistance of the cell and improves ionic conductivity.

[0022] In some embodiments, the heat-sealed border region comprises a regularly repeating intermittent seal around the entire periphery of the stack. In some embodiments, the regularly repeating intermittent seal comprises a plurality of sealed sections distributed around the entire periphery of the stack, each having a substantially identical length, with any two neighbouring sealed sections being separated by an unsealed section, wherein all unsealed sections around the entire periphery of the stack are of substantially identical length. In some embodiments, an unsealed section has a length shorter than a sealed section. In some embodiments, the length of an unsealed section is from 30 to 80%, for example from 30 to 70% or from 30 to 50% of the length of a sealed section. In this way, a majority of the periphery of the stack is sealed, while maintaining an even distribution of unsealed portions around the entire periphery of the stack which make up a minority of the total peripheral length. In this way, a strong and reliable seal is formed while maintaining fluid communication with the outside of the stack to facilitate the ingress of liquid electrolyte.

[0023] In some embodiments, the distance from the inner edge of the heat-sealed border region to the peripheral edge of the first electrode layer is less than 5 mm, for example less than 4 mm, less than 3 mm or less than 2 mm. Providing a seal as close to the first electrode as possible ensures more secure holding of the first electrode within the “envelope” formed by the first and second gel separator layers and minimises the risk that the first electrode may slide relative to the first and second gel separator layers. In some embodiments, the distance from the inner edge of the heat-sealed border region to the peripheral edge of the first electrode layer is at least 0.5 mm, for example at least 1 mm. Ensuring such minimum spacing between the first electrode and the seal reduces the risk of inadvertently forming a seal between the first electrode and one of the gel separator layers, and reduces the risk of one of the gel separator layers tearing or thinning due to increased shear forces caused by proximity of the seal to the first electrode layer. Shear-thinning of the separator layer could arise if the seal is too close to the first electrode, which could lead to a short circuit during use. As such, in some embodiments, the distance from the inner edge of the heat-sealed border region to the peripheral edge of the first electrode layer is from 0.5 to 5 mm, for example from 1 to 5 mm, from 1 to 4 mm, from 1 to 3 mm or from 1 to 2 mm.

[0024] In some embodiments, the heat-sealed border region extends from the edge of the first and second gel separator layers. In other words, in some embodiments there is no unsealed region between the heat-sealed border region and the peripheral edge of the first and second gel separator layers, and the edges of the first and second gel separator layers are sealed together as part of the heat-sealed border region. This could be achieved, for example, by ensuring that heat is applied during the sealing procedure at the very edges of the gel separator layers, or by forming a heat-sealed border region before trimming away the edges of the stack to ensure that the heat-sealed border region extends all the way to the trimmed edge.

[0025] The shapes of the first electrode layer and first and second gel separator layers are not particularly limited. In some embodiments, the first electrode layer has a shape which is substantially the same as the first and second gel separator layers. In some embodiments, the first electrode layer has a shape which is geometrically similar to that of each of the first and second gel separator layers, i.e. the shape and size of the first electrode layer can be achieved by uniformly scaling the shape and size of either of the gel separator layers. In some embodiments, scaling down one of the gel separator layers by 60 to 90% provides the shape of the first electrode layer, for example 65 to 90%, 70 to 90% or 75 to 85%.

[0026] In some embodiments, the first and second gel separator layers are quadrilateral-shaped films of substantially identical geometry such that the peripheral edge of the first gel separator layer is aligned with the peripheral edge of the second gel separator layer. In some embodiments, the first gel separator layer and second gel separator layer are adhered to one another by heat-seals positioned at each comer of the quadrilateral. In some embodiments, one or more further heat-seals are positioned along at least one of the edges between the corners of the quadrilateral while retaining at least one unsealed portion along that edge. By adhering the first and second gel separator layers using a heat-seal in each comer, a secure seal is provided which effectively prevents any movement of the gel separator layers relative to one another and effectively encapsulates the first electrode layer between the first and second gel separator layers to restrict its movement. Providing unsealed portions between any two corners provides a means for the ingress of liquid electrolyte into the stack.

[0027] Providing further heat-seals along the edges between two heat-sealed corners while retaining at least one unsealed portion further strengthens the bond between the first and second gel separator layers and further limits any movement of the first electrode layer, while facilitating the ingress of liquid electrolyte.

[0028] When the first electrode layer and / or the gel separator layers are quadrilateral films, the comers may be square or rounded.

[0029] In some embodiments, the first electrode layer is not adhered to either of the first gel separator layer or second gel separator layer. By ensuring that the first electrode layer is not adhered to either of the first gel separator layer or second gel separator layer, a more homogeneous amount of contact between the first electrode layer and each of the first and second gel separator layers is achieved across the entire surface of the first electrode layer. Furthermore, a bond between the first electrode layer and the gel separator layers is not necessary, since the first electrode layer is effectively prevented from lateral movement relative to the gel separator layers by the heat-seal. Avoiding a heat-seal or other bond between the first electrode layer and gel separator layers reduces the risk of damage to the operational area of the layers caused by the exposure to heat during the sealing process.

[0030] In some alternative embodiments, the first electrode layer may be adhered to one or both of the first and second gel separator layers. For example, a face of the first electrode layer may be adhered to either the first or second gel separator layer. As mentioned above, the first electrode layer may have a “film-like” structure, having a third dimension which is significantly shorter than the other two dimensions. Thus, the first electrode layer may be generally planar in form, defining two opposing faces. In such embodiments, a lamination process may be used to adhere the face of the first electrode layer to the gel separator layer. The lamination process may involve applying a heated roller to heat the face of the first electrode layer such that a part or a whole of the face bonds to the gel separator layer. Adhering the first electrode layer to one or both of the first and second gel separator layers may help to further prevent sliding movement of the first electrode layer between the gel separator layers.

[0031] In some embodiments, the first and second gel separator layers are adhered to one another without the presence of any adhesive composition between the first and second gel separator layers. In some embodiments, the heat-seal between the first and second gel separator layers does not comprise any adhesive composition, and the bond between the first and second gel separator layers arises entirely through melt-bonding of the first and second gel separator layers during a heat-sealing procedure. In this way, additional adhesive compositions, which could affect the structure or performance of the stack over time and create added cost and complexity for the manufacturing process, are avoided.

