Reduced compressibility separator

Abstract

A process for manufacturing a separator for an electrochemical element is presented, comprising the following steps: providing an aqueous suspension comprising fibrillable fibers of regenerated cellulose; fibrillating the fibrillable fibers of regenerated cellulose in the aqueous suspension by milling to a Schopper-Riegler fineness of at least 75°SR and at most 95°SR; flowing the aqueous suspension from the headbox onto a rotating wire of a Fourdrinier paper machine to form a fiber web, wherein the rotational speed of the wire and the speed at which the aqueous suspension flows from the headbox are matched such that the rotational speed of the wire is at least 0.0% and at most 4.0% higher than the said speed at which the aqueous suspension flows from the headbox; calendering the fiber web.wherein at least 80% of the mass of the separator is formed by fibrillated fibers of regenerated cellulose, and the separator has a fiber structure that gives the separator a plastic compressibility of less than 12%.

Description

FIELD OF THE INVENTIONThe invention relates to a separator for electrochemical elements which is formed substantially by fibers of regenerated cellulose and, by virtue of its particular fiber structure, gives an electrochemical element produced therefrom an increased cycle life, and to a method for producing the same.BACKGROUND AND PRIOR ARTAn electrochemical element typically includes at least one positive electrode, a negative electrode, an electrolyte, a separator, a housing, and current collectors. The separator is impregnated with the electrolyte and has the task of electrically separating the two electrodes. However, it should also allow the most unhindered possible flow of ions between the electrodes and from the electrolyte to the electrodes, so that the electrochemical element has favorable properties, in particular rapid charging and the possibility of taking high current intensities.These requirements for the separator mean that it should be as thin as possible so that the path of ions from one electrode to the other through the pores of the separator is short and a high volumetric energy density of the electrochemical element is achieved, and that it should have a high porosity. The pore volume takes up the electrolyte, and therefore a large pore volume is favorable for rapid ion transport. Furthermore, it is favorable if the pores in the separator are formed by a multiplicity of small pores.The separator should be chemically resistant to the electrolyte, since electrochemical elements can be recharged several times and are usually in use for several years. The separator must therefore also be stable in oxidative and reductive environments.For safety reasons, the separator should have good thermal stability in order to limit the risk of fire if the electrochemical element is damaged.In electrochemical elements, especially accumulators, the capacity decreases slowly with each charging and discharging process. After a certain number of charging cycles, the capacity is ultimately so low that the electrochemical element is no longer usable for the intended application. Since the replacement of such elements is complicated and expensive and is in some cases impossible at all, there is an interest in the capacity decreasing as little as possible over many charging cycles, i.e. the cycle life is as high as possible.There is therefore an interest in increasing the cycle life of an electrochemical element as much as possible without degrading other essential properties of the electrochemical element.SUMMARY OF THE INVENTIONThe object of the invention is therefore to provide a separator for electrochemical elements which gives an electrochemical element produced therefrom a high cycle life. Furthermore, the object of the invention is to specify a method for producing a separator according to the invention.This object is achieved by a method for manufacturing an electrochemical element separator according to claim 1, an electrochemical element separator according to claim 16, and an electrochemical element comprising this separator according to claim 45. Advantageous further developments are specified in the dependent claims.The inventors have found that the separator in an electrochemical element can make a substantial contribution to cycle life. The inventors' considerations are based on the finding that the electrodes of electrochemical elements deform during charging and discharging. For example, the porous anode of a lithium ion battery expands upon charging due to the inclusion of lithium ions and shrinks again upon discharging. This effect is particularly pronounced in graphite anodes having a higher proportion of silicon. Due to the usually quite rigid housing of an electrochemical element, the pressure in the interior increases during charging. In particular, the electrodes exert a high pressure on the separator and a portion of the electrolyte is displaced from the separator and the space between the electrodes. During discharge, the pressure falls again, but according to the findings of the inventors, the electrolyte no longer fully returns into the separator and the space between the electrodes, because the deformation of the separator after compression is plastic and therefore no longer fully reversible. This effect can be demonstrated by increased aging in the edge regions of the electrochemical element. During many charging and discharging cycles, the capacity of the electrochemical element thus decreases until it has finally reached the end of its service life.The inventors have now recognized that the separator may help mitigate this effect. In particular, the inventors have recognized that fiber-based separators are particularly well suited for this because they can be provided with a fiber structure that reduces the plastic compressibility of the separator by a specific manufacturing process. Such a separator according to the invention deforms less under the pressure of the electrodes, whereby less electrolyte is pressed out of the space between the electrodes and, after the load, returns for the most part to its original shape, so that it can more completely resume the electrolyte. Thus, the capacity of the electrochemical element decreases less in each charge and discharge cycle, and the cycle life increases.One aspect of the invention relates to a method whereby separators of unusually low compressibility can be made which allow the cycle life of an associated electrochemical element to be increased. The method according to this aspect of the invention comprises the following steps A to F: A - providing an aqueous suspension comprising fibrillated fibers of regenerated cellulose B - fibrillation of the fibrillated fibers of regenerated cellulose in the aqueous suspension of step A by grinding to a freeness according to Schopper-Riegler according to ISO 5267-1:1999 of at least 75°SR and at most 95°SR, C - providing the aqueous suspension comprising the fibrillated fibers of regenerated cellulose from step B in a headbox of a Fourdrinier paper machine, D - flowing the aqueous suspension from step C from the headbox onto a revolving screen of the Fourdrinier paper machine to form a fiber web, wherein the revolving speed at which the screen