Separator for electrochemical elements with reduced self-discharge rate, and method for producing same

Abstract

Disclosed is a separator for electrochemical elements, said separator comprising fibrillated fibers of regenerated cellulose, with the fibrillated fibers of regenerated cellulose making up at least 75% of the mass of the separator. The weight per unit area of the separator is at least 8 g / m2 and at most 17 g / m2, and the mean individual sheet thickness of the separator is at least 12 pm and at most 40 pm. With regard to its thickness and fiber structure, the separator exhibits homogeneity that is characterized by the simultaneous implementation of the following features (i) and (ii): (i) the ratio of the mean individual sheet thickness of the separator to the individual sheet thickness of the separator is at least 0.88 and at most 1.04, and (ii) a contrast value C determined by image analysis is at least 1000 and at most 1600.

Description

[0001] SEPARATOR FOR ELECTROCHEMICAL ELEMENTS WITH REDUCED SELF-DISCHARGE RATE AND METHOD FOR ITS PRODUCTION

[0002] FIELD OF THE INVENTION

[0003] The invention relates to a separator for electrochemical elements which is essentially formed by cellulose fibres and which imparts a reduced self-discharge rate to an electrochemical element made therefrom without significantly increasing the equivalent series resistance.

[0004] BACKGROUND AND STATE OF THE ART

[0005] An electrochemical cell typically comprises at least one positive electrode, one negative electrode, an electrolyte, a separator, a housing, and current collectors. The separator is impregnated with the electrolyte and serves to electrically separate the two electrodes. It should also allow the unhindered flow of ions between the electrodes and from the electrolyte to the electrodes, so that the electrochemical cell has favorable properties, particularly rapid charging and the ability to draw high currents.

[0006] These requirements for the separator mean that it should be as thin as possible to ensure a short path for ions from one electrode to the other through the separator's pores, achieving a high volumetric energy density of the electrochemical element. It should also have a high porosity. The pore volume absorbs the electrolyte, which is why a large pore volume is beneficial for rapid ion transport. Furthermore, it is advantageous if the pores in the separator are formed by a large number of small pores.

[0007] The separator must be chemically resistant to the electrolyte, as electrochemical cells can be recharged multiple times and are typically in use for several years. The separator must therefore also be resistant to oxidative and reductive environments.

[0008] For safety reasons, the separator should have good thermal stability to limit the risk of fire in case of damage to the electrochemical element.

[0009] Electrochemical elements such as electrolytic capacitors, double-layer capacitors, and accumulators exhibit a certain self-discharge rate. This means that a slow discharge occurs without a consumer drawing the stored energy, thus resulting in the loss of the energy stored in the electrochemical element. Self-discharge of electrochemical elements is therefore undesirable and is partly influenced by the separator in the electrochemical element.

[0010] Another important parameter of an electrochemical cell is its equivalent series resistance. This resistance significantly influences the performance parameters of the electrochemical cell and should be as low as possible.

[0011] Separators can, for example, be formed from thin plastic films with a defined porosity. For reasons of safety, thermal stability, and the lower equivalent series resistance of the resulting electrochemical element, separators are often made of cellulose fibers, which are inferior to separators made of plastic films in some aspects, including the self-discharge rate. On the other hand, separators made of cellulose fibers exhibit better wettability for the electrolyte than separators made of plastic films.

[0012] There is therefore an interest in having a separator made of cellulose fibres that allows the production of electrochemical cells with a reduced self-discharge rate without significantly increasing the equivalent series resistance.

[0013] SUMMARY OF THE INVENTION

[0014] The invention is therefore based on the object of providing a separator for electrochemical elements comprising cellulose fibers, which allows the production of electrochemical elements that have at least approximately the same self-discharge rate as can be achieved with separators made of plastic films. The equivalent series resistance of the electrochemical element produced therefrom should not be significantly increased.

[0015] This object is achieved by a separator for electrochemical elements according to claim 1, an electrochemical element comprising this separator according to claim 28, and a method for producing a separator for electrochemical elements according to claim 30. Advantageous further developments are specified in the dependent claims.

[0016] In Lewandowski et al.: Self-discharge of electrochemical double-layer capacitors, Phys. Chem. Chem. Phys., 2013, 15, 8692, DOI: 10.1039 / C3CP44612C, a self-discharge model is described that shows that self-discharge is caused by multiple, complex processes operating on different time scales. However, there is no indication of how the self-discharge rate of electrochemical elements can be improved using separators. Standardized measurement methods for easily quantifying self-discharge that do justice to these different discharge processes are also still lacking. However, the inventors were able to demonstrate that the homogeneity of the separator is an essential property for positively influencing the self-discharge rate of the electrochemical element fabricated from it.A special calendering process makes it possible to improve homogeneity without significantly increasing the equivalent series resistance of an electrochemical element produced from it. The fiber structure of the separator according to the invention obtained with this process can be characterized with regard to its homogeneity using two parameters that describe different aspects of homogeneity and are both necessary together to reduce the self-discharge rate in an electrochemical element while maintaining virtually unchanged equivalent series resistance.

[0017] The separator according to the invention comprises fibrillated fibers of regenerated cellulose, wherein the fibrillated fibers of regenerated cellulose make up at least 75% of the mass of the separator. The basis weight of the separator is at least 8 g / m 2 and a maximum of 17 g / m 2, and the average single sheet thickness of the separator, according to ISO 534:2011, is at least 12 pm and at most 40 pm.

[0018] Furthermore, the separator has a homogeneity in terms of its thickness and fiber structure, which is characterized by the simultaneous realization of the following features (i) and (ii):

[0019] (i) the ratio of the mean single sheet thickness of the separator according to ISO 534:2011 to the single sheet thickness of the separator according to ISO 534:2011 is not less than 0,88 and not more than 1,04 and

[0020] (ii) an image-analytically determined contrast value is at least 1000 and at most 1600, the image-analytically determined contrast value being determined from the two-dimensional power spectrum of an image of the separator taken in transmitted light in 256 grey levels with a resolution of 0.0423 mm per pixel (600 dpi), a square section of the image with an area of ​​approximately 8.62 cm x 8.62 cm (2048 x 2048 pixels) being used to calculate the two-dimensional power spectrum.

[0021] When reference is made below to the "homogeneity" of the separator, this refers in particular to the simultaneous realization of the above-mentioned features (i) and (ii). The values ​​for the ratio of the average single sheet thickness to the single sheet thickness from feature (i) and the contrast value determined by image analysis from feature (ii) serve to quantify the homogeneity within the meaning of the present disclosure. The inventors have determined that one aspect of the homogeneity of the separator can be assessed according to the invention based on the thickness. This aspect is manifested in feature (i). The thickness is determined according to ISO 534:2011. ISO 534:2011 stipulates that the thickness of thin papers, such as separators, can be measured on ten superimposed layers. The measured thickness of ten superimposed layers is divided by ten, thus obtaining the "average single sheet thickness" (ISO 534:2011, Definition 3.2).Alternatively, the thickness can also be measured on a single sheet according to ISO 534:2011, yielding the "single sheet thickness" (ISO 534:2011, definition 3.1). The single sheet thickness and the average single sheet thickness are usually not the same.

[0022] According to the inventors' findings, the difference is partly due to the fact that the separator exhibits inhomogeneities in terms of thickness. The individual sheet thickness of the separator is primarily determined by the maximum thickness within the measuring area. At the average individual sheet thickness, the inhomogeneities of the individual layers partially compensate for each other, so that the measured value corresponds more closely to the average thickness of the separator. In general, therefore, the average individual sheet thickness of the separator is less than the individual sheet thickness. According to the inventors' findings, the extent of the deviation is a key indicator of the homogeneity of the separator for the invention. The greater the deviation, i.e. the smaller the thickness ratio, the greater the local inhomogeneities in terms of thickness.A thickness that is as uniform as possible maximizes the contact area between the separator and the electrodes, which has a positive effect on the self-discharge rate and other properties of the resulting electrochemical element.

[0023] For conventional separators made of cellulose fibers, the ratio of the average individual sheet thickness to the individual sheet thickness is approximately 0.80 to 0.85. However, this ratio can be increased using a special calendering process, which is described in more detail below and represents a further aspect of the invention. The calendering process allows for a rearrangement of the fibers in the separator through moistening of the separator and low line loads, so that a more favorable ratio can be achieved than for conventional separators made of cellulose fibers. This process is explained further below.

[0024] According to the inventors' findings, another aspect of separator homogeneity can be assessed using an image analysis method and is quantified according to feature (ii). Although the measurement of paper homogeneity, for example based on formation, is not standardized, certain methods have been established and are widely used. For this purpose, an image of the separator is captured in transmitted light in grayscale and subjected to a two-dimensional, discrete Fourier transformation. Formation parameters such as contrast, cloudiness, or fiber orientation are derived from the results of the Fourier transformation. The principles of these methods are described in ZELLCHEMING - Information Sheet: Online Measurement of Paper Formation - Theory and Fundamentals, July 28, 2010, Section 5.2 (available at https: / / www.zellcheming.com).de / publikationen), and a variant of this method was implemented in a system distributed by the Paper Technology Foundation under the name DOMASmultispec (Digital Optical Measurement and Analysis System). For this invention, a variant of this method was used to calculate the contrast value determined by image analysis, which is explained in detail below.

