Electrode with a nanostructure template comprising a plurality of multi-generation nanostructures, and method of making the electrode

Abstract

An electrode for an electrochemical energy storage device, such as lithium-ion batteries or sodium-ion batteries, is configured to use a reversible intercalation of ions into the electrode's material to store energy. The electrode comprises: a substrate, such as a semiconductor-based or metal substrate. It further comprises a nanostructure template comprising a plurality of multigeneration nanostructures, wherein each of the nanostructures is formed on a surface of the substrate and comprises one or more first- generation substructures, each being rooted to the substrate, and one or more substructures of at least one further generation extending directly from a respective substructure of the next lower generation. The electrode further comprises a (first) layer, which coats at least a portion of the respective surface of each nanostructure and serves as an electrochemically active material of the electrode. Further provided are an electrochemical energy storage device comprising such an electrode, and a method of making such an electrode.

Description

[0001] ELECTRODE WITH A NANOSTRUCTURE TEMPLATE COMPRISING A PLURALITY OF MULTI-GENERATION NANOSTRUCTURES, AND METHOD OF MAKING THE ELECTRODE

[0002] The present invention relates to the field of electrodes, particularly electrodes serving as negative electrodes, for electrochemical energy storage devices, such as lithium-ion batteries or sodium-ion batteries, being configured to use a reversible intercalation of ions into the electrode’s material to store energy.

[0003] Specifically, the invention is directed to an electrode of this type, to an electrochemical energy storage device comprising such an electrode, and to a method of making such an electrode.

[0004] Electrochemical energy storage devices (often referred to as “batteries”) of different types, including particularly accumulators exhibiting a high-energy storage capacity, have become a ubiquitous element of many small and large electrically powered devices and systems. Examples of such devices and systems include hearing aids, portable computers, mobile communication devices and electrically powered vehicles, such as battery-electric vehicles (BEV) or hybrid electric vehicles (HEV), just to name a few.

[0005] An important category of such energy storage devices are rechargeable batteries, such as lithium-ion batteries or sodium-ion batteries, which use a reversible intercalation of ions (such as lithium ions or sodium ions, respectively) into the material of their electrodes of one polarity, esp. negative electrodes, to store energy. Known types of such electrodes include carbon-based electrodes (particularly graphite-based electrodes) and silicon-based electrodes.

[0006] Generally, in battery technology, the negative electrode of a battery cell (which is the anode when the cell is discharging) and the positive electrode (which is the cathode when the cell is discharging) are prevented from shorting by a separator. The negative and positive electrodes swap their electrochemical roles (anode and cathode) when the cell is charged. However, frequently the negative electrode of a rechargeable cell is often just called "the anode" and the positive electrode "the cathode". This common nomenclature is adopted herein. Accordingly, an electrode of the present invention may particularly be used as an anode of an electrochemical energy storage cell using a reversible intercalation of ions into the electrode’s material. In comparison to conventional carbon-based anodes, silicon-based anodes typically exhibit a much higher intercalation capacity (e.g., approx. 10x in the case of lithium-ion absorption in graphite anodes) for absorbing the ions when the cell is charging. However, when absorbing such high amounts of ions, silicon swells dramatically, e.g. in some examples as high as by 400%, which typically results in a gradual destruction of the anode and hence a shortened battery lifetime.

[0007] It is an object of the present invention to provide an improved electrode being configured to use a reversible intercalation of ions into the electrode’s material to store energy and a method of making same. Specifically, it is an object to provide an electrode which can improve one or more of a battery life, a battery capacity, a battery charging speed, an initial coulombic efficiency, and an output power of a battery comprising the electrode, e.g. as its anode.

[0008] A solution to this problem is provided by the teaching of the independent claims. Various preferred embodiments of the present solution are provided by the teachings of the dependent claims.

[0009] A first aspect of the present solution is directed to an electrode, such as an anode, for an electrochemical energy storage device, such as a lithium-ion cell or a sodium-ion cell, being configured to use a reversible intercalation of ions into the electrode’s material to store energy.

