Electrochemical cell

Abstract

An electrochemical cell 400 comprises a pouch 10, and a plurality of stacked electrode layers 21; and a separator layer 22 interposed between each pair of adjacent electrode layers; wherein at least o

Description

B ACKGROUND To ensure high performance of electrochemical cells, for example pouch cells containing gel polymer components such as gel polymer electrodes and / or gel polymer separators, good contact between the components of the cell is desired to minimise internal resistance and reduce the risk of a short circuit. There is a risk of movement of the components in the electrochemical cell after assembly of the stack, which can impact the performance and functioning of the cell. In particular, shearing of the stack (i.e. relative movement of the layers of the stack) is a possibility if the electrochemical cell is exposed to shocks. SUMMARY In a first aspect there is provided an electrochemical cell comprising: a pouch; and a stack received in the pouch, the stack having a laminate structure comprising: a plurality of stacked electrode layers; and a separator layer interposed between each pair of adjacent electrode layers; wherein at least one separator layer is bonded to an inner layer of the pouch by a first separator-pouch seal. Some attempts to limit shearing of the stack due to shocks have focussed on sealing adjacent separator layers of the stack to each other and compressing the stack inside a bag under vacuum. However, sealing of adjacent separator layers of the stack to each other still does not restrict movement of the entire stack within the pouch, and sealing the stack inside a bag under vacuum, whilst initially providing some further resistance to shearing, can become less effective as gases are generated by chemical reactions in the stack, these gases reducing the vacuum that the bag is under. The sealing of at least one of the separator layers to the inside surface of the pouch may further restrict the ability of the stack to move within the pouch, and thus reduce the magnitude of the shocks that can be exerted on the pouch and the amount of shearing that the stack may undergo during use. This mechanism may not be affected by gas generation in the electrochemical cell. Herein, the term “bonded”, with reference to two or more elements of the electrochemical cell, may be understood to encompass joining of those elements such as to be affixed to one another, as opposed to there merely being contact between those elements. “Bonded” may incorporate direct joining of these elements, e.g. via a heat seal, or indirect joining of these element, e.g. via an adhesive or other intermediate substance. Herein, the term “pouch” may be understood to be a container, envelope, housing, or enclosure for the stack of the electrochemical cell. The pouch may be understood to be a structural component of the electrochemical cell that encloses the stack when the pouch has been sealed. The pouch typically has a third dimension which is significantly smaller than the first and second dimensions, such that the pouch is a substantially planar structure. By way of example, the third dimension may be more than 10 times smaller than the first or second dimension. The pouch is typically a thin, flexible structure formed from a sheet of pouch material. The sheet of pouch material may have a thickness between 10 micrometres and 500 micrometres. The stack may comprise a first plurality of separator layers. The first plurality of separator layers may be bonded together to form a first separator-separator seal. Even where the separator-separator seal is not also bonded to the inner layer of the pouch, such a separatorseparator seal can more securely hold the electrode layer(s) between the bonded separator layers in position compared to if the separator layers were not bonded to each other. The separator layer(s) and the electrode layers may be “film-like” structures, having a third dimension which is significantly smaller than the other two dimensions (the third dimension may tend to have a length in the 1-1000 micrometre range, compared with lengths in the 1-30 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. 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 of the layer, i.e. first and second dimensions. The term “peripheral edge of the stack” refers to an edge of the stack which lies parallel with the plane perpendicular to the direction in which the electrode layers are stacked (i.e. lying parallel to the plane including the first and second dimensions of an electrode layer). 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, opposing edges will be either the first pair of parallel edges, or the second pair of parallel edges. 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 layers and the separator layer(s) which lies between them. Each electrode layer may comprise one or more peripheral edges. The, or each, separator layer may extend beyond a peripheral edge of the adjacent electrode layers. The portion of the separator layer extending beyond said peripheral edge may be referred to as the extension portion. In other words, the separator layer may be larger than an adjacent electrode layer in at least one of the first and second dimensions. This can make bonding the separator layer(s) in a separator-pouch seal and / or a separator-separator seal easier, since the extension portion can be manipulated to bridge the distance between the separator layer and the pouch, or between adjacent separator layers, respectively. The separatorpouch seal and / or separator-separator seal may be provided in the extension portion. In some examples, each separator layer may be larger than the adjacent electrode layers in both the first and second dimensions. In this way, it is possible to stack the electrode layers between separator layers in a way which ensures that a border of each separator layer protrudes around the entire periphery of the adjacent electrode layer. This may allow two adjacent separator layers to contact one another around the entire periphery of the interposed electrode layer, forming an uninterrupted “envelope” around the interposed electrode layer. When a separator layer provides such a border around the entire periphery of the first electrode layer, two separator-pouch seals may be formed by bonding the separator layer to the inner layer of the pouch along opposing edges of the adjacent electrode layer. In some examples, separator-pouch seals are present along each of a first pair of opposing edges of the adjacent electrode layer and a second pair of opposing edges of the adjacent electrode layer. In this way, movement of the stack may be limited in both the first and second dimensions by the two pairs of separator-pouch seals that effectively circumscribe the stack. However, in some examples, the first separator-separator seal may be bonded to the inner layer of the pouch by the first separator-pouch seal. In this way, greater support for the stack can be provided by the separator-pouch