Three-dimensional network of metal fibers and method of manufacture

Abstract

A three-dimensional (3D) network of metal fibers comprising a plurality of metal fibers fixed to one another, where the plurality of metal fibers comprises a first metal fiber and a second metal fiber, where the first metal fiber and the second metal fiber differ from one another, in particular in terms of their mechanical properties.

Description

Technical Field

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

[0002] 3D structures, such as networks of metal fibers, have advantageous properties in a number of technical fields, such as filters and electrodes, especially electrodes for batteries. In particular, very fine fibers, such as microfibers having a thickness and / or width in the range of 50 μm or less, provide a very large surface area to volume ratio and a corresponding surface area, which has been found to be advantageous, for example, for electrode applications. In addition, when such fine fibers have a non-circular cross-section (such as a ribbon cross-section), the surface area is even higher. Furthermore, for filter applications, it has been found that networks of fibers having such non-circular cross-sections (such as ribbon cross-sections) have better filtering properties, which may be due to the turbulence caused by the shape of the fibers having such non-circular cross-sections.

[0003] To manufacture such 3D networks, various methods (dry and wet) known to those skilled in the art can be used to lay metal fibers into a non-woven fabric that is electrically conductive and mechanically strong. For the bonding of such non-woven fabrics, processes such as sintering at a temperature close to or above the melting point are used. The manufacture of such metal fiber networks is known to those skilled in the art, for example, from WO2020 / 016240A1, the entire content of which is incorporated herein by reference. Metal fibers can be manufactured by melt spinning, as described, for example, in WO2016 / 020493A1 and WO2020 / 229400A1, the entire content of these references being incorporated herein by reference.

[0004] However, especially when these networks are based on fine fibers, shrinkage of the network thickness (z-axis) can be observed. In addition, when made from relatively short and fine fibers, such non-woven networks can only be handled with very fine precautions to prevent them from spreading and the thickness (z-axis) of the net from shrinking, for example, during sintering or during coating with an active electrode material. This hinders the roll-to-roll process for manufacturing 3D networks of fine metal fibers.

[0005] Furthermore, even after the 3D network is completed by sintering the metal fibers of the non-woven fabric to each other, the resulting network may have problems in terms of its mechanical stability against compression. Especially for microfiber networks having a thickness and / or width in the range of 50 μm or less, even after sintering the metal fibers to each other, the stability is not sufficient for large-scale production processes, such as the roll-to-roll process for loading electrode active material onto the metal fiber network.

[0006] A 3D network of metal fibers with conductive connectors (serving as an electron collector in a lithium-ion battery) can enable very thick electrodes when they are filled with the corresponding active materials. Such thick electrodes can provide an areal load of 100 mAh / cm 2 or greater. Such electrodes constructed with a 3D metal fiber network can achieve high currents in three dimensions towards the power outlet, and also involve the ion mobility within the electrode itself (also described in the unpublished patent application PCT / EP2021 / 062443 filed on May 11, 2021). This is of particular interest when the electrode is a battery electrode and the battery is subjected to fast charge or discharge cycles.

[0007] Especially when using very fine fibers, due to the small conductor cross-section, each fiber can only handle a certain amount of current. When the current is too high, it is possible to damage a single fiber. Repeated use may cause increasing damage to the network of metal fibers, thereby gradually deteriorating its function. In addition, when used in a battery, high temperatures may occur at a single fiber with a high current load. This may lead to the degradation of the active material and / or the electrolyte.

[0008] Therefore, the problem is that existing network structures have a limited lifespan when repeatedly subjected to current (such as in a battery electrode), and when manufacturing such a metal fiber network or loading such a metal fiber network with an active electrode material, such a metal fiber network is fragile and difficult to process, for example, in a roll-to-roll process. Summary of the Invention

[0009] This application solves this problem by providing a three-dimensional network structure according to claim 1. In particular, the three-dimensional (3D) network of metal fibers includes a plurality of metal fibers fixed to each other, wherein the plurality of metal fibers (14) includes a first type of metal fiber and a second type of metal fiber, and wherein the first type of metal fiber and the second type of metal fiber (14) are different from each other, especially in terms of their mechanical properties.

[0010] According to the present invention, the 3D network includes a plurality of first type of metal fibers and a plurality of second type of metal fibers, and the first type of metal fiber and the second type of metal fiber are different from each other. The first type of metal fiber can be considered as a stable type of fiber, while the second type of metal fiber can be considered as a functional type of fiber. Compared with the first type of metal fiber, the second type of metal fiber may have a larger surface area per unit weight of the metal fiber. This means that the first type of metal fiber can also provide desired functions, such as a filtering effect or an available surface area for an electrochemical process, even to a lesser extent.