[0010] The electrode comprises:

[0011] (i) a substrate, such as a semiconductor-based or metal substrate;

[0012] (ii) a nanostructure template comprising a plurality of multigeneration nanostructures, wherein each of the nanostructures is formed on a surface of the substrate and comprises:

[0013] - one or more first-generation substructures, each being rooted to the substrate; and

[0014] - one or more substructures of at least one further generation extending directly from a respective substructure of the lower generations.

[0015] The substrate and the nanostructure template are each made from a respective electrically conductive material.

[0016] In an electrode of this kind, any forces resulting from an expansion or contraction of the substrate and / or nanostructure template particularly during the use of the electrode in an electrochemical storage device, and / or the weight of an active material provided on the nanostructure, if any, can be distributed over multiple generations of substructures and not only over the first-generation substructures. The substructures of the second and higher generations (if any), most of which will typically be located in the intermediate space between neighboring first-generation substructures. Thus, they can provide a more even distribution of the substructures across the nanostructure template so that the three- dimensional scaffold defined by the set of substructures will typically have a higher density when compared to a template comprising first-generation-only substructures, such as mere nanowires.

[0017] Accordingly, this enables a high degree of mechanical robustness of the electrode and / or an increased capability of the electrode to efficiently supply and collect electric currents (upon charging or discharging, respectively) across its surface being populated by the nanostructures, particularly also in the case of high current loads. Specifically, the high mechanical robustness may lead to a reduced degradation of the electrode, particularly to a reduced risk of cracks in the active material or in the nanostructure template or in the substrate. This may thus have a positive effect on the battery life and / or the achievable charging / discharging speeds (hence output / input power) of a battery having such an electrode, e.g. anode.

[0018] Consequently, such an electrode may also be used to increase the battery capacity as it allows for a greater (mass) amount of active material to be used in the electrode without (significantly) adversely affecting the reliability, lifetime, and / or available current loads of the battery.

[0019] Terms

[0020] Some terms used herein to define the present solution are explained below in more detail:

[0021] The terms "electrochemical energy storage device" and “battery” variations of this term, as used herein, refer to a device which can store energy in an electrochemical manner. Without limitation, such a device may comprise a plurality of electrically interconnected galvanic cells, each having an anode, a cathode and a separator in-between. Specifically, electrochemical energy storage devices may be designed as rechargeable or non- rechargeable storage devices. Without thereby limiting the scope of the present solution as defined by the appended claims, the specific examples and embodiments described herein are particularly suitable for use with rechargeable batteries. The term "electrode" and variations of this term, as used herein, refer particularly to an electron conductor that interacts with a counter electrode (anode - cathode) with a medium located between the two electrodes. Electrodes are used to connect non-electron- conducting areas, such as an electrolyte, with electron-conducting areas, for example metal terminals, in electrochemical elements.

[0022] The term "intercalation" and variations of this term, as used herein, generally refer to the insertion of molecules, ions, or rarely also atoms into chemical compounds, whereby these compounds do not significantly change their structure during the insertion process, but typically, their volume, as discussed further above.

[0023] The term "substrate" and variations of this term, as used herein, refer to a homogeneous or heterogeneous body of material, e.g. a material layer, being present in a solid state in the operating temperature range of the electrode, particularly at temperatures below 100° C or more specifically below 60°C. In particular, the substrate can have a plate-like or foil-like shape, at least in sections. Specifically, a substrate may be a body of a semiconductor material, such as silicon or a Ill-V-semiconductor material (e.g., GaAs). Specifically, the semiconductor material may be on of a single-crystal, a poly-crystal, or amorphous. A substrate may also be a body comprising one or more of Zinc oxide, a metal (such as Zinc or Nickel), and any other conductive material.

[0024] The term “electrically conductive” and variations of this term, as used herein, refer to a material's ability to conduct electric current. Specifically, a material is deemed “electrically conductive” if the material exhibits an electrical conductivity a at a temperature of 20°C of a > 10 S / m. Specifically, a > 102S / m, or even a > 103S / m are particularly good choices.

