SE2151603A1

SE2151603A1 - Assembling device for a secondary cell

Info

Publication number: SE2151603A1
Application number: SE2151603A
Authority: SE
Inventors: Jeong Keun Kwon; Yong Kyoung Kim
Original assignee: Northvolt Ab
Priority date: 2021-12-22
Filing date: 2021-12-22
Publication date: 2023-06-23
Also published as: WO2023117692A1

Abstract

A device (100) and corresponding method for assembling an inner cell structure (110) of a secondary battery cell is disclosed. The device comprises an assembly stage (130) for arranging a separator film between sheet electrodes, and a guiding arrangement (140) for guiding the separator film from a separator film supply (141) to the assembly stage. Further, the guiding arrangement comprises a heating roller (150) configured to be arranged in thermal contact with the separator film to transfer heat to the separator film.

Description

ASSEMBLING DEVICE FOR A SECONDARY CELL Technical field The present disclosure generally relates to secondary battery cells, and more specifically to a device and a method for assembling an inner cell structure of such a cell.

Background ln addressing climate change there is an increasing demand for rechargeable batteries. Rechargeable batteries may for instance be employed to enable electrification of transportation and to supplement renewable energy. Currently, lithium-ion batteries are becoming increasingly popular.

A rechargeable battery typically comprises one or more secondary battery cells, which in the context of the present disclosure also may be referred to as simply a "cell". Each cell may comprise an inner cell structure of sheet-like electrodes interleaved with a separator film. The sheet electrodes are, for example, inserted into a zigzag folded separator film to form a prismatic cell or wound together with the separator film to form a prismatic cell.

The performance of the separator film is a critical factor for the operation of the resulting battery cell as well as the quality of the assembling process. A poorly functioning or defective separator film may lead to a reduced production yield, short-circuiting of the electrode sheets and faulty battery cells. With the ever-growing demand for rechargeable batteries, there is therefore an increasing interest in improved and more efficient technologies for assembling the battery cells.

Summary One object of the disclosure is to propose methods and devices for assembling an inner cell structure of a secondary battery cell. This and other objectives are achieved by the invention as defined by the independent claims. The dependent claims provide embodiments of the invention.

According to a first aspect, there is provided a device for assembling an inner cell structure of a secondary battery cell, wherein the inner cell structure comprises sheet electrodes inter|eaved with a separator film. The device comprises an assembly stage for arranging the separator film between the sheet electrodes, and a guiding arrangement for guiding the separator film from a separator film supply to the assembly stage. Further, the guiding arrangement comprises a heating roller configured to be arranged in thermal contact with the separator film to transfer heat to the separator film.

According to a second aspect, there is provided a method for assembling an inner cell structure of a secondary battery cell, wherein the inner cell structure comprises sheet electrodes inter|eaved with a separator film. The method comprises: guiding the separator film from a separator film supply to an assembly stage; heating the separator film by means of a heating roller; followed by: arranging, by the assembly stage, the separator film between the sheet electrodes.

The present invention is based on the realisation that alignment errors of the separator film, that is, errors in the relative position between the separator film and the electrode sheets, often may be caused by defects and irregularities of the separator film. Examples of defects and irregularities may include creases, folds and curls, eventually leading to edge placement issues of the separator film and potential short-circuiting of the resulting cell. Such defects may in turn be caused by variations in the thickness of the separator film. As the separator film commonly may be wound onto a supply roll, relatively small defects and thickness variations may accumulate and propagate through the successive layers of the supply roll. A relatively small deviation or irregularity in an inner layer of the supply roll may hence be amplified into a relatively large defect at the outer layer of the supply roll. lt is one of the merits of the invention that at least some of the alignment issues can be overcome or alleviated by the heating roller heating the separator film on its way from the separator film supply to the assembly stage. The heat treatment allows for defects such as creases, folds and curls to be reduced and may hence be employed to increase the surface flatness of the separator film. ln other words, the heating roller may be used to, at least partly, relax residual stresses and restore the quality of the separator film to a state it had prior to being wound onto the supply roll.

