Separator for a lithium-ion battery cell

Abstract

Separator (2) for a lithium-ion battery cell (4), comprising - two electrically insulating layers (8), and - an electrically conductive layer (6) arranged between the two electrically insulating layers (8), wherein the electrically conductive layer (6) is formed from a polymer (10) in which electrically conductive particles (12) are incorporated, - wherein each of the electrically insulating layers (8) has two laterally projecting contact sections (16a, 16b).

Description

[0001] The invention relates to a separator for a lithium-ion battery cell. Furthermore, the invention relates to methods for producing such a separator and to a lithium-ion battery cell comprising such a separator.

[0002] An electrically powered motor vehicle typically has a traction battery (high-voltage battery, HV battery) that supplies energy to an electric motor to drive the motor vehicle. An electrically powered motor vehicle is understood to mean, in particular, an electric vehicle that stores the energy required for propulsion solely in the traction battery (BEV, battery electric vehicle), an electric vehicle with a range extender (REEV, range extended electric vehicle), a hybrid vehicle (HEV, hybrid electric vehicle), a plug-in hybrid vehicle (PHEV, plug-in hybrid electric vehicle), and / or a fuel cell vehicle (FCEV, fuel cell electric vehicle), which temporarily stores the electrical energy generated by a fuel cell in the traction battery.

[0003] Such a traction battery, designed as a lithium-ion battery, comprises a number of interconnected lithium-ion battery cells (Li-ion battery cells). Each of the lithium-ion battery cells has at least one anode and at least one cathode, with a separator arranged between the anode and the cathode.

[0004] During operation of the battery cell, especially during charging, so-called dendrites made of lithium can form on the anode. These can penetrate the separator and contact the cathode. Dendrite formation therefore poses the risk of a short circuit between the anode and cathode, and thus the risk of the battery cell overheating. Furthermore, a burr or sharp edge of the anode or cathode, especially of their active material, can penetrate the separator, which can also result in a short circuit. Such a burr results, for example, from a laser cutting process during the manufacture of the respective anode or cathode.

[0005] The invention is based on the object of providing a particularly suitable separator. In particular, such penetration of the separator—preferably from both the anode and cathode sides—should be detectable. Furthermore, a method and an apparatus for producing such a separator, a battery cell with such a separator, and a method for operating this battery cell are to be provided.

[0006] EP 2 731 169 B1 discloses a three-layer separator for a battery cell in which an electrically conductive layer is arranged between two polymer layers.

[0007] EP 3 333 938 A1 discloses a battery cell comprising a separator with two electrically insulating layers, with a layer containing an electrical circuit arranged between these two layers. External contacts are connected to the electrical circuit to detect an internal short circuit.

[0008] US 5 952 120 A and US 2015 / 0 056 492 A1 each teach the manufacture of a three-layer separator.

[0009] With regard to the separator, the object is achieved according to the invention by the features of claim 1. With regard to the method, the object is achieved according to the invention by the features of claim 3 and claim 5, and with regard to the device for producing the separator by the features of claim 4. With regard to the battery cell, the object is achieved according to the invention by the features of claim 6, and with regard to the method for operating the battery cell by the features of claim 8. Advantageous embodiments and further developments are the subject of the dependent claims.

[0010] The separator is intended and configured for a lithium-ion battery cell with a liquid electrolyte (liquid electrolyte). The separator has two, expediently foil-shaped, electrically insulating (i.e., electrically non-conductive) layers, with an electrically conductive (electrically conductive) layer arranged between these two layers, which is expediently also foil-shaped. In other words, one of the electrically insulating layers is arranged on each side of the electrically conductive layer. In summary, a layered structure is formed from the electrically conductive layer and the electrically insulating layers. The electrically insulating layers form the outer layers of the separator.

[0011] The electrically insulating layers expediently cover the electrically conductive layer completely with respect to a direction (separator vertical direction, separator thickness direction, structure vertical direction) perpendicular to a plane spanned by one of the layers (in particular those formed in the form of a film).

[0012] The electrically conductive layer is formed from a polymer, in particular plastic, into which electrically conductive particles are incorporated. Preferably, an electrically non-conductive polymer is used. Thus, the electrically conductive layer is a polymer layer. In particular, the conductive particles are a filler for the polymer. The polymer and the electrically conductive particles thus form a one-piece, monolithic layer of the separator. Thus, the electrically conductive particles are integrated into the polymer or polymer layer.