[0025] Using an Epson V750 Pro scanner, model number J221A, serial number G78W054203, images of the separators were captured in transmitted light at 256 gray levels with a resolution of 0.0423 mm per pixel (600 dpi), using square pixels. A square section of the image measuring 8.62 cm x 8.62 cm (2048 x 2048 pixels) was used for image analysis.

[0026] The image can be captured using any other scanner with a CCD sensor in transmitted light. However, the scanner must always be calibrated using the IT8.7 / 1 color standard defined in ISO 12641-1:2016 before capturing the images. Additionally, the scanner must be configured so that no manipulation of the image beyond calibration is performed, and the image data is provided uncompressed or losslessly compressed for further calculations.

[0027] The image section is then defined by a two-dimensional array of gray values ​​gj,k with j = 0, 1, 2, ..., 2047 and k = 0, 1, 2, ..., 2047, where gj,k can assume integer values ​​from 0 (black) to 255 (white). In a first step, the image section is normalized to a mean gray value of 1. This allows fluctuations or aging of the illumination and differences between different scanners to be compensated. For the contrast value determined by image analysis, only the relative differences within the image section are important. The normalization is performed by 2047 with The normalized gray values ​​are subjected to a discrete two-dimensional Fourier transformation, resulting in the Fourier coefficients f m ,n with m = 0, 1, 2, ..., 2047 and n = 0, 1, 2, ..., 2047. The discrete Fourier transform is given by defined in the usual way. Since the number of pixels in each dimension is a power of 2, the calculation can be performed efficiently using the two-dimensional variant of the fast Fourier transform. Furthermore, the Fourier coefficients exhibit symmetry properties because the gray values ​​are real numbers, so later only the indices m = 0, 1, 2, ..., 1023 and n = 0, 1, 2, ..., 1023 are considered.

[0028] Each of the indices m and n corresponds to a wavelength X m or X n , which results from the resolution of the image and by

[0029] 2048

[0030] A m = 0.0423 - m

[0031] 2048

[0032] A n = 0.0423 - n in mm, with m = 1, 2, 3, ..., 1023 and n = 1, 2, 3, ..., 1023.

[0033] The contrast value C is then calculated from the average signal energy for wavelengths from 2 mm to 64 mm and is given by where the sum is to be formed over all pairs (m,n) with m = 1, 2, 3, ..., 1023 and n = 1, 2, 3, ..., 1023, for which

[0034] 2 < A 2 n + A 2 < 64.

[0035] According to the inventors' findings, the contrast value determined by image analysis is another key indicator of separator homogeneity for the invention. Unlike the thickness ratio, the contrast value determined by image analysis provides more information about the homogeneity of the mass distribution in the separator. The contrast value determined by image analysis is therefore used as a further parameter alongside the thickness ratio explained above to ensure that the separator exhibits high homogeneity and can be used to manufacture an electrochemical element with a reduced self-discharge rate and a not significantly increased equivalent series resistance. A low contrast value determined by image analysis is an indication of a homogeneous structure of the separator, especially with regard to density.According to the inventors' findings, a homogeneous density allows a particularly uniform distribution of the charge in the pores of the separator filled with the electrolyte, which has a positive effect on the self-discharge rate of an electrochemical element made from it.

[0036] For conventional separators made of cellulose fibers, the contrast value determined by image analysis in this way is above the above-mentioned upper limit of 1600, and usually has values ​​up to about 2200. As with the thickness ratio, the contrast value determined by image analysis can be improved by the special calendering process according to the invention.

[0037] To achieve the inventive effect of a low self-discharge rate in an electrochemical element made from the separator according to the invention, it is necessary that both the thickness ratio and the contrast value determined by image analysis lie within the respective intervals according to the invention. As the exemplary embodiments described below demonstrate, these are two independent parameters that only together achieve the desired effect.

[0038] The separator according to the invention is formed to at least 75% of its mass from fibrillated fibers of regenerated cellulose. These fibers create a pore structure and strength favorable for a separator. However, the proportion of fibrillated fibers of regenerated cellulose is preferably higher, amounting to at least 80% and at most 100%, and particularly preferably at least 80% and at most 95% of the separator's mass.

[0039] The fibrillated fibers of regenerated cellulose are preferably formed from solvent-spun, fibrillable fibers of regenerated cellulose that are additionally fibrillated. Such fibers are sold, for example, under the name Lyocell.

[0040] The linear density of the fibrillable fibers of regenerated cellulose is important for the fibrillation of the fibers. Preferably, the average linear density of the fibrillated fibers of regenerated cellulose before fibrillation is at least 0.8 g / 10,000 m (0.8 dtex) and at most 3.0 g / 10,000 m (3.0 dtex), and more preferably at least 1.0 g / 10,000 m (1.0 dtex) and at most 2.8 g / 10,000 m (2.8 dtex).

[0041] The length of the fibrillated fibers of regenerated cellulose before fibrillation is primarily important for the strength of the separator, with longer fibers resulting in greater strength but also requiring more energy during fibrillation. The average length of the fibrillated fibers of regenerated cellulose before fibrillation is preferably at least 2 mm and at most 8 mm, and particularly preferably at least 3 mm and at most 6 mm. Fibrillation of the regenerated cellulose fibers can be achieved, for example, by means of refining units such as those known in the art for paper production. The freeness of the fibrillated fibers of regenerated cellulose is preferably at least 70°SR and at most 95°SR, particularly preferably at least 75°SR and at most 90°SR. The Schopper-Riegler freeness can be determined according to ISO 5267-1:1999.

[0042] In addition to the fibrillated fibers of regenerated cellulose, the separator according to the invention may comprise further cellulose fibers.

[0043] Preferably, the further cellulose fibers can be formed entirely or partly by non-fibrillated fibers of regenerated cellulose.

[0044] Preferably, the additional cellulose fibers can be formed entirely or partially from cellulose fibers or mixtures thereof, with the cellulose fibers preferably being obtained from coniferous woods, hardwoods, or other plants such as hemp, flax, jute, ramie, kenaf, kapok, coconut, abaca, sisal, bamboo, cotton, or esparto grass, or from waste paper pulp. Mixtures of cellulose fibers from various sources can also be used to produce the separator. The cellulose fibers are particularly preferably obtained from hardwoods or softwoods.

[0045] Particularly preferably, the cellulose fibers are at least partially microfibrillated cellulose fibers, nanofibrillated cellulose fibers, or cellulose fibers with an average length-weighted length of at most 0.2 mm, preferably of at most 0.15 mm. These types of cellulose fibers are particularly well suited for providing the separator with a small average pore size and improving porosity homogeneity. They therefore also contribute to a reduced self-discharge rate of an electrochemical element made from the separator according to the invention.

[0046] The mean length-weighted length of the cellulose fibers can be determined by optical analysis according to ISO 16065-2:2014.

[0047] In addition to the fibrillated fibers of regenerated cellulose and other optional cellulose fibers, the separator according to the invention can also contain other fibers. These preferably include fibers made from cellulose derivatives, glass fibers, plastic fibers, such as fibers made from polyolefins such as polyethylene or polypropylene; from polyesters such as polyethylene terephthalate or polylactic acids; from polyarylates such as poly-(4-hydroxybenzoic acid-co-6-hydroxy-2-naphthoic acid); from polyethers, polysulfones, polyurethanes, polyamides, aromatic polyamides such as poly-(p-phenylene terephthalamide); polyimides, polyvinyl alcohol, polyacrylates such as polyacrylonitrile or poly-(acrylonitrile-co-methylacrylate); polyphenylene sulfide, or from poly-(ethylene-co-vinyl acetate). Preferably, the proportion of fibers other than the fibrillated fibers of regenerated cellulose taken together is at most 25%, more preferably at most 20% and most preferably at most 15% of the mass of the separator.

[0048] The separator according to the invention can contain further components which the person skilled in the art can choose according to his experience to suit the manufacturing process, these include, for example, polyvinyl alcohol, polyethylene glycol, polyvinylidene fluoride, guar gum, starch, carboxymethylcellulose, methylcellulose, dialdehydes such as glyoxal, and inorganic fillers such as kaolin, titanium dioxide (TiO2), silicon dioxide (SiO2), aluminum oxide (ΔI2O3), zirconium dioxide (ZrO2) or calcium carbonate (CaCO3).

[0049] The amount of inorganic fillers in the separator is preferably at most 20%, more preferably at most 15% and most preferably at most 5% of the mass of the separator.

[0050] The separator according to the invention has a basis weight of at least 8 g / m 2 and a maximum of 17 g / m 2 , preferably at least 9 g / m 2 and a maximum of 16 g / m 2 and particularly preferably at least 10 g / m 2and a maximum of 15 g / m 2 The basis weight can be determined according to ISO 536:2019. The basis weight influences the thickness of the separator, the strength of the separator, the material requirements for manufacturing the separator, and also the homogeneity of the separator, with higher basis weights tending to result in a more homogeneous separator. The basis weight, through the thickness, has a major influence on the performance parameters of an electrochemical element made from the separator, and generally, one wants to keep the basis weight as low as possible. The specified inventive and preferred intervals allow a favorable balance between these requirements.

