Separator with reduced compressibility

Abstract

The invention relates to a method for producing a separator for an electrochemical element, having the following steps: providing an aqueous suspension comprising fibrillatable fibers of regenerated cellulose; fibrillating the fibrillatable fibers of regenerated cellulose in the aqueous suspension by grinding to a freeness number, according to the Schopper-Riegler test, of at least 75 °SR and at most 95 °SR; flowing the aqueous suspension from the headbox onto a circulating screen of a Fourdrinier machine in order to form a fiber web, the circulating speed at which the screen circulates and the speed at which the aqueous suspension flows from the headbox being adapted to one another such that the circulating speed of the screen is at least 0.0% and at most 4.0% greater than the aforementioned speed at which the aqueous suspension flows from the headbox; and calendering the fiber web, at least 80% of the mass of the separator being formed by fibrillated fibers of regenerated cellulose, and the separator having a fiber structure which provides the separator with a plastic compressibility of less than 12%.

Description

[0001] Reduced compressibility separator

[0002] AREA OF INVENTION

[0003] The invention relates to a separator for electrochemical elements, which is essentially formed by fibers of regenerated cellulose and which, due to its special fiber structure, gives an electrochemical element made therefrom an increased cycle lifetime, and to a method for its production.

[0004] BACKGROUND AND STATE OF THE ART

[0005] An electrochemical cell typically comprises at least one positive electrode, one negative electrode, an electrolyte, a separator, a casing, and current collectors. The separator is saturated with the electrolyte and serves to electrically separate the two electrodes. At the same time, it should allow as unimpeded a flow of ions as possible between the electrodes and from the electrolyte to the electrodes, so that the electrochemical cell has favorable properties, in particular rapid charging and the ability to draw high currents.

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

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

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

[0009] In electrochemical cells, especially batteries, the capacity gradually decreases with each charge and discharge cycle. After a certain number of charge cycles, the capacity eventually becomes so low that the electrochemical cell is no longer usable for its intended application. Since replacing such cells is complex and expensive, and in some cases even impossible, there is an interest in minimizing the capacity decrease over many charge cycles, thus maximizing the cycle life.

[0010] There is therefore an interest in increasing the cycle life of an electrochemical element as much as possible without impairing other essential properties of the electrochemical element.

[0011] SUMMARY OF THE INVENTION

[0012] The invention is therefore based on the objective of providing a separator for electrochemical elements that gives a long cycle life to an electrochemical element manufactured therefrom. Furthermore, the invention is based on the objective of providing a method for manufacturing a separator according to the invention.

[0013] This problem is solved by a method for producing a separator for electrochemical elements according to claim 1, a separator for electrochemical elements according to claim 16, and an electrochemical element comprising this separator according to claim 45. Advantageous embodiments are specified in the dependent claims.

[0014] The inventors have discovered that the separator in an electrochemical cell can significantly contribute to its cycle life. Their reasoning is based on the understanding that the electrodes of electrochemical cells deform during charging and discharging. For example, the porous anode of a lithium-ion battery expands during charging due to the incorporation of lithium ions and shrinks again during discharge. This effect is particularly pronounced in graphite anodes with a higher silicon content. Due to the typically rigid casing of an electrochemical cell, the internal pressure increases during charging. In particular, the electrodes exert high pressure on the separator, and some of the electrolyte is displaced from the separator and the space between the electrodes.During discharge, the pressure drops again, but according to the inventors, the electrolyte does not fully return to the separator and the space between the electrodes because the separator's deformation after compression is plastic and therefore not completely reversible. This effect can be detected by accelerated aging in the outer regions of the electrochemical cell. Over many charge and discharge cycles, the electrochemical cell's capacity decreases until it finally reaches the end of its service life.

[0015] The inventors have now recognized that the separator can help mitigate this effect. In particular, they have found that fiber-based separators are especially well-suited for this purpose because, through a special manufacturing process, they can be equipped with a fiber structure that reduces the separator's plastic compressibility. Such a separator according to the invention deforms less under the pressure of the electrodes, resulting in less electrolyte being squeezed out of the space between the electrodes, and largely returns to its original shape after the load is removed, allowing it to reabsorb the electrolyte more completely. Consequently, the capacity of the electrochemical cell decreases less in each charge and discharge cycle, and the cycle life increases.

[0016] One aspect of the invention relates to a method by which separators with unusually low compressibility can be produced, which makes it possible to increase the cycle life of an associated electrochemical element. The method according to this aspect of the invention comprises the following steps A to F:

[0017] A - Providing an aqueous suspension containing fibrillable fibers of regenerated cellulose

[0018] B - Fibrillation of the fibrillable fibers of regenerated cellulose in the aqueous suspension from step A by milling to a Schopper-Riegler grade according to ISO 5267-1:1999 of at least 75°SR and at most 95°SR,

[0019] C - Providing the aqueous suspension comprising the fibrillated fibers of regenerated cellulose from step B in a headbox of a Fourdrinier paper machine,

[0020] D - Flow of the aqueous suspension from step C from the headbox onto a rotating wire of the 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 speed at which the aqueous suspension flows from the headbox.