[0013] The electrically insulating layers of the separator exhibit, for example, ionic conductivity for lithium ions. These layers are preferably permeable to lithium ions. In particular, the electrically insulating layers are porous for this purpose, so that a liquid electrolyte can penetrate into the pores. The electrically insulating layers are preferably made of (preferably porous) polypropylene, in particular polypropylene films. Such (porous) polypropylene layers are known from the prior art. Alternatively, polyethylene, for example, is used for the electrically non-conductive (insulating) layers.

[0014] For example, an acrylic resin, polyamide, or polyester can also be used for the electrically conductive layer. However, polyethylene is preferably used as the polymer for the electrically conductive layer. The electrically conductive layer is preferably also porous, so that it can be impregnated with an electrolyte. The pore diameters of the electrically insulating layers and / or the electrically conductive layer are preferably between 20 nm and 40 nm on average.

[0015] The electrically conductive particles are preferably carbon particles. For example, the carbon particles are carbon fibers. For example, these have an (average) diameter between 5 nm and 50 nm, in particular 10 nm, and an (average) length between 10 nm and 1000 nm, in particular between 50 nm and 500 nm, preferably 100 nm. Furthermore, for example, a granulate of carbon particles with an average diameter between 10 nm and 100 nm can be used.

[0016] Carbon particles are advantageous because they are comparatively light and easily miscible with the polymer.

[0017] Other alternatives to carbon particles as electrically conductive particles are graphite or aluminum nanoparticles.

[0018] Advantageously, the electrically insulating layers and the electrically conductive layer each have a layer thickness between 1 µm and 10 µm, in particular between 2 µm and 7 µm.

[0019] This layered structure of the separator makes it particularly advantageous to detect the penetration of a lithium dendrite or a ridge of the anode or cathode into the separator, whereby mechanical and / or electrical contact with the electrically conductive layer is formed upon penetration. For this purpose, for example, the electrical resistance of one of the electrically insulating layers is determined—as described comparatively precisely below in the course of the operating method of a battery cell comprising such a separator.

[0020] Furthermore, each of the electrically insulating layers has two laterally projecting contact sections, namely a first contact section and a second contact section. The contact sections extend in a plane defined by the respective electrically insulating layer. The contact sections project beyond the electrically conductive layer. Thus, with respect to the vertical direction of the separator, no electrically conductive layer is arranged below or above these contact sections.

[0021] These contact sections serve to connect a measuring device for determining the electrical resistance of the electrically insulating layer or layers. In a lithium-ion battery cell, the measuring device can thus be connected at a distance from its anodes and cathodes. For comparatively simple connection of the measuring device, the contact sections are preferably led out of a housing or casing of the lithium-ion battery cell (battery cell), enabling a connection on the outside of the battery cell.

[0022] Preferably, the contact sections of each of the electrically insulating layers are arranged on opposite sides. Preferably, the contact sections of the two electrically insulating layers are aligned with each other in the vertical direction of the separator.

[0023] According to an advantageous embodiment, the side of at least one of the electrically insulating layers facing away from the electrically conductive layer is provided with a ceramic layer. In other words, the ceramic layer is applied to a top side and / or a bottom side of the layered structure, i.e., an upper or lower side in the vertical direction of the separator. For example, the ceramic layer is formed from aluminum oxide or comprises aluminum oxide. This ceramic layer increases the (mechanical) penetration resistance of a lithium dendrite or the anode or cathode into the separator.

[0024] A further aspect of the invention relates to a (first) method for producing a separator in one of the variants presented above.

[0025] According to the process, a polymer, in particular a polymer material, preferably in the form of granules, and the electrically conductive particles are first fed into an extruder. The polymer material and the electrically conductive particles are mixed in the extruder. Polyethylene is preferably used as the polymer, and carbon fibers are preferably used as the conductive particles.

[0026] The molten polymer and the conductive particles mixed with it are extruded, i.e., dispensed from the extruder in a shaped manner, specifically, pressed out. The electrically conductive layer is expediently formed into a film. In summary, the electrically conductive layer is the extrudate. Due to the expedient continuous discharge of the electrically conductive layer from the extruder, it has a ribbon shape.

[0027] For example, the electrically conductive layer is subsequently heat-treated ("annealed"). In particular, the heat treatment increases the crystallinity of the electrically conductive layer and / or softens it. The foil-shaped electrically conductive layer is conveniently wound onto a supply roll for further processing.

[0028] The electrically conductive layer is then laminated on both sides, i.e., on opposite base sides, with electrically non-conductive layers. Examples of electrically non-conductive layers include polypropylene films, particularly porous polypropylene films. For lamination, the electrically conductive layer and the electrically non-conductive layers are unrolled, for example, from a respective supply roll and positioned above or below the electrically conductive layer. Using a laminating device, these three layers are joined together to form the (band-shaped) layered structure. The layered structure is preferably wound onto a supply roll for further processing.