[0051] The separator according to the invention has an average individual sheet thickness, according to ISO 534:2011, of at least 12 pm and at most 40 pm, preferably at least 14 pm and at most 38 pm, particularly preferably at least 16 pm and at most 35 pm. The thickness should be as small as possible, but is limited from the bottom up by the requirements regarding reliable electrical separation of the electrodes and strength, and from the top up by the expectations regarding the performance parameters of the electrochemical element manufactured therefrom. The specified inventive and preferred intervals allow a favorable balance between these requirements. The average individual sheet thickness results in particular from the calendering process, as explained further below.

[0052] The special properties of the separator according to the invention with regard to homogeneity can be achieved through the calendering process described in more detail below. This process means that the fiber web has passed through at least one roll nip during the production of the separator, in which mechanical pressure was exerted on the fiber web in the thickness direction. Calendering reduces the thickness and shrinks the pores, but also reduces the overall porosity of the separator. According to the inventors' findings, the special calendering process, which represents a further aspect of the invention, leads to a homogeneous structure of the separator, and this homogeneous structure is essential for imparting a reduced self-discharge rate to an electrochemical element made from it compared to the prior art, while not significantly increasing the equivalent series resistance.The homogeneity resulting from the special calendering process is characterized, as already explained above, by the thickness ratio and by the contrast value determined by image analysis.

[0053] According to the invention, the ratio of the average single sheet thickness to the single sheet thickness of the separator is at least 0.88 and at most 1.04, preferably at least 0.89 and at most 1.02 and most preferably at least 0.91 and at most 1.00, wherein the average single sheet thickness and the single sheet thickness are measured according to ISO 534:2011.

[0054] According to the invention, the contrast value determined by image analysis is at least 1000 and at most 1600, preferably at least 1100 and at most 1550, particularly preferably at least 1200 and at most 1500.

[0055] Particularly preferably, the separator according to the invention has a structure such that the ratio of the average individual sheet thickness to the individual sheet thickness of the separator is at least 0.88 and at most 1.04 and that the contrast value determined by image analysis is at least 1100 and at most 1550.

[0056] Most preferably, the separator according to the invention has a structure such that the ratio of the average individual sheet thickness to the individual sheet thickness of the separator is at least 0.88 and at most 1.04 and that the contrast value determined by image analysis is at least 1200 and at most 1500.

[0057] Preferably, the separator according to the invention has a structure such that the ratio of the average individual sheet thickness to the individual sheet thickness of the separator is at least 0.89 and at most 1.02 and that the contrast value determined by image analysis is at least 1000 and at most 1600.

[0058] Particularly preferably, the separator according to the invention has a structure such that the ratio of the average individual sheet thickness to the individual sheet thickness of the separator is at least 0.89 and at most 1.02, and that the contrast value determined by image analysis is at least 1100 and at most 1550. Very particularly preferably, the separator according to the invention has a structure such that the ratio of the average individual sheet thickness to the individual sheet thickness of the separator is at least 0.89 and at most 1.02, and that the contrast value determined by image analysis is at least 1200 and at most 1500.

[0059] Preferably, the separator according to the invention has a structure such that the ratio of the average individual sheet thickness to the individual sheet thickness of the separator is at least 0.91 and at most 1.00 and that the contrast value determined by image analysis is at least 1000 and at most 1600.

[0060] Particularly preferably, the separator according to the invention has a structure such that the ratio of the average individual sheet thickness to the individual sheet thickness of the separator is at least 0.91 and at most 1.00 and that the contrast value determined by image analysis is at least 1100 and at most 1550.

[0061] Most preferably, the separator according to the invention has a structure such that the ratio of the average individual sheet thickness to the individual sheet thickness of the separator is at least 0.91 and at most 1.00 and that the contrast value determined by image analysis is at least 1200 and at most 1500.

[0062] The aforementioned structures of the separator according to the invention can be achieved through the special calendering process described below. The stated values ​​for the resulting parameters (ratio of the average individual sheet thickness to the individual sheet thickness, contrast value determined by image analysis) can serve as a guideline for the actual implementation of the calendering process, allowing the process to be adapted accordingly.

[0063] In addition to the thickness ratio and the contrast value determined by image analysis, the roughness of the separator can also be used to characterize the effect of the calendering process and thus the homogeneity of the separator, at least of its surface. Roughness is thus an additional feature of the separator according to the invention, which contributes to imparting a low self-discharge rate to an electrochemical element made from it. However, it is not sufficient for characterization on its own; it is only useful in combination with the thickness ratio and the contrast value determined by image analysis.

[0064] The roughness of the separator can be different on both sides. The values ​​refer to an average roughness, which results from the mean roughness of the two sides of the separator and was determined according to Bendtsen in accordance with ISO 8791-2:2013. The average roughness according to Bendtsen is preferably at least 15 ml / min and at most 80 ml / min, more preferably at least 20 ml / min and at most 75 ml / min, and most preferably at least 22 ml / min and at most 70 ml / min. Since the line loads are low in the calendering process according to the invention, a lower roughness is achieved than with separators without calendering, but the values ​​are also not as low as those achieved with calendering with the usual line loads. This is important because, as shown below, the line load in the calendering process according to one aspect of the invention must be within a narrow interval to achieve optimal results.

[0065] The density of the separator is an important parameter for characterizing its porosity. The density is expressed as dry density. To do this, the moisture content of the separator is first determined according to ISO 287:2017. It is expressed as a percentage and indicates the proportion of the separator mass that is made up of water. Furthermore, the basis weight of the separator is determined according to ISO 536:2019, with the separator conditioned according to ISO 187:2022. The average single-sheet thickness of the separator is measured according to ISO 534:2011.

[0066] The dry density can then be calculated according to the formula

[0067] (- 1 - 10ö) ' m p = - d - can be calculated, where p is the dry density in kg / m 3 , c is the moisture content in % according to ISO 287:2017, m is the basis weight in g / m 2 according to ISO 536:2019, and d is the mean single sheet thickness in mm according to ISO 534:2011.

[0068] Preferably, the dry density is at least 350 kg / m 3 and a maximum of 700 kg / m 3 , particularly preferably at least 400 kg / m 3 and a maximum of 700 kg / m 3 and most preferably at least 450 kg / m 3 and a maximum of 650 kg / m 3 As with the roughness, the dry density is lower than usual for calendered separators due to the low line load in the calendering process according to the invention.

[0069] The separator's mechanical properties are important for processing it into an electrochemical cell. These include, for example, tensile strength and elongation at break. The mechanical properties depend on the direction in which a sample was taken from the separator. A distinction is usually made between the machine direction, which is the direction in which the separator moves through the machine during production, and the transverse direction, which is the direction orthogonal to the machine direction in the plane of the separator.

[0070] The tensile strength according to ISO 1924-2:2008 in the machine direction of the separator according to the invention is, based on the width, preferably at least 0.3 kN / m and at most 2.0 kN / m, particularly preferably at least 0.4 kN / m and at most 1.5 kN / m. Based on the cross-sectional area, calculated from the width of the test strip and the average individual sheet thickness according to ISO 534:2011, the tensile strength in the machine direction of the separator according to the invention is preferably at least 15 MPa and at most 60 MPa, particularly preferably at least 25 MPa and at most 50 MPa.

[0071] The tensile strength according to ISO 1924-2:2008 in the machine direction of the separator according to the invention is preferably higher than the tensile strength in the transverse direction. Preferably, the ratio of the tensile strength in the machine direction to that in the transverse direction is at least 1.0:1.0 and at most 3.0:1.0, more preferably at least 1.2:1.0 and at most 2.2:1.0, and most preferably at least 1.5:1.0 and at most 1.8:1.0.

[0072] The elongation at break according to ISO 1924-2:2008 in the machine direction of the separator according to the invention is preferably at least 0.5% and at most 5.0%, particularly preferably at least 1.0% and at most 4.0%.

[0073] A key feature for the safety of the electrochemical element made from the separator according to the invention is the shrinkage of the separator at elevated temperatures. Preferably, the shrinkage of the separator according to the invention after heating to 150°C for 3 hours is at least 0.5% and at most 2.0%, particularly preferably at least 0.5% and at most 1.5%.

[0074] The pore structure is of great importance for the properties of an electrochemical element manufactured from the separator according to the invention. The pore structure can be characterized by both the overall porosity and the pore size distribution.

[0075] The porosity of a separator is the ratio of the pore volume to the total volume of the separator and is usually expressed as a percentage. The porosity of the separator can be estimated from the average individual sheet thickness, measured according to ISO 534:2011, the basis weight, measured according to ISO 536:2019, the moisture content of the separator, measured according to ISO 287:2017, and the density of the fibers, where a density of 1500 kg / m 3 Under these assumptions, the porosity p can be approximately determined as the ratio of the pore volume to the total volume of the separator by where m is the basis weight in g / m 2, d is the average individual sheet thickness in pm, and c is the moisture content of the separator in %. The porosity is thus obtained as a value between 0 and 1 and can be converted into a percentage by multiplying it by 100. The porosity should be as high as possible, but is primarily limited by the necessary mechanical strength and the requirement that the pores be as small as possible. The porosity is preferably at least 35% and at most 75%, particularly preferably at least 40% and at most 70%.

