Battery cell, and method for manufacturing a battery cell

Abstract

An electrode assembly 200 for a battery includes positive electrode layers (14a-c, Figure 1); and negative electrode layers (12a-d, Figure 1). The positive electrode layers and negative electrode laye

Description

TECHNICAL FIELD The present disclosure relates to a battery cell. In particular, the disclosure relates to a battery cell comprising an electrode assembly in which electrode tabs are formed to extend substantially parallel to an end surface of the electrode assembly. A method for manufacturing a battery cell, and an electrode assembly, are also disclosed. BACKGROUND Rechargeable battery cells are used in various applications, including in electric vehicles (EVs), consumer electronics, and storage systems for renewable energy. Three cell geometries are generally available for these applications: prismatic cells, cylindrical cells, and pouch cells. The choice of cell geometry depends on the specific requirements of the application and the trade-offs between energy density, safety, and cost. Electrode assemblies in battery cells generally consist of so-called “jelly rolls”, which consist of several layers of anodes and cathodes separated by a separator. These layers may be stacked together to form a stacked jelly roll, or they may be wound or rolled up to form a wound jelly roll. The jelly roll design is a compact structure aimed at maximising the electrode surface area. Prismatic cells are named after their shape, which is typically rectangular or square. In prismatic cells, anodes and cathodes are stacked or wound, with separators interposed between anodes and cathodes, and are enclosed in a container of prismatic shape. Generally, prismatic cells have comparatively higher energy density (due to their flat and efficient packaging), good heat dissipation, higher packaging efficiency (facilitating integration into battery packs) and enhanced safety (due to the typically rigid structure of the container / casing). However, because of the way the electrodes, and the respective electrode tabs, are arranged in prismatic cells, electrically connecting the electrodes to terminals is challenging. Currently, electrical connection designs are bulky, and result in high resistance which increases heat generation and causes hot spots, thus limiting a maximum charge current. This may in particular limit the suitability of prismatic cells in the automotive industry, where fast charging (e.g. at a power of 7kW to 22 kW, up to 80 kW, or at rates of 2C to 10C, or similar) is becoming increasingly desirable. The above considerations also apply to other cell geometries, in particular pouch cells, in which electrical connection designs also take up a lot of space. Manufacturing battery cells, including prismatic cells, and in particular electrically connecting electrodes of prismatic cells to busbars, is complex and requires several manufacturing steps, resulting in delays and increased manufacturing costs. For example, in prismatic cells, electrode tabs for connecting cathodes and anodes to an external circuit are clinched and then ultrasonically welded together. They are then welded to a busbar which may then be welded onto a lid of the battery cell. The inventors have appreciated the need for a battery cell allowing for a more compact electrical connection, and the need for a method of manufacturing battery cells which is more efficient, e.g. by having fewer steps than prior art methods. SUMMARY OF THE DISCLOSURE The present disclosure provides a battery cell, a method for manufacturing a battery cell, and an electrode assembly, as defined in the appended independent claims, to which reference should now be made. Preferred or advantageous features of the disclosure are set out in the dependent sub-claims. According to a first aspect of the present disclosure, there is provided a battery cell. The battery cell comprises an electrode assembly. The electrode assembly comprises a plurality of positive electrode layers and a plurality of negative electrode layers. The positive electrode layers and negative electrode layers are arranged alternately, with a separator layer interposed between adjacent positive and negative electrode layers. Positive electrode tabs protrude from the plurality of positive electrode layers forming at least one stack of positive electrode tabs and negative electrode tabs protrude from the plurality of negative electrode layers forming at least one stack of negative electrode tabs. The electrode tabs are formed to extend substantially parallel to an end surface of the electrode assembly. By providing an electrode assembly in which the electrode tabs are formed to extend substantially parallel to an end surface of the electrode assembly, the design of the battery cell of the present disclosure may be more compact, that is, it may take up less space. By taking up less space, the battery cell (and consequently, battery packs including the battery cell) may have a higher volumetric energy and power density. It is noted that in prior art battery cells, in particular in prior art prismatic and pouch cells, electrode tabs extend perpendicular to an end surface of the electrode assembly, and are then clinched and ultrasonically welded together. They are then welded to a busbar which may then be welded onto a lid of the battery cells. The electrical connection, and in particular the electrode tabs, in prior art prismatic battery cells thus may take up more space than in a battery cell according to the present disclosure. The end surface of the electrode assembly may be a “virtual” end surface of the electrode assembly. As set out above, the electrode assembly is made up of alternating layers of positive electrodes (or a positive electrode) and negative electrodes (or a negative electrode) with a separator layer interposed between each pair of adjacent positive and negative electrode layers. Thus, the electrode assembly may not have a physical end surface, but may simply be defined by the aligned ends of the layers of the electrode assembly. The electrode tabs being formed to extend “substantially parallel” to an end surface of the electrode assembly may refer to the tabs being formed to extend in a plane which diverges from being exactly parallel to the end surface by up to about 10 degrees, or by up to about 8 degrees, or by up to about 5 degrees, or by up to about 3 degrees, or by up to about 2 degrees, or by up to about 1 degree. Preferably, the electrode tabs do not extend beyond the end surface of the electrode assembly. Advantageously, this may allow for the battery cell to be more resilient. In particular, if the entire electrode tab overlaps the electrode assembly, no portion of the electrode tab protrudes beyond the edge of the end surface, which would risk that portion of the electrode tab being more prone to damage. The electrode tabs may extend substantially in a normal direction of the layers of the layered electrode assembly. In other words, the electrode tabs may extend substantially in a direction in which the electrode layers are stacked, or wound to be adjacent one another. This may be referred to as a width wise direction of the electrode assembly. In some embodiments, the electrode tabs of at least one of the stacks of electrode tabs are formed to extend in a first direction, and the electrode tabs of at least one other one of the stacks of electrode tabs are formed to extend in a second direction which is substantially opposite the first direction. Advantageously, by having tabs of different stacks extend in opposing directions, the tabs of each stack may be longer, permitting for a larger interface area for conducting an electric current, resulting in fewer hot spots. This may enable improved performance, especially when fast charging, as a maximum charge current may be increased. The battery cell may in preferred embodiments be a prismatic battery cell. A prismatic battery cell may comprise a rigid casing. The rigid casing may be rectangular. The electrode assembly may be contained within the rigid casing. Advantageously, the tabs extending in opposing directions and the battery cell being a prismatic battery cell may allow for reduced resistance and heat generation which may increase a maximum current with which the battery cell may be charged. This may enable the prismatic battery cell to be used for high performance applications, such as in the automotive industry, where fast charging is becoming increasingly important. In other embodiments, the battery cell may be a pouch battery cell. A pouch battery cell may comprise a flexible housing. The electrode assembly may be contained within the flexible housing. Preferably, the first direction and the second direction may be transverse directions. Transverse directions may alternatively be referred to as width-wise directions, or horizontal directions. That is, the first direction and the second direction may be perpendicular to a longitudinal plane of the electrode assembly. In particular, the first direction and the second direction may extended along a width wise direction of the electrode assembly. Optionally, the tabs of half of the stacks extend in the first direction, and the tabs of the other half of the stacks extend in the second direction. Advantageously, by having the tabs of half of the stacks extend in the first direction, and the other half of the tabs extend in the second direction, an interface area of the tabs available for conducting an electric current may be maximised, resulting in fewer hot spots and a higher maximum current for charging. In some embodiments, the tabs of at least one of the stacks extend, along a width wise direction of the electrode assembly, toward another one of the stacks. In preferred embodiments, the tabs of at least one stack extend, in the width wise direction, toward another one of the stacks, and the tabs of the another one of the stacks extend, in the width wise direction, toward the at least one stack. The stacks of electrode tabs may be offset along a dimension of the electrode assembly. This may allow for an interface area of the tabs available for conducting an electric current to be maximised, resulting in fewer hot spots. It may also prevent tabs of different stacks from overlapping. Preferably, the stacks of electrode tabs do not overlap one another. This is particularly important if only one stack of positive electrode tabs and one stack of negative electrode tabs are provided as otherwise the battery cell would be inoperable. If multiple stacks of each polarity is provided, ensuring that the stacks of electrode tabs do not overlap may ensure that an interface area available for conducting an electric current is maximised for each stack. The dimension of the electrode assembly is preferably a length of the electrode assembly. This may mean that a width of each tab may be maximised, as the length is the