seal than if a single separator layer is bonded to the pouch, since any load applied to the stack can be transferred to the pouch across a plurality of separator layers. Alternatively, a plurality of separator layer may be individually bonded to the inner layer of the pouch by individual separator-pouch seals. This requires a greater number of separator-pouch seals to be formed, but may still provide support for the stack and distribute any forces exerted on the pouch by the stack over a plurality of separator-pouch seals. A second plurality of separator layers may be bonded together to form a second separatorseparator seal. The first separator-pouch seal may be at a first location; and the second separator-separator seal may be bonded to the inner layer of the pouch by a second separator-pouch seal at a second location. The first location may be different to the second location, for example, the first location may be spaced from the second location (e.g. along on opposite sides of the stack). The first location and second location may be along different peripheral edges of the stack, or may be spaced apart on the same peripheral edge of the electrode layer. In this way, any forces exerted on the pouch by the stack may be shared across the first location and the second location at which the stack is attached to the pouch, thereby reducing the point loading of the pouch and the likelihood of the pouch failing due to forces exerted on it by the stack. The first plurality of separator layers and the second plurality of separator layers may comprise the same separator layers or different separator layers. All the separator layers in the laminate structure may be bonded together in the first separator-separator seal. In this way, any force applied to the stack can be shared across all of the separator layers whilst only needing to form a single separator-pouch seal within the cell to bond the separator-separator seal to the pouch. All the separator layers in the laminate structure may be bonded together in the second separator-separator seal. This can allow all of the separator layers to be bonded to the pouch twice whilst only forming two separator-pouch seals. In some examples, the separator-separator seal(s) and / or the separator-pouch seal(s) may extend around substantially the entire periphery of the stack. Such seals may be continuous, providing an uninterrupted seal around substantially the entire periphery of the stack. By “substantially the entire periphery” it is meant that the seal(s) extend around the entire periphery apart from one or more small breaks in the seal(s) which may be present to provide cell functionality, for example to allow an electrical connector to pass into the stack to provide electrical connection to the electrode layers. In other examples, the separator-separator seal(s) and / or the separator-pouch seal(s) may be discontinuous, for example comprising at least one unsealed portion or section. The unsealed portion may provide fluid communication between the electrode layers and the outside of the stack. In this way, liquid electrolyte added to the stack after forming the one or more separator-pouch seals and / or separator-separator seals may be able to more easily pass into the stack in the space between the two separator layers surrounding an electrode layer. In some examples, the separator-separator seal(s) and / or the separator-pouch seal(s) may comprise a regularly repeating intermittent seal around the entire periphery of the stack. In some examples, the regularly repeating intermittent seal may comprise a plurality of sealed sections distributed around the entire periphery of the stack. Each of the sealed sections may be substantially identical in length. Any two neighbouring sealed sections may be separated by an unsealed section. Two or more of the unsealed sections may be of substantially identical length. For example, all unsealed sections around the entire periphery of the stack may be of substantially identical length. In some examples, an unsealed section has a length shorter than a sealed section. In some examples, 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 may be 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. Thus, a strong and reliable seal may be formed while maintaining fluid communication with the outside of the stack to facilitate the ingress of liquid electrolyte. In some examples, the distance from the separator-pouch seal(s) to the peripheral edge of the stack may be less than 10 mm, for example less than 7mm, less than 5mm, less than 4 mm, less than 3 mm or less than 2 mm. Providing a separator-pouch seal as close to the stack as possible may ensure better securing of the stack to the pouch and minimise the amount by which the stack can move within the pouch. In some examples, the distance from the separator-pouch seal(s) to the peripheral edge of the stack is at least 0.5 mm, for example at least 1 mm. Ensuring such minimum spacing between the stack and the separator-pouch seal may reduce the risk of inadvertently forming a seal directly between one of the electrode layers and the pouch. As such, in some examples, the distance from the separator-pouch seal(s) to the peripheral edge of the stack may be from 0.5 to 10 mm, for example from 1 to 7 mm, from 1 to 5 mm, from 1 to 3 mm or from 1 to 2 mm. The shapes of the electrode layers and the separator layer(s) are not particularly limited. In some examples, the plurality of electrode layers have a shape which is substantially the same as each other. In some examples, the electrode layers may have a shape which is geometrically similar to that of the or each separator layer, i.e. the shape and size of the electrode layers can be achieved by uniformly scaling the shape and size of the separator layer(s). In some examples, scaling down a separator layer by 60 to 90% may provides the shape of the electrode layers, for example 65 to 90%, 70 to 90% or 75 to 85%. In some examples, the separator layers may be quadrilateral-shaped films of substantially identical geometry to each other such that the peripheries of the separator layers are aligned in the stack. In some examples, separator-separator seals and / or separator-pouch seals may be positioned at each comer of the quadrilateral. In some examples, one or more further separator-separator seals and / or separator-pouch seals may be positioned along at least one of the edges between the comers of the quadrilateral. At least one unsealed portion along that edge may be retained. By providing separator-separator seals and / or separator-pouch seals in each comer, a secure seal may be provided which effectively prevents any movement of the separator layers relative to one another and / or relative to the pouch. Providing unsealed portions between any two corners may provide a means for the ingress of liquid electrolyte into the stack. Providing further separator-separator seals and / or separator-pouch seals along the edges between two bonded corners further strengthens