[0076] The pore structure can be simply characterized by the Gurley air permeability. Air permeability is also a good measure of how quickly the separator can absorb the electrolyte. A high absorption rate is beneficial for productivity in the production of electrochemical cells. The Gurley air permeability can be determined according to ISO 5636-5:2013 and is preferably at least 2 s and at most 15 s, more preferably at least 3 s and at most 12 s, and most preferably at least 4 s and at most 12 s. A low Gurley value indicates high air permeability.

[0077] The separator can be used in electrochemical cells.

[0078] An electrochemical element according to the invention comprises two electrodes, an electrolyte, and a separator according to one of the embodiments described above. The electrochemical element is preferably a capacitor, a hybrid capacitor, a double-layer capacitor, or a storage battery, and particularly preferably the electrochemical element is a double-layer capacitor.

[0079] Preferably, the electrochemical element is a double-layer capacitor having a nominal voltage of at least 1.0 V and at most 4.0 V and a capacitance of at least 1 F and at most 5000 F.

[0080] A further aspect of the invention relates to a method by which separators according to one of the embodiments described above can be produced. The method according to this aspect of the invention comprises the following steps A to E.

[0081] A - Producing a fibrous web comprising cellulose fibers,

[0082] B - Fully moistening the fiber web with water,

[0083] C - Drying the fiber web to a target moisture content,

[0084] D - Calendering of the fiber web,

[0085] E - Rolling up the fiber web forming the separator, wherein in step A the quantity and type of cellulose fibers are selected such that at least 75% of the mass of the separator in step E is formed by fibrillated fibers of regenerated cellulose and the basis weight of the separator from step E is at least 8 g / m 2 and a maximum of 17 g / m 2and in step B, the full-surface moistening is carried out with such an amount of water that the moisture content of the fibrous web after step B is at least 25% and at most 125% based on the absolutely dry mass of the fibrous web after step B, and in step B, the full-surface moistening of the fibrous web is carried out in such a way that the water penetrates the structure of the fibrous web, and wherein in step C of drying, the target value of the moisture content of the fibrous web after step C is at least 8% and at most 12% based on the absolutely dry mass of the fibrous web after step C, and in step D, the fibrous web is calendered in such a way that the average individual sheet thickness of the separator from step E is at least 12 pm and at most 40 pm, and in step D, the fibrous web runs through one or more roll gaps,wherein all roll gaps are formed by soft nips and the line load in at least one of the roll gaps is at least 10 kN / m and in none of the roll gaps is more than 70 kN / m, and in which the calendering process comprising steps B, C and D is carried out in such a way that the separator in step E has a homogeneity with regard to its thickness and fiber structure, which is characterized by the simultaneous realization of the following features (i) and (ii):

[0086] (i) the ratio of the mean single sheet thickness of the separator according to ISO 534:2011 to the single sheet thickness of the separator according to ISO 534:2011 is at least 0.88 and at most 1.04, and

[0087] (ii) an image-analytically determined contrast value is at least 1000 and at most 1600, whereby the image-analytically determined contrast value is to be determined from the two-dimensional power spectrum of an image of the separator taken in transmitted light in 256 grey levels with a resolution of 0.0423 mm per pixel (600 dpi), whereby a square section of the image with an area of ​​approximately 8.62 cm x 8.62 cm (2048 x 2048 pixels) is used to calculate the two-dimensional power spectrum.

[0088] In particular, in the roll gap or the roll gaps, the temperature or temperatures and the line load or line loads can be adjusted so that the homogeneity manifested in features (i) and (ii) is achieved.

[0089] According to the inventors' findings, the homogeneity of a separator and thus the self-discharge rate of an electrochemical element produced therefrom can be improved by a special calendering process comprising steps B, C and D of the method according to the invention.

[0090] In step B, the entire surface of the fiber web is moistened with water before calendering. The moistening must be carried out in such a way that the water also penetrates the structure of the fiber web. Unlike what is usually the case in the prior art, for example, by spraying water or vaporizing with water, the moistening affects not only the surface but the entire paper structure by increasing the moisture content of the fiber web significantly above the usual level. This partially breaks the hydrogen bonds and remobilizes the fibers throughout the structure of the fiber web so that they can be rearranged with greater homogeneity in the subsequent calendering step D. This process step is critical because moistening also reduces the strength of the fiber web; in practice, it is barely higher than what is necessary for further processing of the fiber web.

[0091] In step C, the fiber web is pre-dried to a specific target moisture content. This brings the fiber web into a state favorable for calendering in step D, so that the fibers remain sufficiently mobile, but the strength of the fiber web is also well suited for calendering. According to the inventors' findings, the desired mobilization of the fibers does not occur if the fiber web is directly adjusted to the target moisture content after step C, without first being intensively moistened over its entire surface with water in step B. According to the inventors' findings, this is crucial for ensuring that the water is evenly distributed throughout the fiber web and that the fibers are also mobilized within the fiber web.

[0092] In step D, the fiber web is calendered. It passes through at least one roll nip, where pressure is exerted on the fiber web. The pressure is not particularly high, and the number of roll nips should be comparatively small to prevent the separator structure from becoming overly compacted due to the still high mobility of the fibers, which reduces porosity. In contrast to conventional calendering with standard line loads, the thickness is reduced to a lesser extent, the smoothness is increased to a lesser extent, but the homogeneity of the fiber web is improved.

[0093] In step D of calendering, all roll gaps should be formed by soft nips. The term "soft nip" is familiar to those skilled in the art in this context. A soft nip is present in any case if the surface hardness of at least one of the two rolls forming the roll gap has a Shore D hardness of at least 65 and at most 95. For example, with soft nips, at least one of the rolls has a polymer composite coating, whereas with a hard nip, the two rolls forming the roll gap are typically uncoated steel rolls.

[0094] As the following examples show, there is a small range of settings that are actually suitable for improving the homogeneity of the separator without significantly impairing other properties.

[0095] According to the invention, in step A, the type and amount of cellulose fibers are selected such that at least 75%, preferably at least 80% and at most 100% and particularly preferably at least 80% and at most 95% of the mass of the separator in step E is formed by fibrillated fibers of regenerated cellulose.

[0096] The fibrillated fibers of regenerated cellulose in step A are preferably solvent-spun, fibrillatable fibers of regenerated cellulose.

[0097] Preferably, the average linear density of the fibrillated fibers of regenerated cellulose from step A before fibrillation is at least 0.8 g / 10000 m (0.8 dtex) and at most 3.0 g / 10000 m (3.0 dtex) and more preferably at least 1.0 g / 10000 m (1.0 dtex) and at most 2.8 g / 10000 m (2.8 dtex).

[0098] Preferably, the average length of the fibrillated fibers of regenerated cellulose from step A before fibrillation is at least 2 mm and at most 8 mm, and more preferably at least 3 mm and at most 6 mm.

[0099] Preferably, the freeness of the fibrillated fibers of regenerated cellulose in step A is at least 70°SR and at most 95°SR, particularly preferably at least 75°SR and at most 90°SR. The Schopper-Riegler freeness can be determined according to ISO 5267-1:1999.

[0100] In addition to the fibrillated fibers of regenerated cellulose, the fiber web in step A may comprise further cellulose fibers.

[0101] Preferably, the further cellulose fibers can be formed wholly or partly by non-fibrillated fibers of regenerated cellulose or by pulp fibers.

[0102] Particularly preferably, the cellulose fibers are at least partially microfibrillated cellulose fibers, nanofibrillated cellulose fibers or cellulose fibers with an average length-weighted length of at most 0.2 mm, preferably of at most 0.15 mm.

[0103] The separator from step E has a basis weight of at least 8 g / m 2 and a maximum of 17 g / m 2 , preferably at least 9 g / m 2 and a maximum of 16 g / m 2 and particularly preferably at least 10 g / m 2 and a maximum of 15 g / m 2 The basis weight can be determined according to ISO 536:2019. The separator according to the invention from step E has an average individual sheet thickness, according to ISO 534:2011, of at least 12 pm and at most 40 pm, preferably at least 14 pm and at most 38 pm, particularly preferably at least 16 pm and at most 35 pm.

[0104] According to the invention, the ratio of the average single sheet thickness to the single sheet thickness of the separator from step E is at least 0.88 and at most 1.04, preferably at least 0.89 and at most 1.02 and very particularly preferably at least 0.91 and at most 1.00, wherein the average single sheet thickness and the single sheet thickness of the separator are measured according to ISO 534:2011.

[0105] According to the invention, the contrast value of the separator from step E determined by image analysis is at least 1000 and at most 1600, preferably at least 1100 and at most 1550, particularly preferably at least 1200 and at most 1500.

[0106] Preferably, the separator from step E, as a result of the calendering process comprising steps B, C and D, has a structure such that the ratio of the average individual sheet thickness to the individual sheet thickness of the separator is at least 0.88 and at most 1.04 and that the contrast value determined by image analysis is at least 1100 and at most 1550.

[0107] Particularly preferably, the separator from step E, as a result of the calendering process comprising steps B, C and D, has a structure such that the ratio of the average individual sheet thickness to the individual sheet thickness of the separator is at least 0.88 and at most 1.04 and that the contrast value determined by image analysis is at least 1200 and at most 1500.