[0021] E - Dewatering of the fiber web by mechanical pressure and the application of heat,

[0022] F - Calendering the fiber web in a calender, wherein the fiber web passes through at least 3 and at most 5 roll gaps, and wherein the temperature of the rolls forming the roll gaps is at least 105°C and at most 115°C, and the line load in the roll gaps is at least 150 N / mm and at most 400 N / mm 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 the separator produced in steps A to F has a fiber structure that gives the separator a plastic compressibility of less than 12% at 23°C and 50% relative humidity, wherein the plastic compressibility is the relative thickness reduction in percent of a stack of 5 layers of the separator before and after the stack of 5 layers of the separator is subjected to a pressure of 40 MPa in the thickness direction for one minute was exposed.

[0023] The inventors have found that fibrillated fibers of regenerated cellulose are particularly well suited for the separator according to the invention. In particular, lyocell fibers can be readily fibrillated for this purpose by milling in devices such as those used in the paper industry. The special manufacturing process of the invention makes it possible to adapt the fiber structure of the separator so that it exhibits a comparatively low plastic compressibility.

[0024] The plastic compressibility is determined by first measuring the thickness d0 of a stack of 5 layers of separator according to ISO 534:2011. The stack of 5 layers of separator is then subjected to a pressure of 40 MPa in the thickness direction of the stack for one minute across its entire surface in a suitable device, for example, a universal testing machine from ZwickRoell. Immediately afterward, the thickness di of the same 5-layer stack of 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. The plastic compressibility C is then calculated from the two thicknesses d0 and di. The plastic compressibility is calculated as the irreversible, relative decrease in the thickness of the stack of 5 layers of separator before and after compression, expressed as a percentage. According to the inventors, a low value for plastic compressibility proves advantageous for cycle life.

[0025] The manufacturing process of the invention is based on a specific combination of various features of the manufacturing process. A first feature of this combination is the specific degree of refining of the fibrillated fibers of regenerated cellulose from step B. In this range of refining, the fibers are largely fibrillated, but there is still a sufficient number of areas where the fiber bundles are not fibrillated. These thicker parts of the fibers ensure greater stability of the fiber structure.

[0026] Another characteristic is the small difference between the speed at which the aqueous suspension flows from the headbox onto the rotating wire of the Fourdrinier paper machine in step D and the speed of the rotating wire. The ratio of these speeds is known in the field as the wire-to-jet ratio and is usually displayed by the Fourdrinier paper machine's process control system. Speeds for determining the wire-to-jet ratio can also be measured, for example, with the pSpeed ​​laser encoder system from Elovis GmbH, Germany. This small speed difference results in the fibers being less aligned with the direction of travel of the rotating wire than would be the case with a larger speed difference, which is typical in the prior art.This is particularly important with regard to the thicker, non-fibrillated parts of the fibers that may be present due to the selected grinding grade, which, due to their more random orientation resulting from the small difference in speed, contribute significantly to reducing the plastic compressibility of the separator.

[0027] A third feature is the calender settings. According to the inventors, the number of roll gaps, the line load, and especially the temperature during calendering are selected so favorably that the fiber structure formed in step D is strengthened but not destroyed. The inventors believe that the combination of all three features in the inventive process is crucial for achieving a suitable reduction in the separator's plastic compressibility. As will be explained below with reference to exemplary embodiments, the individual features alone are insufficient to produce a separator with the low plastic compressibility required by the invention.Rather, according to the inventors' findings, there is a functional, synergistic relationship between the degree of grinding according to the invention, the special sieve-jet ratio, i.e. the difference according to the invention between the speed at which the aqueous suspension flows from the headbox onto the rotating sieve and the rotational speed of the sieve, and the design and settings of the calender with regard to line load in the roll gaps and the temperature of the rolls.

[0028] Preferably, the plastic compressibility of the separator produced in steps A to F is at most 10%, more preferably at most 9%, and most preferably at most 8%. The lower the plastic compressibility, the less electrolyte is displaced when charging the electrochemical element produced from the separator, and the more electrolyte returns to the separator during discharge. This allows for a longer cycle life. In this respect, the present invention is characterized by the upper limit of the plastic compressibility, which also defines a significant difference from conventional separators. For the purposes of the invention, particularly for increasing the cycle life, there is generally no lower limit to the plastic compressibility; however, arbitrarily low compressibilities cannot be achieved with reasonable effort.In preferred embodiments, the plastic compressibility is therefore at least 1%, preferably at least 2% and in particular at least 2.5%, although it should be noted that for the function of the invention the upper limit, not the lower limit, of the plastic compressibility is decisive.

[0029] Preferably, in the separator produced in steps A to F, the fibrillated fibers of regenerated cellulose constitute at least 90%, particularly preferably at least 95%, and most preferably at least 99% of the separator's mass. This is therefore the mass-related proportion of regenerated cellulose fibers in the finished product. "Fibrillated" here means that the fibers have passed through at least one grinding unit capable of at least partially separating the fibrils bound together in a fiber bundle. It is neither necessary nor within the scope of the invention that all fibers are completely fibrillated, but only that they have passed through this grinding unit and are fibrillated to the extent required to achieve a specific degree of refining.

