Methods and system for determining alignment of an electrode stack

Abstract

There is disclosed herein a computer-implemented method for determining alignment of a stack of electrode sheets comprising obtaining a first and second image of the stack, determining distances between apparent edges of the imaged sheets, and determining an alignment based on these distances, and the known angles along which the first and second images were taken. A further method is disclosed, additionally comprising the capture of the first and second images. A system is also disclosed, for carrying out the methods.

Description

METHODS AND SYSTEM FOR DETERMINING ALIGNMENT OF AN ELECTRODE STACK   Technical Field

[0001] The present disclosure relates to a method and apparatus for testing or inspecting the quality of manufacture of a battery cell. In particular, the present disclosure relates to a method and apparatus for determining an alignment of stacked electrode sheets. Background

[0002] Rechargeable or “secondary” battery cells find widespread use as electrical power supplies and energy storage systems, for example in automobiles. Several different form factors exist for such battery cells depending on their intended application field. In automotive applications, the most common cell form factors are cylindrical, prismatic and pouch cells.

[0003] A battery cell stores electrical energy in an electrode assembly, which may be comprised of stacked electrode sheets, which may be folded and / or rolled and referred to as an “electrode roll” or a “jelly roll”. The electrode sheets may include an anode sheet, a cathode sheet, and a separator sheet arranged between the anode sheet and the cathode sheet.

[0004] During or after the manufacture of battery cells, tests and inspections may be carried out to assess the quality of the battery cells as a whole or constituent components thereof. A particularly important quality to be assessed is the relative position or alignment of stacked electrode sheets.

[0005] Conventional techniques for assessing such a quality may involve the use of industrial radiography such as X-rays or computed tomography scanning, or ‘CT scanning’, which utilizes X-ray equipment to image components externally and / or internally. Such techniques are preferred for assessing the manufacturing quality of battery cells because they are non- destructive and can be performed ‘in-line’ during a manufacturing process for the battery cells.Summary

[0006] The present disclosure aims to provide a rapid, accurate, and repeatable method for carrying our quality control on battery cells having stacked electrode assemblies.

[0007] It is a prejudice in the art that conventional X-ray inspection techniques, involving the analysis of an X-ray image taken of a corner region, are not suitably accurate for detecting misalignments in electrode stacks. Therefore, there has been a trend in the art towards the generation of a CT scan, and then the taking of ‘slices’ therefrom, to determine electrode stack alignment.

[0008] However, it is realized as a part of the present disclosure that carrying out CT scans can be excessively time consuming, as 200 to 400 X-ray images may be required. Moreover, the subsequent consolidation of the 100 or more X-ray images to form the CT scan is not only further time consuming but also computationally taxing. Furthermore, CT scanning arrangements are bulky as they require a holder that can rotate the cell or electrode stack under inspection about a number of axes. As a further drawback, CT scans are very sensitive to even small misalignments of the electrode stack, as the slices taken therefrom represent small effective areas.

[0009] Accordingly, the presently disclosed techniques may realize the accuracy of CT scans, without the associated drawbacks, through the taking of only a pair of X-ray images.

[0010] More particularly, according to an aspect of the present disclosure, there is provided a computer-implemented method for determining alignment of a stack of electrode sheets, wherein the electrode sheets are planar sheets extending in a first axis and a second axis, and the electrode sheets are stacked along a third axis perpendicular to the first axis and the second axis.

[0011] The first and second axes may be perpendicular x and y axes, and the third axis may be a z-axis, and the general geometry of the sheets may be rectangular so as to form a substantially cuboidal shape when stacked.

[0012] The method comprises obtaining a first X-ray image of a corner region of the stack, taken along a first beam path in the plane of the electrode sheets (e.g., an x-y plane), with a first angle from the first axis, and obtaining asecond X-ray image of the corner region of the stack, taken along a second beam path in the plane of the electrode sheets, with a second angle from the first axis.

[0013] Here, a ‘beam path’ may be considered as being the primary direction of the X-ray source. An X-ray beam may have an amount of spread, centered around some central path, and this central path may be considered as being the beam path. Ideally, the X-ray source and detector are substantially aligned along the beam path.

[0014] With the obtained X-ray images (also referred to herein as simply ‘image(s)’), the method further comprises determining a first distance between a first apparent edge of the first electrode sheet and a first apparent edge of the second electrode sheet, along the plane of the electrode sheets in the first image, and determining a second distance between a second apparent edge of the first electrode sheet and a second apparent edge of the second electrode sheet, along the plane of the electrode sheets in the second image.

[0015] These determinations may be carried out using edge detection and / or other image processing techniques understood by those skilled in the art. It will be appreciated that the ‘apparent’ edge is referred to so as to distinguish from the ‘actual’ edge of the electrode sheet, as the edge detection process may have some accompanying error due to low signal-to-noise ratio (SNR) or other imprecisions in the imaging process.

[0016] The distances are measured along the plane of the electrode sheets so that the distances may be determined in the same direction as the overhang, e.g., substantially perpendicular to a stacking direction. In some examples, a bending away of an edge of a sheet from the general plane of extension of said sheet may be accounted for in the determinations.

[0017] The method then further comprises determining an alignment between the first electrode sheet and the second electrode sheet, based on the first distance, the second distance, the first angle, and the second angle. Accordingly, it can be seen that the presently disclosed technique is, among other things, particularly robust to misalignments as it may only be required tocapture the corner region of the stack within the field of view of the X-ray imaging apparatus.

[0018] Determining an alignment between the first electrode sheet and the second electrode sheet may comprise determining a first displacement of the second electrode sheet from the first electrode sheet along the first axis, and determining a second displacement of the second electrode sheet from the first electrode sheet along the second axis, the second axis being perpendicular to the first axis. These displacements may be referred to as an ‘x-overhang’ and a ‘y-overhang’ and thus the alignment may be characterized as an (x, y) co-ordinate for a first sheet, wherein the x-y axes are aligned relative to a second sheet for which alignment with the first sheet is desired, and wherein the (0, 0) origin of the axes may be set as the identified corner of said second sheet.

[0019] The determination of alignment may be carried out according to any geometric assessment, depending on, e.g., the form in which an alignment is to be expressed. In a specific example, where the alignment may be expressed as an (x, y) co-ordinate, as discussed above, determining an alignment between the first electrode sheet and the second electrode sheet may be based on the equations: , and , wherein ^^ is the first angle, ^^′ is the second angle, ^^1 is the first distance, ^^2 is the second distance, ^^ is the first displacement, and y is the second displacement.

[0020] In some cases, it may be desired to establish an error on the determination of the alignment. Thus, according to some examples, the method further comprises determining an error on the determination of the alignment. The error may be a standard deviation, a variance, or some other metric suitable for characterizing the certainty with which the alignment has been determined. Specifically, a statistical tool such as Gage R&R may be used to determine the Gage capability (Cg) or the actual capability (Cgk). The error may be advantageously used to place a lower confidence limit on thealignment, and / or to determine the precision of the image taking. For example, a high error value may be indicative of a poor SNR or poor calibration of the X-ray apparatus.

[0021] In the example provided above, where the alignment can be expressed as an (x, y) co-ordinate, and the alignment can be determined according to the above described equations (or mathematically equivalent equations), determining an error on the determination of the alignment may comprise determining a first standard deviation of the first displacement and determining a second standard deviation of the second displacement based on the equations:, and, wherein ^^ is the first angle, ^^′ is the second angle, ^^1 is the first distance, ^^2 is the second distance, Δ^^ is the standard deviation of the first displacement, and Δy is the standard deviation of the second displacement.

