CELLULOSE FIBER-BASED SEPARATOR FOR ELECTROCHEMICAL ELEMENTS

Description

FIELD OF THE INVENTION

[0001] The invention relates to a separator for electrochemical cells formed by a fiber substrate consisting essentially of fibrillated fibers of regenerated cellulose and pulp with a high fines content. Such a separator exhibits particularly favorable properties, particularly with regard to pore size distribution. 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 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, so that the electrochemical cell has favorable properties, particularly rapid charging and the ability to draw high currents.

[0003] These requirements for the separator mean that it should be as thin as possible to ensure that the path of the ions from one electrode to the other through the separator's pores is short, achieving a high volumetric energy density of the electrochemical element, and that it should have a high porosity. Particularly when the electrochemical element is a rechargeable 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, particularly dendrites, on the electrodes. These crystals can short-circuit the rechargeable battery and thus reduce its service life and performance. A very large number of small, but as uniformly sized, pores is desired, i.e., a pore size distribution with a small standard deviation.

[0004] In addition, the separator must also 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.

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

[0006] Finally, despite its small thickness, the separator needs sufficient mechanical strength so that electrochemical elements can be easily manufactured from it, and it should absorb the electrolyte as quickly as possible during production and into its entire pore volume in order to achieve high ionic conductivity.

[0007] According to the current state of the art, this multitude of requirements can be met primarily with thin plastic films, which can be produced with very consistent quality. However, the plastics used, typically polyolefins, are mostly thermoplastic and often lack sufficient thermal resistance, resulting in problems with the fire safety of the electrochemical elements made from them, primarily because the plastics shrink at high temperatures and no longer prevent large-area contact between the electrodes.

[0008] Attempts to use fiber substrates as separators for electrochemical cells, particularly for lithium-ion batteries, have so far been largely unsuccessful because fiber substrates of sufficient strength are often too thick and, due to the raw materials and manufacturing process, the pores are too large and the standard deviation of the pore size distribution is too high. Cellulose fiber separators, in particular, have proven particularly difficult in this regard because they are based on natural raw materials such as cellulose fibers, which themselves vary considerably in length, thickness, and shape, although cellulose fibers would offer advantages in terms of safety aspects, particularly dimensional stability at high temperatures, and environmental aspects.

[0009] Despite this unfavourable starting position, there is interest in the industry in having separators for electrochemical cells available that are essentially made of cellulose fibres and have favourable properties for use in electrochemical cells. SUMMARY OF THE INVENTION

[0010] The invention is therefore based on the object of providing a separator for electrochemical elements which is essentially made of cellulose fibres and which meets the requirements regarding thickness, pore size distribution, strength and chemical resistance, so that the advantages in terms of safety and ecology can also be used in an economically viable manner.

[0011] This object is achieved by a separator for electrochemical elements according to claim 1 and a method for producing a separator for electrochemical elements according to claim 12. Advantageous further developments are specified in the dependent claims.

[0012] The inventors have found that this object can be achieved by a separator for electrochemical elements, wherein at least 70% and at most 95% of the mass of the separator is formed by fibrillated fibers of regenerated cellulose and at least 3% and at most 30% of the mass of the separator is formed by cellulose with a high fines content, wherein of the fibrillated fibers of the regenerated cellulose which have a length of at least 1 mm, at least 10% by number have a branched structure and wherein in the cellulose with a high fines content the proportion of fibers with a length of less than 0.2 mm is at least 70% by the total length of the fibers in the cellulose with a high fines content.

[0013] A major challenge in the production of fiber substrates containing fines is keeping the fines in the fiber substrate so that they are not lost during the production of the fiber substrate and in subsequent processing steps. The inventors were able to overcome this difficulty through a special milling process that imparts a particular morphology to the regenerated cellulose fibers. In this process, the fibers made from regenerated cellulose are primarily fibrillated rather than cut, resulting in branched structures in at least some of the fibers. According to the inventors' findings, these structures contribute significantly to keeping the fines in the fiber substrate. The branched structures are characterized primarily by the fact that the fibrils are not completely separated from one another but remain connected at one end to form a thicker fiber.The branched structures connect with each other through hydrogen bonds, forming a network that contributes to high strength and provides the basis for holding additional fibers with a branched structure. This creates a sufficiently dense network that can absorb the fine particles. The fine particles serve primarily to reduce the pore size and create a pore size distribution with a small standard deviation.

[0014] Fibrillation of the regenerated cellulose to produce the branched structures can be achieved particularly favorably in a colloid mill.

