Dimensionally stable separator for electrochemical elements

Description

FIELD OF THE INVENTION

[0001] The invention relates to a separator for electrochemical elements, which is essentially formed by cellulose fibers and which achieves high dimensional stability through a special manufacturing process. BACKGROUND AND STATE OF THE ART

[0002] An electrochemical cell typically comprises at least one positive electrode, one negative electrode, an electrolyte, a separator, a casing, and current collectors. The separator is saturated with the electrolyte and serves to electrically separate the two electrodes. At the same time, it should also allow as unimpeded a flow of ions as possible between the electrodes, so that the electrochemical cell has favorable properties, in particular rapid charging and the ability to draw high currents.

[0003] These requirements for the separator mean that it should be as thin as possible so that the path of the ions from one electrode to the other through the separator's pores is short, thus achieving a high volumetric energy density of the electrochemical element, and that it should possess high porosity. Particularly when the electrochemical element is a battery, the porosity should not be formed by a few large pores but by a multitude of small pores, because small pores inhibit the growth of crystals, especially dendrites, on the electrodes. These crystals can short-circuit the battery, thereby reducing its lifespan and performance. Furthermore, the porosity should be as constant as possible across the entire surface of the separator.

[0004] The separator must be chemically resistant to the electrolyte, as electrochemical elements can be recharged multiple times and are typically in use for several years. Therefore, the separator must also be resistant to oxidative and reductive environments.

[0005] For safety reasons, the separator should have good thermal stability in order to limit the risk of fire in case of damage to the electrochemical element.

[0006] However, the production of electrochemical elements sometimes presents the problem that their performance parameters fluctuate, which is due to fluctuating quality of the separator.

[0007] There is therefore an interest in having a separator available that allows the production of electrochemical elements of high and consistent quality. SUMMARY OF THE INVENTION

[0008] The invention is therefore based on the objective of providing a separator for electrochemical elements that makes it possible to produce electrochemical elements with good performance parameters from it with high productivity.

[0009] This problem is solved by a separator for electrochemical elements according to claim 1, an electrochemical element comprising this separator according to claim 40, and a method for producing a separator for electrochemical elements according to claim 41. Advantageous embodiments are specified in the dependent claims.

[0010] The inventors have determined that fluctuations in the performance of electrochemical cells are not only caused by variations in the manufacturing process, but especially by mechanical stresses on the separator during the manufacturing process. In this context, not only is the separator's breaking load important, but it is essential that the separator is dimensionally stable, meaning that it does not deform plastically (i.e., irreversibly) under the stresses encountered during the electrochemical cell's production. Such plastic deformation can adversely alter the separator's pore structure, leading to poorer performance parameters of the electrochemical cell or increasing the number of rejects in production.

[0011] The inventors have found that the aforementioned problem can be solved by a separator for electrochemical elements in which at least 50% of the mass is formed by fibrillated fibers of regenerated cellulose and in which, including the fibrillated fibers of regenerated cellulose, at least 70% and at most 100% of the mass of the separator is formed by cellulose fibers, wherein the separator is furthermore calendered and, under tensile stress in the machine direction according to ISO 1924-2:2008, achieves its 0.1% yield strength at an elongation of not less than 0.5% and not more than 2.0%.

[0012] The fact that the 0.1% yield strength is only reached at an elongation of 0.5% or above is a measure of the aforementioned dimensional stability, which was recognized as essential for solving the problem.

[0013] According to the inventors' findings, the fibrillated fibers of regenerated cellulose allow the separator to be equipped with a homogeneous pore structure and high porosity. Unlike cellulose fibers, the fibrillated fibers of regenerated cellulose are less variable in their geometry and thus create a homogeneous pore structure, but contribute less to the strength than cellulose fibers. Chemical purity is a further advantage of fibrillated fibers of regenerated cellulose over cellulose fibers.

[0014] Although, according to the inventors, advantageous separators can already be obtained with a proportion of 50% fibrillated fibers of regenerated cellulose, the total proportion of cellulose fibers, i.e., including the fibers of regenerated cellulose, should be at least 70%, based on the mass of the separator, in order to provide the separator with suitable mechanical strength. Furthermore, cellulose fibers offer advantages over plastic films with regard to fire safety, thermal stability, and environmental aspects.

[0015] A separator made from these components can, in principle, be produced using papermaking processes known from the prior art, but without measures taken during production to achieve dimensional stability, it will not exhibit the desired properties. When separators known from the prior art are subjected to tensile testing, the deformations are initially linearly elastic. This means that the deformations are proportional to the applied force and completely disappear after the force is released. With further elongation in the tensile test, a small transition range leads to non-linear elastic deformation, in which the applied force is no longer proportional to the elongation, but the deformation still completely disappears after the force is released. At even higher elongation, plastic deformation occurs, i.e., an irreversible deformation that does not disappear even after the force is released.Such irreversible deformation is undesirable because it adversely alters the separator's pore structure and can create microscopic cracks, thereby impairing the properties of any electrochemical cell manufactured from it. Furthermore, it can increase the number of rejects during electrochemical cell production.

[0016] During the production of an electrochemical cell, stresses can occur that leave irreversible deformations in the separator. This can happen, for example, if the separator is subjected to impacts, high accelerations, or velocity differences during the manufacturing process, such as when producing cylindrical cells. Since irreversible deformations occur at loads far below the separator's breaking point, they usually go unnoticed, but generally impair the properties of the electrochemical cell.

[0017] The elastic and plastic behavior of separators can be assessed in a tensile test. Such a tensile test can be carried out according to ISO 1924-2:2008. A 15 mm wide test strip is stretched at a constant speed of 20 mm / min until fracture. During the stretching process, the strain and force are recorded, and a stress-strain diagram is calculated from this data.

[0018] Fig 1 Figure 1 shows an exemplary stress-strain diagram of a separator according to the invention. The stress-strain curve 1 is plotted in a diagram, with the horizontal axis 2 representing the strain and the vertical axis 3 the tensile stress. Starting from a nearly stress- and strain-free state 4, the strain is increased at a constant rate of 20 mm / min until the test strip tears at the strain 5 and the corresponding tensile stress 6. Initially, a linear elastic deformation occurs, indicated by the dashed line 7. The slope of this line 7 is the modulus of elasticity, which can also be determined from the tensile test according to ISO 1924-2:2008. It is common practice to determine the 0.1% yield strength from the stress-strain curve 1 and the line of linear elastic behavior 7.Line 7 is shifted parallel to the horizontal axis 2 until it intersects the horizontal axis 2 at a strain of 0.1%, thus obtaining line 8. The intersection point 9 of this shifted line 8 with the stress-strain curve 1 indicates the 0.1% yield strength, which is characterized by the corresponding strain 10 and the tensile stress 11. It is generally assumed that irreversible deformations occur at strains beyond the intersection point 9, while largely elastic behavior persists at strains below this point.

[0019] For the purposes of this invention, the 0.1% yield strength is therefore always understood to mean point 9 as described above, consisting of the corresponding elongation 10 and tensile stress 11. Devices for testing tensile strength according to ISO 1924-2:2008 are often able to determine the 0.1% yield strength automatically.

[0020] The tensile stress is calculated as force per area (MPa), based on the cross-sectional area, but it can also be multiplied by the thickness of the separator, so that it is only based on the width and is then given in kN / m.

[0021] For the production of an electrochemical element from a separator, its mechanical properties in the machine direction are of primary importance. The machine direction is the direction in which the fiber web runs through the machine during separator production. The direction orthogonal to this, lying in the plane of the fiber web, is the transverse direction.