may further limit any movement of the stack and / or its constituent layers. As discussed above, a separator-pouch seal may incorporate any one or more of the above separator-separator seals to restrict movement of the stack within the pouch. One or more of the separator-pouch seals, e.g. the first separator-pouch seal, may be a heat seal. One or more of the separator-separator seals may be a heat seal. A heat seal can provide an easy manner in which to form the separator-pouch seal and / or the separatorseparator seal, since the heat seal can be made by simply heating a portion of the pouch from the outside of the pouch. The separator-pouch heat seal may be provided by heating an area of the pouch in contact with one or more separator layers to a temperature between 100°C and 200°C, e.g. about 150°C. The separator-separator heat seal may be provided by heating a portion of a separator layer in contact with one or more other separator layers to a temperature between 80°C and 100°C, e.g. about 90°C. Where a separator-separator seal is formed simultaneously with a separator-pouch seal, a heat seal may be provided by heating an area of the pouch in contact with one or more separator layers to a temperature between 100°C and 200°C, e.g. about 150°C, i.e. to a temperature in excess of that used to form a separator-separator seal in isolation. The skilled person can appreciate that the temperature that the inner layer of the pouch should be heated to is dependent on the material the inner layer of the pouch is formed of. Typically, the inner layer of the pouch should be heated to around the melting point of the material it is formed of. The inner layer of the pouch may comprise a thermoplastic. In some examples, the thermoplastic may be selected from the group consisting of: polypropylene, polyethylene terephthalate, polyethylene, polyvinyl acetate, polyvinyl chloride, polystyrene, low-density polyethylene, and high-density polyethylene. In this way, it may be possible to form the separator-pouch seal without adding any additional material (e.g. adhesive) into the pouch. The pouch may be a laminate of a metal foil and the inner layer. The metal foil may comprise aluminium or an aluminium alloy. The pouch may further comprise an outer layer comprising a thermoplastic. The pouch may comprise a sheet of pouch material having a first portion and a second portion with a fold therebetween. The stack may be sandwiched between the first and second portions. This can simplify the fabrication of the pouch, because the presence of the fold between the first and second portions can eliminate the need to seal the pouch along the folded edge. In this case, the inner layer of the pouch may encompass the inner layer of the sheet of pouch material that extends across the first portion and the second portion (i.e. the ‘inner layer of the pouch’ is not necessarily restricted to the inner layer of solely the first portion or the inner layer of solely the second portion). Alternatively, the pouch may comprise a first sheet of pouch material and a second sheet of pouch material (which are initially separate when fabricating the electrochemical cell) that the stack is interposed between. In this case, the inner layer of the pouch may encompass the inner layer of the first sheet and the inner layer of the second sheet (i.e. the ‘inner layer of the pouch’ is not necessarily restricted to the inner layer of just one sheet). The pouch may comprise opposing portions of pouch material (e.g. the first and second portions of pouch material, or first and second sheets of pouch material), and a given separator-pouch seal may be formed between one or more separator layers and just one of the two opposing portions of pouch material that the one or more separator layers are interposed between. This is in contrast to other separator-pouch seals where the separatorpouch seal may be formed between one or more separator layers and both of the opposing portions of pouch material that the one or more separator layers are interposed between. A separator-pouch seal (e.g. the first or second separator-pouch seal) may be formed along the fold. In this way, the void within the pouch that is present along the fold can be occupied by the separator-pouch seal. The electrochemical cell may further comprise a pouch-pouch seal between two opposed portions of the inner layer of the pouch. The pouch-pouch seal can seal at least part of a periphery of the pouch shut (e.g. such that electrolyte contained within the pouch cannot leak out). The pouch-pouch seal may be a heat seal. A heat seal provides an easy manner in which to provide the pouch-pouch seal, since the heat seal can be made by simply heating a portion of the pouch from the outside of the pouch. A pouch-pouch heat seal may be provided by heating an area of the pouch where two opposing portions of the inner layer of pouch are in contact to a temperature between 100°C and 200°C, e.g. about 150°C. The skilled person can appreciate that the temperature that the inner layer of the pouch should be heated to is dependent on the material the inner layer of the pouch is formed of. Typically, the inner layer of the pouch should be heated to around the melting point of the material it is formed of. In some examples, the pouch-pouch seal incorporates a separator-pouch seal (e.g. the first or second separator-pouch seal). It can be understood that this involves at least one separator layer being bonded to the inner layer of the pouch between the two opposed portions of the inner layer of the pouch, such that a layered structure of pouch-separator(s)-pouch is provided at the seal. In this way, a single seal location can be used both to bond the separator layers to the pouch and seal the pouch shut, thereby simplifying manufacture of the electrochemical cell. Alternatively, the pouch-pouch seal may be at an offset position from a separator pouch seal (e.g. the first separator-pouch seal). That is, the pouch-pouch seal may not incorporate a separator-pouch seal. Instead, an adjacent separator-pouch seal may be provided inboard of the pouch-pouch seal. For example, a separator-pouch seal may be formed along a substantially parallel path to the pouch-pouch seal. An inboard direction in the electrochemical cell may be defined as being away from a peripheral edge of the pouch and towards the stack and an outboard direction may be defined as the opposite direction to the inboard direction. Where the pouch is formed by folding the second portion of the sheet of pouch material over the stack, the pouch-pouch seal may be formed between the first portion and the second portion. The, or each, separator layer may be a gel separator layer. The or each gel separator layer may 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 ethoxyethoxidej-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 (Li AMPS), 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, and PEO terpolymer. One or more (e.g. each) of the electrode layers may be any type of electrode