[0011] By using different kinds of metal fibers, one kind of metal fiber (i.e., the first kind of metal fiber as described above) can act as a mechanical stabilizing component and act as a more elastic electrical conductor through the thickness of the 3D network of the second kind of metal fiber (most importantly in the direction of the z-axis). In particular, the first kind of metal fiber and the second kind of metal fiber form an integrated network of metal fibers, which means that the first kind of metal fiber and the second kind of metal fiber are preferably sintered directly to each other to form a single 3D network including both kinds of metal fibers. In other words, the first kind of metal fiber and the second kind of metal fiber do not form separate networks. Moreover, by using different kinds of metal fibers, they constitute a hierarchically structured network, where the finer fibers form a fine mesh supported by the stronger fibers, for example to collect current within small volume elements, while the coarser fibers ensure further transport of the locally collected electrons by maintaining a higher current due to their higher conductive cross-section, thus acting as conductive paths not only in the x-y plane of the network but also in the z-axis of the porous network of metal fibers. Therefore, using two different kinds of metal fibers allows an even further reduction of the electrical load on the individual fibers, i.e., one of the metal fiber kinds provides a current path in the z-axis direction and the second layer collects current in the x-y plane.

[0012] In addition to providing electrical stability, the mixture of two different types of fibers also protects the metal fiber network from undesired densification in the z-direction (especially during sintering the metal fibers together and when loading the 3D network with active material to prepare a functional electrode for a lithium battery). It has been observed that when combining two different kinds of fibers (especially during sintering the metal fibers to form a network), thickness shrinkage occurs when only one kind of metal fiber is used, i.e., shrinkage in the z-axis. Especially when these metal fibers have a thickness and / or width equal to or less than 80 μm. By using two different kinds of metal fibers, this shrinkage can be reduced or even completely avoided. Thus, one of the two types (i.e., the first kind of metal fiber as described above) acts as a mechanical stabilizer and prevents shrinkage. When preparing metal fibers for sintering, they are usually first placed on a carrier to form, for example, a net-like non-woven fabric. The weight of the fibers may cause compression, and the compression causes a porosity gradient, which is not desired. This compression can occur especially when the fibers soften due to the elevated temperature during sintering. In addition, when the fibers are placed from a liquid dispersion and the liquid is drawn through a porous support structure, the hydrodynamic pressure also causes the web to be compressed in the z-axis direction. This can be remedied by mixing in larger fibers with greater mechanical stability (i.e., coarser fibers, fibers with different melting points, or corrugated fibers).

[0013] Each of the first metal fiber and the second metal fiber may have a distribution with respect to their properties such as width, thickness, length, etc. In the meaning of this specification, the difference means that such distributions of the first metal fiber and the second metal fiber are different from each other. The second metal fiber should not be understood as part of the distribution of the first metal fiber. Mixing the second metal fiber and the first metal fiber should give at least a bimodal distribution of these properties.

[0014] Furthermore, the present invention relates to a method for producing a 3D network of metal fibers, which comprises the following steps:

[0015] Step A: Mix the first metal fiber and the second metal fiber with each other, thereby providing a plurality of metal fibers including a mixture of the first metal fiber and the second metal fiber.

[0016] Step B: Sinter the first metal fiber and the second metal fiber with each other to form a network of metal fibers according to the present invention.

[0017] As described above, by mixing two metal fibers, one metal fiber (for example, the first metal fiber) can act as a mechanically and electrically stable component. In addition, during sintering step B, the metal fibers soften due to being heated. In the absence of a mixture of two metal fibers, gravity compresses the metal fibers, thereby reducing the thickness and porosity of the metal fiber network.

[0018] In addition, the present invention relates to a battery, which comprises at least one electrode having a network structure according to the present invention.

[0019] Based on the combination of mechanical stability and electrical stability, the mechanical load and current load on individual fibers are reduced. Therefore, the life of the battery can be increased, especially when subjected to rapid charge and discharge cycles.

[0020] Preferred embodiments of the present invention are described below:

[0021] In the 3D network of metal fibers of the present invention, open pores are between the metal fibers. The fibers are the connections of the network. In particular, the 3D network is composed of a plurality of metal fibers that are fixed to each other, especially sintered or brazed to each other.

[0022] According to the present invention, preferably, the second metal fiber among the plurality of metal fibers is a fiber having 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. By using such a fine fiber, a large surface area can be provided, resulting in favorable electrochemical properties, and the growth of lithium dendrites can be suppressed 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 metal fiber is not particularly limited. In a preferred embodiment of the present invention, it may be 1.0 μm or more. Fibers with a lower thickness may be difficult to manufacture, for example, by melt spinning, and difficult to handle.

[0023] In the 3D network of the present invention, further preferably, the second metal fiber has 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. By using such a fine fiber, a large surface area can be provided, resulting in favorable electrochemical properties, and the growth of lithium dendrites can be suppressed 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 metal fiber is not particularly limited. In a preferred embodiment of the present invention, it may be 1.0 μm or more. Fibers with a lower thickness may be difficult to manufacture, for example, by melt spinning, and difficult to handle.