[0030] The fibers should not be completely fibrillated because it has proven advantageous for the desired fiber structure if the fiber bundles remain partially connected, as these thicker fiber portions can impart increased compression resistance to the fiber structure. The degree of fibrillation can be determined by the degree of refining according to ISO 5267-1:1999 and expressed in degrees Schopper-Riegler (°SR). Preferably, the degree of refining of the fibrillated fibers of regenerated cellulose in step B is at least 78°SR and at most 92°SR, and particularly preferably at least 80°SR and at most 90°SR.

[0031] The linear density of the fibrillated fibers of regenerated cellulose is important for the fibrillation of the fibers. Preferably, the mean linear density of the fibrillable 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 particularly preferably at least 0.8 g / 10000 m (0.8 dtex) and at most 1.8 g / 10000 m (1.8 dtex).

[0032] The length of the fibrillated fibers of regenerated cellulose before fibrillation is particularly important for the strength of the separator; longer fibers lead to higher strength but also require more energy for fibrillation. Preferably, the average length of the fibrillable 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.

[0033] The fibrillable fibers of regenerated cellulose in step A are preferably solvent-spun fibers. 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 with a mean length-weighted length of at most 0.20 mm, preferably at most 0.15 mm, or mixtures thereof.

[0034] In a preferred embodiment of the process, providing the aqueous suspension in step C includes adding nanofibrillated and / or microfibrillated cellulose 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 cellulose fibers.

[0035] Preferably, the speed of the circulating screen 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, particularly preferably at least 0.5% and at most 3.5% and most particularly preferably at least 1.0% and at most 2.0%.

[0036] The dewatering of the fiber web in step E can be carried out according to the methods commonly used in papermaking and preferably includes dewatering in the press section and drying in the dryer section of a paper machine, in particular a Fourdrinier paper machine.

[0037] Preferably, during calendering in step F, the fiber web passes through exactly 4 roll gaps, and the temperature of the rolls forming the four roll gaps is at least 108°C and at most 112°C, and the line load of the four roll gaps is at least 170 N / mm and at most 350 N / mm.

[0038] The separator produced in steps A to F preferably has a basis weight of at least 6 g / m². 2 and at most 22 g / m² 2 , particularly preferably of at least 7 g / m³ 2 and at most 21 g / m² 2 and especially preferably of at least 8 g / m² 2 and at most 20 g / m² 2 The basis weight can be determined according to ISO 536:2019.

[0039] The separator produced in steps A to F preferably has a mean thickness of a single sheet, according to ISO 534:2011, of at least 8 pm and at most 50 pm, preferably at least 10 pm and at most 45 pm, and particularly preferably at least 12 pm and at most 40 pm.

[0040] Preferably, the MacMullin number of the separator produced in steps A to F is at least 2 and at most 14, particularly preferably at least 3 and at most 12, and most preferably at least 4 and at most 10. The significance of the MacMullin number of the separator and its determination are explained in more detail below. Preferably, after compression of a stack of 5 layers of the separator at 23°C and 50% relative humidity for one minute, the separator produced in steps A to F has a MacMullin number that is less than 50%, particularly preferably less than 30%, and most preferably less than 25% higher than the MacMullin number before compression.

[0041] Further preferred properties of the separator produced in steps Ab to F may correspond to those 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 Gurley air permeability.

[0042] This further aspect of the invention relates to a separator for an electrochemical element comprising at least 80% fibrillated fibers of regenerated cellulose by mass and having a fiber structure that gives the separator a plastic compressibility of less than 12% at 23°C and 50% relative humidity, wherein the plastic compressibility is the relative thickness reduction in percent of a stack of 5 layers of the separator before and after the stack of 5 layers of the separator has been subjected to a pressure of 40 MPa in the thickness direction for one minute.

[0043] According to the inventors, the low plastic compressibility of the separator according to the invention is crucial for giving an electrochemical element manufactured therefrom a longer cycle life than is possible with conventional separators. Such a separator is particularly obtainable by manufacturing it using a process according to one of the embodiments described above, which allows the production of a separator with the low plastic compressibility according to the invention. However, according to this aspect of the invention, the separator is defined only by the aforementioned properties, in particular the low plastic compressibility according to the invention, and not by the method of its manufacture.

[0044] Preferably, the plastic compressibility is particularly low and amounts to at most 10%, particularly preferably at most 9% and most preferably at most 8%.

[0045] 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 most preferably at least 99% of the separator mass is formed by fibrillated fibers of regenerated cellulose. Preferably, the fineness of the fibrillated fibers of regenerated cellulose is at least 75°SR and at most 95°SR, particularly preferably at least 78°SR and at most 92°SR, and most preferably at least 80°SR and at most 90°SR.

[0046] Preferably, the mean 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 particularly preferably at least 0.8 g / 10000 m (0.8 dtex) and at most 1.8 g / 10000 m (1.8 dtex).

[0047] Preferably, the mean length of the fibrillated fibers of regenerated cellulose before fibrillation is at least 2 mm and at most 8 mm, and particularly preferably at least 3 mm and at most 6 mm.