[0029] Subsequently, the layered structure is cold-stretched. This means that the laminated layers are not further heated for cold-stretching. For example, the layered structure has a temperature between 40°C and 10°C, especially room temperature. During cold-stretching, the layered structure is subjected to tensile stress, causing it to stretch. The layered structure is preferably stretched both in a longitudinal (strip) direction and in a transverse direction oriented perpendicular to this (strip transverse direction). This results in defects known as failure points, such as cracks or breaks, in the electrically conductive layer and / or in the electrically insulating layers.

[0030] For cold stretching, a cold stretching roll is preferably used, along which the layered structure is guided. Advantageously, several cold stretching rolls are used, with the ribbon-shaped layered structure being guided, in particular in a meandering manner, along the cold stretching rolls.

[0031] For example, the cold stretching roll(s) are used as tensioning roll(s) or to generate a corresponding tensile stress in the longitudinal direction of the strip. Alternatively, a separate tensioning roll is provided to create this tension.

[0032] For example, the roller body of the respective cold stretching roller is curved in an arcuate manner so that the layer structure rolling on the cold stretching roller is stretched in the transverse direction, i.e., in the direction of the axis of the cold stretching roller. Alternatively and preferably to this curvature, the respective cold stretching roller has a structure on its outer surface which causes tensile stress. For example, the structure is designed as a protrusion on the outer surface extending spirally along the roller axis. The spiral-shaped protrusion has, for example, a height between 20 µm and 500 µm. For example, this protrusion and / or the outer surface is made of polyurethane. Such rollers, also referred to as spreader rollers or spreading rollers, are already known from the prior art.

[0033] Furthermore, the layer structure is pressed in during cold drawing to create defect points, in particular in the electrically conductive layer. The defect points are preferably created at a large number of locations. For this purpose, the cold drawing roller or alternatively a further roller expediently has, in particular, needle-shaped extensions which project upwards towards the outer surface and / or the spiral elevation of the cold drawing roller and / or the outer surface of the further roller. These extensions are preferably arranged regularly on the outer surface. As the layer structure is conveyed, these extensions press in the electrically insulating layer and at least the electrically conductive layer. In this way, a comparatively large number of defect points are introduced into the electrically conductive layer and / or the electrically insulating layers in a targeted manner.For example, the layered structure is covered on both sides with a protective film, i.e., the protective film is inserted between the cold-stretching rollers and the layered structure. The protective film is pressed in by the projections, which in turn causes the layered structure to be indented. The projections and the layered structure are therefore not in direct contact with each other. The protective film prevents unwanted damage to the layered structure, particularly tearing or perforation.

[0034] The needle-shaped extensions are formed, for example, by coating the lateral surface and / or the spiral elevation with an aluminum oxide ceramic. The applied ceramic particles form the extensions, which preferably have an (average) diameter between 20 nm and 40 nm and / or a length of 2 µm to 10 µm, in particular 5 µm.

[0035] Following cold stretching, the layered structure, i.e. the laminated layers, is hot-stretched. For this purpose, the layered structure is heated to a temperature between 100°C and 130°C. One or more hot-stretching rollers are expediently used for hot stretching. The hot-stretching roller is preferably designed as a spreader roller, analogous to the cold-stretching roller, so that the layered structure is subjected to tensile stress in the transverse direction. Hot stretching enlarges the defect points introduced during cold stretching, so that pores form in the electrically conductive layer and / or in the electrically non-conductive layers. The tensile forces are preferably adjusted such that the average pore size is between 20 nm and 40 nm.

[0036] For example, the hot stretching roll(s) are used as tensioning rolls or to generate a corresponding tensile stress in the longitudinal direction of the strip. Alternatively, a separate tensioning roll is provided to create this tension.

[0037] The cold stretching and / or hot stretching is preferably carried out using a roll-to-roll process.

[0038] The tensile stress during hot stretching is, for example, between 10 N / mm 2 and 80 N / mm 2 , preferably between 20 N / mm 2 and 40 N / mm 2 . The tensile stress during cold drawing is, for example, between 25 N / mm 2 and 100 N / mm 2 , preferably 50 N / mm 2 .

[0039] If a protective film was used, it is then removed, and the strip-shaped layer structure is cut to size, forming individual separators. If contact sections are provided for the electrically insulating layers, electrically non-conductive layers are used in the production of the layer structure that extend beyond the electrically conductive layer in the transverse direction. For example, the contact sections are cut out of the areas that protrude beyond the electrically conductive layer in or against the transverse direction ("notching").