[0022] Here, Var(d1) is the variance on the determination of the first distance, which may be an output from an edge detection process (which may be implemented by artificial intelligence or other means). That is, there may be some level of uncertainty in the identification of the apparent edges, e.g., due to the SNR of the X-ray images, and thus the distances between these apparent edges will have some uncertainty carried through. This uncertainty may thus be expressed as Var(d1) and Var(d2).

[0023] These equations may be particularly advantageous when it is only possible to get one distance because of practical limitations. In such cases, it can be assumed that the error of the (x, y) coordinate can be obtained by assuming Var(d1) is the same as Var(d2) because θ and (π / 2 – θ) would have substantially the same level of image quality.

[0024] Moreover, these equations may be particularly advantageous when it is only possible to get two difference distances independently, so matching d1 and d2 may not be possible. In such cases, it is possible to calculate Var(d1) and Var(d2) and apply them to these equations.

[0025] The method may further comprise comparing the determined alignment, and optionally the error on the determination of the alignment, to a desired alignment for the stack to make a quality determination for the stack.

[0026] The quality determination may be binary, i.e., either approved or rejected. This binary determination may be a result of a comparison of the determined alignment to some desired alignment. The desired alignment may be a value for an entire stack, e.g., an average of inter-sheet alignment, a value for each inter-sheet alignment, or some combination thereof. In some examples, the worst case may be assumed when factoring in the error on the determination of the alignment, such that it can be assured that every stack that does not fulfill the desired quality characteristics is rejected.

[0027] The preferred ranges for the first angle and the second angle may be dependent on a number of factors, such as a standard deviation of a determined alignment resulting from use of particular angles, the ease of capture of images with beam paths at said angles, etc. If the electrode sheets comprise tabs, especially tabs at the corner region, then the tabs may also be considered so that they do not impede the capture of a useful image.

[0028] It is realized as part of the present disclosure that, to reduce the mathematical error on the determination of the alignment, the first angle should be closer to substantially 90 degrees, and the second angle should be closer to substantially 0 degrees. In this way, the standard deviation may be reduced small, and thus the repeatability of the determination is improved.

[0029] Put another way, as much as the first angle and second angle are close to 90 and 0 degrees, the first distance and second distance can be as close to direct measurements of an x and an x and y displacement, separately, such that the error introduced by subsequent calculations can be decreased.

[0030] Taking into account the image quality, it is further realized as a part of the present disclosure that a better-quality image may be more difficult to obtain if the first angle is substantially more than 80 degree or less than 10 degrees.

[0031] Thus, the first angle may be selected from a range of 55 to 90 degrees, preferably 75 to 85 degrees, and the second angle may be selected from arange of 20 degrees or less, preferably 10 degrees or less. In a most preferred example, the first angle is 80 degrees, and the second angle is 10 degrees. An optimal angle may be deduced through iterative testing in particular implementations.

[0032] The above-described method may be performed by a processing device, e.g., implemented as an alignment determination unit, which may be local to or remote from the system that captures the images, depending on the implementation.

[0033] Viewed from a wider perspective, and according to a further aspect of the present disclosure, there is provided a method for determining alignment of a stack of electrode sheets, comprising arranging the stack at a first angle relative to a first X-ray imaging apparatus, capturing a first X-ray image of at least a corner region of the stack with the X-ray imaging apparatus, arranging the stack at a second angle relative to the first or a second X-ray imaging apparatus, and capturing a second X-ray image of at least the corner region of the stack with the first or the second X-ray imaging apparatus.

[0034] The method then further comprises determining alignment of a stack of electrode sheets by performing the method as described above, i.e., using the images captured at the first and second angles.

[0035] The arranging and capturing steps may be performed in any sequence or simultaneously, depending on the implementation and / or the configuration of the X-ray imaging apparatus.

[0036] The method may further comprise obtaining the calibration X-ray image by capturing an X-ray image of a calibration medium with the X-ray apparatus, thus advantageously ensuring that values for the first and second angles, used in the determination of the alignment, can be relied upon, or at least a further source of error in the determination can be determined and accounted for.

[0037] That is, from the perspective of the alignment determination unit, the method performed thereby may further comprise obtaining a calibration X-ray image, and determining a calibration of the X-ray imaging apparatus based on the calibration image.

[0038] The calibration may be performed once during the installation and commission of the system, or calibration may be performed at intervals as part of a maintenance procedure.

[0039] The calibration medium may be configured in any suitable way for determining that an X-ray beam can be aligned at the first angle and the second angle, relative to the stack.

[0040] For example, the calibration medium may be an elongated metal cylinder, having an axis of extension along the first angle relative to a stack (or a holder therefor), such that an X-ray image captured of the cylinder indicates an extent to which the X-ray source is aligned with the center of the cylinder, and thus the extent to which the beam path of the X-ray source is directed along the desired angle. It will be appreciated that this could also be implemented as a cylindrical hole in a metal block.

[0041] As a further example, the calibration medium may be a stack-like test block that has been manufactured according to high tolerances, such that it is known what the relative overhangs between different layers of the block are, with high precision. Thus, the X-ray source(s) may be adjusted until the determined overhangs correspond to the known overhangs.

[0042] When determining the alignment of the electrode stack, it may be advantageous to assess a plurality of corner regions of the same stack. This may be carried out using further X-ray sources and detectors, or the method may further comprise rotating the electrode stack at least once to present a different corner region of the stack to the X-ray imaging apparatus, and further determining alignment of the stack of electrode sheets by performing the above-described method for alignment determination for said different corner region.

[0043] By determining an alignment for a plurality of corner regions, and preferably at least opposite corners (i.e., top-right and bottom-left, or top-left and bottom-right), a more thorough and complete determination of the alignment of the electrode stack may be obtained, thus allowing for a consideration of, for example, anomalous sizes of sheets.

[0044] Capturing an X-ray image of at least the corner region of the stack with the X-ray imaging apparatus may comprise capturing an X-ray image of aplurality of corner regions, each corner region belonging to a different stack of a plurality of stacks, and determining alignment of each stack of electrode sheets as described above for said each corner region. Put another way, a number of stacks may be arranged together and imaged together, thereby further accelerating the rate at which an alignment determination can be made. Two, four, or more stacks may be arranged together for simultaneous imaging.

[0045] The X-ray apparatus may be configured to scan along the plurality of corner regions, for example the X-ray apparatus may be configured as a line- scanning X-ray camera.

[0046] According to a further aspect of the present disclosure, there is provided a system for determining alignment of a stack of electrode sheets, comprising, an X-ray imaging apparatus comprising at least one X-ray source and at least one X-ray detector, a holder configured to hold the stack, and an alignment determination unit configured to perform the method described above for determining alignment.

[0047] The system further comprises a controller coupled to at least the X-ray imaging apparatus and the alignment determination unit, configured to control at least the X-ray imaging apparatus and the alignment determination unit to perform the method described above in respect of the capture and processing of the images.

[0048] That is, the system may comprise a controller to control the operations of the X-ray imaging apparatus to obtain images, and the controller may pass these images to the alignment determination unit to determine the alignment of the stack(s). The controller may thus be implemented as a processing device that is installed in a same machine as the X-ray imaging apparatus, or the controller may be at least partially remote from said machine.

[0049] In some examples, the X-ray apparatus comprises two X-ray sources and two X-ray detectors arranged with a predefined angular difference between their beam paths, the predefined angular difference being the difference between the first angle and the second angle. In this way, the arranging of the stack may be simplified such that the stack may only bearranged correctly relative to one of the X-ray sources, and it will thereby be ensured that the stack is aligned relative to the other X-ray source.