[0108] Preferably, the separator from step E, as a result of the calendering process comprising steps B, C and D, has a structure such that the ratio of the average individual sheet thickness to the individual sheet thickness of the separator is at least 0.89 and at most 1.02 and that the contrast value determined by image analysis is at least 1000 and at most 1600.

[0109] Particularly preferably, the separator from step E, as a result of the calendering process comprising steps B, C and D, has a structure such that the ratio of the average individual sheet thickness to the individual sheet thickness of the separator is at least 0.89 and at most 1.02 and that the contrast value determined by image analysis is at least 1100 and at most 1550.

[0110] Most preferably, the separator from step E, as a result of the calendering process comprising steps B, C and D, has a structure such that the ratio of the average individual sheet thickness to the individual sheet thickness of the separator is at least 0.89 and at most 1.02, and that the contrast value determined by image analysis is at least 1200 and at most 1500. Preferably, the separator from step E, as a result of the calendering process comprising steps B, C and D, has a structure such that the ratio of the average individual sheet thickness to the individual sheet thickness of the separator is at least 0.91 and at most 1.00, and that the contrast value determined by image analysis is at least 1000 and at most 1600.

[0111] Particularly preferably, the separator from step E, as a result of the calendering process comprising steps B, C and D, has a structure such that the ratio of the average individual sheet thickness to the individual sheet thickness of the separator is at least 0.91 and at most 1.00 and that the contrast value determined by image analysis is at least 1100 and at most 1550.

[0112] Most preferably, the separator from step E, as a result of the calendering process comprising steps B, C and D, has a structure such that the ratio of the average individual sheet thickness to the individual sheet thickness of the separator is at least 0.91 and at most 1.00 and that the contrast value determined by image analysis is at least 1200 and at most 1500.

[0113] Preferably, the Bendtsen roughness averaged over both sides of the separator from step E is at least 15 ml / min and at most 80 ml / min, particularly preferably at least 20 ml / min and at most 75 ml / min, very particularly preferably at least 22 ml / min and at most 70 ml / min.

[0114] Preferably, the production of a fibrous web in step A comprises producing the fibrous web on a paper machine, particularly preferably an inclined wire machine or a fourdrinier machine or a combination of an inclined wire machine and a fourdrinier machine, wherein the combination of an inclined wire machine and a fourdrinier machine is particularly advantageous.

[0115] Preferably, the full-surface moistening of the fiber web with water in step B comprises moistening in a printing press, particularly preferably a gravure printing press, or in the size press or film press of a paper machine. These processes are suitable for allowing the water to penetrate the structure of the fiber web and thus increase the mobility of the fibers across the entire thickness of the fiber web. Other processes that primarily act on the surface of the fiber web, such as spraying or steaming, are not suitable.

[0116] Preferably, the full-surface moistening of the fibrous web with water in step B is carried out such that the moisture content of the fibrous web after step B is at least 50% and at most 90%, particularly preferably at least 60% and at most 80%, based on the absolutely dry mass of the fibrous web after step B. The person skilled in the art can easily select a specific value to suit the manufacturing process, so that the strength of the separator is not reduced more than is just necessary for further processing, but the water can still penetrate the entire structure of the fibrous web. Preferably, the target moisture content during drying in step C is at least 9% and at most 11%, particularly preferably at least 9.5% and at most 10.5%, in each case based on the absolutely dry mass of the fibrous web after step C.

[0117] Preferably, the drying of the fiber web in step C is carried out by contact with at least one heated drying cylinder.

[0118] Preferably, the calendering of the fiber web in step D is carried out such that the fiber web passes through a maximum of three, particularly preferably a maximum of two, and most preferably exactly one roll gap.

[0119] When calendering the fiber web in step D, the line load of all roll gaps is preferably at least 10 kN / m and at most 70 kN / m, particularly preferably at least 10 kN / m and at most 60 kN / m, and most preferably at least 20 kN / m and at most 50 kN / m.

[0120] Preferably, during calendering of the fiber web in step D, the surface temperature of the rolls forming the roll gap or gaps is at least 95°C and at most 120°C, more preferably at least 100°C and at most 115°C, and most preferably at least 107°C and at most 112°C.

[0121] The elevated temperature during calendering in step D may be sufficient to dry the fibrous web sufficiently to allow it to be rolled up in step E, but preferably, the process according to the invention comprises an additional step of drying the fibrous web between step D and step E. The additional drying step is particularly preferably carried out by contact with heated drying cylinders, contact with hot air, infrared radiation, or microwave radiation. Most preferably, the additional drying step is carried out by contact with heated drying cylinders.

[0122] SHORT DESCRIPTION OF THE CHARACTERS

[0123] Figure 1 shows, by way of example, the thickness ratio and the contrast value determined by image analysis as a function of the line load during calendering in step D for some separators according to the invention and not according to the invention.

[0124] Figure 2 shows, by way of example, the thickness ratio and the contrast value determined by image analysis as a function of the line load during calendering in step D for further separators according to the invention and not according to the invention.

[0125] Figure 3 shows, by way of example, the self-discharge curves of double-layer capacitors comprising separators according to the invention and not according to the invention. DESCRIPTION OF SOME PREFERRED EMBODIMENTS AND COMPARISON WITH EMBODIMENTS NOT ACCORDING TO THE INVENTION

[0126] In the following, some preferred embodiments of separators according to the invention as well as separators not according to the invention are described as a comparative example.

[0127] For the separators according to the invention, Lyocell fibers with a linear density of 1.4 g / 10,000 m (1.4 dtex) to 1.7 g / 10,000 m (1.7 dtex) and a length of 4 mm were used. The fibers were fibrillated by refining them to a degree of beading between 79°SR and 82°SR. Separators with a basis weight of approximately 12.5 g / m were produced from 100% of these fibrillated Lyocell fibers. 2 and about 14.5 g / m 2 produced according to the inventive process step A on a paper machine. The further steps of the inventive process were carried out on a separate device.

[0128] In step B, the fiber web was fully moistened with water using a gravure printing process, resulting in a moisture content of approximately 75% based on the absolutely dry mass of the fiber web. Subsequently, in step C, the fiber web was dried by contact with several heated drying cylinders to a moisture content of 10% based on the absolutely dry mass of the fiber web. Preliminary tests have shown that with these moisture content settings, the calendering in step D can be carried out very well and that the thickness ratio and the contrast value determined by image analysis can be favorably influenced.

[0129] In the following step D, the fiber web was calendered in exactly one roll gap formed by a soft nip, with line loads between 10 kN / m and 70 kN / m and a temperature of 0°C being selected so that several separators according to the invention could be produced.

[0130] For comparison, separators not according to the invention were produced from the same Lyocell fibers with the same degree of grinding, which had a basis weight of about 12.5 g / m 2 and about 14.5 g / m 2 The fiber webs after step A were the same as those of the separators according to the invention.

[0131] Thereafter, either the moistening in step B, the drying in step C, and the calendering in step D were omitted entirely, or the moistening in step B, the drying in step C, and the calendering in a roll gap in step D were carried out, but the line load in step D was selected to be above 70 kN / m, so that the process was not in accordance with the invention. As the measurements showed, the separators produced in this way were also not in accordance with the invention. For further analyses, a prior art separator manufactured under the designation delfort 3280 with production number 82142 and commercially available from delfortgroup AG, Fabrikstrasse 20, 4050 Traun, Austria, was also investigated. The separator was made of 100% fibrillated lyocell fibers with a linear density of 1.7 g / 10,000 m (1.7 dtex), which were refined to a freeness of 80°SR.

[0132] Separator was not calendered and is listed as Example Si in Table 1 as a non-inventive example.

[0133] Another commercially available separator, designated TF4030 from Nippon Kodoshi Corp., was also analyzed. It consisted of 100% lyocell fibers; the freeness and exact fiber data were not determined. The properties of the separator are listed as Example S2 in Table 1, also as a non-inventive example.

[0134] The data of the separators according to the invention and not according to the invention are given in Table 1. In Table 1, BW denotes the basis weight according to ISO 536:2019, LL denotes the line load in one roll gap during calendering in step D, Dio denotes the average single sheet thickness according to ISO 534:2011, Di the single sheet thickness according to ISO 534:2011, Dio:Di the ratio of the average single sheet thickness to the single sheet thickness of the separator, and CON denotes the contrast value determined by image analysis.

[0135] Table 1 - Data of various separators Separators Si and S2 demonstrate that commercially available separators with the same composition, comparable basis weight, and other properties do not exhibit the homogeneity according to the invention, as characterized jointly by the thickness ratio Dio:Di and the contrast value determined by image analysis. As will be shown further below, a double-layer capacitor made from them also does not have as favorable self-discharge properties as the separators according to the invention.

[0136] Separators Pi, P2, P3, P4, P5, P6, and P7 are separators according to the invention, while separators Qi, Q2, Q3, and Q4 are not according to the invention. For separators Qi and Q2, steps B, C, and D of the inventive process were omitted. For separators Q3 and Q4, steps B, C, and D were performed, but the linear load during calendering in step D was greater than 70 kN / m, so they were not produced by the inventive process.