[0022] According to the inventors' findings, it is therefore crucial that the 0.1% yield strength of the separator according to the invention lies at high stresses and strains in the machine direction. Such a separator is able to absorb a large amount of deformation energy without irreversible deformation occurring. It thus possesses a high elastic energy absorption capacity and therefore high dimensional stability. The elastic energy absorption capacity is determined according to Fig. 1 by the area bounded by the stress-strain curve 1, the horizontal axis 2, and the line connecting points 9 and 10. As a good approximation, the elastic energy absorption capacity can also be calculated using the area of ​​the triangle formed by points 4, 9, and 10.

[0023] The elastic energy absorption capacity is a volumetric quantity and therefore has the unit energy per volume (kJ / m³). In practical applications with separators, however, it is sometimes multiplied by the thickness of the separator, so that it can be expressed as energy per area of ​​the separator in J / m².

[0024] The elastic energy absorption capacity is to be distinguished from the total energy absorption capacity ("tensile energy absorption", TEA), because the latter describes the total deformation energy absorbed up to the point of fracture and not only that which can be absorbed up to the point of irreversible deformation.

[0025] The fibrillated fibers of regenerated cellulose are essential in the separator according to the invention for creating a favorable pore structure. Fibrillation of the regenerated cellulose fibers increases their surface area and thus the area available for hydrogen bonding. However, due to their round cross-section, they still exhibit some disadvantages with regard to mechanical strength, which can be overcome by suitable manufacturing processes, allowing the 0.1% yield strength of the separator to be shifted to higher strains and stresses.

[0026] Fig. 2 Figure 2 shows two fibrillated fibers of regenerated cellulose, 20 and 21, which intersect. Due to the round cross-section of the fibrillated fibers of regenerated cellulose 20 and 21, they theoretically only touch at a single point 22. Only a few hydrogen bonds can form on the small area 22, resulting in unfavorable strength and, consequently, the 0.1% yield strength of a separator produced in this way. By calendering, the fibrillated fibers of regenerated cellulose 20 and 21 can be flattened, yielding fibers 23 and 24. This creates a larger contact area 25, which allows for more hydrogen bonds and thus leads to higher strength and a better 0.1% yield strength.

[0027] However, according to the inventors, for the purposes of improved dimensional stability, it is not only the change in the fiber cross-section that matters. Fig. 3 Figure 3 shows two cellulose fibers, 30 and 31, which cross each other at approximately a right angle 33. Even when the cellulose fibers 30 and 31 are flattened, the contact area 32 is relatively small. In contrast, the cellulose fibers 34 and 35 cross at a smaller angle 37, resulting in a significantly larger contact area 36 between the fibers and improving the strength and the 0.1% yield strength. The improvement occurs primarily along the longitudinal axes of the cellulose fibers 34 and 35, while in the perpendicular directions the improvement is considerably smaller or even deteriorates. Therefore, the angle 37 between the cellulose fibers should not be arbitrarily small.As explained in more detail below, it is possible to stretch and possibly also plastically deform the cellulose fibers by adjusting the manufacturing process, especially the web tension, temperature and humidity, and on average to orient them at an angle so that the 0.1% yield strength can be significantly shifted to higher stresses and strains.

[0028] A fiber-based separator according to the invention, which makes it possible to produce electrochemical elements with good performance parameters with high productivity, is therefore characterized in particular by improved dimensional stability, which can be quantitatively described by the fact that the separator under tensile stress in the machine direction according to ISO 1924-2:2008 only reaches its 0.1% yield strength at an elongation of 0.5% or above.

[0029] The inventors were able to demonstrate that such a dimensionally stable fiber-based separator can be produced from the aforementioned components if the manufacturing process is appropriately adapted. The influence of certain settings in the manufacturing process, which positively affect the dimensional stability of the fiber web, in particular the influence of web tension, temperature, and humidity, is explained in more detail below and demonstrated experimentally. These measures are based on the inventors' theoretical understanding, which is derived from… Fig. 2 as explained above. However, it is emphasized that the invention is not limited to these specific manufacturing measures and that other modifications to the manufacturing process are also covered by the invention, provided they lead to the 0.1% yield point of the separator according to the invention. Likewise, the theoretical explanation with reference to Fig. 2 , which corresponds to the current understanding of the inventors, any limitation of the invention may be derived.

[0030] The separator according to the invention is formed by fibrillated fibers of regenerated cellulose to at least 50% of its mass. These fibers create a pore structure favorable for a separator. Preferably, however, the proportion of fibrillated fibers of regenerated cellulose is higher, amounting to at least 55% and at most 100%, and particularly preferably at least 60% and at most 95% of the separator's mass.

[0031] The fibrillated fibers of regenerated cellulose are preferably solvent-spun fibers (" solvent-spun ") regenerated cellulose, especially preferred Lyocell® fibers.

[0032] The linear density of the fibrillated fibers of regenerated cellulose before fibrillation is important for the fibrillation process. Preferably, the mean linear density of the fibrillated fibers of regenerated cellulose 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 particularly preferably at least 1.0 g / 10000 m (1.0 dtex) and at most 2.5 g / 10000 m (2.5 dtex).

[0033] The length of the fibrillated fibers of regenerated cellulose before fibrillation is particularly important for the strength of the separator; longer fibers lead to higher strength but also require more energy during fibrillation. Preferably, the average length of the fibrillated fibers of regenerated cellulose before fibrillation is at least 2 mm and at most 8 mm, and particularly preferably at least 3 mm and at most 6 mm.

[0034] Including the fibrillated fibers of regenerated cellulose, the separator according to the invention is formed by cellulose fibers to a minimum of 70% and a maximum of 100% of its mass. Preferably, the proportion of cellulose fibers is at least 75% and a maximum of 95% based on the mass of the separator. This type and quantity of fibers in the separator allow for good strength to be achieved, so that the separator can also be processed into an electrochemical element.

[0035] In addition to fibrillated fibers of regenerated cellulose, the cellulose fibers can also be formed from non-fibrillated fibers of regenerated cellulose, from pulp fibers, or from mixtures thereof. The pulp fibers are preferably obtained from softwoods, hardwoods, or other plants such as hemp, flax, jute, ramie, kenaf, kapok, coconut, abaca, sisal, bamboo, cotton, or esparto grass, or from recycled paper pulp. Mixtures of pulp fibers from different sources can also be used for the production of the separator. Pulp fibers obtained from hardwoods or softwoods are particularly preferred.

[0036] It is particularly preferred that the cellulose fibers are a mixture of regenerated cellulose fibers, i.e., fibrillated and possibly non-fibrillated regenerated cellulose fibers, and pulp fibers. In a particularly preferred embodiment, the ratio of the masses of regenerated cellulose fibers to pulp fibers is at least 1:1 and at most 30:1, preferably at least 2:1 and at most 20:1, wherein, furthermore, at least 50% of the mass of the separator must be formed by fibrillated regenerated cellulose fibers, and the cellulose fibers in the separator must constitute at least 70% and at most 100% of the separator's mass.

[0037] Particularly preferred are the cellulose fibers, at least partially microfibrillated, nanofibrillated, or with a mean length-weighted length of at most 0.2 mm, preferably at most 0.15 mm. These types of cellulose fibers are especially well suited to equipping the separator with a small mean pore size and a small standard deviation of the pore size distribution.

[0038] The separator according to the invention can contain other fibers besides cellulose fibers. These can include, for example, fibers made of cellulose derivatives, glass fibers, plastic fibers such as fibers made of polyolefins like polyethylene or polypropylene; fibers made of polyesters such as polyethylene terephthalate or polylactic acid; fibers made of polyarylates such as poly(4-hydroxybenzoic acid-co-6-hydroxy-2-naphthoic acid); fibers made of polyethers, polysulfones, polyurethanes, polyamides, aromatic polyamides such as poly(p-phenylene terephthalamide); polyimides, polyvinyl alcohol, polyacrylates such as polyacrylonitrile or poly(acrylonitrile-co-methyl acrylate); polyphenylene sulfide or poly(ethylene-co-vinyl acetate).