[0032] In some embodiments, the width of the heat-sealed border region, measured in a direction from the edge of the first and second gel separator layers towards the centre of the first and second gel separator layers, is from 5 to 15% of the entire width of the first and second gel separator layers, for example from 6 to 15% or from 7 to 12%. In this way, a relatively narrow sealed “band” is provided around the periphery of the first and second gel separator layers. A thickness of this band within the above ranges provides a good balance of seal strength and durability while minimising any detrimental effects on the structure of the layers which may be caused by the application of heat during the sealing process.

[0033] In some embodiments, the first and second gel separator layers each have a length, in the first or second dimension, of from 3 to 10 cm, for example from 3 to 8 cm or from 4 to 6 cm.

[0034] In some embodiments, the first and second gel separator layers each have an area (defined in the first and second dimensions) of from 10 to 100 cm2, for example from 10 to 80 cm2, from 10 to 60 cm2or from 14 to 50 cm2. In some embodiments, the first and second gel separator layers each have an area (defined in the first and second dimensions) of from 30 to 70 cm2, for example from 40 to 60 cm2or from 45 to 55 cm2. In some embodiments, the first and second gel separator layers have an area (defined in the first and second dimensions) identical with one another.

[0035] In some embodiments, the first and second gel separator layers each independently have a thickness of from 10 to 30 pm, for example from 12 to 28 pm or from 14 to 25 pm. Such thicknesses enable an effective heat-seal between the two layers without the layers being damaged by the heat-sealing process.

[0036] The first electrode layer may be any type of electrode layer, for example a solvent-cast (solid) electrode layer or a gel electrode layer.

[0037] In some embodiments, the first electrode layer is a gel electrode layer, comprising a polymer and liquid electrolyte together forming a polymer-electrolyte gel matrix phase. Gel electrode layers may be preferred to solvent-cast electrode layers, because they offer a natural “tackiness” which serves to further strengthen adhesion between the first electrode layer and the neighbouring gel separator layers within the stack. Such gel electrodes are known. The gel electrode may further comprise a dispersed solid phase within the polymer-electrolyte gel matrix phase. The dispersed solid phase may comprise an electrochemically active material. The dispersed solid phase may further comprise one or more of a binder and a conductive additive.

[0038] The exact nature of any electrochemically active material, binder or conductive additive is not relevant to the functioning of the invention. The skilled person is aware of suitable choices of electrochemically active material, binder and conductive additive.

[0039] In some embodiments, the first electrode layer is a cathode layer. In some embodiments, the first electrode layer is an anode layer.

[0040] The cathode layer may comprise a positive active material. The cathode layer may further comprise one or more of a conductive additive and a binder. The anode layer may comprise a negative active material. The anode layer may further comprise one or more of a conductive additive and a binder.

[0041] In some embodiments, the first electrode layer is a gel cathode layer. In some embodiments, the first electrode layer is a gel anode layer.

[0042] The first electrode layer may comprise a current collector. For example, the first electrode layer may comprise a current collector layer.

[0043] When the first electrode layer is a cathode layer, a positive active material may be applied to one or both sides of the current collector layer. When the first electrode layer is a cathode layer, the current collector may comprise or consist of aluminium. For example, the current collector may comprise or consist of aluminium foil.

[0044] When the first electrode layer is an anode layer, a negative active material may be applied to one or both sides of the current collector layer. When the first electrode layer is an anode layer, the current collector may comprise or consist of copper. For example, the current collector may comprise or consist of copper foil.

[0045] In some embodiments, the first electrode layer is a gel electrode layer comprising one or more 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 [bi s(m ethoxy ethoxyethoxide)-phosphazene], poly(dimethylsiloxane) (PDMS), polyacene, polydisulfide, polystyrene, polystyrene sulfonate, polypyrrole, polyaniline, polythiophene, polythione, polyvinyl pyridine (PVP), polyvinyl chloride (PVC), polyaniline, poly(3,4-ethylenedioxythiophene) (PEDOT), poly(p-phenylene), poly(triphenylene), polyazulene, polyfluorene, polynaphthalene, polyanthracene, polyfuran, polycarbazole, tetrathiafulvalene- substituted polystyrene, ferrocene- substituted polyethylene, carbazole-substituted 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-Acryl ami do-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.

[0046] In some embodiments, the first electrode layer comprises one or more of poly(vinylidene difluoride) (PVdF), poly(vinylidene fluoride-co-hexafluoropropylene) (PvDF-HFP) and poly(methyl methacrylate) (PMMA). In some embodiments, the first electrode layer comprises poly(vinylidene difluoride) (PVdF).

[0047] In some embodiments, the first electrode layer further comprises a liquid electrolyte and an electrochemically active material.

[0048] The liquid electrolyte causes gelation of the polymer to form the gel electrode layer. In some embodiments, the liquid electrolyte comprises a linear or cyclic carbonate solvent and a lithium salt. In some embodiments the solvent comprises or consists of one or more of ethylene carbonate (EC) propylene carbonate (PC), vinylene carbonate (VC) and fluoroethylene carbonate (FEC).

[0049] In some embodiments, the lithium salt comprises one or more of LiPFe, LiBF4, lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), lithium bis(fluorosulfonyl)imide (LiFSI), lithium 4,5-dicyano-2-(trifluoromethyl)imidazolide (LiTDI), lithium difluoro(oxalato)borate (LiDFOB), lithium difluorophosphate and lithium bis(oxalato) borate. In some embodiments, the liquid electrolyte comprises a mixture of two or more different lithium salts.

[0050] In some embodiments, the first and second gel separator layers each independently comprise one or more 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 [bi s(m ethoxy ethoxyethoxide)-phosphazene], poly(dimethylsiloxane) (PDMS), polyacene, polydisulfide, polystyrene, polystyrene sulfonate, polypyrrole, polyaniline, polythiophene, polythione, polyvinyl pyridine (PVP), polyvinyl chloride (PVC), polyaniline, poly(3,4-ethylenedioxythiophene) (PEDOT), poly(p-phenylene), poly(triphenylene), polyazulene, polyfluorene, polynaphthalene, polyanthracene, polyfuran, polycarbazole, tetrathiafulvalene- substituted polystyrene, ferrocene- substituted polyethylene, carbazole-substituted 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-Acryl ami do-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.

[0051] In some embodiments, the first and second gel separator layers each independently comprise one or more of poly(vinylidene difluoride) (PVdF), poly(vinylidene fluoride-co- hexafluoropropylene) (PvDF-HFP) and poly(m ethyl methacrylate) (PMMA). In some embodiments, the first and second gel separator layers each independently comprise poly(vinylidene difluoride) (PVdF).