[0137] Accordingly, it also becomes apparent that, with regard to the properties of the separator itself, the separators Qi, Q2, Q3 and Q4 are not in accordance with the invention because the thickness ratio Dio:Di is too low or the contrast value CON determined by image analysis is too high.

[0138] Furthermore, it can be seen from Table 1 that the thickness ratio Dio:Di and the contrast value CON determined by image analysis characterize different aspects of the separator's homogeneity. In the non-inventive separator Qi, steps B, C, and D were omitted, and although the thickness ratio Dio:Di of 0.894 is just within the inventive interval, the contrast value CON determined by image analysis is high at 1780 and indicates an inhomogeneous mass distribution. In contrast, the non-inventive separator Q3 was calendered with a line load of 90 kN / m, i.e. outside the inventive interval, and the thickness ratio Dio:Di of 0.868 is no longer within the inventive range, although the contrast value CON determined by image analysis would be favorable for the invention. This shows that the two parameters are influenced independently by steps B, C, and D.

[0139] The diagram in Figure 1 shows key data of separators Qi, Pi, P2, P3, P4 and P5, i.e. those separators with a basis weight of approximately 14.5 g / m 2 The x-axis 101 shows the line load during calendering in step D in kN / m. The left y-axis 102 shows the values ​​for the thickness ratio Dio:Di, and the right y-axis 103 shows the values ​​for the contrast value CON determined by image analysis. Line 104 shows the thickness ratio Dio:Di, with each square representing one of the separators Qi, Pi, P2, P3, P4, and P5. Line 105 shows the contrast value CON determined by image analysis, with each circle representing one of the separators Qi, Pi, P2, P3, P4, and P5. The diagram in Figure 2 shows key data for separators Q2, P6, P7, Q3, and Q4, i.e., those separators with a basis weight of approximately 12.5 g / m 2The line load during calendering in step D is plotted in kN / m on the x-axis 201. The left y-axis 202 shows the values ​​for the thickness ratio Dio:Di, and the right y-axis 203 shows the values ​​for the contrast value CON determined by image analysis. Line 204 shows the thickness ratio Dio:Di, with each square representing one of the separators Q2, P6, P7, and Q3. Line 205 shows the contrast value CON determined by image analysis, with each circle representing one of the separators Q2, P6, P7, and Q3. Due to the high line load, the data for separator Q4 are no longer included in the diagram for the sake of clarity, but the course of lines 204 and 205 points in the direction of the data points for separator Q4.

[0140] Table 1 and the derived Figures 1 and 2 show that favorable separator homogeneity can only be achieved at line loads from 10 kN / m to 70 kN / m. In particular, the thickness ratio reaches a maximum and the image-analytically determined contrast value a minimum at a line load of approximately 20 kN / m to approximately 50 kN / m. This shows that optimal separator homogeneity can be achieved within these line load ranges. This interval is therefore particularly preferred.

[0141] Further data on the separators according to the invention and non-inventive ones are given in Table 2, where the designations of the separators correspond to those in Table 1 and again the separators Pi, P2, P3, P4, P5, P6 and P7 are according to the invention, the separators Qi, Q2, Q3 and Q4 are not according to the invention and the separators Si and S2 are non-inventive separators from the prior art. In Table 2, p denotes the dry density calculated from the basis weight, moisture content and average single sheet thickness, AP the air permeability according to Gurley in accordance with ISO 5636-5:2013, RO the roughness according to Bendtsen in accordance with ISO 8791-2:2013 averaged over the two sides of the separator and TS-MD the tensile strength in the machine direction in accordance with ISO 1924-2:2008.

[0142] Table 2 - Further data of the separators

[0143] The data in Table 2 show that the separators according to the invention also fall within a range favorable for the production of electrochemical cells with regard to other technical parameters. The special calendering process in steps B, C, and D of the process according to the invention therefore does not lead to a deterioration of other properties of the separator.

[0144] Double-layer capacitors in the form of pouch-type EDLC (Electric Double Layer Capacitor) cells with a capacitance of approximately 25 F and a nominal voltage of 2.7 V were fabricated from the inventive separators P6 and P7, as well as from the non-inventive separators Q2 and Q3. The electrodes were made of activated carbon on aluminum-based current collectors. The electrode area was approximately 18 cm 2 and 1M tetraethylammonium tetrafluoroborate in acetonitrile was used as electrolyte.

[0145] Double-layer capacitors have proven suitable for this study because the self-discharge rate of these electrochemical elements is particularly critical and meaningful results can be obtained in a comparatively short time. However, the results can be transferred to other electrochemical elements, especially rechargeable batteries.

[0146] The equivalent series resistance (ESR) of the EDLC cells was determined by electrochemical impedance spectroscopy and is given in Table 3 for the EDLC cells made from separators Q2, P6, P7 and Q3.

[0147] Table 3 - Equivalent series resistance of EDLC cells

[0148] Table 3 shows that the equivalent series resistances of the EDLC cells made from the inventive separators P6 and P7 are not significantly increased compared to those of an EDLC cell with the non-inventive separator Q2. The non-inventive separator Q3 was calendered with a higher line load than the inventive one, resulting in a more compact pore structure of the separator Q3 and an increased equivalent series resistance of the EDLC cell made from it. The properties of the EDLC cell made from the non-inventive separator Q3 are therefore significantly worse in terms of maximum current draw and charging speed.

[0149] The self-discharge of the EDLC cells was monitored for 24 hours at 21°C, and the results are shown in Fig. 3. In the diagram in Fig. 3, the x-axis 301 shows the time in hours, and the y-axis 302 shows the voltage of the EDLC cell in volts. The four self-discharge curves are assigned to the EDLC cells, each comprising one of the separators Q2, P6, P7, and Q3, as follows: Curve 303 corresponds to separator Q2, curve 304 to separator P6, curve 305 to separator P7, and curve 306 to separator Q3.

[0150] From Fig. 3, it can be seen that the self-discharge rates of the EDLC cells made from the inventive separators P6 (curve 304) and P7 (curve 305) are significantly reduced compared to those of the EDLC cell made from the non-inventive separator Q2 (curve 303). They are in the range of the EDLC cell made from the non-inventive separator Q3 (curve 306), which, however, has significantly poorer performance parameters due to the higher equivalent series resistance. Due to the great similarity between the separators Si and Q2, similar results regarding the self-discharge rate are to be expected for EDLC cells made from the Si separator.

[0151] This shows that the homogeneity of the separator has a significant influence on the self-discharge rate of an electrochemical element made from it and, in particular, that the thickness ratio and the contrast value determined by image analysis are only together sufficient to achieve a low self-discharge rate with a not significantly increased equivalent series resistance in an electrochemical element made from the separator according to the invention.

Claims

delfortgroup AG CLAIMS 1. Separator for electrochemical cells comprising fibrillated fibers of regenerated cellulose, wherein the fibrillated fibers of regenerated cellulose constitute at least 75% of the mass of the separator, wherein the basis weight of the separator is at least 8 g / m 2 and a maximum of 17 g / m 2 and the average individual sheet thickness of the separator, according to ISO 534:2011, is at least 12 pm and at most 40 pm, and wherein the separator has a homogeneity with respect to its thickness and fiber structure which is characterized by the simultaneous realization of the following features (i) and (ii): (i) the ratio of the mean single sheet thickness of the separator according to ISO 534:2011 to the single sheet thickness of the separator according to ISO 534:2011 is at least 0.88 and at most 1.04, and (ii) a contrast value C determined by image analysis is at least 1000 and at most 1600, where the contrast value C determined by image analysis is to be determined from the two-dimensional power spectrum of an image of the separator taken in transmitted light in 256 grey levels with a resolution of 0.0423 mm per pixel (600 dpi), where a square section of the image with an area of ​​approximately 8.62 cm x 8.62 cm, corresponding to 2048 x 2048 pixels, is used to calculate the two-dimensional power spectrum, to which a two-dimensional field of grey values ​​g k with j = o, 1, 2, ..., 2047 and k = o, 1, 2, ..., 2047, where g k can take integer values ​​from 0 (black) to 255 (white), where the contrast value C is to be calculated according to the following rule: 1 c = 1024 ■ 1024 where the sum is to be formed over all pairs (m,n) with m = i, 2, 3, 1023 and n = 1, 2, 3, 1023, for which with A, '„ n = 0.0423 and where f m n are the Fourier coefficients of a discrete two-dimensional Fourier transform of the two-dimensional field of grey values, where the mean grey value is normalised to 1, and are to be calculated as follows: with 2047 and 2. Separator according to claim 1, wherein the proportion of fibrillated fibers of regenerated cellulose is at least 80% and at most 100%, preferably at least 80% and at most 95% of the mass of the separator.

3. Separator according to claim 1 or 2, wherein the fibrillated fibers of regenerated cellulose are formed by fibrillatable fibers of regenerated cellulose spun in a solvent, which are additionally fibrillated.

4. Separator according to one of the preceding claims, wherein the average linear density of the fibrillated fibers of regenerated cellulose before fibrillation is at least 0.8 g / 10,000 m (0.8 dtex) and at most 3.0 g / 10,000 m (3.0 dtex), preferably at least 1.0 g / 10,000 m (1.0 dtex) and at most 2.8 g / 10,000 m (2.8 dtex).