[0039] Preferably, the proportion of fibers other than the combined cellulose fibers is at most 30%, and particularly preferably at most 20%, of the separator mass. Such fibers generally do not bond via hydrogen bonds and therefore cannot contribute as effectively to a high 0.1% yield strength as cellulose fibers. An exception to this are fibers made of polyvinyl alcohol, which can also form hydrogen bonds and are therefore particularly preferred.

[0040] The separator according to the invention can contain further components which the person skilled in the art can select according to their experience to suit the manufacturing process, these include, for example, polyvinyl alcohol, polyethylene glycol, polyvinylidene fluoride, guarana, starch, carboxymethylcellulose, methylcellulose, dialdehydes such as glyoxal, and inorganic fillers such as kaolin, titanium dioxide (TiO2), silicon dioxide (SiO2), aluminum oxide (Al2O3), zirconium dioxide (ZrO2) or calcium carbonate (CaCO3).

[0041] The amount of inorganic fillers in the separator is at most 30%, preferably at most 20% and particularly preferably at most 15% of the mass of the separator.

[0042] The separator according to the invention is calendered, meaning that the fiber web has passed through at least one rolling nip during the manufacture of the separator, in which mechanical pressure in the thickness direction was exerted on the fiber web. Calendering reduces the thickness and the size of the pores, but also reduces the overall porosity of the separator. A particular effect of calendering according to the inventive process is also the flattening of the fibrillated fibers of regenerated cellulose, as described with reference to Fig. 2 This has already been explained. Certain settings are advantageous during calendering, which are described in more detail below in connection with the inventive method.

[0043] Under tensile stress in the machine direction according to ISO 1924-2:2008, the separator according to the invention achieves its 0.1% yield strength at an elongation of not less than 0.5% and not more than 2.0%, preferably of not less than 0.55% and not more than 2.0% and particularly preferably of not less than 0.6% and not more than 1.0%.

[0044] Under tensile loading in the machine direction according to ISO 1924-2:2008, the separator according to the invention preferably reaches its 0.1% yield strength at a width-related tensile stress of at least 0.1 kN / m and at most 2.0 kN / m, particularly preferably at least 0.15 kN / m and at most 1.6 kN / m. Based on the cross-sectional area, the tensile stress in the machine direction at the 0.1% yield strength is preferably at least 15 MPa and at most 30 MPa, particularly preferably at least 18 MPa and at most 28 MPa.

[0045] The modulus of elasticity in the machine direction according to ISO 1924-2:2008 is preferably at least 1 GPa and at most 8 GPa, and particularly preferably at least 2 GPa and at most 6 GPa. A high modulus of elasticity is advantageous because the separator deforms less under load; however, an excessively high modulus of elasticity is disadvantageous because, for example, strain caused by speed differences leads to high forces in the separator and can cause it to crack.

[0046] The elastic energy absorption capacity per unit area in the machine direction is preferably at least 0.05 J / m² and at most 0.8 J / m², and particularly preferably at least 0.10 J / m² and at most 0.60 J / m². Per unit volume, the elastic energy absorption capacity in the machine direction is preferably at least 4 kJ / m³ and at most 15 kJ / m³, and particularly preferably at least 5 kJ / m³ and at most 13 kJ / m³. High elastic energy absorption is achieved through high stress and strain at the 0.1% yield strength and allows for the compensation of large mechanical loads during separator processing without causing irreversible deformation of the separator.

[0047] 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, more preferably at least 0.5 kN / m and at most 1.5 kN / m. Based on the cross-sectional area, the tensile strength in the machine direction of the separator according to the invention is preferably at least 20 MPa and at most 60 MPa, more preferably at least 30 MPa and at most 50 MPa.

[0048] 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%.

[0049] Surprisingly, the inventors discovered that the separator according to the invention also exhibits favorable mechanical properties in the transverse direction, in particular a high elongation at break, which cannot be achieved with separators from the prior art. The high elongation at break results from the setting of a small average angle at which the cellulose fibers cross, as described in the description of Fig. 3 The angle can be influenced by the web tension and other settings, such as temperature or humidity, during the manufacture of the separator, which are described further below in connection with the method according to the invention.

[0050] As a result, the separator according to the invention is also more dimensionally stable in the transverse direction and can withstand higher mechanical loads during the production of an electrochemical element without undergoing irreversible deformation. Furthermore, it possesses an exceptionally high elongation at break in the transverse direction. All of this is surprising because, according to the invention, the stretching of the fibers during the production of the separator occurs primarily in the machine direction.

[0051] Under transverse tensile stress according to ISO 1924-2:2008, the separator according to the invention preferably reaches its 0.1% yield strength at an elongation of not less than 0.4% and not more than 2.0%, particularly preferably at not less than 0.45% and not more than 1.0%. Typical separators from the prior art reach their 0.1% yield strength in the transverse direction at an elongation of only 0.2% to approximately 0.3%.

[0052] Under transverse tensile stress according to ISO 1924-2:2008, the separator according to the invention preferably reaches its 0.1% yield strength at a width-related tensile stress of at least 0.1 kN / m and at most 0.8 kN / m, particularly preferably at least 0.15 kN / m and at most 0.6 kN / m. Based on the cross-sectional area, the transverse tensile stress at the 0.1% yield strength is preferably at least 8 MPa and at most 15 MPa, particularly preferably at least 10 MPa and at most 13 MPa.

[0053] The modulus of elasticity in the transverse direction according to ISO 1924-2:2008 is preferably at least 1 GPa and at most 6 GPa, particularly preferably at least 1.5 GPa and at most 5 GPa.

[0054] The elastic energy absorption capacity per unit area in the transverse direction is preferably at least 0.04 J / m² and at most 0.25 J / m², and particularly preferably at least 0.05 J / m² and at most 0.20 J / m². Per unit volume, the elastic energy absorption capacity in the transverse direction is preferably at least 1.5 kJ / m³ and at most 5.0 kJ / m³, and particularly preferably at least 2.0 kJ / m³ and at most 4.0 kJ / m³.

[0055] The tensile strength according to ISO 1924-2:2008 in the transverse direction of the separator according to the invention, based on the width, is preferably at least 0.3 kN / m and at most 2.0 kN / m, more preferably at least 0.5 kN / m and at most 1.5 kN / m. Based on the cross-sectional area, the tensile strength in the transverse direction of the separator according to the invention is preferably at least 20 MPa and at most 60 MPa, more preferably at least 30 MPa and at most 50 MPa.

[0056] The elongation at break according to ISO 1924-2:2008 in the transverse direction of the separator according to the invention is preferably at least 1.0% and at most 8.0%, particularly preferably at least 2.0% and at most 7.0%.

[0057] A feature important for the safety of the electrochemical element manufactured 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 30 minutes is at least 0.4% and at most 1.2%, more preferably at least 0.45% and at most 1%.

[0058] The separator according to the invention should be thin so that the ions flowing in the electrolyte only have to travel a short distance through the pores of the separator between the two electrodes, and the electrochemical element produced therefrom has a high volumetric energy density. On the other hand, a certain thickness is required to reliably insulate the electrodes from each other electrically and to achieve good separator strength. Therefore, the thickness of the separator according to the invention is preferably at least 10 µm and at most 55 µm, and particularly preferably at least 12 µm and at most 35 µm. The thickness of the separator can be determined on a single sheet according to ISO 534:2011. It is significantly influenced by the settings during the calendering of the separator and the basis weight.

[0059] The areal weight of the separator ensures good strength; however, the thickness and material requirements also increase with the areal weight. Therefore, the areal weight of the separator according to the invention is preferably at least 8 g / m² and at most 30 g / m², and particularly preferably at least 12 g / m² and at most 25 g / m². The areal weight can be determined according to ISO 536:2019.