As already mentioned, the heating roller is configured to be arranged in thermal contact with the separator film. The thermal contact may for example be provided by a direct, mechanical contact between an outer surface of the heating roller and a surface of the passing separator film. During operation, the interaction between the two surfaces may be a pure rolling, without any slipping or skidding at the points of contact between the separator film and the heating roll. Alternatively, there may be a difference in velocity between the points of contact on the surface of the separator film and their counterparts on the surface of the heating roller, so that the separator at least partly slides on the surface of the roller. The roller may for example rotate with a slightly higher velocity than the passing film to induce a stretching of the film. Thus, the heating roller may be supported by a bearing allowing for a free rotation around the length axis of the roller, and/or a torque transferring means, for instance coupled to a motor, for controlling the rotational speed of the heating roller.

The temperature of the roller may, for instance, be controlled by means of electrical heating, or by means of a passing fluid such as a heating gas or liquid. As will be discussed in further detail in the following, the temperature of the outer surface of the heating roller may be maintained at a level allowing the separator film to be heated to 30-90°, such as for instance 40-60°C.

Further, as will be readily understood by the skilled person, the present inventive concept is not limited to only one heating roller. On the contrary, it can be advantageous to use two or more heating rollers on the feeding path between the separator film supply and the assembly stage. Further, it will be appreciated that one or more additional stations may be provided, in which the film may be exposed to heat and/or stretching. Thus, a heating system may be provided, which in addition to the above-mentioned heating roller(s) may comprise further means for heating the separator film. Such further means may for instance include a heating chamber, through which the separator film may be fed on its way to the assembly stage.

According to an embodiment, the guiding arrangement may comprise at least one guiding roller assisting in the guiding of the separator film from the film supply to the assembly stage. The guiding roller and the heating roller may be configured to cooperate to stretch the separator film. The stretching is for example provided by adjusting a separating distance between the two rollers in the feeding direction of the separator film. Thus, by increasing the distance between the heating roller and the guiding roller, the tension in the feeding direction of the separator film may be increased. Conversely, the tension may be reduced by reducing the distance between the two rollers. The guiding roller and the heating roller may be operated to provide, and preferably maintain, a predetermined stretch ratio, or extension ratio of the separator film. Alternatively, or additionally, the guiding roller and the heating roller may be operated to provide, and preferably maintain, a predetermined tension or pulling force transmitted along the feeding direction of the separator film.

The stretching and/or tensioning of the separator film may for example be provided by varying the position of the guiding roller relative to the heating roller, and further by varying the torque transmitted from the heating and/or guiding roller to the separator film. A stretching or tensioning of the separator film may for example achieved by moving the outer surface of at least one of the heating and guiding rollers faster than the surface of the passing separator film. This may cause the separator film to slide on the outer surface of the roller(s), allowing for torque to be transmitted by means of frictional forces between the roller(s) and the separator film.

The stretching or tensioning of the separator film may be controlled to a level at which an increased flatness of the separator film is observed. Thus, the level of stretching or tensioning may be adjusted until at least some of the defects, such as folds, curls and creases, are reduced.

The stretching and/or tensioning of the separator film may hence be combined with the heating to further reduce the defects and irregularities of the separator film.

The guiding arrangement may comprise a plurality of guiding means, such as the above-mentioned guiding roller, to further improve the flatness of the separator film and reduce the risk for misalignment when interleaved with the sheet electrodes. The guiding means may be arranged upstream from the heating roller relative the feeding direction, that is, between the heating roller and the separator film supply, as well as downstream from the heating roller (between the heating roller and the assembly stage).

The distance between the heating roller and the assembly stage, as well as the relative separation between a guiding roller and the heating roller, may in some examples be defined in terms of the time it takes for a point on the separator film to travel between the heating roller and the assembly stage or guiding roller. The travelling time may for instance affect the cooling of the separator film, which may experience a reducing temperature with an increasing travelling time between the heating roller and the assembly stage. Thus, the distance between the heating roller and the assembly stage (as measured along the feeding path of the separator film) may be determined by a cooling rate and a speed by which the separator film is fed, so that a predetermined travelling time (or cooling) can be obtained on the way to the assembly stage. ln addition to this, the travelling time may further affect mechanical properties of the separator film material, wherein an increased travelling time for instance may allow for the material of the separator film to start relaxing or recovering after the heating. The distance between the heating roller and the assembly stage may thus be determined also by a relaxation rate or recovery time of the material of the separator film.