[0025] The term "nanostructure" and variations of this term, as used herein, refer to a structure of intermediate size between microscopic and molecular structures. Particularly, structures having at least one dimension that has an extension of 1 micron or less are deemed nanostructures. This dimension could be, for example, a diameter or thickness of the nanostructure or a substructure thereof, or some other dimension of the nanostructure. Notwithstanding this limitation, other dimensions of the nanostructure may exceed this size limitation. Accordingly, the number of dimensions in the volume of an object on the nanoscale, i.e. a nanostructure, may vary. For example, (i) nanotextured surfaces typically have only one dimension on the nanoscale, i.e., only the thickness of the surface of an object is between 0.1 and 1000 nm; (ii) nanotubes typically have two dimensions on the nanoscale, i.e., the diameter of the tube is between 0.1 and 1000 nm while its length can be far more; and spherical nanoparticles have three dimensions on the nanoscale, i.e., the particle is between 0.1 and 1000 nm in each spatial dimension.

[0026] The term "nanowire" and variations of this term, as used herein, refer to a specific kind of elongated nanostructures having an aspect ratio AR, i.e., a ratio of its longest dimension to its shortest dimension, with AR >1 above 1 , typically with AR > 2 or even AR > 4. In some instances, even ratios of AR > 10 or AR > 100 are achieved.

[0027] The term "nanostructure template" and variations of this term, as used herein, refer to a material or structure with features on the nanoscale (typically between 1-1000 nanometers) that can be used as a scaffold or mold to direct the growth, assembly, or organization of other nanoscale materials or structures (i.e., nanostructures). Herein, the term "template" refers to the idea that the nanostructure provides a framework or pattern for the formation of other nanostructures, such as nanowires, much like a template in a craft project provides an outline or shape for a craft item. Nanostructure templates can be made from a variety of materials, including metals, semiconductors, polymers, and biomolecules. They can take various forms, such as nanoparticles, nanowires, nanotubes, or 2D / 3D structures like membranes, sheets, or frameworks. The specific properties of the template material and its surface features can influence the formation and properties of the nanostructures that grow on or inside it.

[0028] The term "multi-generation nanostructure" and variations of this term, as used herein, refer to a type of nanostructure that undergoes multiple stages of self-assembly or synthesis, resulting in the formation of complex and hierarchical structures at various length scales. Herein, the term "multi-generation" indicates that the nanostructure has been created through multiple generations of growth or assembly, with each generation involving a different level of complexity or organization.

[0029] In the context of multi-generation nanostructures, the term "substructure" of a specific generation, as used herein, refers to a smaller structure forming a part of a larger nanostructure and pertaining to said specific generation in the hierarchy of generations of the nanostructure to which it pertains. For example, in a tree-shaped nanostructure, the “stem portion” of the nanostructure forms a first-generation substructure and any branches extending directly from the stem each form a second-generation substructure, and so forth.

[0030] The term “semiconductor-based”, as used herein, refers to a material that is predominantly (e.g., by mass-percentage) a semiconductor material. Particularly, a semiconductor-based material may be substantially made from a crystalline or amorphous semiconductor material, such as silicon, but may in addition comprise a minor amount of one or more other materials, such as a dopant.

[0031] The terms “electrochemically active material” or its equivalent “active material”, as used herein, refer to a substance in an electrode for an electrochemical storage device that undergoes chemical reactions during charging and discharging of the storage device (battery or battery cell). These reactions involve the transfer of ions between the active material and the electrolyte of the storage device, resulting in the storage and release of electrical energy. The active material is typically a solid or semi-solid substance that is dispersed within or on another material, such as a porous matrix, of the electrode.

[0032] The terms “first”, “second”, “third” and the like in the description and in the claims, are used for distinguishing between similar elements and not necessarily for describing a sequential or chronological order. It is to be understood that the terms so used are interchangeable under appropriate circumstances, and that the embodiments of the present solution described herein are capable of operation in other sequences than described or illustrated herein.

[0033] Unless the context requires otherwise, where the term "comprising" or “including” or a variation thereof, such as “comprises” or “comprise” or “include”, is used in the present description and claims, it does not exclude other elements or steps and are to be construed in an open, inclusive sense, that is, as "including but not limited to".

[0034] Where an indefinite or definite article is used when referring to a singular noun, e.g. "a" or "an", "the", this includes a plural of that noun unless something else is specifically stated.