[0052] These polymers have been found to provide the combined benefits of good electrochemical properties alongside good heat-sealing properties.

[0053] In some embodiments, the first and second gel separator layers and the first electrode layer each independently comprise one or more 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). In some embodiments, the first and second gel separator layers and the first electrode layer each independently comprise one or more polymers independently selected from poly(vinylidene difluoride) (PVdF), poly(vinylidene fluoride-co-hexafluoropropylene) (PvDF-HFP) and poly(methyl methacrylate) (PMMA). In some embodiments, the first and second gel separator layers and the first electrode layer each independently comprise one or more polymers independently selected from poly(vinylidene difluoride) (PVdF) and poly(vinylidene fluoride-co-hexafluoropropylene) (PvDF-HFP).

[0054] In some embodiments, the first and second gel separator layers each independently further comprise a liquid electrolyte and an electrochemically active material.

[0055] The liquid electrolyte causes gelation of the polymer to form the first and second gel separator layers. In some embodiments, the liquid electrolyte comprises a linear or cyclic carbonate solvent and a lithium salt. In some embodiments the solvent comprises or consists of one or more of ethylene carbonate (EC) propylene carbonate (PC), vinylene carbonate (VC) and fluoroethylene carbonate (FEC).

[0056] In some embodiments, the lithium salt comprises one or more of LiPFe, LiBF4, lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), lithium bis(fluorosulfonyl)imide (LiFSI), lithium 4,5-dicyano-2-(trifluoromethyl)imidazolide (LiTDI), lithium difluoro(oxalato)borate (LiDFOB), lithium difluorophosphate and lithium bis(oxalato) borate. In some embodiments, the liquid electrolyte comprises a mixture of two or more different lithium salts.

[0057] In some embodiments, the first electrode layer has a thickness of 40 to 150 pm.

[0058] A second aspect of the invention is an electrochemical secondary cell comprising: a stack according to the first aspect; and a second electrode layer in contact with the first gel separator layer. The second electrode layer is positioned on the side of the first gel separator layer opposite the first electrode layer. In other words, the first gel separator layer lies interposed between the first electrode layer and the second electrode layer. This creates a complete electrochemical cell comprising first and second electrode layers separated by the first gel separator layer. Stacking of layers can then be continued with alternating stacks according to the first aspect, each separated by further electrode layers to create a battery of cells (i.e. an electrochemical secondary cell stack).

[0059] In some embodiments, the first electrode layer is a cathode layer and the second electrode layer is an anode layer. In some embodiments, the first electrode layer is a gel cathode layer and the second electrode layer is a gel anode layer.

[0060] In some embodiments, the first electrode layer is a gel cathode layer and the second electrode layer is a solid, solvent-cast anode layer.

[0061] In some embodiments, the first electrode layer is an anode layer and the second electrode layer is a cathode layer. In some embodiments, the first electrode layer is a gel anode layer and the second electrode layer is a gel cathode layer.

[0062] In some embodiments, the first electrode layer is a solid, solvent-cast anode layer and the second electrode layer is a gel cathode layer.

[0063] In some embodiments, the first electrode layer is a gel anode layer and the second electrode layer is a solid, solvent-cast cathode layer.

[0064] The second electrode layer may comprise a current collector. For example, the second electrode layer may comprise a current collector layer.

[0065] When the second electrode layer is a cathode layer, a positive active material may be applied to one or both sides of the current collector layer. When the first electrode layer is a cathode layer, the current collector may comprise or consist of aluminium. For example, the current collector may comprise or consist of aluminium foil. When the second electrode layer is an anode layer, a negative active material may be applied to one or both sides of the current collector layer. When the first electrode layer is an anode layer, the current collector may comprise or consist of copper. For example, the current collector may comprise or consist of copper foil.

[0066] In some embodiments, the electrochemical secondary cell further comprises a third electrode layer in contact with the second gel separator layer.

[0067] In some embodiments, one or both of the second electrode layer and the third electrode layer is an anode layer. In some embodiments, each of the second electrode layer and the third electrode layer are anode layers.

[0068] In some embodiments, one or both of the second electrode layer and the third electrode layer is a gel anode layer. In some embodiments, each of the second electrode layer and the third electrode layer are gel anode layers.

[0069] In some embodiments, the electrochemical cell is a lithium-ion electrochemical secondary cell.

[0070] A third aspect of the invention is an electrochemical secondary cell stack comprising first and second electrochemical secondary cells each according to the second aspect of the invention, wherein the adhered first and second gel separator layers of the stack of the first electrochemical secondary cell and the adhered first and second gel separator layers of the stack of the second electrochemical secondary cell are further adhered to one another by an inter-stack heat-seal.

[0071] In some embodiments, the adhered portion or portions of the first and second gel separator layers of the stack of the first electrochemical secondary cell and the adhered portion or portions of the first and second gel separator layers of the stack of the second electrochemical secondary cell may be further adhered to one another by an inter-stack heat-seal. In some embodiments, the electrochemical secondary cell stack may comprise a stack according to the first aspect of the invention in contact with the second electrode layer of the second electrochemical secondary cell. The adhered first and second gel separator layers of the stack according to the first aspect of the invention may be adhered to the adhered first and second gel separator layers of the stack of the second electrochemical secondary cell by a further inter-stack heat-seal.

[0072] A fourth aspect of the invention is an electrochemical secondary cell stack comprising a plurality of electrochemical secondary cells each according to the second aspect of the invention, wherein the adhered first and second gel separator layers of two or more stacks are adhered to one another by one or more inter-stack heat-seals.

[0073] In some embodiments, the adhered portion or portions of the first and second gel separator layers of two or more stacks are adhered to one another by one or more inter-stack heatseals.

[0074] In such arrangements, the second electrode layer of an outermost (i.e. the uppermost or lowermost) electrochemical secondary cell in the electrochemical secondary cell stack may be exposed (i.e. may not be in abutment with a further electrochemical secondary cell). Thus, in some embodiments, the electrochemical secondary cell stack may comprise a stack according to the first aspect of the invention in contact with the second electrode layer of the uppermost or lowermost electrochemical secondary cell.

[0075] The sealed first and second gel separator layers of the stack according to the first aspect of the invention may be adhered to the adhered first and second gel separator layers of the stack of the uppermost or lowermost electrochemical secondary cell by a further inter-stack heat-seal.

