Three-dimensional metal fiber network and manufacturing method

Abstract

A three-dimensional (3D) network of metal fibers comprising a plurality of metal fibers fixed to one another, the plurality of metal fibers comprising a first type of metal fibers and a second type of metal fibers, the first type of metal fibers and the second type of metal fibers being different from one another, in particular with respect to mechanical properties.

Description

[Technical Field]

[0001] The present invention relates to a three-dimensional (3D) network of metal fibers and a method for producing the network of metal fibers. [Background technology]

[0002] 3D structures such as metal fiber networks have advantageous properties in several technical fields, such as filters and electrodes, especially battery electrodes. In particular, it has been found that ultrafine fibers, such as microfibers, having thicknesses and / or widths in the range of 50 μm or less, offer a very large surface-to-volume ratio and corresponding surface area, which is advantageous for electrode applications. Furthermore, if such fine fibers have a non-circular cross section, e.g., a ribbon-like cross section, the surface area is even greater. It has also been found that in filter applications, such networks using fibers with non-circular cross sections, e.g., ribbon-like cross sections, have better filtration properties, likely due to the turbulence caused by the shape of the fibers with such non-circular cross sections.

[0003] To produce such a 3D network, metal fibers can be laid into a nonwoven fabric with high electrical conductivity and mechanical strength using various processes (dry and wet) known to those skilled in the art. To bond such nonwoven fabrics, processes such as sintering at temperatures close to or above the melting point are used. The production of such metal fiber networks is known to those skilled in the art, for example, from International Publication No. WO 2020 / 016240 (A1), the entire contents of which are incorporated herein by reference. Metal fibers can be produced by melt spinning, as described, for example, in International Publication No. WO 2016 / 020493 (A1) and International Publication No. WO 2020 / 229400 (A1), the entire contents of which are incorporated herein by reference.

[0004] However, shrinkage of the network thickness (z-axis) can occur, especially when these networks are based on fine fibers. Also, such nonwoven networks, when made from relatively short and thin fibers, cannot be handled without very careful precautions to prevent them from falling apart or shrinking the web thickness (z-axis) during sintering or coating with electrode active material, etc. This hampers roll-to-roll processes for producing 3D networks of metal fine fibers.

[0005] Furthermore, even after completing the 3D network by sintering the nonwoven metal fibers together, the resulting network may have problems with mechanical stability against compression. Particularly for microfiber networks with thicknesses and / or widths in the 50 μm or less range, even after sintering the metal fibers together, the stability is not sufficient for large-scale production processes, such as roll-to-roll processes, to fill the metal fiber network with electrode active material.

[0006] The 3D network of metal fibers with conductive interconnects used as the electron current collector in lithium-ion batteries allows for very thick electrodes with high active material loading. Such thick electrodes can reach capacities of 100 mAh / cm. 2 Such electrodes constructed with a 3D metal fiber network not only allow for high current flow toward the power socket in three dimensions, but also allow for high current flow in terms of ion mobility within the electrode itself (as described in unpublished patent application PCT / EP2021 / 062443, filed May 11, 2021). This is particularly interesting when the electrode is a battery electrode and the battery is subjected to rapid charge or discharge cycles.

[0007] Especially when using ultra-fine fibers, each fiber can only handle a certain current due to its small conductor cross-section. If the current is too high, individual fibers may be destroyed. Repeated use can cause more and more damage to the metal fiber network, which can gradually reduce its functionality. Furthermore, when used in batteries, high current loads can cause high temperatures in individual fibers. This can lead to degradation of the active material and / or electrolyte. Summary of the Invention [Problem to be solved by the invention]

[0008] Therefore, existing network structures have a limited lifespan when repeatedly exposed to electric current, such as in battery electrodes, and when manufacturing such metal fiber networks or filling such metal fiber networks with electrode active materials, there is a problem in that the metal fiber networks are brittle and difficult to process using a roll-to-roll process or the like. [Means for solving the problem]

[0009] This problem is solved by the present application by providing a three-dimensional network structure as set forth in claim 1. In particular, the three-dimensional (3D) network of metal fibers comprises a plurality of metal fibers fixed to one another, said plurality of metal fibers (14) comprising a first type of metal fibers and a second type of metal fibers, the first type and the second type of metal fibers (14) differing from one another, in particular with regard to mechanical properties.

[0010] According to the present invention, the 3D network comprises a first type of metal fiber and a second type of metal fiber, the first and second types of metal fibers being different from each other. The first type can be considered a stabilizing type of fiber, while the second type can be considered a functional type of fiber. The second type can have a higher surface area per weight of metal fiber compared to the first type. This means that the first type of metal fiber can also provide desired functionality, such as available surface area for electrochemical processes or a filtering effect, albeit to a lesser extent.

