Method for manufacturing a battery cell using compressive forces

Abstract

A method for manufacturing a battery cell is performed during one or more manufacturing stages comprising at least one of: a dry stack time interval, electrolyte introduction, soaking, and precharge. The method comprises: providing a substantially rigid cell can (102) having a cell stack (302) disposed therein, the cell stack comprising a plurality of sheetlike cell layers arranged in a stack-like or roll configuration; and applying a supplemental, actively controllable compressive force to the cell stack (302) directly or indirectly by one or more inner walls of the cell can (102), wherein the supplemental compressive force is applied in addition to any forces resulting from electrolyte introduction into the cell can (102), and the supplemental compressive force is applied in a uniaxial direction that is substantially normal to the sheetlike cell layers, causing the sheetlike cell layers to have a denser configuration in the uniaxial direction compared to when no supplemental compressive force is applied.

Description

METHOD FOR MANUFACTURING A BATTERY CELL USING COMPRESSIVE FORCESBACKGROUND

[0001] The present invention relates to battery cell manufacturing. Battery cells, such as lithium-ion battery cells, are manufactured through a series of process steps that involve assembling different components. Typically, the manufacturing of a lithium-ion battery cell involves the following steps:1- Electrode Preparation: A mixture of metal, metal oxide, or metal phosphate compounds (such as lithium cobalt oxide, lithium iron phosphate, lithium nickel manganese oxide, lithium manganese oxide, or lithium nickel manganese cobalt oxide) is coated onto a metal foil (typically aluminum) to form the cathode. Anode active material such as graphite or silicon compounds are coated onto another metal foil (typically copper) to form the anode.2. Separator Preparation: A thin, porous separator material is placed between the cathode and anode to prevent them from coming into direct contact with each other and causing a short circuit. The separator material allows the lithium ions to pass through while blocking the flow of electrons.3- Cell Can Assembly: The prepared cathode, anode, and separator are assembled into a cell can, leaving room for the electrolyte to be filled. The cell can is typically a rigid, prismatic, container, but a variety of shapes exist, as will be described in further detail below.4. Electrolyte Filling: The electrolyte, which is typically a lithium salt dissolved in a solvent (for example, a mixture of ethylene carbonate and diethyl carbonate), is injected into the cell can. The electrolyte facilitates the movement of lithium ions between the cathode and anode during charge and discharge. Electrolyte filling can be facilitated by the use of pressure, vacuum, or both.5- Cell Can Sealing: Once the cell can is filled with electrolyte, it is sealed to prevent leaks and ensure the integrity of the cell.6. Soaking, Precharge, and Formation: The newly assembled battery cell is allowed to soak, so that electrolyte can fully penetrate all porosity in the cell, both in separators and electrodes. The cell undergoes an initial charging and discharging cycle called "precharge and formation," or a combination thereof. Precharge and formation stabilize the battery celland optimizes its performance. Subsequently, the cell is held (typically at an elevated temperature) for a number of hours in the “aging” process, to allow the cell to equilibrate and reach a stable output voltage.7. Quality Control: After formation, the battery cell undergoes rigorous testing and quality control measures to ensure the battery cell meets safety and performance standards.8- Module and Pack Assembly: Lithium-ion battery cells are then assembled into modules, which are groups of battery cells arranged in a specific configuration to achieve the desired voltage and capacity. Multiple modules can be combined to form a battery pack, along with the necessary electronics (such as a battery management system) for monitoring and controlling the battery pack's performance.

[0002] It is known that the stack of electrodes in a lithium-ion cell experience expansion during electrolyte filling due to a soaking mechanism. Further expansion occurs during the precharge, formation, and aging steps, as chemical compounds in the electrolyte react at the electrode surfaces to form solid passivating layers. It is also known that expansion and contraction, both reversible and irreversible, occur during the cycling of the battery cell. This is mainly due to a volume change experienced by the cathode active material and anode active material during charge and discharge. The volume change of the intercalation or alloying anode active material will usually be higher than the one of the insertion cathode active material.

[0003] These phenomena cause the stack volume at top of charge to be higher than the volume at bottom of charge, leading to what is commonly referred to as “cell breathing.” In addition to these mechanisms, irreversible swelling can result from the decomposition of the electrolyte at the surface of the electrodes. This creates the formation of a stable interface layer called SEI (Solid Electrolyte Interphase) at the anode, and CEI (Cathode Electrolyte Interphase) at the cathode. Irreversible swelling can also be caused by reorganization of the particles of active material into a less compact configuration. Other processes, such as lithium trapping in the active materials or solvent intercalation between graphene layers of different carbon-based electrode materials, can also lead to swelling of the stack.

[0004] The rigid case of the conventional prismatic battery cell format provides benefits in packaging efficiency, mechanical stability, heat transfer, and safety. However, as the cell stack swells, the cell can side surfaces are prone to flex, due to the pressure exerted by the cell stack on the inner walls of the cell can. This may lead to various problems during assembly and use of the battery packs, and if the cell can needs to maintain its original shape, it may be necessaryto reduce the size of the cell stack, thus leading to a lower battery capacity. Thus, it would be desirable to have a solution that minimizes, or at least reduces, issues associated with cell stack expansion and contraction, both during manufacturing and during regular use of the battery cell.SUMMARY

[0005] In one aspect, a method for manufacturing a battery cell, the method being performed during one or more manufacturing stages includes at least one of: electrolyte introduction, soaking, precharge, formation and aging. The method includes providing a substantially rigid cell can having a cell stack disposed therein. The cell stack includes a plurality of sheetlike cell layers arranged in a stack-like or roll configuration. A supplemental, actively controllable compressive force is applied to the cell stack directly or indirectly by one or more inner walls of the cell can. The supplemental compressive force is applied in addition to any forces resulting from electrolyte introduction into the cell can, and the supplemental compressive force is applied in a uniaxial direction that is substantially normal to the sheetlike cell layers, causing the sheetlike cell layers to have a denser configuration in the uniaxial direction compared to when no supplemental compressive force is applied.

[0006] In some embodiments, the compressive force is uniformly distributed across the cell layer surface.

[0007] In some embodiments, the supplemental compressive force is variable across the cell layer surface.

[0008] In some embodiments, the compressive force is variable over time during the manufacturing stages.

[0009] In some embodiments, the cell can in is a prismatic cell can.

[0010] In some embodiments, the cell can is a cylindrical cell can and the cell stack includes a roll of sheetlike cell layers.

[0011] In some embodiments, applying the supplemental compressive force includes applying external compression to one or more outer walls of the cell can to displace a surface section of the cell can.

[0012] In some embodiments, the cell can includes a flexural feature on a cell can front surface, a cell can back surface, and / or a cell can side surface, allowing the cell can front surface and / or cell can back surface to translate in a direction normal to the cell can frontsurface and / or cell can back surface to apply the supplemental compressive force to the cell stack, and the flexural feature is arranged so that the supplemental compressive force is applied uniformly across a side of the cell stack.

[0013] In some embodiments, a rigid element is provided between the side surface of the cell stack and a respective inner wall of the cell can, the rigid element being arranged to distribute an externally applied pressure evenly across the side surface of the cell stack.

[0014] In some embodiments, the interior of the cell can further includes one or more fluid- tight bags in communication with the environment outside the cell can, and applying a supplemental compressive force to the cell stack includes adjusting a fluid pressure inside or outside the one or more fluid-tight bags to cause compression of the cell stack as a result of direct or indirect contact with the inner wall of the cell can.

[0015] In some embodiments, one or more swell pads are included in the cell stack, and applying the supplemental compressive force to the cell stack is achieved by the one or more swell pads pushing a cell stack layer against one or more of the inner walls of the cell can, directly or indirectly.

[0016] In some embodiments, one or more electrolyte-filled bags are inserted into the cell stack, where the one or more electrolyte-filled bags are configured to release their contents as a result of the cell stack swelling to a size at which compression against the inner wall of the cell can is achievable without using the electrolyte-filled bag.

[0017] In some embodiments, a dissolvable or meltable shim is placed into the cell can together with the cell stack, and the shim dissolved or melted into the electrolyte as the cell stack swells.

[0018] In some embodiments, the substantially rigid cell can is made from metal, plastic, or a combination thereof.