layer, for example a solvent-cast (solid) electrode layer or a gel electrode layer. Where an electrode layer is a gel electrode layer, it may comprise a polymer and a liquid electrolyte together forming a polymer-electrolyte gel matrix phase. Gel electrode layers offer a natural “tackiness” which can serve 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 polymerelectrolyte 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. Where a gel electrode layer is a cathode layer, it may comprise a positive active material. The cathode layer may further comprise one or more of a conductive additive and a binder. Where a gel electrode layer is an anode layer, it may comprise a negative active material. The anode layer may further comprise one or more of a conductive additive and a binder. The liquid electrolyte causes gelation of the polymer to form the gel electrode layer. In some examples, the liquid electrolyte may comprise a linear or cyclic carbonate solvent and a lithium salt. In some examples the solvent may comprise or consist of one or more of ethylene carbonate (EC) propylene carbonate (PC), vinylene carbonate (VC) and fluoroethylene carbonate (FEC). In some examples, the lithium salt may comprise 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 examples, the liquid electrolyte may comprise a mixture of two or more different lithium salts. 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. The or each gel separator layer and the or each gel electrode layer may each independently comprise one or more polymers independently selected from poly(vinylidene difluoride) (PVdF), poly(vinylidene fluori de-co-hexafluor opropylene) (PvDF-HFP), poly(methyl methacrylate) (PMMA), poly(ethylene oxide) (PEO), poly-L-lactic acid (PLA) and polystyrene (PS). These polymers have been found to provide the combined benefits of good electrochemical properties alongside good heat-sealing properties. The, or each, gel separator layer may further comprise a linear or cyclic carbonate liquid electrolyte and one or more lithium salts. In some examples, the lithium salt may comprise 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 examples, the liquid electrolyte may comprise a mixture of two or more different lithium salts. In some examples, the gel separator layers may each comprise a liquid electrolyte and an electrochemically active material. The liquid electrolyte causes gelation of the polymer to form the first and second gel separator layers. In some examples, the liquid electrolyte may comprise a linear or cyclic carbonate solvent and a lithium salt. In some examples, the solvent comprises or consists of one or more of ethylene carbonate (EC) propylene carbonate (PC), vinylene carbonate (VC) and fluoroethylene carbonate (FEC). In some examples, each electrode layer may have a thickness of 40 to 150 pm. In some examples, the, or each, separator layer (e.g. a gel separator layers) may have a thickness of from 10 to 30 pm, for example from 12 to 28 pm or from 14 to 25 pm. Such thicknesses may enable an effective heat-seal between separator layers and / or the separator layer and the inner layer of the pouch without the layers being damaged by the heat-sealing process. In some examples, the, or each, separator layer may 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. In some examples, the, or each, separator layer may 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 cm2 or from 14 to 50 cm2. In some examples, the, or each, separator layer may have an area (defined in the first and second dimensions) of from 30 to 70 cm2, for example from 40 to 60 cm2 or from 45 to 55 cm2. In some examples, the plurality of separator layers may have an area (defined in the first and second dimensions) identical to each other. In a second aspect, there is provided a method of manufacturing the electrochemical cell according to the first aspect, the method comprising: laminating the plurality of electrode layers together into the stack with a separator layer interposed between each pair of adjacent electrode layers; positioning the stack inside the pouch; forming the first separator-pouch seal. Any one or more of the optional features set out in respect of the first aspect is equally applicable to the second aspect, except where such a combination is clearly impermissible or expressly avoided. The stack may comprise a plurality of separator layers bonded together to form the first separator-separator seal; and the method may comprise forming the first separatorseparator seal simultaneously with forming the first separator-pouch seal. In this way, the number of bonding operations in manufacturing the electrochemical cell can be reduced. Alternatively, the stack may comprise a plurality of separator layers bonded together to form the first separator-separator seal; and the method may comprise forming the first separator-separator seal, and subsequently, forming the first separator-pouch seal by bonding the first separator-separator seal to the inner layer of the pouch. In this way, greater control over the arrangement of the separator layers within the separator-separator seal and thus the separator-pouch seal, can be provided, as the separator-separator seal can be formed prior to positioning the stack inside the pouch. The method may further comprise trimming the separator-separator seal prior to forming the first separator-pouch seal. In this way, the separator-separator seal can be easily formed across a large portion of the separators, and subsequently can be trimmed such as to allow the stack to fit into the pouch. The stack may comprise a periphery (e.g. one or more peripheral edges of the stack); and the method may comprise forming the first separator-pouch seal along a first portion of the periphery (e.g. one or more peripheral edges or portions thereof) to leave a second portion of the periphery unsealed. In this way, it is still possible to access the electrode layers from outside the stack after forming the first separator-pouch seal. The method may then further comprise, subsequent to forming the first separator-pouch seal, adding an electrolyte solution into the pouch through the second portion. The method may then subsequently comprise forming a further separator-pouch seal along the second portion of the periphery. In this way, the stack may not be completely sealed to the pouch at the point of adding the electrolyte solution to the pouch, and thus permeation of the electrolyte solution into the stack may be improved. The periphery of the stack may comprise a plurality of peripheral edges of the stack. The first portion of the periphery may comprise one or more of the peripheral edges. The second portion may comprise the remaining one or more peripheral edges. Alternatively, the stack may comprise a periphery (e.g. one or more peripheral edges of the stack); and the method may comprise: forming one or more separator-pouch seals along the entire periphery (e.g. along all of the peripheral edges) to seal the stack to the pouch; and