[0050] Such an arrangement, with two X-ray sources, is further advantageous in that a movement of the stack and / or the X-ray imaging apparatus may not be required to arrange the stack at the first and second angles relative to the X-ray imaging apparatus. Hence, there is less risk of an incorrect alignment resulting from imprecise movement of the stack or X-ray imaging apparatus.

[0051] In such an example, the X-ray sources may be respectively arranged to be incident upon different edges of the corner region of the stack. It will be understood that a corner region of an electrode stack comprises two edges converging at a corner. Thus, arranging the different X-ray sources to be incident upon different edges may thus allow the X-ray detectors to be arranged along different edges, and hence the detectors can be better spaced from one another without interfering with each other. Hence, a more compact system can be provided.

[0052] The holder may be any suitable gripper or mount for the stack, or a plurality of stacks. In a simple example, the holder may be a pair of raised walls on a placement surface such that the stack can be arranged to abut said raised walls.

[0053] In some examples, the holder may be configured to hold the stack by holding a battery cell in which the stack is installed. For example, the holder may be configured to grip a casing of one or more battery cells. Brief Description of the Drawings

[0054] One or more example implementations of the present disclosure will be described, by way of example only, and with reference to the following figures, in which:

[0055] Figure 1A schematically shows a top view of a stack of electrode sheets;

[0056] Figures 1B and 1C show different side views of the stack of electrode sheets with enlarged sections;

[0057] Figure 2A schematically shows an X-ray apparatus capturing an X-ray image of a corner region of the stack of electrode sheets shown in figures 1A to 1C, according to a known technique;

[0058] Figure 2B shows an example resultant X-ray image from the X-ray capture shown in figure 2A;

[0059] Figures 2C and 2D demonstrate a shortcoming of the known technique shown in figure 2A;

[0060] Figures 3A to 3D schematically show an X-ray apparatus capturing an X-ray image of a corner region of a stack of electrode sheets, and the resultant X-ray images, according to an example implementation of the present disclosure;

[0061] Figure 4 illustrates a method for determining alignment of a stack of electrode sheets, according to an embodiment of the present disclosure; and

[0062] Figure 5 schematically shows a system for determining alignment of a stack of electrode sheets, according to an embodiment of the present disclosure. Detailed Description

[0063] The present disclosure is described in the following by way of a number of illustrative examples. It will be appreciated that these examples are provided for illustration and explanation only and are not intended to be limiting on the scope of the present disclosure. Instead, the scope of the present disclosure is defined by the appended claims.

[0064] Furthermore, although examples may be presented individually for the sake of focused discussion of particular features, it will be recognized that the present disclosure also encompasses combinations of the examples described herein.

[0065] Figure 1A schematically shows the top view of a stack 100 of electrode sheets 102, 104. The electrode sheets 102, 104 may also be referred to as, e.g., sheets, or layers. The stack 100 may optionally have tabs 106, which may enable a connection of the sheets 102 and / or 104 to respective current collectors.

[0066] The stack 100 may be a component for the manufacture of a battery cell. That is, the sheet 102 may be an anode sheet (e.g., a coated or uncoated metal foil or the like) and the sheet 104 may be a cathode sheet (e.g., a coated or uncoated metal foil or the like), in which case a separator (not shown) may be placed between the sheets 102, 104 to prevent electrical contact between the cathode and the anode sheets 102, 104. The optional tabs 106 may be an integral part of the uncoated metal foil of a sheet. The tabs of sheet 104 may extend beyond the edges of the larger sheet 102, as shown in figure 1A. These tabs are not shown in figures 1B and 1C.

[0067] The top-view, or the top-down view, is the top view of a plane that is defined by the x-axis and the y-axis. The x-axis and the y-axis are perpendicular to each other in this example, as the geometry of the (pre- formed) electrode sheets 102, 104 is substantially rectangular, with two pairs of parallel sides meeting at right-angles. The x-axis and the y-axis do not need to be perpendicular; they may have any other relative (non-parallel) configuration.

[0068] The two uppermost electrode sheets 102, 104 are visible in figure 1A. A first electrode sheet 102 is sized larger than the second electrode sheet 104. It will be appreciated that, in some examples, the sizes of the sheets 102, 104, may instead be the same. The electrode sheets 102, 104 are planar, extending in the plane defined by the x-axis and the y-axis.

[0069] As mentioned above, each sheet 102, 104 is rectangular and thereby comprises four edges 102a-d, 104a-d and four corners where the respective relevant edges of the same sheet meet. A corner region can be seen as a region around at least one corner, including the at least one corner. A corner region thus encompasses corresponding corners of the different sheets 102, 104, e.g., the bottom-left corner of the sheets 102 and 104 as shown in figure 1A.

[0070] The sheets 102, 104 are rectangular in the figure, but they may instead have any other planar shape, such as square, parallelogrammatic, circular, triangular, or trapezoidal. It is further not required that the sheets 102 and 104 have the same shape as each other. If any sheet 102, 104 has any other shape than rectangular, the person skilled in the art should understand thatthe items in the description relating to the rectangular shape shall be adapted to apply to the, of the sheet, embodied shape.

[0071] The sheets 102 and 104 are arranged relative to each other. The figures show the sheets 102 and 104 arranged concentric as seen from the top-down view, but they may be arranged differently, for example they may be eccentric, meaning that a displacement between the center of the sheets 102 and 104 in the top-down view may occur. The sheets 102, 104 being arranged concentric or eccentric from one view means that the two- dimensional representation of the sheets 102, 104 from that view are concentric or eccentric. It does not mean that the center of the sheets 102, 104 need to be concentric or eccentric.

[0072] It will be understood that the sheets 102 and 104 (and other sheets in the stack 100) have a preferred alignment. That is, there may be a range of tolerance for the displacement between corresponding edges of the sheets 102 and 104 in the x-direction, and in the in the y-direction, such as 1 mm, 2 mm, or some other amount, depending on the cell design. A substantial deviation from this preferred alignment may lead to a poor performance of the cell or, in some cases, failure of the cell.

[0073] Figures 1B and 1C shows side views of the stack 100. The optional tabs 106 are not included in these figures. The z-axis, which may also be viewed as a ‘stacking axis’ or a ‘stacking direction’ is perpendicular to the x- axis and perpendicular to the y-axis in this example. As seen by the coordinate system, figure 1B is showing the stack 100 along the y-direction. Figure 1B is showing the edges with reference 102c, 104c in figure 1A. As seen by the coordinate system, figure 1C is showing the stack 100 along the x-direction. Figure 1C is showing the edges with reference 102b, 104b in figure 1A.

[0074] The illustrated stack 100 has eight sheets of the type of sheet 102 and eight sheets of the type of sheet 104, placed so that every other sheet is a sheet like sheet 102 shown in figure 1A (e.g., an anode sheet) and every other sheet is a sheet like sheet 104 shown in figure 1A (e.g., a cathode sheet). A stack 100 may have any number of sheets 102, 104 and a personskilled in the art will understand that the number of sheets 102, 104 is not limited by the illustrated number of sheets 102, 104 in the figures.

[0075] Figures 1B and 1C also show enlarged sections 110. The enlarged sections 110 show an enlarged side view of a corner region of the stack 100. This side view shows two sheets 102, 104. The enlarged section 110 of figure 1B shows the extension of the sheets 102 and 104 in the x-direction along the x-axis. This extension may be referred to as an ‘x overhang’. The difference of extension between 102 and 104 in the x-direction is labeled as dx. The enlarged section 110 of figure 1C shows the extension of the sheets 102 and 104 in the y-direction along the y-axis. This extension may be referred to as an ‘y overhang’. The difference of extension between 102 and 104 in the y- direction is labeled as dy.