[0040] A further aspect of the invention relates to a device for producing a separator according to the first method. In summary, the device comprises the extruder for forming the electrically conductive layer as an extrudate, and the laminating device for laminating the electrically conductive layer to the electrically insulating layers on both sides. Furthermore, the device comprises at least one cold-stretching roller for cold-stretching the laminated layers. The cold-stretching roller preferably has, on its outer surface, particularly needle-shaped extensions for generating defect points in the electrically conductive layer. Furthermore, the device comprises at least one hot-stretching roller for hot-stretching the cold-stretched layers.

[0041] To heat the layer structure for hot stretching, the device expediently comprises a heating device, for example, an infrared radiator. Additionally or alternatively, the hot stretching rollers can be heated.

[0042] A further aspect of the invention relates to an alternative (second) method for producing a separator in one of the variants presented above.

[0043] According to this second process, the polymer (material), preferably in the form of granules, the electrically conductive particles, and a plasticizer are fed into the extruder. Here, polyethylene is preferably used as the polymer, and carbon fibers are used as the conductive particles.

[0044] Naphtha or a linear carbonate such as dimethyl carbonate is used as a plasticizer, for example. The plasticizer typically has a melting point between 50°C and 90°C. When such a plasticizer evaporates, the corresponding pores are created in the conductive layer. Alternatively, a salt, particularly a lithium-based salt, preferably LiTFSi (lithium bis(trifluoromethylsulfonyl)amide), is used as a plasticizer. The salt dissolves in a solvent, creating the pores in the layer. Even if the salt is not completely dissolved during the separator's production, it is advantageously dissolved in the battery cell's electrolyte.

[0045] The extrudate produced is a film-shaped, in particular ribbon-shaped, electrically conductive layer, i.e. a polymer layer with conductive particles and plasticizer incorporated or absorbed therein.

[0046] The plasticizer is then removed from the electrically conductive layer. To do this, the electrically conductive layer is heated, for example, to a temperature between 40°C and 70°C, so that the plasticizer evaporates. Alternatively, or in addition to this, the plasticizer is dissolved from the electrically conductive layer using a solvent. Removing the plasticizer creates the pores in the electrically conductive layer.

[0047] The electrically conductive layer is then laminated on both sides with an electrically non-conductive layer, forming the layered structure. For example, porous polypropylene films, especially those with electrically non-conductive layers, are used for this purpose.

[0048] If contact sections are provided on the electrically non-conductive layers, electrically non-conductive layers are used in a manner analogous to the first method, which project beyond the electrically conductive layer on both sides in the transverse direction.

[0049] Finally, the band-shaped layer structure is cut out and / or trimmed to form individual separators.

[0050] Preferably, the separator is manufactured in one of the variants described above using the first method or the second method. In particular, the statements regarding the separator described above apply analogously to a separator manufactured using one of these methods.

[0051] A further aspect of the invention relates to a lithium-ion battery cell, also referred to as a battery cell for short. This battery cell has at least one separator that is designed according to one of the variants described above and / or was manufactured according to one of the methods described above.

[0052] The lithium-ion battery cell comprises at least one anode and at least one cathode, with such a separator arranged between the anode and the cathode, or between the anodes and the cathodes. Thus, each separator serves to electrically and / or mechanically separate the corresponding anode and the cathode. Furthermore, the lithium-ion battery cell comprises a liquid electrolyte.

[0053] According to an expedient embodiment of the lithium-ion battery cell, the contact sections of the insulating layers are directed to the outside of the battery cell and thus protrude from the housing or shell. Particularly preferably, only the contact sections of the respective separator protrude to the outside of the battery cell. In other words, the contact sections extend from an inner cell region enclosed by the housing or shell through the housing or shell into an outer cell region. In this way, the contact sections can be contacted with a measuring device with comparatively little effort for measuring or determining the electrical resistance of the electrically insulating layer or layers. Advantageously, the electrically conductive layer of the insulator is arranged entirely within the inner cell region, so that it is protected from damage or contact.

[0054] The housing or shell expediently has a first opening and a second opening. For each of the electrically insulating layers, a first contact section thereof extends through the first opening and the second contact section thereof extends through the second opening to the outside of the housing. The first contact sections of the electrically insulating layers, in particular of all electrically insulating layers, are preferably joined to one another, for example by ultrasonic welding. Analogously, the second contact sections of the electrically insulating layers, in particular of all electrically insulating layers, are preferably joined to one another, for example by ultrasonic welding. With this configuration, the total electrical resistance of the electrically insulating layers can be determined.

[0055] In particular, if the battery cell is designed as a prismatic cell with a rigid housing, for example made of a metal sheet, the contact sections are alternatively arranged in the cell interior, wherein the first contact sections are electrically connected to a first measuring contact (terminal) and the second contact sections are electrically connected to a second measuring contact.