[0024] Preferably, the thickness of the first metal fiber 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 metal fiber. Also preferably, the width of the first metal fiber 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 metal fiber. When the thickness and / or width of the first metal fiber is at least 1.5 times the thickness and / or width of the second metal fiber, a stable effect can be obtained. Therefore, the thickness and / or width of the first metal fiber is at least 50% (at least 1.50 times), especially at least 100% (at least 2.0 times) greater than the thickness and / or width of the second metal fiber. Thus, mechanical and electrical stability can be easily achieved.

[0025] Furthermore, preferably, the ratio of the difference between the arithmetic means of the thicknesses and / or widths of the first and second metal fibers to the product of the variances of the first and second metal fibers is at least 5, particularly at least 7, and further particularly at least 10.

[0026] Preferably, the first metal fiber and the second metal fiber have a length 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. When the length of the metal fiber meets the above length specifications, the mechanical stability of the 3D network of metal fibers is improved because, due to the increase in the length of the metal fiber, each metal fiber can have a number of contact points with other metal fibers of the network, where each metal fiber is fixed to a corresponding other metal fiber to form a highly mechanically strong and conductive connection therebetween. Therefore, when one connection between metal fibers breaks, this does not compromise the overall structural integrity of the network or detach the metal fibers from the network because a number of other connections between the fibers are available to hold the network together and provide the desired conductivity. Preferably, the fiber length should be in the range of 1.0 to 20 cm, more preferably in the range of 3.0 to 15 cm, even more preferably in the range of 4.0 to 10.0 cm, because it is easily possible to arrange the fibers by carding in this way.

[0027] The thickness, width and length of each metal fiber can follow a distribution. In this case, the values indicated herein refer to the arithmetic mean.

[0028] Even more preferably, the first metal fiber and the second metal fiber have different cross-sectional forms, and / or different melting points before sintering with each other, and / or different longitudinal shapes, and / or different widths and / or thicknesses, and / or different metal alloys (especially in terms of alloy addition to the same base metal). The longitudinal shape describes how the fiber extends along its length, for example in a corrugated manner or in a straight manner. The corrugated manner includes sudden changes in the fiber direction, while the straight manner includes no change or only smooth changes. A sudden change should be considered when a change in direction is observed in a length segment smaller than the fiber width. A change in direction extending over a distance longer than the fiber width is a smooth change.

[0029] In a preferred embodiment, at least the second metal fiber has a non-circular cross-section, especially a ribbon shape. This non-circular cross-section provides a large surface area, which may be advantageous for filtration applications due to the turbulence caused by the non-circular cross-section in the fluid flowing through the filter. When used in a battery electrode, the fiber with a non-circular cross-section increases the surface area of the electrode material, thus improving the electrochemical properties of the electrode and suppressing lithium dendrite growth in, for example, lithium metal batteries.

[0030] Preferably, the second metal fiber has a non-circular cross-sectional shape, i.e., the second metal fiber is ribbon-shaped, while the first fiber has a substantially circular cross-section. The ribbon-shaped fibers provide a large surface area per unit mass, thus providing the desired function. In contrast, the circular fibers have a lower surface area per unit mass but higher mechanical stability. However, even though the circular fibers have a lower surface area per unit mass, they provide a surface that contributes to the desired function.

[0031] In another preferred embodiment, the melting points of the first metal fiber and the second metal fiber are different. This is possible even when these types of metal fibers are of the same material. When manufacturing metal fibers by melt spinning, the melt of the fiber material is rapidly cooled. Therefore, the melt solidifies extremely quickly, not allowing the material to arrange itself into a thermodynamically preferred state, as detailed in WO2020 / 016240A1. After manufacturing by melt spinning, the metal fibers can be annealed or aged, so that the material relaxes and arranges itself in a thermodynamically preferred state. It has been observed that the melting point of the metal fibers is higher after such an annealing treatment, even though the material composition remains unchanged. The melting point difference between the first metal fiber and the second metal fiber can be at least 1 K, and preferably at least 5 K, more preferably at least 6 K, even more preferably at least 7 K, even more preferably at least 8 K. Another criterion for the difference between the first and second fibers resulting from the rapid cooling during melt spinning is the degree of recrystallization, which can be observed when the fibers are heated to melting, as a measure of their metastable state. For example, the onset of recrystallization can occur several hundred degrees below the melting point.

[0032] There is no specific upper limit for the melting point difference. Only for practical reasons, the melting point difference is typically equal to or less than 20 K, preferably equal to or less than 15 K.

[0033] Particularly preferably, the metal fibers are sintered directly to each other 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 fiber. Therefore, it is further preferred that the metal fibers are fixed to each other without a polymer binder, since such a polymer binder typically has poor electrical conductivity and high-temperature properties. In the case where the metal fibers are sintered directly to each other, solder materials, etc. can also be omitted in the network according to the present invention.