[0011] Due to the use of different types of metal fibers, one type of metal fiber, i.e., the first type mentioned above, can function as a mechanical stabilizing component and as a more elastic conductor along the thickness of the 3D network of the second type of metal fibers, most importantly in the z-axis direction. In particular, the first type of metal fibers and the second type of metal fibers form an integral network of metal fibers, meaning that the first type of metal fibers and the second type of metal fibers are preferably sintered directly to each other to form a single 3D network containing both types of metal fibers. In other words, the first type of metal fibers and the second type of metal fibers do not form separate networks. Furthermore, due to the use of different types of metal fibers, they constitute a hierarchically structured network, in which thin fibers form a fine mesh and are supported by stronger fibers, for example, to collect current within a small volume element, while thicker fibers ensure further transport of locally collected electrons by maintaining a higher current due to their higher conductive cross-section, thereby functioning as a conductive path not only in the x-y plane of the network but also in the z-axis of the porous network of metal fibers. As a result, the use of two different types of metal fibers allows for even further reduction of the electrical load on each individual fiber: one type of metal fiber provides a current path in the z-axis direction, while the second layer collects the current in the xy-plane.

[0012] In addition to providing electrical stabilization, a mixture of two different fiber types protects the metal fiber network from undesired densification in the z-direction, particularly when the metal fibers are sintered together, as well as when the 3D network is filled with active material to prepare a functional electrode for a Li-ion battery. It has been observed that combining two different fiber types, particularly during sintering of metal fibers to form a network, results in thickness shrinkage, i.e., shrinkage in the z-axis, compared with using only one type of metal fiber, particularly when these metal fibers have a thickness and / or width of 80 μm or less. Such shrinkage can be reduced or even completely avoided by using two different types of metal fibers. In this way, one of the two types, i.e., the first type mentioned above, acts as a mechanical stabilizer and prevents shrinkage. When preparing metal fibers for sintering, they are typically first deposited on a support, for example, to form a web-like nonwoven fabric. The weight of the fibers can cause compression, leading to undesirable porosity gradients. Such compression can occur, especially when the fibers soften due to elevated temperatures during sintering. Also, when fibers are deposited from a dispersion and the liquid drains from the porous support structure, hydrodynamic pressure causes web compression in the z-axis. This can be improved by incorporating bulkier fibers with greater mechanical stability, i.e., thicker fibers, fibers with different melting points, or corrugated fibers.

[0013] Each of the first and second types of metal fibers may have a distribution of properties such as width, thickness, length, etc. This difference in meaning means that such distributions of the first and second types are different from each other. The second type should not be understood as part of the distribution of the first type. Mixing the second type with the first type should result in at least a bimodal distribution of these properties.

[0014] The present invention further provides a method for producing a 3D network of metal fibers, comprising the steps of: Step A: mixing a first type of metal fiber and a second type of metal fiber with each other to provide a plurality of metal fibers comprising a mixture of the first type of metal fiber and the second type of metal fiber; Step B: Sintering the first type of metal fibers and the second type of metal fibers together to form a metal fiber network according to the present invention. The present invention relates to a method comprising:

[0015] As mentioned above, by mixing two types of metal fibers, one type, e.g., the first type, can function as a mechanical and electrical stabilizing component. Furthermore, in sintering step B, the metal fibers are heated and softened. Without a mixture of two types of metal fibers, gravity compresses the metal fibers, reducing the thickness of the metal fiber network and decreasing the porosity.

[0016] Furthermore, the present invention relates to a battery comprising at least one electrode having a network structure according to the present invention.

[0017] The combination of mechanical and electrical stabilization reduces the mechanical and current loads on the individual fibers, which can extend the life of the battery, especially when subjected to rapid charge and discharge cycles.

[0018] Preferred embodiments of the present invention will now be described.

[0019] In the 3D network of metal fibers of the present invention, open pores are present between the metal fibers. The fibers are the connecting parts of the network. In particular, the 3D network is composed of a plurality of metal fibers that are fixed to each other, in particular sintered or soldered to each other.

[0020] According to the present invention, the second type of metal fibers preferably have a thickness of 50 μm or less, more preferably 30 μm or less, even more preferably 10 μm or less, and most preferably 5.0 μm or less. Such thin fibers provide a large surface area, resulting in advantageous electrochemical properties and suppressing lithium dendrite growth when the 3D network is used as an electrode in a lithium-ion battery. In the 3D network of the present invention, the thickness of the second type of metal fibers is not particularly limited. In a preferred embodiment of the present invention, the thickness limit may be 1.0 μm or more. Thinner fibers may be difficult to manufacture, for example, by melt spinning, and may be difficult to handle.

[0021] In the 3D network of the present invention, it is further preferred that the second type of metal fibers have a width of 80 μm or less, more preferably 70 μm or less, even more preferably 40 μm or less, and most preferably 5.0 μm or less. Such thin fibers provide a large surface area, resulting in advantageous electrochemical properties and suppressing lithium dendrite growth when the 3D network is used as an electrode in a lithium-ion battery. In the 3D network of the present invention, the lower limit of the thickness of the second type of metal fibers is not particularly limited. In a preferred embodiment of the present invention, the lower limit of the thickness may be 1.0 μm or more. Thinner fibers can be difficult to produce by melt spinning or the like and can be difficult to handle.