[0019] In some embodiments, a cross section of the prismatic cell can is substantially square, substantially rectangular, substantially hexagonal, or substantially octagonal.

[0020] In some embodiments, a fluid-tight bag is inserted to form a center cylinder of the roll, and where applying the supplemental compressive force includes pressurizing the fluid-tight bag to apply a radial outwards pressure onto the roll.

[0021] In some embodiments, applying external compression to one or more of the outer walls of the cell can is done by stamp tool having a compression plate that is smaller than the cell can front surface and having an outline that substantially corresponds to the cell stack layer.

[0022] In some embodiments, the cell can includes a ductile area allowing the surface section of the cell can to translate in a direction normal to the surface section to apply the supplemental compressive force to the cell stack.

[0023] In some embodiments, the ductile area is provided after construction of the cell can, such as by local heating of the cell can.

[0024] In some embodiments, the flexural feature includes a corrugated section on the cell can.

[0025] In some embodiments, the rigid element includes an inner surface and an outer surface, the outer surface being arranged to define a desired final bulge shape of the cell can after swelling.

[0026] In some embodiments, one or more fluid-tight bags are included in the cell stack, and the compression is achieved by adjusting the differential pressure between the inside of the one or more the fluid-tight bags and the pressure inside the cell can.

[0027] In some embodiments, the environment inside the cell can is pressurized, outside the one or more fluid-tight bags, based on the state of one or more fluid-tight bags, to reduce the risk of the one or more fluid-tight bags breaking due to a pressure difference between the inside and outside of the one or more fluid-tight bags.

[0028] In some embodiments, at least one of the one or more fluid-tight bags is a single bag surrounding the cell stack, and where the compression is achieved by pressurizing a volume between the fluid-tight bag and the inner wall of the cell can with a pressurized medium.

[0029] In some embodiments, the pressurized medium is released when the stack swelling is sufficient to apply the supplemental compressive force to the cell stack by the cell stack touching the inner wall of the cell can through the fluid-tight bag.

[0030] In some embodiments, the dissolvable shim is made from one of: LiPF6 salt, frozen electrolyte carbonate solvents, hydrocarbon wax, or extruded polystyrene foam.

[0031] Other technical features may be readily apparent to one skilled in the art from the following figures, descriptions, and claims.BRIEF DESCRIPTION OF THE DRAWINGS

[0032] In the following, the invention will be described in detail, with reference to exemplifying embodiments of the invention and to the enclosed drawings, wherein:

[0033] FIG. 1 illustrates a prismatic cell can, in accordance with some embodiments.

[0034] FIG. 2 illustrates a prismatic cell can, in accordance with some embodiments.

[0035] FIG. 3 illustrates a prismatic cell can having a flexural feature, in accordance with some embodiments.

[0036] FIG. 4 illustrates a prismatic cell can having a flexural feature, in accordance with some embodiments.

[0037] FIG. 5 illustrates a prismatic cell can including a stiff plate, in accordance with some embodiments.

[0038] FIG. 6 illustrates a prismatic cell can including a fluid-tight bag, in accordance with some embodiments.

[0039] FIG. 7 illustrates a prismatic cell can including a fluid-tight pouch cell, in accordance with some embodiments.

[0040] FIG. 8 illustrates a prismatic cell can including a swell pad, in accordance with some embodiments.

[0041] FIG. 9 illustrates a cell stack with components facilitating insertion into a cell can, in accordance with some embodiments.

[0042] FIG. 10 illustrates a prismatic cell can including an electrolyte-filled bag, in accordance with some embodiments.

[0043] FIG. 11 illustrates a prismatic cell can including an internal dissolvable shim, in accordance with some embodiments.

[0044] FIG. 12 illustrates a compression plate for applying a compressive force, in accordance with some embodiments.

[0045] FIG. 13 illustrates how a compressive force can be applied to the cell stack during a manufacturing process, in accordance with some embodiments.

[0046] FIG. 14 illustrates how a compressive force can be applied to the cell stack during a manufacturing process, in accordance with some embodiments.

[0047] FIG. 15 illustrates how a compressive force can be applied to the cell stack during a manufacturing process, in accordance with some embodiments.

[0048] FIG. 16 illustrates how a compressive force can be applied to the cell stack during a manufacturing process, in accordance with some embodiments.

[0049] FIG. 17 illustrates cylindrical battery cell, in accordance with some embodiments.

[0050] FIG. 18 illustrates schematically how the thickness of the cell stack varies over time during the different manufacturing stages, in accordance with some embodiments.

[0051] Like reference numerals in the drawings indicate like features.DETAILED DESCRIPTION

[0052] The various embodiments described herein relate to techniques for manufacturing battery cells (hereinafter simply referred to as cells). In particular, the various embodiments of the present invention relate to methods for subjecting the electrode layers in battery cells in a substantially rigid cell can to a compressive force during cell manufacturing, specifically, from the time electrolyte is first introduced into the cell to the completion of the formation and aging stages of the manufacturing process. The inventors have realized that providing such a compressive force during manufacturing, typically corresponding to approximately 1-10 bar, may result in various performance and structural advantages to the cell, which can avoid not only the above-mentioned issues, but also provide further advantages, such as:• Improved volumetric energy density, which enables smaller cells with maintained capacity, or alternatively same size of cells with larger capacity.• Better-controlled, more-ordered surface interphase layers (like anode SEI), which benefits cycle life.• Better rate capability due to maintained minimal distance between active material and current collector.• Better expulsion of gas bubbles, and therefore better anode-cathode coupling during early cycles of the cell, which helps minimize local lithium plating and improves cell durability.• Greater determinism and positional control over where electrode edges end up as active layers undergo volume changes during electrolyte filling and formation; improving yield.• Minimizing the risk of bunching, creasing, and wrinkling of layers in the cell stack.• Quicker and more optimal electrolyte distribution during electrolyte filling.

[0053] As will be described in further detail below, the compressive force can be controlled throughout manufacturing and can be constant or vary continuously and / or stepwise over time to optimize the efficiency of the various manufacturing steps and the performance of the final module or battery pack. For example, during the electrolyte filling step, variable stack compression (e.g., over time, stack surface, and strength) can be used to optimize filling speed and the distribution of electrolyte in the cell. The application of the compressive force can also be combined with a general constant or variable hydrostatic pressure applied via liquid electrolyte filled into the cell can during the manufacturing of the cell.

[0054] The operating principles of the invention will be described by way of example and with reference to three main categories of embodiments.

[0055] The first category of embodiments pertains to a prismatic cell can design that enables a greater range of cell can surface displacement during the application of an external compressive force to an outer wall of the cell can. The displaced cell surface(s) applies a controllable compressive force to the cell stack, such as during electrolyte filling, formation, and aging.

[0056] The second category of embodiments pertains to designs and methods for applying a controllable compressive force to the cell stack inside a prismatic cell can by manipulating the size and / or position of one or more components installed in or next to the cell stack inside the cell can, such as during electrolyte filling, formation, and aging.

[0057] The third category of embodiments pertains to methods for compactly installing the cell stack during assembly, to enable a sufficient compressive force to be applied to the cell stack during electrolyte filling, formation, and aging, by using an internal swell pad.

[0058] As was noted above, the first category of embodiments pertains to a cell design that allows a force to be applied to one or more of the outer walls of the cell can, in order to displace one or more outer wall sections, while maintaining cell can integrity. As a result of the displacement of the outer wall section(s), the corresponding inner wall section(s) will be displaced and subject the cell stack to a compressive force. The compressive force is directed substantially perpendicularly to a cell layer surface of the cell stack and may vary over time during various stages of the manufacturing process. The magnitude of force to apply and when to apply it can be selected based on the desired outcome and particular situation at hand, but as a general principle, it may be advantageous to cycle the compressive force, such as pulse a magnitude of the compressive force, for example, during the electrolyte filling process, as thismay speed up the processes of getting rid of bubbles (for example, by squeezing) and letting electrolyte soak into the electrodes (for example, by relaxing).