[0015] The fibers of the regenerated cellulose are milled in such a way that some of the fibers have a branched structure. According to the invention, at least 10% of all fibers of the fibrillated regenerated cellulose with a length of at least 1 mm have such a branched structure. Preferably, the proportion of fibers with a branched structure is higher, amounting to at least 15% and particularly preferably at least 20%, each based on the number of fibers of the fibrillated regenerated cellulose with a length of at least 1 mm.

[0016] The separator according to the invention is made up of at least 70% and at most 95%, preferably at least 75% and at most 90%, fibrillated fibers of regenerated cellulose, based on the mass of the separator. This type and quantity of fibers in the separator allow for good strength, so that the separator can also be processed into an electrochemical element.

[0017] The fibers of regenerated cellulose are preferably solvent-spun fibers (" solvent-spun "), particularly preferred Lyocell ®< fibers.

[0018] The linear density of the regenerated cellulose fibers before fibrillation is important for fiber refining. Preferably, the average linear density of the regenerated cellulose fibers 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 2.5 g / 10,000 m (2.5 dtex).

[0019] The length of the regenerated cellulose fibers before fibrillation is primarily important for the strength of the separator. Longer fibers result in greater strength but also require more energy during refining. The average length of the regenerated cellulose fibers 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.

[0020] The separator according to the invention is formed from at least 3% and at most 30%, preferably at least 5% and at most 20%, based on the mass of the separator, of high-fines pulp. The high-fines pulp produces a pore size distribution with a small standard deviation and high porosity. A higher proportion of high-fines pulp complicates dewatering of the fiber web during production of the separator on a paper machine. In addition, high-fines pulp is comparatively complex to produce and expensive. The specified intervals therefore allow a particularly favorable combination of porosity, strength, cost, and dewatering time. The fibrillation of the fibers of the regenerated cellulose also forms fines, which can sometimes be present in the separator in even larger quantities than the fines of the high-fines pulp.According to the inventors' findings, however, the fines of regenerated cellulose have a coarser structure and are therefore not as well suited to achieving high porosity with a small average pore size and a low standard deviation of the pore size distribution. The fines of pulp with a high fines content, on the other hand, have a finer structure and therefore allow the average pore size and the standard deviation of the pore size distribution to be reduced much more effectively, even with a low total content in the separator. Furthermore, by even more intensive fibrillation of the fibers of regenerated cellulose, the proportion of fines can hardly be increased without destroying the fibers with a branched structure, so that within the scope of the invention, the proportion of fines is adjusted by adding the fines from the pulp with a high fines content.

[0021] According to the invention, the high-fines pulp is produced from pulp, preferably from coniferous wood, hardwood, 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 pulps from various sources can also be used to produce the high-fines pulp. Particularly preferably, the pulps are obtained from hardwood or softwood.

[0022] According to the invention, the pulp with a high fines content is characterized in that the proportion of pulp fibers with a length of less than 0.2 mm is at least 70% based on the total length of the pulp fibers. This means that the sum of the lengths of the pulp fibers with a length of less than 0.2 mm is at least 70% of the sum of the lengths of all pulp fibers in the pulp with a high fines content. The fines contribute to further reducing the pores and creating a pore size distribution with a small standard deviation and high porosity. The content of fines in the pulp with a high fines content is therefore preferably higher, so that the proportion of pulp fibers with a length of less than 0.2 mm is at least 80%, particularly preferably at least 90%, in each case based on the total length of the pulp fibers in the pulp with a high fines content.This content of fines can be determined using an image analysis method according to ISO 16065-2:2014.

[0023] Nanofibrillated pulp or microfibrillated pulp can be well-suited as pulp with a high fines content and is commercially available, for example under the name Exilva-F-01, from Borregaard.

[0024] In preferred embodiments, the properties of the separator can be further improved by adjusting the length and thickness of the fines even more precisely during the production process of the high-fines pulp, so that even shorter and thinner fibers are produced, the so-called secondary fines. Secondary fines are fibers whose length L is less than 100 µm and whose thickness D in µm satisfies the inequality D ≤ 50 − 0,3 ⋅ L where L is to be entered in µm. The proportion of secondary fines in high-fines pulp is preferably at least 40%, particularly preferably at least 60%, based in each case on the total fiber length in the high-fines pulp. The proportion of secondary fines can also be determined using an image analysis method according to ISO 16065-2:2014. For example, the L&W Fiber Tester Plus measuring device from Lorentzen & Wettre can be used to determine fiber lengths and thicknesses and their distribution.

[0025] The separator according to the invention can contain further components which the person skilled in the art can select according to his experience to suit the production 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 (TiO 2 ), silicon dioxide (SiO 2 ), aluminum oxide (Al 2 O 3 ), zirconium dioxide (ZrO 2 ) or calcium carbonate (CaCO 3 ).