[0076] A fifth aspect of the invention is an electrochemical energy storage device comprising an electrochemical cell according to the second aspect, or an electrochemical secondary cell stack according to the third or fourth aspects. A sixth aspect of the invention is a method of manufacturing the stack according to the first aspect comprising: positioning a first electrode layer between a first gel separator layer and a second gel separator layer, such that portions of the respective first and second gel separator layers extend beyond a peripheral edge of the first electrode layer; and heat-sealing the portions of the respective first and second gel separator layers which extend beyond a peripheral edge of the first electrode layer, thereby forming a heatseal between the first gel separator layer and the second gel separator layer.

[0077] In some embodiments, heat-sealing the portions of the respective first and second gel separator layers comprises applying a temperature of from 70 to 90 °C to the outer surfaces of the first and second gel separator layers. In some embodiments, heat-sealing the portions of the respective first and second gel separator layers comprises applying a temperature of from 70 to 90 °C to the outer surfaces of the first and second gel separator layers for a period of time sufficient to create a heat-seal between the first and second gel separator layers. In some embodiments heat is applied for a period of at least 200 ms, for example at least 500 ms or at least 1 s. In some embodiments heat is applied for a period of from 200 ms to 5 s, for example from 200 ms to 3 s or from 500 ms to 2 s.

[0078] In some embodiments, heat-sealing the portions of the respective first and second gel separator layers comprises applying a temperature of about 80 °C to the outer surfaces of the first and second gel separator layers.

[0079] In some embodiments, heat-sealing the portions of the respective first and second gel separator layers is performed without the use of any adhesive composition between the layers.

[0080] In some embodiments, heat sealing is carried out using a bar sealer. In some embodiments, heat sealing is carried out using an impulse heat sealer. Impulse heat sealers tend to include a metallic element through which a current us passed to create heat for a predetermined period of time which is adjustable. In some embodiments, a thermocouple is used to monitor the temperature between the jaws of the impulse heat sealer and the heating time is adjusted accordingly to achieve a maximum temperature read-out from the thermocouple within the ranges described above.

[0081] In some embodiments, a protective sheet is placed between the stack and the heat sealer to avoid direct contact between the first and second gel separator layers and the heat-sealing elements of the heat sealer. In some embodiments the protective sheet comprises or consists of mylar. In some embodiments the protective sheet has a thickness of from 20 to 100 pm, for example from 30 to 80 pm or about 50 pm.

[0082] In some embodiments, the method comprises, after forming the heat-seal between the first gel separator layer and the second gel separator layer, trimming a portion of the first and second gel separator layers away from the edge of the stack along at least one edge of the stack. In some embodiments, trimming the portion of the first and second gel separator layers comprising cutting away a portion of the first and second gel separator layers by cutting along a line contained within the heat-sealed region, such that the heat-sealed region extends up to the end of the stack after the portion of the first and second gel separator layers has been cut away.

[0083] In some embodiments, the method comprises applying heat at intermittent locations around a peripheral edge of the stack to form an intermittently heat-sealed stack comprising a plurality of unsealed portions regularly distributed around the heat-sealed border region. This provides a means for liquid electrolyte to pass between the gel separator layers at points around the entire periphery of the first electrode layer, as described above under the first aspect.

[0084] In some embodiments, after forming the heat-seal between the first gel separator layer and the second gel separator layer, the method comprises exposing the stack to a liquid electrolyte to allow the liquid electrolyte to soak into the space between the two gel separator layers surrounding the first electrode layer. When the stack comprises one or more unsealed regions around its periphery as described under the first aspect, the time required for the liquid electrolyte to effectively soak into the stack is reduced and the manufacturing process is more efficient.

[0085] In some embodiments, exposing the stack to the liquid electrolyte comprises submerging the stack in the liquid electrolyte for a predetermined period of time.

[0086] In some embodiments, exposing the stack to the liquid electrolyte comprises submerging the stack in the liquid electrolyte for at least an hour, for example at least 2 hours, at least 3 hours, at least 5 hours, at least 10 hours, at least 12 hours, or for about 12 hours. In some embodiments, exposing the stack to the liquid electrolyte comprises submerging the stack in the liquid electrolyte for 1 to 15 hours, for example 2 to 15 hours, 3 to 15 hours, 5 to 15 hours, 10 to 15 hours or 10 to 12 hours.

[0087] In some embodiments, exposing the stack to the liquid electrolyte comprises assembling the stack with other components to form a complete cell, filling the cell with the liquid electrolyte and then vacuum-sealing the cell.

[0088] BRIEF DESCRIPTION OF THE DRAWINGS

[0089] Figure 1 shows schematic representations of various embodiments of stacks according to the invention, adopting different heat-seal architecture.

[0090] Figure 2 shows further schematic representations of embodiments of stacks according to the invention, adopting different heat-seal architecture.

[0091] Figure 3 shows a schematic cross-section of a stack both before and after heat-sealing.

[0092] Figure 4 shows a schematic cross-section of an assembly of heat-sealed stacks, each separated by a second electrode layer, thereby forming electrochemical cells.

[0093] Figure 5 shows an electrochemical impedance spectrum (EIS) for a cell containing a stack according to the invention, after soaking in liquid electrolyte for (a) 1 hour, and (b) 12 hours. Figure 6 shows an electrochemical impedance spectrum (EIS) for three different stacks: (a) an unlaminated (unsealed) comparative stack; (b) a comparative stack with the gel separator layers fully face-sealed against one another and the inner electrode layer; and (c) a stack sealed along two opposing edges according to the invention.

[0094] Figure 7 shows discharge curves showing the rate performance of three different stacks: (a) an unlaminated (unsealed) comparative stack; (b) a comparative stack with the gel separator layers fully face-sealed against one another and the inner electrode layer; and (c) a stack sealed along two opposing edges according to the invention.

[0095] Figure 8a shows a schematic cross-section of an assembly of heat-sealed stacks, each separated by a second electrode layer, thereby forming electrochemical cells.

[0096] Figure 8b shows a schematic cross-section of the assembly of Figure 8a in which the heat- sealed layers of the individual heat-sealed stacks are further heat-sealed together.

[0097] Figure 8c shows a schematic cross-section of the assembly of Figure 8b in which the further heat-sealed together layers are trimmed down.

[0098] DETAILED DESCRIPTION

[0099] Figure 1(a) shows an assembled stack 1 for an electrochemical cell, before any heat-sealing procedure has been carried out.