longest dimension of the electrode assembly. In other embodiments, the dimension may be a height of the electrode assembly. In these other embodiments, the electrode tabs may extend substantially parallel to a longitudinal end surface of the electrode assembly. In some embodiments, the positive electrode tabs form a plurality of stacks of positive electrode tabs, and the negative electrode tabs form a plurality of stacks of negative electrode tabs. In some embodiments in which the positive electrode tabs form a plurality of stacks of positive electrode tabs, the stacks of positive electrode tabs are distributed along a width of the electrode assembly. Similarly, in some embodiments in which the negative electrode tabs form a plurality of stacks of negative electrode tabs, the stacks of negative electrode tabs are distributed along a width of the electrode assembly. In other words, the stacks of positive electrode tabs are arranged in different width wise positions along the electrode assembly and the stacks of negative electrode tabs are arranged in different width wise positions along the electrode assembly. Forming stacks in different width wise positions along the electrode assembly may allow each electrode layer to have an electrode tab, while still preventing different stacks from overlapping. Optionally, the stacks of positive electrode tabs are adjacent one another along a, or the, dimension of the electrode assembly, and the stacks of negative electrode tabs are adjacent one another along a, or the, dimension of the electrode assembly. Preferably, stacks of electrode tabs of the same polarity being adjacent one another may enable an electrical connection to be made more easily and in a matter which may be more compact. It may also allow a single component to be welded to all stacks of a polarity. Optionally, the tabs of one stack of positive electrode tabs and the tabs of one stack of negative electrode tabs extend in a, or the, first direction; and the tabs of the other stack of positive electrode tabs and the tabs of the other stack of negative electrode tabs extend in a, or the, second direction which is substantially opposite the first direction. Advantageously, in such a battery cell having two stacks of positive electrode tabs and two stacks of negative electrode tabs, an interface area of the tabs available for conducting an electric current may be maximised, which may result in an increased maximum charge current. Optionally, the tabs of half of the stacks of positive electrode tabs, and the tabs of half of the stacks of negative electrode tabs, extend in a, or the, first direction; and the tabs of the other half of the stacks of positive electrode tabs, and the tabs of the other half of the stacks of negative electrode tabs, extend in a, or the, second direction which is substantially opposite the first direction. Optionally, the electrode assembly comprises, or consists of, at least a first electrode assembly and a second electrode assembly, each of the first and second electrode assembly comprising at least one of the stacks of positive electrode tabs and at least one of the stacks of negative electrode tabs. As such, each of the first and second electrode assemblies may have only one stack of negative electrode tabs and one stack of positive electrode tabs. This may facilitate manufacturing of the first and second electrode assemblies, and may allow the electrode assemblies to be manufactured in a known manner. It may further allow each electrode layer to comprise a tab, and thus may allow the first and second electrode assemblies to be provided as stacked jelly rolls. The first electrode assembly and the second electrode assembly may differ in a position of the stacks of electrode tabs along a, or the, dimension of the respective electrode assembly. Advantageously, this may allow for each of the first and second electrode assemblies to be manufactured more easily, having electrode tabs for each polarity in a single (e.g., longitudinal) position, but a position which is different between the first electrode assembly and the second electrode assembly. In other words the first electrode assembly and the second electrode assembly may be substantially identical, except for the position (along the dimension of the electrode assembly) of the stacks of electrode tabs. Optionally, at least some of the electrode tabs of the first electrode assembly overlap the second electrode assembly, and at least some of the electrode tabs of the second electrode assembly overlap the first electrode assembly. Tabs of one electrode assembly overlapping the other electrode assembly may allow for a more compact design. The stacks of the first electrode assembly and the stacks of the second electrode assembly may be arranged alternately along a, or the, dimension of the electrode assemblies. In some embodiments, each of the tabs has a length which is less than a width of the respective one of the first electrode assembly and the second electrode assembly. This may allow no portion of any electrode tab to extend beyond the end surface, thus reducing risk of damage to the electrode tabs. Optionally, each of the tabs has a length which is less than half a width of the electrode assembly. Advantageously, if the electrode tabs overlap the electrode assembly, but a length of each electrode tab is half a width of the electrode assembly, this may mean that no portion of the electrode tabs protrudes beyond a width-wise end of the electrode assembly, thus reducing risk of any damage. Optionally, a length of each tab is substantially identical. Each tab having a same length may simplify manufacturing. In particular, if the electrode assemblies are stacked electrode assemblies, stacking layers having electrode tabs of identical length may be easier than stacking layers having electrode tabs of varying lengths in a specific order. In some embodiments, a length of at least some of the tabs may differ. For example, the length may increase gradually from an innermost tab of each stack to an outermost tab, or vice versa. In some embodiments, the battery cell comprises at least one further electrode assembly adjacent the electrode assembly. The, or each, further electrode assembly may be substantially identical to the electrode assembly. Advantageously, this may facilitate mass production of the battery cell, as substantially identical electrode assemblies may be produced for combination in the battery cell. Optionally, the, or each, electrode assembly is a stacked jelly roll. Alternatively, the, or each, electrode assembly is a wound jelly roll. In embodiments in which the, or each, electrode assembly is a wound jelly roll, the, or each, electrode assembly may comprise only one stack of positive electrode tabs and only one stack of negative electrode tabs. In such embodiments, the tabs of each stack may extend in a same direction. The tabs may not extend beyond a width-wise end of the electrode assembly. Preferably, each of the stacks may be formed so that each tab in the respective stack is accessible from a side facing away from the electrode assembly. For example, if the electrode assembly is wound with regard to a height wise, or “z”, axis, the stacks may be formed so that a portion of a surface of each tab faces “upwards”, in the height wise, or “z”, direction, and hence away from the electrode assembly. In other words, at least a portion of a surface area of each of the tabs may be accessible from a side facing away from the electrode assembly. In this manner, an electrical connection between the electrode layers in the electrode assembly and, e.g., a busbar, may be improved, and making the electrical collection may be facilitated. In particular, this arrangement may lead to shorter current paths, lower resistance, and fewer hot spots. The battery cell may further comprise at least one of: a negative weld plate welded to each tab in the, or each, stack of negative electrode tabs, or to each tab in one of the stacks of negative electrode tabs, from the side facing away from the electrode assembly; and a positive weld plate welded to each tab in the, or each, stack of positive electrode tabs, or to each tab in one of the stacks of positive electrode tabs, from a side facing away from the electrode assembly. The battery cell comprising such weld plates may result in fewer hot spots, as a larger interface area between the respective stack(s) of tabs and the respective weld plate may be provided. The battery cell may comprise a negative weld plate per stack of negative electrode tabs, and a positive weld plate per stack of positive electrode tabs. Optionally, the battery cell may further comprise at least one of: a positive busbar welded to the, or each, positive weld plate; and a negative busbar welded to the, or each, negative weld plate. Further optionally, the, or each, busbar is coupled to a lid of the battery cell. Yet further optionally, the, or each, busbar is bent around so that a portion of the busbar is welded to the respective weld plate and a portion of the busbar coupled to the lid are substantially parallel. In some embodiments, the battery cell further comprises a non-conductive plate covering the, or a, end surface of the, or each, electrode assembly. Advantageously, a non-conductive plate covering the end surface may prevent electrical shorts. Indeed, it may also provide a surface for folding the electrode tabs onto, and on which to press the electrode tabs to flatten them. The non-conductive plate may comprise a polymer. In some embodiments, the non-conductive plate may be a polymer plate. Advantageously, a polymer plate may be easy to manufacture and inexpensive. The non-conductive plate may alternatively, or additionally, comprise a ceramic material. In some embodiments, the non-conductive plate may be a ceramic plate. Advantageously, a ceramic plate may be heat-resistant. Thus a ceramic plate may be unaffected by in-situ welding heat, and may protect the electrode assembly (or electrode assemblies) from in-situ welding heat. In some embodiments, the non-conductive plate may be a polymer plate comprising a ceramic material. Preferably, the ceramic material may be provided adjacent a weld zone to protect the polymer plate, and underlying electrode assembly, from welding heat. Such a non-conductive plate may be inexpensive, while at the same time incorporating ceramic material which is heat-resistant. In some further embodiments, the non-conductive plate comprises cut-outs for the stacks of electrode tabs. In this manner, the tabs may project from the electrode layers, extend beyond the non-conductive plate, and then extend, substantially parallel to the end surface (and parallel to the non-conductive plate), along the non-conductive plate. In some yet further embodiments, the cut-outs are arranged such that at least some of the tabs in at least one of the stacks of electrode tabs are curved around the