[0026] In addition to the fibrillated fibers of regenerated cellulose and the high-fines pulp, the separator according to the invention can also contain other fibers. These can include, for example, fibers made of cellulose derivatives, non-fibrillated fibers of regenerated cellulose, glass fibers, plastic fibers, such as fibers made of polyolefins such as polyethylene or polypropylene; fibers made of polyesters such as polyethylene terephthalate or polylactic acids; fibers made of polyethers, polysulfones, polyurethanes, polyamides, polyimides, polyvinyl alcohol, polyacrylonitrile, polyphenylene sulfide, or ethylene-vinyl acetate copolymers.

[0027] Preferably, however, the proportion of other fibers taken together amounts to no more than 10%, particularly preferably no more than 5% of the mass of the separator.

[0028] 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 resulting electrochemical element has a high volumetric energy density. On the other hand, a certain thickness is required to reliably electrically insulate the electrodes from each other 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, 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.

[0029] The basis weight of the separator ensures good strength, but the thickness and material requirements also increase with the basis weight. Therefore, the basis weight of the separator according to the invention is preferably at least 8 g / m 2 and at most 30 g / m 2 , particularly preferably at least 12 g / m 2 and at most 25 g / m 2 . The basis weight can be determined according to ISO 536:2012.

[0030] 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, each measured according to ISO 534:2011, and the density of the fibers, whereby a density of 1500 kg / m 3 can be chosen for the fibers. Under these assumptions, the porosity µ can be approximately expressed as the ratio of the pore volume to the total volume of the separator by μ = 1 − 2 3 m d where m is the basis weight in g / m 2 and d is the thickness in µm, and the porosity is 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 30% and at most 85%, particularly preferably at least 35% and at most 75%.

[0031] The pore size distribution, the mean flow pore size, and the 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. This method determines the flow rate of a medium through the separator with increasing pressure difference. This measurement method is particularly suitable for separators because it only measures 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.

[0032] The pores in the separator should not exceed a certain size to limit the growth of dendrites on the electrodes, and they should be uniform in size, i.e., have a pore size distribution with a small standard deviation. Therefore, the average flow pore size is preferably at least 40 nm and at most 1000 nm, preferably at least 50 nm and at most 800 nm.

[0033] Typically, the pore sizes in separators according to the invention are distributed monomodally, so that the breadth 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 200 nm, particularly preferably at least 3 nm and at most 100 nm. Alternatively or in addition to the standard deviation of the mean flow pore size, the pore size distribution can also be characterized by the flow pore size D 90 , where D 90 is determined such that 90% of the flow occurs through pores whose flow pore sizes do not exceed the value D 90 . The flow pore size D 90 is preferably at least 100 nm and at most 1500 nm, particularly preferably at least 200 nm and at most 1000 nm.

[0034] The strength of the separator is important for processing the separator into an electrochemical element. Strength can be characterized by tensile strength and determined according to ISO 1924-2:2008. Due to the nature of the manufacturing process and the fibers' branched structure, tensile strength does not depend particularly strongly on the direction in which the sample was taken from the separator. Therefore, the requirements are considered met if they are met in at least one direction. The tensile strength of the separator according to the invention is preferably at least 0.3 kN / m and at most 2 kN / m, particularly preferably at least 0.5 kN / m and at most 1.5 kN / m. Strength can be increased by more intensive refining of the fibers from regenerated cellulose; however, this requires greater energy consumption and the fibers are shortened more significantly, so strength cannot be increased arbitrarily.

[0035] The separator's elongation is also important for the automated further processing of the separator into an electrochemical cell. Elongation can be described by the elongation at break and measured according to ISO 1924-2:2008. Like tensile strength, elongation at break also depends on the direction in which the sample was taken from the separator. However, this dependence is not very pronounced, so the requirements are met if they are met in at least one direction. The elongation at break of the separator according to the invention is preferably at least 0.5% and at most 4.0%, particularly preferably at least 1.0% and at most 3.5%.

[0036] The elasticity of the separator is also important. It can be characterized by the elastic modulus, which results from measuring the force-strain curve according to ISO 1924-2:2008. The elastic modulus of the separators according to the invention also depends only slightly on the direction in which the sample was removed from the separator, so that the elastic modulus, regardless of the direction, is preferably at least 1 GPa and at most 8 GPa, particularly preferably at least 2 GPa and at most 6 GPa.

[0037] Since measuring the pore size distribution using capillary flow porosimetry is complex, the pore structure of the separator can also be characterized simply 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 10 s and at most 450 s, preferably at least 40 s and at most 300 s.

[0038] The separator can be used in electrochemical elements. An electrochemical element according to the invention comprises two electrodes, an electrolyte, and the separator according to the invention. The electrochemical element is preferably a capacitor, a hybrid capacitor, a supercapacitor, or an accumulator, and the electrochemical element is particularly preferably a lithium-ion battery.