[0076] In the illustrated example, all corresponding edges 102a-d, 104a-d are aligned along the z-axis in the z-direction with all other edges with the same reference number. This means, for example, that an edge 102a of a sheet 102 is aligned along the z-axis with the corresponding edge 102a of all other sheets 102 of the stack 100. A perfect alignment is seen in the figures 1B and 1C as the edges of all sheets 102, 104 of a corresponding type are aligned with each other in the direction of the z-axis. This may be a preferred state for a stack 100 of electrode sheets 102, 104.

[0077] However, during assembly of a battery cell and, namely, the stack 100 of electrode sheets 102, 104, some sheets 102, 104 may be misaligned – e.g., translationally and / or rotationally relative to each other. Accordingly, the operational life of the resultant battery cell may not be as long as a battery cell having better aligned electrode sheets 102, 104. For example, an anode sheet may be misaligned relative to a (smaller underlying or overlying) cathode sheet in a stack of electrode sheets 102, 104 such that their edges are closer together than a minimum safe distance. It is preferred that any such malformed stacks are identified in an accurate and rapid manner.

[0078] Accordingly, figure 2A schematically shows a known process for determining an alignment of a stack 100 of electrode sheets 102, 104, using an X-ray imaging device 200.

[0079] In particular, figure 2A shows a stack 100 of electrode layers 102, 104, and an X-ray imaging device 200. The stack 100 may be the stack of electrode sheets shown in figures 1A to 1C. The X-ray imaging device 200 comprises an X-ray source 202 and a detector 210. The X-ray source 202 emits an X-ray beam 204 in the beam direction 206 towards the detector 210. The detector 210 is positioned so as to be able to detect at least a part of the X-ray beam 204. In other words, an X-ray image (shown in figure 2B) is taken along a beam direction 206.

[0080] In the illustrated example, the alignment of a stack 100 of electrode sheets 102, 104, is determined by analyzing the overhang of the sheets 102, 104. This is done by utilizing the X-ray imaging device 200.

[0081] The X-ray beam 204 of the X-ray source 202 of the X-ray imaging device 200 has a conical spread, as is understood by those skilled in the art. It is to be appreciated that, although the illustrated figure is showing the X-ray beam 204 having flat shape, the actual shape may extend in three dimensions. The cross-section of the X-ray beam 204 along the beam direction 206 may, for example, be circular, oval, obround, rectangular, or any other shape.

[0082] To be able to distinguish the sheets overhang, the X-ray imaging device 200 is positioned to capture an X-ray image, or image, from a side view of the stack 100. It is preferred that the side view is along the plane of the electrode sheets 102, 104, but the side view could be any view where the sheets do not substantially overlap with each other, as seen from the beam direction 206, which may also depend on, e.g., the resolution of the X-ray imaging device 200.

[0083] The X-ray imaging device 200 is further positioned so that the X-ray beam 204 from the X-ray source 202 at least partially penetrates a corner region of the stack 100 before the X-ray beam 204 reaches the detector 210. This corner region encompasses an expected position, and a region around the expected position, of one corner from each of sheets 102, 104 of the stack 100. The expected position of the corners are encompassed in the corner region rather than the corners themselves since the misalignment of sheets 102, 104 could have move the actual corners to be outside of the X-ray beam 204, or some corners may be folded, and thus the region captured should be configured with this in mind. In the illustrated example, the corner region that the X-ray beam 204 penetrates is the bottom left corner of the sheets 102, 104 as shown in figure 2A, and it contains not only the corner of the two topmost sheets 102, 104, i.e. the visible sheets 102, 104, but also the corners of the sheets below the visible sheets in the stack 100. It will be appreciated that although the provided examples only cover the use of one stack 100, this is not intended to be limiting. Assuming a suitable setup, any number of stacks 100 may be scanned, e.g., with a line-scanning X-ray camera.

[0084] To capture information both about the x-overhang, like the x-overhang dx shown in figure 1B, and the y-overhang, like the y-overhang dy shown in figure 1C, of the sheets 102, 104 as shown in figure 2A, the X-ray beam 204 is incident at an angle to the stack 100, creating an angle ^ between the path of the X-ray beam 204 exiting the stack 100, and the x-axis of the stack 100. In the illustrated example, the angle ^ is the angle between the stack 100 and the path of the beam direction 206 and is dependent on, e.g., the angle of incidence of the X-ray beam 204 to the sheets 100.

[0085] Consequently, the X-ray beam 204 carries information relating to the alignment of stack 100 when it reaches the detector 210. More specifically, the X-ray beam 204 carries information about the x-overhang, like the x- overhang dx shown in figure 1B, jointly with the y-overhang, like the y- overhang dy shown in figure 1C, as seen from an X-ray beam 204 with a beam direction 206 with an angle ^.

[0086] An image is created by the X-rays being incident on the detector 210. This image depicts the structure / alignment of the corner region of the stack 100, obtained using the chosen relative positions of the X-ray imaging device and the stack 100. Due to the nature of X-rays and using X-ray technology the sheets 102, 104 of the stack 100 may, in the image, be depicted as unsharp or blurry. Edge detection processing may preferably be applied to the image so as to identify the apparent position of the edges of the sheets 102, 104.

[0087] The apparent positions of the edges of the sheets 102, 104 may be where the edge detection processing identifies the edges of the sheets 102, 104 to be, from the image. Apparent edges are edges with the position of the apparent positions of the edge of the sheets 102, 104, so that the apparent edges represent the real edges of the sheets 102, 104 as well as the edge detection will allow during the circumstances. These circumstances include, for example, the resolution of the X-ray imaging device 200 and the relative positions of the stack 100 and the X-ray imaging device. It is preferred that the apparent edges match the actual edges as accurately as possible. The apparent edges, or the corresponding apparent corners, may be used for computational analysis, representing the actual edges, or corners.

[0088] Figure 2B schematically illustrates an image portion 220 containing the edges of two sheets like the sheets 102 and 104 from figure 2A. This image portion is a portion of the image created by the X-ray imaging device 202 from figure 2A. In the image portion 220, the image plane corresponds to the z- axis, and is along a stacking direction and the f(x,y)-axis represents a plane orthogonal to the beam direction 206, which can be understood as having a geometric relationship to the x and y axes defined by, e.g., the sheet 102.

[0089] The difference of extension d in the image portion 220 is the distance between the edge of a sheet like the sheet 102 from figure 2A and the edge of a sheet like the sheet 104 from figure 2A, along the f(x,y) axis. Since the beam direction 206 is at an angle ^, the distance d contains information about the x-overhang dx shown in figure 1B, jointly with the y-overhang, like the y- overhang dy shown in figure 1C, of the sheets 102, 104. The angle ^ decides if the x-overhang and the y-overhang are represented equally, or if one is overrepresented, in d.

[0090] Because only one distance d represents both the x-overhang and the y-overhang, this results in an undetermined system. This means that misalignment along the beam direction 206 can occur without being represented in the distance d. Figure 2C illustrates an example of this shortcoming.

[0091] In particular, figure 2C illustrates a stack 100 containing sheets 102, 104. Only one sheet of the type of sheet 102 is present, but it is to beunderstood that a sheet of the type of sheet 102 is between the sheets of the type of sheet 104. The sheets of the type of sheet 102 may be aligned with other sheets of the type of sheet 102. In figure 2C, the visible sheets of the type of sheet 104 are misaligned in relation to each other. More precisely, in figure 2C, the corner of the sheets of type of sheet 104 are, in relation to the back most visible sheet of the type of sheet 104, shifted down and to the right, so that the relative misalignment has caused the corner in the corner region of the sheet of the type of sheet 104 to move along the beam direction 204.