revolves and a speed, where the speed at which the aqueous suspension flows from the headbox is matched to one another in such a way that the rotational speed of the wire is at least 0.0% and at most 4.0% higher than the said speed at which the aqueous suspension flows from the headbox, E - dewatering of the fibrous web by mechanical pressure and application of heat, F - calendering of the fibrous web in a calender, where the fibrous web passes through at least 3 and at most 5 nips and where the temperature of the rolls forming the nips is at least 105° C. and at most 115° C. and the line load in the nips is at least 150 N / mm and at most 400 N / mm, in order to obtain the separator, wherein at least 80% of the mass of the separator produced in steps A to F are formed by fibrillated fibers of regenerated cellulose, and the separator produced in steps A to F has a fiber structure that imparts a plastic compressibility of less than 12% to the separator at 23° C. and 50% relative humidity, wherein the plastic compressibility is the relative decrease in thickness in percent of a stack of 5 plies of the separator before and after the stack of 5 plies of the separator was subjected to a pressure of 40 MPa in the thickness direction for one minute.The inventors have found that fibrillated fibers of regenerated cellulose are particularly well suited for the separator according to the invention. Lyocell fibres in particular can be fibrillated well for this purpose by grinding in apparatuses as used in the paper industry. The specific manufacturing method of the invention allows the fibrous structure of the separator to be adjusted to have a comparatively low plastic compressibility.The plastic compressibility is determined in this case by first measuring the thickness d 0 of a stack of 5 layers of the separator according to ISO 534:2011. Thereafter, the stack of 5 layers of the separator is subjected to a pressure of 40 MPa in the thickness direction of the stack over its entire surface for one minute in a suitable apparatus, for example, with a ZwickRoell universal testing machine. Directly thereafter, the thickness d 1 of the same stack of 5 layers of the separator is measured again. The entire process is carried out under the climate specified in ISO 187:2022 of 23° C. and 50% relative humidity. From the two thicknesses d 0 and d 1 the plastic compressibility C is calculated by. Plastic compressibility is thus the irreversible relative decrease in thickness of the stack of 5 layers of separator before and after compression, expressed in percent. According to the findings of the inventors, a low value for the plastic compressibility proves to be favorable for the cycle life.The manufacturing method of the invention is based on a specific combination of different features of the manufacturing method. A first feature of this feature combination is the specific freeness of the fibrillated fibers of regenerated cellulose from step B. In this range of freeness, the fibers are substantially fibrillated, but there are still a sufficient number of locations where the fiber bundles are not fibrillated. These thicker parts of the fibers provide for a higher stability of the fiber structure.Another feature is the slight difference between the speed at which the aqueous suspension flows from the headbox onto the rotary screen of the Fourdrinier paper machine in step D and the speed of the rotary screen. The ratio of these speeds is referred to in the art as the wire-to-jet ratio and is usually indicated by the process control system of the Fourdrinier paper machine. Speeds for determining the screen-beam ratio can also be measured, for example, using the μSpeed Laser Encoder System from Elovis GmbH, Germany. This small speed difference results in the fibers being oriented less in the direction of travel of the rotating screen than would be the case with a higher speed difference customary in the prior art. This is of particular importance with regard to the thicker, non-fibrillated parts of the fibers which are present entirely owing to the selected freeness and which, owing to their more random orientation owing to the small speed difference, contribute particularly to reducing the plastic compressibility of the separator.A third feature is finally the settings of the calender. According to the findings of the inventors, the number of roll gaps according to the invention, the line load and in particular the temperature during calendering are chosen to be just so favorable that the fiber structure formed in step D is consolidated but not destroyed. According to the findings of the inventors, the combination of all of these three features in the method according to the invention is especially decisive for the fact that the plastic compressibility of the separator can be lowered in a suitable manner. As will be explained further below with reference to exemplary embodiments, the individual features, on the other hand, are not sufficient per se to produce a separator having the low plastic compressibility according to the invention. Rather, according to the inventors' findings, there is a functional, synergistic relationship between the freeness according to the invention, the particular wire jet ratio, i.e. the difference according to the invention between the speed at which the aqueous suspension flows from the headbox onto the circulating wire, and the circulating speed of the wire and the structure and the settings of the calender with respect to line load in the rolling nips and the temperature of the rolls.The plastic compressibility of the separator produced in steps A to F is preferably at most 10%, particularly preferably at most 9% and very particularly preferably at most 8%. The lower the plastic compressibility, the less electrolyte is displaced during charging of the electrochemical element manufactured from the separator and the more electrolyte returns into the separator during discharging. As a result, the cycle life can be longer. In this respect, the present invention is distinguished by the upper limit of the plastic compressibility, which also defines a significant difference from conventional separators. Downward, the plastic compressibility for the purposes of the invention, in particular the increase of the cycle life, is fundamentally not restricted, but with reasonable cost it is not possible to realize arbitrarily low compressibilitys. In preferred embodiments, the plastic compressibility is therefore at least 1%, preferably at least 2% and in particular at least 2.5%, wherein it should be noted that the upper limit, not the lower limit of the plastic compressibility, is decisive for the function of the invention.Preferably, in the separator produced in steps A to F, the fibrillated fibers constitute regenerated cellulose, at least 90%, more preferably at least 95%, and most preferably at least 99% of the mass of the separator. This is thus the mass-related proportion of the fibers of regenerated cellulose in the finished product. In this context, "fibrillated" means that the fibers have passed through at least one grinding unit which is capable of separating at least some of the fibrils connected in a fiber to form a fiber bundle from one another. This is not important and is also not within the meaning of