According to an embodiment, the guiding arrangement may comprise a folding means configured to zigzag fold the separator film. Further, the assembly stage may comprise an electrode insertion means configured to insert the sheet electrodes into the zigzag folded separator film to form a zigzag stacked structure comprising the sheet electrodes interleaved with the separator film. The folding means, which also may be referred to as a folding device, may for example comprise a separator guiding means for the separator film and a stack table configured to move back and forth relative to the separator guiding means. After an electrode sheet has been placed on the separator film on the stack table by means of the electrode insertion means, the stack table may be moved relative to the separator guiding means to allow the separator film to be folded over the electrode sheet. Thereafter, another electrode sheet may be placed on top of the folded separator film covering the previous electrode sheet, and the stack table moved back to the initial position. By repeating this process, a Z-stacking may be achieved, in which a plurality of sheet electrodes is stacked in an alternating manner in a zigzag folded separator sheet by means of the folding means and the electrode insertion means. The resulting Z-stack may be used as the inner cell structure of a prismatic cell.

According to an embodiment, the guiding arrangement may comprise an electrode guiding means, a separator guiding means and a winding means. The guiding means for the separator film and the electrodes may be configured to form a stacked structure in which the sheet electrodes are interleaved with the separator film, whereas the winding means may be configured to wind the stacked structure into a coil or roll. The coil, which also may be referred to as a jellyroll, may be used to form a cylindrical cell.

Generally, the separator film may provide spatial and electrical isolation of the electrodes, while admitting ion exchange between the electrodes. Advantageously, the separator film may comprise an organic film fabricated from a variety of materials, such as microporous polymer films or nonwoven fabrics. Examples of microporous polymeric films include polyolefin films, such as for example a polyethylene film or a polypropylene film. Further, the separator film may be a laminate of two or more films, such as a polyethylene film and a polypropylene film.

Further, the separator film may be a composite film, comprising a coating arranged on one or both sides of the film. The coating may for example be a ceramic coating, which advantageously may improve the mechanical, dimensional, and thermal stability of the separator and thus ensure a maintained isolation of the electrodes also at elevated temperatures. Examples of ceramic coatings include oxides and hydroxides of non-metallic inorganic materials as well as metallic materials. Thus, the coating may comprise for example aluminium oxide, aluminium hydroxide, aluminium oxide hydroxide, silicon oxide, magnesium oxide, or barium sulphate. The coating, or ceramic layer, may for instance have a thickness of about 0.5-5 um.

The coating may be formed by adding ceramic particles to a binder to form a dispersion, such as a slurry or paste, which can be provided to the surface of the separator film. The dispersion may for example be added to the separator film by spraying, printing, roll coating or other techniques known in the art. Examples of binders include polyvinylidene fluoride, polyvinylidene difluoride, acrylate, polyacrylamide, polyvinyl acetate amide, polyvinlyalcohol, polyvinylpyrrolidone, polyacrylic acid and combinations thereof.

For a laminate film comprising a polyethylene film and a polypropylene film and at least one ceramic coating layer of 0.5-5 um, experiments have shown that an improved surface flatness of the separator film may be achieved when heated to temperatures of about 30°C and above. Further, damages to the separator film were observed for temperatures of 90°C and above. Hence, in may be advantageous to heat the separator film to temperatures in the interval of 30-90°C. lt will be appreciated that the above-mentioned optional additional features and effects of the device according to the first aspect, when applicable, apply to the method according to the second aspect as well, and vice versa.

A further scope of applicability of the present invention will become apparent from the detailed description given below. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the scope of the invention, as defined by the appended claims, will become apparent to those skilled in the art from this detailed description. Hence, it is to be understood that this invention is not limited to the particular component parts of the device described or acts of the methods described, as such device and method may vary.

Brief description of drawinqs The above and other aspects of the present invention will now be described in more detail, with reference to the appended figures. The figures should not be considered limiting but are instead used for explaining and understanding.

Figure 1 shows a device for assembling an inner cell structure of a secondary battery cell according to an embodiment.

Figure 2 shows a device for assembling an inner cell structure according to another embodiment.

Figures 3a and b show heating ro||ers and guiding ro||ers according to some embodiments.

Figures 4a and b are cross sections of an inner cell structure for a prismatic cell and a cy|indrica| cell, respectively.

Figures 5a and b show examples of irregu|arities and defects of a separator film.

Figure 6 is a perspective view of a portion of a separator film.

Figure 7 is a flowchart of a method according to an embodiment of the invenﬁon.

Detailed description The present invention will now be described hereinafter with reference to the accompanying drawings, in which currently preferred, exemplary embodiments of the invention are illustrated.