[0034] According to a preferred embodiment of the present invention, the first metal fiber and the second metal fiber are different in metal alloy, especially in terms of alloy addition to the same base metal. Minor differences in alloying additions can have a significant impact on the melting temperature. Therefore, fibers of higher melting point alloy can stabilize the network structure during sintering. During sintering, the fibers of lower melting point are heated to near or even above their melting points, whereby these fibers are significantly softened. However, due to the fibers of higher melting point, such as fibers of different alloys of the same base metal, shrinkage of the entire network structure in the z-axis direction can be reduced or even avoided.

[0035] Although the first metal fiber and the second metal fiber can be different from each other at least in the above aspects, it is preferred that the first metal fiber and the second metal fiber are of substantially the same material, i.e., the elemental composition. Thus, an integrated network of metal fibers can be easily fabricated because the first metal fiber and the second metal fiber can be easily bonded to each other.

[0036] Furthermore, it is preferred when the first metal fiber can be obtained from the second metal fiber, for example, by heat treatment. Herein, the second metal fiber can preferably be obtained by melt spinning, and the first metal fiber can preferably be obtained by heat treatment of the second metal fiber. Heat treatment can allow the metal fiber material to approach its thermodynamic equilibrium, thereby increasing the melting point. In addition, by heat treatment, the cross-sectional shape of the second metal fiber can be changed from a non-circular cross-section (i.e., a ribbon cross-section) to a substantially circular cross-section.

[0037] The ratio of the first metal fiber and the second metal fiber can vary according to the desired degree of electrical and mechanical stability. The weight ratio of the first metal fiber and the second metal fiber 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 further more preferably in the range of 40:60 to 60:40. The weight fraction of the stabilizing fiber (i.e., the first one) can exceed the weight fraction of the second fiber. However, due to the larger surface area of the second fiber, the desired function is still provided. The provision of the desired function also comes from the first one, although possibly to a lesser extent. Preferably, the ratio of the first metal fiber and the second metal fiber is set such that the amount of the first metal fiber is higher than the percolation threshold. Thus, the first metal fiber in the sintered 3D network extends throughout the network structure, thereby providing improved mechanical and electrical stability in the z-axis direction as well as in the x-y plane.

[0038] 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 further more preferably at least 1,000 μm. The thickness of the first layer can even be 2,000 μm or higher. There is no particular limitation on the upper limit of the thickness of the first layer, and it can be 10,000 μm or less, 8,000 μm or less, 6,000 μm or less, or 4,000 μm or less.

[0039] The present invention also relates to a composite structure comprising the 3D network of metal fibers of the present invention. Hereinafter, the 3D network will also be referred to as the first layer. In the composite structure of the present invention, 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 surface contact with each other.

[0040] The second layer serves the purpose of mechanically supporting the 3D network of the first layer. The mechanical support is achieved by disposing the first layer on the second layer. Thus, when a mechanical load is applied, it does not act directly on the individual fibers. Instead, the second layer acts as a carrier for the porous structure, which absorbs the mechanical load, thereby preventing individual metal fibers from being damaged during the processing of the 3D network (e.g., during the manufacture of an electrode comprising such a network). Therefore, this embodiment provides a 3D network with increased mechanical strength because the second layer is in surface contact with the first layer and provides mechanical and electrical stability.

[0041] In addition, the increased mechanical stability makes it easier to further process the network structure, such as incorporating electroactive materials into the porous structure. The mechanical strength is high enough such that a roll-to-roll process is possible.

[0042] The second layer can be a material substantially the same as the first metal fiber and / or the second metal fiber. This makes it easy to form a stable fixation between the 3D network and the second layer. According to a preferred embodiment of the present invention, the second layer is a conductive layer.

[0043] Due to the electrical coupling of the first layer and the second layer (where the second layer is conductive), the current conducted through the thickness of the network (i.e., the porous structure) only needs to cross the z-plane through the metal network, while the second layer acts as a surface current collector in the x-y plane, i.e., the first layer and the second layer are in surface contact with each other. Thus, the electrical load on each section of the porous structure (e.g., foam struts or fibers) is reduced. Therefore, the present invention provides a 3D network with increased mechanical and electrical stability because the second layer is in surface contact with the first layer and provides mechanical and electrical stability.

[0044] According to another embodiment, the second layer is made of a material different from the first and second metal fibers, for example, made of a polymer material. In this case. In other words, the material of the second layer is different from the materials of the first and second metal fibers in the 3D network. In this case, the fixing of the second layer 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 damaging the structure of the 3D network. When filling the network with an electrode active material, the second layer can be used to mechanically stabilize and protect the 3D network. Thereafter, it may no longer be necessary, and the removal of the second layer reduces the areal weight of the network structure.

[0045] 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-metal layer. The porous metal layer can be a perforated metal foil, or a woven or non-woven metal mesh, and the porous non-metal layer can be a polymer layer, such as a perforated polymer sheet, or a woven or non-woven structure of polymer filaments.