[0092] Figure 2D schematically illustrates an image portion 220 containing edges of two sheets like the sheets 102 and 104 from figure 2C, along the f(x,y)-axis. The difference of extension d is the distance between the edge of a sheet like the sheet 102 from figure 2C and the edge of a sheet like the sheet 104 from figure 2C, along the f(x,y) axis.

[0093] If the corner of a sheet 102, 104 would have a misalignment in the direction of the beam 204, as compared to the intended position of the corner of the sheet 102, 104, there would be no difference of extension d in the corresponding image portion 220 as compared to if the sheet 102, 104 would not have been misaligned, assuming the adjacent sheet 102, 104 that the difference of extension d is measured in reference to, has the same position in both cases.

[0094] For example, assume that the (visible or omitted) sheets of the type of sheet 102 in figure 2C have the same apparent edge as each other, excluding the difference in position along the z-axis. Since the visible sheets of the type of sheet 104 are misaligned in relation to each other along the direction of the beam 204, all of the visible (in figure 2C) sheets of the type 104 will have the same apparent edge and the sheets of the type of sheet 104 will therefore have the same difference in extension d as each other despite having substantially different (mis)alignments.

[0095] The relative misalignment of the corner of the sheets 102, 104 could be the result of a on translation and / or rotation of the sheets 102, 104 in relation to the other sheets 102, 104.

[0096] Therefore, it is realized as a part of the present disclosure that an improved technique for determining alignment of the stack 100 is desired.However, as discussed above, approaches involving CT scans are costly, and consume substantial time, space, and processing power, thus limiting the rate at which stacks can be analyzed.

[0097] Figure 3A schematically shows a process for determining alignment of a stack 100 of electrode sheets 102, 104, according to an example implementation of the present disclosure, using X-ray imaging.

[0098] In particular, figure 3A illustrates a stack 100 and two X-ray imaging devices 200-1, 200-2. Although the illustrated example shows two separate devices, the same device with different relative positions to the stack 100 may be used, in some examples. The different relative positions between the stack 100 and an X-ray device 200-1, 200-2 may be achieved due to repositioning of the stack 100, repositioning of the X-ray device 200-1, 200-2, or a combination thereof.

[0099] The stack 100 may be the same as illustrated in figures 1A to 1C The individual X-ray imaging devices 200-1, 200-2 may have the same setup as the X-ray device 200 illustrated in Figures 2A to 2D. However, by capturing two X-ray images from different angles, as illustrated, the system may no longer be underdetermined, and information both about the x-overhang and about the y-overhang may be obtained.

[0100] The X-ray imaging devices 200-1, 200-2 consist of X-ray sources 202- 1, 202-2, and detectors 210-1, 210-2. The X-ray sources 202-1, 202-2 emit X- ray beams 204-1, 204-2 in beam directions 206-1, 206-2 towards the respective detectors 210-1, 210-2. The detectors 210-1, 210-2 are adapted to receive the respective X-ray beam 202-1, 202-2 and produce an image therefrom. The X-ray beams 204-1, 204-2 are directed to at least partially go through a corner region of the stack 100 before reaching the respective detector 210-1, 210-2. The components that are denoted with the identifier -1 can be seen as components or sub-components related to the X-ray imaging device 200-1. The components denoted with the identifier -2 can be seen as components or sub-components related to the X-ray imaging device of the type 200-2. In other words, an X-ray image (shown in figure 3C) of a corner region of the stack 100 is taken along a beam direction 206-1, and an X-ray image (shown in figure 3D) of the same corner region is taken along thebeam direction 206-2. These images may be taken simultaneously, sequentially, or during overlapping timeframes.

[0101] As is true for the X-ray beam 204 in figure 2A, the X-ray beams 204-1, 204-2 of figure 3A are illustrated as being flat, but the actual shape may extend in three dimensions. The cross section of the X-ray beams 206-1, 206- 2 along the respective beam direction 204-1, 204-2 may, for example, be circular, oval, obround, rectangular, or any other shape.

[0102] The alignment of a stack 100 of electrode sheets 102, 104 is determined by analyzing the overhang of the sheets 102, 104 in multiple directions. This is done by analyzing the X-ray images of multiple X-ray imaging devices 200-1, 200-2.

[0103] To be able to distinguish the sheets 102, 104 overhang, the X-ray imaging devices 200-1, 200-2 are positioned to capture images from side views of the stack 100. It is preferred that the side views are along the plane of the electrode sheets 102, 104, as seen in figures 1B and 1C, but the side views could be any view where the sheets do not substantially overlap with each other, as seen from the respective beam directions 206-1, 206-2, which may also depend on, e.g., the respective resolution of the X-ray imaging devices 200-1, 200-2. The X-ray imaging devices 200-1, 200-2 may have different positions relative to the plane of the electrode sheets 102, 104 as compared to each other.

[0104] The X-ray imaging devices 200-1, 200-2 are further positioned so that the X-ray beams 204-1, 204-2 from the X-ray sources 202-1, 202-2 at least partially penetrate corner regions of the stack 100 that at least partially overlap with each other, before the X-ray beams 204-1, 204-2 reaches the detector 210-1, 210-2. These corner regions encompass the expected positions, and a region around the expected positions, of one corner from each of the sheets 102, 104 of the stack 100. The expected positions of the corners are encompassed in the corner region rather than the corners themselves since the misalignment of sheets 102, 104 could have move the actual corners to be outside of the X-ray beams 204-1, 204-2, or some corners may be folded, and thus the region captured should be configured with this in mind.

[0105] In short, to obtain information about the corners of the sheets 102, 104, it is desired that the corners of the sheets 102, 104 that are intended to be examined are in the path of the X-ray beams 204-1, 204-2. In the illustrated example, the corner region that the X-ray beams 204-1, 204-2 penetrates contains the bottom left corners of the sheets 102, 104 as shown in figure 3A, and it contains not only the corner of the two topmost sheets 102, 104, i.e. the visible sheets 102, 104, but also the corners of the sheets below the visible sheets in the stack 100. As mentioned before, it will be appreciated that although the provided examples only cover the use of one stack 100, this is not intended to be limiting. Assuming a suitable setup, any number of stacks 100 may be scanned, e.g., with a line-scanning X-ray camera.

[0106] In the illustrated example, the X-ray imaging devices 200-1, 200-2 are positioned so that the point of incidence of the respective X-ray beams 204- 1, 204-2 are on different sides of the stack 100. Since information regarding the internal structure of a corner region of a stack 100 could be obtained from any relevant side view of the stack 100, it is to be understood that the X-ray image devices 200-1, 200-2 could be positioned so that the point of incidence of the respective X-ray beams 204-1, 204-2 are on the same or separate sides of the stack 100 as each other. The angles ^^, ^^are therefore measured between the respective beam paths, which may be directionless and along to the respective beam directions 206-1, 206-2, and one side of the stack 100. Figure 3B illustrates these angles ^^, ^^.

[0107] More specifically, figure 3B schematically shows the beam directions 206-1, 206-2 at a corner region of the sheets 102, 104 of the stack 100 from the top view of a plane that is defined by the x-axis and the y-axis. There are two beam directions 206-1, 206-2 from each X-ray imaging device 202-1, 202-2, illustrating where a portion of the X-ray beams 204-1, 204-2 from figure 3A may intersect the corner region.