[0048] The fibrillated fibers of regenerated cellulose are preferably solvent-spun fibers. For example, lyocell fibers are particularly well suited for the separator according to the invention.

[0049] In addition to the fibrillated fibers of regenerated cellulose, the separator according to the invention can also comprise further cellulose fibers.

[0050] Preferably, the additional cellulose fibers can be formed entirely or partially from non-fibrillated fibers of regenerated cellulose.

[0051] Preferably, the additional cellulose fibers can be formed wholly or partially from pulp fibers, the pulp fibers preferably being obtained from softwoods, hardwoods, or other plants such as hemp, flax, jute, ramie, kenaf, kapok, coconut, abaca, sisal, bamboo, cotton, or esparto grass, or from recycled paper pulp. Mixtures of pulp fibers from different sources can also be used for the production of the separator. Pulp fibers obtained from hardwoods, softwoods, or cotton are particularly preferred.

[0052] Particularly preferred are the cellulose fibers, at least partially microfibrillated, nanofibrillated, or with a mean length-weighted length of no more than 0.20 mm, preferably no more than 0.15 mm. These types of cellulose fibers are especially well suited for providing the separator with a small mean pore size.

[0053] The mean length-weighted length of cellulose fibers can be determined by optical analysis according to ISO 16065-2:2014.

[0054] Preferably, the proportion of additional cellulose fibers is at least 1% and at most 10%, particularly preferably at least 2% and at most 10%, and most 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 most preferably at least 3% and at most 7% of the mass of the separator are formed by nanofibrillated cellulose fibers or microfibrillated cellulose fibers or cellulose fibers with a mean length-weighted length of at most 0.20 mm, preferably at most 0.15 mm.

[0055] In a particularly preferred embodiment, the separator contains or consists of fibrillated fibers of regenerated cellulose and nanofibrillated and / or microfibrillated cellulose fibers.

[0056] The separator according to the invention can contain, in addition to the fibrillated fibers of regenerated cellulose and other optional cellulose fibers, other fibers as well. These preferably include fibers made of cellulose derivatives, glass fibers, plastic fibers such as fibers made of polyolefins like polyethylene or polypropylene; of polyesters such as polyethylene terephthalate or polylactic acid; of polyarylates such as poly(4-hydroxybenzoic acid-co-6-hydroxy-2-naphthoic acid); of polyethers, polysulfones, polyurethanes, polyamides, aromatic polyamides such as poly(p-phenylene terephthalamide); polyimides, polyvinyl alcohol, polyacrylates such as polyacrylonitrile or poly(acrylonitrile-co-methyl acrylate); polyphenylene sulfide or poly(ethylene-co-vinyl acetate).

[0057] The separator according to the invention can preferably comprise, in addition to the fibrillated fibers of regenerated cellulose, a filler. The filler is particularly 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 a mixture thereof, wherein the filler constitutes at least 1% and at most 10%, and particularly preferably at least 1% and at most 7%, of the mass of the separator.

[0058] The separator according to the invention may contain further components which the person skilled in the art can select according to their experience to suit the manufacturing process, including, for example, polyvinyl alcohol, polyethylene glycol, polyvinylidene fluoride, guarana, starch, carboxymethylcellulose, methylcellulose, dialdehydes such as glyoxal, sizing agents such as alkyl ketene dimer (AKD) and alkenylsuccinic anhydride (ASA), or wet-strength agents such as polyamide-polyamine-epichlorohydrin resin.

[0059] The separator according to the invention preferably has an basis weight of at least 6 g / m². 2 and at most 22 g / m² 2 , particularly preferably of at least 7 g / m³ 2 and at most 21 g / m² 2 and especially preferably of at least 8 g / m² 2 and at most 20 g / m² 2The basis weight can be determined according to ISO 536:2019. The basis weight influences the separator's thickness, strength, and material requirements. Due to its thickness, the basis weight significantly impacts the performance parameters of an electrochemical cell manufactured from the separator, and therefore, it is generally desirable to keep the basis weight as low as possible. The specified preferred ranges allow for a favorable balance between these requirements.

[0060] The separator according to the invention preferably has a mean thickness of a single sheet, according to ISO 534:2011, of at least 8 pm and at most 50 pm, preferably at least 10 pm and at most 45 pm, and particularly preferably at least 12 pm and at most 40 pm. The thickness should be as small as possible, but is limited from below by the requirements regarding reliable electrical separation of the electrodes and strength, and from above by the expected performance parameters of the electrochemical cell manufactured therefrom. The specified preferred intervals allow for a favorable balance between these requirements.

[0061] In preferred embodiments, the density of the separator is at least 500 kg / m³. 3 and at most 1000 kg / m² 3 , particularly preferably at least 550 kg / m² 3 and at most 900 kg / m² 3 and especially preferably at least 600 kg / m² 3 and at most 850 kg / m² 3The density can be determined, for example, by the ratio of the basis weight according to ISO 536:2019 and the mean thickness of a single sheet according to ISO 534:2011. This density is comparatively high, which is beneficial for achieving low plastic compressibility.