[0022] The first type of metal fibers preferably have a thickness at least 1.50 times, more preferably at least 2.0 times, and even more preferably at least 2.5 times the width of the second type of metal fibers. Also preferably, the width of the first type of metal fibers is at least 1.50 times, more preferably at least 2.0 times, and even more preferably at least 2.5 times the width of the second type of metal fibers. A stabilizing effect can be achieved when the thickness and / or width of the first type of metal fibers is at least 1.5 times that of the second type of metal fibers. Therefore, the thickness and / or width of the first type of metal fibers is at least 50% (at least 1.50 times), and especially at least 100% (at least 2.0 times) greater than the thickness and / or width of the second type of metal fibers. This facilitates mechanical and electrical stabilization.

[0023] Furthermore, it is preferred that the ratio of the difference in the arithmetic mean values ​​of the thickness and / or width of the first type of metal fibers and the second type of metal fibers to the product of the variances of the first type of metal fibers and the second type of metal fibers is at least 5, in particular at least 7, and even more in particular at least 10.

[0024] The first and second types of metal fibers preferably have lengths of 2.0 mm or greater, more preferably 10 mm or greater, even more preferably 20 mm or greater, and even more preferably 70 mm or greater. Metal fiber lengths that meet the above length specifications improve the mechanical stability of the 3D metal fiber network because, due to the increased length, each metal fiber can have several contact points with other metal fibers in the network, where each metal fiber is anchored to each of the other metal fibers to form electrically conductive connections with high mechanical strength between them. Therefore, if one connection between metal fibers is severed, several other connections between fibers are available to hold the network together and provide the desired electrical conductivity, preventing the overall structural integrity of the network from being compromised or the metal fibers from separating from the network. Preferably, the fiber length should be in the range of 1.0 to 20 cm, more preferably 3.0 to 15 cm, and even more preferably 4.0 to 10.0 cm, since this allows for easier fiber placement by carding.

[0025] The thickness, width and length of each type of metal fiber may follow a distribution, in which case the values ​​given herein refer to arithmetic mean values.

[0026] More preferably, the first and second types of metal fibers have different cross-sectional morphologies, and / or different melting points before sintering, and / or different longitudinal shapes, and / or different widths and / or thicknesses, and / or different metal alloys, particularly with respect to alloy additions to the same base metal. The longitudinal shape refers to how the fibers extend along their length, e.g., wavy or straight. Wavy refers to abrupt changes in fiber direction, while straight refers to no changes or only smooth changes. Abrupt changes in direction are considered when the change in direction is observed over a length shorter than the fiber width. A smooth change in direction is considered when the change in direction extends over a distance greater than the fiber width.

[0027] In a preferred embodiment, at least the second type of metal fiber has a non-circular cross-section, particularly a ribbon-like shape. Such a non-circular cross-section provides a large surface area, which can be advantageous in filtration applications because the non-circular cross-section induces turbulence in the fluid flowing through the filter. When used in battery electrodes, fibers with a non-circular cross-section increase the surface area of ​​the electrode material, thereby improving the electrochemical properties of the electrode and improving the suppression of lithium dendrite growth, such as in lithium metal batteries.

[0028] Preferably, the second type of metal fibers have a non-circular cross-sectional shape, i.e., the second type of metal fibers are ribbon-shaped, while the first type of fibers have a substantially circular cross-section. Ribbon-shaped fibers have a large surface area per mass, providing the desired functionality. In contrast, circular fibers have a smaller surface area per mass, but are more mechanically stable. However, even though the circular fibers have a smaller surface area per mass, they still contribute to the desired functionality by providing such a surface.

[0029] In a further preferred embodiment, the first type of metal fibers and the second type of metal fibers have different melting points. This is possible even if these types of metal fibers are made of the same material. When metal fibers are produced by melt spinning, the melt of the fiber material cools rapidly. This causes the melt to solidify so quickly that the material does not settle into a thermodynamically favorable state, as explained in detail in WO 2020 / 016240 A1. After production by melt spinning, the metal fibers can be subjected to an annealing or aging treatment, which allows the material to relax and settle into a thermodynamically favorable state. It has been observed that after such an annealing treatment, the melting point of the metal fibers increases, even if the material composition remains unchanged. The difference in melting point between the first type of metal fibers and the second type of metal fibers can be at least 1 K, preferably at least 5 K, more preferably at least 6 K, even more preferably at least 7 K, and even more preferably at least 8 K. Another measure of the difference between the first and second types of fibers resulting from rapid cooling during melt spinning is the degree of recrystallization that can be observed when the fiber is heated to melt, as a measure of metastable conditions. For example, the onset of recrystallization can occur several hundred degrees below the melting point.

[0030] There is no particular upper limit to the difference in melting points. For practical reasons only, the difference in melting points is typically no more than 20K, preferably no more than 15K.

[0031] It is particularly preferred that the metal fibers be sintered directly to one another without the need for an additional binder, such as a polymer binder. Most preferably, the fixation of one metal fiber to another is achieved by the material of the metal fibers. Therefore, it is even more preferred that the metal fibers be fixed to one another without a polymer binder, as such polymer binders often have poor electrical conductivity and high-temperature performance. When the metal fibers are sintered directly to one another, it is also possible to omit solder materials, etc., in the network according to the present invention.