[0039] The separator according to the invention can be produced by the following process according to the invention, which comprises the following steps, A - Producing an aqueous suspension of fibrillable regenerated cellulose fibers, B - Fibrillating the regenerated cellulose fibers from step A, C - Introducing the aqueous suspension of fibrillated regenerated cellulose fibers from step B into a headbox, D - Applying the aqueous suspension from step C to a rotating screen to form a fiber web, E - Dewatering the fiber web on the rotating screen, F - Drying the fiber web in a first drying device, G - Drying the fiber web in a second drying device, H - Rolling up the fiber web forming the separator, wherein the fibers of regenerated cellulose are fibrillated in step C such that, of the fibers having a length of at least 1 mm, at least 10% of the fibers, based on the number, have a branched structure, and wherein high-fines pulp is added in at least one of the following steps: (a) in step A by adding it to the aqueous suspension, (b) in step C by adding it to the headbox, (c) in step D by applying it from another headbox to the fiber web formed on the rotating wire, (d) between steps E and F by applying it to the fiber web in an application device, or (e) between steps G and H by applying it to the fiber web in an application device, and wherein in the high-fines pulp, at least 70% of the fibers, based on the total length of the fibers, have a length of less than 0.2 mm,and wherein at least 70% and at most 95% of the mass of the separator after drying in step G is formed by fibrillated fibers of regenerated cellulose and at least 3% and at most 30% of the mass of the separator is formed by high fines content pulp.

[0040] In a preferred embodiment of the process according to the invention, step B is carried out in such a way that the fibers of regenerated cellulose are fibrillated more and cut less, and particularly preferably, step B is carried out in a colloid mill. The inventors have found that the formation of the branched structures is related to the fact that the fibers are cut to a lesser extent and a significant portion of the fibrillation occurs through fiber-to-fiber friction. This type of fibrillation can be carried out in various milling units, but a colloid mill has proven particularly effective.

[0041] In a preferred embodiment, step B is carried out such that the Schopper Riegler bead freeness (°SR), measured according to ISO 5267-1:1999, is at least 70 °SR and at most 100 °SR, particularly preferably at least 80 °SR and at most 95 °SR. More intensive beading, and thus a higher Schopper Riegler bead freeness, leads to more fibrils and greater strength and a finer pore structure. However, since the energy expenditure is considerable and the fibers are shortened with increasing beading intensity, the specified intervals represent a favorable compromise.

[0042] Refining the regenerated cellulose in step B can produce fibers with a length of less than 0.2 mm, but their proportion should not be very high, because the fibers of the regenerated cellulose are primarily intended to form a network that holds the fibers of the pulp with a high fines content. Therefore, step B is preferably carried out such that in the fibrillated fibers of the regenerated cellulose after step B, at least 30% and at most 70%, particularly preferably at least 40% and at most 65%, of the total fiber length is made up of fibers with a length of less than 0.2 mm. This proportion of fibers with a length of less than 0.2 mm can be determined according to ISO 16065-2:2014.

[0043] In a preferred embodiment of the process according to the invention, at least steps C to G are carried out on a paper machine.

[0044] The drying devices of steps F and G may be different or the same and may preferably be formed by one or more heated drying cylinders.

[0045] In a preferred embodiment of the process according to the invention, the fibrous web can be calendered between steps G and H. The fibrous web is passed through at least one roll nip, whereby mechanical pressure is exerted on the fibrous web. Particularly preferably, the number of roll nips through which the fibrous web is passed is at least 2 and at most 14, particularly preferably at least 5 and at most 10. The line load exerted on the fibrous web in all or at least some of the roll nips is preferably at least 20 kN / m and at most 600 kN / m, preferably at least 60 kN / m and at most 400 kN / m. Calendering is helpful for reducing the thickness of the separator and densifying the structure to create smaller pores. If step (e) is performed, calendering preferably takes place between steps (e) and H.

[0046] In a preferred embodiment of the process according to the invention, at least part of the pulp with a high fines content is applied in step (d) by a film press or a coating unit.

[0047] In a preferred embodiment of the process according to the invention, at least a portion of the high-fines pulp is applied in step (e) by printing or spraying. In this preferred embodiment, the high-fines pulp can be applied to one or both sides, and particularly preferably, it is applied to both sides.

[0048] The separator from step H of the process according to the invention is preferably formed from fibrillated fibers of regenerated cellulose to an extent of at least 75% and at most 90% based on the mass of the separator.

[0049] The fibrillatable fibers of regenerated cellulose from step A of the process according to the invention are preferably fibers spun in a solvent (" solvent-spun "), particularly preferred Lyocell ®< fibers.