the invention that actually all the fibers are completely fibrillated, but only that they have passed through this grinding unit and have been fibrillated to the extent necessary to achieve a specific freeness.The fibers should also not be completely fibrillated because it has proven favorable for the desired fiber structure if the fiber bundles remain partially bonded because these thicker parts of the fibers can impart increased stability to the fiber structure with respect to compression. How strongly the fibers are fibrillated can be determined by the freeness according to ISO 5267-1:1999 and expressed in degrees Schopper-Riegler (°SR). Preferably, the freeness of the fibrillated fibers of regenerated cellulose in step B is at least 78° and at most 92°SR, and more preferably at least 80°SR and at most 90°.The linear density of the fibrillated fibers of regenerated cellulose is important for fibrillation of the fibers. Preferably, the average linear density of the fibrillatable fibers of regenerated cellulose in step A is at least 0.6 g / 10000 m (0.6 dtex) and at most 3.0 g / 10000 m (3.0 dtex), and more preferably at least 0.8 g / 10000 m (0.8 dtex) and at most 1.8 g / 10000 m (1.8 dtex).The length of the fibrillated fibers of regenerated cellulose before fibrillation is important above all for the strength of the separator, with longer fibers leading to a higher strength, but also meaning more energy expenditure during fibrillation. Preferably, the average length of the fibrillatable fibers of regenerated cellulose in step A is at least 2 mm and at most 8 mm and particularly preferably at least 3 mm and at most 6 mm.The fibrillated fibers of regenerated cellulose in step A are preferably fibers spun in a solvent.The aqueous suspension in step C may comprise further fibers. These fibers are preferably selected from the group consisting of non-fibrillated fibers of regenerated cellulose, cellulose fibers, nanofibrillated cellulose fibers, microfibrillated cellulose fibers and cellulose fibers having an average length-weighted length of at most 0.20 mm, preferably of at most 0.15 mm, or mixtures thereof.In a preferred embodiment of the method, the provision of the aqueous suspension in step C comprises the addition of nanofibrillated and / or microfibrillated pulp fibers, such that the separator produced in steps A to F contains or consists of fibrillated fibers of regenerated cellulose and nanofibrillated and / or microfibrillated pulp fibers.Preferably, the speed of the rotating wire of the Fourdrinier paper machine in step D is at least 0.3% and at most 4.0% higher than the speed at which the aqueous suspension flows from the headbox, more preferably at least 0.5% and at most 3.5% and most preferably at least 1.0% and at most 2.0%.Dewatering of the fibrous web in step E can be carried out by the processes customary in papermaking and preferably comprises dewatering in the press section and drying in the dryer section of a paper machine, in particular a Fourdrinier paper machine.Preferably, the fiber web passes exactly 4 nips during calendering in step F, and the temperature of the rolls forming the four nips is at least 108°C and at most 112°C, and the line load of the four nips is at least 170 N / mm and at most 350 N / mm.The separator produced in steps A to F preferably has a basis weight of at least 6 g / m 2 and at most 22 g / m 2, more preferably of at least 7 g / m 2 and at most 21 g / m 2 and most preferably of at least 8 g / m 2 and at most 20 g / m 2. The basis weight can be determined according to ISO 536:2019.The separator produced in steps A to F preferably has an average thickness of an individual sheet, according to ISO 534:2011, of at least 8 μm and at most 50 μm, preferably at least 10 μm and at most 45 μm, particularly preferably at least 12 μm and at most 40 μm.The MacMullin number of the separator produced in steps A to F is preferably at least 2 and at most 14, more preferably at least 3 and at most 12 and most preferably at least 4 and at most 10.Preferably, the separator produced in steps A to F has a MacMullin number at 23° C. and 50% relative humidity after compression of a stack of 5 layers of the separator with a pressure of 40 MPa for one minute, which is less than 50%, particularly preferably less than 30% and very particularly preferably less than 25%, higher than the MacMullin number before compression of the separator produced in steps A to F.Further preferred properties of the separator produced in steps A to F may correspond to those as disclosed below in connection with the preferred embodiments of a separator according to a further aspect of the invention, in particular with regard to composition, density, tensile strength, elongation at break, shrinkage, porosity, pore structure and air permeability according to Cucumbery.This further aspect of the invention relates to a separator for an electrochemical element comprising by mass at least 80% fibrillated fibers of regenerated cellulose and having a fiber structure that imparts a plastic compressibility of less than 12% to the separator at 23° C. and 50% relative humidity, wherein the plastic compressibility is the relative decrease in thickness in percent of a stack of 5 layers of the separator before and after the stack of 5 layers of the separator was subjected to a pressure of 40 MPa in the thickness direction for one minute.The low plastic compressibility of the separator according to the invention is decisive according to the findings of the inventors for it imparting a higher cycle life to an electrochemical element produced therefrom than is possible with conventional separators. Such a separator is obtainable in particular by production in a method according to one of the embodiments described above, which allows a separator having the low plastic compressibility according to the invention to be produced. However, the separator according to this aspect of the invention is defined only by the above-mentioned properties, in particular the low plastic compressibility according to the invention, not by the method of its production.The plastic compressibility is preferably particularly low and is at most 10%, particularly preferably at most 9% and very particularly preferably at most 8%.The separator according to the invention comprises fibrillated fibers of regenerated cellulose, wherein at least 80%, preferably at least 90%, particularly preferably at least 95% and very particularly preferably at least 99% of the mass of the separator is formed by fibrillated fibers of regenerated cellulose. Preferably, the freeness of the fibrillated fibers of regenerated cellulose is at least 75° and at most 95°, more preferably at least 78°SR and at most 92°SR and most preferably at least 80°SR and at most 90°.Preferably, the average linear density of the fibrillated fibers of regenerated cellulose before fibrillation is at least 0.6 g / 10000 m (0.6 dtex) and at most 3.0 g / 10000 m (3.0 dtex), and more preferably at least 0.8 g / 10000 m (0.8 dtex) and at most 1.8 g / 10000 m (1.8 dtex).Preferably, the average length of the fibrillated fibers of regenerated cellulose before fibrillation is at least 2 mm and at most 8 mm and more preferably at least 3 mm and at most 6 mm.The fibrillated fibers of regenerated cellulose are preferably