subsequently, adding an electrolyte solution into the pouch. Fully sealing the stack to the pouch with the separator layers prior to adding electrolyte solution to the pouch can reduce the number of steps in the manufacturing method, since all of the separator-pouch seals can be formed simultaneously, rather than conducting a final bonding step after adding the electrolyte solution. Particularly where the separator layers are gel separator layers, the electrolyte solution can still permeate into the stack through the separator layers even when the stack has been fully sealed to the pouch. Each of the electrode layers may comprise a periphery, and the periphery of the stack may be defined by the aligned peripheries of one or more of the peripheries of the electrode layers. Prior to positioning the stack inside the pouch, the pouch may comprise an unsealed periphery to allow the stack to be positioned inside the pouch. Subsequent to positioning the stack inside the pouch, the method may further comprise sealing the pouch along its unsealed periphery to form a pouch-pouch seal. The step of forming the pouch-pouch seal to seal the pouch may be conducted subsequent to adding electrolyte solution into the pouch. The step of positioning the stack inside the pouch may comprise: providing a sheet of pouch material; positioning the stack on a first portion of the sheet; and folding a second portion of the sheet over the stack, thereby forming a fold between the first portion and the second portion of the sheet. A separator-pouch seal may be formed along the fold. Alternatively, the step of positioning the stack inside the pouch may comprise: providing a first sheet of pouch material and a second sheet of pouch material; positioning the stack on the first sheet of pouch material, overlaying the second sheet of pouch material to interpose the stack between the first sheet and the second sheet. A pouch-pouch seal to seal the pouch may be formed after positioning the stack inside the pouch, for example, after forming the separator-pouch seal along the fold. Alternatively, the pouch-pouch seal may be formed simultaneously with a separator-pouch seal (e.g. the first or second separator-pouch seal), for example, if the pouch-pouch seal incorporates the separator-pouch seal. The pouch-pouch seal may further optionally be formed simultaneously with a separator-separator seal (e.g. the first separator-separator seal). The pouch-pouch seal may provide a hermetically sealed pouch. BRIEF DESCRIPTION OF THE DRAWINGS Figures 1A - IC show schematic views of a prior-art electrochemical cell; Figures 2A - 2C show schematic views of a first embodiment of an electrochemical cell according to the first aspect; Figure 2D provides a flow chart for a method according to the second aspect for manufacturing the first embodiment electrochemical cell; Figures 3 A - 3C show schematic views of a second embodiment of an electrochemical cell according to the first aspect; Figure 3D provides a flow chart for a method according to the second aspect for manufacturing the second embodiment electrochemical cell; Figures 4A - 4C show schematic views of a third embodiment of an electrochemical cell according to the first aspect; and Figure 4D provides a flow chart for a method according to the second aspect for manufacturing the second embodiment electrochemical cell. DETAILED DESCRIPTION Figure 1A shows a plan view of a prior art electrochemical cell 100. Figure IB shows a section view of the prior art electrochemical cell 100 along the line B-B in Figure 1A. Figure IC shows a section view of the prior art electrochemical cell 100 along the line C-C in Figure 1A. The electrochemical cell 100 comprises a stack 20 positioned within a pouch 10. The stack 20 comprises a plurality of electrode layers 21 that are separated by separator layers 22 interposed between adjacent electrode layers 21. The electrode layers 21 comprise alternating anode layers and cathode layers. Also present within the electrochemical cell 100 are first and second electrical connectors 30a, 30b that provide electrical contact with the anode layers and cathode layers, respectively, and extend from the stack 20 out of the pouch 10 for ultimate connection to a circuit within a device to be powered by the electrochemical cell 100. As can be seen in Figures IB and IC, adjacent separator layers 22 are bonded together to form separator-separator seals 2. In particular, the stack 20 in Figures 1A - IC has a rectangular cross-section in plan view and there are separator-separator seals 2 provided along all four peripheral edges of the stack 20. This is facilitated by each of the separator layers 22 extending beyond the peripheral edges of the electrode layers 21 to form an extension portion beyond each peripheral edge. Each separator-separator seal 2 is provided in the respective extension portion. These separator-separator seals 2 can reduce the amount of shearing that the stack 20 can undergo by restricting the possible range of motion of the electrode layers 21 between two adjacent separator layers 22 sealed by a separator-separator seal 2. As illustrated by Figure IC, the pouch 10 comprise a sheet of pouch material having a first portion underlying the stack 20 and a second portion overlying the stack 20 with a fold 15 therebetween. In order to seal the pouch 10, pouch-pouch seals 1 are provided along the three peripheral edges of the pouch 10 that are not sealed (i.e. all the edges of the pouch 10 other than the edge where the fold 15 is present), as shown in Figure 1A. No seals are present between the stack 20 and the pouch 10, and thus the stack 20 is able to move inside the pouch 10. Figure 2A shows a plan view of a first embodiment of an electrochemical cell 200. Figure 2B shows a section view of the first embodiment electrochemical cell 200 along the line B-B in Figure 2A. Figure 2C shows a section view of the first embodiment electrochemical cell 200 along the line C-C in Figure 2A. The electrochemical cell 200 in Figures 2A to 2C is similar to that of shown in Figures 1A -IC. Where features of the prior art electrochemical cell 100 and the first embodiment electrochemical cell 200 are the same, reference may be made back to the description of Figures 1A - IC. The first embodiment electrochemical cell 200 differs from that in Figures 1A - IC in that, rather than each separator layers 22 solely being bonded to an adjacent separator layer 22 in a separator-separator bond 2 as in the prior art electrochemical cell 100, all the separator layers 22 in the first embodiment electrochemical cell 200 are bonded to the inner layer of the pouch 10 by first separator-pouch seals 3. This is illustrated in Figures 2B and 2C, where an extension portion of each of the separator layers 21 extends to the inner layer of the pouch 10. The first embodiment electrochemical cell 200 has a pouch 10 constructed in a similar manner to that of the prior art electrochemical cell 100 in Figures 1A - IC, where a second portion of a