[0046] In the composite structure of the present invention, the second layer (such as a metal or 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 further more preferably at least 20 μm. In particular, the polymer foil can be very thin and can be easily removed from the 3D network of the metal foil, for example, by peeling. Examples of suitable polymeric materials for such foils are fluorinated polymeric materials such as PTFE.

[0047] Further 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 fixing of the metal fibers to the second layer is achieved through the materials of the porous structure and the second layer. Therefore, it is further preferred that the metal fibers are fixed to the second layer without a polymer binder, since such polymer binders generally have poor electrical conductivity and high-temperature performance. In the case where the metal fibers are directly sintered onto the second layer, solder materials, etc. can also be omitted in the resulting layered structure.

[0048] 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. Since the metal of the first layer and the second layer are substantially the same material, stable fixing is achieved by sintering these layers to each other. As used herein, substantially the same material means that these materials are based on the same element, such as copper or aluminum. Herein, being based on the same element indicates that the materials include the said element to an extent of at least 80 wt.-%, particularly at least 90 wt.-%, and the remainder can be other elements and can be different for the first and second metal fibers. In one embodiment, the materials of the two metal fibers are the same.

[0049] 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 pore-free, in particular a foil. A 2D structure is a structure that is substantially free of functional pores in the thickness direction (i.e., the z-axis direction). Functional pores are such pores that contribute to the necessary technical functions of the network structure, i.e., the filtering effect in the case of a filter application or participate in the electrochemical process of a battery electrode. In another preferred aspect of this embodiment, the second layer has a porous structure, for example in the form of a woven mesh or an irregularly laid layer of sintered metal fibers having a higher mechanical stability compared to the first layer, or in the form of a perforated polymer sheet. Due to the pores or perforations of the second layer, the first layer can be loaded with an active material that penetrates into the first layer via the second layer. By introducing the active material via 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.

[0050] For lightweight applications, it may be desirable to remove the second layer after loading the first layer with the active material. In this case, the second layer preferably has a different material from the first layer, for example from a polymer material or a non-adhesive metal such as molybdenum, which can be peeled off from the first layer.

[0051] 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 when the porous metal structure of the first layer has been completed even before bringing it into contact with the second layer. From a manufacturing perspective, even when the pores of the second layer are larger than the length of the metal fibers, a plurality of loose metal fibers can be placed on the porous second layer. However, it is preferred when the size of the pores of the second layer is equal to or less than the length of the metal fibers. The size of the pores herein corresponds to the opening diameter of the pores at the surface of the second layer in contact with the first layer. In the case of a metal mesh, the pore size corresponds to the mesh width.

[0052] Further preferably, according to the present invention, the thickness of the first layer is greater than the thickness of the second layer. The main part of the function 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 can be minimized, thus providing a large capacity per weight of the network structure in the case where the network is a battery electrode.

[0053] By arranging a 3D network on the second layer, a composite structure of a 3D structure and a 2D substrate such as a film, a screen fabric or a 2D grid structure is achieved.

[0054] Preferably, the 3D network of the present invention is part of a battery electrode.

[0055] In the present document, all the preferred embodiments and features described above or in the claims with respect to the 3D network itself apply, mutatis mutandis, to the method of manufacturing such a 3D network and to the method of manufacturing an electrode.

[0056] For example, for the method of manufacturing a 3D network according to the invention, it is also preferred that the first metal fiber and the second metal fiber are of at least substantially the same material. Thus, a stable bond between the two fibers can be obtained and the desired mechanical and electrical stability can be achieved. Further details of the first metal fiber and the second metal fiber have been described in detail above. Thus, preferably, the first layer and the second layer are fixed to each other such that an electrical contact is formed between these layers. The fixing of the first layer and the second layer can be achieved in different ways, such as sintering, brazing, ultrasonic welding or by adhesion.

[0057] Whether or not the network of metal fibers has been sintered before being placed on the second layer, the second layer is preferably a porous structure. The porous structure of the second layer may be advantageous for subsequent processing steps, which should avoid compression of the first layer.

[0058] According to the invention, the method of manufacturing a 3D network of metal fibers is preferably carried out in a roll-to-roll process. Thereby, large-scale manufacturing can be achieved.

[0059] As described above, the invention also relates to a method of manufacturing an electrode, in particular a battery electrode. The method of manufacturing an electrode uses a 3D network according to the invention. In this method, the electrode material is preferably provided as a slurry having a thixotropic behavior.

[0060] In a preferred aspect of the method of manufacturing an electrode according to the invention, the second layer is porous. Thereby, the first layer can be loaded with the active material (in particular the active electrode material) by passing the active material through the pores of the second layer. A person skilled in the art can select the pore size of the second layer according to the particle size and / or the fluid behavior of the active material.