[0035] Appearances of the phrases “in some embodiments”, "in one embodiment" or "in an embodiment", if any, in the description are not necessarily all referring to the same embodiment. Furthermore, the particular features, structures, or characteristics may be combined in any suitable manner in one or more embodiments.

[0036] Further, unless expressly stated to the contrary, "or" refers to an inclusive or and not to an exclusive or. For example, a condition A or B is satisfied by any one of the following: A is true (or present) and B is false (or not present), A is false (or not present) and B is true (or present), and both A and B are true (or present). By the terms "configured" or "arranged" to perform a particular function, (and respective variations thereof) as they may be used herein, it is to be understood that a relevant device or component is already in a configuration or setting in which it can perform the function, or it is at least adjustable - i.e., configurable - in such a way that it can perform the function after appropriate adjustment. In this context, the configuration can be carried out, for example, through a corresponding setting of parameters of a process sequence or of hardware (HW) or software (SW) or combined HW / SW- switches or the like for activating or deactivating functionalities or settings. In particular, the device may have a plurality of predetermined configurations or operating modes so that the configuration can be performed through a selection of one of these configurations or operating modes.

[0037] In the following, without thereby limiting the invention as set forth in the appended claims, various embodiments of the electrode of the first aspect are described, which embodiments can be arbitrarily combined with each other or with other aspects of the present solution, unless such combination is explicitly excluded or technically impossible.

[0038] In some embodiments, each nanostructure in at least a subset of the nanostructures has a hierarchical branched tree shape comprising as a first-generation substructure a stem portion formed as a nanowire and rooted in the substrate and as second-generation substructure multiple first branches extending from the stem portion. An advantage of such a hierarchical branched tree shape is the possibility to create a “nanoforest” of many neighboring similarly shaped nanostructures have a rather high spatial penetration of the first layer when the latter is disposed on the nanoforest. This can be used to provide very mechanically robust electrodes with a high electrochemical capacity.

[0039] In some embodiments, the hierarchical branched tree shape further comprises as respective further-generation substructures multiple second or higher-level branches, each extending from a branch of a respective next lower generation.

[0040] Such branches of the second or higher generations help to further distribute pressure caused by the weight, expansion and / or contraction of the first layer among neighbor nanowires on the stem and branch level (i.e. substructures of the second and any higher generations, if any) and also help to increase the electrical connection between the nanostructure template and the first layer, as already discussed above. A higher number of generations provided in the nanostructures, i.e., three or more generations, will hereby typically improve the mechanical and electrical anchoring of the first layer to the nanostructure template and hence the mechanical and electrical properties of the electrode.

[0041] In some embodiments, for each hierarchical branched-tree-shaped nanostructure in at least a subset of two or more of the nanostructures, the average spatial extension L of its first branches, when measured for each such branch between its proximal end at the stem portion and its distal end, is L = D z, wherein z >0.25 and D is the average distance between the stem portion of such nanostructure and the stem portion of its nearest neighboring nanostructure. Specifically, the value of z may be chosen so as to satisfy z > 1 , z > 2 or even z > 3. The higher the value of z, the more likely will more second-generation substructures (branches) per stem arise when the nanostructure template is built, particularly when a growth process for growing the nanostructures is involved. The higher the average number of second-generation substructures (branches) per first-generation substructure (stem portion) the more likely will these second-generation substructures be arranged at different heights at the stem so that the first layer will be distributed over multiple levels defined by the different heights of the branches. Accordingly, the volume or mass of the first layer and thus the active material may be increased, and a higher battery capacity can thus be enabled.

[0042] In some embodiments, the respective stem portion of each nanostructure in at least a subset of the tree-shaped nanostructures has a maximum diameter d, measured e.g., in a direction being orthogonal to the spatial dimension along which the stem portion has its largest spatial extension, with d > 50 nm. Specifically, a good choice of a diameter d of the stem portion may be anywhere in the range of 50 nm to 1000 nm, but it may even be greater than 1000 nm. Such a strong nanowire provides a particularly strong mechanical support to the first layer. Furthermore, the second-generation substructures may be created as nanowires as branches extending from the stem portion.