[0056] A further aspect of the invention relates to a method for operating a lithium-ion battery cell in one of the variants described above. In particular, the method serves to detect penetration of the separator with conductive material, in particular by a lithium dendrite or by an anode or cathode.

[0057] According to the method, the electrical resistance of the respective separator or separators (particularly if the first contact sections are joined together and / or lead out of the cell interior through a common opening) is determined. A current measurement is preferably used for this purpose. In this case, the current between the first contact section and the second contact section or between the first contact sections and the second contact sections is expediently recorded.

[0058] Based on the measured electrical resistance, the condition of the separator(s) is determined. The electrical resistance determined from the current measurement is compared with a threshold value, such as 1 MΩ. If the determined electrical resistance falls below the threshold, penetration of the separator is considered to have occurred. In this case, for example, operation of the lithium-ion battery cell is terminated and / or a warning is issued to the battery cell user.

[0059] A further aspect of the invention relates to an electrically powered motor vehicle. Its traction battery comprises at least one battery cell that has a separator in one of the variants described above and / or is operated according to the method described above.

[0060] In the following, exemplary embodiments of the invention are explained in more detail with reference to a drawing. In the drawings: Fig. 1 schematically shows a separator for a lithium-ion battery cell in side view, wherein the separator has an electrically conductive layer arranged between two electrically non-conductive layers, Fig. 2 schematically shows the separator in a plan view, wherein each of the electrically insulating layers has a contact section on opposite sides, Fig. 3 schematically shows an electrode stack with anodes and electrodes in a perspective view, wherein a separator is arranged between the anodes and the cathodes, Fig. 4 schematically shows a lithium-ion battery cell designed as a pouch cell with the electrode stack, wherein the contact sections of the separators are guided to the outside of the battery cell for determining the electrical resistance, Fig. 5 shows, in a flow diagram, a first method for producing the separator, wherein a film-shaped extrudate forming an electrically conductive layer is laminated on both sides with a non-conductive layer, and wherein the laminated layers are cold-stretched and subsequently hot-stretched, Fig. 6 schematically shows an apparatus for producing the separator according to the method according to Fig. 5, Fig. 7 schematically shows in perspective a section of a cold stretching roll which has needle-shaped projections on its outer surface, and Fig. 8 shows, by way of a flow diagram, a second method for producing the separator, wherein an extrudate is produced with a polymer, conductive particles and plasticizer, wherein the plasticizer is removed from the extrudate, and wherein the film-shaped extrudate is laminated on both sides with a non-conductive layer.

[0061] Corresponding parts and sizes are always provided with the same reference symbols in all figures.

[0062] In the Fig. 1 shows a separator 2 for a lithium-ion battery cell 4 (battery cell 4). The separator 2 is constructed in a sandwich design. It comprises an electrically conductive layer 6 arranged between two electrically non-conductive layers 8. The electrically non-conductive layers 8 are joined to the electrically conductive layer 6.

[0063] The vertical direction of the separator Z is understood to be the direction in which the layers 6, 8 are arranged one above the other. In other words, the vertical direction of the separator Z is oriented perpendicular to a plane spanned by one of the layers 6, 8.

[0064] The electrically non-conductive layers 8 are formed from a larger polypropylene film. The electrically conductive layer 6 is a polymer layer made of polyethylene as the polymer 10, into which electrically conductive particles 12, such as carbon fibers, are incorporated.

[0065] In the example of Fig. 1, a ceramic layer 14 is applied to the side of one of the electrically insulating layers 8 facing away from the electrically conductive layer 6. This layer is formed, in particular, from aluminum oxide.

[0066] According to an alternative of the separator 2 (not shown in further detail), the separator 2 does not have a ceramic layer 14. According to a further alternative of the separator 2 (not shown in further detail), ceramic layers 14 are arranged on the side of both electrically insulating layers 8 facing away from the electrically conductive layer 6.

[0067] Especially in the Fig. 2, the electrically insulating layers 8 project beyond the electrically conductive layer 6 in a plane perpendicular to the separator vertical direction Z. In other words, the electrically non-conductive layers 8 completely cover the electrically conductive layer 6 with respect to the separator vertical direction Z. For the purpose of better comprehensibility, the circumference of the electrically conductive layer 6 (covered by the electrically insulating layer 8) is shown by dashed lines.