non-conductive plate. In this manner, the non-conductive plate may overlap at least some of the electrode assembly adjacent to the stack of electrode tabs. This may allow the non-conductive plate to shield the electrode assembly, e.g. during laser welding, thus preventing damage to the electrode assembly. The battery cell may be a lithium-ion battery cell. In some embodiments, the battery cell may be a sodium-ion battery cell, or the battery cell may be of any other suitable battery chemistry. Each positive electrode layer may comprise a current collector and an active material layer coated onto the current collector. In particular, the current collector may comprise an aluminium foil, and the active material layer may comprise a positive electrode active material. The positive electrode active material may be selected from lithium iron phosphate (LFP), lithium manganese iron phosphate (LMFP), lithium nickel manganese cobalt oxide (NMC), and lithium nickel cobalt aluminium oxide (NCA). Other positive electrode active materials may be selected. The active material layer may further comprise a binder such as polyvinylidene fluoride (PVDF) or sodium alginate. The active layer may further comprise a conductive material, such as carbon. Positive electrode layers may be referred to as cathode layers. In preferred embodiments, each positive electrode layer is a two-sided cathode layer, i.e. an active material layer is coated on each side of the current collector. Each negative electrode layer may comprise a current collector and an active material layer coated onto the current collector. In particular, the current collector may comprise a copper foil, and the active material layer may comprise a negative electrode active material. The negative electrode active material may be selected from graphite, lithium, silicon, titanium oxide, iron oxide, non-graphitic carbon, or the like. The active material layer may further comprise a binder such as PVDF or sodium alginate. The active layer may further comprise a conductive material, such as carbon. Negative electrode layers may be referred to as anode layers. In preferred embodiments, each negative electrode layer is a two-sided anode layer, i.e. an active material layer is coated on each side of the current collector. The electrode tabs may be made up of an uncoated portion of the current collector. In particular, the positive electrode tabs may comprise, or may be made up of, an uncoated portion of aluminium foil, and the negative electrode tabs may comprise, or may be made up of, an uncoated portion of copper foil. Each separator layer may comprise a polymeric membrane. The polymeric membrane may comprise polypropylene. In other examples, the separator layer may comprise a gel polymer electrolyte, or a solid state electrolyte. The electrode tabs being formed to extend substantially parallel to an end surface of the electrode assembly may refer to the electrode tabs extending substantially parallel to, and substantially adjacent to, the end surface of the electrode assembly. In some embodiments, the electrode tabs of each stack may extend parallel to, and substantially adjacent to, the same end surface. The end surface may be a vertical end surface or a longitudinal end surface. As used herein, the term “longitudinal” may refer to a length wise direction of the electrode assembly, which may also be referred to as the “x” direction. As used herein, the term “vertical” may refer to a height wise direction of the electrode assembly, i.e. a direction which is perpendicular to the longitudinal direction in the plane of each electrode layer, which may also be referred to as the “z” direction. As used herein, the term “horizontal” or “transverse” may refer to a width wise direction of the electrode assembly, i.e. a direction which is perpendicular to the plane of the electrode layers, which may be referred to as the “y” direction. In other words, “longitudinal” may refer to a longest dimension of each electrode layer, “vertical” may refer to a second longest dimension of each electrode layer, and “horizontal” may refer to the direction in which adjacent layers are stacked / wound and which is substantially perpendicular to a plane of each electrode layer. The end surface preferably is a vertical end surface of the electrode assembly. The end surface may alternatively be a longitudinal end surface of the electrode assembly. In some embodiments, the electrode tabs of at least one stack may be formed to extend substantially parallel to a first end surface of the electrode assembly, and the electrode tabs of at least one other stack may be formed to extend substantially parallel to a second end surface of the electrode assembly. In some embodiments, the first end surface and the second end surface are opposite end surface of the electrode assembly. For example, the first end surface may be a first vertical end surface of the electrode assembly, and the second end surface may be a second vertical end surface of the electrode assembly. In another example, the first end surface may be a longitudinal end surface of the electrode assembly, and the second end surface may be a second longitudinal end surface of the electrode assembly. In some embodiments, the first end surface is substantially perpendicular to the second end surface. For example, the first end surface may be a vertical end surface of the electrode assembly, and the second end surface may be a longitudinal end surface of the electrode assembly. Preferably, each of the electrode layers may have an electrode tab. In some embodiments, at least some of the electrode layers comprise more than one electrode tabs. The tabs of the at least some electrode layers comprising more than one electrode tabs may be formed to extend substantially parallel to the same end surface, or they may be formed to extend substantially parallel to different end surfaces. An electrode layer having more than one electrode tab may be capable of accommodating a higher charge current, as a larger interface area for conducting current is provided. As used herein, terms such as “first” and “second”, and terms such as “upper”, “lower”, and “above”, are for descriptive purposes only and should not be interpreted as limiting. According to a second aspect of the present disclosure, there is provided a method for manufacturing a battery cell. The method comprises providing an electrode assembly comprising a plurality of positive electrode layers; and a plurality of negative electrode layers, wherein the positive electrode layers and negative electrode layers are arranged alternately, with a separator layer interposed between adjacent positive and negative electrode layers, wherein positive electrode tabs protrude from the plurality of positive electrode layers forming at least one stack of positive electrode tabs, and wherein negative electrode tabs protrude from the plurality of negative electrode layers forming at least one stack of negative electrode tabs. The method further comprises forming the electrode tabs to extend substantially parallel to an end surface of the electrode assembly. By forming the electrode tabs to extend substantially parallel to an end surface of the electrode assembly, the method of the second aspects may allow for a more compact battery cell to be manufactured. A more compact battery cell may allow for higher volumetric energy and power densities to be achieved. The method according to the second aspect of the present disclosure may be a method for manufacturing a prismatic battery cell. Alternatively, the method may be a method for manufacturing a pouch battery cell. The step of providing an electrode assembly as set out above may refer to providing a pre-made electrode assembly. Alternatively, the step of providing an electrode assembly may comprise manufacturing the electrode assembly. Manufacturing the electrode assembly may comprise stacking a plurality of negative electrodes having negative electrode tabs protruding from the plurality of negative electrode layers, a plurality of positive electrodes having positive electrode tabs protruding from the plurality of positive electrode layers, and a plurality of separator layers in the order negative electrode layer, separator, positive electrode layer, separator, to form a stacked jelly roll. Alternatively, manufacturing the electrode tabs may comprise providing a stack of an elongate negative electrode layer, an elongate separator layer, an elongate positive electrode layer, and a further elongate separator layer, and winding the stack to form a wound jelly roll. The positive electrode tabs are arranged to form at least one stack of positive electrode tabs, and the negative electrode tabs are arranged to form at least one stack of negative electrode tabs. Preferably, the stacks of electrode tabs are offset along a dimension of the electrode assembly. Optionally, forming the electrode tabs comprises at least one of: ultrasonically kneading the tabs; and folding and pressing the tabs. Ultrasonic kneading is known in the manufacture of cylindrical battery cells. Ultrasonic kneading may flatten the stacks of electrode tabs, creating a surface of the stack to which a further component of the battery cell may be welded. Folding and pressing the tabs may comprise pressing the tabs using a force of about 0.05 N to about 5 N, or of about 0.5 N to about 2 N, or of about 1 N. Folding and pressing the tabs preferably comprises folding and pressing each tab in the stack time simultaneously. Optionally, forming the electrode tabs comprises forming the electrode tabs of at least one of the stacks of electrode tabs to extend in a first direction, and forming the electrode tabs of at least one other of the stacks of electrode tabs to extend in a second direction which is substantially opposite the first direction. In some embodiments, forming the electrode tabs further comprises forming the tabs of half of the stacks to extend in the first direction, and forming the tabs of the other half of the stacks to extend in the second direction. Optionally, the positive electrode tabs protruding from the plurality of positive electrode layers form a plurality of stacks of positive electrode tabs, and the negative electrode tabs protruding from the plurality of negative electrode layers form a plurality of stacks of negative electrode tabs. In some embodiments in which the method comprises stacking a plurality of layers to form a stacked jelly roll and in which the electrode assembly comprises a plurality of stacks of negative electrode tabs and a plurality of stacks of positive electrode tabs, the two sets of positive electrode layers and two sets of negative electrode layers may be provided. The sets may differ in a position of the tabs, for example the position of the tabs along a, or the, dimension of the electrode assembly. In one example, the positive electrode tabs may be provided at a first