[0032] According to a preferred embodiment of the present invention, the first type of metal fibers and the second type of metal fibers are made of different metal alloys, particularly with respect to alloy additions to the same base metal. Small differences in alloy additions can have a significant effect on the melting temperature. Therefore, fibers with a higher melting point alloy can stabilize the network structure during sintering. During sintering, fibers with a lower melting point are heated near or even above their melting point, which significantly softens these fibers. However, fibers with a higher melting point, such as a different alloy of the same base metal, can reduce or even prevent shrinkage of the entire network structure in the z-axis direction.

[0033] Although the first and second types of metal fibers can differ from one another at least in the manner described above, it is still preferred that the first and second types of metal fibers be of substantially the same material, i.e., of substantially the same elemental composition, so that the first and second types of metal fibers can be easily bonded to one another, thereby facilitating the production of a unitary network of metal fibers.

[0034] Furthermore, it is preferred if the first type of metal fiber can be obtained from the second type of metal fiber by, for example, heat treatment. In this context, the second type of metal fiber can preferably be obtained by melt spinning, and the first type of metal fiber can preferably be obtained by heat treatment of the second type of metal fiber. Heat treatment can increase the melting point by allowing the metal fiber material to approach its thermodynamic equilibrium. Furthermore, heat treatment can change the cross-sectional shape of the second type of metal fiber to a non-circular cross section, i.e., from a ribbon-like cross section to a substantially circular cross section.

[0035] The ratio of the first type of metal fibers to the second type of metal fibers can vary depending on the degree of electrical and mechanical stabilization required. The weight ratio of the first type to the second type is preferably in the range of 0.10:99.90 to 99.90:0.10, more preferably in the range of 15:85 to 85:15, even more preferably in the range of 30:70 to 70:30, and even more preferably in the range of 40:60 to 60:40. The weight portion of the stabilizing fibers, i.e., the first type, may exceed the weight portion of the second type of fibers. However, the larger surface area of ​​the second type of fibers still provides the desired functionality. The desired functionality can also be provided, although perhaps to a lesser extent, by the first type. Preferably, the ratio of the first type of metal fibers to the second type of metal fibers is set so that the amount of the first type exceeds the percolation threshold. This allows the first type of metal fibers in the sintered 3D network to extend throughout the entire network structure, improving mechanical and electrical stabilization not only in the z-axis direction but also in the xy plane.

[0036] Preferably, the 3D network of metal fibers has a thickness of at least 200 μm, more preferably at least 500 μm, even more preferably greater than 550 μm, even more preferably at least 750 μm, and even more preferably at least 1,000 μm. The thickness of the first layer can even be 2,000 μm or more. The upper limit of the thickness of the first layer is not particularly limited, and may be 10,000 μm or less, 8,000 μm or less, 6,000 μm or less, or 4,000 μm or less.

[0037] The present invention also relates to a composite structure comprising a 3D network of metal fibers of the present invention, hereinafter also referred to as a first layer, in which the 3D network of metal fibers, i.e., the first layer, is disposed on a second layer different from the 3D network, such that the first layer and the second layer are in face-to-face contact with each other.

[0038] The second layer has the purpose of providing mechanical support to the 3D network of the first layer. This mechanical support is achieved by placing the first layer on the second layer, so that when mechanical loads are applied, they do not act directly on the individual fibers. Instead, the second layer acts as a carrier for the porous structure that absorbs the mechanical loads, thereby preventing damage to the individual metal fibers when handling the 3D network, for example, when fabricating an electrode containing the network. Thus, this embodiment provides a 3D network with improved mechanical strength, since the second layer is in face-to-face contact with the first layer and provides mechanical and electrical stabilization.

[0039] Furthermore, the improved mechanical stability makes it easier to further process the network structure, such as for the purpose of incorporating electrically active materials into the porous structure. The mechanical strength is high enough to allow roll-to-roll processing.

[0040] The second layer may be made of substantially the same material as the first and / or second metal fibers, which facilitates the formation of a stable connection between the 3D network and the second layer. According to a preferred embodiment of the present invention, the second layer is a conductive layer.

[0041] Due to the electrical coupling between the first and second layers (the second layer being electrically conductive), current conducted along the thickness of the network, i.e., the porous structure, only needs to traverse the z-plane through the metal network, while the second layer acts as a surface current collector in the xy-plane, i.e., the first and second layers are in surface contact with each other. This reduces the electrical load on individual parts of the porous structure, such as foam struts or fibers. Thus, the present invention provides a 3D network with improved mechanical and electrical stability, since the second layer is in surface contact with the first layer and provides mechanical and electrical stabilization.

[0042] In another embodiment, the second layer is made of a different material, such as a polymer material, from the first and second types of metal fibers. In other words, the material of the second layer is different from the material of the first and second types of metal fibers in the 3D network. In such cases, the second layer's attachment to the 3D network is not permanent; that is, the second layer can be removed from the 3D network of metal fibers, for example, by peeling, without destroying the structure of the 3D network. The second layer may be used to mechanically stabilize and protect the 3D network when filling the network with the electrode active material. After that, the second layer may no longer be needed, and removing the second layer reduces the basis weight of the network structure.