sheet of pouch material is folded over a first portion of the pouch material with the stack 20 sandwiched therebetween. As illustrated in Figure 2C, a first separator-pouch seal 3 is formed along the fold 15, the first separator-pouch seal 3 extending along substantially the entire peripheral edge of the stack 20 adjacent the fold 15, as is illustrated by Figure 2A. The first separator-pouch seal 3 is not incorporated into a pouch-pouch seal 1, as the fold 15 means that a pouch-pouch seal 1 is not required along this edge of the pouch 10. The electrochemical cell 200 further comprises a second separator-pouch seal 3 that extends along the remaining three peripheral edges of the stack 20. The second separatorpouch seal 3 is incorporated into the pouch-pouch seal 1 that extends around the peripheral edge of the pouch 10 by the separator layers 22 extending to be interposed between the two opposed portions of the inner layer of the pouch 10 that are bonded together to provide the pouch-pouch seal 1. Accordingly, at the pouch-pouch seal 1, a layered structure of pouch-separators-pouch is provided. Figure 2D provides a flow chart for a method according to the second aspect for manufacturing the first embodiment electrochemical cell 200. At step SI00, a plurality of electrode layers 21 are laminated together into a stack 20 that comprises a separator layer 22 interposed between each pair of adjacent electrode layers 21. Subsequently, at step S200 the stack 20 is positioned in the pouch 10. In the case of the first embodiment electrochemical cell 200, step S200 is conducted by providing a sheet of pouch material, positioning the stack 20 on a first portion of the sheet, and folding a second portion of the sheet over the stack 20 to provide the pouch 10 comprising the fold 15. At step S300, the first separator-pouch seal 3 is formed along the fold 15 by heating the pouch 10 along this peripheral edge of the stack 20 such as to bond all the separator layers 22 to the inner layer of the pouch 10 with a heat seal. Subsequently, at step S400a, the pouch-pouch seal 1 to hermetically seal the pouch 10 is formed along the three unsealed peripheral edges of the pouch 10 simultaneously with forming the second separator-pouch seal 3. Step S400a can be conducted by heating the pouch 10 from both sides (i.e. on both the first portion and second portion of the sheet of pouch material) at a position where the extension portions of the separator layers 22 are interposed between the first portion and second portion of the pouch 10. Figure 3A shows a plan view of a second embodiment of an electrochemical cell 300. Figure 3B shows a section view of the second embodiment electrochemical cell 300 along the line B-B in Figure 3A. Figure 3C shows a section view of the second embodiment electrochemical cell 300 along the line C-C in Figure 3 A. The electrochemical cell 300 in Figures 3A to 3C is similar to that of shown in Figures 2A - 2C. Where features of the first embodiment electrochemical cell 200 and the second embodiment electrochemical cell 300 are the same, reference may be made back to the description of Figures 2 A - 2C. The second embodiment electrochemical cell 300 differs from the first embodiment electrochemical cell 200 in that, rather than incorporating the second separator-pouch seal 3 into the pouch-pouch seal 1, all the separator-pouch seals 3 are provided separately to the pouch-pouch seals 1. As in the first embodiment, a first separator-pouch seal 3 is provided along the fold 15 in the pouch 10, as illustrated in Figure 3C. However, in contrast to the first embodiment, in the second embodiment electrochemical cell 300, the separator-pouch seals 3 around the remainder of the periphery of the stack 20 are offset from the pouch-pouch seal 1 in an inboard direction around the periphery of the pouch 10. This can be seen in Figure 3A, where the hatching illustrating the separator-pouch seal 3 is positioned inboard of the hatching indicating the pouch-pouch seal 1. This inboard offsetting of the second separator-pouch seal 3 is also shown in Figures 3B and 3C, where the separator layers 22 are shown extending to a point on the inner layer of the pouch 10 that is in between the electrode layers 21 and the pouch-pouch seal 1. Figure 3 A illustrates how the offset separator-pouch seal 3 along the three peripheral edges that are not located in the fold 15 of the pouch extends along a substantially parallel path to the pouch-pouch seal 1 along the periphery of the pouch 10. Figures 3B and 3C also illustrate how, in the second embodiment, the second separatorpouch seal 3 along the three peripheral edges that are not located in the fold 15 of the pouch 10 is formed only with the first portion of the pouch 10 that underlies the stack 20 in the orientation shown in Figures 3B and 3C and not with the second portion of the pouch 10 that has been folded over the top of the stack 20. In other words, the pouch comprises two opposing portions of its inner layer and said second separator-pouch seal 3 is only formed with one of said portions. Figure 3D provides a flow chart for a method according to the second aspect for manufacturing the second embodiment electrochemical cell 300. Steps SI00 to S300 in Figure 3D are the same as those in Figure 2D, and reference may be made back to their description above in relation to Figure 2D. Following step S300, the separator layers 22 are sealed to the pouch 10 along the fold 15 and also around the rest of the periphery of the stack 20, such that the stack 20 is fully sealed. At this stage, the pouch 10 is not sealed along its three peripheral edges other than the fold 15. Subsequently, at step S400, two pouch-pouch seals 1 are formed to seal the two shorter peripheral edges of the pouch 10 from which the electrical connectors 30a, 30b extend (see Figure 3A). Step S400 in Figure 3D differs from step S400a in Figure 2D in that the pouch-pouch seals 1 formed in step S400 do not incorporate the separator-pouch seal 3, and the pouch-pouch seals 1 formed in step S400 do not fully seal the pouch 10, since the peripheral edge of the pouch 10 opposite the fold 15 is not sealed by a pouch-pouch seal in step S400. Although not illustrated in the figures, such electrochemical cells typically comprise an electrolyte solution within the stack 20. It may be desired to add additional electrolyte solution to the stack 20 after it has been positioned in the pouch 10 to ensure it is fully wetted. Therefore, prior to fully sealing the pouch 10, the method in Figure 3D further comprises step S500 of filling the pouch 10 with electrolyte solution through the final unsealed peripheral edge of the pouch 10 that extends parallel to the fold 15 and is positioned on the opposite side of the stack 20 to the fold. Although the stack 20 has been fully sealed by the separator-pouch seals 3 formed in step S300, it is still possible for the electrolyte solution to permeate into the stack 20 through the separator layers 22, particularly where the separator