[0050] The average length of the fibrillatable fibers made of regenerated cellulose in step A of the process according to the invention is at least 2 mm and at most 8 mm, and particularly preferably at least 3 mm and at most 6 mm. The average length of the fibers can be determined according to ISO 16065-2:2014.

[0051] The separator from step H of the process according to the invention is preferably formed from pulp with a high fines content to an extent of at least 5% and at most 20%, based on the mass of the separator.

[0052] The high-fines pulp added in at least one of steps (a) to (e) is produced according to the invention from pulp, wherein the pulp is preferably 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 pulps from different sources can also be used to produce the high-fines pulp. The pulps are particularly preferably obtained from coniferous woods or hardwoods.

[0053] According to the invention, the high-fines pulp from at least one of steps (a) to (e) is characterized in that the proportion of fibers with a length of less than 0.2 mm is at least 70%, based on the total length of the fibers in the high-fines pulp. Preferably, the proportion of fibers with a length of less than 0.2 mm is at least 80%, particularly preferably at least 90%, in each case based on the total length of the fibers in the high-fines pulp. This fines content can be determined using an image analysis method according to ISO 16065-2:2014.

[0054] The high-fines pulp from at least one of steps (a) to (e) may preferably contain secondary fines. Secondary fines are fibers whose length L is less than 100 µm and whose thickness D in µm satisfies the inequality D ≤ 50 − 0,3 ⋅ L where L is to be entered in µm. The proportion of secondary fines in the high-fines pulp is preferably at least 40%, particularly preferably at least 60%, based in each case on the total fiber length in the high-fines pulp. The proportion of secondary fines can also be determined using an image analysis method according to ISO 16065-2:2014. SHORT DESCRIPTION OF THE CHARACTERS

[0055] Fig. 1 shows in the Figures 1a to 1i exemplary fibrillated fibers of regenerated cellulose with a branched structure after dissolving a separator according to the invention in water. Fig. 2 shows in the Figures 2a to 2c Example of fibrillated fibers of regenerated cellulose with a branched structure at the tear-off edge of a separator according to the invention by taking pictures with a light microscope. Fig. 3 shows in the Figures 3a and 3bFor example, fibrillated fibers of regenerated cellulose that do not have a branched structure. DESCRIPTION OF SOME PREFERRED EMBODIMENTS

[0056] In the following, some preferred embodiments of separators according to the invention and of the process according to the invention as well as separators not according to the invention are described as a comparative example.

[0057] An aqueous suspension of 4 mm long fibrillar fibers of regenerated cellulose (Lyocell ®< ) with 1.7 dtex was prepared (step A) and ground in a colloid mill to a freeness of 82 °SR or 93 °SR, measured according to ISO 5267-1:1999. The suspension was then transported to a headbox (step C), where high-fines pulp was added to the headbox (step b). A fiber web was then formed, dried, and rolled up on a paper machine using steps D to H.

[0058] The amounts of regenerated cellulose fibers and high-fines pulp were selected such that the separator was formed from 85% to 100% regenerated cellulose fibers and 0%, 10%, or 15% high-fines pulp, with the percentages referring to the mass of the finished and dried separator. A total of three separators S1, S2, and S3 according to the invention and two non-inventive separators P1 and P2 were produced in this way. A fourth separator S4 according to the invention was produced by calendering S1, and another non-inventive separator P3 was produced by calendering P1.The properties of S1, S2, S3, S4 and P1, P2, P3 are summarized in Tables 1 and 2, where the mass of lyocell fibers, the mass of high fines pulp (HFP), the freeness (DR), the basis weight (BW), the thickness (TH), the tensile strength in the machine direction (TS-MD) and the modulus of elasticity in the machine direction (MoE-MD) are given in Table 1 and for the same separators the porosity (PV), the air permeability (AP), the mean flow pore size (M-PS), the standard deviation of the mean flow pore size (SD-PS) and the flow pore size D 90 (90%-PS) defined above are given in Table 2. Table 1 Lyocell ®< HFP DR BW TH TS-MD MoE-MD % % °SR g / m 2 µm kN / m GPa S1 90 10 93 14,3 34,5 0,82 2,79 S2 90 10 93 15,0 35,1 1,04 2,98 S3 85 15 82 15,6 52,9 0,85 1,85 S4 90 10 93 14,3 18,6 0,67 4,86 P1 100 0 93 13,5 33,9 0,76 2,40 P2 100 0 82 15,5 59,9 0,53 1,29 P3 100 0 93 13,5 19,4 0,65 3,76 Table 2 PV AP M-PS SD-PS 90% horsepower % s nm nm nm S1 72 62,0 175 108 365 S2 72 89,4 S3 80 18,1 340 446 971 S4 45 164,7 109 69 264 P1 73 17,4 275 146 426 P2 83 2,6 879 545 1149 P3 52 55,6 174 79 319

[0059] The air permeability (AP) can serve as a measure of pore size, and it can be seen that the separators S1 and S2 according to the invention, compared to P1, and S3 compared to P2, each exhibit significantly lower air permeability, i.e., higher Gurley values, at comparable porosity. This indicates that the average flow pore size is smaller in the separators according to the invention, which is why these separators are better suited for use in electrochemical cells than the separators not according to the invention.