[0061] Preferably, the active material is introduced into the first layer in the form of a slurry, in particular a thixotropic slurry. A thixotropic slurry reduces its viscosity when subjected to sheering forces. After the sheering forces no longer act on the slurry, its viscosity increases again. If desired, in order to adjust this rheological behavior, a person skilled in the art can add a corresponding commercially available rheological additive. Here, it is preferred when the thixotropic slurry is filled into the void portion of the 3D network structure (i.e., the first layer) through the porous second layer. This can be achieved by using a doctor blade. The thixotropic slurry is subjected to sheering forces when contacting the doctor blade. Therefore, the viscosity of the slurry decreases, and the slurry can flow through the pores of the second layer into the first layer. Thus, no mechanical load is applied to the first layer, thereby protecting it from any compression. After the thixotropic slurry penetrates into the first layer, it no longer undergoes sheering forces, so its viscosity increases, and thus it does not flow out of the first layer but remains in the first layer.

[0062] In a preferred embodiment of the present invention, at least the active electrode material is loaded into the first layer through the pores of the second layer as a roll-to-roll process. Even more preferably, the method of manufacturing the electrode includes the method of manufacturing the 3D network of the present invention, i.e., as described herein.

[0063] The present invention also relates to a battery, which includes 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, especially when the thickness of the 3D network is 200 μm or greater, as preferably described above, the battery can be especially a mono-cell battery. As mentioned herein, a mono-cell battery is not a stack of many battery bodies, but a single battery body having an anode, a cathode, and a separator therebetween. Of course, the battery can also be a multi-body battery.

[0064] Furthermore, the present invention relates to a filter, which includes at least one filter material having a 3D network of metal fibers according to the present invention or a composite structure according to the present invention. Description of the Drawings

[0065] The present invention will now be described in more detail by way of example only, with reference to the drawings and diagrams and various examples of the network and method of the present invention. Shown in the drawings are:

[0066] Figure 1 is a schematic diagram showing the layers of the composite network structure of the present invention and naming the z-axis and the x-y plane,

[0067] Figure 2 is a schematic diagram showing the method of manufacturing the composite network structure according to the present invention.

[0068] Figure 3is a schematic diagram showing a method of manufacturing an electrode according to the present invention.

[0069] Figure 4 is a scanning electron microscope image of a 3D network of metal fibers according to Example 1.

[0070] Figure 5 is a scanning electron microscope image of a 3D network of metal fibers according to Example 2.

[0071] Figure 6 is a composite network structure according to Example 3 of the present invention.

[0072] Figure 7 is a composite network structure according to Example 4 of the present invention. Detailed Description

[0073] For clarity, in some of the figures, not all reference numerals are shown or reference numerals are not shown.

[0074] In Figure 1 a composite network structure 1 of the present invention is schematically shown. The shown 3D composite structure 1 has a porous first layer 10 of a 3D network of metal fibers and a conductive second layer 20. Each of the first layer and the second layer extends in the x-y plane, as shown by the coordinate system in Figure 1 The first layer 10 and the second layer 20 are in direct surface contact with each other. In the figure of Figure 1 the surface contact between these layers extends parallel to the x-y plane. As shown in Figure 1 the first layer 10 is a 3D network of two kinds of metal fibers sintered to each other. In the drawing of Figure 1 only this network is schematically shown, so the individual fibers 12 are not denoted by reference numerals (see Figure 2 ). The second layer 20 is a support layer, and the fibers of the network of the first layer 10 are directly fixed to this support layer by sintering. Thus, the fibers in the network are in direct electrical contact with the conductive second layer 20. When an electrochemical process provides electrons in the first layer, these electrons only have to travel through the network in the direction of the z-axis before reaching the second layer 20 (which can be used as a current collector), which is supported by the first kind of metal fibers in the 3D network. Therefore, through the interaction between the first kind of metal fibers and the conductive second layer, the electrical load on the individual fibers is reduced because the electrons do not have to pass through the metal fiber network in the x-y plane to reach an electrical connector (not shown in the figure), and the electron transport in the z-axis direction is supported by the first kind of metal fibers.

[0075] In Figure 2 a method of preparing a 3D network is shown. In step A, a mixture of two kinds of metal fibers is provided. In Figure 2In the schematic diagram, the mixture contains metal fibers 14 that are loosely entangled with each other, i.e., not yet sintered to each other. In Figure 2 , only seven metal fibers are shown. In addition, for clarity, the schematic diagram does not show different types of metal fibers (even though the present invention requires two different types of such metal fibers). In addition, it should be understood that in reality, the number of metal fibers is much higher than Figure 2 the number of metal fibers in the schematic diagram. The conductive second layer is provided from roller 21. Roller 21 rotates in the direction shown by the white arrow. In step C, the mixture of metal fibers 14 (i.e., the loosely arranged metal fibers) is placed on the second layer 20. In step D, the first layer 10 is fixed to the second layer 20 by sintering. Thereafter, the network structure 1 is wound onto roller 22, which rotates in the direction shown by the white arrow. This shows how the present invention for manufacturing a 3D network structure is implemented as a roll-to-roll process. Although the winding process is shown in Figure 2 , it is clear that the process with steps A to D as described above can also be implemented without rollers 21 and 22.