5. Separator according to one of the preceding claims, wherein the average length of the fibrillated fibers of regenerated cellulose before fibrillation is at least 2 mm and at most 8 mm, preferably at least 3 mm and at most 6 mm.

6. Separator according to one of the preceding claims, wherein the fibrillated fibers of regenerated cellulose have a freeness of at least 70°SR and at most 95°SR, preferably of at least 75°SR and at most 90°SR. 7- Separator according to one of the preceding claims, which comprises further cellulose fibers in addition to the fibrillated fibers of regenerated cellulose.

8. Separator according to claim 7, wherein the further cellulose fibers are formed wholly or partly by non-fibrillated fibers of regenerated cellulose.

9. Separator according to claim 7 or 8, wherein the further cellulose fibers are formed wholly or partly by cellulose fibers, wherein the cellulose fibers are preferably obtained from coniferous wood, hardwood or other plants, in particular hemp, flax, jute, ramie, kenaf, kapok, coconut, abaca, sisal, bamboo, cotton or esparto grass, or from waste paper pulp, or are a mixture of two or more cellulose fibers of different of the aforementioned origins.

10. Separator according to claim 9, wherein the cellulose fibers are at least partially microfibrillated cellulose fibers, nanofibrillated cellulose fibers or cellulose fibers having an average length-weighted length of at most 0.2 mm, preferably of at most 0.15 mm.

11. Separator according to one of the preceding claims, which, in addition to the fibrillated fibers of regenerated cellulose, comprises fibers made of cellulose derivatives, glass fibers, plastic fibers, in particular fibers made of polyolefins, preferably polyethylene or polypropylene; of polyesters, such as polyethylene terephthalate or polylactic acids; of polyarylates, preferably poly-(4-hydroxybenzoic acid-co-6-hydroxy-2-naphthoic acid); of polyethers, polysulfones, polyurethanes, polyamides, aromatic polyamides, preferably poly-(p-phenylene terephthalamide); polyimides, polyvinyl alcohol, polyacrylates, polyacrylonitrile or poly-(acrylonitrile-co-methylacrylate); polyphenylene sulfide or of poly-(ethylene-co-vinyl acetate).

12. Separator according to one of the preceding claims, in which a proportion of fibers other than the fibrillated fibers of regenerated cellulose taken together amounts to at most 25%, preferably at most 20% and particularly preferably at most 15% of the mass of the separator.

13. Separator according to one of the preceding claims, which further contains one or more components which are selected from the group consisting of polyvinyl alcohol, polyethylene glycol, polyvinylidene fluoride, guar gum, starch, carboxymethylcellulose, methylcellulose, dialdehydes, in particular glyoxal, and inorganic fillers, in particular kaolin, titanium dioxide (TiO2), silicon dioxide (SiO2), aluminum oxide (AlO3), zirconium dioxide (ZrO2), calcium carbonate (CaCO3). 14- Separator according to claim 13, wherein the amount of inorganic fillers in the separator is at most 20%, preferably at most 15% and particularly preferably at most 5% of the mass of the separator.

15. Separator according to one of the preceding claims, whose basis weight is at least 9 g / m 2 and a maximum of 16 g / m 2 , preferably at least 10 g / m 2 and a maximum of 15 g / m 2 amounts.

16. Separator according to one of the preceding claims, which has an average individual sheet thickness, according to ISO 534:2011, of at least 14 pm and at most 38 pm, preferably of at least 16 pm and at most 35 pm.

17. Separator according to one of the preceding claims, wherein the ratio of the average single sheet thickness to the single sheet thickness of the separator is at least 0.89 and at most 1.02 and preferably at least 0.91 and at most 1.00, wherein the average single sheet thickness and the single sheet thickness are measured according to ISO 534:2011.

18. Separator according to one of the preceding claims, wherein the contrast value determined by image analysis is at least 1100 and at most 1550, preferably at least 1200 and at most 1500.

19. Separator according to one of the preceding claims, wherein the separator has a structure such that the ratio of the average single sheet thickness to the single sheet thickness of the separator is at least 0.88 and at most 1.04 and the image-analytically determined contrast value is at least 1100 and at most 1550, wherein preferably the ratio of the average single sheet thickness to the single sheet thickness of the separator is at least 0.88 and at most 1.04 and the image-analytically determined contrast value is at least 1200 and at most 1500, or the ratio of the average single sheet thickness to the single sheet thickness of the separator is at least 0.89 and at most 1.02 and the image-analytically determined contrast value is at least 1000 and at most 1600, or wherein preferably the ratio of the average single sheet thickness to the single sheet thickness of the separator is at least 0.89 and at most 1,02 and the contrast value determined by image analysis is at least 1100 and at most 1550, wherein particularly preferably the ratio of the average single sheet thickness to the single sheet thickness of the separator is at least 0.89 and at most 1.02 and the, the contrast value determined by image analysis is at least 1200 and at most 1500, or the ratio of the average single sheet thickness to the individual sheet thickness of the separator is at least 0.91 and at most 1.00 and the contrast value determined by image analysis is at least 1000 and at most 1600, wherein preferably the ratio of the average single sheet thickness to the individual sheet thickness of the separator is at least 0.91 and at most 1.00 and the contrast value determined by image analysis is at least 1100 and at most 1550, wherein particularly preferably the ratio of the average single sheet thickness to the individual sheet thickness of the separator is at least 0.91 and at most 1.00 and the contrast value determined by image analysis is at least 1200 and at most 1500.

20. Separator according to one of the preceding claims, wherein an average roughness resulting from the mean value of the roughness of the two sides of the separator determined according to Bendtsen in accordance with ISO 8791-2:2013 is at least 15 ml / min and at most 80 ml / min, preferably at least 20 ml / min and at most 75 ml / min, and particularly preferably at least 22 ml / min and at most 70 ml / min.

21. Separator according to one of the preceding claims, whose dry density is at least 350 kg / m 3 and a maximum of 700 kg / m 3 , preferably at least 400 kg / m 3 and a maximum of 700 kg / m 3 and particularly preferably at least 450 kg / m 3 and a maximum of 650 kg / m 3 where the dry density is given by the formula (- 1 - 10ö) ' m p = - d - is to be calculated, where p is the dry density in kg / m 3 , c is the moisture content in % according to ISO 287:2017, m is the basis weight in g / m 2according to ISO 536:2019, and d is the mean single sheet thickness in mm according to ISO 534:2011.

22. Separator according to one of the preceding claims, whose tensile strength according to ISO 1924-2:2008 in the machine direction, based on the width, is at least 0.3 kN / m and at most 2.0 kN / m, preferably at least 0.4 kN / m and at most 1.5 kN / m, and / or whose tensile strength in the machine direction, based on the cross-sectional area, is at least 15 MPa and at most 60 MPa, preferably at least 25 MPa and at most 50 MPa. 23- Separator according to one of the preceding claims, wherein the ratio of the tensile strength in the machine direction to that in the transverse direction is at least 1.0 : 1.0 and at most 3.0 : 1.0, preferably at least 1.2 : 1.0 and at most 2.2 : 1.0 and more preferably at least 1.5 : 1.0 and at most 1.8 : 1.

0.

24. Separator according to one of the preceding claims, wherein the elongation at break according to ISO 1924-2:2008 in the machine direction is at least 0.5% and at most 5.0%, preferably at least 1.0% and at most 4.0%.

25. Separator according to one of the preceding claims, the shrinkage of which after heating to 150°C for 3 hours is at least 0.5% and at most 2.0%, particularly preferably at least 0.5% and at most 1.5%.

26. Separator according to one of the preceding claims, the porosity of which is at least 35% and at most 75%, preferably at least 40% and at most 70%, wherein the porosity p is determined according to the formula where m is the basis weight in g / m measured according to ISO 536:2019 2 , d is the mean single sheet thickness in pm measured according to ISO 534:2011, and c is the moisture content of the separator in % measured according to ISO 287:2017.

27. Separator according to one of the preceding claims, whose air permeability according to ISO 5636-5:2013 according to Gurley is at least 2 s and at most 15 s, preferably at least 3 s and at most 12 s and particularly preferably at least 4 s and at most 12 s.

28. An electrochemical element comprising at least two electrodes, an electrolyte and at least one separator according to any one of claims 1 to 27, wherein the electrochemical element is preferably formed by a capacitor, a hybrid capacitor, a double-layer capacitor or an accumulator.

29. Electrochemical element according to claim 28, which is formed by a double-layer capacitor with a nominal voltage of at least 1.0 V and at most 4.0 V and a capacitance of at least 1 F and at most 5000 F.