fibers spun in a solvent. Lyocell fibres, for example, are particularly well suited for the separator according to the invention.In addition to the fibrillated fibers of regenerated cellulose, the separator according to the invention can also comprise further cellulose fibers.Preferably, the further cellulose fibers can be formed in total or in part by non-fibrillated fibers of regenerated cellulose.The further cellulose fibers can preferably be formed in total or in part by pulp fibers, wherein 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 espartogras, or from waste paper stock. Mixtures of cellulose fibers of different origin can also be used for the production of the separator. Particularly preferably, the pulp fibers are obtained from hardwoods, softwoods or cotton.The pulp fibers are particularly preferably at least partially microfibrillated pulp fibers, nanofibrillated pulp fibers or pulp fibers having an average length-weighted length of at most 0.20 mm, preferably of at most 0.15 mm. These types of pulp fibers are particularly well suited to provide the separator with a small average pore size.The average length-weighted length of the cellulose fibers can be determined by means of optical analysis according to ISO 16065-2:2014.The proportion of further cellulose fibers is preferably in total at least 1% and at most 10%, particularly preferably at least 2% and at most 10% and very particularly preferably at least 3% and at most 7%, in each case based on the mass of the separator.In a preferred embodiment of the separator, at least 1% and at most 10%, particularly preferably at least 2% and at most 10% and very particularly preferably at least 3% and at most 7% of the mass of the separator are formed by nanofibrillated pulp fibers or microfibrilated pulp fibers or pulp fibers having an average length-weighted length of at most 0.20 mm, preferably of at most 0.15 mm.In a very particularly preferred embodiment, the separator contains or consists of fibrillated fibers of regenerated cellulose and nanofibrillated and / or microfibrillated pulp fibers.The separator according to the invention can also contain other fibers in addition to the fibrillated fibers of regenerated cellulose and further optional cellulose fibers. These preferably include fibers made from cellulose derivatives, glass fibers, plastic fibers, such as fibers made from polyolefins, such as polyethylene or polypropylene; from polyesters, such as polyethylene terephthalate or polylactic acids; from polyarylates, such as poly(4-hydroxybenzoic acid-co-6-hydroxy-2-naphthoic acid); from polyethers, polysulfones, polyurethanes, polyamides, aromatic polyamides, such as poly(p-phenylene terephthalamide); polyimides, polyvinyl alcohol, polyacrylates, such as polyacrylonitrile or poly(acrylonitrile-co-methyl acrylate); polyphenylene sulfide or from poly(ethylene-co-vinyl acetate).The separator according to the invention can also preferably comprise filler in addition to the fibrillated fibers of regenerated cellulose. Particularly preferably, the filler is selected from the group consisting of kaolin, titanium dioxide (TiO2), silicon dioxide (SiO2), aluminum oxide (Al2O3), zirconium dioxide (ZrO2), calcium carbonate (CaCO3) and zinc oxide (ZnO) or a mixture thereof, wherein the filler makes up at least 1% and at most 10%, particularly preferably at least 1% and at most 7%, of the mass of the separator.The separator according to the invention can contain further components which the skilled person can choose according to his experience to suit the production process, these include, for example, polyvinyl alcohol, polyethylene glycol, polyvinylidene fluoride, guaran, starch, carboxymethylcellulose, methylcellulose, dialdehydes, such as glyoxal, sizing agents, such as alkyl ketene dimer (AKD) and alkenyl succinic anhydride (ASA), or wet strength agents, such as polyamide-polyamine-epichlorohydrin resin.The separator according to the invention preferably has a weight per unit area of at least 6 g / m 2 and at most 22 g / m 2, particularly preferably of at least 7 g / m 2 and at most 21 g / m 2 and very particularly preferably of at least 8 g / m 2 and at most 20 g / m 2. The basis weight can be determined according to ISO 536:2019. The basis weight influences the thickness of the separator, the strength of the separator, and the material requirement for producing the separator. The weight per unit area has a great influence over the thickness on the performance parameters of an electrochemical element manufactured from the separator and in general the weight per unit area therefore would be desired to be selected as low as possible. The preferred intervals indicated allow a favourable balance between these requirements.The separator according to the invention preferably has an average thickness of an individual sheet, according to ISO 534:2011, of at least 8 μm and at most 50 μm, preferably at least 10 μm and at most 45 μm, particularly preferably at least 12 μm and at most 40 μm. The thickness should be as small as possible, but is limited from below by the requirements with regard to a secure electrical separation of the electrodes and the strength and from above by the expectation of the performance parameters of the electrochemical element produced therefrom. The preferred intervals indicated allow a favourable balance between these requirements.In preferred embodiments, the density of the separator is at least 500 kg / m 3 and at most 1000 kg / m 3, particularly preferably at least 550 kg / m 3 and at most 900 kg / m 3 and very particularly preferably at least 600 kg / m 3 and at most 850 kg / m 3. The density can be determined, for example, by the ratio of the basis weight according to ISO 536:2019, and the average thickness of an individual sheet according to ISO 534:2011. This density is comparatively high, which is conducive to achieving the low plastic compressibility.For the processing of the separator to form an electrochemical element, mechanical properties of the separator are important. These include, for example, the tensile strength and the elongation at break. The mechanical properties depend on the direction in which a sample was taken from the separator. A distinction is usually made between the machine direction, that is to say the direction in which the separator runs through the machine during production, and the transverse direction, which denotes the direction lying in the plane of the separator and being orthogonal to the machine direction.The tensile strength according to ISO 1924-2:2008 in the machine direction of the separator according to the invention is, based on the width, preferably at least 0.3 kN / m and at most 2.0 kN / m, particularly preferably at least 0.4 kN / m and at most 1.5 kN / m. Based on the cross-sectional area, calculated from the width of the test strip and the average thickness of an individual sheet according to ISO 534:2011, the tensile strength in the machine direction of the separator according to the invention is preferably at least 15 MPa and at most 75 MPa, particularly preferably at least 25 MPa and at most 60 MPa.The tensile strength according to ISO 1924-2:2008 in the machine direction of the separator according to the invention is preferably higher than the tensile strength in the transverse direction. Preferably, the ratio of the tensile strength in the machine direction to that in the transverse direction is at least 1.0 and at most 3.0, more preferably at least 1.2 and at most 2.2 and most preferably at least 1.5 and at most 1.8.