[0043] The second layer can be a porous or non-porous layer. Preferably, the second layer is a porous layer, particularly a porous metal layer or a porous non-metallic layer. The porous metal layer can be a perforated metal foil or a woven or non-woven metal mesh, and the porous non-metallic layer can be a polymer layer, such as a perforated polymer sheet or a woven or non-woven polymer filament structure.

[0044] In the composite structure of the present invention, the second layer, e.g., a metal foil or a non-metal foil, preferably has a thickness of at least 1 μm, more preferably at least 5 μm, even more preferably at least 10 μm, and even more preferably at least 20 μm. In particular, polymer foils are very thin and can be easily removed from the 3D network of the metal foil, for example, by peeling. An example of a suitable polymer material for such a foil is a fluorinated polymer material such as PTFE.

[0045] More preferably, the 3D network of metal fibers is directly fixed to the second layer without the need for an additional binder, such as a polymer binder. Most preferably, the metal fibers are fixed to the second layer by the porous structure and the material of the second layer. Therefore, it is even more preferable that the metal fibers are fixed to the second layer without a polymer binder, as such polymer binders often have poor electrical conductivity and high-temperature performance. If the metal fibers are sintered directly to the second layer, it is also possible to omit solder materials, etc., from the resulting laminate structure.

[0046] According to a particularly preferred embodiment of the present invention, the metal material of the first layer and the material of the second layer are substantially the same material. When both the metals of the first and second layers are substantially the same material, a stable fixation is achieved by sintering the layers together. As used herein, "substantially the same material" means that these materials are based on the same element, for example, copper or aluminum. In this specification, "based on the same element" indicates that the material contains at least 80 wt.-%, in particular at least 90 wt.-%, of the element, the remainder of which may be other elements and may differ between the first and second types of metal fibers. In one embodiment, the material of both types of metal fibers is the same.

[0047] According to another preferred embodiment, the first layer has a different structure compared to the second layer. In particular, the second layer can have a two-dimensional (2D) structure and can be non-porous, in particular a foil. A 2D structure is one that is substantially devoid of functional pores in the thickness direction, i.e., the z-axis direction. Functional pores are pores that contribute to the essential technical function of the network structure, i.e., filtration in filter applications or participation in the electrochemical process of a battery electrode. In a further preferred aspect of this embodiment, the second layer has a porous structure, such as a randomly laid layer or woven mesh of sintered metal fibers, which has higher mechanical stability compared to the first layer, or a perforated polymer sheet. The pores or perforations in the second layer allow the first layer to be filled with active material, which then penetrates through the second layer into the first layer. By introducing the active material through the second layer, the first layer is protected from mechanical loads during the introduction of the active material. The active material is, in particular, an electrode active material.

[0048] In light-duty applications, it may be desirable to remove the second layer after loading the first layer with active material. In such cases, it is preferable to have the second layer be made of a different material than the first layer, such as a polymeric material or a non-adherent metal such as molybdenum that can be peeled off from the first layer.

[0049] The porous structure of the second layer is preferably a metal mesh or expanded metal. The pore size of the second layer is not particularly limited, especially if the porous metal structure of the first layer is already completed before contacting the second layer. From a manufacturing standpoint, it is possible to deposit multiple discrete metal fibers on the porous second layer, even if the pores of the second layer are larger than the length of the metal fibers. However, it is preferable that the pore size of the second layer is equal to or smaller than the length of the metal fibers. In this specification, the pore size corresponds to the opening size of the pores at the surface of the second layer that is in contact with the first layer. In the case of a metal mesh, the pore size corresponds to the mesh width.

[0050] More preferably, according to the present invention, the thickness of the first layer is greater than the thickness of the second layer. The majority of the functionality of the network structure is provided by the first layer. By making the thickness of the first layer greater than the thickness of the second layer, the weight contribution of the second layer is minimized, thereby increasing the capacity per weight of the network structure when the network is used as a battery electrode.

[0051] By placing a 3D network on the second layer, a composite structure of the 3D structure and a 2D substrate (such as a film, screen cloth, or 2D mesh structure) is realized.

[0052] Preferably, the 3D network of the present invention is part of a battery electrode.

[0053] All preferred embodiments and features described above or claimed herein for the 3D networks themselves apply mutatis mutandis to the methods for producing such 3D networks as well as to the methods for producing electrodes.

[0054] For example, in the method for producing a 3D network according to the present invention, it is also preferred that the first type of metal fibers and the second type of metal fibers are at least substantially made of the same material. This allows for a stable bond between these two types of fibers, achieving the desired mechanical and electrical stabilization. Further details of the first type of metal fibers and the second type of metal fibers are described in detail above. Therefore, it is preferred that the first and second layers are fixed to each other so that electrical contact is formed between them. Fixing the first and second layers can be achieved by various means, such as sintering, soldering, ultrasonic welding, or adhesive bonding.

[0055] Regardless of whether the metal fiber network is already sintered before being placed on the second layer, the second layer is preferably a porous structure, which may be advantageous in subsequent processing steps where compression of the first layer should be avoided.