[0062] For the processing of the separator into an electrochemical element, the separator's mechanical properties are important. These include, for example, tensile strength and elongation at break. The mechanical properties depend on the direction in which a sample was taken from the separator. A distinction is usually made between the machine direction, which is the direction in which the separator moves through the machine during manufacturing, and the transverse direction, which is the direction orthogonal to the machine direction and lies in the plane of the separator.

[0063] 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 mean thickness of a single 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.

[0064] 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, particularly preferably at least 1.2 and at most 2.2, and most preferably at least 1.5 and at most 1.8.

[0065] 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%.

[0066] 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%, more preferably at least 0.5% and at most 1.5%.

[0067] The pore structure is of great importance for the properties of an electrochemical element manufactured from the separator according to the invention. The pore structure can be characterized by the overall porosity as well as by the pore size distribution.

[0068] 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 mean thickness of a single sheet, measured according to ISO 534:2011, the basis weight, measured according to ISO 536:2019, the moisture content of the separator, measured according to ISO 287:2017, and the density of the fibers, where the density p0 for the fibers is 1500 kg / m³. 3 can be chosen, and p denotes the dry density of the separator. Under these assumptions, the porosity p can be approximated as the ratio of the pore volume to the total volume of the separator by to be calculated, where the basis weight m in g / m² is used in the above equation. 2The mean thickness of a single sheet, d, in pm, and the moisture content, c, of the separator in % are to be used. This yields the porosity as a value between 0 and 1, which can be converted into a percentage by multiplying by 100. The porosity should be as high as possible, but is primarily limited by the necessary mechanical strength and the requirement that the pores should be as small as possible. Preferably, the porosity is at least 35% and at most 75%, and particularly preferably at least 40% and at most 70%. The pore structure can be characterized, in simplified terms, by the Gurley air permeability. The air permeability is also a good measure of how quickly the separator can absorb the electrolyte. A high absorption rate is advantageous for productivity in the production of electrochemical cells.The Gurley air permeability can be determined according to ISO 5636-5:2013 and is preferably at least 2 s and at most 60 s, particularly preferably at least 5 s and at most 50 s and most preferably at least 10 s and at most 35 s, where a lower Gurley value means a high air permeability.

[0069] The pore structure of the separator according to the invention is an essential feature that influences the transport of ions through the separator. Those skilled in the art typically assess the pore structure of a separator using the MacMullin number. This number is the ratio of the specific electrical conductivity of the pure electrolyte to the specific electrical conductivity 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 special fiber structure of the separator according to the invention can favorably influence the MacMullin number.The inventors assume that the manufacturing process particularly promotes the formation of pores with low tortuosity, such that the MacMullin number is preferably at least 2 and at most 14, particularly preferably at least 3 and at most 12, and most particularly preferably at least 4 and at most 10. The MacMullin number is practically independent of the specific choice of electrolyte and is therefore a property of the separator itself. To determine the MacMullin number for a specific separator, it is advantageous to use the same electrolyte as is used in an electrochemical cell in which the separator is to be used. Within the scope of the present disclosure, for example, an electrolyte consisting of 1 mol / L lithium hexafluorophosphate (LiPF₆) can be used. (l ) in a volumetric 1:1:1 mixture of ethylene carbonate, dimethyl carbonate and diethyl carbonate to determine the MacMullin number.

[0070] A particularly advantageous and preferred feature of the separator according to the invention is that the MacMullin number does not increase significantly after compression of the separator. This ensures that the internal resistance of the electrochemical cell remains low even after repeated compression, and that the performance parameters of the electrochemical cell are more stable over its service life. A higher internal resistance would lead to higher temperatures during charging and discharging of the electrochemical cell, which would accelerate the undesirable thermal degradation of the electrolyte. Preferably, 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, the separator exhibits a MacMullin number that is less than 50%, more preferably less than 30%, and most preferably less than 25% higher than the MacMullin number before compression.

[0071] The procedure for compressing the stack of the 5 layers of the separator is the same as that explained for determining the plastic compressibility C.

[0072] The separator can be used in electrochemical elements.

[0073] An electrochemical element according to the invention comprises two electrodes, an electrolyte, and a separator according to one of the embodiments described above. Preferably, the electrochemical element is a battery, and particularly preferably, the electrochemical element is a lithium-ion battery or a sodium-ion battery.

[0074] BRIEF DESCRIPTION OF THE FIGURES

[0075] Figure 1 shows in a diagram the course of the capacity over standardized charge and discharge cycles of accumulator cells with separators according to the invention and those not according to the invention.

[0076] DESCRIPTION OF SOME PREFERRED EXECUTIONS AND COMPARISON WITH NON-INDUCTIVE EXECUTIONS

[0077] In the following, some preferred embodiments of separators according to the invention, as well as separators not according to the invention, are described as a comparative example.