layers 22 are gel separator layers 22. Having filled the pouch 10 with electrolyte solution, at step S400b the final pouch-pouch seal 1 is formed along the peripheral edge of the pouch 10 opposite the fold 15 to fully seal the pouch 10. Figure 4A shows a plan view of a third embodiment of an electrochemical cell 400. Figure 4B shows a section view of the third embodiment electrochemical cell 400 along the line B-B in Figure 4A. Figure 4C shows a section view of the second embodiment electrochemical cell 400 along the line C-C in Figure 4A. The electrochemical cell 400 in Figures 4A to 4C is similar to that of shown in Figures 2A - 2C and 3 A - 3C. Where features of the first and / or second embodiment electrochemical cells 200, 300 and the third embodiment electrochemical cell 400 are the same, reference may be made back to the description of Figures 2A - 2C and / or 3A - 3C, respectively. The third embodiment electrochemical cell 400 firstly differs from the first and second embodiments in that, rather than the pouch 10 being provided by two portions of a sheet of pouch material that have been folded over each other, the pouch 10 comprises a first sheet of pouch material that is positioned below the stack 20, and a second sheet of pouch material that is overlayed on the stack 20 and first sheet of pouch material such as to interpose the stack 20 between the first sheet and the second sheet, as illustrated by Figures 4B and 4C. Accordingly, there is no fold present in the pouch 10 in the fourth embodiment and in its place is a pouch-pouch seal 1 along the peripheral edge of the pouch 10 to attach the first and second sheets together. Additionally, in contrast to the first and second embodiments, the third embodiment comprises a combination of separator-pouch seals 3 that are separate from the pouch-pouch seals 1 in the electrochemical cell 400 and separator-pouch seals 3 that are incorporated into the pouch-pouch seals 1. The pouch 10 in Figure 4A has a rectangular cross section in plan view, comprising a pair of shorter peripheral edges and a pair of longer peripheral edges. Figure 4B illustrates how, along the two shorter peripheral edges, separator-pouch seals 3 are provided inboard of the respective pouch-pouch seals 1 along those edges. Along each of the shorter peripheral edges, there are two separator-pouch seals 3, a first of which is between the inner layer of the first sheet of pouch material below the stack 20 and a first plurality of separator layers 22 (i.e. the separator layers positioned closer to the first sheet of pouch material than the second sheet of pouch material) and a second of which is between the inner layer of the second sheet of pouch material above the stack 20 and a second plurality of separator layers 22 (i.e. the separator layers positioned closer to the second sheet of pouch material than the first sheet of pouch material). That is, each separator-pouch seal 3 along these shorter peripheral edges is only formed with one of the two sheets of pouch material that the stack 20 is interposed between, and not all of the separator layers 22 are bonded to the same sheet of pouch material along these shorter peripheral edges. Figure 4C then illustrates how, along one of the longer peripheral edges of the pouch 10 on one side of the stack 20, all of the separator layers 22 are bonded together to form a separator-separator seal 2a, and that this separator-separator seal 2a is then bonded to the inner layer of the second sheet of pouch material to form a further separator-pouch seal 3 a. This separator-pouch seal 3a is not incorporated into the pouch-pouch seal la along this peripheral edge of the pouch 10, with the pouch-pouch seal la instead being formed outboard of the separator-pouch seal 3a and extending along a path that is offset from, but substantially parallel to, the separator-pouch seal 3a, as illustrated in Figure 4A. Figure 4C shows, along the final longer peripheral edge of the pouch 10, all of the separator layers 22 in the stack 20 are interposed between the two portions of the inner layer of the pouch 10 that are bonded together to form the pouch-pouch seal lb along this peripheral edge. In this way, the pouch-pouch seal lb incorporates all the separator layers 22 and thus provides a separator-pouch seal 3b along this peripheral edge. Figure 4D provides a flow chart for a method according to the second aspect for manufacturing the third embodiment electrochemical cell 400. Steps SI00 and S200 in Figure 4D are the same as those in Figures 2D and 3D, and reference may be made back to their description above in relation to Figures 2D and 3D. Prior to conducting step S200, the method comprises step SI50 of forming a separatorseparator seal 2a along a long edge of the stack. This separator-separator seal will subsequently be bonded to the inner layer of the pouch 10 to provide a separator-pouch seal 3a in step S300. Following step S200, at step S300, separator-pouch seals 3, 3a are formed along three of the peripheral edges of the pouch 10. The separator-separator seal 2a formed at step SI50 is bonded to the second sheet of pouch material along one of the long peripheral edges of the pouch 10, and also the first and second pluralities of separator layers 22 are separately bonded to the first and second sheets of pouch material, respectively, along both of the short peripheral edges of the pouch 10. Following step S300, the stack 20 is bonded to the pouch 10 along three of its four peripheral edges. Subsequently, at step S400, pouch-pouch seals 1, la are formed between the opposed inner layers of the pouch 10 along the same three peripheral edges of the pouch 10 that the separator-pouch seals 3, 3a were formed at step S300. Each of the pouch-pouch seals 1, la are positioned outboard of, but substantially parallel to, the respective separator-pouch seals 3, 3a along those peripheral edges (i.e. offset from the respective separator-pouch seals 3, 3a), as illustrated in Figure 4A. Thus, following step S400, the stack 20 is bonded to the pouch 10 along three of its four peripheral edges, and the pouch 10 is sealed along those three peripheral edges, leaving one edge of the pouch 10 along which the separator layers 22 are not sealed to each other or the pouch 10, and along which the pouch 10 is not sealed to itself. Accordingly, at step S500, electrolyte solution is added to the stack 20 through the 5 unsealed peripheral edge of the pouch 10, along which edge the separator layers 22 have also not been bonded to each other or the pouch, thereby facilitating permeation of the electrolyte solution into the stack 20. Having filled the pouch 10 with electrolyte solution at step S500, at step S400a the final 10 pouch-pouch seal lb is formed along the final peripheral edge of the pouch 10 to seal the pouch 10 shut. The final pouch-pouch seal lb incorporates, and is formed simultaneously with, the final separator-pouch seal 3b that incorporates all of the separator layers 22, as illustrated in Figure 4C. 15