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%.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, conditioned according to ISO 187:2022, after heating to 150° C. for 3 hours is at least 0.5% and at most 2.0%, particularly preferably at least 0.5% and at most 1.5%.For the properties of an electrochemical element manufactured from the separator according to the invention, the pore structure is of great importance. The pore structure can be characterized by the porosity as a whole and by the pore size distribution.The porosity of a separator is the ratio of the pore volume to the total volume of the separator and is usually expressed as a percentage. The porosity of the separator can be estimated from the average thickness of a single sheet, measured according to ISO 534:2011, the weight per unit area, measured according to ISO 536:2019, the moisture content of the separator, measured according to ISO 287:2017, and the density of the fibers, wherein a density ρ 0 of 1500 kg / m 3 can be selected for the fibers, and ρ denotes the dry density of the separator. Approximately, under these assumptions, the porosity μ can be calculated as the ratio of the pore volume to the total volume of the separator by using, in the above equation, the weight per unit area m in g / m 2, the average thickness of a single sheet d in μm and the moisture content c of the separator in %. Thus, the porosity is obtained as a value between 0 and 1 and can be converted to a percentage by multiplication by 100. The porosity should be as high as possible, but is limited from above primarily by the necessary mechanical strength and the requirement that the pores should be as small as possible. The porosity is preferably at least 35% and at most 75%, particularly preferably at least 40% and at most 70%.The pore structure can be characterized in a simplified manner by the air permeability according to Cucumbery. Air permeability is also a good measure of how quickly the separator can absorb the electrolyte. A high absorption speed is advantageous for productivity in the production of electrochemical elements. The Cucumber air permeability can be determined according to ISO 5636-5:2013 and is preferably at least 2 s and at most 60 s, particularly preferably at least 5 s and at most 50 s and very particularly preferably at least 10 s and at most 35 s, wherein a low value according to Cucumber means a high air permeability.The pore structure of the separator according to the invention is an essential feature that influences the transport of ions through the separator. The pore structure of a separator is usually assessed by the person skilled in the art on the basis of the MacMullin number. This number is the ratio of the specific electric conductance of the pure electrolyte and the specific electric conductance of the electrolyte-impregnated separator in the thickness direction. Indirectly, the MacMullin number thus provides information about the porosity and tortuosity of the pores. High porosity and low tortuosity result in a low MacMullin number, which is generally desirable for separators. The particular fiber structure of the separator according to the invention can favorably influence the MacMullin number. The inventors assume that the production method particularly promotes the formation of pores with low tortuosity, such that the MacMullin number is preferably at least 2 and at most 14, more preferably at least 3 and at most 12 and most preferably at least 4 and at most 10. The MacMullin number is virtually independent of the specific choice of electrolyte and is in this respect a property of the separator as such. For determining the MacMullin number for a specific separator, the same electrolyte as is used in an electrochemical element in which the separator is to be used can expediently be used. For example, an electrolyte consisting of 1 mol / L lithium hexafluorophosphate (LiPF 6) in a 1:1:1 volumetric mixture of ethylene carbonate, dimethyl carbonate and diethyl carbonate may be used in the present disclosure to determine the MacMullin number.A particularly favorable, preferred feature of the separator according to the invention is that the MacMullin number does not increase significantly after the compression of the separator. As a result, even after multiple compressions, the internal resistance of the electrochemical element remains low and the performance parameters of the electrochemical element are more stable over its useful life. A higher internal resistance would lead to higher temperatures during charging and discharging of the electrochemical element, which accelerates the undesired thermal degradation of the electrolyte. Preferably, the separator has a MacMullin number at 23° C. and 50% relative humidity after compression of a stack of 5 layers of the separator at a pressure of 40 MPa for one minute, which is less than 50%, particularly preferably less than 30% and very particularly preferably less than 25% higher than the MacMullin number before compression of the separator.The method for compressing the stack of the 5 layers of the separator is the same as explained for determining the plastic compressibility C.The separator can be used in electrochemical elements.An electrochemical element according to the invention comprises two electrodes, an electrolyte and a separator according to one of the above-described embodiments. Preferably, the electrochemical element is a rechargeable battery and particularly preferably the electrochemical element is a lithium-ion rechargeable battery or a sodium-ion rechargeable battery.BRIEF DESCRIPTION OF THE FIGURESFIG. 1 shows a graph of the course of the capacity over standardized charging and discharging cycles of rechargeable battery cells with separators according to the invention and not according to the invention.DESCRIPTION OF SOME PREFERRED EMBODIMENTS AND COMPARISON WITH NONINVENTIVE EMBODIMENTSSome preferred embodiments of separators according to the invention and separators not according to the invention are described below as a comparative example.Lyocell fibres with a linear density of 1.7 g / 10000 m (1.7 dtex) and a length of 4 mm were used for the separators S1 and S2 according to the invention. The fibers were fibrillated by grinding to a freeness between 80°SR and 89°SR. In some embodiments, nanofibrillated cellulose was added to the fibrillated lyocell fibers such that the separators of the invention consisted of either about 95% fibrillated lyocell fibers and 5% nanofibrillated cellulose or 100% of these fibrillated lyocell fibers. The separators were made on a Fourdrinier paper machine having a basis weight of from about 14.1 g / m 2 to about 15.8 g / m 2 in accordance with the process of the present invention.At this time, in step D, the speed of the revolving wire indicated by the process control system of the Fourdrinier paper machine was about 1.3% higher than the speed of the suspension flowing out of the headbox. The screen-to-jet ratio was thus 1.013.In Step F, the fibrous web was calendered in a calender