[0100] The stack 1 has a laminate structure made up of a first electrode layer 2 sandwiched between a first gel separator layer 3 and a second gel separator layer 4 (not shown in Figure 1(a)). The laminate structure of the stack and the arrangement of the layers is best seen in Figure 3, described below.

[0101] An electrical connector 5 is also sandwiched between the first gel separator layer 3 and second gel separator layer 4 and provides electrical contact with the first electrode layer 2 for ultimate connection to a circuit within a device to be powered by a cell containing the stack. As best seen in Figure 3, which is a cross-sectional view of the stack along the dotted line marked “A” in Figure 1(b), the stack is made up of an assembly of the first electrode layer

[0102] 2 sandwiched between the first gel separator layer 3 and the second gel separator layer 4. The first gel separator layer 3 and the second gel separator layer 4 each have a larger area (larger length and larger width) than the first electrode layer 2, resulting in an area outside the peripheral edge of the first electrode layer 2, around the entire periphery of the first electrode layer 2, where the first gel separator layer 3 and second gel separator layer 4 overlap and face one another with no layer between them.

[0103] Figure 3(a) shows the stack before any heat seal is applied, with the first gel separator layer

[0104] 3 and second gel separator layer 4 lying on either side of the first electrode layer 2. The stack of the invention is shown in Figure 3(b) after the application of a heat-seal to adhere the first gel separator layer 3 and second gel separator layer 4 together by a heat-seal 6 adjacent to opposing edges of the first electrode layer. The heat-seal 6 extends around the entire periphery of the first electrode layer 2 in a heat-sealed border region 7. The result is the “encapsulation” of the first electrode layer 2 within an envelope or pouch formed by the creation of the heat-seal between the first gel separator layer 3 and second gel separator layer 4. There is no heat seal between either first gel separator layer 3 or second gel separator layer 4 and the intermediate first electrode layer 2. The first electrode layer 2 is immobilised between the first gel separator layer 3 and second gel separator layer 4 not by any heat-seal or other physical adhesion between the first electrode layer 2 and the separator layers, but only by the restriction on the movement of the first electrode layer 2 caused by the proximity of the heat-seal 6 to the periphery of the first electrode layer 2.

[0105] Returning to Figure 1, the same stack is shown in Figures 1 (b)-(f) but in each case having a different architecture of heat-seal. In each of Figures 1 (a)-(f), the stack is shown as a front view, looking at the first gel separator layer 3. Although the first electrode layer 2 would not be visible behind the first gel separator layer 3 from such a viewpoint, its outline is nevertheless shown in Figures 1 (b)-(f) to demonstrate its position within the stack and its size relative to the gel separator layers. Figure 1(b) shows a stack with a heat-sealed border region around substantially the entire periphery, where the first gel separator layer 3 and second gel separator layer 4 are sealed together by an unbroken seal around the entire periphery of the stack.

[0106] The electrical connector 5 is sandwiched between the first gel separator layer 3 and second gel separator layer 4 to provides electrical contact with the first electrode layer 2. There is therefore a small break in the heat-sealed border region between the first gel separator layer 3 and second gel separator layer 4 where the electrical connector 5 passes between them, hence the heat-sealed border region being around “substantially” the entire periphery. However this small break in the seal is not detrimental to the performance of the stack.

[0107] The heat-seal 6 forms a border region of substantially uniform width around the entire periphery of the stack. The heat-sealed border region extends inwards from the edge of the stack. In other words, there is no unsealed region between the heat-sealed border region and the peripheral edge of the stack, and the edges of the first and second gel separator layers 3, 4 are sealed together as part of the heat-sealed border region.

[0108] The internal edge of the heat-seal 6 lies outward of the peripheral edge of the first electrode layer 2, such that during the heat-sealing procedure the first electrode layer 2 does not become adhered to either of the first or second gel separator layers 3, 4. The extension of the heat-seal 6 around the entire periphery of the stack provides effective restriction of any movement of the first electrode layer 2 relative to the first and second gel separator layers 3, 4.

[0109] Figure 1(c) shows an alternative embodiment of a stack where the heat-seal between the first and second gel separator layers 3, 4 is made up of two separate heat-sealed strips 7a, 7b. The first gel separator layer 3 and second gel separator layer 4 are adhered to one another by a first heat-sealed strip 7a along an upper edge of the stack and a second heat- sealed strip 7b along a lower edge of the stack which is opposite the upper edge of the stack. The first gel separator layer 3 and second gel separator layer 4 are unsealed along the two edges which lie perpendicular to the upper and lower edges of the stack, i.e. unsealed along the vertical edges as depicted in Figure 1(c). The heat-sealed strips 7a and 7b offer effective prevention of movement of the first electrode layer 2 relative to the first and second gel separator layers 3, 4, while the two unsealed edges facilitate the ingress of liquid electrolyte between the first and second gel separator layers 3, 4 when the stack is exposed to liquid electrolyte. Since relatively long, uninterrupted unsealed portions of the periphery of the stack are provided, liquid electrolyte can more easily penetrate the space around the first electrode layer 2 between the first and second gel separator layers 3, 4, reducing the length of any soaking time needed to introduce liquid electrolyte.

[0110] Figure 1(d) shows an alternative embodiment of a stack which is similar to that shown in Figure 1(c), expect that the heat-sealed and unsealed edges are interchanged. In the stack of Figure 1(d), the first gel separator layer 3 and second gel separator layer 4 are adhered to one another by a first heat-sealed strip 8a along the left-hand vertical edge of the stack and a second heat-sealed strip 8b along the right-hand vertical edge of the stack which is opposite the left-hand edge of the stack. The first gel separator layer 3 and second gel separator layer 4 are unsealed along the two edges which lie perpendicular to the left- and right-hand edges of the stack, i.e. unsealed along the upper and lower edges as depicted in Figure 1(d).

[0111] Figure 1(e) shows an alternative embodiment of a stack in which a heat-sealed border region 9a, 9b is sealed around the entire periphery of the stack with the exception of two unsealed sections 9c, 9d, each making up a portion of the length of an edge of the stack. The two unsealed sections 9c, 9d lie within respective opposite edges of the stack, and create a heat-sealed border region which is divided into a continuous upper portion 9a and a continuous lower portion 9b. The two unsealed sections 9c, 9d lie within respective parallel edges of the stack, and the remaining pair of parallel edges are completely sealed.