[0078] For the separators Si and S2 according to the invention, lyocell fibers with a linear density of 1.7 g / 10,000 m³ (1.7 dtex) and a length of 4 mm were used. The fibers were fibrillated by milling them to a fineness between 80°SR and 89°SR. In some embodiments, nanofibrillated cellulose was added to the fibrillated lyocell fibers, so that the separators according to the invention consisted either of approximately 95% fibrillated lyocell fibers and 5% nanofibrillated cellulose or of 100% of these fibrillated lyocell fibers. The separators were produced on a Fourdrinier paper machine with a basis weight of approximately 14.1 g / m². 2up to approximately 15.8 g / m² 2 produced according to the inventive method.

[0079] In step D, the speed of the rotating wire, as displayed by the process control system of the Fourdrinier paper machine, was approximately 1.3% higher than the speed of the suspension flowing from the headbox. The wire-to-jet ratio was therefore 1.013. In step F, the fiber web was calendered in a calender with four nips at a temperature of no°C and a line load of 150 N / mm² to approximately 400 N / mm².

[0080] For comparison, non-inventive separators Zi, Z2 and Z3 were produced from the same lyocell fibers, but the settings of the manufacturing process were partially chosen outside the intervals according to the invention.

[0081] Finally, a non-inventive, commercially available, fiber-based separator Z4 and a non-inventive, commercially available, film-based separator Z5 were also used for comparison.

[0082] The separator data are summarized in Table 1, where "BW" represents the basis weight according to ISO 536:2019, "TH" the thickness according to ISO 534:2011, "LY" the percentage of fibrillated lyocell fibers based on the mass of the separator, "NFC" the percentage of nanofibrillated cellulose based on the mass of the separator, and "COMP" the plastic compressibility, which was determined using the previously described method on a stack of 5 layers at a pressure of 40 MPa for one minute. The pressure is applied over the entire surface of the stack, with the compressive force directed in the thickness direction of the stack. A ZwickRoell universal tensile and compression testing machine was used for this measurement.

[0083] Table 1 - Data for separators according to the invention and those not according to the invention

[0084] The separators Si and S2 according to the invention demonstrate that fiber-based separators with particularly low plastic compressibility can be produced using the inventive method. The non-inventive separator Zi had the same composition and otherwise comparable properties as the inventive separator S2, but was calendered at a lower line load of 35 N / mm², while Si was calendered at a line load of 150 N / mm² and S2 at a line load of 400 N / mm².

[0085] The separator Z2, which is also not according to the invention, was essentially manufactured according to the inventive method and is similar in its data to the separator Si according to the invention, but the grinding degree was not chosen between 80°SR and 89°SR as with the separators according to the invention, but was somewhat lower at 73°SR.

[0086] The non-inventive separator Z3 was produced essentially according to an identical process on a Fourdrinier paper machine and is also similar in data to the inventive separator Si, but in step D the speed of the circulating screen was 10% higher than the speed of the suspension flowing from the headbox.

[0087] A comparison of the plastic compressibility of the separators Si and S2 according to the invention with the non-inventive separators Zi, Z2 and Z3, which are all fiber-based separators and, apart from the described deviations, were produced according to an identical process, shows that only the inventive combination of the grinding degree in step B, the speed difference in step D and the conditions during calendering in step F leads to separators that also have a particularly low plastic compressibility.

[0088] The non-inventive separator Z4 was a commercially available, fiber-based separator whose composition was not analyzed in detail, but which had a considerably higher plastic compressibility of 16.0%, which can be considered quite common for fiber-based separators from the prior art.

[0089] The non-inventive separator Z5 was a commercially available, film-based, multilayer separator with a plastic compressibility of 12.1%. This demonstrates that fiber-based separators according to the invention are capable of achieving a lower plastic compressibility than film-based separators.

[0090] Further parameters were determined for the two separators Si and S2 according to the invention, which are given in Table 2. In Table 2, “POR” represents the porosity, “PS” the mean pore size, “GUR” the Gurley air permeability according to ISO 5636-5:2013, “TS-MD” the tensile strength in the machine direction according to ISO 1924-2:2008, “MM” the MacMullin number before compression, and “MM-C” the MacMullin number after compression of a stack of 5 layers of the separator with 40 MPa for one minute at 23°C and 50% relative humidity. The MacMullin number was measured according to the method described in “Application Note: Determination of MacMullin Numbers: The Stacking-Method”, 05.2021, rhd instruments GmbH & Co. KG. Lithium hexafluorophosphate (LiPF₆) was used as the electrolyte at a concentration of 1 mol / L. (l ) used in a volumetric 1:1:1 mixture of ethylene carbonate, dimethyl carbonate and diethyl carbonate.

[0091] These data show that Si and S2 separators are very well suited as separators in electrochemical elements. In particular, the MacMullin number increases only very slightly due to compression, whereas, for example, with film-based separators such as Z5, the MacMullin number can increase from 5 to over 30.

[0092] Table 2 - Further data on the separators according to the invention

[0093] Accumulator cells of type 18650 with an initial capacity of approximately 2800 mAh and a nominal voltage of 3.6 V were produced from the separators Si and S2 according to the invention, as well as from the separators Zi, Z2 and Z3 not according to the invention. The electrolyte used was 1 mol / L lithium hexafluorophosphate (LiPF₆). (l ) in a volumetric 1:1:1 mixture of ethylene carbonate, dimethyl carbonate, and diethyl carbonate. The cells were repeatedly charged and discharged at 23°C in a standard cycle with i,oC.