[0068] Furthermore, the two electrically insulating layers 8 each have two laterally projecting contact sections, namely a first contact section 16a and a second contact section 16b. These are arranged on opposite sides of the respective electrically non-conductive layer 8 and extend in a direction perpendicular to the separator vertical direction Z. In the separator vertical direction Z, the contact sections 16a of the two electrically non-conductive layers 8 are aligned with one another. Furthermore, the contact sections 16b of the two electrically non-conductive layers 8 are aligned with one another.

[0069] In the Fig. Figure 3 shows a schematic perspective view of an electrode arrangement 18 for the lithium-ion battery cell 4. According to the electrode arrangement 18 shown here, it is designed as an electrode stack in which anodes 20 and cathodes 22 are arranged alternately one above the other. A separator 2 in one of the variants shown above is arranged between the anodes 20 and the cathodes 22.

[0070] According to the Fig. 3, contact sections 24 of the anodes 20 and one of the contact sections 16, the contact sections 26 of the cathodes 22 are arranged on opposite sides of the electrode stack and are each aligned in a stack direction Z S with each other. Regarding the stacking direction Z SThe contact sections 16a of the separators 2 are also aligned with each other in the direction of the electrode stack. Similarly, the contact sections 16b are aligned with each other in this direction.

[0071] According to a variant of the electrode arrangement 18 not shown in detail, this is designed as an electrode winding, wherein a separator 2 is arranged between its anode and its cathode.

[0072] In the Fig. 4, the lithium-ion battery cell 4, also referred to as battery cell 4 for short, is shown schematically in a plan view. Here, the battery cell 4 is designed as a pouch cell. However, the following explanations also apply analogously to a so-called prismatic battery cell. The casing 27 of the battery cell 4 has four openings, namely a first opening 28a, a second opening 28b, a third opening 28c and a fourth opening 28d. The first contact sections 16a of the separators 2, which are joined to one another, for example by means of an ultrasonic welding process, are guided through the first opening 28a from a cell interior 30 enclosed by the casing 27 to the outside of the battery cell. The first opening 28a is expediently closed in a fluid-tight manner with the first contact sections 16a received therein.In a similar manner, the joined second contact sections 16b extend from the cell interior 30 to the outside of the battery cell through the second opening 28b. The electrically conductive layers 6 are arranged entirely within the casing 27. Thus, they do not protrude to the outside of the battery cell.

[0073] A first current collector 32, which is joined to the contact sections 24 of the anodes 20 in the cell interior 30, for example, by ultrasonic welding or laser beam welding, is routed to the outside of the battery cell through the third opening 28c. Similarly, a second current collector 34, which is joined to the contact sections 26 of the cathodes 22 in the cell interior 30, is routed to the outside of the battery cell through the fourth opening 28d.

[0074] During operation of the battery cell 4, the electrical resistance of its separators 2 is determined. For this purpose, the outwardly directed contact sections 16a, 16b are connected to a current or voltage source 36. The contact sections 16a, 16b are connected to the current or voltage source 36 on the outside of the battery cell. To determine the electrical resistance, the current between the current or voltage source 36 and the contact sections 16b is measured using an ammeter 38. If the electrical resistance of the separators 2, determined from the measured current, falls below and / or exceeds a respective threshold value, penetration of the separator 2, at least one of its electrically non-conductive (insulating) layers 8, is deemed to have been detected by a lithium dendrite or a ridge of one of the anodes 20 or the cathodes 22.This is based on the consideration that when a lithium dendrite or a ridge of one of the anodes 20 or the cathodes 22 penetrates one of the electrically non-conductive layers 8, a corresponding electrical current flows from the anode 20 or the cathode 22 into or through the electrically conductive layer 6 to the contact section 16a or 16b connected to the current measuring device 38. This effectively results in a reduction in the electrical resistance of the separator and can be detected based on the current measurement by the current measuring device 38. In summary, the state of the separators 2 of the battery cell 4 is determined based on the determined electrical resistance.

[0075] In the Fig. 4, the cover 27 is shown transparent for the purpose of better recognition.

[0076] In the Fig. 5 shows a flow chart illustrating a first method for producing a separator 2 according to the explanations for the Fig. 1 and Fig. 2. A corresponding device 40 for carrying out the method, i.e. for producing such a separator 2, is shown in Fig. 6. In a first step 1a of the method, the polymer 10 in the form of granules and the electrically conductive particles 12, in particular in the form of a powder or granules, are fed to an extruder 42 of the device 40. The extruder melts the polymer 10, in particular polyethylene, and mixes with the conductive particles 12, in particular carbon fibers. The polymer 10 mixed with the conductive particles 12 is extruded, with the extrudate expediently being shaped into a film, in particular a strip. The extrudate thereby forms the electrically conductive layer 6.