Figure 1 is a schematic illustration of a device 100 for assembling an inner cell structure 110 of a secondary battery cell. The device 100, which also may be referred to as a stacker or stacking device, comprises an assembly stage 130 for arranging a separator film 120 between sheet electrodes 111, 112 of the inner cell structure, as well as a guiding arrangement 140 for guiding the separator film 120 from a film supply 141 to the assembly stage 130.

The guiding arrangement 140 may comprise a plurality of ro||ers 143 for controlling the feeding of the separator film 120. Rollers 143 may for example be provided to control the unwinding of the separator film 120 from the film supply 141 and well as the tension and stretching of the separator film 120 on its way to the assembly stage. Examples of rollers 143 include dancer rollers, idler rollers and feeding rollers.

The guiding arrangement 140 may further comprise a heating roller 150. The heating roller 150 may be configured to be arranged in thermal contact with the separator film 120 so as to transfer heat to the separator film 120. The heating roller 150 may be arranged along the path between the film supply 141 and the assembly station 130, and preferably in mechanical contact with a passing surface of the separator film 120. ln some examples the heating roller 150 may serve the combined purpose of heating and mechanically tensioning or stretching the separator film 120. As indicated in the present figure, the heating roller 150 may be formed as a cylinder rotating around its length axis as the separator film is fed to the assembly stage 130. The outer surface of the cylinder 150 may for instance be a metal surface adapted to facilitate heat transfer between the cylinder 150 and the surface of the separator film 120 abutting the cylinder 150.

A guiding roller 142 may be arranged to control the tension in the separator film 120, and/or to stretch the separator film 120. The guiding roller 142, which also may be referred to as a dancer roller, may for instance be coupled to a preloaded idler arm for maintaining consistent tensioning in the separator film 120. The guiding roller 142 may be moveable relative to the heating roller 150, as seen in the feeding direction of the separator film 120, to enable a stretching of the separator film 120. Alternatively, or additionally the heating roller 150 may be movable relative to the guiding roller 142.

The tension and/or stretching may further be controlled by varying the line speed between drive rollers of the guiding arrangement 140. The rotational speed of a downstream drive roller may for example be slightly higher than the rotational speed of an upstream drive roller to generate a tensioning force in the separator film 120.

The guiding roller 142 and/or the heating roller 150, as well as any drive rollers, may be controlled by a control system (not shown), using for instance an open-loop or closed loop control.

The guiding arrangement 140 may further comprise a folding means 144 configured to fold the separator film 120 into a zigzag fold, or Z-fold. As illustrated in the present figure, the Z-fold may be provided by moving the separator film 120 alternatingly back and forth over a stack table 136 to form a Z-fold stack into which the electrodes 111, 112 can be inserted. ln alternative configurations, the stack table is moved back and forth instead of the separator film 120, and in further examples both the stack table 136 and the separator film 120 are moved relative to each other.

The assembly stage 130 may thus comprise a stack table 136 for supporting the inner cell structure 110 as it is being assembled. The assembly stage 130 may further comprise an electrode insertion means 132, such as one or several robotic arms 132, configured to pick up sheet electrodes 111, 112 from a supply stack and place them on the folded separator film 120 on the stack table 136. The assembly stage 130 may for example comprise a first and a second robotic arm 132 taking turns placing a sheet electrode 111, 112 on each fold of the separator film 120.

The separator film 120 may for instance comprise a laminate of a first and a second polymer film, such as a polyethylene film and a polypropylene film (as is discussed in greater detail in the following with reference to figure 6). The separator film 120 may be provided from a separator film supply 141, such as a roll 141, from which the separator film may be unwound by the guiding arrangement 140. ln a typical example, the separator film 120 may be about 150 mm wide and wound onto a supply roll in about 1500 layers or more.

While the device in figure 1 may be used to form an inner cell structure 110 for prismatic cell, the device in figure 2 may be used to form an inner cell structure 110 for a cylindrical cell. The device 100 of figure 2 may be similarly configured as the embodiment disclosed in connection with figure 1, with the difference that the guiding arrangement 140 may comprise an electrode guiding means 146 and a separator guiding means 148 configured to form a stacked structure comprising the sheet electrodes 111, 112 separated by two separator films 120. The stacked structure may be fed to a winding means 134 for winding the structure into a coil 110, forming an inner cell structure for a cylindrical cell. 11 Figure 3a is a schematic cross section of a heating roller 150, which may be similarly configured as the heating rollers 150 discussed above in connection with figures 1 and 2. The heating roller 150 may be a cylindric roller having an outer surface 151 on which the separator film 120 may run as it is fed in the feeding direction F towards the assembly stage 130. The heating roller 150 may be heated by a heating means, such as an electric heater 152 schematically indicated in the present figure. Other heating means are however conceivable, for example utilising a heated air flow or liquid flow passing through the heating roller 150.