[0112] Figure 1(e) shows the two unsealed sections 9c, 9d located along opposite (parallel), vertically extending edges but in other embodiments (not shown) the two unsealed sections 9c, 9d may be located along any other pair of edges of the stack. For example, the two unsealed sections 9c, 9d may be located along opposite, horizontally extending edges; or the two unsealed sections 9c, 9d may be located along any two adjacent (perpendicular) edges. However location of the two unsealed sections 9c, 9d along opposite (parallel) edges may be preferred because this may help to provide a more even diffusion of liquid electrolyte around the first electrode layer 2.

[0113] The two unsealed sections 9c, 9d facilitate the ingress of liquid electrolyte between the first and second gel separator layers 3, 4 when the stack is exposed to liquid electrolyte. At the same time, since the heat-sealed border region 9a, 9b includes sections along each of the four edges of the stack, adjacent each of the four corners of the stack, the first electrode layer 2 is very effectively immobilised between the first and second gel separator layers 3, 4, while retaining the ability of the stack to effectively facilitate the ingress of liquid electrolyte through the unsealed sections 9c, 9d.

[0114] Figure 1(f) shows an alternative embodiment of a stack in which a heat-sealed border region is made up of four heat-sealed corner sections 10a, 10b, 10c and lOd, located at each of the respective corners of the stack, each one extending part-way along each of the two edges of the stack in which the comer lies. Between any two adjacent corner sections along the edge that separates them lies an unsealed section which extends along a central portion of the length of the edge of the stack, creating four separate unsealed sections lOe, lOf, 10g, lOh.

[0115] The four unsealed sections lOe, lOf, 10g, lOh facilitate the ingress of liquid electrolyte between the first and second gel separator layers 3, 4 when the stack is exposed to liquid electrolyte. At the same time, since the heat-sealed border region includes four heat-sealed comer sections 10a, 10b, 10c , lOd located at each of the four comers of the stack, the first electrode layer 2 is very effectively immobilised between the first and second gel separator layers 3, 4, while retaining the ability of the stack to effectively facilitate the ingress of liquid electrolyte through the unsealed sections lOe, lOf, 10g, lOh.

[0116] Figure 2(a) shows an alternative embodiment of a stack in which a heat-sealed border region 11 is sealed around the entire periphery of the stack with the exception of an unsealed section I la making up a portion of the length of a single edge of the stack. The unsealed section lies substantially centrally along the edge of the stack (the left-hand edge in Figure 2(a)). In other embodiments (not shown), the unsealed section I la may be located in any one of the other three edges of the stack instead. Additionally, it is not necessary that the unsealed section 1 la be located centrally or substantially centrally along an edge and it may instead be offset from the centre of an edge, lying closer to one end of the edge than the other end.

[0117] Figure 2(b) shows an alternative embodiment of a stack in which a heat-sealed border region 12 comprises a regularly repeating intermittent seal around the entire periphery of the stack. The heat-sealed border region 12 is therefore discontinuous, comprising a plurality of unsealed sections. The regularly repeating intermittent seal comprises a plurality of sealed sections distributed around the entire periphery of the stack, each having a substantially identical length, with any two neighbouring sealed sections being separated by an unsealed section, wherein all unsealed sections around the entire periphery of the stack are of substantially identical length.

[0118] A majority of the periphery of the stack is sealed by the intermittent heat-sealed border region 12, while an even distribution of unsealed portions is maintained around the entire periphery of the stack. In this way, a strong and reliable seal is formed to hold the first electrode layer 2 while maintaining fluid communication with the outside of the stack to facilitate the ingress of liquid electrolyte.

[0119] Figure 4 shows an assembly 100 of four identical stacks 1 having a structure as described for the stack of Figure 3, each separated by a second electrode layer 13. The assembly 100 defines an electrochemical secondary cell stack.

[0120] In a first arrangement, first electrode layer 2 is a cathode and second electrode layer 13 is an anode. In an alternative arrangement, first electrode layer 2 is an anode and second electrode layer 13 is a cathode.

[0121] The assembly 100 shown in Figure 4 is manufactured by alternately arranging stacks and second electrode layers. The result is an electrochemical cell, or a plurality of electrochemical cells, containing alternating cathode and anode layers separated from one another by gel separator layers.

[0122] Figure 8a shows an assembly 200 of four identical stacks 201 each separated by a second electrode layer 213.

[0123] Each stack 201 is made up of an assembly of a first electrode layer 202 sandwiched between a first gel separator layer 203 and a second gel separator layer 204. In each stack 201, the first gel separator layer 203 and the second gel separator layer 204 are adhered together by first and second heat-seals 206a, 206b adjacent to opposing edges of the first electrode layer 202. The first heat-seals 206a form first heat-sealed border regions 207a, and the second heat-seals form second heat-sealed border regions 207b.

[0124] The first electrode layer 202 of each stack 201 comprises a current collector layer 220. In this example, the first electrode layer 202 is a cathode and so includes a positive active material. In this example, the current collector layer 220 is an aluminium film and is coated on both sides with a layer of positive active material in order to form the first electrode layer 202.

[0125] Each second electrode layer 213 also comprises a current collector layer 230. In this example, each second electrode layer 213 is an anode and so includes a negative active material. In this example, the current collector 230 is a copper film and is coated on both sides with a layer of negative active material in order to form the second electrode layer 213.

[0126] The assembly 200 shown in Figure 8a is manufactured by alternately arranging stacks 201 and second electrode layers 213.

[0127] As illustrated in Figure 8b, an assembly 300 can be manufactured from the assembly 200 show in Figure 8a by further heat-sealing together the four first heat-seals 206a of the four identical stacks 201. This can be achieved using heat-sealing bars 250a and 250b, which position the four first heat-seals 206a together and heat them to form a first heat-sealed bundle 216a. At the same time, or separately, the four second heat-seals 206b of the four identical stacks 201, are also further heat-sealed together. This can be achieved, for example, using a second set of heat-sealing bars 252a and 252b, which position the four second heat-seals 206b together and heat them to form a second heat-sealed bundle 216b.

[0128] By further heat-sealing together the first heat-seals 106a to form the first heat-sealed bundle 216a, and further heat-sealing together the second heat-seals 106b to form the second heat-sealed bundle 216b, movement of the first and second electrode layers 202, 213 relative to each other is further restricted in the assembly 300.