having 4 nips ("nips") at a temperature of 110°C and a line load of 150 N / mm to about 400 N / mm.For comparison, separators Z 1, Z 2 and Z 3 not according to the invention were produced from the same lyocell fibres, but the settings of the production method were partly chosen outside the intervals according to the invention.Finally, a commercially available fiber-based separator Z 4 not according to the invention and a commercially available film-based separator Z 5 not according to the invention were also used for comparison.The data of the separators are summarized in Table 1, wherein "BW" denotes the basis weight according to ISO 536:2019, "TH" denotes the thickness according to ISO 534:2011, "LY" denotes the proportion of fibrillated lyocell fibers in percent based on the mass of the separator, "NFC" denotes the proportion of nanofibrillated cellulose in percent based on the mass of the separator, and "COMP" denotes the plastic compressibility, which was determined according to the method described above using a stack of 5 layers at a pressure of 40 MPa for one minute. In this case, the pressure is applied over the entire surface of the stack, wherein the pressure force is directed in the thickness direction of the stack. For this measurement, a universal testing machine for tensile and compression tests from ZwickRoell was used. Table 1 - Data of Inventive and Non-Inventive Separators Table 1 - Data of Inventive and Non-Inventive Separatorsg / m 2μm%%%S115,819,710002,8S214,119,29554,5Z114,524,095515,7Z215,420,0100013,8Z315,519,8100014,2Z414,316,316,0Z511,818,812,1The separators S 1 and S 2 according to the invention show that fiber-based separators can be produced with the method according to the invention, the plastic compressibility of which is particularly low. The separator Z1 not according to the invention had the same composition and otherwise comparable properties as the separator S2 according to the invention, but was calendered at a lower line load of 35 N / mm, while S1 was calendered at a line load of 150 N / mm and S2 was calendered at a line load of 400 N / mm.The separator Z2, which is likewise not according to the invention, was produced substantially by the process according to the invention and is similar in its data to the separator S1 according to the invention, but the freeness was not selected between 80° SR and 89° SR as in the separators according to the invention, but somewhat lower at 73° SR.Separator Z3 not according to the invention was essentially produced by an identical method on a Fourdrinier paper machine and is also similar in data to separator S1 according to the invention, but in step D the speed of the revolving wire was 10% higher than the speed of the suspension flowing from the headbox.A comparison of the plastic compressibility of the separators S 1 and S 2 according to the invention with the separators Z 1, Z 2 and Z 3 which are not according to the invention and which are all fibre-based separators and have been produced according to an identical method apart from the described deviations shows that only the combination according to the invention of the freeness in step B, the speed difference in step D and the conditions during calendering in step F leads to separators which also have a particularly low plastic compressibility.The separator Z4 not according to the invention was a commercially available, fiber-based separator whose composition was not analyzed in any more detail, but which had a considerably higher plastic compressibility of 16.0%, as can be considered quite customary for fiber-based separators from the prior art.The separator Z5 not according to the invention was a commercially available, film-based, multilayer separator having a plastic compressibility of 12.1%. It is thus found that fiber-based separators according to the invention are capable of achieving a lower plastic compressibility than film-based separators.Of the two separators S1 and S2 according to the invention, further parameters were determined which are given in Table 2. In Table 2, "POR" means porosity, "PS" means mean pore size, "GUR" means Cucumber air permeability according to ISO 5636-5:2013, "TS-MD" means machine direction tensile strength according to ISO 1924-2:2008, "MM" means MacMullin number before compression, and "MM-C" means MacMullin number after compression of a stack of 5 layers of the separator at 40 MPa for one minute at 23° C. and 50% relative humidity. The measurement of the MacMullin number was carried out according to the method described in "Application Note: Determination of MacMullin Numbers: The Stacking-Method", 05.2021, rhd instruments GmbH & Co. KG. As electrolyte, 1 mol / L lithium hexafluorophosphate (LiPF 6) in a 1:1:1:1 volumetric mixture of ethylene carbonate, dimethyl carbonate and diethyl carbonate was used.These data show that separators S1 and S2 are very well suited as separators in electrochemical elements. In particular, the MacMullin number increases only very little due to the compression, whereas, for example, in the case of film-based separators such as Z5, the MacMullin number can increase from 5 to above 30. Table 2 - Further data of the separators of the invention Table 2 - Further data of the separators of the inventionPOR%4852PSμm0,3860,302GURs12,931,9TS-MDkN / m0,7060,687MM-7,128,24MM-C-7,528,40Battery cells of type 18650 having an initial capacity of approximately 2800 mAh and a rated voltage of 3.6 V were produced from the separators S1 and S2 according to the invention and from the separators Z1, Z2 and Z3 not according to the invention. As electrolyte, 1 mol / L lithium hexafluorophosphate (LiPF 6) in a 1:1:1:1 volumetric mixture of ethylene carbonate, dimethyl carbonate and diethyl carbonate was used. The cells were repeatedly charged and discharged at 23°C in a standard cycle of 1.0C.FIG. 1 shows the curve of the capacity of the cells over 200 standard cycles. In the diagram in FIG. 1, the number of standard cycles is plotted on the x-axis 101 and the capacity of the cell in mAh is plotted on the y-axis 102. Curve 103 shows the capacity curve for separator S 1 according to the invention, and curve 104 shows the capacity curve for separator S 2 according to the invention. The three curves 105 show the capacity profile for the separators Z 1, Z 2 and Z 3 which are not according to the invention. The graph shows that separators S 1 and S 2 according to the invention impart a substantially higher cycle life to the cell, since the capacity decreases significantly less over 200 cycles than in the cells with separators not according to the invention. As a result, the low plastic compressibility of the fiber-based separators of the present invention is an essential factor for prolonging the cycle life of an electrochemical element.References included in the specificationThis list of documents cited by the applicant has been produced in an automated manner and is only included for the better information of the reader. The list is not part of the German patent application or utility model application. The DPMA does not take any adhesion for any faults or omissions.Cited Non-Patent LiteratureISO 5267-1:1999

[0020] ISO 16065-2:2014

[0045] ISO 1924-2:2008 [0057, 0081]ISO 5636-5:2013