longitudinal position and at a second longitudinal position, and the negative electrode tabs may be provided at a third longitudinal position and at a forth longitudinal position. Stacking may thus comprise first stacking the first set of positive electrodes and the first set of negative electrode tabs, so that their respective tabs form a first stack of positive electrode tabs and a first stack of negative electrode tabs, and then stacking the second set of positive electrodes and the second set of negative electrode tabs, so that their respective tabs form a second stack of positive electrode tabs and a second stack of negative electrode tabs. Optionally, the method further comprises providing at least one further electrode assembly, in particular providing at least one further electrode assembly adjacent the electrode assembly. In some embodiments, the further electrode assembly and the electrode assembly are substantially identical. Forming the electrode tabs may comprise forming the tabs so that each tab in each stack is accessible from a side facing away from the electrode assembly. In some embodiments, the method further comprises at least one of: welding, preferably laser welding, a positive weld plate to each tab in the, or each, stack of negative electrode tabs, or to each tab in one of the stacks of negative electrode tabs, from the side facing away from the electrode assembly; and welding, preferably laser welding, a negative weld plate to each tab in the, or each, stack of positive electrode tabs, or to each tab in one of the stacks of positive electrode tabs, from a side facing away from the electrode assembly. Advantageously, by welding a weld plate to each tab in the, or each, stack, a battery cell having shorter current paths and fewer hot spots may be achieved, leading to a battery capable of being charged / discharged at higher rates. It is noted that in prior art methods for manufacturing prismatic battery cells, electrode tabs extend perpendicular to an end surface of the electrode assembly, and are then clinched and ultrasonically welded together. They are then welded to a busbar which may then be welded onto a lid of the battery cells. Thus, the method of the second aspect of the present disclosure may have fewer steps than prior art methods, and may thus be more efficient. Laser welding may comprise generating a laser beam having a wavelength of about 400 nm to about 1200 nm, or of about 900 nm to about 1200 nm, or of about 1000 nm to about 1100 nm, or of about 1070 nm. Laser welding may comprise generating a laser beam having a power of about 50 W to about 2,000 W, or of about 70 W to 1,000 W, or of about 100 W to about 600 W, or of about 150 W. In one example, laser welding comprises generating a laser beam having a power of about 150 W and a wavelength of about 1070 nm. Laser welding may comprise moving the laser beam across a weld zone at a speed of about 20 mm / s to about 500 mm / s, or of about 50 mm / s to about 100 mm / s. The method may further comprise, before laser welding, providing a plate between the stack of electrode tabs and the electrode assembly for preventing the laser penetrating into the electrode assembly. Advantageously, by preventing the laser penetrating into the electrode assembly, damage to the electrode assembly may be prevented. In a further aspect of the present disclosure, a battery cell may be provided. The battery cell comprises an electrode assembly. The electrode assembly comprises a plurality of positive electrode layers and a plurality of negative electrode layers. The positive electrode layers and negative electrode layers are arranged alternately, with a separator layer interposed between adjacent positive and negative electrode layers. Positive electrode tabs protrude from the plurality of positive electrode layers forming at least one stack of positive electrode tabs and negative electrode tabs protrude from the plurality of negative electrode layers forming at least one stack of negative electrode tabs. Each electrode tab is formed to extend substantially perpendicular to a plane containing a portion of the respective electrode layer connected to the respective electrode tab. In another further aspect of the present disclosure, a battery cell may be provided. The battery cell comprises an electrode assembly. The electrode assembly comprises a plurality of positive electrode layers and a plurality of negative electrode layers. The positive electrode layers and negative electrode layers are arranged alternately, with a separator layer interposed between adjacent positive and negative electrode layers. Positive electrode tabs protrude from the plurality of positive electrode layers forming at least one stack of positive electrode tabs and negative electrode tabs protrude from the plurality of negative electrode layers forming at least one stack of negative electrode tabs. Each electrode tab is formed to extend in a direction in which the layers of the electrode assembly are stacked. According to a yet further aspect of the present disclosure, there is provided an electrode assembly. The electrode assembly comprises a plurality of positive electrode layers and a plurality of negative electrode layers. The positive electrode layers and negative electrode layers are arranged alternately, with a separator layer interposed between adjacent positive and negative electrode layers. Positive electrode tabs protrude from the plurality of positive electrode layers forming at least one stack of positive electrode tabs and negative electrode tabs protrude from the plurality of negative electrode layers forming at least one stack of negative electrode tabs. The electrode tabs are formed to extend substantially parallel to an end surface of the electrode assembly. Optionally, the electrode assembly may be a jelly roll. It will be appreciated that features described in relation to one aspect of the present disclosure may also be applied equally to all of the other aspects of the present disclosure. Features described in relation to the first aspect of the present disclosure may be applied equally to the second aspect of the present disclosure and vice versa. For example, features of the battery cell described in relation to the first aspect may be applied, mutatis mutandis, to the method of the second aspect or the battery cell of the further, or the another further, aspect or the electrode assembly of the yet further aspect, and vice versa. In particular, the features of the electrode assembly of the battery cell described in relation to the first aspect may be applied to the electrode assembly of the yet further aspect. It will further be appreciated that particular combinations of the various features described and defined in any aspects of the invention may be implemented and / or supplied and / or used independently. BRIEF DESCRIPTION OF DRAWINGS The disclosure will be further described, by way of example only, with reference to the accompanying drawings, in which: Figure 1a shows a perspective view of an electrode assembly and Figure 1b shows a partial cross sectional view of the electrode assembly; Figure 2a shows a perspective view of an electrode assembly comprising a pair of the electrode assembly of Figures 1a and 1b; Figures 2b and 2c show partial cross sectional views of alternative examples of the electrode assembly of Figure 2a; Figure 3 shows a perspective view of the electrode assembly of Figure 2a, with formed electrode tab stacks; Figure 4a is an enlarged portion of Figure 3; Figures 4b and 4c show perspective views of an electrode assembly comprising a pair of the electrode assembly according to Figure 3; Figure 5 shows a perspective view of the electrode assembly of Figure 3, further comprising a non-conductive plate; Figure 6 shows a perspective view of the electrode assembly of Figure 5, further comprising a weld plate per formed electrode tab stack; Figures 7 and 8 show a perspective view and a front view of a portion of the electrode assembly of Figure 6, further comprising a lid having terminals; Figures 9a to 9d show perspective views of example electrode assemblies made up of wound jelly rolls; Figure 10 shows a schematic cross-section of one example of a prismatic battery cell; Figure 11a shows a schematic cross-section of a further example of a prismatic battery cell; Figure 11b shows a perspective view of a further example electrode assembly; Figure 11c shows a perspective view of another further example electrode assembly; and Figures 12 to 14 show flow diagrams of different example methods for manufacturing a battery cell. DETAILED DESCRIPTION OF DRAWINGS Figures 1a and 1b shows an electrode assembly 100 which is a stacked jelly roll made up of alternating negative electrode layers 12a...12d (or “anode” layers) and positive electrode layers 14a... 14c (or “cathode” layers). Interposed between each pair of adjacent negative and positive electrode layer (e.g., pair 12a, 14a, or pair 14a, 12b, or pair 14c, 12d) is a respective separator layer 16. In other words, the electrode assembly 100 is made up of a stack of layers in the repeating order negative electrode layer, separator layer, positive electrode layer, and separator layer. Each negative electrode layer 12a... 12d consists of a current collector 18 coated on both sides with a negative electrode active material layer 20, 22. Each negative electrode layer 12a...12d has a protruding negative electrode tab 102a...102d, which is a portion of the current collector 18 which is not coated with an active material layer 20, 22. The current collector 18 of each negative electrode layer comprises a copper foil on which the positive electrode active material layers (typically consisting of an anode active material with a conductive carbon and a binder) are provided. Preferably, a layer of anode active material is provided on both sides of the copper foil of each current collector 18, however single sided negative electrode layers may be preferred in some embodiments. Each positive electrode layer 14a... 14c consists of a current collector 24 coated on both sides with a positive electrode active material layer 26, 28. Each positive electrode layer 14a... 14c has a protruding positive electrode tab 104a... 104c, which is a portion of the current collector 24 which is not coated with an active material layer 26, 28. The current collector 24 of each positive electrode layer comprises an aluminium foil on which the positive electrode active material layers (typically consisting of a cathode active material with a conductive carbon and a binder) are coated. Preferably, a layer of cathode active material is provided on both sides of the aluminium foil of each current collector 24, however single sided positive electrode layers may be preferred in some embodiments. The negative electrode tabs 102a...102d are arranged so as to form a stack 102 of negative electrode tabs, extending in