[0108] The angle ^^is between the beam path, along a beam direction like the beam direction 206-1, and a side of the stack 100. The angle ^^^is between the beam path, along a beam direction like the beam direction 206-2, and the same side of the stack 100 that ^^is measured relative to. The side of the stack may extend along the x-axis, as illustrated in figure 1A, and thereforethe angles ^^, ^^may in these cases be seen as the angles between the respective beam paths and the x-axis.

[0109] To capture information both about the x-overhang, like the x-overhang dx shown in figure 1B, and the y-overhang, like the y-overhang dy shown in figure 1C, of the sheets 102, 104 as shown in figure 3A and 3B, the X-ray beams 204-1, 204-2 are incident at separate angles ^^, ^^^to the stack 100. In other words, to avoid the system being undetermined, the angles ^^, ^^are not of the same size. Consequently, the relative angle ^^is non-zero.

[0110] It is preferred that the first angle ^^is selected from a range of 55 to 90 degrees, more preferably 75 to 85 degrees. The second angle ^^is preferably 20 degrees or less, or more preferably by 10 degrees or less. In a most preferred example, the first angle ^^^is 80 degrees and the second angle^^^^is 10 degrees.

[0111] The top-view of the corner region illustrated in figure 3B shows coordinates (0,0), (x1, y1) for a corner each of the sheets 102, 104. The co- ordinate (x1, y1) will be appreciated as a way of expressing the alignment of the sheet 104 relative to the sheet 102.

[0112] The distances d1, d2 are the overhang that may be visible in the images obtained by the respective X-ray imaging devices 200-1, 200-2 along the respective beam path. The image portions 220 presented in figures 3C and 3D are portions of the images obtained by the X-ray imaging devices 200-1, 200-2. The axes f1(x,y), f2(x,y), orthogonal to the respective beam directions 206-1, 206-2, are effectively the horizontal axis of the images taken along said beam paths.

[0113] Figure 3C and 3D illustrate the overhang of the sheets 102, 104 as seen from the X-ray imaging devices 200-1, 200-2 that are placed along the respective beam directions 206-1, 206-2. In the images, and the image portions 220, the image plane corresponds to the plane defined by the z-axis, which is along a stacking direction, and an orthogonal axis f1(x,y), f2(x,y), orthogonal to the beam direction 206-1, 206-2, which can be understood as having a geometric relationship to the x and y axes defined by, e.g., the sheet 102.

[0114] Figure 3C illustrates the distance d1 in the image portion 220 obtained from the X-ray imaging device like the X-ray imaging device 200-1. For this representation, the axis orthogonal to the beam direction like the beam direction 206-1 is along the f1(x,y)-axis.

[0115] Figure 3D illustrates the distance d2 in the image portion 220 obtained from the X-ray imaging device like the X-ray imaging device 200-2. For this representation, the axis orthogonal to the beam direction like the beam direction 206-2 is along the f2(x,y)-axis.

[0116] The x-overhang and the y-overhang can be calculated from the distances d1, d2 using trigonometric equations. These trigonometric equations could, for example, be:The variables in the trigonometric equation are represented in figures 3A to 3D. ^^ as used here is the first angle ^^, between the beam path 206-1 and the stack 100, and ^^′ is the second angle ^^, between the beam path that 206-2 and the stack 100. ^^1 from the trigonometric equation may be the distance d1 from figure 3C, between the edges of the sheets 102, 104 as seen from an X- ray imaging device 200-1 and ^^2 from the trigonometric equation may be the distance d2 from figure 3D, between the edges of the sheets 102, 104. The ^^ in the trigonometric equation corresponds to a determined x-overhang, and the y in the trigonometric equation corresponds to the determined y-overhang. In short, using this trigonometric equation, the x-overhang and the y-overhang of a sheet 102, 104, can be calculated from the distances d1, d2 and the first and second angles ^^^and ^^.

[0117] However, knowing the x-overhang and the y-overhang may not give an entirely comprehensive overview of the alignment of a sheet 102, 104. As mentioned before, some sheets 102, 104 may be misaligned – e.g., translationally and / or rotationally relative to each other. In order to better determine the existence of translational and rotational misalignment, multiple (adjacent or non-adjacent) corner regions of the same stack 102, 104 can be examined. These corner regions may encompass different corners of thesame sheets 102, 104 as each other. The calculated positions of different corners of the same sheet 102, 104 may be examined in relation to each other to get a more comprehensive view of the (mis)alignment of the sheet 102, 104.

[0118] As mentioned previously, some corners may be folded. One of the reasons it could be beneficial to examine many corners of the same sheet 102, 104 is to get information about whether or not the corners are folded. It may also be beneficial to examine more than two corners, or to take multiple measurements of the same corner, to reduce the possibilities for error and to obtain a smaller margin of error.

[0119] When the alignment of a sheet is decided, the determination of the alignment may include an error. This error may be determined. A method of determining this error is to determine the standard deviations of the x- overhang and the y-overhang. The standard deviation of the x-overhang can be calculated the, and the standard deviation of the y-overhang can be calculated using the following equation:. The variables in the equation are the same as those used above. Δ^^ is the standard deviation of the x-overhang, and Δy is the standard deviation of the y-overhang.

[0120] In short, using this equation, the standard deviations of the x-overhang and the y-overhang of a sheet 102, 104, can be calculated from the distances d1, d2 and the first and second angles ^^^and ^^.

[0121] The obtained x-overhang, y-overhang, and possible standard deviations thereof can be compared to desired values in order to assess the quality of alignment of the stack 100.

[0122] A method 400 of determining alignment is illustrated in figure 4. This method 400 is for determining the alignment of a stack of electrode sheets, which may be the stack 100 illustrated in figure 1A to 1C. This method may,for example, be performed with a system like the system illustrated in figures 3A to 3D.

[0123] As illustrated in figure 4, the method 400 comprises obtaining 410 a first X-ray image and obtaining 420 a second X-ray image of a corner region of an electrode stack. The first and second X-ray images are taken along respective first and second beam paths, having respect first and second angles relative to the stack.

[0124] The method 400 further comprises determining 430 a first distance between the apparent edge of a first sheet and the apparent edge of a second sheet, along the plane of the sheets, as seen in the first image taken along the first beam path.

[0125] The method 400 further comprises determining 440 a second distance between the apparent edge of the first sheet and the apparent edge of the second sheet, along the plane of the sheets, as seen in the second image taken along the second beam path.

[0126] One method of obtaining the apparent edges is to use edge detection processing on the image in question. Edge detection processing (e.g., implemented with AI methods) used on the image may identify the apparent position of the edges of the sheets. It is preferable that any apparent edges match the actual edges as accurately as possible. However, the accuracy of the apparent edge can vary depending on, for example, the resolution of the X-ray imaging device used to obtain the image in question, and the relative positions of the stack and the X-ray imaging device used to obtain the image in question.

[0127] The illustrated method 400 concludes with determining 450 an alignment between the first sheet and the second sheet, based on the first distance, the second distance, the first angle, and the second angle.

[0128] The steps 410, 420, 430, 440, 450 of this method 400 are, by the arrows in figure 4, illustrated in a specific order. It will be appreciated that the order of the steps 410, 420, 430, 440, 450 are provided for illustration and explanation only and are not intended to be limiting on the scope of the present disclosure. Where applicable, the steps 410, 420, 430, 440, 450 may be taken in any order. For example, in figure 4, the step with the reference430 may be taken before or after the step with the reference 420. It will also be appreciated that other steps, not illustrated in the figure, may be taken in between the illustrated steps 410, 420, 430, 440, 450.