[0060] In particular, the separator P2 not according to the invention has a very high air permeability, i.e. a low Gurley value, and thus large pores, which is why there is a risk that in an electrochemical element, in particular a lithium-ion battery, dendrites can be formed over time starting from the electrodes with this separator, which reduces the service life and performance of the electrochemical element.

[0061] A comparison of the separators S1, S2 and S3 according to the invention with the non-inventive separators P1 and P2 without high-fines pulp also shows the positive effect of the high-fines pulp on the strength of the separator.

[0062] The pore size distribution of separators S1 and P1 was 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.

[0063] For S1, the mean flow pore size was 175 nm with a standard deviation of the mean flow pore size of approximately 108 nm, while the non-inventive separator P1 had a mean flow pore size of 275 nm with a standard deviation of the mean flow pore size of approximately 146 nm. The pulp with a high fines content in the inventive separator S2 thus results in a smaller mean flow pore size and a pore size distribution with a smaller standard deviation, both of which have a positive effect on the properties of an electrochemical element manufactured from it.

[0064] The Figures 1a to 1i show examples of fibrillated fibers of regenerated cellulose with a length of at least 1 mm and a branched structure, wherein in each of the Figures 1a to 1iThe same reference numerals also refer to similar objects. For this purpose, a separator according to the invention was dissolved in water and images of the fibers were taken using the L&W Fiber Tester Plus from Lorentzen & Wettre. The branched structure is characterized by the fact that several fibrils 12 are bound to the fiber 11, thus forming branches of the fiber 11. In addition, there are also fibrils 13 that are no longer connected to the fiber 11. The length 14 in each of the Figures 1a to 1i 1 mm each and shows that fiber 11 is longer than 1 mm. The Figures 1a to 1i are only an example and fibrillated fibers of regenerated cellulose, as they occur in the separator according to the invention, can also have a substantially different shape, provided that the essential elements 11 and 12 of the branched structure are present and the fibers have a length of at least 1 mm.

[0065] The Figures 2a to 2cshow examples of fibrillated fibers of regenerated cellulose with a length of at least 1 mm and a branched structure, wherein in each of the Figures 2a to 2c The same reference numerals also refer to similar objects. For this purpose, a separator according to the invention was torn into two parts and images of the fibers were taken at the tear-off edge using a light microscope. The branched structure is characterized by the fact that several fibrils 22 are bound to the fiber 21, thus forming branches of the fiber 21. The length 24 in each of the Figures 2a to 2c 200 µm each and shows that fiber 21 is longer than 1 mm. Figures 2a to 2c are only an example and fibrillated fibers of regenerated cellulose, as they occur in the separator according to the invention, can also have a substantially different shape, provided that the essential elements 21 and 22 of the branched structure are present and the fibers have a length of at least 1 mm.

[0066] The Figures 3a and 3b show examples of fibrillated fibers of regenerated cellulose that do not exhibit a branched structure. The images were also taken with the L&W Fiber Tester Plus from Lorentzen & Wettre. The length 34 is in the Figures 3a and 3b 500 µm each. In Figure 3a Examples of fibrillated fibers 35 are shown, which are produced by refining regenerated cellulose. Refining primarily results in fiber shortening. Such fibers do not exhibit a branched structure because the fibrils are not sufficiently detached from the fiber 35. The fibers 35 do not form a sufficiently dense network to retain the fines within the fiber network.

[0067] In Figure 3bExamples of fibrillated fibers 36 are shown, which are produced by intensive refining of regenerated cellulose, whereby the refining has led to a complete separation of the fibrils from the fiber. The fibrillated fibers 36 also do not have a branched structure and are therefore not suitable for forming a sufficiently dense fiber network.

[0068] From the Figures 1 to 3 It is evident that the grinding of the fibres is of great importance for the fibre morphology and only with a suitable choice of grinding method, for example in a colloid mill, can the Figures 1 and 2 represented fibers with a branched structure and a length of at least 1 mm can be obtained in sufficient quantity.