[0060] 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 thickness and the basis weight, measured according to ISO 534:2011 or ISO 536:2019, and the density of the fibers, where a density of < 1500 kg / m³ can be chosen for the fibers. Under these assumptions, the porosity µ can be approximated as the ratio of the pore volume to the total volume of the separator by μ = 1 − 2 3 m d The porosity is calculated where m is the basis weight in g / m² and d is the thickness in µm, and the porosity is obtained as a value between 0 and 1, which can be converted into a percentage by multiplying by 100. The porosity should be as high as possible, but is primarily limited by the required mechanical strength and the requirement that the pores should be as small as possible. Preferably, the porosity is at least 30% and at most 85%, more preferably at least 35% and at most 75%.

[0061] The pore size distribution, mean flow pore size, and standard deviation of the mean flow pore size can be determined by capillary flow porosimetry according to ASTM F316- 03(2019) Standard Test Methods for Pore Size Characteristics of Membrane Filters by Bubble Point and Mean Flow Pore Test The flow rate of a medium through the separator is determined by measuring it as the pressure difference increases. This measurement method is particularly well-suited for separators because it only detects pores that pass through the separator, and the narrowest point of each pore determines the flow rate. These pore characteristics are also important for the conduction of ions through the separator.

[0062] The pores in the separator should not exceed a certain size to limit dendrite growth on the electrodes, and they should be of uniform size, i.e., exhibit a pore size distribution with a small standard deviation. Preferably, the mean flow pore size is therefore at least 40 nm and at most 1000 nm, more preferably at least 50 nm and at most 800 nm.

[0063] Typically, the pore sizes in separators according to the invention are monomodally distributed, so that the width of the pore size distribution can be well characterized by the standard deviation of the mean flow pore size. The standard deviation of the mean flow pore size in the separator according to the invention is therefore preferably at least 3 nm and at most 300 nm, and more preferably at least 3 nm and at most 200 nm.

[0064] Since measuring the pore size distribution using capillary flow porosimetry is complex, the separator's pore structure can be simplified and characterized by its Gurley air permeability. Air permeability is also a good measure of how quickly the separator can absorb the electrolyte. A high absorption rate is advantageous 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 10 s and at most 450 s, more preferably at least 20 s and at most 300 s.

[0065] The separator can be used in electrochemical cells. An electrochemical cell according to the invention comprises two electrodes, an electrolyte, and the separator according to the invention. Preferably, the electrochemical cell is a capacitor, a hybrid capacitor, a supercapacitor, or an accumulator, and particularly preferably, the electrochemical cell is a lithium-ion battery.

[0066] The separator according to the invention can be produced by a method according to the invention, which comprises the following steps. A - Production of a fiber web comprising cellulose fibers, B - Calendering of the fiber web from step A, C - Winding of the fiber web forming the separator, wherein the quantity and type of cellulose fibers in the fiber web in step A are selected such that at least 50% of the mass of the separator in step C is formed by fibrillated fibers of regenerated cellulose and, including the fibrillated fibers of regenerated cellulose, at least 70% and at most 100% of the mass of the separator in step C is formed by cellulose fibers, and wherein the production of the fiber web in step A or the calendering of the fiber web in step B takes place at least in part at a web tension that is at least 20% and at most 50% of the width-related tensile strength in the machine direction that the fiber web has immediately before step B, and wherein the separator obtained in step C, under tensile stress in the machine direction according to ISO 1924-2:2008, reaches its 0.1% yield strength at an elongation of not less than 0.5% and not more than 2.0%.

[0067] According to the inventors' findings, the increase in the 0.1% yield strength can be achieved, among other things, by subjecting the fiber web to a high tensile load in the machine direction during manufacturing in step A or during calendering in step B. This load is between 20% and 50% of the width-related tensile strength (kN / m) of the fiber web immediately before step B in the machine direction. The inventors hypothesize that this load stretches the fibers in the machine direction, causing them to cross at smaller angles on average. As a result, the finished separator reacts elastically to tensile loads up to higher elongations, as described above with reference to... Fig. 3 was explained.

[0068] The tensile strength of the fiber web per unit width immediately before step B in the machine direction has proven to be a favorable reference value, according to the inventors' findings. Immediately before step B of the inventive process, the tensile strength of the fiber web is already well developed, thus representing a representative value.

[0069] Preferably, the tensile stress of the fiber web in the machine direction during at least part of step A or step B is at least 25% and at most 40%, and particularly preferably at least 25% and at most 35%, of the width-related tensile strength in the machine direction that the fiber web exhibits immediately before step B. The tensile stress of the fiber web can be substantially influenced by the torque applied to the rollers for transporting the fiber web through the device for production or calendering.

[0070] Naturally, during the first execution of the inventive process, the width-related tensile strength in the machine direction, which the fiber web exhibits immediately before step B, will not yet be precisely known. However, a person skilled in the art can, based on their experience and considering the fiber composition, the fibrillation of the fibers, and the basis weight of the fiber web, make a relatively accurate estimate of the tensile strength of the fiber web immediately before calendering in step B and use this as a basis for determining the web tension. If the process runs stably, the web tension can be adjusted even more precisely to the actual measured tensile strength of fiber web samples taken immediately before calendering in step B. This step can be repeated several times.

[0071] The described tensile stress on the fiber web can occur during the production of the fiber web in step A or during calendering in step B.

[0072] Preferably, the production of the fiber web in step A takes place in a paper machine and comprises the following steps A.1 to A.5, A.1 - Providing an aqueous suspension comprising cellulose fibers, A.2 - Fibrillating at least some of the cellulose fibers in the suspension, A.3 - Dewatering the suspension on a circulating sieve to form a fiber web, A.4 - Dewatering the fiber web by mechanical pressure, A.5 - Drying the fiber web.

[0073] In this preferred embodiment of the inventive method, the fiber web can preferably be subjected to the described web tension in a drying section, step A.5, or a press section, step A.4, of the paper machine. The variant in which the fiber web is subjected to the aforementioned web tension in the press section, step A.4, is particularly preferred. While the structure of the fiber web has already partially solidified in the drying section, step A.5, it can be influenced even more easily in the press section, step A.4.

[0074] In some embodiments of the invention, it is possible to subject the fiber web to the aforementioned web tension on a separate device, distinct from the paper machine, and thus achieve the desired dimensional stability. This can preferably be done before or simultaneously with calendering in step B.

[0075] The person skilled in the art therefore has a large number of process variants available to increase the 0.1% yield strength of the separator according to the invention and thus to obtain a separator with high dimensional stability.

[0076] The inventors assume that increased moisture content in the fiber web enhances fiber mobility, thus making it easier to stretch the fibers under tensile stress. This applies particularly to fibrillated fibers of regenerated cellulose, which are highly crimped in their undeformed state. Preferably, the average moisture content of the fiber web during tensile stress (to generate the specified web tension) in step A or B is between 20% and 50% of the width-related tensile strength in the fiber web immediately before step B in the machine direction, therefore at least 4% and at most 15%, and particularly preferably at least 5% and at most 12%. If the moisture content is selected in the upper range of the specified intervals, then it is possible to select a lower tensile stress on the fiber web.In a particularly preferred embodiment of the method according to the invention, the mean moisture content of the fiber web during a tensile load in step A or B, which is between 20% and 30% of the width-related tensile strength in the fiber web directly before step B in the machine direction, is at least 8% and at most 15%.

[0077] Preferably, the moisture content of the fiber web is homogeneous across its width, but in order to meet specific requirements of the manufacturing process, a specific moisture profile can be created across the width of the fiber web using means known from the prior art, for example a spray bar.