[0076] Figure 3 A schematic diagram of the manufacturing of the electrode 30 according to the present invention is shown in Figure 3 . In step E, the 3D network 1 according to the present invention is provided from roller 23. The 3D network has a composite structure having a first layer 10 and a second layer 20. Roller 23 rotates in the direction shown 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 includes a network of metal fibers 14 sintered to each other. The second layer 20 is a porous layer. The network structure 1 is conveyed from roller 23 to a doctor blade 40, where the thixotropic slurry 42 of the active electrode material is subjected to shear force and is pressed through the second layer 20 into the first layer 10, thereby forming the electrode 30 after a drying step that is not shown in Figure 3 . Since the doctor blade only acts on the second layer 20, the first layer does not undergo any pressure, thus avoiding the undesired compaction of the first layer 10. If circumstances permit, the drying step can be carried out directly after filling the pores with the slurry 42, or at another stage, or omitted completely. As Figure 3 shown, the electrode is then wound onto roller 24, which rotates in the direction shown by the white arrow. Thus, the manufacturing of the electrode according to the present invention is implemented as a roll-to-roll process.

[0077] In principle, the Figure 2 and Figure 3 roll-to-roll processes can be carried out as a single integrated process, replacing winding the network structure 1 onto roller 22 and instead guiding it to the doctor blade 40, as Figure 3 shown.

[0078] The following examples further illustrate the present invention:

[0079] Examples 1 and 2 and Comparative Examples 1 and 2 describe the manufacture of a 3D network of metal fibers according to the present invention. Also according to the present invention, such a 3D network can be used as the first layer of a composite network structure.

[0080] Example 1: For this example, ribbon-shaped flat fibers obtained by melt spinning (about 100 μm wide and 5 μm thick, equivalent radius about 12.6 μm) are mixed with nearly circular fibers (radius about 45 μm) of the same material composition in a 50:50 ratio.

[0081] The fibers are sintered at 620 °C (average heating rate of 200 K / min, argon atmosphere, holding time of 60 seconds, natural cooling, i.e., removed from the furnace and allowed to cool to room temperature by itself). From Figure 4 the scanning electron microscope images, it can be recognized that the fibers are bonded together and the shape of the fibers has not changed. The network has a thickness of 950 μm after sintering.

[0082] Comparative Example 1 is carried out in the same manner as Example 1. However, only ribbon-shaped flat fibers are used without using circular fibers. The thickness of the network after sintering is only about 300 μm.

[0083] The comparison between Example 1 and Comparative Example 1 shows that using two different types of fibers, one having a ribbon shape with a non-circular cross-section and the other having a circular cross-section, results in improved mechanical stability during the sintering process, thus avoiding the compaction of metal fibers.

[0084] Example 2: For this example, ribbon-shaped flat fibers obtained by melt spinning (about 80×5 μm) are used. Half of the fibers (by weight) are aged at 400 °C for 60 minutes in an inert gas and sintered with the untreated fibers at 620 °C as described in Example 1 above. The fibers are firmly bonded together after sintering, and the thickness of the sintered network is 750 μm, as Figure 5 shown.

[0085] After the aging treatment, the aged fibers do not change their shape. However, according to DSC measurements, the melting temperature of the aged fibers increases by about 7 K compared to the unaged fibers.

[0086] Comparative Example 2 is carried out in the same manner as Example 2. However, only untreated ribbon-shaped flat fibers, i.e., fibers without aging, are used. The final network has a thickness of only 250 μm.

[0087] The comparison between Example 2 and Comparative Example 2 shows that using two fibers with different melting points improves the mechanical stability during the sintering process, thus avoiding the compaction of metal fibers.

[0088] Examples 3 and 4 illustrate the preparation of the composite network structure according to the present invention.

[0089] Example 3: A network of sintered metal fibers (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 itself to room temperature)) was placed on a copper foil (99.9 +, 10 μm thickness) and sintered together using the same 920 °C temperature program as for the initial sintering of the network of metal fibers. A photograph of the resulting composite network structure is shown in Figure 6 in.

[0090] Example 4: Unsintered fibers were placed in a loose manner on a fine mesh copper screen 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 itself to room temperature). The fibers within itself and with the copper network were firmly sintered together. A photograph of the resulting composite network structure is shown in Figure 7 in.

[0091] The thickness and width of the metal fibers as mentioned herein can be determined by scanning electron microscopy or by micro-CT. The melting point of the metal fibers can be measured by DSC.