[0069] The inventive separator S1 and the non-inventive separator P1 were also calendered using a calender with 8 roll gaps at a line load of 150 kN / m, thus obtaining another inventive separator S4 from S1 and a non-inventive separator P3 from P1. The pore size distributions of the separators S4 and P3 were determined using capillary flow porosimetry. A comparison of S1 with S4 and P1 with P3 shows that calendering can reduce the average flow pore size, and in parallel, the air permeability also decreases.

[0070] Lithium-ion batteries were produced from the separators S1, S2 and S3 according to the invention, and their basic function was determined, so that the separators are in any case suitable for use in lithium-ion batteries or other electrochemical elements.

[0071] The separators P1 and P2 not according to the invention are also suitable in principle for lithium-ion batteries or other electrochemical elements, but do not have properties as good as those of the separators according to the invention.

Claims

1. Separator for an electrochemical element, wherein at least 70% and at most 95% of the mass of the separator is formed by fibrillated fibers of regenerated cellulose and at least 3% and at most 30% of the mass of the separator is formed by pulp having a high fines content, and wherein in the pulp having a high fines content, the proportion of fibers with a length of less than 0.2 mm is at least 70% with respect to the total length of the fibers in the pulp having a high fines content, characterized in that of the fibrillated fibers of regenerated cellulose with a length of at least 1 mm, at least 10% with respect to number, have a branched structure.

2. Separator according to claim 1, wherein of the fibrillated fibers of regenerated cellulose with a length of at least 1 mm, at least 15%, preferably 20%, each with respect to number have a branched structure, and / or wherein at least 75% and at most 90% of the separator with respect to its mass is formed from fibrillated fibers of regenerated cellulose.

3. Separator according to one of the preceding claims, wherein the fibers of regenerated cellulose are solvent-spun fibers and are in particular formed by Lyocell® fibers, and / or wherein the mean linear density of the 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), preferably at least 1.0 g / 10000 m (1.0 dtex) and at most 2.5 g / 10000 m (2.5 dtex).

4. Separator according to one of the preceding claims, wherein the mean length of the 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, and / or which is formed by at least 5% and at most 20% of pulp having a high fines content with respect to its mass, and / or wherein the pulp having a high fines content is sourced from coniferous woods, deciduous woods or other plants, in particular hemp, flax, jute, ramie, kenaf, kapok, coconut, abaca, sisal, bamboo, cotton or esparto grass, from recycled pulp, or from mixtures of one or more of said sources.

5. Separator according to one of the preceding claims, wherein the proportion of pulp fibers with a length of less than 0.2 mm is at least 80%, preferably at least 90% with respect to the total length of the fibers in the pulp having a high fines content, and / or wherein the pulp having a high fines content is formed by nano-fibrillated pulp or micro-fibrillated pulp.

6. Separator according to one of the preceding claims, wherein the pulp having a high fines content contains secondary fines which are formed by fibers the length L of which in µm is less than 100 and the thickness D in µm of which satisfies the inequality D ≤ 50 − 0.3 ⋅ L , wherein the proportion of secondary fines in the pulp having a high fines content is at least 40%, preferably at least 60%, each with respect to the total length of the fibers in the pulp having a high fines content.

7. Separator according to one of the preceding claims, which further contains one or more components selected from the group consisting of polyvinyl alcohol, polyethylene glycol, polyvinylidene fluoride, guarana, starch, carboxymethyl cellulose, methylcellulose, dialdehydes, in particular glyoxal, and inorganic fillers, in particular kaolin, titanium dioxide (TiO2), silicon dioxide (SiO2), aluminum oxide (Al2O3), zirconium dioxide (ZrO2) or calcium carbonate (CaCO3), and / or which, in addition to said fibrillated fibers of regenerated cellulose and the pulp having a high fines content contains further fibers which are selected from the group consisting of fibers from cellulose derivatives, non-fibrillated fibers from regenerated cellulose, glass fibers and plastic fibers, wherein the plastic fibers are in particular fibers from polyolefins, preferably polyethylene or polypropylene; fibers from polyesters, preferably polyethylene terephthalate or polylactic acids; fibers from polyethers, polysulfones, polyurethanes, polyamides, polyimides, polyvinyl alcohol, polyacrylonitrile, polyphenylene sulfide or from ethylene-vinylacetate co-polymers, wherein the total proportion of these further fibers is preferably at most 10%, particularly preferably at most 5%, of the mass of the separator.