A thermal contact may be provided between the separator film 120 and the outer surface 151 of the heating roller 150, allow heat to be transferred into the passing separator film 120. The heat generated by the electric heater 152 may be controlled so as to allow the passing separator film 120 to be heated to a temperature in the interval of 30-90°C, which has shown to reduce irregularities of the separator film 120 and increase its flatness.

To further increase the smoothening of the separator film 120 prior to assembly, one or several guiding rolls 142 may be arranged to stretch the separator film 120 or at least increase the tension in the separator film. The applied tension and/or stretching of the separator film 120 may be controlled by varying the position of the guiding rollers 142 along the path of the separator film 120, as indicated by the arrows in the present figure. Thus, by applying a tension force by means of the guiding rollers 142, the separator film may be both heated and stretched when passing over the heating roller 150.

Further, it will be appreciated that more than one heating roller 150 may be provided. This is illustrated in figure 3b, showing an example embodiment wherein the separator film 120 passes an additional heating roller 150' on its way to the assembly stage. The additional heating roller 150' may be similarly configured as the heating roller 150 disclosed in figure 3a.

Figures 4a and b are cross sections of a prismatic inner cell structure and a cylindrical inner cell structure, respectively. The inner cell structure 110 of figure 4a may be the result of the assembling process disclosed in connection with figure 1, whereas the inner cell structure 110 of figure 4b may 12 be the outcome of the assembling process discussed with reference to figure 2.

Figure 4a shows a Z-folded separator film 120 in which sheet electrodes 111, 112 have been interleaved to form a stack. The sheet electrodes 111, 112 may be arranged in an alternating manner, with every second sheet electrode being a cathode 111 and every second electrode being an anode 112.

Figure 4b shows an alternative configuration, wherein a stacked structure of alternating layers of sheet electrodes 111, 112 and separator fi|ms 120 are wound into a coi|ed structure or roll 110. The coi|ed structure may also be referred to as a jellyroll. While the sheet electrodes 111, 112 in the stacked structure of figure 4a may be electrically contacted from the sides of the stack by means of contacting tab structures, the sheet electrodes 111, 112 in the coi|ed structure in figure 4b may be contacted from above and below (i.e., at the top and bottom end portion of the roll 110). There are alternatives to how to electrically contact the electrodes which are known to the skilled person and will thus not be described here.

Figures 5a and b illustrate different examples of irregularities and defects of the separator film 120 prior to heating according any of the above- mentioned embodiments. The present invention hence aims at reducing or even eliminating such irregularities and defects. Figure 5a shows an example of a surface of the separator film 120 having a certain waviness or creases, which risk affecting the dimensional stability of the separator film (i.e., the width) during the assembly, thereby causing alignment errors and possibly short-circuiting of the sheet electrodes 111, 112. The waviness and creases may for example be caused by defects and thickness variations in the separator film 120, which may be accumulated and amplified through successive layers of the supply roll 141. Due to the winding onto the supply roll, a relatively small defect or irregularity in an inner layer of the coil may lead to relatively large defects or distortions of the outer layers of the coil. This has in particular been observed for separator fi|ms comprising polyethylene. The polyethylene may to a large extent be crystalline, with some amorphous regions. lnitial thickness variations, which may be amplified 13 when the film is wound on the supply roll, may cause the amorphous regions may be stretched and deformed. The local stretching of these amorphous regions may cause wrinkles, skewing and wavy edges of the film. lt has further been observed that by heating the separator film, such as the above- mentioned comprising polyethylene, the material of the separator film 120 may be relaxed and at least partly restored, thereby reducing at least some of these defects or distortions.

Figure 5b shows an example wherein an edge of the separator film 120 may be curled, which risks leading to folds and misalignments when stacked with the sheet electrodes. Similar to the above, the curls may be at least partly removed by means of the heating roller 150, resulting in the separator film 120 having an increased flatness when arriving at the assembly stage 130.