[0129] A system 500 for determining the alignment of an electrode stack is presented in figure 5. More specifically, figure 5 illustrates a system 500 for determining an alignment of a stack 512, where the stack 512 might be like the stack 100 of electrode sheets 102, 104 illustrated in figure 1A to 1C, or the stack 512 might be in a cell containing one or more such stacks. In further examples, multiple stacks or cells may have their alignments determined simultaneously by the system 500.

[0130] The system 500 includes a controller 502, an alignment determination unit 504 and a measuring system 510. The measuring system 510 includes two X-ray imaging devices 200-1, 200-2, and a holder 514. The holder 514 is arranged to hold a stack 512.

[0131] The X-ray imaging devices 200-1, 200-2 may be the same as illustrated in figure 2A to 2D. The X-ray imaging devices 200-1, 200-2 have their respective X-ray sources 202-1, 202-2, and their respective detectors 210-1, 210-2. The X-ray sources 202-1, 202-2 may emit their respective X-ray beams in their respective X-ray beam directions 206-1, 206-2, towards the respective detectors 210-1, 210-2. The paths of the X-ray beams of the X-ray imaging devices 200-1, 200-2 are arranged to intersect a corner region of a stack 512 when it is held in the holder 514.

[0132] In the illustrated example, the point of incidence of the respective X-ray beams along the X-ray beam direction 206-1, 206-2 are on separate sides of the stack 512, and thus it can be seen that the detectors 210-1 and 210-2 do not obstruct one another as they may otherwise do, where the beams directed to be incident upon the same side. However, it is not required that the X-ray sources 202-1, 202-2 are on different sides of the stack 512 as each other.

[0133] In some examples, only one X-ray imaging device 200-1, 200-2 may be used if the relative position of the stack 512 and the X-ray imaging device 200-1, 200-2 could change. Being able to change the relative position of an X-ray imaging device 200-1, 200-2 and the stack 512 would allow for a cornerregion to be intersected by multiple different beam paths from the same X-ray imaging device 200-1, 200-2.

[0134] It is also possible that if more than one imaging device is used, they do not have to be arranged to measure overlapping corner regions. For example, if the X-ray imaging devices 200-1, 200-2 would be arranged to measure corner regions that are on opposite sides of a stack 512 in a first direction, a relative rotation of the stack 512 and the X-ray imaging devices 200-1, 200-2 along a specific axis may allow for the X-ray imaging devices 200-1, 200-2 to measure the same corner region as they measured previously, but from a second direction, thus allowing two corner regions of the same stack of a stack 512 to be measured simultaneously.

[0135] The X-ray imaging devices 200-1, 200-2 may be arranged with the intention to be stationary, or they may be arranged so that one or both could be repositioned. The X-ray imaging devices 200-1, 200-2 may be arranged so that they can be reposition in order to change their angle or to measure another corner region of the stack of the stack 512.

[0136] Additionally or alternatively, the relative position of the X-ray imaging devices 200-1, 200-2 and the stack 512 could be altered by arranging the holder 514 so that it is able to move the stack 512. If the stack 512 would be arranged to move, it could allow for obtaining information about corner regions containing the (same or different) corners of the same sheet of the stack 512. The stack 512 may be arranged to be moved translationally and / or rotationally. The movement of the stack 512 may be due to movement of the holder 514. The movement of the stack 512 and / or the holder 514 may be controlled by a controller 502.

[0137] The dotted lines in figure 5 symbolize the communication between the components of the system 500. The communications may be of any form, including, for example utilizing wired or wireless methods. The dotted lines illustrate that the controller 502 and the X-ray imaging devices 200-1, 200-2 can communicate through connections between the controller 502 and the X- ray sources 202-1, 202-2, and / or between the controller 502 and the detectors 210-1, 210-2. The controller 502 can communicate with the holder 514 and the alignment determination unit 504. The dotted lines in figure 5 arean example of possible paths of communication in the system 500. There may be other paths of communications in the system 500.

[0138] Using, for example, the connections illustrated in figure 5, the controller 502 may communicate with the components of the measuring system 510. The controller 502 may obtain information from and give instructions to, for example, the X-ray imaging devices 200-1, 200-2 and the holder 514. The controller 502 may control when the X-ray imaging devices 200-1, 200-2 are to obtain images. The controller 502 may control any repositioning of the X- ray imaging devices 200-1, 200-2. The controller 502 may control any repositioning of the stack 512 and / or the holder 514. The controller 502 may communicate with the alignment determination unit 504.

[0139] The alignment determination unit 504 may obtain information from the controller 502. The alignment determination unit 504 may utilize the method described in relation to figures 3A to 3D, and figure 4 to, for example, determine an alignment and possibly determine an error in the determined alignment. The controller 502 or the alignment determination unit 504 may also compare the determined alignment to a preferred alignment to assess the quality of alignment of the stack 512.

[0140] Hence, the system 500 so described is compact and efficient, and able to rapidly process stacks to determine their alignments, and thereby provide a rapid quality control process that, in turn, accelerates the speed of manufacture of battery cells.

[0141] While the present disclosure is susceptible to various modifications and alternative forms, specific embodiments are shown and described above by way of example in relation to the drawings, with a view to clearly explaining the various advantageous aspects of the present disclosure. It should be understood, however, that the detailed description herein and the drawings attached hereto are not intended to limit the disclosure to the particular form disclosed. Rather, the intention is to cover all modifications, equivalents, and alternatives falling within the scope of the following claims.