a height-wise direction (H) of the electrode assembly 100 at a first position along the length (L) of the electrode assembly 100, from a height-wise, or vertical, end surface 106 of the electrode assembly 100. Similarly, the positive electrode tabs 104a... 104c are arranged so as to form a stack 104 of positive electrode tabs, extending in the height-wise direction H at a second position along the length (L) of the electrode assembly 100, from the vertical end surface 106 of the electrode assembly 100. As further shown in Figure 1a, the length wise, or longitudinal, direction may be referred to as the “x” direction, the width wise, or transverse, direction may be referred to as the “y” direction, and the height wise, or vertical, direction may be referred to as the “z” direction. In other words, each electrode layer may extend in the “x” direction and the “z” direction, and the electrode layers are stacked, with the interposed separator layers, in the “y” direction. The electrode assembly 100 further has opposing longitudinal end surfaces 108 and opposing width-wise (or horizontal) end surfaces 110. As best appreciated in Figure 1b, the layers 12, 14, 16 are stacked in a width wise direction (W) of the electrode assembly 100. The length (L) and height (H) of the electrode assembly 100 are defined by the length and height of each electrode layer. Figures 2a and 2b show an electrode assembly 200 made up of two electrode assemblies 100a, 100b (e.g. stacked jelly rolls) similar to the electrode assembly 100 shown in Figure 1. Each of the two electrode assemblies 100a, 100b is a stacked jelly roll made up of alternating negative electrode layers and positive electrode layers separated by a separator layer. Each negative electrode layer, and each positive electrode layer, has a protruding electrode tab, which is not coated with electrode material in order to make an electrical connection to the respective electrode layer. The negative current collector tabs and the positive electrode tabs of the electrode assembly 100a are arranged as a stack 102a of negative electrode tabs and a stack 104a of positive electrode tabs, respectively. Similarly, the negative current collector tabs and the positive electrode tabs of the other electrode assembly 100b are arranged as a stack 102b of negative electrode tabs and a stack 104b of positive electrode tabs, respectively. The stack 102a of negative electrode tabs of the electrode assembly 100a and the stack 102b of negative electrode tabs of the other electrode assembly 100b are arranged adjacent one another, but offset from one another in a longitudinal direction (L) of the electrode assembly 200. In other words, the stacks 102a, 102b are provided in different longitudinal positions, i.e. in different locations / positions along the length (L) of the electrode assembly 100a. Because the electrode assemblies 100a, 100b are arranged adjacent one another in a width-wise (transverse) dimension of the electrode assembly 200, the stacks 102a, 102b of negative electrode tabs are at different width-wise positions along the width of the electrode assembly 200. Similarly, the stack 104a of positive electrode tabs of the electrode assembly 100a and the stack 104b of negative electrode tabs of the other electrode assembly 100b are arranged adjacent one another, but offset from one another in a longitudinal direction (L) of the electrode assembly 200. In other words, the stacks 104a, 104b are provided in different longitudinal positions, i.e. in different locations / positions along the length (L) of the electrode assembly 100a. In other words, the electrode assembly 100a and the other electrode assembly 100b are substantially identical and symmetrical, except for the positioning of the stacks 104a, 104b of positive electrode tabs and the stacks 102a, 102b of negative electrode tabs, which are provided at different longitudinal positions along the electrode assembly 200. Because the electrode assemblies 100a, 100b are arranged adjacent one another in a width-wise (transverse) dimension of the electrode assembly 200, the stacks 104a, 104b of positive electrode tabs are at different width-wise positions along the width of the electrode assembly 200. A partial cross-sectional view of the electrode assembly 200 is shown in Figure 2b, which shows that two separate stacked jelly rolls 100a, 100b (or a first and second electrode assemblies) make up electrode assembly 200. Figures 2c shows an alternative example 200c of the electrode assembly 200, which is substantially the same as the example of Figures 2a and 2b. However, rather than being made up of two separate jelly rolls 100a, 100b which differ in the positioning (along a length of the electrode assembly) of the stacks of electrode tabs, the electrode assembly 200c of Figure 2c is made up of a single stacked electrode assembly, or jelly roll. Such a jelly roll may be stacked in a similar manner to the two separate jelly rolls 100a, 100b. All that is required is that sets of electrode layers with tabs in one longitudinal position are stacked on a set of electrode layers with tabs in another longitudinal position. Figure 3 shows the electrode assembly 200 of Figures 2a-2c (whether made up on one or two jelly rolls), in which the stacks 102a, 102b of negative electrode tabs have been folded so that they extend, as folded stacks 302a, 302b of negative electrode tabs, in a width wise direction (W) of the electrode assembly 200, substantially parallel to a vertical end surface 106 of the electrode assembly 200. In other words, the folded stacks 302a, 302b extend substantially perpendicular to the height-wise direction (H) of the electrode assembly 200 in which the unfolded stacks of negative electrode tabs 102a, 102b extend. The folded stacks 302a, 302b of electrode tabs extend in opposing directions along the width wise direction (W), as shown in Figure 3, but towards one another in the width wise direction. As such, the folded stacks 302a, 302b overlap, or extend over, the electrode assembly 200. It is noted that the folded stacks 302a, 302b are folded and flattened. As such, they may also be referred to as flattened stacks. However, for ease of reference, they will be referred to as folded stacks 302a, 302b. As shown in the enlarged section of Figure 4a, the folded stack 302a of negative electrode tabs comprises a stack of fifty-eight negative electrode tabs 306a-1 ...306a-58 of the electrode assembly 100a. The folded stack 302a of negative electrode tabs is arranged so that at least a portion of each negative electrode tab 306a-1 ...306a-58 is visible in Figure 4a, and thus accessible for welding from “above”, or, in other words, from a side of the folded stack 302a of negative electrode tabs facing away from the electrode assembly 200. Each negative electrode tab 306a-1 ...306a-58 of the electrode assembly 100a has a length which is less than a width of the electrode assembly 100a. In other words, as the two electrode assemblies 100a, 100b are arranged adjacent one another, the “innermost” negative electrode tab 306a-58 of the electrode assembly 100a when bent, or folded, onto the other electrode assembly 100b does not extend all the way to an opposite width-wise end of the other electrode assembly 100b. As such, in other words, a length of each negative electrode tab 306a-1 ...306a-58 may be less than half the width of the electrode assembly 200 comprising the two electrode assemblies 100a, 100b, so that when bent / folded, there remains a transverse gap 303 between the folded stack 302a of negative electrode tabs. The above also applies for the other example electrode assembly 300c, only that a length of each tab is less than half a width of the entire electrode assembly 300c. It is noted that the electrode assembly may be made up of electrode assemblies of unequal width, in which case the length of each tab may be less than the width of the other electrode assembly over which the stack of tabs extends. The length of each negative electrode tab 306a-1 ...306a-58 is apparent from the “outermost” negative electrode tab 306a-1 shown in Figure 4a, an entire side of which faces away from the electrode assembly 200. This “outermost” negative electrode tab 306a-1 may also be referred to as the “uppermost” negative electrode tab. As already set out above with regards to the folded stack 302a of negative electrode tabs of the electrode assembly 100a, the folded stack 302b of negative electrode tabs of the other electrode assembly 100b also comprises a stack of fifty-eight negative electrode tabs 306b-1 ...306b-58. The folded stack 302b of negative electrode tabs is arranged so that at least a portion of each negative electrode tab 306b-1 ...306b-58 is visible in Figure 4a facing away from the electrode assembly 200, and thus accessible for welding from “above”, or, in other words, from a side of the folded stack 302b of negative electrode tabs facing away from the electrode assembly 200. Each negative electrode tab 306b-1 ...306b-58 of the other electrode assembly 100b, like each negative electrode tab 306a-1 ...306a-58 of the electrode assembly 100a, has a length which is less than a width of the electrode assembly 100b. In other words, as the two electrode assemblies 100a, 100b are arranged adjacent one another, the “innermost” negative electrode tab 306b-58 of the other electrode assembly 100b when bent, or folded, onto the electrode assembly 100a, and thus in in an opposite direction to the folded stack 302a of negative electrode tabs of the electrode assembly 100a, does not extend all the way to an opposite width-wise end of the electrode assembly 100a. There is thus a transverse gap 303 between the folded stack 302b of negative electrode tabs and the opposite width-wise end of the electrode assembly 100a. As already set out above regarding the stacks 102a, 102b of negative electrode tabs, the folded stacks 302a, 302b of negative electrode tabs are offset from one another in a longitudinal direction (L) of the electrode assembly 200, or, in other words, they are provided in different longitudinal positions. Indeed, the longitudinal positions are separated sufficiently so that the folded stacks 102a, 102b, which extend in opposite transverse directions but towards one another, do not overlap. The longitudinal positions may further be separated sufficiently so that there is a longitudinal gap 400 between the folded stacks 102a, 102b. For the avoidance of repetition, it is noted that the arrangement of the folded stack 304a of positive electrodes of the electrode assembly 100a and the folded stack 304b of positive electrodes of the other electrode assembly 100b is substantially the same as the arrangement of the stacks 302a, 302b of negative electrode tabs of the electrode assemblies 100a, 100b. The folded stacks 304a, 304b are provided at different longitudinal positions than the folded stacks 302a, 302b, and are provided at different