[0059] In each of the embodiment categories, the compressive force can be exerted in a controllable manner in terms of compressive force magnitude as a function of time. In some embodiments, the force can be an at least section-wise constant force over time, and in some embodiments the force is arranged to vary at least section-wise continuously over time. Figs. 18-x illustrate various examples of how the compressive force can be varied to achieve various goals during electrolyte filling, formation and / or aging of the cell. For each of the embodiment categories with respect to the cell itself, reference is made to FIG. 18 for information regarding how the compressive force can be varied over time.

[0060] In this first category of embodiments, as is also the case in the other categories described herein, the compressive force is applied at least, substantially, or completely perpendicularly to a cell layer surface of the cell stack. In particular, the cell layer surface can be curved but is generally a sheet-like two-dimensional surface. For instance, in a prismatic cell the cell layer surface can be flat; whereas in a cylindrical cell the cell layer surface can be curved along one of its dimensions of extension. In some embodiments of the present invention, the compressive force is only, or at least substantially only, applied normally to the cell layer surface. This is in contrast to a force mediated via a hydrostatic pressure generally prevailing in the filled liquid electrolyte, for instance by pressing electrolyte liquid into the cell can at an overpressure, hence resulting in a corresponding hydrostatic overpressure inside the cell can. Such a hydrostatic pressure will of course exert a force onto the cell layer surface that is normal to the cell layer surface, but it will also exert a force onto an end side of the cell layer, which for instance can be perpendicular to the force applied normal to the cell layer surface. Also, an electrolyte in which the cell stack is immersed will provide both a force and a counterforce to each cell layer, hence not resulting in any net compression of the cell stack.

[0061] The compressive force applied by the various mechanisms described herein can be applied in a uniaxial manner, where the force vector field representing the compressive force only contains force vectors that are locally normal to the two-dimensional cell layer surface. In the example of a prismatic cell, the force vectors will all be parallel, or at least substantially parallel. At the edges of the cell layer surface, the compressive force will then not, like a hydrostatic pressure, apply a significant force that is non-parallel or perpendicular to the compressive force applied normal to the cell layer surface. It may be the case that thecompressive force field contains force field vectors near the edges of the cell layer surface that are not normal to the cell layer surface in their local vicinity, but such force field vectors are then generally of smaller magnitude as compared to the cell layer surface-normal force field vectors of the compressive force field. Generally, the compressive force applied in either of the ways described herein will not lead to any significant compression of any cell layers in the cell stack 302 in a direction that is perpendicular to the two-dimensional cell layer surface of the cell layer in question in the local vicinity to the application point of the compressive force.

[0062] In some embodiments however, the pressure (force per surface unit) onto side edges of the cell stack due to the compressive force can be comparable, or even the same as, the pressure applied onto the cell layer surface. However, this still differs from the case in which a hydrostatic pressure of the electrolyte in which the cell stack is immersed applies a pressure onto the cell stack layers, since the compressive force generally will compress the cell stack. Generally, the cell stack can have a larger, such as twice or even five times larger, cell layer surface size as compared to a surface of the cell stack formed by side edges of individual cell layers. This will provide a desired compression of the cell stack even in case of a non-trivial net pressure force onto its side edges.

[0063] In case the cell stack is fully immersed in electrolyte, a hydrostatic pressure of the electrolyte also does not compress the cell stack, since the entire cell layer of the cell stack is subjected to the same hydrostatic pressure. In contrast thereto, the compressive force provided in accordance with any of the approaches described herein, the cell stack is compressed so as to become thinner in a direction perpendicular to the individual cell stack layer and its cell layer surface.

[0064] Common to all embodiments described herein, the compressive force is achieved by the direct or indirect, but active, application of a pressure force onto the cell layer surface in question, normal to the cell layer surface.

[0065] Another way to characterize the application of the compressive force according to the solutions described herein is that an external or internal component part having a defined geometric extension is arranged to apply a directed force onto the cell surface layer of one or more cell layers of the cell stack. This is in contrast to any electrolyte hydrostatic pressure, that merely applies a general pressure throughout the cell can.

[0066] Generally, in the various embodiments described herein the force field of surfacenormal force vectors is achieved by applying a uniaxial force onto an object that extends acrossthe cell layer surface, pressing the object onto the cell layer surface to achieve a uniform and normal compressive force across the cell layer surface; and / or by providing an pressurized object inside the cell can that exerts the compressive force in a uniform and possibly uni-axial manner, for instance by the pressurized object being generally flat, of homogenous thickness and extending along the cell layer surface, and / or by the object surrounding at least part of the cell stack.

[0067] Furthermore, generally, the compressive force can be uniformly applied across at least 50%, such as across at least 75%, such as across at least 90%, of the cell layer surface that is compressed. The “cell layer surface” in question can in turn be arranged to cover the entire cell stack, or at least 90% of the entire cell stack, as viewed in a projection of the cell stack along the direction of the compressive force vector.

[0068] As used herein, the term “uniform pressure” can mean that the pressure varies by less than 20% across the cell layer surface onto which it is applied.

[0069] There are many ways in which a cell can may be designed to achieve this functionality. A few of these designs according to the first category of embodiments will now be described by way of example and with reference to the drawings. Here, the object being pressed against the cell surface layer is an outside wall of the cell can, such as a compression plate which will be discussed in further detail with respect to FIG. 12. However, it should be realized that this is by no means an exhaustive set of embodiments and that there are many alternative embodiments that fall within the scope of the appended claims.

[0070] FIG. 1, shows a prismatic cell can 102, having a front surface 104, a corresponding back surface (not shown), side surfaces 106, a top surface 108, and a corresponding bottom surface (not shown). Typically, the prismatic cell can 102 is made from aluminum or another metal having similar properties. Prismatic cell cans 102 are commonly used in the industry, although it should be realized that other shapes may also be available, such as cylindrical cell cans, which will be discussed in further detail with respect to FIG. 17.

[0071] The prismatic cell can 102 illustrated in FIG. 1 has an outer surface rim 110 that joins the front surface 104 to the top, bottom, and side surfaces, respectively. Generally, the side surface of the outside wall can be translated towards the interior of the cell can, compressing the cell can uniaxially along an axis being perpendicular to the side surface plane. This can be achieved by pressing onto the outside wall using a pressing tool, such as a compression plate, arranged to apply homogenous pressure across the side surface and / or by the parts of the sidesurface that are to be translated are sufficiently rigid so as to prevent any significant building of the translated side wall across such parts.

[0072] However, in the example shown in FIG. 1, the outer surface rim 110 is a locally weakened, such as ductile, area, formed, for example, by local annealing, causing the outer surface rim to be weaker or more ductile compared to the rest of the prismatic cell can 102. Annealing is a well-known concept in the metallurgy field, and makes use of the fact that metals and alloys often have defects or irregularities in their crystal structure due to rapid cooling or mechanical processing. Annealing involves heating the material to a specific temperature (in the case of aluminum, typically 400 degrees Celsius) and then allowing it to cool slowly, often in a controlled environment. This slow cooling allows the material's atoms to rearrange into a differently organized crystal structure, which results in improved ductility, reduced hardness, and greater resistance to fracture. Common annealing techniques include bulk heat treatment, induction annealing, and laser annealing. Annealing may be applied locally (to only highly stressed areas, such as the outer surface rim 110 described above) or globally, to the entire cell can. The annealing results in a local ductility increase in a region along the outer surface rim 110 so that the cell can wall surface that is surrounded by the outer surface rim 110 can translate towards the cell layer surface of the cell stack contained inside the cell can by the region flexing. The cell can wall may be rigid enough so that it moves without significant bulging when being pressed inwards, and / or the cell can wall can be pressed inwards using a stamp tool or similar, arranged to apply a homogenous pressure across the majority or entire cell can surface so as to avoid any bulging of the cell can wall.

[0073] In some embodiments, a part of the cell can wall that is arranged to be pressed against the cell layer surface can be manufactured from aluminum or steel. The wall thickness can vary between the different sides due to manufacturing methods (e.g., deep drawing, impact extrusion), and can be in the range of 0.3-2.0 mm thick, such as between 0.5-1.0 mm thick.