[0056] According to the present invention, the method for producing a 3D network of metal fibers is preferably carried out as a roll-to-roll process, which allows large-scale production to be achieved.

[0057] As already mentioned above, the present invention also relates to a method for producing an electrode, in particular a battery electrode, using a 3D network according to the invention, in which the electrode material is preferably provided as a slurry having thixotropic behavior.

[0058] In a preferred embodiment of the method for producing an electrode of the present invention, the second layer is porous. This allows the first layer to be filled with an active material, particularly an electrode active material, by passing the active material through the pores of the second layer. Those skilled in the art can select the pore size of the second layer depending on the particle size and / or fluid behavior of the active material.

[0059] Preferably, the active material is introduced into the first layer in the form of a slurry, particularly a thixotropic slurry. The viscosity of the thixotropic slurry decreases when subjected to a shear force. After the shear force is no longer acting on the slurry, its viscosity increases again. To adjust this rheological behavior as needed, those skilled in the art can add commercially available rheological additives. In this context, it is preferable for the thixotropic slurry to fill the 3D network structure, i.e., the voids of the first layer, through the porous second layer. This can be achieved by using a doctor blade. The thixotropic slurry is subjected to a shear force when it comes into contact with the doctor blade. This reduces the viscosity of the slurry, allowing it to flow through the pores of the second layer into the first layer. This prevents mechanical stress on the first layer and its compression. After penetrating the first layer, the thixotropic slurry is no longer subjected to a shear force, and therefore its viscosity increases, so it remains there instead of flowing out of the first layer.

[0060] In a preferred embodiment of the present invention, the filling of the electrode active material into the first layer through the pores of at least the second layer is carried out as a roll-to-roll process. Even more preferably, the method of manufacturing an electrode comprises the method of manufacturing the 3D network of the present invention, i.e., the 3D network described herein.

[0061] The present invention also relates to a battery comprising at least one electrode having a 3D network of metal fibers according to the present invention. The 3D network of the present invention can be used in many different types of batteries. However, in the case of a 3D network thickness of 200 μm or more, as described above as preferred herein, the battery can be a single-cell battery. The single-cell battery referred to herein is not a stack of many battery cells, but a single battery cell having one anode, one cathode, and a separator therebetween. Of course, the battery can also be a multi-cell battery.

[0062] Furthermore, the present invention relates to a filter comprising at least one filter material having a 3D network of metal fibers according to the invention or a composite structure according to the invention.

[0063] The present invention will now be described in more detail, by way of example only, with reference to the accompanying drawings and figures and by way of various examples of networks and methods of the present invention. [Brief explanation of the drawings]

[0064] [Figure 1] FIG. 1 is a schematic diagram showing the layers of a composite network structure of the present invention and the designations of the z-axis and xy-planes. [Figure 2] 1 is a schematic diagram illustrating a method for producing a composite network structure according to the present invention. [Figure 3] 1 is a schematic diagram illustrating a method for manufacturing an electrode according to the present invention. [Figure 4] 1 is a scanning electron microscope image of a 3D network of metal fibers according to Example 1. [Figure 5]1 is a scanning electron microscope image of a 3D network of metal fibers according to Example 2. [Figure 6] 3 shows a composite network structure according to a third embodiment of the present invention. [Figure 7] 4 shows a composite network structure according to a fourth embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0065] For clarity, not all or no reference signs are shown in some figures.

[0066] FIG. 1 shows a schematic representation of a composite network structure 1 of the present invention. The 3D composite structure 1 shown includes a porous first layer 10 of a 3D network of metal fibers and a conductive second layer 20. The first and second layers each extend in the xy plane, as shown in the coordinate system of FIG. 1. The first layer 10 and the second layer 20 are in direct surface contact with each other. In the view of FIG. 1, the surface contact between these layers extends parallel to the xy plane. The first layer 10, as shown in FIG. 1, is a 3D network of two types of metal fibers sintered together. In the view of FIG. 1, this network is only shown schematically, and therefore individual fibers 12 (see FIG. 2) are not labeled. The second layer 20 is a support layer to which the fibers of the network of the first layer 10 are directly fixed by sintering. This places the fibers in the network in direct electrical contact with the conductive second layer 20. When an electrochemical process provides electrons in the first layer, these electrons only need to travel through the network in the z-axis direction, assisted by the first type of metal fibers in the 3D network, before reaching the second layer 20, which can act as a current collector. Thus, the interaction of the first type of metal fibers with the conductive second layer reduces the electrical load on the individual fibers, because the electrons do not have to pass through a network of metal fibers in the xy plane to reach an electrical connection (not shown in the drawings), but rather the electron transport in the z-axis direction is assisted by the first type of metal fibers.