[0043] In some embodiments, wherein for at least one generation of substructures of the nanostructures, the respective longest spatial extensions of such substructures are distributed in such a way, that lengths of these extensions can be binned, in the sense of a statistical data binning, in two or more bins, each bin defining a limited range for the lengths, so that the smallest difference between two lengths from any two different neighboring bins is greater by at least a factor y > 1 ,2 than the largest difference between any two lengths from a same one of these neighboring bins. For example, nanowires can be grown to have sizes distributed among multiple bins according to size (long, medium short). Particularly, generation of first-generation substructures according to such a binning approach, significantly increases the likelihood or can even ensure that the locations at which the second-generation branches extend from the first-generation stems will be distributed across different height levels along the first-generation stems with the related advantages regarding higher battery capacity etc., as discussed above.

[0044] In some embodiments, one or more of the nanostructures in the plurality of nanostructures comprises a nanowire, a nanotube, a nanoparticle, a nanosphere, a nanorod, a nanowhisker. Specifically, a nanostructure may comprise a mix of two or more of the different structural components, e.g. at different levels of the hierarchy of substructure generations. For example, the first-generation substructure of a nanostructure might be a nanowire or nanotube, a second-generation substructure might be a nanorod and third- generation substructure might be a nanoparticle, such as a nanosphere. Specifically, such a hierarchical approach may involve using stronger types of structures, esp. such with a greater diameter, for the lower-generation substructures, particularly for first-generation substructures, and use lighter types of structures, esp. one having a smaller diameter or more generally smaller dimensions for the higher generations. Particularly, in some examples, the dimension of each next generation is smaller than that of the next lower generation. This helps to achieve a very high mechanical robustness of the nanostructure template itself because the weight of the higher-generation substructures which in their majority will typically tend to extend away from the first-generation substructures can thus be reduced and any mechanical forces or torques acting back on the first-generation substructures can be effectively limited.

[0045] In some embodiments, the electrode further comprises a first layer, which coats at least a portion of the respective surface of each nanostructure and serves as an electrochemically active material of the electrode. The first layer is preferably made of conductive material but might comprise one or more semiconductor materials and / or non-conductive materials instead or in addition. The first layer can help to bridge gaps between neighboring nanostructures to provide an electrical and mechanical coupling therebetween. The active material can thus be distributed more homogeneously across the nanostructure template, and therefore the thickness of the first layer may be kept more limited. Thus, a higher degree of penetration of the scaffold being formed by the nanostructures into the first layer can be achieved, particularly in a more homogenous manner so that the weight of the first layer is more evenly distributed within the nanostructure template and currents can be supplied and collected more efficiently and homogeneously. In some embodiments, each nanostructure in at least a subset of the nanostructures is physically interconnected with one or more neighboring nanostructures through respective interconnections formed by the first layer and / or any one or more further layers being disposed on the first layer (if any, such as the second or third layers described further below). In this way, the first layer (and / or further layers) forms bridges between neighboring nanostructures which helps to further strengthen the mechanical robustness and the overall electrical conductivity of the electrode.

[0046] Furthermore, the interconnections may define a plurality of pores as intermediate hollow spaces being located between neighboring nanostructures. Such pores may particularly serve as spaces into which the active material may “breathe”, i.e. expand or contract during a charge / discharge cycle (in the case of a rechargeable battery). Accordingly, the pores allow the first (and further) layers to expand or move into these spaces, at least partially, when stressed and thus serve as stress-relief means, hence as means for increasing the robustness, reliability and lifetime of batteries equipped with such an electrode.

[0047] In some embodiments, at each nanostructure of at least a subset of the nanostructures, the first layer coats, at least in parts, a respective surface of each of at least two generations of the nanostructure. This provides for a particularly strong anchoring of the first layer on the nanostructure template and for a high degree of penetration of the first layer by the nanostructure template. This, in turn, helps to provide good electrical conditions, i.e. a low overall electrical resistance between the first layer and the nanostructure template.

[0048] In some embodiments, the nanostructure template comprises or consists of one or more of the following materials: a semiconductor material (doped or undoped), such as silicon- based or Ill-V-type semiconductor material (e.g., GaAs-based or InP-based), gold, copper silver, nickel, zinc, zinc oxide, aluminum. All of them have a high electrical conductivity (in the case of the semiconductor material, at least when properly doped) and are at the same time suitable for effectively growing nanostructures, particularly of the multi-generation type. The semiconductor material is provided in one of the following forms: single-crystal, polycrystal, amorphous.