[0078] In step B, mechanical pressure is applied to the fiber web from step A in at least one rolling gap, perpendicular to the fiber web, so that it is compressed in the thickness direction. With appropriate pressure and temperature settings in the rolling gap(s), the fibers are flattened and the contact area between the fibrillated fibers of regenerated cellulose is, as in Fig. 2 The image is shown enlarged accordingly. This shifts the 0.1% yield strength to higher stresses and strains, making the separator more dimensionally stable.

[0079] Preferably, the number of roll gaps in step B is higher and is at least 2 and at most 14, particularly preferably at least 3 and at most 10.

[0080] According to the inventors' findings, it has proven advantageous for carrying out the calendering process if the rolls forming the roll gap(s) have an elevated temperature. Preferably, the average temperature of all or some of these rolls in step B is therefore at least 25°C and at most 140°C, and particularly preferably at least 50°C and at most 140°C, and most preferably at least 80°C and at most 140°C. At temperatures above 80°C, covalent crosslinking and fixation of the fiber structure can also occur, which can further improve the 0.1% yield strength.

[0081] The mechanical pressure in all or part of the roll gap in step B is at least 80 kN / m and at most 400 kN / m, preferably at least 160 kN / m and at most 320 kN / m. The preferred intervals allow for a particularly favorable combination of several effects. These include increasing the contact area between the fibrillated fibers of regenerated cellulose, reducing the thickness, and decreasing the mean pore size without excessively reducing the porosity.

[0082] The amount of cellulose fibers in step A is selected such that the separator in step C is formed by fibrillated fibers of regenerated cellulose to at least 50% of its mass. Preferably, however, the proportion of fibrillated fibers of regenerated cellulose is higher, amounting to at least 55% and at most 100%, and particularly preferably at least 60% and at most 95% of the mass of the separator in step C.

[0083] The fibrillated fibers of regenerated cellulose are preferably solvent-spun fibers (" solvent-spun ") regenerated cellulose, especially preferred Lyocell® fibers.

[0084] The mean linear density of the fibrillated fibers of regenerated cellulose 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 particularly preferably at least 1.0 g / 10000 m (1.0 dtex) and at most 2.5 g / 10000 m (2.5 dtex).

[0085] The length of the fibrillated fibers of regenerated cellulose before fibrillation is at least 2 mm and at most 8 mm, and particularly preferably at least 3 mm and at most 6 mm.

[0086] Including the fibrillated fibers of regenerated cellulose, the separator in step C is formed by cellulose fibers to a minimum of 70% and a maximum of 100% of its mass. Preferably, the proportion of cellulose fibers is at least 75% and a maximum of 95% based on the mass of the separator in step C.

[0087] In addition to fibrillated fibers of regenerated cellulose, the cellulose fibers can also be formed from non-fibrillated fibers of regenerated cellulose, from pulp fibers, or from mixtures thereof. The pulp fibers are preferably obtained from softwoods, hardwoods, or other plants such as hemp, flax, jute, ramie, kenaf, kapok, coconut, abaca, sisal, bamboo, cotton, or esparto grass, or from recycled paper pulp. Mixtures of pulp fibers from different sources can also be used for the production of the separator in step A. Pulp fibers obtained from hardwoods or softwoods are particularly preferred.

[0088] It is particularly preferred that the cellulose fibers are a mixture of regenerated cellulose fibers, i.e., fibrillated and optionally non-fibrillated regenerated cellulose fibers, and pulp fibers. In a particularly preferred embodiment, the ratio of the masses of regenerated cellulose fibers to pulp fibers is at least 1:1 and at most 30:1, preferably at least 2:1 and at most 20:1, wherein, furthermore, at least 50% of the mass of the separator in step C must be formed by fibrillated regenerated cellulose fibers, and the cellulose fibers in the separator must constitute at least 70% and at most 100% of the mass of the separator in step C.

[0089] Particularly preferred are the cellulose fibers being at least partially microfibrillated cellulose fibers, nanofibrillated cellulose fibers or cellulose fibers with a mean length-weighted length of at most 0.2 mm, preferably at most 0.15 mm.

[0090] The separator in step C may contain fibers other than cellulose fibers. These may include, for example, fibers made from cellulose derivatives, glass fibers, plastic fibers such as polyolefins like polyethylene or polypropylene; polyesters like polyethylene terephthalate or polylactic acid; polyarylates like poly(4-hydroxybenzoic acid-co-6-hydroxy-2-naphthoic acid); polyethers, polysulfones, polyurethanes, polyamides, aromatic polyamides like poly(p-phenylene terephthalamide); polyimides, polyvinyl alcohol, polyacrylates like polyacrylonitrile or poly(acrylonitrile-co-methyl acrylate); polyphenylene sulfide, or poly(ethylene-co-vinyl acetate).

[0091] Preferably, the proportion of fibers other than the cellulose fibers taken together is at most 30%, particularly preferably at most 20% of the mass of the separator in step C.

[0092] Under tensile stress in the machine direction according to ISO 1924-2:2008, the separator reaches its 0.1% yield strength in step C at an elongation of not less than 0.5% and not more than 2.0%, preferably of not less than 0.55% and not more than 2.0% and particularly preferably of not less than 0.6% and not more than 1%.

[0093] Under tensile loading in the machine direction according to ISO 1924-2:2008, the separator preferably reaches its 0.1% yield strength in step C at a tensile stress of at least 0.1 kN / m and at most 2.0 kN / m, particularly preferably at least 0.15 kN / m and at most 1.6 kN / m. Based on the cross-sectional area, the tensile stress in the machine direction at the 0.1% yield strength is preferably at least 15 MPa and at most 30 MPa, particularly preferably at least 18 MPa and at most 28 MPa.

[0094] The elastic energy absorption capacity in the machine direction is preferably at least 0.05 J / m² and at most 0.8 J / m², and particularly preferably at least 0.10 J / m² and at most 0.6 J / m². Based on volume, the elastic energy absorption capacity in the machine direction is preferably at least 4 kJ / m³ and at most 15 kJ / m³, and particularly preferably at least 5 kJ / m³ and at most 13 kJ / m³.

[0095] Under tensile stress in the transverse direction according to ISO 1924-2:2008, the separator reaches its 0.1% yield strength in step C preferably at an elongation of not less than 0.4% and not more than 2.0%, particularly preferably at not less than 0.45% and not more than 1%.

[0096] Under transverse tensile stress according to ISO 1924-2:2008, the separator preferably reaches its 0.1% yield strength in step C at a tensile stress of at least 0.1 kN / m and at most 0.8 kN / m, particularly preferably at least 0.15 kN / m and at most 0.6 kN / m. Based on the cross-sectional area, the transverse tensile stress at the 0.1% yield strength is preferably at least 8 MPa and at most 15 MPa, particularly preferably at least 10 MPa and at most 13 MPa.

[0097] The elastic energy absorption capacity in the transverse direction is preferably at least 0.04 J / m² and at most 0.25 J / m², and particularly preferably at least 0.05 J / m² and at most 0.20 J / m². Based on volume, the elastic energy absorption capacity in the transverse direction is preferably at least 1.5 kJ / m³ and at most 5 kJ / m³, and particularly preferably at least 2 kJ / m³ and at most 4 kJ / m³.

[0098] Further parameters of the separator from step C, such as the modulus of elasticity in the machine and transverse directions, the tensile strength in the machine and transverse directions, the elongation at break in the machine and transverse directions, the shrinkage, the basis weight, the thickness, the porosity, the mean flow pore size, the standard deviation of the mean flow pore size and the air permeability, apply in the specified and preferred intervals as disclosed above for the separator according to the invention. BRIEF DESCRIPTION OF THE FIGURES

[0099] Fig. 1 shows an exemplary stress-strain diagram for determining the 0.1% yield strength. Fig. 2 shows an example of the contact area of ​​intersecting fibrillated fibers of regenerated cellulose before and after calendering in step B. Fig. 3 shows an example of the influence of the crossing angle between two cellulose fibers on the size of the contact area. DESCRIPTION OF SOME PREFERRED EXECUTION FORMS

[0100] 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. Example 1

[0101] According to step A of the inventive process, a fiber web was produced on a paper machine which, based on the mass of the finished separator, consisted of 77% fibrillated fibers of regenerated cellulose, 13% nanofibrillated pulp fibers and 10% fibers of polyethylene terephthalate (PET).