[0094] Figure 1 shows the capacity curve of the cells over 200 standard cycles. In the diagram in Figure 1, the number of standard cycles is plotted on the x-axis 101 and the cell capacity in mAh on the y-axis 102. Curve 103 shows the capacity curve for the separator Si according to the invention, and curve 104 shows the capacity curve for the separator S2 according to the invention. The three curves 105 show the capacity curve for the separators Zi, Z2, and Z3 not according to the invention. The diagram shows that the separators Si and S2 according to the invention give the cell a significantly longer cycle life, since the capacity decreases considerably less over 200 cycles than in the cells with the separators not according to the invention. This indicates that the low plastic compressibility of the fiber-based separators according to the invention is a key factor in extending the cycle life of an electrochemical cell.

Claims

Claims i. Method for manufacturing a separator for an electrochemical element, comprising the following steps A to F: A - Providing an aqueous suspension containing fibrillable fibers of regenerated cellulose B - Fibrillation of the fibrillable fibers of regenerated cellulose in the aqueous suspension from step A by milling to a Schopper-Riegler grade 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 - Flow of the aqueous suspension from step C from the headbox onto a rotating wire of the 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 speed at which the aqueous suspension flows from the headbox. E - Dewatering of the fiber web by mechanical pressure and the application of heat, F - Calendering the fiber web in a calender, wherein the fiber web passes through at least 3 and at most 5 roll gaps, and wherein the temperature of the rolls forming the roll gaps is at least 105°C and at most 115°C, and the line load in the roll gaps is at least 150 N / mm and at most 400 N / mm 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 produced in steps A to F has a fiber structure which gives the separator a plastic compressibility of less than 12% at 23°C and 50% relative humidity, wherein the plastic compressibility is the relative thickness reduction in percent of a stack of 5 layers of the separator before and after the stack of 5 layers of the separator is subjected to a pressure of 40 MPa in the thickness direction for was suspended for one minute.

2. The method according to claim 1, wherein the plastic compressibility of the separator from step F is at most 10%, particularly preferably at most 9% and most preferably at most 8%.

3. Method according to claim 1 or 2, wherein the fibrillated fibers of regenerated cellulose constitute at least 90%, particularly preferably at least 95% and most preferably at least 99% of the mass of the separator produced in steps A to F.

4. Method according to any of the preceding claims, wherein the degree of refining 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°SR.

5. A method according to any of the preceding claims, wherein the mean linear density of the fibrillable 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 particularly preferably at least 0.8 g / 10000 m (0.8 dtex) and at most 1.8 g / 10000 m (1.8 dtex).

6. Method according to any of the preceding claims, wherein the mean length of the fibrillable fibers of regenerated cellulose in step A is at least 2 mm and at most 8 mm and preferably at least 3 mm and at most 6 mm.

7. Method according to any of the preceding claims, wherein the fibrillable fibers of regenerated cellulose in step A are fibers spun in a solvent.

8. A 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, cellulose fibers, nanofibrillated cellulose fibers, microfibrillated cellulose fibers and cellulose fibers with a mean length-weighted length of at most 0.20 mm, preferably at most 0.15 mm, or mixtures thereof.

9. A method according to any of the preceding claims, wherein the provision of the aqueous suspension in step C comprises the addition of nanofibrillated and / or microfibrillated cellulose 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 cellulose fibers.

10. Method according to one of the preceding claims, wherein the speed of the circulating sieve in step D is at least 0.3% and at most 4.0%, preferably at least 0.5% and at most 3.5% and particularly preferably at least 1.0% and at most 2.0% higher than the rate at which the aqueous suspension flows out of the headbox.

11. Method according to one of the preceding claims, wherein the fiber web passes through exactly 4 roll gaps during calendering in step F and the temperature of the rolls forming the four roll gaps is at least 108°C and at most 112°C and the line load of the four roll gaps is at least 170 N / mm and at most 350 N / mm.

12. Method according to one of the preceding claims, wherein the separator produced in steps A to F has an basis weight of at least 6 g / m². 2 and at most 22 g / m² 2 , preferably of at least 7 g / m³ 2 and at most 21 g / m²2 and especially preferably of at least 8 g / m² 2 and at most 20 g / m² 2 exhibits.

13. Method according to any of the preceding claims, wherein the separator produced in steps A to F has a mean thickness of a single sheet, according to ISO 534:2011, of at least 8 pm and at most 50 pm, preferably at least 10 pm and at most 45 pm, and particularly preferably at least 12 pm and at most 40 pm.

14. Method according to any 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 particularly preferably at least 4 and at most 10.

15. A method according to any of the preceding claims, wherein the separator produced in steps A to F, after compression in a stack of 5 layers of the separator at 23°C and 50% relative humidity at a pressure of 40 MPa for one minute, exhibits a MacMullin number that is less than 50%, preferably less than 30% and particularly preferably less than 25% higher than the MacMullin number before compression.