[0142] Moreover, the present disclosure may be better understood through consideration of the following numbered clauses:1. A computer-implemented method for determining alignment of a stackof electrode sheets, wherein the electrode sheets are planar sheets extending in a first axis (x) and a second axis (y), and the electrode sheets are stacked along a third axis (z) perpendicular to the first axis and the second axis; the method comprising: obtaining a first X-ray image of a corner region of the stack, taken along a first beam path in the plane (x-y) of the electrode sheets, with a first angle from the first axis (x), obtaining a second X-ray image of the corner region of the stack, taken along a second beam path in the plane (x-y) of the electrode sheets, with a second angle from the first axis; determining a first distance (d1) between a first apparent edge of the first electrode sheet and a first apparent edge of the second electrode sheet, along the plane (x-y) of the electrode sheets in the first image; determining a second distance (d2) between a second apparent edge of the first electrode sheet and a second apparent edge of the second electrode sheet, along the plane (x-y) of the electrode sheets in the second image; and determining an alignment ((x,y)) between the first electrode sheet and the second electrode sheet, based on the first distance, the second distance, the first angle, and the second angle.2. The method according to clause 1, wherein the first angle is selectedfrom a range of 55 to 90 degrees, preferably 75 to 85 degrees, and the second angle is selected from a range of 20 degrees or less, preferably 10 degrees or less.3. The method according to clause 1 or clause 2, wherein determining analignment between the first electrode sheet and the second electrode sheet comprises determining a first displacement of the second electrode sheet from the first electrode sheet along the first axis, and determining a second displacement of the second electrode sheet from the first electrode sheet along the second axis, the second axis being perpendicular to the first axis.4. The method according to any preceding clause, wherein determiningan alignment between the first electrode sheet and the second electrode sheet comprises determining the alignment based on the equations: , and , wherein ^^ is the first angle, ^^′ is the second angle, ^^1is the first distance, ^^2is the second distance, ^^ is the first displacement, and y is the second displacement.5. The method according to any preceding clause, further comprisingdetermining an error on the determination of the alignment.6. The method according to clause 5, wherein determining an error on thedetermination of the alignment comprises determining a first standard deviation of the first displacement and determining a second standard deviation of the second displacement based on the equations: , and , wherein ^^ is the first angle, ^^′ is the second angle, ^^1 is the first distance, ^^2 is the second distance, Δ^^ is the standard deviation of the first displacement, and Δy is the standard deviation of the second displacement.7. The method according to any preceding clause, further comprisingcomparing the determined alignment, and optionally an error on the determination of the alignment, to a desired alignment for the stack to make a quality determination for the stack.8. The method according to clause 1 or clause 2, further comprising obtaining a calibration X-ray image, and determining a calibration of the X-ray imaging apparatus based on the calibration image. 9. A method for determining alignment of a stack of electrode sheets, comprising: arranging the stack at a first angle relative to a first X-ray imaging apparatus; capturing a first X-ray image of at least a corner region of the stack with the X-ray imaging apparatus; arranging the stack at a second angle relative to the first or a second X-ray imaging apparatus; capturing a second X-ray image of at least the corner region of the stack with the first or the second X-ray imaging apparatus; and determining alignment of a stack of electrode sheets by performing the method according to any preceding clause. 10. The method according to clause 8 and clause 9, further comprising obtaining the calibration X-ray image by capturing an X-ray image of a calibration medium with the X-ray apparatus. 11. The method according to clause 9 or clause 10, further comprising rotating the electrode stack at least once to present a different corner region of the stack to the X-ray imaging apparatus, and further determining alignment of the stack of electrode sheets by performing the method according to any of claims 1 to 8 for said different corner region. 12. The method according to any of clauses 9 to 11, wherein capturing an X-ray image of at least the corner region of the stack with the X-ray imaging apparatus comprises capturing an X-ray image of a plurality of corner regions, each corner region belonging to a different stack of a plurality of stacks, and determining alignment of each stack of electrode sheets by performing the method according to any of claims 1 to 8 for said each corner region.13. A system for determining alignment of a stack of electrode sheets,comprising: an X-ray imaging apparatus comprising at least one X-ray source and at least one X-ray detector; a holder configured to hold the stack; and an alignment determination unit configured to perform the method of any of clauses 1 to 8; and a controller coupled to at least the X-ray imaging apparatus and the alignment determination unit, configured to control at least the X-ray imaging apparatus and the alignment determination unit to perform the method of any of clauses 9 to 12.14. The system according to clause 13, wherein the X-ray apparatuscomprises two X-ray sources and two X-ray detectors arranged with a predefined angular difference between their beam paths, the predefined angular difference being the difference between the first angle and the second angle. 15. The system according to clause 14, wherein the X-ray sources are respectively arranged to be incident upon different edges of the corner region of the stack.16. The system according to any of clauses 13 to 15, wherein the X-rayapparatus is a line-scanning X-ray camera.17. The system according to any of clauses 13 to 16, wherein the holder isconfigured to hold the stack by holding a battery cell in which the stack is installed.

Claims

1 CLAIMS 1. A computer-implemented method for determining alignment of a stack of electrode sheets, wherein the electrode sheets are planar sheets extending in a first axis (x) and a second axis (y), and the electrode sheets are stacked along a third axis (z) perpendicular to the first axis and the second axis; the method comprising: obtaining a first X-ray image of a corner region of the stack, taken along a first beam path in the plane (x-y) of the electrode sheets, with a first angle from the first axis (x), obtaining a second X-ray image of the corner region of the stack, taken along a second beam path in the plane (x-y) of the electrode sheets, with a second angle from the first axis; determining a first distance (d1) between a first apparent edge of the first electrode sheet and a first apparent edge of the second electrode sheet, along the plane (x-y) of the electrode sheets in the first image; determining a second distance (d2) between a second apparent edge of the first electrode sheet and a second apparent edge of the second electrode sheet, along the plane (x-y) of the electrode sheets in the second image; and determining an alignment ((x,y)) between the first electrode sheet and the second electrode sheet, based on the first distance, the second distance, the first angle, and the second angle.

2. The method according to claim 1, wherein the first angle is selected from a range of 55 to 90 degrees, preferably 75 to 85 degrees, and the second angle is selected from a range of 20 degrees or less, preferably 10 degrees or less.

3. The method according to claim 1 or claim 2, wherein determining an alignment between the first electrode sheet and the second electrode sheet2 comprises determining a first displacement of the second electrode sheet from the first electrode sheet along the first axis, and determining a second displacement of the second electrode sheet from the first electrode sheet along the second axis, the second axis being perpendicular to the first axis.

4. The method according to any preceding claim, wherein determining an alignment between the first electrode sheet and the second electrode sheet comprises determining the alignment based on the equations:, and , wherein ^^ is the first angle, ^^′ is the second angle, ^^1 is the first distance, ^^2 is the second distance, ^^ is the first displacement, and y is the second displacement.

5. The method according to any preceding claim, further comprising determining an error on the determination of the alignment.

6. The method according to claim 5, wherein determining an error on the determination of the alignment comprises determining a first standard deviation of the first displacement and determining a second standard ns:, and, wherein ^^ is the first angle, ^^′ is the second angle, ^^1 is the first distance, ^^2 is the second distance, Δ^^ is the standard deviation of the first displacement, and Δy is the standard deviation of the second displacement.3 7. The method according to any preceding claim, further comprising comparing the determined alignment, and optionally an error on the determination of the alignment, to a desired alignment for the stack to make a quality determination for the stack.

8. The method according to claim 1 or claim 2, further comprising obtaining a calibration X-ray image, and determining a calibration of the X-ray imaging apparatus based on the calibration image.

9. A method for determining alignment of a stack of electrode sheets, comprising: arranging the stack at a first angle relative to a first X-ray imaging apparatus; capturing a first X-ray image of at least a corner region of the stack with the X-ray imaging apparatus; arranging the stack at a second angle relative to the first or a second X-ray imaging apparatus; capturing a second X-ray image of at least the corner region of the stack with the first or the second X-ray imaging apparatus; and determining alignment of a stack of electrode sheets by performing the method according to any preceding claim.

10. The method according to claim 8 and claim 9, further comprising obtaining the calibration X-ray image by capturing an X-ray image of a calibration medium with the X-ray apparatus.

11. The method according to claim 9 or claim 10, further comprising rotating the electrode stack at least once to present a different corner region of the stack to the X-ray imaging apparatus, and further determining alignment of the stack of electrode sheets by performing the method according to any of claims 1 to 8 for said different corner region.4 12. The method according to any of claims 9 to 11, wherein capturing an X-ray image of at least the corner region of the stack with the X-ray imaging apparatus comprises capturing an X-ray image of a plurality of corner regions, each corner region belonging to a different stack of a plurality of stacks, and determining alignment of each stack of electrode sheets by performing the method according to any of claims 1 to 8 for said each corner region.

13. A system for determining alignment of a stack of electrode sheets, comprising: an X-ray imaging apparatus comprising at least one X-ray source and at least one X-ray detector; a holder configured to hold the stack; and an alignment determination unit configured to perform the method of any of claims 1 to 8; and a controller coupled to at least the X-ray imaging apparatus and the alignment determination unit, configured to control at least the X-ray imaging apparatus and the alignment determination unit to perform the method of any of claims 9 to 12.

14. The system according to claim 13, wherein the X-ray apparatus comprises two X-ray sources and two X-ray detectors arranged with a predefined angular difference between their beam paths, the predefined angular difference being the difference between the first angle and the second angle.

15. The system according to claim 14, wherein the X-ray sources are respectively arranged to be incident upon different edges of the corner region of the stack.5 16. The system according to any of claims 13 to 15, wherein the X-ray apparatus is a line-scanning X-ray camera.

17. The system according to any of claims 13 to 16, wherein the holder is configured to hold the stack by holding a battery cell in which the stack is installed.

Images