[0049] In some embodiments, the substrate and the nanostructures are made of a same material. This is particularly advantageous regarding the manufacturing process, where the nanostructures can either be grown directly from the material of the substrate or, if added, do not cause material incompatibility issues. In the alternative, in some embodiments the substrate is made from a different material than at least one of the nanostructures. This allows for an optimal choice of material for each of the substrate and the at least one nanostructure individually.

[0050] In some embodiments, the substrate is a GaAs-based substrate (e.g., in a 110, 100, or 111 B orientation) and at least one of the first layer and a nanostructure (of any generation) of the template comprises nanowires. Accordingly, in this case the nanostructures may particularly be made from Gallium-Arsenide (GaAs) as well. Specifically, a tilted nanowire may be rooted in the substrate as a first-generation nanostructure. Accordingly, in this case the nanostructure(s) may particularly be made from Gallium-Arsenide (GaAs) as well.

[0051] In some embodiments, each nanostructure of at least a subset of the nanostructures is rooted to the substrate at a location that is defined by a respective seed nanoparticle, such as a gold nanoparticle, being provided at such location and comprising a material being different from that of the nanostructure. For example, the seed nanoparticle may be a nanoparticle of a suitable metal and the nanostructure may be made from a semiconductor material, such as GaAs. Accordingly, the seeds are useful to define the location and support the growth of first-generation substructures on the substrate and / or of higher-generation substructures on a respective substructure of the next lower generation.

[0052] In some embodiments, each nanostructure of at least a subset of the nanostructures is rooted to the substrate in a seedless manner. An advantage of this is that during the process of forming the template, the relative location of such a nanostructure on the substrate can be adjusted before the nanostructure is fixed to the substrate.

[0053] In some embodiments, each nanostructure of at least a subset of the nanostructures is rooted to the substrate at a location that is defined through a coating of the substrate and / or of higher-generation substructures on a respective substructure of the next lower generation. Accordingly, the coating is structured as a mask, similarly as in semiconductor lithography, so as to defining the locations by openings in the coating.

[0054] In some embodiments, the electrode further comprises a semiconductor-based second layer that coats the first layer, at least in parts.

[0055] Specifically, the first layer may have a first average mass density and the second layer has a second average mass density being higher than the first average mass density. The difference in density may particularly be caused (at least in part) by the presence of more voids within the material of the first layer, even in cases where both layers are made from the same material, e.g. from silicon. The conductivity of the semiconductor-based material of the second layer may particularly be based on a dopant provided in the material. The material of the second layer may particularly comprise silicon or a SiFx material with 0 <x < 1 , wherein F is one of nitrogen, carbon, boron, phosphorus, oxygen, magnesium, aluminum, germanium, tin, nickel, copper, titanium, zinc or a zinc oxide, and combinations thereof. The second layer may comprise electrochemically active material so that it complements the electrochemical capacity already provided by the first layer.

[0056] Specifically, the nanostructure template may comprise a surface portion which is not coated by the first layer, but wherein the second layer further coats this surface portion in whole or in part. This is particularly useful, when a deposition of the first layer is effected during production in such a way that the first-generation substructure is not completely coated, e.g. at its lower part next to the substrate, by the first layer. The second layer thus adds further electrochemical capacity to the electrode, i.e. to a battery equipped with it.

[0057] In some embodiments, the electrode further comprises a supplementary third layer that coats the nanostructure template, at least in part, so that at least a section of the first layer is sandwiched between the third layer and the second layer. The third layer has, at a temperature of 20°C, a higher electrical conductivity than the first layer and may particularly be a metal layer. It may thus provide a higher conductivity of the electrode, specifically at the interface of the nanostructure template and the active material of the first layer and improve resistance to mechanical stress. It may also be used to provide an improved adhesion interface between the nanostructure template and the first layer. The third layer may particularly be formed from a material comprising metals or conductive materials, for example, Ti, Ni, Si, or alike or any combination of two or more of the foregoing. The deposition of such materials, typically as one or more thin films forming the third layer, may particularly involve magnetron sputtering deposition or a thermal evaporation method, for example.