Claims

1. An electrochemical cell comprising:a pouch; anda stack received in the pouch, the stack having a laminate structure comprising:a plurality of stacked electrode layers; anda separator layer interposed between each pair of adjacent electrode layers; wherein at least one separator layer is bonded to an inner layer of the pouch by a first separator-pouch seal.

2. The electrochemical cell according to claim 1, wherein: the stack comprises a first plurality of separator layers; and the first plurality of separator layers are bonded together to form a first separatorseparator seal.

3. The electrochemical cell according to claim 2, wherein the first separator-separator seal is bonded to the inner layer of the pouch by the first separator-pouch seal.

4. The electrochemical cell according to claim 3, wherein:a second plurality of separator layers are bonded together to form a second separator-separator seal;the first separator-pouch seal is at a first location; andthe second separator-separator seal is bonded to the inner layer of the pouch by a second separator-pouch seal at a second location.

5. The electrochemical cell according to any one of claims 2 to 4, wherein all the separator layers in the laminate structure are bonded together in the first separatorseparator seal.

6. The electrochemical cell according to any one of the preceding claims, wherein the first separator-pouch seal is a heat seal.

7. The electrochemical cell according to any one of the preceding claims, wherein the inner layer of the pouch comprises a thermoplastic.

8. The electrochemical cell according to any one of the preceding claims, wherein the, or each, separator layer is a gel separator layer.

9. The electrochemical cell according to any preceding claim, wherein:the pouch comprises a sheet of pouch material having a first portion and a second portion with a fold therebetween; andthe stack is sandwiched between the first portion and the second portion.

10. The electrochemical cell according to claim 9, wherein the first separator-pouch seal is formed along the fold.

11. The electrochemical cell according to any preceding claim, further comprising a pouch-pouch seal between two opposed portions of the inner layer of the pouch.

12. The electrochemical cell according to claim 11, wherein the pouch-pouch seal incorporates a separator-pouch seal.

13. The electrochemical cell according to claim 11, wherein the pouch-pouch seal is at an offset position from the first separator-pouch seal.

14. A method of manufacturing the electrochemical cell according to any one of claims1 to 13, the method comprising:laminating the plurality of electrode layers together into the stack with a separator layer interposed between each pair of adjacent electrode layers;positioning the stack inside the pouch;forming the first separator-pouch seal.

15. The method according to claim 14, wherein:the stack comprises a plurality of separator layers bonded together to form the first separator-separator seal; andthe method comprises forming the first separator-separator seal simultaneously with forming the first separator-pouch seal.

16. The method according to claim 14, wherein:the stack comprises a plurality of separator layers bonded together to form the first separator-separator seal; andthe method comprises forming the first separator-separator seal, and subsequently, forming the first separator-pouch seal by bonding the first separator-separator seal to the inner layer of the pouch.

17. The method according to any one of claims 14 to 16, wherein:the stack comprises a periphery; andthe method comprises forming the first separator-pouch seal along a first portion of the periphery to leave a second portion of the periphery unsealed.

18. The method according to claim 17, wherein the method comprises:subsequent to forming the first separator-pouch seal, adding an electrolyte solution into the pouch through the second portion; andsubsequently, forming a further separator-pouch seal along the second portion of the periphery.

19. The method according to any one of claims 14 to 16, wherein:the stack comprises a periphery; andthe method comprises:forming one or more separator-pouch seals along the entire periphery to seal the stack to the pouch; andsubsequently, adding an electrolyte solution into the pouch.

20. The method according to any one of claims 14 to 19, wherein:prior to positioning the stack inside the pouch, the pouch comprises an unsealed periphery to allow the stack to be positioned inside the pouch; andsubsequent to positioning the stack inside the pouch, the method further comprises sealing the pouch along the unsealed periphery to form a pouch-pouch seal.

521. The method according to any of claims 14 to 20, wherein the step of positioning the stack inside the pouch comprises:providing a sheet of pouch material;positioning the stack on a first portion of the sheet; and10 folding a second portion of the sheet over the stack, thereby forming a fold betweenthe first portion and the second portion of the sheet.

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