[0067] FIG. 2 illustrates a process for preparing a 3D network. In step A, a mixture of two types of metal fibers is provided. In the schematic diagram of FIG. 2, this mixture includes metal fibers 14 that are loosely intertwined with each other, i.e., not yet sintered to each other. Although only seven metal fibers are shown in FIG. 2, and for clarity, this schematic diagram does not show different types of metal fibers, the present invention requires two such different types of metal fibers. It should be understood that in practice, the number of metal fibers is much greater than shown in the schematic diagram of FIG. 2. A second conductive layer is provided from roll 21, which rotates in the direction indicated by the white arrow. In step C, a mixture of metal fibers 14, i.e., loosely arranged metal fibers, is deposited on second layer 20. In step D, first layer 10 is fixed on second layer 20 by sintering. The network structure 1 is then wound onto roll 22, which rotates in the direction indicated by the white arrow. This illustrates how the present invention for producing a 3D network structure can be implemented as a roll-to-roll process. Although a rolling process is shown in FIG. 2, it is clear that the process having steps A to D as described above can be realized without rolls 21 and 22.

[0068] A schematic diagram of the fabrication of an electrode 30 according to the present invention is shown in FIG. 3. In step E, a 3D network 1 according to the present invention, having a composite structure with a first layer 10 and a second layer 20, is provided from a roll 23 rotating in the direction indicated by the white arrow. The network structure 1 has a first layer 10 and a second layer 20, as described above. The first layer 10 comprises a network of metal fibers 14 sintered together. The second layer 20 is a porous layer. The network structure 1 is conveyed from the roll 23 to a doctor blade 40, where a thixotropic slurry 42 of the electrode active material is subjected to a shear force and forced through the second layer 20 and into the first layer 10, thereby forming the electrode 30 after a drying step (not shown in FIG. 3). Because the doctor blade acts only on the second layer 20, the first layer is not subjected to any pressure, thereby avoiding undesired compression of the first layer 10. The drying step may be performed immediately after filling the pores with the slurry 42 or at a separate stage, or, if circumstances permit, may be omitted entirely. 3, the electrode is then wound onto a roll 24 rotating in the direction indicated by the white arrow, thereby realizing the manufacture of the electrode according to the present invention as a roll-to-roll process.

[0069] In principle, each of the roll-to-roll processes of Figures 2 and 3 can be performed as a single integrated process instead of winding the network structure 1 onto a roll 22 and directing it to a doctor blade 40 as shown in Figure 3.

[0070] The following examples further illustrate the present invention.

[0071] Examples 1 and 2 and Comparative Examples 1 and 2 illustrate the production of 3D networks of metal fibers according to the present invention. Such 3D networks can also be used as the first layer of composite network structures according to the present invention.

[0072] Example 1: In this example, ribbon-shaped flat fibers (width approximately 100 μm, thickness 5 μm, equivalent radius approximately 12.6 μm) obtained by melt spinning were mixed 50:50 with approximately circular fibers (radius approximately 45 μm) of the same material composition.

[0073] The fibers were sintered at 620 °C (average heating rate of 200 K / min, argon atmosphere, 60 s holding time, natural cooling, i.e., removed from the furnace and allowed to cool to room temperature). Scanning electron microscope images in Figure 4 show that the fibers were bonded together and that the fiber morphology remained unchanged. The network was 950 μm thick after sintering.

[0074] Comparative Example 1 was carried out in the same manner as Example 1, except that only ribbon-shaped flat fibers were used without using circular fibers. The thickness of the network after sintering was only about 300 μm.

[0075] Comparison of Example 1 and Comparative Example 1 shows that the use of two different types of fibers, one with a ribbon-like shape with a non-circular cross section and the other with a circular cross section, improves the mechanical stabilization during the sintering process, thereby avoiding compression of the metal fibers.

[0076] Example 2: In this example, ribbon-like flat fibers (approximately 80 × 5 μm) obtained by melt spinning were used. Half of the fibers (by weight) were aged at 400 °C for 60 minutes under inert gas and sintered with untreated fibers at 620 °C as described above for Example 1. As shown in Figure 5, the fibers were firmly bonded together after sintering, and the thickness of the sintered network was 750 μm.

[0077] After aging, the shape of the aged fibers did not change, but DSC measurements showed that the aged fibers had a melting temperature about 7 K higher than that of the unaged fibers.

[0078] Comparative Example 2 was carried out in the same manner as Example 2, except that only untreated ribbon-like flat fibers were used, i.e., without aged fibers, and the final network thickness was only 250 μm.

[0079] Comparison of Example 2 and Comparative Example 2 shows that the use of two types of fibers with different melting points improves the mechanical stabilization during the sintering process, thereby avoiding compaction of the metal fibers.

[0080] Examples 3 and 4 demonstrate the preparation of composite network structures according to the present invention.

[0081] Example 3: An already sintered metal fiber network (sintered at 920°C (average heating rate of 200 K / min, argon atmosphere, 60 s holding time, natural cooling, i.e., removed from the furnace and allowed to cool to room temperature) was placed on a copper foil (99.9+, 10 μm thick) and sintered together using the same 920°C temperature program used for the initial sintering of the metal fiber network. A photograph of the resulting composite network structure is shown in Figure 6.

[0082] Example 4: Unsintered fibers were placed loosely on a fine-mesh copper mesh and sintered at 920°C (average heating rate of 200 K / min, argon atmosphere, 60 s holding time, natural cooling, i.e., removed from the furnace and allowed to cool to room temperature). The fibers were firmly sintered to each other, within themselves, and to the copper network. A photograph of the resulting composite network structure is shown in Figure 7.