[0058] Specifically, in some embodiments the first layer is a semiconductor-based layer (such as silicon, the second layer is a semiconductor-based, e.g. silicon-based, layer, and / or the third layer is a metal layer. According to a particularly preferred selection, the first layer is a layer of amorphous silicon, the second layer is a SiNx layer with 0.01 < x < 0.85.

[0059] A second aspect of the present solution is directed to an electrochemical energy storage device, such as a lithium-ion cell or a sodium-ion cell, having an electrode, such as an anode, of the first aspect and being configured to use a reversible intercalation of ions into the electrode’s material to store energy.

[0060] A third aspect of the present solution is directed to a method of making an electrode of any one of the preceding claims. The method comprises:

[0061] (i) providing a substrate;

[0062] (ii) building, on the substrate, a nanostructure template comprising a plurality of multigeneration nanostructures, wherein each of the multigeneration nanostructures is formed on a surface of the substrate and comprises a first-generation substructure being rooted to the substrate and respective substructures of one or more further generations being rooted to a respective substructure of the next lower generation; wherein each of the substrate and the nanostructure template are made from a respective electrically conductive material.

[0063] In some embodiments, the method further comprises coating at least a portion of the surface of the nanostructure template with a first layer serving as an electrochemically active material of the electrode, wherein the coating comprises coating at least a portion of the respective surface of each nanostructure with the first layer.

[0064] In the following, without thereby limiting the invention as set forth in the appended claims, various embodiments of the method of the third aspect are described, which embodiments can be arbitrarily combined with each other or with other aspects of the present solution, unless such combination is explicitly excluded or technically impossible.

[0065] In some embodiments, the first layer is formed with a material having a first average mass density and the method further comprises coating the first layer, at least in parts, with a second electrically conductive layer having a second average mass density being higher than the first average mass density.

[0066] In some embodiments, providing the substrate comprises cleaning a surface of the substrate and building the nanostructure template comprises:

[0067] (i) growing a buffer layer of material on the cleaned surface of the substrate as a base material of the nanostructure template; (ii) depositing a plurality of nanoparticles on the buffer layer as seeds for a growth of a first generation of substructures of the nanostructures on the substrate;

[0068] (iii) applying environmental conditions to the substrate to cause a growth of first-generation substructures of the multi-generation nanostructures from the buffer layer at the location of the seeds; and

[0069] (iv) for each further generation of substructures of the multi-generation nanostructures: depositing a plurality of nanoparticles on the substructures of the next lower generation as seeds for a growth of said further generation of substructures of the nanostructures on the substructures of the next lower generation; and applying environmental conditions to the substrate to cause a growth of the substructures of said further generation of the multi-generation nanostructures from the substructures of the next lower generation at the location of the seeds of said further generation.

[0070] In some embodiments, at least a section of a wafer of a semiconductor- based material (e.g., Si or GaAS) or metal-based material is used as the substrate.

[0071] In some embodiments, the plurality of particles used as seeds comprises nanoparticles made of one or more of gold, silver, germanium. Specifically, gold nanoparticles are particularly suitable to promote growth of nanowires and they do not react with the supplied materials (gold rather can act as a catalyst).

[0072] In some embodiments, the substructures of at least one of the generations of the multigeneration nanostructures comprise nanowires.

[0073] In some embodiments, growing the substructures of at least one of the generations of the multi-generation nanostructures comprises application of one or more of the following processes:

[0074] - a molecular beam epitaxy, MBE;

[0075] - applying hydrothermal environmental conditions to the substrate;

[0076] - Electrodeposition

[0077] - Chemical Vapor Deposition, CVD;

[0078] - Plasma Enhanced Chemical Vapor Deposition, PECVD

[0079] - Causing the growth in the absence of air or another oxidant at the location of growth. The detailed explanations, features, examples, and advantages provided above regarding the electrode of the first aspect apply mutatis mutandis to the energy storage device of the second aspect and the method of the third aspect.