[0129] The first and / or second heat-sealed bundles 216a, 216b may be formed at or towards the middle of the stack, i.e. at or towards the middle layer (or layers) of the stack, as shown in Figure 8b. Alternatively, the first and / or second heat-sealed bundles 216a, 216b may be formed at or towards an end of the stack. The ends of the stack may be defined by the outermost layers (i.e. uppermost and lowermost layers) of the stack. In some examples, one of the first and / or second heat-sealed bundles 216a, 216b may be formed at or towards the middle of the stack, and the other of the first and / or second heat-sealed bundles 216a, 216b may be formed at or towards an end of the stack.

[0130] In some examples, the assembly 200 may be wrapped in a layer of gel separator material, and / or sandwiched between two layers of gel separator material, prior to heat-sealing together the four first heat-seals 206a and four second heat-seals 206b. The layer or layers of gel separator material may be heat-sealed together with the four first heat-seals 206a to form part of the first heat-sealed bundle 216a. Additionally, or alternatively, the layer or layers of gel separator material may be heat-sealed together with the first the four second heat-seals 206b to form part of the second heat-sealed bundle 216b.

[0131] The assembly 400 shown in Figure 8c can be manufactured from the assembly 300 shown in Figure 8b by trimming down the first and second heat-sealed bundles 216a, 216b to form first and second trimmed heat-sealed bundles 226a, 226b. Trimming the first and second heat-sealed bundles 216a, 216b may suitably reduce the footprint / area of the assembly 400, thereby increasing the volumetric energy density of the assembly 400. Further second electrode layers 213 may be arranged on the uppermost and / or lowermost layers of the assembly 400. Alternatively, further second electrode layers 213 may be arranged on the uppermost and / or lowermost layers of the assembly 200 prior to heatsealing together the heat-seals 206a, 206b.

[0132] EXAMPLES

[0133] A cathode slurry composition was prepared comprising NMC 811 active material, PVDF- HFP, conductive carbon additive, ethylene carbonate, propylene carbonate, vinylene carbonate, fluoroethylene carbonate, LiFSI and LiTDI. The slurry was coated onto a current collector to 180 g / m2, to a density of 3.56 g / m3.

[0134] An anode slurry composition was prepared comprising graphite, silicon oxide, conductive carbon additive and polyacrylic acid binder. The slurry was coated onto a current collector to 66 g / m2, to a density of 1.6 g / m3.

[0135] Gel separators were prepared by mixing PVDF-HFP, ethylene carbonate, propylene carbonate, vinylene carbonate, fluoroethylene carbonate, LiFSI and LiTDI, before extruding sandwiched between two 50 pm-thick mylar sheets to form a film structure. The thickness of each gel separator film between the mylar sheets was 20 pm.

[0136] The cathode and anode sheet materials prepared as above were each punched using a die cutter to produce individual electrode pieces. The anode pieces punched out in this way were larger than the cathode pieces in such a way that when the cathode is placed on the anode, a border of anode 1 mm wide extends beyond the edge of the cathode.

[0137] Gel separator pieces between mylar sheets were cut from the film manufactured as described above using a punch and swing arm press. An electrode stack was then made by removing one of the two supporting mylar sheets from each separator, placing one piece of separator behind and one piece of separator on top of the cathode, ensuring that the separator edges were aligned with one another, sandwiching the cathode between them with an equal border of separator material surrounding the edges of the cathode. This assembly, with a supporting mylar sheet remaining on each face, was then heat-sealed using a sealer (Audionvac VMS 163), with seal settings -0.4 bar, 1 second, temperature recorded to be 80 °C using a thermocouple. The assembly was heat-sealed first along a top edge and then along an opposing bottom edge of the cathode, leaving two remaining side edges unsealed (i.e. the sealing pattern shown in Figure 1(c)). This assembly was then placed on top of the anode, aligning the cathode centrally so that the larger anode formed an even border around the cathode, and the separator formed an even border around the anode.

[0138] To build a multilayer cell, this process was repeated, placing heat-sealed separatorcathode-separator stacks against one another with an anode between them, until the desired number of cathode layers are achieved, finishing with an anode on top. An aluminium tab was welded onto the cathode foil current collector using a Branson L20 Ultrasonic welder, such that it protruded from the top of the separator layers as depicted in Figure 1. A nickel tab was welded to the anode foil current collectors using a Branson L20 Ultrasonic welder, such that it also protruded from the top of the separator layers.

[0139] The electrode stack was then sealed within pouch bag material using a sealer (Star Universal Platen Heater). Liquid electrolyte was then added to the stack in the pouch using a pipette before vacuum sealing (Audionvac VMS 163).

[0140] Cells were then clamped in a jig between two pieces of foam over each side of the cell (Rogers Corp, BISCO HT-840) and compressed to 100 kPa. Cells were placed into a temperature-controlled oven (Binder KMF 115) set at 30 °C and connected to a potentiostat (VMP3e). Cells were soaked for 12 hours to allow liquid electrolyte to wet into the stack, PEIS was recorded every hour to monitor wetting of electrolyte (10 mV amplitude, frequency range from 500 kHz to 0.1 Hz, with 10 points per decade).

[0141] Cells were formed using the same equipment and temperature as the soaking step.

[0142] Cells were charged for 1 hr at C / 20, then at C / 10 up to 4.2 V, then a CV hold was used until the current reached C / 40. Cells were discharged to 2.5 V at C / 5. This process was repeated an additional two times, however the charging step was conducted at C / 10 in the two repeat steps.

[0143] Rate testing was conducted on cells at 30 °C on VMP3e. Test conditions C / 5, D / 5, C / 3, ID, C / 3, 2D, C / 3, 5D, C / 3, 7D, C / 3, 10D. CV hold at 4.2 V until current decay was C / 40. Operating voltage between 4.2 V and 2.5 V.

[0144] Figure 5 shows the EIS spectra obtained from a cell manufactured as set out above, after 1 hour of soaking in liquid electrolyte, and again after 12 hours. The spectra show that as liquid electrolyte wets into the stack over time, diffusional resistance decreases. The change in series resistance between the 1 hour and 12 hour time points was minimal. Series resistance is a measure of the ionic conductivity of the liquid / gel separator mix. The minimal change shows that the liquid electrolyte wets very quickly into the gel separator matrix. By contrast the diffusional resistance represents the resistance of ion diffusion through the “porous” network of the electrodes - as the liquid electrolyte wets in, the diffusional resistance drops.