[0074] A similar effect may be achieved with cell can material selection and manufacturing method. Some cell can materials and alloys, as processed and formed into the final net shape of the cell, may already possess sufficient local ductility, with the rim being more ductile than the central parts of the side surface 106, to allow this functionality without additional heat treatment or annealing. In this case, the net shape of the cell can, including wall thicknesses, bend radii, and heat-affected zones from welding, is designed to accommodate deflection without net-shape geometric damage, such as fractures, ruptures or the like, to the cell.

[0075] A result of having the annealed outer surface rim 110 is that when the front surface 104 is subjected to a force from the outside, the outer surface rim 110 plastically deforms and allows the front surface 104, such as at least a major part thereof or the entirety thereof, to translate towards the interior of the prismatic cell can 102, causing the inner wall of the prismatic cell can 102 to subject the cell stack inside the prismatic cell can 102 to a compressive force that is directed essentially perpendicularly to a surface of the cell stack layer(s), without the shape of the front surface undergoing more than an insignificant change.

[0076] FIG. 2 shows a prismatic cell can 102, similar to the one shown in FIG. 1. However, rather than having an outer surface rim 110, the prismatic cell can 102 has one or more flexural features built into the prismatic cell can 102, having the shape of a recessed surface allowing the front surface 104 to translate towards the center of the prismatic cell can 102 when subjected to an outside force. The one or more flexural features can be positioned on the rim of the front surface 104 and / or back surface, respectively, and may be accomplished using hydro forming or similar techniques. This may result in a “bistable plateau” where the front surface 104 has two states; one in which it “bulges out" a few millimeters away from the center of the prismatic cell can 102, and one where it “sinks in” a few millimeters towards the center of the prismatic cell can 102.

[0077] FIG. 3 shows a prismatic cell can 102 in which a flexural feature 304 has been built into the prismatic cell can 102. The view in FIG. 3 is a horizontal cross-sectional view when looking down the prismatic cell can 102 from the top towards the bottom, and shows the cell stack 302 being placed in the center of the prismatic cell can 102. In this embodiment, the flexural feature 304 is embodied as a corrugated section, which allows the front surface 104 of the prismatic cell can 102 to translate perpendicularly towards the center of the prismatic cell can 102 when subjected to an outside force. The flexural feature 304 can either be positioned along the front surface 104 and / or back surface of the prismatic cell can 102, or be positioned on the rim of the side surfaces 106.

[0078] FIG. 4 shows a similar view to that of FIG. 3, with a prismatic cell can 102 having a flexural feature 304. The flexural feature 304 can be formed as an integrated part of the cell can wall material, such as the cell can material locally being bent. In the embodiment shown in FIG. 4, the flexural feature 304 is positioned along the side surfaces 106, but the end result is similar, that is, the front surface 104 and / or back surface of the prismatic cell can 102 to translate perpendicularly towards the center of the prismatic cell can 102 when subjected to anoutside force. It should be noted that flexural features shown are schematic only for illustration purposes, and may in actuality comprise several smaller, shallower, or strictly planar features, as can be realized by a person having ordinary skill in the art, based on the particular situation at hand.

[0079] Hence, as exemplified in FIG. 2 and FIG. 3, the locally weakened or flexural features 304 can be provided at the periphery of the side of the prismatic cell can 102 that is to be translated in order to apply the compressive force and / or around said side but along adjacent sides of the prismatic cell can 102. What is important is that the locally weakened or flexural features 304 allow the compressive force to be applied to the cell stack 302 by the side in question being translated towards the cell stack 302, perpendicularly to the cell layer surface without the overall shape of the prismatic cell can 102 changing apart from the translation of the side in question.

[0080] The various examples shown in FIG. 2 , FIG. 3 and FIG. 4 can be combined in any way, for instance by combining a locally ductile peripheral part of the translated side with flexural features 304 of adjacent side walls encircling the translated side.

[0081] FIG. 5 shows a similar cross-sectional view to those of FIGs. 3 and 4 of the prismatic cell can 102 and the cell stack 302. However, in contrast to the above embodiments, where the front surface is translated without significantly changing its shape, in this embodiment, the front surface can change shape. Therefore, in addition to the cell stack 302, the prismatic cell can 102 also contains one or more rigid elements, such as stiff plates 502, located between one or more of the inner walls of the prismatic cell can 102 and the cell stack 302. Each stiff plate 502 has a flat surface that is in contact with the cell stack 302, and is arranged to extend across the cell surface layer covering at least 80% of the cell surface layer. The surface of the stiff plate 502 can have a slightly curved surface that is in contact with the inner wall of the front surface 104 and / or back surface, respectively, of the prismatic cell can 102. The purpose of the stiff plate 502 is to disperse the outside compressive force evenly on the cell stack 302, such that if the outside compressive force is applied only on one or a few discrete points on the front surface 104, it will still be distributed evenly across the cell stack 302.

[0082] The stiff plates 502 used in this design allow for an even pressure onto the cell stack 302 without the need to compress the front surface 104 evenly over the most of, or the entire, front surface 104. Further, the outer side of the stiff plate 502 that faces the inner wall of the prismatic cell can 102 may be rounded, as described, to match cell surface curvature afterproduction. The slight curvature can, in some embodiments, have a radius of curvature of at least 10 times a largest surface size dimension of the stiff plate 502. However, it should be realized that this is not a requirement and that an even force distribution on the cell stack 302 can be achieved even with a stiff plate 502 that has two flat sides.

[0083] The stiff plate 502 is typically manufactured from a non-reactive rigid material, which does not interact with the electrochemical processes that occur inside the cell. Some examples of suitable materials include ceramic, plastic, aluminum, and steel. The specific choice of a suitable material can be easily made by a person having ordinary skill in the art.

[0084] In all embodiments shown in FIG. 1-FIG. 5, the applied compressive force is substantially uniformly distributed across an internal surface of the cell stack, and the externally applied force compresses the cell stack along one dimension. Expressed differently, all force vectors causing the cell stack to be compressed are substantially parallel. This is in contrast to, for example, a fluid pressure that acts in all directions. The compressive force can generally cause a pressure in the range of 0-15 bar, such as 10 bar.

[0085] As was mentioned above, the second category of embodiments pertains to designs and methods for applying a controllable compressive force to the cell stack inside a prismatic cell can by manipulating the size and / or position of one or more components installed in or next to the cell stack inside the cell can. That is, rather than applying an external force to displace a surface section of the prismatic cell can, the shape of the prismatic cell can may remain unaltered, and instead the internal environment of the prismatic cell can may be changed to apply pressure to the cell stack, for example by pushing the cell stack up against one or more of the inner walls of the prismatic cell can. In particular, such internally provided pressure can act to decrease a total space for the cell stack layers in a direction perpendicular to the cell layer surface between opposite internal walls of the cell can, hence compressing the material of the cell stack in said direction. For the sake of clarity, it should be noted that this category of embodiments still requires application of external compressive force to the exterior of the cell can, but only as a reaction force to the element or cell stack inside that is applying force to the inside surface of the cell can. This may be implemented, for example, by embedding a device into the cell can, which device is capable of expansion, contraction, and / or moving in a controlled way in order to apply pressure to the cell stack, as will be discussed in further detail below.

[0086] As mentioned above, the built-in object providing the compressive force has a generally flat constitution, with a force-exerting side surface that extends across at least a majority of the cell layer surface onto which the compressive force is to be delivered. However, the built-in object does generally not extend past a peripheral side edge of said cell layer surface, whereby no or substantially no compressive force is applied to a side of the cell layer surface.

[0087] FIG. 6 shows a prismatic cell can 102, in which the device is embodied as a substantially fluid-tight flat bag or bladder 602, and has been placed inside the prismatic cell can 102 together with the cell stack 302. It should be noted that for ease of illustration and explanation, only one fluid-tight flat bag 602 is illustrated in FIG. 6, but in a real-world scenario, there may be more than one fluid-tight flat bag 602 that are included as part of the cell stack 302, such as at least two, at least three or at least four fluid-tight flat bags 602. In case there are more than one fluid-tight flat bags 602 they can be arranged with parallel main extension planes and uniformly distributed with respect to translational position in a direction along the applied compressive force vector . Furthermore, while the fluid-tight flat bag 602 is illustrated as being placed in the center of the cell stack 302, it could be placed essentially anywhere inside the prismatic cell can 102, for example, between one of the inner walls of the prismatic cell can 102 and the cell stack 302. The fluid-tight bag can be made of a range of materials with a range of manufacturing methods, including heat-sealable blown film, heat- sealable rolled film, blow-fill-seal pouches, blow molding, or rotational molding. In some embodiments, the material is a flexible polymeric material, such as polyethylene. Other materials include silicone, styrene-butadiene rubber (SBR), Fluroelastomers, latex, natural rubber. In some embodiments, the material is less than 0,2 mm thick.