[0077] In a second step IIa, the electrically conductive layer 6 is heat-treated ("annealed") using a first heating device 44. The electrically conductive layer 6 is then wound onto a supply roll (not shown).

[0078] In a third step IIIa, the electrically conductive layer 6 is unwound and placed between two electrically non-conductive, i.e., electrically insulating, layers 8. For this purpose, for example, the three layers are unwound from a respective supply roll and arranged one above the other. The electrically conductive layer 6 and the electrically insulating layers 8 are terminated, i.e., joined, to one another using a laminating device 46. In particular, for this purpose, the layers 6, 8 are pressed against one another and heated in the process. The terminated layers 6, 8 are expediently rewound onto a supply roll.

[0079] In a fourth step IVa, a protective film 48 is arranged on both sides of the laminated layers 6, 8, i.e. the layered structure formed by these layers 6, 8. The layered structure with the protective films is cold-stretched in this step, wherein the layered structure has a temperature between 40°C and 10°C, in particular room temperature. For this purpose, the device comprises 40 rollers, by means of which the film is guided and / or conveyed. At least one of these rollers is a cold-stretching roller 50, which is designed as a spreader roller. Such a cold-stretching roller 50 is shown by way of example in the Fig. 7. To generate a tensile stress in the direction of the axis of the cold stretching roller 50, i.e., in the transverse direction of the layered structure, the latter has a protrusion 52 on its outer surface 54, extending spirally along its axis. According to an alternative to the cold stretching roller 50 (not shown in further detail), the roller is curved in an arcuate manner to generate the tensile stress in the transverse direction. For example, a tensile stress transverse to the transverse direction, i.e., in the (strip) longitudinal direction of the strip-shaped layered structure, is generated by the cold stretching roller 50 and, additionally or alternatively, by further rollers such as a friction roller or a stretching roller.

[0080] The or each of the cold stretching rollers 50 has needle-shaped extensions 56 on its outer surface 54 and preferably additionally on its elevation 52. According to an alternative not shown in detail, a further roller in addition to the cold stretching roller 56 has the needle-shaped extensions 56 on its outer surface. When the layer structure is conveyed by the cold stretching roller 56 or by the cold stretching roller and the further roller, the protective film and thus the layer structure, i.e. the interconnected layers 6, 8, are pressed in due to the extensions 56. In other words, the extensions 56 press into the protective film 48, so that error points, i.e. local defects, arise in the layer structure, i.e. in the electrically conductive layer 6 and / or in one or both electrically insulating layers 8.

[0081] In the Fig. 7, the projections 56 are shown comparatively large for the purpose of better recognition. For example, the projections 56 are formed by an aluminum oxide coating, which is expediently applied by so-called jet deposition of aluminum oxide particles onto the lateral surface 54 or onto the elevation 52. These particles have an average length of between 2 µm and 10 µm, in particular 5 µm. The elevation, on the other hand, protrudes between 20 µm and 500 µm from the lateral surface 54. For the purpose of better recognition, the projections 56 and the elevation 52 in the Fig. 7 only shows the projections 56 on the lateral surface 54. However, according to this preferred embodiment, such projections 56 are also arranged on the spiral elevation 52.

[0082] The layer structure provided with defect points is then hot-stretched (step Va). In other words, the layer structure is first heated, for example to a temperature between 110°C and 130°C, and then subjected to tensile stress, preferably both in the transverse direction and in the longitudinal direction. For this purpose, at least one hot-stretching roller 58 of the device 40 is used, which is expediently designed as a spreader roller. For this purpose, the device 40 comprises a second heating device 60, for example an infrared radiator, by means of which the layer structure can be heated. Additionally or alternatively, the hot-stretching roller 58(s) can be heated. Due to the hot-stretching, the defect points expand spatially, so that pores are formed in the respective layer 6, 8.

[0083] The cold stretching and the hot stretching are expediently designed as a roll-to-roll process.

[0084] In a sixth step Vla, the protective films 48 are removed from the layer structure and separators 2 are separated using a cutting device 62.

[0085] In the Fig. 8 is a flow chart showing an alternative second method for producing the separator 2 according to one of the Fig. 1 or Fig. 2. In a first step 1b of this second process, the polymer 10, expediently in the form of granules, the electrically conductive particles 12, and a plasticizer 64 are fed to the extruder 42. Here, polyethylene is preferably used as the polymer 10, and carbon fibers 12 are used as the conductive particles.

[0086] The film-shaped, in particular band-shaped, electrically conductive layer 6, i.e. a polymer layer with conductive particles 12 and plasticizer 64 introduced or absorbed therein, is produced as an extrudate.