16. Separator for an electrochemical element comprising at least 80% fibrillated fibers of regenerated cellulose by mass and having a fiber structure that gives the separator a plastic compressibility of less than 12% at 23°C and 50% relative humidity, wherein the plastic compressibility is the relative thickness reduction in percent of a stack of 5 layers of the separator before and after the stack of 5 layers of the separator has been subjected to a pressure of 40 MPa in the thickness direction for one minute. 17- Separator according to claim 16, wherein the plastic compressibility is at most 10%, preferably at most 9% and particularly preferably at most 8%.

18. 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.

19. Separator according to any one of claims 16 to 18, wherein the degree of refining 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°SR.

20. Separator according to any one of claims 16 to 19, wherein the mean linear density of the fibrillated fibers of regenerated cellulose prior to 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).

21. 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.

22. Separator according to any one of claims 16 to 21, wherein the fibrillated fibers are regenerated cellulose spun fibers in a solvent, in particular lyocell fibers.

23. Separator according to one of claims 16 to 22, comprising further cellulose fibers in addition to the fibrillated fibers of regenerated cellulose.

24. Separator according to claim 23, wherein the aforementioned further cellulose fibers are formed wholly or partly by non-fibrillated fibers of regenerated cellulose.

25. Separator according to claim 23 or 24, wherein the further cellulose fibers mentioned are formed wholly or partly by pulp fibers, wherein the pulp fibers are preferably obtained from coniferous trees, deciduous trees or other plants, in particular hemp, flax, jute, ramie, kenaf, kapok, coconut, abaca, sisal, bamboo, cotton or esparto grass, or from recycled paper pulp, or are a mixture of pulp fibers of two or more different sources mentioned.

26. Separator according to claim 25, wherein the cellulose fibers are at least partially microfibrillated cellulose fibers, nanofibrillated cellulose fibers or cellulose fibers with a mean length-weighted length of at most 0.2 mm, preferably at most 0.15 mm.

27. Separator according to any one of claims 23 to 26, wherein the proportion of the aforementioned further cellulose fibers is 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%, in each case based on the mass of the separator.

28. 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 cellulose fibers or microfibrillated cellulose fibers or cellulose fibers with a mean length-weighted length of at most 0.20 mm, preferably of at most 0.15 mm.

29. Separator according to any one of claims 16 to 28, wherein the separator consists of fibrillated fibers of regenerated cellulose and nanofibrillated and / or microfibrillated cellulose fibers.

30. Separator according to any one of claims 16 to 28, comprising further fibers selected from the group consisting of fibers made of cellulose derivatives, glass fibers, and plastic fibers.

31. Separator according to claim 30, wherein the plastic fibers are selected from the group consisting of fibers made of polyol olefins, 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).

32. Separator according to 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. 33- Separator according to one of claims 16 to 32, having an basis weight of at least 6 g / m² 2 and at most 22 g / m² 2 , preferably of at least 7 g / m³ 2 and at most 21 g / m² 2 and especially preferably of at least 8 g / m² 2 and at most 20 g / m² 2 exhibits.

34. Separator according to any one of claims 16 to 33, having a mean thickness of a single sheet, according to ISO 534:2011, of at least 8 pm and at most 50 pm, preferably at least 10 pm and at most 45 pm, and particularly preferably at least 12 pm and at most 40 pm.

35. Separator according to one of claims 16 to 34, the density of which is at least 500 kg / m³ 3 and at most 1000 kg / m² 3 , preferably at least 550 kg / m² 3 and at most 900 kg / m² 3 and especially preferably at least 600 kg / m² 3 and at most 850 kg / m² 3 amounts.

36. Separator according to any one of claims 16 to 35, the tensile strength of which, 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.

37. Separator according to any one of claims 16 to 36, the tensile strength in the machine direction, based on the cross-sectional area, being at least 15 MPa and at most 75 MPa, preferably at least 25 MPa and at most 60 MPa.

38. 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.

39. Separator according to any 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%.

40. 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%.

41. Separator according to any one of claims 16 to 40, the porosity p of which is at least 35% and at most 75%, preferably at least 40% and at most 70%, wherein the porosity p is determined according to the formula to be calculated, where the basis weight m measured according to ISO 536:2019 is in g / m² 2 , the mean thickness of a single sheet d in µm as measured according to ISO 534:2011, and the moisture content c of the separator in % as measured according to ISO 287:2017 are to be used.

42. Separator according to any one of claims 16 to 41, the air permeability of which, as determined according to ISO 5636-5:2013 according to Gurley, 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.

43. 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 particularly preferably at least 4 and at most 10.

44. Separator according to any one of claims 16 to 43, which, after compression in a stack of 5 layers of the separator at 23°C and 50% relative humidity for one minute, exhibits a MacMullin number that is less than 50%, preferably less than 30% and particularly preferably less than 25% higher than the MacMullin number before compression of the separator.

45. Electrochemical element comprising at least two electrodes, an electrolyte and at least one separator according to any one of claims 16 to 44.

46. ​​Electrochemical element according to claim 45, wherein the electrochemical element is a battery, preferably a lithium-ion battery or a sodium-ion battery.

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