[0088] The fluid-tight flat bag 602 can comprise more than one compartment for a contained fluid extending across a main extension plane of the fluid-tight flat bag 602, the compartments then possibly being or not being in fluid communication between each other, but in some embodiments the fluid-tight flat bag 602 only has one single compartment.

[0089] The fluid-tight flat bag 602 has at least one fluid path from inside the prismatic cell can 102 to the outside of the prismatic cell can 102, allowing the or each compartment of the fluid-tight flat bag 602 to be filled with a pressurized fluid medium 604, such as a suitable liquid or gas. In concrete examples, the medium can be Argon, or various carbonate solvents (such as Ethylene Carbonate (EC), Ethyl Methyl Carbonate (EMC), Dimethyl Carbonate(DMC), or Diethyl Carbonate (DEC)). The electrolyte 606 is added through a separate fluid path, such that it does not mix with the pressurized medium 604 inside the prismatic cell can 102. Once the cell stack 302 and fluid-tight flat bag 602 have been put in the prismatic cell can 102, a differential pressure may be applied between the interior of the fluid-tight bag 602 and the internal volume of the prismatic cell can 102, which causes a compressive force to be applied to the cell stack 302. Hence, the pressure inside the fluid-tight flat bag 602 can be higher than a generally hydrostatic pressure of the electrolyte 606 inside the prismatic cell can 102 to achieve the compressive force. In some embodiments, the differential pressure can be at least 10 bars, such as at least 6 bars.

[0090] It is realized that the compressive force in embodiments involving a fluid-tight flat bag 602 pressing against one or more cell layers apply the compressive force with the inside wall of the prismatic cell can 102 as counterforce providing support. So, a prevailing electrolyte 606 hydrostatic pressure inside the prismatic cell can 102 will provide at least the same pressure inside the fluid-tight flat bag 602 as the hydrostatic pressure of the electrolyte 606.

[0091] The pressure of the pressurized medium 604 can be independently controlled using a per se conventional pressure-control valve; whereas the hydrostatic pressure of the electrolyte 606 can be independently controlled by another per se conventional pressure-control valve.

[0092] Again, as can be deduced from the physical arrangement in FIG. 6, the compressive force will be evenly distributed across the cell layer surface of the cell stack 302, normal to the cell layer surface, and its magnitude can be controlled by controlling the pressure differential between the pressure of pressurized medium 604 that is let into the fluid-tight flat bag 602 and the pressure of the pressurized medium inside the prismatic cell can 102. A higher pressure of pressurized medium 604 causes the fluid-tight flat bag 602 to fill up more, thus pushing the cell stack 302 uniaxially against the inner walls of the prismatic cell can 102 with higher compressive force. Similarly, by releasing or evacuating pressurized medium 604 from the fluid-tight flat bag 602, the compressive force can be reduced, thus enabling fine grained control of the compressive force on the cell stack 302 over time. The fluid-tight flat bag 602 can be generally flat (at least 10 times wider and higher than thick) and be arranged along, such as between, layers in the cell stack. The fluid-tight flat bag 602 can be arranged to extend across at least 80% of a surface area of the cell stack layers between which it sits. As a result, the compressive force achieved by the fluid-tight flat bag 602 onto the cell stack 302 acts in substantially one single dimension, and the resulting force vectors are parallel. Thecompressive force can generally cause a pressure of 10 bar or less. The fluid-tight flat bag 602 can in some embodiments not extend, in its main extension plane, past a peripheral edge of the cell layer surface onto which the compressive force is applied.

[0093] In some embodiments, in addition to adding electrolyte 606 to the inside of the prismatic cell can 102, pressurized fluid is also added, such that the pressure differential between the pressurized medium 604 inside the fluid-tight flat bag 602 and the interior of the prismatic cell can 102 is reduced, thereby lowering the risk of bursting the fluid-tight flat bag 602 as it is filled with pressurized medium 604. The pressure differential between the pressurized medium 604 inside the fluid-tight flat bag 602 and the surrounding volume inside the prismatic cell can 102 in the embodiment illustrated in FIG. 6 amounts to approximately 8- 10 bar. However, it should be realized that this may vary depending on the particular situation at hand, for example, based on what material is used for the fluid-tight flat bag 602, and so on. Again, these are parameter values that can be easily determined through experimentation by a person having ordinary skill in the art. It is noted that the fluid-tight flat bag 602 exerts said uni-axial compressive force onto the cell layer surface, whereas the pressurized fluid provides a counter-pressure active at the peripheral edges of the fluid-tight flat bag to prevent the fluid- tight flat bag 602 from bursting at its edge. This counter-pressure does not generally decrease the compressive force.

[0094] FIG. 7 shows an embodiment of a prismatic cell can 102, in which a fluid-tight pouch cell 702 has been placed inside the prismatic cell can 102. Pouch cells are well known in the field, and in essence, they use a flexible, flat pouch as its packaging instead of a rigid metal casing as is the case with prismatic cell cans. That is, the cell stack and electrolyte are placed inside the pouch, which is typically made of a lightweight, polymer material that provides mechanical flexibility and thus makes the battery cell more compact compared to a prismatic cell can. The pouch cell can be manufactured from similar materials as the fluid-tight flat bag 602 described above. Pouch cells enable several advantages, such as high energy density, they are lightweight and compact, cost-effective to produce, they have customizable shape, and good thermal performance, making them widely used in the industry.

[0095] Placing a fluid-tight pouch cell 702 inside a prismatic cell can 102, as is shown in FIG. 7, makes it possible to add a pressurized medium 604 in the space between the interior walls of the prismatic cell can 102 and the outside of the fluid-tight pouch cell 702, and by adjusting the pressure of the pressurized medium 604, the compressive force on the cell stack 302 can bevaried. The pressurized medium 604 can be evacuated after the cell stack 302 swells enough to generate a compressive force between the cell stack 302 and the inner walls of the prismatic cell can 102.

[0096] In some embodiments, in order to limit the compressive force applied onto the sides of the cell stack, the cell stack 302 encapsulated by the fluid-tight pouch cell 702 can have a larger cell layer surface (as defined above) as compared to a side surface (made up of individual cell layer side edges), such as at least twice or even at least five times the surface size.

[0097] The fluid-tight pouch cell 702 can be arranged to encapsulate the entire cell stack 302 or only a part of the cell stack 302. More than one fluid-tight pouch cell 702 can be used so that all cell layers are encapsulated by at least one fluid-tight pouch cell 702.

[0098] It is realized that the fluid-tight pouch cell 702 encapsulating the cell layer(s) and the fluid-tight flat bag 602 represent two different ways of introducing an internal pressure into the cell can, in addition to any prevailing electrolyte hydrostatic pressure, so as to give rise to the compressive force. It is also foreseeable that in-between type solutions are also possible, such as a fluid-tight pouch cell 702 that only partly surrounds one, many, or all of the cell layers of the cell stack, such as a fluid-tight pouch cell 702 having an open configuration with a through hole or similar. It is also possible to combine one or more fluid-tight flat bags 602 with one or more fluid-tight pouch cells 702, depending on the detailed effect to be achieved.

[0099] Turning now to the third category of embodiments, some examples of designs and methods for compactly installing the cell stack during assembly, to enable a sufficient compressive force to be applied to the cell stack during electrolyte filling, formation, and aging, will now be presented. These are generally based on the concept that as the cell stack swells, a homogenous compressive force is generated by the cell stack pushing towards the inner walls of the cell can. In order to further facilitate these mechanisms, further devices, such as swell pads, may be incorporated inside the cell can to allow for a more consistent compressive force, as will be described below with reference to the figures. Such devices are then not arranged to receive a pressurized fluid from a point external to the cell can, but interact with the internal components of the cell can to achieve the compressive force described herein. Various additional components may also be introduced to facilitate insertion of the cell stack into the cell can.