[0145] Figure 6 shows the EIS spectra for a different stacks according to the invention having a heat-seal structure where the top and bottom edges of the gel separator layers are heat- sealed (with the remaining two edges unsealed), as depicted in Figure 1(c), and two comparative stacks, a first one without any heat seal between the first and second gel separators, and a second one where the entire inner surfaces of the gel separator layers are sealed against each other and against the inner cathode layer. The spectra in Figure 6 were taken after soaking the stacks in liquid electrolyte for 12 hours. The comparative, unsealed stack and the edged-sealed stack each show a lower diffusional resistance than the comparative fully face-sealed stack. This demonstrates the improved ability for liquid electrolyte to wet into the stack in cases where sections of the periphery of the gel separator layers are left unsealed. The diffusional resistance of the comparative unsealed stack and the edged-sealed stack are similar to one another, demonstrating that sealing along two opposing edges does not hinder the ability of the liquid electrolyte to wet into the stack. The comparative, fully face-sealed stack had low diffusional resistance.

[0146] Figure 7 shows the rate performance testing of cells containing stacks having different heat-seal types (the same three stacks as tested for Figure 6). Cells were all 44 mAh based on active material loading. It can be observed that comparative cells made with full face- sealed lamination show a reduction in rate performance at rates 7C and above.

[0147] Where top and bottom edge seal are used in the inventive stacks, the rate performance results are comparable to cells where no lamination is applied.

[0148] Figures 6 and 7 therefore show that, by edge-sealing the gel separator layers of the stack, the inner electrode layer will be more effectively immobilised (reducing the risk of movement of the electrode layer during use, which could cause a short-circuit) while achieving a diffusional resistance and rate capability equivalent to that of a stack where the separators are not sealed to one another at all.

Claims

CLAIMS1. A stack for an electrochemical cell, having a laminate structure comprising: a first gel separator layer and a second gel separator layer; and a first electrode layer between the first and second gel separator layers; wherein the first gel separator layer and second gel separator layer each extend beyond a peripheral edge of the first electrode layer such that they contact one another beyond the peripheral edge of the first electrode layer; and the first gel separator layer and second gel separator layer are adhered to one another by a heat-seal adjacent to opposing edges of the first electrode layer.

2. The stack according to claim 1, wherein the first gel separator layer and second gel separator layer each extend beyond the peripheral edge of the first electrode layer around the entire peripheral edge of the first electrode layer.

3. The stack according to claim 1 or 2, wherein the first gel separator layer and second gel separator layer are adhered to one another by a heat-sealed border region which extends around substantially the entire periphery of the first electrode layer.

4. The stack according to claim 3, wherein the heat-sealed border region is discontinuous, comprising at least one unsealed portion which extends from the first electrode layer to the edges of the first and second gel separator layers.

5. The stack according to any one of the preceding claims, wherein the first electrode layer is not adhered to either of the first gel separator layer or second gel separator layer.

6. The stack according to any one of the preceding claims, wherein the first electrode layer comprises a current collector.

7. The stack according to any one of the preceding claims, wherein the first electrode layer is a gel cathode layer or a gel anode layer.

8. The stack according to claim 7, wherein the first electrode layer comprises one or more 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 [bi s(m ethoxy ethoxyethoxide)-phosphazene], poly(dimethylsiloxane) (PDMS), polyacene, polydisulfide, polystyrene, polystyrene sulfonate, polypyrrole, polyaniline, polythiophene, polythione, polyvinyl pyridine (PVP), polyvinyl chloride (PVC), polyaniline, poly(3,4-ethylenedioxythiophene) (PEDOT), poly(p-phenylene), poly(triphenylene), polyazulene, polyfluorene, polynaphthalene, polyanthracene, polyfuran, polycarbazole, tetrathiafulvalene- substituted polystyrene, ferrocene- substituted polyethylene, carbazole-substituted 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-Acryl ami do-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.

9. The stack according to claim 8, wherein the first electrode layer further comprises a linear or cyclic carbonate liquid electrolyte, one or more lithium salts and an electrochemically active material.

10. The stack according to any one of the preceding claims, wherein the first and second gel separator layers each independently comprise one or more 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), polyethylene 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.

11. The stack according to any one of the preceding claims, wherein the first and second gel separator layers and the first electrode layer each independently comprise one or more 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).

12. The stack according to claim 10 or 11, wherein the first and second gel separator layers further comprise a linear or cyclic carbonate liquid electrolyte and one or more lithium salts.

13. The stack according to any one of the preceding claims, wherein the first and second gel separator layers each independently have a thickness of from 10 to 30 pm.

14. The stack according to any one of the preceding claims, wherein the first electrode layer has a thickness of 40 to 150 pm.

15. The stack according to any one of the preceding claims, wherein the distance between a peripheral edge of the first electrode layer and the heat-seal is less than 5 mm.

16. An electrochemical secondary cell comprising: a stack according to any one of claims 1 to 15; and a second electrode layer in contact with the first gel separator layer.

17. The electrochemical secondary cell according to claim 16, wherein the second electrode layer comprises a current collector.

18. The electrochemical secondary cell according to claim 16 or 17, further comprising a third electrode layer in contact with the second gel separator layer.

19. The electrochemical secondary cell according to any one of claims 16 to 18, wherein one or both of the second electrode layer and the third electrode layer is an anode layer.

20. An electrochemical secondary cell according to any one of claims 16 to 19, which is a lithium-ion electrochemical secondary cell.

21. An electrochemical secondary cell stack comprising: first and second electrochemical secondary cells each according to claim 16; wherein the adhered first and second gel separator layers of the stack of the first electrochemical secondary cell and the adhered first and second gel separator layers of the stack of the second electrochemical secondary cell are further adhered to one another by an inter-stack heat-seal.

22. An electrochemical energy storage device comprising an electrochemical cell according to any one of claims 16 to 20 or an electrochemical secondary cell stack according to claim 21.

23. A method of manufacturing the stack according to any one of claims 1 to 15 comprising: positioning a first electrode layer between a first gel separator layer and a second gel separator layer, such that portions of the respective first and second gel separator layers extend beyond a peripheral edge of the first electrode layer; and heat-sealing the portions of the respective first and second gel separator layers which extend beyond a peripheral edge of the first electrode layer, thereby forming a heatseal between the first gel separator layer and the second gel separator layer.

24. The method according to claim 23, wherein heat-sealing the portions of the respective first and second gel separator layers comprises applying a temperature of from 70 to 90 °C to the outer surfaces of the first and second gel separator layers.

25. The method according to claim 24, wherein the temperature is applied using an impulse sealer.

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