[0062] Application Note: Determination of MacMullin Numbers: The Stacking Method", 05.2021, rhd instruments GmbH & Co. KG

[0081]

Claims

A method for producing a separator for an electrochemical element, comprising the following steps A to F: A - providing an aqueous suspension comprising fibrillated fibers of regenerated cellulose B - fibrillation of the fibrillated fibers of regenerated cellulose in the aqueous suspension from step A by grinding to a freeness according to Schopper-Riegler according to ISO 5267-1:1999 of at least 75°SR and at most 95°SR, C - providing the aqueous suspension comprising the fibrillated fibers of regenerated cellulose from step B in a headbox of a Fourdrinier paper machine, D - flowing the aqueous suspension from step C from the headbox onto a revolving screen of the Fourdrinier paper machine to form a fibrous web, wherein the revolving speed at which the screen revolves, and a speed at which the aqueous suspension flows out of the headbox are matched to one another in such a way that the rotational speed of the wire is at least 0.0% and at most 4.0% higher than the said speed at which the aqueous suspension flows out of the headbox, E - dewatering the fibrous web by mechanical pressure and supply of heat, F - calendering the fibrous web in a calender, wherein the fibrous web passes through at least 3 and at most 5 nips and wherein the temperature of the rolls forming the nips is at least 105° C. and at most 115° C. and the line load in the nips is at least 150 N / mm and at most 400 N / mm in order to obtain the separator, wherein at least 80% of the mass of the separator produced in steps A to F is formed by fibrillated fibers of regenerated cellulose, and wherein the separator prepared in steps A to F has a fiber structure that imparts a plastic compressibility of less than 12% to the separator at 23° C. and 50% relative humidity, wherein the plastic compressibility is the relative decrease in thickness in percent of a stack of 5 layers of the separator before and after the stack of 5 layers of the separator was subjected to a pressure of 40 MPa in the thickness direction for one minute.Method according to Claim 1, in which the plastic compressibility of the separator from step F is at most 10%, particularly preferably at most 9% and very particularly preferably at most 8%.The process according to claim 1 or 2, wherein the fibrillated fibers of regenerated cellulose constitute at least 90%, more preferably at least 95%, and most preferably at least 99% of the mass of the separator produced in steps A to F.The process according to any one of the preceding claims, wherein the freeness of the fibrillated fibers of regenerated cellulose in step B is at least 78°SR and at most 92°SR, and preferably at least 80°SR and at most 90°.The method according to any one of the preceding claims, wherein the average linear density of the fibrillated fibers of regenerated cellulose in step A is at least 0.6 g / 10000 m (0.6 dtex) and at most 3.0 g / 10000 m (3.0 dtex), and more preferably at least 0.8 g / 10000 m (0.8 dtex) and at most 1.8 g / 10000 m (1.8 dtex).Process according to any one of the preceding claims, in which the mean length of the fibrillated fibers of regenerated cellulose in step A is at least 2 mm and at most 8 mm, preferably at least 3 mm and at most 6 mm.The method of any preceding claim wherein the fibrillatable regenerated cellulose fibers in step A are solvent spun fibers.The method according to any of the preceding claims, wherein the aqueous suspension in step C comprises further fibers selected from the group consisting of non-fibrillated fibers of regenerated cellulose, pulp fibers, nanofibrillated pulp fibers, microfibrillated pulp fibers and pulp fibers having an average length-weighted length of at most 0.20 mm, preferably at most 0.15 mm, or mixtures thereof.The method of any preceding claim, wherein providing the aqueous suspension in step C comprises adding nanofibrillated and / or microfibrillated pulp fibers such that the separator prepared in steps A-F contains or consists of fibrillated fibers of regenerated cellulose and nanofibrillated and / or microfibrillated pulp fibers.The method according to any one of the preceding claims, wherein the speed of the rotating screen in step D is at least 0.3% and at most 4.0%, preferably at least 0.5% and at most 3.5% and more preferably at least 1.0% and at most 2.0% higher than the speed at which the aqueous suspension flows from the headbox.Method according to any one of the preceding claims, wherein the fibre web passes through exactly 4 nips during calendering in step F, and the temperature of the rolls forming the four nips is at least 108°C and at most 112°C, and the line load of the four nips is at least 170 N / mm and at most 350 N / mm.The method according to any of the preceding claims, wherein the separator produced in steps A to F has a basis weight of at least 6 g / m 2 and at most 22 g / m 2, preferably of at least 7 g / m 2 and at most 21 g / m 2 and particularly preferably of at least 8 g / m 2 and at most 20 g / m 2.Method according to one of the preceding claims, in which the separator produced in steps A to F has an average thickness of an individual sheet, according to ISO 534:2011, of at least 8 μm and at most 50 μm, preferably at least 10 μm and at most 45 μm, and particularly preferably at least 12 μm and at most 40 μm.Method according to any one of the preceding claims, wherein the MacMullin number of the separator produced in steps A to F is at least 2 and at most 14, preferably at least 3 and at most 12 and more preferably at least 4 and at most 10.The method according to any one of the preceding claims, wherein the separator produced in steps A to F has a MacMullin number at 23°C and 50% relative humidity after compression in a stack of 5 layers of the separator at a pressure of 40 MPa for one minute which is less than 50%, preferably less than 30% and particularly preferably less than 25% higher than the MacMullin number before compression.An electrochemical element separator comprising by mass at least 80% fibrillated fibers of regenerated cellulose and having a fiber structure that imparts a plastic compressibility of less than 12% to the separator at 23°C and 50% RH, wherein the plastic compressibility is the relative decrease in thickness in percent of a stack of 5 plies of the separator before and after the stack of 5 plies of the separator was subjected to a pressure of 40 MPa in the thickness direction for one minute.Separator according to claim 16, wherein the plastic compressibility is at most 10%, preferably at most 9% and particularly preferably at most 8%.Separator according to claim 16 or 17, wherein at least 90%, preferably at least 95%, and particularly preferably at least 99% of the mass of the separator is formed by fibrillated fibers of regenerated cellulose.Separator according to any one of claims 16 to 18, wherein the freeness of the fibrillated fibers of regenerated cellulose is at least 75°SR and at most 95°SR, preferably at least 78°SR and at most 92°SR and particularly preferably at least 80°SR and at most 90°.Separator according to any one of claims 16 to 19, wherein the average linear density of the fibrillated fibers of regenerated cellulose before fibrillation is at least 0.6 g / 10000 m (0.6 dtex) and at most 3.0 