[0100] FIG. 8 shows a schematic cross section of a prismatic cell can 102 with a cell stack 302, similar to what has been shown and described above. In this embodiment, a swell pad 802 has been included as part of the cell stack 302. Generally, the swell pad 802 has the same shape as a cell stack layer, although it may also be larger or smaller in some embodiments, and a thickness in the range of approximately 1-4 mm. It should be noted that while only one swell pad 802 is illustrated in FIG. 8 for ease of illustration and explanation, there can be several swell pads 802 in a real-world application, in a way that can correspond to what has been described above with respect to the fluid-tight flat bags 602. The swell pad 802 may be made from, for example, a polyurethane foam or aerogel, and may be compressible to absorb the swelling of the cell stack 302 during filling, formation, and aging, while applying a spring-like pressure to the cell stack 302, for instance due to capillary pressures as it absorbs electrolyte and / or due to elastic material properties of the swell pad 802 itself.

[0101] In some embodiments, the swell pad 802 is pre-compressed to achieve a tight assembly of the cell stack 302 into the prismatic cell can 102. In those situations, it may also be necessary to quickly insert the cell stack 302 into the prismatic cell can 102, to prevent the swell pad 802 from expanding prior to the swell pad 802 and cell stack 302 being fully inserted into the prismatic cell can 102. FIG. 9 schematically illustrates an embodiment in which the cell stack 302 and swell pad 802 have been wrapped in a protective wrapping 904, and a guiding insulator 902 have been provided at each short end of the cell stack 302. Optionally, a lubricant can be provided on the outside of the protective wrapping 904 to facilitate quick and easy insertion into the prismatic cell can 102. In various embodiments, one or several of these measures 904, 902, 906 can be applied alone or in any combination. Like the fluid-tight flat bag 602, the swell pad 802 will, due to its shape and size, apply a force field of generally parallel force vectors to the cell stack 302, uniformly distributed across the cell stack surface.

[0102] FIG. 10 shows an embodiment in which an electrolyte-filled bag 1002 is included as a part of the cell stack 302. The bag is pre-filled with electrolyte, sealed and included in the cell stack 302, although it should be realized that this is merely one example and that there may be more than one electrolyte-filled bag 1002 and the location may vary inside the prismatic cell can 102, in a way that can correspond to what has been discussed above in relation to the fluid- tight flat bags 602. As the cell stack 302 swells, eventually a point is reached where the compressive force exerted on the electrolyte-filled bag 1002 will be strong enough to cause the electrolyte-filled bag 1002 to break or to open a slow-flow pressure release valve, allowing theelectrolyte to flow out of the electrolyte-filled bag 1002 and surround the cell stack 302 in the interior of the prismatic cell can 102, for example, after formation and aging. With an integral slow-flow pressure release valve attached to the bag with a cracking pressure corresponding to the desired applied compressive force, constant compression can be maintained on the cell stack across a range of parallel force vectors. A smallest pressure difference between the interior of the electrolyte-filled bag 1002 and its surrounding causing the electrolyte to flow out of the electrolyte-filled bag 1002 can be at least 6 bars, such as at least 8 bars; and / or at the most 20 bars, such as at the most 12 bars.

[0103] The electrolyte-filled bag 1002 can be made from the same type of materials that were discussed above in relation to the fluid-tight flat bag 602.

[0104] Yet another embodiment is shown in FIG. 11, where the prismatic cell can 102 includes the cell stack 302 and an internal dissolvable or meltable shim 1102. The dissolvable or meltable shim 1102 is solid or semi-solid when assembled into the prismatic cell can 102 at Time TO, together with the cell stack 302. The prismatic cell can 102 is welded shut and filled with electrolyte.

[0105] During filling, formation and aging the dissolvable or meltable shim 1102 allows the cell stack 302 to be compressed by a compressive force that is caused by the inner walls contacting the cell stack 302. The dissolvable or meltable shim 1102 slowly dissolves or melts into the electrolyte as the cell stack 302 swells, as illustrated at Time Tl, and eventually at Time T2, the dissolvable shim 1102 is completely dissolved or melted and the cell stack 302 pushes up against the inner walls of the prismatic cell can 102. T2 can be between 24 hours and several days, such as at least 14 days or at least 21 days, and is typically based on whether the dissolvable or meltable shim 1102 needs to occupy space only during the early stages of formation, or all the way through formation and aging.

[0106] The dissolvable shim 1102 may be made from a material that is able to dissolve into the electrolyte, such as LiPF6 electrolyte salt or polystyrene foam, just to mention a few examples. Alternatively, the shim may be made of a material with sufficiently low compressive yield strength that it will remain intact up to a certain level of compressive stress, but then yield, pulverize, or break down when the compressive stress exceeds that level. Materials such as silica aerogel, polymer foam, elastomer foam, or low-density ceramic composite are a few examples exhibiting such properties. Again, the particular choice of material to be used for the shim 1102 depends on the specific circumstances at hand.

[0107] In contrast to a dissolvable shim 1102, a meltable shim relies on freezing and then melting the shim material, by the transfer of heat into and out of the meltable shim material, respectively. Materials such as wax and electrolyte solvents (e.g., ethylene carbonate) are examples of suitable meltable shim materials. At the conclusion of manufacturing, the meltable shim’s temperature can be raised above its melting temperature, softening temperature, or glass transition temperature to allow the meltable shim to flow out of the space around the cell stack. In a further variation on this embodiment, the melted or dissolved shim material can be removed from the cell at the end of manufacturing.

[0108] It should be noted that the dissolvable or meltable shim does not need to completely disappear in these embodiments. In some versions, it may be acceptable for a thin layer or small bits of the dissolvable or meltable shim to remain inside the cell even after time T2.

[0109] Everything that has been said above in relation to the fluid-tight flat bag 602 with respect to numbers, sizes, and geometric arrangements, is equally applicable also to the swell pads 802, the electrolyte-filled bags 1002 and the dissolvable shims 1102.

[0110] FIG. 12 shows an embodiment of a compression plate 1202 (side view and bottom view, respectively), which can be used to apply the supplemental compressive force to the cell stack 302, as described above in conjunction with FIG. 1 - FIG. 2. The compression plate 1202 has a plateau 1204 that substantially matches the outline of the cell stack 302 inside the prismatic cell can 102. By pressing the plateau 1204 into the side surface 106 of the prismatic cell can 102, the side wall of the prismatic cell can 102 elastically deforms such that the inner wall applies a compressive force uniformly to the cell stack 302, as described above. In this embodiment, the material of the prismatic cell can 102 is sufficiently ductile, and the shape is designed to allow the final net shape of the prismatic cell can 102 to remain within the geometric specifications for the final finished battery cell. This embodiment allows applying compression to the cell stack 302 without substantially modifying the internal contents of the battery cell, or the design of the prismatic cell can 102. It should be noted that depending on the particular embodiment at hand, there may be only one compression plate 1202 applying the supplemental compressive force to the front surface 104 of the prismatic cell can 102, or there may also be a second compression plate 1202 which applies pressure from the opposite side (i.e., to the back surface of the prismatic cell can 102) at the same time, causing the cell stack 302 to be squeezed in a vise-like manner.

[0111] FIG. 13 - FIG. 16 schematically illustrate how a compressive force can be applied to the cell stacks in some of the embodiments described above, during the manufacturing process. FIG. 13 shows the embodiment of the cell can of FIG. 4 with the flexural feature. As shown in the leftmost part of FIG. 13, the cell stack is inserted into the cell can. A compressive force is then applied to the outside of the cell can, causing the walls of the cell can to translate towards the center and subject the cell stack to an evenly distributed compressive force, as illustrated by the small arrows. The external compressive force is maintained during fill, precharge, formation, and aging, and may optionally vary in magnitude during these steps as discussed above, and as will be discussed in further detail with respect to FIG. 18. The swelling of the cell stack causes the cell stack to expand, as shown in the third stage of FIG. 13, but due to the evenly distributed compressive force along an axis normal to the cell stack layers, the swelling can be controlled such that no bulging of the cell can walls occurs, and a battery cell with straight walls is achieved at the end of manufacturing.