[0102] The fiber web was calendered in a 4-groove calender at a line load of 150 kN / m and a roller temperature of 130°C according to step B. The web was then wound up (step C) to produce the separator. The tensile strength of the fiber web immediately before calendering in step B was approximately 0.4 kN / m in the machine direction, and various web tensions were selected based on this tensile strength immediately before the start of the calendering process. The basis weight of the separator was 15.6 g / m² according to ISO 536:2019, and the thickness of a single sheet, measured according to ISO 534:2011, was approximately 22.3 µm. The tensile strength, elongation at break, modulus of elasticity and stress-strain curve of the separators thus obtained were determined according to ISO 1924-2:2008 and the 0.1% yield strength and the elastic energy absorption capacity were calculated from these values.

[0103] The data are given in Table 1 for the machine direction and in Table 2 for the transverse direction, where "WT / TS" is the ratio of the web tension immediately before the start of calendering to the width-related tensile strength of the fiber web immediately before calendering in the machine direction, "LL" is the mechanical line load exerted in all roll gaps during calendering, "TH" is the thickness, "0.1%-YP" is the 0.1% yield strength with the associated stress in kN / m and MPa and the associated strain in %, "EEA" is the elastic energy absorption capacity in J / m² and kJ / m³, and "YM" is the modulus of elasticity in GPa. Table 1 Code Maschinenrichtung WT / TS LL TH 0,1%-YP EEA YM % kN / m µm kN / m MPa % J / m 2< kJ / m 3< GPa X 10 150 22,5 0,25 11,1 0,40 0,05 2,4 3,6 Y 15 150 22,3 0,29 12,9 0,43 0,07 3,0 3,7 A 20 150 22,1 0,55 24,9 0,69 0,21 9,5 4,2 B 30 150 22,2 0,50 22,6 0,64 0,17 7,9 4,2 C 40 150 22,2 0,46 20,6 0,57 0,14 6,3 4,4 Table 2 Code Querrichtung WT / TS LL TH 0,1%-YP EEA YM % kN / m µm kN / m MPa % J / m 2< kJ / m 3< GPa X 10 150 22,5 0,18 8,2 0,41 0,04 1,8 2,6 Y 15 150 22,3 0,19 8,4 0,40 0,04 1,8 2,6 A 20 150 22,1 0,28 12,5 0,61 0,09 4,2 2,5 B 30 150 22,3 0,25 11,4 0,57 0,08 3,6 2,5 C 40 150 22,2 0,25 11,1 0,58 0,08 3,5 2,4

[0104] Table 1 shows that the two non-inventive separators X and Y reach their 0.1% yield strength at a relatively low strain of less than 0.45%. The elastic modulus in the machine direction of the inventive separators A, B, and C is somewhat higher than that of the non-inventive separators X and Y, so that even at the same strain, the inventive separators A, B, and C can absorb more elastic deformation energy and are therefore more dimensionally stable.

[0105] It is also evident that the elastic behavior can be extended to higher strains if the web tension exceeds approximately 20% of the width-related tensile strength in the machine direction that the fiber web exhibits immediately before step B. Increasing the ratio of web tension to the width-related tensile strength of the separator in the machine direction above 20% is possible, but does not result in any improvements to the 0.1% yield strength.

[0106] The effect in the transverse direction is surprising because it was expected that tensile stress in the machine direction during manufacturing would not significantly change the mechanical properties of the finished separator in the transverse direction, or might even worsen. However, the experiments show that the 0.1% yield strength also shifts in the transverse direction to higher stresses and strains, and therefore the elastic energy absorption capacity in the transverse direction can also be considerably increased.

[0107] The pore structure of separator A according to the invention was determined by capillary flow porosimetry using ASTM F316-03(2019). The mean flow pore size was 173 nm with a standard deviation of 150 nm. The porosity was 45% and the Gurley air permeability, according to ISO 5636-5:2013, was 54 s.

[0108] Due to their mechanical properties and pore structure, the separators A, B and C according to the invention are well suited for the production of electrochemical elements and the experimental production of lithium-ion batteries was easily possible. Ausführungsbeispiel 2

[0109] According to step A of the inventive process, a fiber web was produced on a paper machine which, based on the mass of the finished separator, consisted of 90% fibrillated fibers of regenerated cellulose and 10% nanofibrillated pulp fibers.

[0110] The fiber web was calendered in a calender with 6 roll gaps under various line loads from 80 kN / m to 400 kN / m at a roll gap temperature of 90°C, as per step B. The fiber web was then wound up, step C, to produce several separators. The tensile strength per unit width in the machine direction, which the fiber web exhibited immediately before calendering in step B, was approximately 0.44 kN / m to 0.46 kN / m. Based on this tensile strength, the fiber web was subjected to various web tensions in the drying section in step A, where it was moistened in the size press before entering this drying section. The basis weight of the separator was approximately 15.6 g / m² according to ISO 536:2019.The tensile strength, elongation at break, modulus of elasticity and stress-strain curve of the separators thus obtained were determined according to ISO 1924-2:2008 and the 0.1% yield strength and the elastic energy absorption capacity were calculated from these values.

[0111] The data are given in Table 3 for the machine direction and in Table 4 for the transverse direction, where "WT / TS" is the ratio of the web tension in the drying section of step A to the width-related tensile strength of the fiber web immediately before calendering in the machine direction, "LL" is the mechanical line load exerted in all roll gaps during calendering, "TH" is the thickness, "0.1%-YP" is the 0.1% yield strength with the associated stress in kN / m and MPa and the associated strain in %, "EEA" is the elastic energy absorption capacity in J / m² and kJ / m³, and "YM" is the modulus of elasticity in GPa. Table 3 Code Maschinenrichtung WT / TS LL TH 0,1%-YP EEA YM % kN / m µm kN / m MPa % J / m 2< kJ / m 3< GPa V 10 240 20,3 0,28 13,8 0,47 0,07 3,5 3,8 W 15 240 20,2 0,31 15,5 0,49 0,08 4,1 3,9 D 20 240 20,0 0,47 23,6 0,67 0,17 8,6 4,2 E 30 240 20,1 0,54 26,9 0,65 0,19 9,5 4,7 F 40 240 20,2 0,51 25,5 0,61 0,17 8,4 4,8 G 25 80 20,8 0,55 26,3 0,72 0,21 10,3 4,1 H 25 400 19,4 0,46 23,8 0,56 0,14 7,1 5,0 Table 4 Code Querrichtung WT / TS LL TH 0,1%-YP EEA YM % kN / m µm kN / m MPa % J / m 2< kJ / m 3< GPa V 10 240 20,3 0,16 7,8 0,40 0,03 1,7 2,6 W 15 240 20,2 0,16 7,7 0,39 0,03 1,6 2,5 D 20 240 20,0 0,20 9,9 0,49 0,05 2,7 2,4 E 30 240 20,1 0,21 10,3 0,50 0,06 2,8 2,4 F 40 240 20,2 0,19 9,3 0,48 0,05 2,4 2,2 G 25 80 20,8 0,19 9,3 0,52 0,05 2,6 2,0 H 25 400 19,4 0,18 9,2 0,45 0,04 2,2 2,4

[0112] Table 3 shows that the two non-inventive separators V and W reach their 0.1% yield strength in the machine direction at strains of 0.47% and 0.49%, respectively. The elastic modulus in the machine direction of the inventive separators D, E, F, G, and H is somewhat higher than that of the non-inventive separators V and W, so that even at the same strain, the inventive separators D to H can absorb more elastic deformation energy and are therefore more dimensionally stable.