longitudinal positions from one another, so that they do not overlap and a longitudinal gap is provided between the folded stacks 304 a, 304b. The dimensions of the tabs of the folded stacks 304a, 304b are substantially the same as the dimensions of the tabs of the folded stacks 302a, 302b. The dimensions of the tabs of the folded stacks 304a, 304b and of the tabs of the folded stacks 302a, 302b may differ in some embodiments. It is noted that although the example shown in Figures 3 and 4a was described above as consisting of two electrode assemblies 100a, 100b, the example may instead comprise only a single electrode assembly 200c, as shown in Figure 2c. Figure 4b shows another example electrode assembly 400 according to the present disclosure, which comprises two adjacent electrode assemblies 300c, and is thus made up of two jelly rolls 300c, each having two stacks of positive electrode tabs, and two stacks of negative electrode tabs. The electrode assembly 400 is shown in Figure 4c with the stacks of electrode tabs folded and flattened, as discussed above with reference to electrode assembly 200. Although specific examples are described above with reference to Figures 1 to 4, it is apparent to those skilled in the art that various modifications may be made. For example, at least one of the stacks 102a, 102b, 104a, 104b may be provided not extending in a height wise direction but rather longitudinally, from a longitudinal end surface 108, so that when bent / folded, the respective folded stack 302a, 302b, 304a, 304b extends substantially parallel to the longitudinal end surface 108 of the electrode assembly 200. Similarly, as shown in Figures 4b and 4c, more than two electrode assemblies 102a, 102b may be provided, and the number of electrode tabs in each stack 102a, 102b, 104a, 104b may differ, or the number of electrode tabs in each stack be different to 58. Similarly, at least some of the electrode layers may comprise more than one electrode tab. Figure 5 shows the electrode assembly 200, with a polymer plate 500 covering the vertical end surface 106 of the electrode assembly 200. The polymer plate 500 has cut-outs 502a, 502b, 504a, 504b configured to receive respective ones of the stacks 102a, 102b, 104a, 104b. The material of the polymer plate 500 is non-conductive, and facilitates folding of the stacks 102a, 102b, 104a, 104b onto the height-wise end surface 106 of the electrode assembly 200 and flattening of the stacks 102a, 102b, 104a, 104b. Figure 6 shows the electrode assembly 200 of Figure 5, further having a first negative weld plate 600a laser-welded to the folded stack 302a of negative electrode tabs of the electrode assembly 100a and a second negative weld plate 600b laser-welded to the folded stack 302b of negative electrode tabs of the other electrode assembly 100b. As is apparent from the enlarged view of Figure 6, the negative weld plates 600a, 600b are welded to each tab in the respective folded stack 302a, 302b. This is enabled, as discussed above, by each tab in the respective stacks 302a, 302b having a portion facing away from the electrode assembly 200, and accessible for welding from a side of the folded stack 302b of negative electrode tabs facing away from the electrode assembly 200. As is further shown in Figure 6, the electrode assembly 200 further has a first positive weld plate 602a laser-welded to the folded stack 304a of positive electrode tabs of the electrode assembly 100a and a second positive weld plate 602b laser-welded to the folded stack 304b of positive electrode tabs of the other electrode assembly 100b. As described above for the negative weld plates 600a, 600b, the positive weld plates 602a, 602b are welded to each tab in the respective folded stack 304a, 304b. Figure 7 shows the electrode assembly 200 of Figure 6, having a negative busbar 700 coupled (preferably welded) to the negative weld plates 600a, 600b and a positive busbar 702 coupled (preferably welded) to the positive weld plates 602a, 602b, both busbars 700, 702 being coupled to the respective weld plates on a side facing away from the electrode assembly 200. Connected to the lid 704, and electrically connected with the respective busbars 700, 702, are a negative terminal 706 and a positive terminal 708. The negative and positive busbars 700, 702 are bent into a U shape as shown in Figure 8, having a first straight portion 800 coupled to the respective weld plates, and a second straight portion 802 substantially parallel to the first straight portion 800 coupled (preferably welded) to a lid 704 of a prismatic cell. The first and second straight portions 800, 802 of each current collector 700, 702 are connected by a rounded portion 804. Figure 9a shows a further example electrode assembly 900 according to the present disclosure. Electrode assembly 900 differs from the electrode assemblies discussed above and shown in Figures 1 to 8 in that it is a wound jelly roll rather than a, or a plurality of, stacked jelly rolls. Wound, or rolled, jelly rolls are produced by providing a length of anode, a length of cathode, and two lengths of separators, and winding of the anode, cathode and separator layers together to form alternating anode layers and cathode layers with a separator layer interposed between each pair of anode and cathode layers. Electrode assembly 900 is a vertically wound jelly roll, that is, the electrode layers and separators are wound around a central vertical axis of the jelly roll, so that each layer extends in a substantially longitudinal direction of the electrode assembly and perpendicular to a vertical (height wise) direction. In the wound jelly roll 900, all negative electrodes are formed of the same anode, and thus connected. Similarly, all positive electrodes are formed of the same cathode, and thus connected. Therefore, it is possible, as shown in Figure 9a, to only provide half of the positive electrode layers (e.g. those one side of a central cavity 903 of the wound jelly roll 900) and only half of the negative electrode layers (e.g. those on the other side of the central cavity 903) with electrode tabs. One half of the layers is provided on one side of the central cavity 903, and the other half of layers is provided on the other side of the central cavity 903. In this manner, as shown in Figure 9a, a folded stack 902 of negative electrode tabs extends in a first width wise direction of the electrode assembly 900 to overlap a portion of the electrode assembly 900 not provided with negative electrode tabs. Similarly, a folded stack 904 of positive electrode tabs extends in a second width wise direction, which is opposite to the first width wise direction of the folded stack 902 of negative electrode tabs, to overlap a portion of the electrode assembly 900 not provided with positive electrode tabs. The tabs in both stacks 902, 904 extend substantially parallel to a vertical end surface 906 of the electrode assembly 900. Figure 9b shows a further example electrode assembly 900b according to the present disclosure. Like electrode assembly 900, electrode assembly 900b is a wound jelly roll, rather than a stacked jelly roll. However, unlike in electrode assembly 900, an electrode tab projects from each electrode layer of the electrode assembly 900b. However, like electrode assembly 200 described above, electrode assembly 900b has two folded stacks 902a, 902b of negative electrode tabs, and two folded stacks 904a, 904b of positive electrode tabs. One of each type of stack 902b, 904a is provided on one side of the central cavity 906, and the other of each type of stack 902a, 904b is provided on the other side of the central cavity 906. The folded stacks 902a, 902b of negative electrode tabs are provided adjacent one another along the length of the electrode assembly 900b, and folded stacks 904a, 904b of positive electrode tabs are provided adjacent one another along the length of the electrode assembly 900b. The tabs of each stack 902a, 902b, 904a, 904b extend substantially parallel to the vertical end surface 906 of the electrode assembly 900b. As described above, a length of each tab of each stack is less than half a width of the electrode assembly 900b, so that the folded stacks do not project horizontally beyond the edge of the electrode assembly 900b. Figure 9c shows a further example electrode assembly 900c according to the present disclosure. The electrode assembly 900c consists of two wound jelly rolls 900b as described above, arranged adjacent one another. Figure 9d shows a further example electrode assembly 900d according to the present disclosure. The electrode assembly 900d is similar to the electrode assembly 900, in that it is a wound jelly roll, in which only half of the positive electrode layers, and half of the negative electrode layers, are provided with electrode tabs. As set out above, this is possible because the negative electrode layers are all connected as they are formed from a single anode, and the positive electrode layers are all connected as they are formed from a single cathode. However, unlike in electrode assembly 900, the stack 902 of negative electrode tabs and the stack 904 of positive electrode tabs are provided on the same side of the central cavity 903. In the electrode assembly 900d, the folded stacks 902, 904 extend substantially parallel to the vertical end surface 906 of the electrode assembly 900d. However, unlike in electrode assembly 900, the tabs of the stacks 902, 904 extend in the same (width-wise) direction, and extend over the same portion of the electrode assembly 900d. The stacks in the electrode assemblies 900a-900d may be provided in the desired longitudinal positions by forming, or cutting, the lengths of anode and cathode accordingly. This is in contrast to the stacked jelly rolls, or electrode assemblies, described above, in which sheets, or layers, of electrodes having tabs in different longitudinal positions may be provided. Figure 10 shows a schematic view of an example battery cell 1000, having a prismatic housing 1002 including the lid 704 and an electrode assembly 200 as set out above, comprising jelly rolls 100a and 100b. The electrode assembly 200 comprises a polymer plate 500 having cut outs and covering a vertical end surface of the electrode assembly 200. Electrode tabs are formed to extend parallel to the vertical end surface, on top of the polymer plate 500, to form a folded stack 1004 of electrode tabs. The folded stack 1004 provides a weld zone 1006 for welding to each of the tabs in the folded stack 1004. The weld zone 1006 for the tabs in the folded stack 1004 of the jelly roll 100 extends over, or overlaps, the jelly roll 100. Figure 11a shows a schematic view of a further example battery cell 1100, in which the polymer plate 500, or the cut outs of the polymer plate 500, are arranged differently to battery cell 1000. In particular, the polymer plate 500 is arranged so that some of the electrode tabs 1101 are