[0083] The thickness and width of the metal fibers referred to herein can be determined either by scanning electron microscopy or micro-CT. The melting point of the metal fibers can be determined by DSC. [Explanation of symbols]

[0084] 1. Complex network structure 10 1st layer 14 Metallic Fibers 20 2nd layer 21 rolls 22 rolls 23 rolls 24 rolls 30 electrodes 40 Doctor Blade 42 Slurry

Claims

1. a plurality of metal fibers fixed to one another; the plurality of metal fibers (14) includes a first type of metal fiber and a second type of metal fiber, and the first type and the second type of metal fiber (14) are different from each other, particularly in terms of mechanical properties; Three-dimensional (3D) network of metal fibers.

2. the second type of metal fibers have a thickness of 50 μm or less, more preferably 30 μm or less, even more preferably 10 μm or less, and most preferably 5.0 μm or less; and / or the second type of metal fibers have a width of 80 μm or less, more preferably 70 μm or less, even more preferably 40 μm or less, and most preferably 5.0 μm or less; and / or the first type of metal fibers have a thickness and / or width that is at least several times the thickness and / or width of the second type of metal fibers; 10. A three-dimensional (3D) network of metal fibers according to claim 1.

3. the first type of metal fibers and the second type of metal fibers have different cross-sectional morphologies; and / or the first type of metal fibers and the second type of metal fibers have different melting points before being sintered together; and / or the first type of metal fibers and the second type of metal fibers have different longitudinal shapes; and / or the first type of metal fibers and the second type of metal fibers have different widths and / or thicknesses; and / or the first type of metal fibers and the second type of metal fibers differ in metal alloy, particularly with respect to alloy additions to the same base metal; A three-dimensional (3D) network of metal fibers according to claim 1 or 2.

4. at least said second type of metal fibers have a non-circular cross section, in particular a ribbon-like shape; and / or the first type of metal fibers have a substantially circular cross section; A three-dimensional (3D) network of metal fibers according to claim 1 or 2.

5. the first type of metal fibers and the second type of metal fibers have melting points, before being sintered together, that differ by at least 1 K as determined by DSC measurement; A three-dimensional (3D) network of metal fibers according to claim 1 or 2.

6. the first type of metal fibers and the second type of metal fibers are of substantially the same material; A three-dimensional (3D) network of metal fibers according to claim 1 or 2.

7. the weight ratio of said first type to said second type is in the range of from 1:99 to 99:1, more preferably in the range of from 15:85 to 85:15, even more preferably in the range of from 30:70 to 70:30, and even more preferably in the range of from 40:60 to 60:40; and / or the ratio of the first type of metal fibers to the second type of metal fibers is set so that the amount of the first type is above a percolation threshold; A three-dimensional (3D) network of metal fibers according to claim 1 or 2.

8. The 3D network of metal fibers has a thickness of at least 200 μm, more preferably at least 500 μm, even more preferably more than 550 μm, even more preferably at least 750 μm, and even more preferably at least 1,000 μm; A three-dimensional (3D) network of metal fibers according to claim 1 or 2.

9. A three-dimensional (3D) network of metal fibers according to claim 1, The 3D network is a first layer (10) and is arranged on a second layer (20) different from the first layer (10). Composite structure (1).

10. said second layer (20) being of at least substantially the same material as said first and / or second type of metal fibers (14); and / or the first layer and the second layer are secured to one another such that electrical contact is formed between the layers; The composite structure of claim 9.

11. said second layer (20) being made of a material different from said first and second types of metal fibres, in particular made of a polymer material; and / or the second layer (20) is removable from the 3D network of metal fibers, for example by peeling; The composite structure of claim 9.

12. said second layer (20) is a porous layer, in particular a porous metallic layer or a porous non-metallic layer; and / or the second layer has a thickness of at least 1 μm, preferably at least 5 μm, even more preferably at least 10 μm, and even more preferably at least 20 μm; The composite structure according to any one of claims 9 to 11.

13. The second layer (20) is a porous layer or a non-porous layer. The composite structure according to any one of claims 9 to 11.

14. 1. A method for producing a three-dimensional (3D) network of metal fibers, comprising: Step A: mixing a first type of metal fiber (14) and a second type of metal fiber (14) together to provide a mixture of a plurality of metal fibers (14); Step B: Sintering the metal fibers (14) in the mixture obtained in step A to each other to form a metal fiber network according to claim 1 or 2 or a composite structure (1) according to any one of claims 9 to 11. A method comprising:

15. Providing a composite structure (1) according to claim 12, and introducing a slurry of electrode active material through the porous second layer; and / or 1. A method for producing an electrode, in particular a battery electrode, by introducing a slurry of an electrode active material through pores of a 3D network of metal fibers, wherein the pores of the 3D network of metal fibers are voids between the metal fibers.

16. A battery comprising at least one electrode with a 3D network of metal fibers according to claim 1 or 2, in particular a single cell battery or a multi-cell battery.

17. A filter comprising at least one filter material having a 3D network of metal fibers according to claim 1 or 2 or a composite structure according to any one of claims 9 to 11, in particular wherein said second layer is a porous layer.

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