Figure 6 is a perspective view of a portion of an example separator film 120 as discussed above with reference to figures 1-5. Thus, the separator film 120 may be a composite structure comprising a base film 121, 122 and a coating 124. The base film 121, 122 may be a laminate structure of a first and a second porous polymer film, such as a polyethylene film 121 and a polypropylene film 122. The coating 124, which may be provided on at least one of the surfaces of the base film 121, 122, may be formed of ceramic particles and a binder. ln an example, the coating 134 comprises aluminium particles dispersed in polyvinylidene difluoride. While the separator film 120 may have a relatively high thickness uniformity of for instance 11.5 um, it will be appreciated that also very small thickness variations may give rise to defects, as the variations may be amplified as the separator film is wound into a supply roll.

Figure 7 is a flow chart illustrating a method 200 according to some embodiments. The acts 210-230 of the method 200 may be performed by a device which may be similarly configured as any of the above-described example devices, and the details of such devices are therefore not repeated in the following.

With reference to figure 7, the method 200 may comprise guiding 210 the separator film from the separator film supply to the assembly stage, and 14 heating 220 the separator film by means of the heating roller. ln some examples, the method 200 may further comprise a step of stretching 225 the separator film (the optional character of this step is illustrated by the dotted line in figure 7), preferably by means of a guiding roller arranged to cooperate with the heating roller. Thereafter, the separator film may be arranged 230, by the assembly stage, between the sheet electrodes to form the inner cell structure of the secondary battery cell.

A person skilled in the art realises that the present invention is not limited to the embodiments described above. On the contrary, many modifications and variations are possible within the scope of the appended claims. Such modifications and variation can be understood and effected by a skilled person in practising the claimed invention, from a study of the drawings, the disclosure and the appended claims.

Claims

1. A device (100) for assembling an inner cell structure (110) ofa secondary battery cell, wherein the inner cell structure comprises sheet electrodes (111, 112) interleaved with a separator film (120), the device comprises: an assembly stage (130) for arranging the separator film between the sheet electrodes; and a guiding arrangement (140) for guiding the separator film from a separator film supply (141) to the assembly stage; wherein the guiding arrangement comprises a heating roller (150) configured to be arranged in thermal contact with the separator film to transfer heat to the separator film.

2. The device according to claim 1, wherein the guiding arrangement further comprises a guiding roller (142), and wherein the guiding roller and the heating roller are configured to cooperate to stretch the separator film.

3. The device according to claim 2, wherein the guiding roller and the heating roller are further configured to cooperate to maintain a predetermined tension in the separator film.

4. The device according to any of the preceding claims, wherein: the guiding arrangement comprises a folding means (144) configured to zigzag fold the separator film; and the assembly stage comprises an electrode insertion means (132) configured to insert the sheet electrodes into the zigzag folded separator film, thereby forming a zigzag stacked structure comprising the sheet electrodes interleaved with the separator film.

5. The device according to any of claims 1-3, wherein: the guiding arrangement further comprises an electrode guiding means (146) and a separator guiding means (148), wherein the electrode guidingmeans and the separator guiding means are configured to form a stacked structure comprising the sheet electrodes interleaved with the separator film; and the assembly stage comprises a winding means (134) configured to wind the stacked structure into a coil.

6. A method (200) for assembling an inner cell structure of a secondary battery cell, wherein the inner cell structure comprises sheet electrodes interleaved with a separator film, the method comprises: guiding (210) the separator film from a separator film supply to an assembly stage; heating (220) the separator film by means of a heating roller; followed by: arranging (230), by the assembly stage, the separator film between the sheet electrodes.

7. The method according to claim 6, wherein the separator film comprises a laminate formed ofa polyethylene film (121) and a polypropylene film.

8. The method according to claim 7, wherein the separator film further comprises a ceramic coating.

9. The method according to claim 8, wherein the ceramic coating has a thickness of 0.5-5 um.

10. arranged on one or both sides of the separator film. The method according to claim 8 or 9, wherein the ceramic coating is

11. the separator film to a temperature of 30-90°C, such as 40-60°C. The method according to any one of claims 6-10, comprising heating

12. stretching (225) the separator film. The method according to any one of claims 6-11, further comprising

13. The method according to ciaim 12, wherein a portion of the separator film is stretched and heated simultaneously.