30. A method for producing a separator for an electrochemical cell, comprising the following steps A to E: A - Producing a fibrous web comprising cellulose fibers, B - Fully moistening the fiber web with water, C - Drying the fiber web to a target moisture content, D - Calendering of the fiber web, E - Rolling up the fiber web forming the separator, wherein in step A the quantity and type of cellulose fibers are selected such that at least 75% of the mass of the separator in step E is formed by fibrillated fibers of regenerated cellulose and the basis weight of the separator from step E is at least 8 g / m 2 and a maximum of 17 g / m 2and in step B, the full-surface moistening is carried out with such an amount of water that the moisture content of the fibrous web after step B is at least 25% and at most 125% based on the absolutely dry mass of the fibrous web after step B, and in step B, the full-surface moistening of the fibrous web is carried out in such a way that the water penetrates the structure of the fibrous web, and wherein in step C of drying, the target value of the moisture content of the fibrous web after step C is at least 8% and at most 12% based on the absolutely dry mass of the fibrous web after step C, and in step D, the fibrous web is calendered in such a way that the average individual sheet thickness of the separator from step E is at least 12 pm and at most 40 pm, and in step D, the fibrous web runs through one or more roll gaps,wherein all roll gaps are formed by soft nips and the line load in at least one of the roll gaps is at least 10 kN / m and in none of the roll gaps is more than 70 kN / m, and in which the calendering process comprising steps B, C and D is carried out in such a way that the separator in step E has a homogeneity with regard to its thickness and fiber structure, which is characterized by the simultaneous realization of the following features (i) and (ii): (i) the ratio of the mean single sheet thickness of the separator according to ISO 534:2011 to the single sheet thickness of the separator according to ISO 534:2011 is at least 0.88 and at most 1.04, and (ii) a contrast value C determined by image analysis is at least 1000 and at most 1600, whereby the contrast value C determined by image analysis is to be determined from the two-dimensional power spectrum of an image of the separator taken in transmitted light in 256 grey levels with a resolution of 0.0423 mm per pixel (600 dpi), whereby for the calculation of the two-dimensional power spectrum a square section of the image with an area of ​​approximately 8.62 cm x 8.62 cm, corresponding to 2048 x 2048 pixels, is used, to which a two-dimensional field of grey values ​​g k with j = o, 1, 2, ..., 2047 and k = o, 1, 2, ..., 2047, where g fc can take integer values ​​from 0 (black) to 255 (white), where the contrast value C is to be calculated according to the following rule: 1 C = 1024 ■ 1024 where the sum over all pairs (m,n) with m = 1, 2, 3, ..., 1023 and n = 1, 2, 3, ..., 1023 is to be formed, for which 2 < A 2 ( + A 2 < 64 with A m = 0.0423 and A n = 0.0423 where f m n are the Fourier coefficients of a discrete two-dimensional Fourier transform of the two-dimensional field of grey values, where the mean grey value is normalised to 1, and are to be calculated as follows: with , 9j,kgj,k — - J> k — 0, 1, 2, .. ., 2047 9 and 9 2048 ■ 2048 31. A process according to claim 30, wherein in step A the type and amount of cellulose fibers are selected such that at least 80% and at most 100%, and preferably at least 80% and at most 95%, of the mass of the separator in step E is formed by fibrillated fibers of regenerated cellulose.

32. The method according to claim 30 or 31, wherein the fibrillated fibers of regenerated cellulose in step A are formed by solvent-spun, fibrillatable fibers of regenerated cellulose which are additionally fibrillated.

33. The method according to any one of claims 30 to 32, wherein the average linear density of the fibrillated fibers of regenerated cellulose from step A before fibrillation is at least 0.8 g / 10,000 m (0.8 dtex) and at most 3.0 g / 10,000 m (3.0 dtex), and more preferably at least 1.0 g / 10,000 m (1.0 dtex) and at most 2.8 g / 10,000 m (2.8 dtex), and / or wherein the average length of the fibrillated fibers of regenerated cellulose from step A before fibrillation is at least 2 mm and at most 8 mm, and more preferably at least 3 mm and at most 6 mm.

34. A process according to any one of claims 30 to 33, wherein the freeness of the fibrillated fibers of regenerated cellulose in step A is at least 70°SR and at most 95°SR, preferably at least 75°SR and at most 90°SR.

35. The method according to any one of claims 30 to 34, wherein the fiber web in step A comprises, in addition to the fibrillated fibers of regenerated cellulose, further cellulose fibers, wherein the further cellulose fibers are formed wholly or partly by non-fibrillated fibers of regenerated cellulose or by cellulose fibers, wherein the cellulose fibers are preferably at least partly microfibrillated cellulose fibers, nanofibrillated cellulose fibers or cellulose fibers having an average length-weighted length of at most 0.2 mm, preferably of at most 0.15 mm.

36. A process according to any one of claims 30 to 35, wherein the separator from step E has a basis weight of at least 9 g / m 2and a maximum of 16 g / m 2 , preferably at least 10 g / m 2 and a maximum of 15 g / m 2 has.

37. The method according to any one of claims 30 to 36, wherein the separator from step E has an average single sheet thickness, according to ISO 534:2011, of at least 14 pm and at most 38 pm, preferably of at least 16 pm and at most 35 pm.

38. A method according to any one of claims 30 to 37, wherein the ratio of the average single sheet thickness to the single sheet thickness of the separator from step E is at least 0.89 and at most 1.02, and preferably at least 0.91 and at most 1.00, wherein the average single sheet thickness and the single sheet thickness of the separator are to be measured according to ISO 534:2011.

39. Method according to one of claims 30 to 38, wherein the contrast value of the separator from step E determined by image analysis is at least 1100 and at most 1550, preferably at least 1200 and at most 1500.

40. Method according to one of claims 30 to 39, wherein the separator from step E, as a result of the calendering process comprising steps B, C and D, has a structure such that the ratio of the average single sheet thickness to the single sheet thickness of the separator is at least 0.88 and at most 1.04 and the contrast value determined by image analysis is at least 1100 and at most 1550, wherein preferably the ratio of the average single sheet thickness to the single sheet thickness of the separator is at least 0.88 and at most 1.04 and the contrast value determined by image analysis is at least 1200 and at most 1500, or the ratio of the average single sheet thickness to the single sheet thickness of the separator is at least 0.89 and at most 1.02 and the contrast value determined by image analysis is at least 1000 and at most 1600, or wherein preferably the ratio of the average single sheet thickness to the single sheet thickness of the Separators at least 0.89 and at most 1.02 and the contrast value determined by image analysis is at least 1100 and at most 1550, wherein particularly preferably the ratio of the average single sheet thickness to the single sheet thickness of the separator is at least 0.89 and at most 1.02 and the contrast value determined by image analysis is at least 1200 and at most 1500, or the ratio of the average single sheet thickness to the single sheet thickness of the separator is at least 0.91 and at most 1.00 and the contrast value determined by image analysis is at least 1000 and at most 1600, wherein preferably the ratio of the average single sheet thickness to the single sheet thickness of the separator is at least 0.91 and at most 1.00 and the contrast value determined by image analysis is at least 1100 and at most 1550, wherein particularly preferably the ratio of the average single sheet thickness to the single sheet thickness of the separator is at least 0.91 and at most 1.00 and the contrast value determined by image analysis is at least 1200 and at most 1500.

41. A process according to any one of claims 30 to 40, wherein the Bendtsen roughness averaged over both sides of the separator from step E is at least 15 ml / min and at most 80 ml / min, preferably at least 20 ml / min and at most 75 ml / min, and particularly preferably at least 22 ml / min and at most 70 ml / min.

42. A method according to any one of claims 30 to 41, wherein the production of a fibrous web in step A comprises producing the fibrous web on a paper machine, preferably an inclined wire machine or a fourdrinier machine or a combination of an inclined wire and a fourdrinier machine.

43. A method according to any one of claims 30 to 42, wherein the full-surface moistening of the fiber web with water in step B is carried out in a printing machine, preferably a gravure printing machine, or in a size press or film press of a paper machine.

44. Method according to one of claims 30 to 43, wherein the full-surface moistening of the fiber web with water in step B is carried out such that the moisture content of the fiber web after step B is at least 50% and at most 90%, particularly preferably at least 60% and at most 80%, based on the absolutely dry mass of the fiber web after step B.

45. A process according to any one of claims 30 to 44, wherein the target moisture content during drying in step C is at least 9% and at most 11%, preferably at least 9.5% and at most 10.5%, in each case based on the absolutely dry mass of the fibrous web after step C.

46. ​​A method according to any one of claims 30 to 45, wherein the drying of the fibrous web in step C is carried out by contact with at least one heated drying cylinder.

47. Method according to one of claims 30 to 46, wherein the calendering of the fiber web in step D is carried out such that the fiber web passes through at most three, preferably at most two and particularly preferably exactly one roll gap.

48. A method according to any one of claims 30 to 47, wherein during calendering of the fiber web in step D, the line load of all roll gaps is at least 10 kN / m and at most 70 kN / m, preferably at least 10 kN / m and at most 60 kN / m and particularly preferably at least 20 kN / m and at most 50 kN / m.

49. A method according to any one of claims 30 to 48, wherein during calendering of the fibrous web in step D, the surface temperature of the rolls forming the roll gap or gaps is at least 95°C and at most 120°C, preferably at least 100°C and at most 115°C, and particularly preferably at least 107°C and at most 112°C.

50. A method according to any one of claims 30 to 49, wherein the method comprises an additional step of drying the fibrous web between step D and step E, wherein the additional step of drying is preferably carried out by contact with heated drying cylinders, contact with hot air, infrared radiation or microwave radiation.

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