[0112] FIG. 14 shows the corresponding manufacturing process that was described above with respect to FIG. 13, but for the cell can embodiment shown in FIG. 3, where the flexural feature is embodied as a corrugated section that allows the front surface of the prismatic cell can 102 to translate perpendicularly towards the center of the prismatic cell can when subjected to an outside force. As can be seen in FIG. 13, the same results are achieved, with the only significant difference being that only the front surface of the cell can moves, as a result of the placement of the corrugated section.

[0113] FIG. 15 shows the corresponding manufacturing process that was described above with respect to FIG. 13 and FIG. 14, but for the cell can embodiment shown in FIG. 2 where the flexural features are positioned on the rim of the front surface. As can be seen in FIG. 15, the same results are achieved, with the only significant differences being that only the front surface of the cell can moves, and that at the end of the manufacturing process, the front surface is crimped or welded to fixate its position.

[0114] FIG. 16 shows the corresponding manufacturing process with a compressive force being applied, but for the embodiment shown in FIG. 6 where the cell can includes one or more fluid-tight bags, in the present case two fluid-tight bags, placed between the cell stack and the respective inner walls of the cell can. As noted above, the compressive force on the cell stack is accomplished by filling the bags with a pressurized medium, and as the cell stack swells during filling, precharging, formation and aging, the pressurized medium gets evacuated from the bagsand the compressive force is instead accomplished by the inner walls coming into contact with the cell stack. Again, though, due to the evenly distributed compressive force, the swelling can be controlled such that no bulging of the cell can walls occurs, and a battery cell with straight walls is achieved at the end of manufacturing.

[0115] It should be noted that while all the different embodiments have been described above in the context of a prismatic cell can, the same general principles can also be applied in other geometries. For example, FIG. 17 shows an exploded view of a cylindrical cell can 1702, as one alternative geometry in which the inventive concepts can be applied. The cylindrical cell can 1702 contains a jelly roll cell stack 1704, that includes an anode 1706, a cathode 1708, and separators 1710, wrapped around a core 1712 in a “jelly roll” like fashion. The cylindrical battery cell also contains other components, such as an insulating washer 1714, a seal 1716, a cover 1718, a vent ball 1720, a cap 1722, and a positive tab 1724, which together with the 1702 and jelly roll cell stack 1704 form the battery cell.

[0116] In this configuration, rather than having a substantially fluid-tight flat bag at the center of the cell stack, a counterforce exerting component, such as a cylindrical bag or a rigid cylindrical element, is placed at the core 1712. The cylindrical bag can be arranged to communicate with the outside of the cylindrical cell can 1702 to enable a pressurized medium, such as a suitable liquid or gas to be disposed into the cylindrical bag corresponding to fluid- tight flat bag 602 or electrolyte-filled bag 1002 described above; or it can be in the form of a swell pad or dissolvable shim corresponding to the swell pad 802 or dissolvable shim 1102 described above. As the cylindrical bag expands, or as the counter-pressure exerting component in question imparts a radially outwards-directed counter force that acts against a reacting radially internally-directed force, a radial force pushes the jelly roll cell stack 1704 radially outwards against the inner wall of the cylindrical cell can 1702, which creates a compressive force on the jelly roll cell stack 1704. Since the compressive force is a radially directed force, it is essentially perpendicular to the surface of the jelly roll cell stack 1704 and evenly distributed 360 degrees around the center of the cell stack 1704, (i.e., corresponding to the parallel compressive force vector in the prismatic cell can embodiments). However, in contrast to the prismatic cell can embodiment discussed above, the magnitude of the compressive force decreases with the distance from the central axis of the cylindrical cell can, as the area under pressure increases with the radius. A pouch corresponding to the fluid-tight pouch cell 702 can also be arranged radially outside of the cell stack, to encapsulate the cell stack in a waycorresponding to what has been described above in relation to the fluid-tight pouch cell 702. Also here, the electrolyte is added through a separate fluid path, such that the electrolyte does not mix with the pressurized medium inside the cylindrical bag. Similar to what was described above with respect to FIG. 6, the magnitude of the compressive force can be regulated by controlling the differential pressure between the cylindrical bag and the internal volume of the cylindrical cell can, , thus enabling fine-grained, time variable, control of the compressive force on the jelly roll cell stack 1704 over time.

[0117] In general, all of the compressive force-mediating components described in relation to FIG. 2 - FIG. 16 for a prismatic geometry can be applied also to the cylindrical case, but then applying the compressive force along a radial axis rather than along a Euclidian axis.

[0118] FIG. 18 is a diagram 1800 that schematically illustrates how the thickness of the cell stack varies over time during the different manufacturing stages, including filling and soaking, pre-charging, formation, and cycling, when compression is not applied (upper curve 1802) vs. when compression is applied (lower curve 1804). In the diagram 1800, the horizontal axis represents time, and the vertical axis represents the thickness of the cell stack 302.

[0119] The dry stack time interval 1806 illustrates how the cell stack 302 has been observed to be compressible when exposed to a compressive force, as discussed above (i.e., the lower curve 1804 shows how the cell stack 302 gets thinner when pressure is applied according to one of the methods described above). Typically, the compressive force results in a 3-10% thickness reduction of the cell stack. Expressed differently, the compressive force causes a current width of the cell stack to be less than a nominal width of the cell stack, e.g. during the dry stack time interval 1806, the soaking time interval 1808 and the pre-charging time interval 1810, e.g. at least to some extent during each of these time intervals. The current width can thus be 3-10% less than the nominal width, e.g. at least during the soaking time interval 1808, and possibly to some extent also during the dry stack time interval 1806 and the pre-charging time interval 1810. The nominal width refers to a width of the cell stack can before being exposed to the compressive force, e.g. a width of cell stack can as manufactured. The expressions “cell stack” and “cell stack can” may have been used interchangeably.

[0120] Next, during the soaking time interval 1808, electrolyte is introduced into the cell, which is followed by a soaking period of anywhere between a few hours and several days, often 12-36 hours, to allow the inserted electrolyte to be absorbed by the cell stack, allowing ionconductivity between the electrodes. Typically, both the filling and soaking are done in multiple steps.

[0121] Pressure is an important parameter during the filling- and soaking period. Low filling pressure is commonly applied to speed up and improve the spread of electrolyte into the cell, while moderate wetting pressure (i.e., pressure when the electrolyte is filling the pores of the cell stack) has been shown to be optimal for electrolyte absorption into the porous components of the cell stack. However, an optimal filling and wetting pressure procedure is designed by the cell manufacturer potentially including both over- and under pressure.

[0122] The temperature of the electrolyte and stack is another important factor, since the temperature affects viscosity of the electrolyte. Typically, a temperature in the range of 30-50 degrees Celsius is used, as this provides a decreased viscosity compared to room temperature, thereby improving the wetness (i.e., soaking in of electrolyte into the pores of the cell stack). A 12-24-hour soaking time is normally needed, but varies depending on a variety of factors, such as material selection, cell size, and form factor, just to mention a few. As can be seen in FIG. 18, the wetting of the cell stack has an observed correlation to the thickness of the cell stack. When no compressive force is applied, the cell stack thickness increases, as illustrated by the upper curve 1802. In contrast, by applying a compressive force to the cell stack 302 during the soaking time interval 1808, the cell stack thickness increase can be mitigated, as shown by the lower curve 1804. By synchronizing the control of the applied compressive force and the internal hydrostatic pressure in the cell resulting from the electrolyte introduction, for example, by pulsing the compression of the cell stack in combination with the electrolyte introduction, various benefits to the wetting quality and time can be achieved, for example, as a result of better expulsion of gas bubbles, which allows better wetting.

[0123] Next, during the pre-charging time interval 1810, the battery cell is taken from a zero or close to zero voltage, such as + / - 0.5 V to the bottom of its working voltage window, such as 2.5-2.8 V, depending on anode and cathode materials, respectively. This is preferably done close in time to the electrolyte filling since corrosion inside the battery cell may occur unless a differential voltage is created between the electrodes. As can be seen in the diagram 1800, cell stack swelling occurs during the pre-charging time interval 1810, but as illustrated by the curves, the amount of swelling can be reduced by up to 1 / 3, or possibly more, when a compressive force is applied, as described above, and can improve the volumetric energy density (VED) for the battery cell.