[0113] It is also evident that the elastic behavior can be extended to higher strains if the web tension in the drying section in step A exceeds approximately 20% of the width-related tensile strength in the machine direction that the fiber web exhibits immediately before calendering. Increasing the ratio of web tension to the width-related tensile strength of the fiber web immediately before calendering in the machine direction by more than 20% is possible and, in some cases, results in further improvements in the 0.1% yield strength and the elastic energy absorption capacity.

[0114] A comparison of the separators D, G and H according to the invention shows that the line load during calendering can influence the elastic energy absorption capacity. At high line loads, the elastic energy absorption capacity decreases in the machine direction and transverse direction.

[0115] The tensile test according to ISO 1924-2:2008 showed a transverse elongation at break of 5.5% to 7.3% for the separators D to H according to the invention, which is a very high value. In comparison, the transverse elongation at break for the non-inventive separators V and W was less than 5%. This is a further advantage of the inventive method.

[0116] The air permeability of the Gurley separators according to the invention, as per ISO 5636-5:2013, was between 140 s and 250 s and the mean flow pore size was between 130 nm and 160 nm with a standard deviation of the mean flow pore size of 80 nm to 150 nm, which is why it can be assumed that the pore structure is suitable for use as a separator in electrochemical elements.

[0117] The production of lithium-ion batteries from the separators D to H according to the invention was easily possible.

[0118] The separators X, Y, V and W, which are not according to the invention, are also suitable for the production of electrochemical elements; however, the separators A to H according to the invention have more favorable mechanical properties, so that electrochemical elements with better performance parameters and higher productivity can be produced from them.

Claims

1. Separator for an electrochemical element, in which at least 50% of the mass of the separator is formed by fibrillated regenerated cellulose fibers wherein, including the fibrillated regenerated cellulose fibers, at least 70% and at most 100% of the mass of the separator is formed by cellulose fibers, wherein the separator is calendered, and wherein under tensile load in the machine direction in accordance with ISO 1924-2:2008, the separator reaches its 0.1% yield point at an elongation of no less than 0.5% and no more than 2.0%.

2. Separator according to claim 1, in which the proportion of fibrillated regenerated cellulose fibers is at least 55% and at most 100%, preferably at least 60% and at most 95% of the mass of the separator, and / or in which the fibrillated regenerated cellulose fibers are formed by solvent-spun regenerated cellulose fibers, preferably by Lyocell® fibers, and / or in which the mean linear density of the fibrillated regenerated cellulose fibers before fibrillation is at least 0.8 g / 10000 m (0.8 dtex) and at most 3.0 g / 10000 m (3.0 dtex), preferably at least 1.0 g / 10000 m (1.0 dtex) and at most 2.5 g / 10000 m (2.5 dtex).

3. Separator according to one of the preceding claims, in which the mean length of the fibrillated regenerated cellulose fibers before fibrillation is at least 2 mm and at most 8 mm, preferably at least 3 mm and at most 6 mm, and / or in which the proportion of cellulose fibers, including the fibrillated regenerated cellulose fibers, is at least 75% and at most 95% with respect to the mass of the separator, and / or in which, in addition to the fibrillated regenerated cellulose fibers, said cellulose fibers also contain non-fibrillated regenerated cellulose fibers, pulp fibers, or mixtures thereof, wherein the cellulose fibers preferably are a mixture of regenerated cellulose fibers or pulp fibers, wherein in all cases, the regenerated cellulose fibers contain fibrillated and optionally non-fibrillated regenerated cellulose fibers, and wherein the ratio of the masses of regenerated cellulose fibers to pulp fibers is at least 1:1 and at most 30:1, preferably at least 2:1 and at most 20:1, under the condition that at least 50% of the mass of the separator is formed by fibrillated regenerated cellulose fibers and the cellulose fibers in the separator in total make up at least 70% and at most 100% of the mass of the separator.

4. Separator according to one of the preceding claims, in which the pulp fibers are at least in part micro-fibrillated pulp fibers, nano-fibrillated pulp fibers or pulp fibers with a length-weighted mean length of at most 0.2 mm, preferably at most 0.15 mm, and / or which, in addition to the cellulose fibers, also contains fibers from cellulose derivatives, glass fibers, or plastic fibers, wherein the plastic fibers are preferably formed by polyolefins, in particular polyethylene or polypropylene; by polyesters, in particular polyethylene terephthalate or polylactic acids; by polyarylates in particular poly(4-hydroxybenzoic acid-co-6-hydroxy-2-naphthoic acid); by polyethers, polysulfones, polyurethanes, polyamides, by aromatic polyamides, in particular poly(p-phenylene terephthalamide); by polyimides, polyvinyl alcohol, polyacrylates in particular polyacrylonitrile or poly(acrylonitrile-co-methyl acrylate); by polyphenylene sulfide or by poly(ethylene-co-vinyl acetate), wherein the proportion of fibers other than cellulose fibers in total is preferably at most 30%, particularly preferably at most 20% of the mass of the separator.

5. Separator according to one of the preceding claims, which further contains one or more of the following substances or components: polyvinyl alcohol, polyethylene glycol, polyvinylidene fluoride, guaran, starch, carboxymethyl cellulose, methyl cellulose, dialdehydes, in particular glyoxal, or one or more inorganic fillers, in particular kaolin, titanium dioxide (TiO2), silicon dioxide (SiO2), aluminum oxide (Al2O3), zirconium dioxide (ZrO2) or calcium carbonate (CaCO3), wherein the amount of inorganic fillers is preferably at most 30%, particularly preferably at most 20% and particularly preferably at most 15% of the mass of the separator.

6. Separator according to one of the preceding claims which, under tensile load in the machine direction in accordance with ISO 1924-2:2008, reaches its 0.1% yield point at an elongation of no less than 0.55% and no more than 2.0% and preferably no less than 0.6% and no more than 1.0%, and / or which, under tensile load in the machine direction in accordance with ISO 1924-2:2008, reaches its 0.1% yield point at a width-related tensile stress of at least 0.1 kN / m and at most 2.0 kN / m, preferably at least 0.15 kN / m and at most 1.6 kN / m. and / or which, under tensile load in the machine direction in accordance with ISO 1924-2:2008, reaches its 0.1% yield point at a tensile stress with respect to cross-sectional area of at least 15 MPa and at most 30 MPa, preferably at least 18 MPa and at most 28 MPa.

7. Separator according to one of the preceding claims, with a Young's modulus in the machine direction in accordance with ISO 1924-2:2008 of at least 1 GPa and at most 8 GPa, preferably at least 2 GPa and at most 6 GPa, and / or with an elastic energy absorption with respect to area in the machine direction of at least 0.05 J / m2 and at most 0.80 J / m2, preferably at least 0.10 J / m2 and at most 0.60 J / m2, and / or with an elastic energy absorption with respect to volume in the machine direction of at least 4 kJ / m3 and at most 15 kJ / m3, preferably at least 5 kJ / m3 and at most 13 kJ / m3, wherein the elastic energy absorption with respect to area or volume, respectively, is to be measured by the corresponding method described in the description, and / or with a width-related tensile strength in the machine direction in accordance with ISO 1924-2:2008 of at least 0.3 kN / m and at most 2.0 kN / m, preferably at least 0.5 kN / m and at most 1.5 kN / m, and / or with a tensile strength with respect to the cross-sectional area in the machine direction in accordance with ISO 1924-2:2008 of at least 20 MPa and at most 60 MPa, preferably at least 30 MPa and at most 50 MPa, and / or with an elongation at break in the machine direction in accordance with ISO 1924-2:2008 of at least 0.5% and at most 5.0%, preferably at least 1.0% and at most 4.0%, and / or which, under tensile load in the cross direction in accordance with ISO 1924-2:2008, reaches its 0.1% yield point at an elongation of no less than 0.4% and no more than 2.0%, particularly preferably no less than 0.45% and no more than 1.0%, and / or which, under tensile load in the cross direction in accordance with ISO 1924-2:2008, reaches its 0.1% yield point at a width-related tensile stress of at least 0.1 kN / m and at most 0.8 kN / m, preferably at least 0.15 kN / m and at most 0.6 kN / m, and / or which, under tensile load in the cross direction in accordance with ISO 1924-2:2008, reaches its 0.1% yield point at a tensile stress with respect to cross-sectional area of at least 8 MPa and at most 15 MPa, preferably at least 10 MPa and at most 13 MPa.