[0092] Reference Numerals

[0093] 1 Composite network structure

[0094] 10 First layer

[0095] 14 Metal fiber

[0096] 20 Second layer

[0097] 21 Roller

[0098] 22 Roller

[0099] 23 Roller

[0100] 24 Roller

[0101] 30 Electrode

[0102] 40 Doctor blade

[0103] 42 Slurry

Claims

1. A three-dimensional (3D) network of metallic fibers, comprising a plurality of metallic fibers fixed to each other, Among them, wherein the plurality of metallic fibers (14) comprises a first type of metallic fiber and a second type of metallic fiber, and wherein the first type of metallic fiber and the second type of metallic fiber (14) are different from each other, in particular with respect to their mechanical properties.

2. The three-dimensional (3D) network of metallic fibers according to claim 1, Among them, wherein the second type of metallic fiber has 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 wherein the second type of metallic fiber has 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 wherein the thickness and / or width of the first type of metallic fiber is at least a multiple of the thickness and / or width of the second type of metallic fiber.

3. The three-dimensional (3D) network of metallic fibers according to claim 1 or 2, Among them, wherein the cross-sectional form of the first type of metallic fiber is different from that of the second type of metallic fiber, and / or wherein the melting points of the first type of metallic fiber and the second type of metallic fiber before sintering with each other differ by at least 1 K, the melting points being determined by DSC measurement, and / or wherein the longitudinal shape of the first type of metallic fiber is different from that of the second type of metallic fiber, and / or wherein the width and / or thickness of the first type of metallic fiber is different from that of the second type of metallic fiber, and / or wherein the metallic alloys of the first type of metallic fiber and the second type of metallic fiber are different, in particular with respect to the addition of alloys to the same base metal.

4. The three-dimensional (3D) network of metallic fibers according to any one of the preceding claims, Among them, wherein at least the second type of metallic fiber has a non-circular cross-section, in particular a ribbon shape, and / or wherein the first type of metallic fiber has a substantially circular cross-section.

5. The three-dimensional (3D) network of metallic fibers according to any one of the preceding claims, Among them, wherein the melting points of the first type of metallic fiber and the second type of metallic fiber before sintering with each other differ by at least 1 K, the melting points being determined by DSC measurement.

6. The three-dimensional (3D) network of metallic fibers according to any one of the preceding claims, Among them, wherein the first type of metallic fiber and the second type of metallic fiber are of substantially the same material.

7. The three-dimensional (3D) network of metallic fibers according to any one of the preceding claims, Among them, wherein the weight ratio of the first type of metallic fiber to the second type of metallic fiber is in the range of 1:99 to 99:1, 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 further more preferably in the range of 40:60 to 60:40, and / or wherein the ratio of the first type of metallic fiber to the second type of metallic fiber is set such that the amount of the first type of metallic fiber is higher than the percolation threshold.

8. The three-dimensional (3D) network of metallic fibers according to any one of the preceding claims, Among them, The 3D network of the 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 further more preferably at least 1,000 μm.

9. Composite structure (1), comprising a three-dimensional (3D) network of metal fibers according to any one of the preceding claims, Among them, wherein the 3D network is a first layer (10) and is arranged on a second layer (20), which is different from the first layer (10).

10. Composite structure according to claim 9, Among them, wherein the second layer (20) is of a material that is at least substantially the same as the first metal fiber and / or the second metal fiber (14), and / or wherein the first layer and the second layer are fixed to each other such that an electrical contact is formed between these layers.

11. Composite structure according to claim 9, Among them, wherein the second layer (20) is made of a material different from the first metal fiber and the second metal fiber and is in particular made of a polymer material, and / or wherein the second layer (20) can be removed from the 3D network of the metal fibers, for example by peeling.

12. Composite structure according to any one of claims 9 to 11, Among them, wherein the second layer (20) is a porous layer, in particular a porous metal layer or a porous non-metal layer, and / or wherein 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 further more preferably at least 20 μm.

13. Composite structure according to any one of claims 9 to 12, Among them, wherein the second layer (20) is a porous layer or a non-porous layer.

14. Method for manufacturing a three-dimensional (3D) network of metal fibers, comprising the following steps: Step A: Mixing a first metal fiber (14) and a second metal fiber (14) with each other, thereby providing a mixture of a plurality of metal fibers (14), Step B: Sintering the metal fibers (14) in the mixture obtained in step A with each other to form a network of metal fibers according to any one of claims 1 to 8 or a composite structure (1) according to any one of claims 9 to 13.

15. Method for manufacturing an electrode, in particular a battery electrode, the method being: providing a composite structure (1) according to claim 12, and introducing a slurry of an active electrode material through the porous second layer, and / or Introduce a slurry of active electrode material through the pores of the 3D network of the metal fibers, wherein, The pores of the 3D network of the metal fibers are the voids between the metal fibers.

16. A battery, comprising at least one electrode having a 3D network of metal fibers according to any one of claims 1 to 8, wherein, The battery is in particular a single cell or a multi-cell battery.

17. Filter, comprising at least one filter material having a 3D network of metal fibers according to any one of claims 1 to 8 or a composite structure according to any one of claims 9 to 13, in particular wherein the second layer is a porous layer.

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