[0124] After the pre-charging time interval 1810, the cell is repeatedly cycled during a formation time interval 1812 with the purpose of creating the SEI-layer. The formation is optimized versus SEI-formation as low currents are beneficial for SEI-quality, but longer time drives cost. As can be seen in the diagram 1800, the formation time interval 1812 with the build-up of the SEI-layer impacts the cell stack thickness, and the compressive force on the cell stack can be controlled to affect the SEI-layer’ s thickness and improve VED.

[0125] As the formation process is creating the SEI-layer, gas is created inside the cell as a side reaction. This gas is extracted out of the cell through a process called degassing. Applying a compressive force to the cell stack can speed up this process, and / or improve the extraction quality by mechanically squeezing out the gas from the stack layers.

[0126] Finally, the cell is aged, which is illustrated in FIG. 18 as the cycling time interval 1814. Aging of the cell, or rest periods between process steps is commonly used to allow the cell to reach an equilibrium in terms of temperature, ion diffusion and electrolyte wetting. By using a compressive force, the rest period can be shortened by increasing thermal- and ionic conductivity within the stack, due to its higher density resulting from the compression.

[0127] The increased thermal conductivity at higher compression forces can be used to improve the control of the battery cell’s internal temperature by external heating or cooling during the manufacturing steps, as well as achieve a more homogenous heat spread within the cell. The temperature of the external heating typically ranges from room temperature up to about 50 degrees Celsius, but could in some embodiments extend lower or higher.

[0128] The heat can be conducted into or out of the cell by heating or cooling the external cell can, for example, through any of the cell can surfaces and / or through its positive or negative terminals, respectively. The temperature can be controlled together in a designed schedule together with cell SOC (state of charge) and stack compression force. By ensuring optimal stack temperature, the risks of lithium plating can be mitigated.

[0129] From FIG. 18, it is clear that an overall cell stack size reduction can be accomplished throughout the cell manufacturing process, as illustrated by the lower curve 1804, when a compressive force is applied, compared to a more conventional manufacturing process, as illustrated by the upper curve 1802.

[0130] While this specification contains many implementation details, these should not be construed as limitations on the scope of the invention or of what may be claimed, but as descriptions of features specific to implementations of the invention. Certain features that aredescribed in this specification in the context of separate embodiments can also be implemented in combination in a single embodiment. Conversely, various features that are described in the context of a single embodiment can also be implemented in multiple embodiments separately or in any suitable sub-combination. Moreover, although features may be described above as acting in certain combinations and even initially claimed as such, one or more features from a claimed combination can in some cases be excised from the combination, and the claimed combination may be directed to a sub-combination or variation of a sub-combination. Thus, unless explicitly stated otherwise, or unless the knowledge of one of ordinary skill in the art clearly indicates otherwise, any of the features of the embodiment described above can be combined with any of the other features of the embodiment described above.

[0131] Similarly, while operations are depicted in the drawings in a particular order, this should not be understood as requiring that such operations be performed in the order shown or in sequential order, or that all illustrated operations be performed, to achieve desirable results. In certain circumstances, multitasking and / or parallel processing may be advantageous. Moreover, the separation of various system components in the embodiments described above should not be understood as requiring such separation in all embodiments.

[0132] Thus, embodiments of the invention have been described. Other embodiments are within the scope of the following claims. For example, while the cell cans have been described above as prismatic cell cans made from metal, it should be realized that there may be embodiments in which other substantially rigid materials are used, such as certain types of plastic. There may also be “hybrid” cell can embodiments that use a combination of two or more materials, such as metal and plastic. Essentially any material or combination of materials can be used that allows the cell can to be deformed in a controllable manner without jeopardizing its integrity, and while being able to safely hold the cell stack and electrolyte.

[0133] Further, while the battery cells have been described above as prismatic (“brick shaped”) or cylindrical, it should be realized that other shapes may also be possible, such as hexagonal, or octagonal shapes, for example, as may be suitable for assembly into battery packs by the final user.

[0134] It should also be noted that whereas the examples above have been presented as different categories of embodiments, it is also possible to combine these categories of embodiments. That is, they are non-exclusive. For example, there may be situations in which more fine-grained application of compressive force can be accomplished by combining the displaceable cell cansurfaces of the first category of embodiments with the internal pressure control of the second category of embodiments to achieve a more dynamic variability of the compressive force on the cell stack. Thus, it should be clear that various combinations of the above examples can also be made.

[0135] Thus, many variations to the above examples lie well within the scope of the attached claims and within the capabilities of a person having ordinary skill in the art.

Claims

CLAIMS1. A method for manufacturing a battery cell, the method being performed during one or more manufacturing stages comprising at least one of: a dry stack time interval, electrolyte introduction, soaking, and precharge, the method comprising: providing a substantially rigid cell can (102) having a cell stack (302) disposed therein, the cell stack (302) comprising a plurality of sheetlike cell layers arranged in a stack-like or roll configuration; and applying a supplemental, actively controllable compressive force to the cell stack (302) directly or indirectly by one or more inner walls of the cell can (102), wherein: the supplemental compressive force is applied in addition to any forces resulting from electrolyte (606) introduction into the cell can (102), and the supplemental compressive force is applied in a uniaxial direction that is substantially normal to the sheetlike cell layers, causing the sheetlike cell layers to have a denser configuration in the uniaxial direction compared to when no supplemental compressive force is applied.

2. The method of claim 1 , wherein the applying of the supplemental, actively controllable compressive force is subjected to the cell stack (302) from the time electrolyte is first introduced into the cell can (102) to the completion of formation and aging stages of the manufacturing process.

3. The method of claim 1, wherein the compressive force is uniformly distributed across the cell layer surface.

4. The method of claim 1 , wherein the supplemental compressive force is variable across the cell layer surface.

5. The method of claim 1, wherein the compressive force is variable over time during the manufacturing stages.

6. The method of claim 1, wherein the cell can (102) in is a prismatic cell can (102).

7. The method of claim 6, wherein a cross section of the prismatic cell can (102) is substantially square, substantially rectangular, substantially hexagonal, or substantially octagonal.

8. The method of claim 1, wherein the cell can is a cylindrical cell can (1702) and the cell stack comprises a roll (1704) of sheetlike cell layers.

9. The method of any one of claims 1 to 8, wherein applying the supplemental compressive force comprises: applying external compression to one or more outer walls of the cell can (102) to displace a surface section of the cell can (102).

10. The method of claim 9, wherein applying external compression to one or more of the outer walls of the cell can (102) is done by stamp tool (1202) having a compression plate (1204) that is smaller than the cell can front surface (104) and having an outline that substantially corresponds to the cell stack (302) layer.

11. The method of claim 9, wherein the cell can (102) comprises a ductile area (110) allowing the surface section of the cell can (102) to translate in a direction normal to the surface section to apply the supplemental compressive force to the cell stack (302).

12. The method of claim 11, further comprising providing the ductile area (110) after construction of the cell can (102), such as by local heating of the cell can (102).

13. The method of any one of claims 1-6 or 9, wherein the cell can (102) comprises a flexural feature (304) on a cell can front surface (104), a cell can back surface, and / or a cell can side surface, allowing the cell can front surface (104) and / or cell can back surface to translate in a direction normal to the cell can front surface (104) and / or cell can back surface to apply the supplemental compressive force to the cell stack (302), and wherein the flexural feature (304) is arranged so that the supplemental compressive force is applied uniformly across a side of the cell stack (302).

14. The method of claim 13, wherein the flexural feature (304) comprises a corrugated section on the cell can (102).

15. The method of any one of claims 1-6 or 9-14, further comprising: providing a rigid element (502) between the side surface of the cell stack (302) and a respective inner wall of the cell can (102), the rigid element (502) being arranged to distribute an externally applied pressure evenly across the side surface of the cell stack (302).

16. The method of claim 15, wherein the rigid element (502) comprises an inner surface and an outer surface, the outer surface being arranged to define a desired final bulge shape of the cell can (102) after swelling.

17. The method of any one of claims 1 to 16, wherein the substantially rigid cell can (102) is made from metal, plastic, or a combination thereof.

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