8. Separator according to one of the preceding claims, with a Young's modulus in the cross direction in accordance with ISO 1924-2:2008 of at least 1 GPa and at most 6 GPa, preferably at least 1.5 GPa and most 5 GPa, and / or with an elastic energy absorption with respect to area in the cross direction of at least 0.04 J / m2 and at most 0.25 J / m2, preferably at least 0.05 J / m2 and at most 0.20 J / m2, and / or with an elastic energy absorption with respect to volume in the cross direction of at least 1.5 kJ / m3 and at most 5.0 kJ / m3, preferably at least 2.0 kJ / m3 and at most 4.0 kJ / m3, wherein the elastic energy absorption with respect to area or volume, respectively, is to be measured by the corresponding method described in the description, and / or with a width-related tensile strength in the cross direction in accordance with ISO 1924-2:2008 of at least 0.3 kN / m and at most 2.0 kN / m, preferably at least 0.5 kN / m and at most 1.5 kN / m, and / or with a tensile strength with respect to cross-sectional area in the cross direction in accordance with ISO 1924-2:2008 of at least 20 MPa and at most 60 MPa, preferably at least 30 MPa and at most 50 MPa, and / or with an elongation at break in the cross direction in accordance with ISO 1924-2:2008 of at least 1.0% and at most 8.0%, preferably at least 2.0% and at most 7.0%, and / or with a shrinkage after heating to 150°C for 30 minutes of at least 0.4% and at most 1.2%, preferably at least 0.45% and at most 1.0%, and / or with a thickness, determined on a single sheet in accordance with ISO 534:2011, of at least 10 µm and at most 55 µm, preferably at least 12 µm and at most 35 µm, and / or with a basis weight in accordance with ISO 536:2019 of at least 8 g / m2 and at most 30 g / m2, preferably at least 12 g / m2 and at most 25 g / m2.

9. Separator according to one of the preceding claims, with a porosity µ of at least 30% and at most 85%, preferably at least 35% and at most 75%, wherein the porosity µ is calculated according to μ = 1 − 2 3 m d , wherein m is the basis weight in g / m2 and d is the thickness in µm and the porosity is obtained as a value between 0 and 1 and is converted into a percentage by multiplication by 100, and / or in which the mean flow pore size is at least 40 nm and at most 1000 nm, preferably at least 50 nm and at most 800 nm, and / or in which the standard deviation of the mean flow pore size is at least 3 nm and at most 300 nm, preferably at least 3 nm and at most 200 nm, and / or with a Gurley air permeability determined in accordance with ISO 5636-5:2013 of at least 10 s and at most 450 s, preferably at least 20 s and at most 300 s.

10. Electrochemical element, which comprises two electrodes, an electrolyte and a separator according to one of the preceding claims, wherein the electrochemical element is preferably formed by a capacitor, a hybrid capacitor, a super capacitor or an accumulator, wherein the electrochemical element is particularly preferably a lithium ion battery.

11. Process for manufacturing a separator, with the following steps: A - manufacturing a fibrous web comprising cellulose fibers, B - calendering the fibrous web from step A, C - rolling up the fibrous web forming the separator, wherein the amount and type of cellulose fibers in the fibrous web in step A is selected such that at least 50% of the mass of the separator in step C is formed by fibrillated regenerated cellulose fibers and, including the fibrillated regenerated cellulose fibers, at least 70% and at most 100% of the mass of the separator in step C is formed by cellulose fibers, wherein the manufacture of the fibrous web in step A or the calendering of the fibrous web in step B is carried out at least in part at a web tension which is at least 20% and at most 50% of the width-related tensile strength in the machine direction that the fibrous web has directly before step B, and wherein, under tensile load in the machine direction in accordance with ISO 1924-2:2008, the separator obtained in step C reaches its 0.1% yield point at an elongation of no less than 0.5% and no more than 2.0%.

12. Process according to claim 11, in which a tensile load of the fibrous web in the machine direction for generating said web tension during at least a part of step A or step B is at least 25% and at most 40%, preferably at least 25% and at most 35% of the width-related tensile strength of the fibrous web in the machine direction that the fibrous web has directly before step B, and / or in which the manufacture of the fibrous web in step A is carried out in a paper machine and comprises the following steps A.1 to A.5: A.1 - providing an aqueous suspension comprising cellulose fibers, A.2 - fibrillating at least a part of the cellulose fibers in the suspension, A.3 - de-watering the suspension on a running wire to form a fibrous web, A.4 - de-watering the fibrous web by mechanical pressure, A.5 - drying the fibrous web, wherein the fibrous web preferably is exposed to said web tension during step A.5 in a drying section or during step A.4 in a press section of the paper machine.

13. Process according to one of claims 11 or 12, in which the mean moisture content of the fibrous web under a tensile load in step A or B to generate said web tension, which is between 20% and 50% of the width-related tensile strength of the fibrous web in the machine direction directly before step B, is at least 4% and at most 15%, preferably at least 5% and at most 12%, and / or in which the mean moisture content of the fibrous web under a tensile load in step A or B to generate said web tension, which is between 20% and 30% of the width-related tensile strength of the fibrous web in the machine direction directly before step B, is at least 8% and at most 15%, and / or in which the fibrous web in step B passes through at least 2 and at most 14, preferably at least 3 and at most 10 nips, wherein a mechanical pressure in all or at least a part of the nips in step B is at least 80 kN / m and at most 400 kN / m, preferably at least 160 kN / m and at most 320 kN / m.

14. Process according to one of claims 11 to 13, in which the calendering in step B is carried out by means of a plurality of rolls, wherein the mean temperature of all or a part of these rolls in step B is at least 25°C and at most 140°C, preferably at least 50°C and at most 140°C, particularly preferably at least 80°C and at most 140°C, and / or in which the amount and composition of the cellulose fibers in step A is selected such that the separator in step C is formed by at least 55% and at most 100%, preferably at least 60% and at most 95% of its mass by fibrillated regenerated cellulose fibers.

15. Process according to one of claims 11 to 14, in which the fibrillated regenerated cellulose fibers are formed by solvent-spun regenerated cellulose fibers, and in particular by Lyocell® fibers, and / or in which the mean linear density of the fibrillated regenerated cellulose fibers 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 preferably at least 1.0 g / 10000 m (1.0 dtex) and at most 2.5 g / 10000 m (2.5 dtex), and / or wherein the length of the fibrillated regenerated cellulose fibers before fibrillation is at least 2 mm and at most 8 mm and preferably at least 3 mm and at most 6 mm, and / or in which the separator in step C has a proportion of cellulose fibers, including the fibrillated regenerated cellulose fibers, of at least 75% and at most 95% with respect to the mass of the separator, and / or in which the separator in step C is a separator according to one of claims 1 to 9.