8. Separator according to one of the preceding claims, the thickness of which, determined on a single sheet in accordance with ISO 534:2011, is at least 10 µm and at most 55 µm, preferably at least 12 µm and at most 35 µm, and / or the basis weight of which, determined in accordance with ISO 536:2012, is 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, the porosity µ of which is at least 30% and at most 85%, preferably at least 35% and at most 75%, wherein the porosity µ is determined by 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 to a percentage by multiplying by 100, and / or the mean flow pore size of which, measured by capillary flow porosimetry in accordance with ASTM F316-03(2019), is at least 40 nm and at most 1000 nm, preferably at least 50 nm and at most 800 nm, and / or wherein the standard deviation of the mean flow pore size measured in accordance with ASTM F316-03(2019) is at least 3 nm and at most 200 nm, particularly preferably at least 3 nm and at most 100 nm, and / or which has a value D90 for the distribution of the flow pore size which is at least 100 nm and at most 1500 nm, preferably at least 200 nm and at most 1000 nm, wherein D90 is to be determined such that 90% of the flow is through pores the flow pore sizes of which do not exceed the value D90.

10. Separator according to one of the preceding claims, the tensile strength of which in accordance with ISO 1924-2:2008 in at least one direction is at least 0.3 kN / m and at most 2 kN / m, preferably at least 0.5 kN / m and at most 1.5 kN / m, and / or the elongation at break of which in accordance with ISO 1924-2:2008 in at least one direction is at least 0.5% and at most 4.0%, preferably at least 1.0% and at most 3.5%, and / or the modulus of elasticity of which, determined in a measurement of the force-strain curve in accordance with ISO 1924-2:2008 in at least one direction is at least 1 GPa and at most 8 GPa, preferably at least 2 GPa and at most 6 GPa, and / or the air permeability according to Gurely of which, determined in accordance with ISO 5636-5:2013, is at least 10 s and at most 450 s, preferably at least 40 s and at most 300 s.

11. Electrochemical element which comprises two electrodes, an electrolyte and a separator according to one of claims 1 to 10, wherein the electrochemical element is preferably formed by a capacitor, a hybrid capacitor, a supercapacitor or an accumulator, wherein the electrochemical element is preferably a lithium-ion battery.

12. Process for manufacturing a separator for an electrochemical element, which comprises the following steps, A - manufacturing an aqueous suspension of fibers of regenerated cellulose which can be fibrillated, B - fibrillating the fibers of regenerated cellulose from step A, C - adding the aqueous suspension of fibrillated fibers of regenerated cellulose from step B to a head box, D - applying the aqueous suspension from step C to a running wire to form a fiber web, E - de-watering the fiber web on the running wire, F - drying the fiber web in a first drying device, G - drying the fiber web in a second drying device, H - winding up the separator formed by the fiber web, wherein the fibers of regenerated cellulose in step C are fibrillated such that of the fibers with a length of at least 1 mm, at least 10% of the fibers, with respect to their number, have a branched structure, and wherein the pulp having a high fines content is added in at least one of the following steps, (a) in step A, by addition to the aqueous suspension, (b) in step B, by addition to the head box, (c) in step D, by application to the fiber web formed on the running wire from a further head box, (d) between the steps E and F, by application to the fiber web in an application device, or (e) between steps G and H, by application to the fiber web in an application device, and wherein in the pulp having a high fines content, at least 70% of the fibers with respect to the total length of the fibers have a length of less than 0.2 mm, and wherein at least 70% and at most 95% of the mass of the separator after drying in step G is formed by fibrillated fibers of regenerated cellulose and at least 3% and at most 30% of the mass of the separator is formed by pulp having a high fines content, and wherein the step B is carried out in a colloid mill.

13. Process according to claim 12, wherein the fibrillation in step B is carried out such that the fibers of regenerated cellulose are fibrillated more and cut less, and / or wherein step B is carried out such that the degree of refining according to Schopper Riegler (°SR), measured in accordance with ISO 5267-1: 1999, is at least 70 °SR and at most 100 °SR, preferably at least 80 °SR and at most 95 °SR, and / or wherein the step B is carried out such that in the fibrillated fibers of regenerated cellulose after step B, at least 30% and at most 70%, preferably at least 40% and at most 65% of the total length of the fibers is formed by fibers with a length of less than 0.2 mm, and / or wherein at least the steps C to G are carried out on a paper machine.

14. Process according to one of claims 12 or 13, wherein the fiber web is calendered between steps G and H, wherein the fiber web is passed through at least one nip, preferably at least 2 and at most 14, and particularly preferably through at least 5 and at most 14 nips, wherein mechanical pressure is exerted on the fiber web, and wherein a line load which is exerted on the fiber web in at least a part of the nips is preferably at least 20 kN / m and at most 600 kN / m, preferably at least 60 kN / m and at most 400 kN / m, wherein the calendering is preferably carried out between steps (e) and H.

15. Process according to one of claims 12 to 14, wherein the application of at least a part of the pulp having a high fines content in step (d) is carried out by a film press or a coating device, and / or wherein the application of at least a part of the pulp having a high fines content in step (e) is carried out by printing or spraying, wherein the application of the pulp having a high fines content is preferably on both sides.