[0087] In a second step IIb, the plasticizer 64 is removed from the electrically conductive layer 6. For this purpose, the electrically conductive layer is heated, for example, to a temperature between 40°C and 70°C, so that the plasticizer 64 evaporates. Alternatively or additionally, the plasticizer is dissolved out of the electrically conductive layer 6 using a solvent. Pores may form in the electrically conductive layer 6 due to the removal of the plasticizer 64.

[0088] Subsequently, in a third step IIIb, the electrically conductive layer 6 is laminated on both sides with an electrically non-conductive layer 8. For example, especially porous polypropylene films, i.e., electrically non-conductive layers 8, are used for this purpose.

[0089] Finally, in a fourth step IVb, the band-shaped layer structure is cut to form individual separators 2, optionally including the contact sections 16a, 16b.

[0090] The invention is not limited to the exemplary embodiments described above. Rather, other variants of the invention can also be derived therefrom by those skilled in the art without departing from the subject matter of the invention. In particular, all individual features described in connection with the exemplary embodiments can also be combined with one another in other ways without departing from the subject matter of the invention. List of reference symbols 2 Separator 4 lithium-ion battery cells 8 electrically insulating layer 6 electrically conductive layer 14 ceramic layer 10 polymers 12 conductive particles 16a,16b contact section 18 Electrode arrangement / electrode stack 20 Anode 22 Cathode 24 Contact section of the anode 26 Contact section of the cathode 27 Cover 28a to d opening 30 cell interior 32 first pantograph 34 second pantograph 36 Current or voltage source 38 ammeter 40 Device 42 extruders 44 first heating device 46 Laminating device 48 protective film 50 cold stretching roll 52 Survey 54 lateral surface 56 extension 58 Hot stretching roll 60 second heating device 62 Cutting device 64 plasticizers Z separator vertical direction Z S Stacking direction Ia Extruding IIa Heat treatment IIIa Laminating IVa cold sections Va warm stretches Via cropping Ib Extruding IIb Removing the plasticizer IIIb Laminating IVb Cutting

Claims

[1] Separator (2) for a lithium-ion battery cell (4), comprising - two electrically insulating layers (8), and - an electrically conductive layer (6) arranged between the two electrically insulating layers (8), wherein the electrically conductive layer (6) is formed from a polymer (10) in which electrically conductive particles (12) are incorporated, - wherein each of the electrically insulating layers (8) has two laterally projecting contact sections (16a, 16b). [2] Separator (2) according to claim 1, characterized by that the side of at least one of the electrically insulating layers (8) facing away from the electrically conductive layer (6) is provided with a ceramic layer (14), in particular made of aluminum oxide. [3] A method for producing a separator (2) according to claim 1 or 2, - wherein a polymer (10) and electrically conductive particles (12) are fed to an extruder (42) and the electrically conductive layer (6) is extruded, - wherein the extruded electrically conductive layer (6) is laminated on both sides with an electrically insulating layer (8), - wherein the laminated layers (8, 6) are cold-stretched, - wherein the laminated layers (8, 6) are pressed in during cold stretching to create fault points in the electrically conductive layer (6), and - wherein the laminated layers (8, 6) are hot-stretched. [4] Device (40) for producing a separator (2) according to the method of claim 3, comprising - an extruder (42) for forming the electrically conductive layer (6) from a polymer (10) in which electrically conductive particles (12) are introduced, - a laminating device (46) for laminating the electrically conductive layer (6) on both sides with electrically insulating layers (8), - at least one cold stretching roller (50) for cold stretching the laminated layers (8, 6), wherein the cold stretching roller (50) preferably has on its outer surface (54), in particular needle-shaped, extensions (56) for generating defect points in the electrically conductive layer (6), - at least one hot stretching roller (58) for hot stretching the cold-stretched layers (8, 6). [5] Method for producing a separator (2) according to claim 1 or 2, - wherein a polymer (10), electrically conductive particles (12) and a plasticizer (64) are fed to an extruder (42) and the electrically conductive layer (6) is extruded, - wherein the plasticizer (64) is removed from the electrically conductive layer (6), - wherein the electrically conductive layer (6) is laminated on both sides with an electrically insulating layer (8). [6] Lithium-ion battery cell (4) with a separator (2) which is designed according to claim 1 or 2 and / or manufactured according to the method according to claim 3 or according to the method according to claim 5. [7] Lithium-ion battery cell (4) according to claim 6, characterized by that the contact sections (16a, 16b) are guided to the outside of the battery cell. [8] Method for operating a lithium-ion battery cell (4) designed according to claim 6 or 7, - wherein the electrical resistance of the separator (2) or separators (2) is determined, - wherein a state of the separator (2) is determined based on the determined electrical resistance.

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