g / 10000 m (3.0 dtex), and preferably at least 0.8 g / 10000 m (0.8 dtex) and at most 1.8 g / 10000 m (1.8 dtex).Separator according to any one of claims 16 to 20, wherein the mean length of the fibrillated fibers of regenerated cellulose before fibrillation is at least 2 mm and at most 8 mm and preferably at least 3 mm and at most 6 mm.Separator according to any one of claims 16 to 21, wherein the fibrillated fibers of regenerated cellulose are fibers spun in a solvent, in particular Lyocell fibers.The separator according to any one of claims 16 to 22, which comprises further cellulose fibers in addition to the fibrillated fibers of regenerated cellulose.A separator according to claim 23, wherein said further cellulose fibres are formed in whole or in part by non-fibrillated fibres of regenerated cellulose.Separator according to claim 23 or 24, wherein said further cellulose fibers are formed in total or in part by pulp fibers, wherein the pulp fibers are preferably obtained from softwoods, hardwoods or other plants, in particular hemp, flax, jute, ramie, kenaf, kapok, coconut, abaca, sisal, bamboo, cotton or espartogras, or from waste paper stock, or are a mixture of pulp fibers of two or more different of said hercunft.Separator according to claim 25, wherein the pulp fibers are at least partially microfibrillated pulp fibers, nanofibrillated pulp fibers or pulp fibers having an average length-weighted length of at most 0.2 mm, preferably of at most 0.15 mm.Separator according to one of Claims 23 to 26, in which the proportion of said further cellulose fibres totals at least 1% and at most 10%, preferably at least 2% and at most 10% and particularly preferably at least 3% and at most 7%, based in each case on the mass of the separator.Separator according to any one of claims 16 to 27, wherein at least 1% and at most 10%, preferably at least 2% and at most 10% and particularly preferably at least 3% and at most 7% of the mass of the separator are formed by nanofibrillated pulp fibers or microfibrilated pulp fibers or pulp fibers having an average length-weighted length of at most 0.20 mm, preferably of at most 0.15 mm.The separator of any one of claims 16 to 28, wherein the separator is comprised of fibrillated fibers of regenerated cellulose and nanofibrillated and / or microfibrillated pulp fibers.Separator according to one of claims 16 to 28, which contains further fibers selected from the group consisting of fibers from cellulose derivatives, glass fibers, and plastic fibers.Separator according to claim 30, wherein the plastic fibers are selected from the group consisting of fibers of polyolefins, in particular polyethylene or polypropylene; polyesters, in particular polyethylene terephthalate or polylactic acids; polyarylates, in particular poly-(4-hydroxybenzoic acid-co-6-hydroxy-2-naphthoic acid); polyethers; polysulfones; polyurethanes; polyamides; aromatic polyamides, in particular poly-(p-phenylene terephthalamide); polyimides; polyvinyl alcohol; polyacrylates, in particular polyacrylonitrile or poly-(acrylonitrile-co-methyl acrylate); polyphenylene sulfide; and poly-(ethylene-co-vinyl acetate).The separator according to any one of claims 16 to 28, 30 or 31, wherein the separator comprises filler, wherein the filler is preferably selected from the group consisting of kaolin, titanium dioxide (TiO2), silicon dioxide (SiO2), aluminum oxide (Al2O3), zirconium dioxide (ZrO2), calcium carbonate (CaCO3) and zinc oxide (ZnO), or is formed by a mixture of two or more of these fillers, wherein the filler constitutes at least 1% and at most 10%, preferably at least 1% and at most 7% of the mass of the separator.Separator according to any of claims 16 to 32, which has a weight per unit area of at least 6 g / m 2 and at most 22 g / m 2, preferably of at least 7 g / m 2 and at most 21 g / m 2 and particularly preferably of at least 8 g / m 2 and at most 20 g / m 2.Separator according to any one of claims 16 to 33, which has an average thickness of an individual sheet, according to ISO 534:2011, of at least 8 μm and at most 50 μm, preferably at least 10 μm and at most 45 μm, and particularly preferably at least 12 μm and at most 40 μm.Separator according to any one of claims 16 to 34, the density of which is at least 500 kg / m 3 and at most 1000 kg / m 3, preferably at least 550 kg / m 3 and at most 900 kg / m 3 and particularly preferably at least 600 kg / m 3 and at most 850 kg / m 3.Separator according to one of Claims 16 to 35, whose tensile strength according to ISO 1924-2:2008 in the machine direction, based on the width, is at least 0.3 kN / m and at most 2.0 kN / m, preferably at least 0.4 kN / m and at most 1.5 kN / m.Separator according to any one of claims 16 to 36, the tensile strength of which in the machine direction, based on the cross-sectional area, is at least 15 MPa and at most 75 MPa, preferably at least 25 MPa and at most 60 MPa.Separator according to any one of claims 16 to 37, wherein the ratio of the tensile strength according to ISO 1924-2:2008 in the machine direction to that in the transverse direction is at least 1.0 and at most 3.0, preferably at least 1.2 and at most 2.2 and particularly preferably at least 1.5 and at most 1.8.Separator according to one of Claims 16 to 38, the elongation at break of which according to ISO 1924-2:2008 in the machine direction is at least 0.5% and at most 5.0%, preferably at least 1.0% and at most 4.0%.Separator according to any one of claims 16 to 39, the shrinkage of which, conditioned according to ISO 187:2022, after heating to 150°C for 3 hours, is at least 0.5% and at most 2.0%, preferably at least 0.5% and at most 1.5%.Separator according to one of Claims 16 to 40, the porosity μ of which is at least 35% and at most 75%, preferably at least 40% and at most 70%, the porosity μ being calculated according to the formula μ = 1 - 2 3 ( 1 - c 100) ⋅ m d wherein the weight per unit area m measured according to ISO 536:2019, in g / m 2, the average thickness of a single sheet d measured according to ISO 534:2011, in μm, and the moisture content c of the separator measured according to ISO 287:2017, in % are to be used.Separator according to any of claims 16 to 41, the air permeability of which, determined according to ISO 5636-5:2013 according to Cucumber, is at least 2 s and at most 60 s, preferably at least 5 s and at most 50 s and particularly preferably at least 10 s and at most 35 s.Separator according to any one of claims 16 to 42, wherein the MacMullin number is at least 2 and at most 14, preferably at least 3 and at most 12 and more preferably at least 4 and at most 10.The separator according to any one of claims 16 to 43, which, at 23°C and 50% relative humidity after compression in a stack of 5 layers of the separator at a pressure of 40 MPa for one minute, has a MacMullin number which is less than 50%, preferably less than 30% and particularly preferably less than 25% higher than the MacMullin number before compression of the separator.An electrochemical element comprising at least two electrodes, an electrolyte and at least one separator according to any one of claims 16 to 44.The electrochemical element of claim 45, wherein the electrochemical element is an accumulator, preferably a lithium ion accumulator or a sodium ion accumulator.

Images