bent around an end of the polymer plate 500, or the cut outs of the polymer plate 500. This allows the polymer plate 500 to extend over (or overlap) a portion of the jelly roll 100a. In this way, the weld zone 1006 does not overlap a portion of the jelly roll 100a not covered by the polymer plate 500. Thus, the polymer plate 500 may thus prevent radiation during laser welding from penetrating into the electrode layers, preventing potential damage to the electrode layers. Figure 11b shows a further example electrode assembly 1110 which is similar to electrode assembly 200. The electrode assembly 1110, like electrode assembly 200, is a stacked jelly roll having two folded stacks 1102a, 1102b of negative electrode tabs, and two folded stacks of positive electrode tabs (not shown in Figure 11b). However, in contrast to electrode assembly 200, the two folded stacks 1102a, 1102b of negative electrode tabs of electrode assembly 1110 extend in a width wise direction (W) of the electrode assembly 1110 substantially parallel to, and adjacent to, a longitudinal end surface 108 of the electrode assembly 1110. In other words, the folded stacks 1102a, 1102b extend substantially perpendicular to a length-wise direction of the electrode assembly 1100. It is noted that before folding, the negative electrode tabs extend in a longitudinal direction, whereas in the stacked jelly roll 200, before folding, the negative electrode tabs extend in a vertical direction, as shown in Figure 2a. That is, while the stacked jelly roll 200 is made up of electrode layers in which tabs are formed to project in a vertical direction, the stacked jelly roll 1110 is made up of electrode layers in which tabs are formed to project in a longitudinal direction. The folded stacks 1102a, 1102b extend in opposing directions along the width wise direction (W), as shown in Figure 11b, but towards one another in the width wise direction. As such, the folded stacks 1102a, 1102b overlap, or extend over, the electrode assembly 1110. Two folded stacks of positive electrode tabs of electrode assembly 1110, not visible in Figure 11b because of the perspective shown, extend in a width wise direction (W) of the electrode assembly 1110 substantially parallel to, and adjacent to, a second longitudinal end surface (not shown) opposite the first longitudinal end surface 108 of the electrode assembly 1110, similar to folded stacks 1102a, 1102b. In other words, the folded stacks of positive electrode tabs extend substantially perpendicular to a length-wise direction of the electrode assembly 1100. It is noted that before folding, the positive electrode tabs extend in an opposite longitudinal direction, and from a different longitudinal end surface, than the negative electrode tabs, whereas in the stacked jelly roll 200, before folding, the positive electrode tabs extend in the same (vertical) direction, and from the same (vertical) end surface, as the negative electrode tabs, as shown in Figure 2a. The folded stacks of positive electrode tabs extend in opposing directions along the width wise direction (W), but towards one another in the width wise direction, similar to the folded stacks 1102a, 1102b of the negative electrode tabs. As such, the folded stacks of positive electrode tabs overlap, or extend over, the electrode assembly 1110. Figure 11c shows a further example electrode assembly 1120 which is similar to electrode assembly 900b. The electrode assembly 1120, like electrode assembly 900b, is a wound jelly roll having two folded stacks 1102a, 1102b of negative electrode tabs, and two folded stacks of positive electrode tabs (not shown in Figure 11c). However, there are two main differences between the electrode assembly 1120 and electrode assembly 900b. The electrode assembly 1120 is a longitudinally wound jelly roll, that is, the electrode layers and separators are wound around a central longitudinal axis of the jelly roll, so that each layer extends in a substantially vertical direction of the electrode assembly and perpendicular to a horizontal (width wise) direction. Further, in contrast to electrode assembly 900b, the two folded stacks 1102a, 1102b of negative electrode tabs of electrode assembly 1120, extend in a width wise direction (W) of the electrode assembly 1120 substantially parallel to, and adjacent to, a longitudinal end surface 108 of the electrode assembly 1120, similar to the two folded stacks 1102a, 1102b of negative electrode tabs of electrode assembly 1110 shown in Figure 11b. Two folded stacks of positive electrode tabs of electrode assembly 1120, not visible in Figure 11c because of the perspective shown, extend in a width wise direction (W) of the electrode assembly 1120 substantially parallel to, and adjacent to, a second longitudinal end surface (not shown) opposite the first longitudinal end surface 108 of the electrode assembly 1120 similar to the two folded stacks 1102a, 1102b, and as set out above for the folded stacks of positive electrode tabs of electrode assembly 1110 shown in Figure 11b. An example method 1200 for manufacturing a battery cell is described below and shown in Figure 12. The example method comprises a step of providing 1202 an electrode assembly such as electrode assembly 200 or 400. The electrode assembly has a plurality of positive electrode layers; and a plurality of negative electrode layers, wherein the positive electrode layers and negative electrode layers are arranged alternately, with a separator layer interposed between adjacent positive and negative electrode layers, wherein positive electrode tabs protrude from the plurality of positive electrode layers forming at least one stack of positive electrode tabs, and wherein negative electrode tabs protrude from the plurality of negative electrode layers forming at least one stack of negative electrode tabs. Providing 1202 may refer to simply providing a pre-made electrode assembly, or the method 1200 may comprise providing by manufacturing an electrode assembly as set out above. The method 1200 further comprises a step of forming 1204 the electrode tabs to extend substantially parallel to an end surface of the electrode assembly. In particular, the electrode tabs may be formed to extend substantially parallel to a vertical end surface of the electrode assembly. Forming 1204 the electrode tabs may alternatively be referred to as flattening the electrode tabs (relative to a height-wise direction of the electrode assembly), so that they extend parallel to an end surface of the electrode assembly. Before flattening, the electrode tabs project from the electrodes in a vertical, or height-wise, direction of the electrode assembly. Forming 1204 may comprise ultrasonically kneading the tabs to flatten them. Alternatively, forming 1204 comprises, as set out in example method 1300 shown in Figure 13, folding 1304 the tabs and pressing 1308 the tabs. In one particular example, the tabs may be pressed with a force of 1 N. In this particular example, all tabs may be folded simultaneously, or the tabs of different stack may be folded consecutively. In one example of the methods 1200 and 1300, the step of forming 1204 further comprises forming the electrode tabs of at least one of the stacks of electrode tabs to extend in a first direction, and forming the electrode tabs of at least one other of the stacks of electrode tabs to extend in a second direction which is substantially opposite the first direction. In one example of the methods 1200 and 1300, the step of forming 1204 further comprises forming the tabs of half of the stacks to extend in the first direction, and forming the tabs of the other half of the stacks to extend in the second direction. In some examples of the methods 1200 and 1300, the positive electrode tabs protruding from the plurality of positive electrode layers form a plurality of stacks of positive electrode tabs, and the negative electrode tabs protruding from the plurality of negative electrode layers form a plurality of stacks of negative electrode tabs. Additionally or alternatively, some examples of the methods 1200 and 1300 further comprise a step of providing at least one further electrode assembly. As set out above with regards to the electrode assembly, providing may comprise providing a pre-made further electrode assembly, or manufacturing a further electrode assembly. In some examples of the methods 1200 and 1300, the step of forming 1204 further comprises forming the tabs so that each tab in each stack is accessible from a side facing away from the electrode assembly. In this manner, a better electrical connection may be achieved, and fewer hot spots will be generated during charging, in particular during fast charging. As shown in Figure 14, a further example method 1400 comprises steps 1202 and 1204 of example method 1200, and further comprises a step of laser welding 1400 a positive weld plate to each tab in the, or each, stack of negative electrode tabs, or to each tab in one of the stacks of negative electrode tabs, from the side facing away from the electrode assembly; and / or laser welding 1400 a negative weld plate to each tab in the, or each, stack of positive electrode tabs, or to each tab in one of the stacks of positive electrode tabs, from a side facing away from the electrode assembly. In one particular example, the step of laser welding 1400 may comprise generating a laser beam having a wavelength of 1070 nm, and a power of 150 W, and moving the laser beam across a weld zone, such as weld zone 1006, at a speed of 50 mm / s. In some examples of the method 1400, the method further comprises at least one of: welding, preferably laser welding, a positive weld plate to each tab in the, or each, stack of positive electrode tabs, or to each tab in one of the stacks of positive electrode tabs, from the side facing away from the electrode assembly; and welding, preferably laser welding, a negative weld plate to each tab in the, or each, stack of negative electrode tabs, or to each tab in one of the stacks of negative electrode tabs, from a side facing away from the electrode assembly. As set out above, by forming 1204 the electrode tabs to extend substantially parallel to an end surface of the electrode assembly, and laser welding 1400 to a side of (each) stack of electrode tabs facing away from the electrode assembly, the electrode assembly may be electrically connected to a lid in a more compact manner. Indeed, in this way, an electrode assembly and / or a battery cell may be manufactured in fewer steps with fewer welds, as the electrode tabs do not need to be clinched and ultrasonically welded together. In some examples of the method 1400, before laser welding, the method may comprise a step of providing a plate between the stack of electrode tabs and the electrode assembly for preventing the laser penetrating into the electrode assembly. This may prevent the laser damaging the electrode assembly during laser welding.

Claims

1. A battery cell comprising:an electrode assembly comprising:a plurality of positive electrode layers; anda plurality of negative electrode layers,wherein the positive electrode layers and negative electrode layers are arranged alternately, with a separator layer interposed between adjacent positive and negative electrode layers,wherein positive electrode tabs protrude from the plurality of positive electrode layers forming at least one stack of positive electrode tabs and negative electrode tabs protrude from the plurality of negative electrode layers forming at least one stack of negative electrode tabs, andwherein the electrode tabs are formed to extend substantially parallel to an end surface of the electrode assembly.

2. A battery cell according to claim 1, wherein the electrode tabs of at least one of the stacks of electrode tabs are formed to extend in a first direction, and wherein the electrode tabs of at least one other one of the stacks of electrode tabs are formed to extend in a second direction which is substantially opposite the first direction.

3. A battery cell according to claim 2. wherein the tabs of half of the stacks extend in the first direction, and wherein the tabs of the other half of the stacks extend in the second direction.

4. A battery cell according to 1,2, or 3, wherein the stacks of electrode tabs are offset along a dimension of the electrode assembly, and preferably wherein the stacks of electrode tabs do not overlap one another.

5. A battery cell according to any preceding claim, wherein the positive electrode tabs form a plurality of stacks of positive electrode tabs, and wherein the negative electrode tabs form a plurality of stacks of negative electrode tabs.

6. A battery cell according to claim 5, wherein the stacks of positive electrode tabs are adjacent one another along a, or the, dimension of the electrode assembly, and thestacks of negative electrode tabs are adjacent one another along a, or the, dimension of the electrode assembly.

7. A battery cell according to claim 5 or 6, wherein the tabs of one stack of positive electrode tabs, or of half of the stacks of positive electrode tabs, and the tabs of one stack of negative electrode tabs, or of half of the stacks of negative electrode tabs, extend in a, or the, first direction; and wherein the tabs of the other stack of positive electrode tabs, or of the other half of the stacks of positive electrode tabs, and the tabs of the other stack of negative electrode tabs, or of the other half of the stacks of negative electrode tabs, extend in a, or the, second direction which is substantially opposite the first direction.

8. A battery cell according to claim 5, 6, or 7, wherein the electrode assembly is composed of at least a first electrode assembly and a second electrode assembly, each of the first and second electrode assembly comprising at least one of the stacks of positive electrode tabs and at least one of the stacks of negative electrode tabs.

9. A battery cell according to claim 8, wherein the first electrode assembly and the second electrode assembly differ in a position of the stacks of electrode tabs along a, or the, dimension of the respective electrode assembly.

10. A battery cell according to claim 8 or 9, wherein at least some of the electrode tabs of the first electrode assembly overlap the second electrode assembly, and wherein at least some of the electrode tabs of the second electrode assembly overlap the first electrode assembly.

11. A battery cell according to claim 8, 9, or 10, wherein the stacks of the first electrode assembly and the stacks of the second electrode assembly are arranged alternately along a, or the, dimension of the electrode assemblies; and / or wherein each of the tabs has a length which is less than a width of the first electrode assembly and / or a width of the second electrode assembly.

12. A battery cell according to any preceding claim, wherein each of the tabs has a length which is less than half a width of the electrode assembly; and / or wherein a length of each tab is substantially identical.

13. A battery cell according to any preceding claim, comprising at least one further electrode assembly adjacent the electrode assembly.

14. A battery cell according to claim 13, wherein the, or each, further electrode assembly is substantially identical to the electrode assembly.

15. A battery cell according to any preceding claim, wherein the, or each, electrode assembly is a stacked jelly roll, or a wound jelly roll.

16. A battery cell according to any preceding claim, wherein each of the stacks is formed so that each tab in the respective stack is accessible from a side facing away from the electrode assembly.

17. A battery cell according to claim 16, further comprising at least one of:a negative weld plate welded to each tab in the, or each, stack of negative electrode tabs, or to each tab in one of the stacks of negative electrode tabs, from the side facing away from the electrode assembly; anda positive weld plate welded to each tab in the, or each, stack of positive electrode tabs, or to each tab in one of the stacks of positive electrode tabs, from a side facing away from the electrode assembly.

18. A battery cell according to claim 17, further comprising at least one of:a positive busbar welded to the, or each, positive weld plate; anda negative busbar welded to the, or each, negative weld plate, optionally wherein the, or each, busbar is coupled to a lid of the battery cell, and further optionally wherein the, or each, busbar is bent around so that a portion of the busbar is welded to the respective weld plate and a portion of the busbar coupled to the lid are substantially parallel.

19. A battery cell according to any preceding claim, further comprising a non-conductive plate covering the, or a, end surface of the, or each, electrode assembly, preferably a polymer plate, comprising cut-outs for the stacks of electrode tabs, and optionally wherein the cut-outs are arranged such that at least some of the tabs in at least one of the stacks of electrode tabs are curved around the non-conductive plate.

20. A method for manufacturing a battery cell, comprising:providing an electrode assembly comprising:a plurality of positive electrode layers; and a plurality of negative electrode layers, wherein the positive electrode layers and negativeelectrode layers are arranged alternately, with a separator layer interposed between adjacent positive and negative electrode layers, wherein positive electrode tabs protrude from the plurality of positive electrode layers forming at least one stack of positive electrode tabs, and wherein negative electrode tabs protrude from the plurality of negative electrode layers forming at least one stack of negative electrode tabs; andforming the electrode tabs to extend substantially parallel to an end surface of the electrode assembly, andoptionally wherein forming the electrode tabs comprises at least one of:ultrasonically kneading the tabs; and folding and pressing the tabs.

21. A method according to claim 20, wherein forming the electrode tabs comprises forming the electrode tabs of at least one of the stacks of electrode tabs to extend in a first direction, and forming the electrode tabs of at least one other of the stacks of electrode tabs to extend in a second direction which is substantially opposite the first direction, and preferably wherein forming the electrode tabs further comprises forming the tabs of half of the stacks to extend in the first direction, and forming the tabs of the other half of the stacks to extend in the second direction.

22. A method according to claim 20 or 21, wherein the positive electrode tabs protruding from the plurality of positive electrode layers form a plurality of stacks of positive electrode tabs, and wherein the negative electrode tabs protruding from the plurality of negative electrode layers form a plurality of stacks of negative electrode tabs; and / or further comprising providing at least one further electrode assembly.

23. A method according to claim 20, 21, or 22, wherein forming the electrode tabs comprises forming the tabs so that each tab in each stack is accessible from a side facing away from the electrode assembly.

24. A method according to any of claims 20 to 23, further comprising at least one of: welding, preferably laser welding, a positive weld plate to each tab in the, or each, stack of negative electrode tabs, or to each tab in one of the stacks of negative electrode tabs, from the side facing away from the electrode assembly; andwelding, preferably laser welding, a negative weld plate to each tab in the, or each, stack of positive electrode tabs, or to each tab in one of the stacks of positive electrode tabs, from a side facing away from the electrode assembly.

25. A method according to claim 24, further comprising:before laser welding, providing a plate between the stack of electrode tabs and the electrode assembly for preventing the laser penetrating into the electrode assembly.

26. An electrode assembly for a battery cell, comprising: a plurality of positive electrode layers; and a plurality of negative electrode layers, wherein the positive electrode layers and negative electrode layers are arranged alternately, with a separator layer interposed between adjacent positive and negative electrode layers, wherein positive electrode tabs protrude from the plurality of positive electrode layers forming at least one stack of positive electrode tabs and negative electrode tabs protrude from the plurality of negative electrode layers forming at least one stack of negative electrode tabs, and wherein the electrode tabs are formed to extend substantially parallel to an end surface of the electrode assembly.

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