[0080] BRIEF DESCRIPTION OF THE DRAWINGS

[0081] Further advantages, features and applications of the present solution are provided in the following detailed description and the appended figures, wherein:

[0082] Fig. 1 schematically illustrates a basic design of an electrochemical energy storage device with two electrodes;

[0083] Fig. 2 schematically illustrates an exemplary single (hierarchical branched-tree-shaped) multi-generation nanostructure rooted to a substrate;

[0084] Fig. 3 schematically illustrates an exemplary nanostructure template provided on a substrate and having multiple (hierarchical branched-tree-shaped) multi-generation nanostructures of Fig. 2, thus forming a nanostructure forest;

[0085] Fig. 4 shows a scanning electron microscope image of an exemplary real nanostructure template in the form of a nanostructure forest of tree-shaped nanostructures;

[0086] Fig. 5 schematically illustrates an exemplary single (tree-shaped) multi-generation nanostructure of Fig. 2 and a coating thereof with a first and a second layer;

[0087] Fig. 6 schematically illustrates an exemplary electrode comprising the nanostructure forest of Fig. 3 and a coating thereof with a first and a second layer;

[0088] Fig. 7 shows a scanning electron microscope image of an exemplary real coated nanostructure template, i.e. the surface of a real electrode corresponding to the scheme of Fig. 6; and

[0089] Figs. 8A-8D schematically illustrate an exemplary embodiment of a method of making an electrode, e.g. the electrode of Fig. 6 or Fig. 7.

[0090] In the figures, identical reference signs are used for the same or mutually corresponding elements of the devices, arrangements, and methods described herein. When the following refers to a "step" or "steps" of the method, this does not mean that the associated action must necessarily take place in a single coherent operation. Rather, it is also possible that a "step" is composed of several individual operations in the sense of a process and thus corresponds to a sub-process of the method.

[0091] DETAILED DESCRIPTION OF EMBODIMENTS

[0092] Fig. 1 shows the well-known basic design of an electrochemical energy storage device 1 . Such devices are often also referred to as battery, battery cell, or galvanic cell, and may particularly be rechargeable, depending on their design. Prominent examples of rechargeable batteries are lithium-ion batteries and sodium-ion batteries. Energy storage device 1 comprises, within a battery housing 2, two electrodes 3 and 4 of opposite electrical polarity (which is the same for charging and discharging). The electrodes 3 and 4 are separated from each other by a separator 5 and are each in contact with an electrolyte 6 in which ions, such as lithium ions Li+in the case of a lithium battery, can move freely.

[0093] The separator 5 is permeable for the ions but not for electrons. Electrode 3 has a positive polarity and is usually referred to as “cathode” while electrode 4 has a negative polarity and is usually referred to as “anode”. When charged, the energy storage device 1 delivers a voltage V between the two electrodes 3 and 4 which results in a usable electric current, when an (electron conducting) electrical circuit between the electrodes is closed (outside the battery) and the battery discharges to deliver the current.

[0094] The following discussion focuses on the design and manufacturing of a specific proposed design of an electrode for an electrochemical energy storage device 1 , wherein the electrode may particularly be an anode (negative electrode 4). More specifically, the following discussion focuses on electrodes being configured to use a reversible intercalation of ions into the electrode’s material to store energy in the associated electrochemical energy storage device 1 .

[0095] Fig. 2 schematically illustrates, according to embodiments, an exemplary view 7 of a single hierarchical branched (tree-shaped) multi-generation nanostructure 9 rooted to a substrate 8. An electrode of the proposed design will typically comprise high number of nanostructures arranged next to each other to form together a highly complex overall structure at the nanoscale. Fig. 2 serves to discuss a single one of these nanostructures in more detail, while more complex structures will be discussed further below with reference to the further figures. The substrate 8 may particularly be a semiconductor substrate, such as a section of a semiconductor wafer, e.g. a GaAs-Wafer. Specifically, the substrate 8 may be, without limitation, a 111 B-oriented Gallium-Arsenide-based substrate.

[0096] The multi-generation nanostructure 9 has an exemplary hierachical branched shape of a tree (other shapes are possible as well), wherein a first-generation substructure 10 of the nanostructure 9 defines a stem of the tree, several second-generation substructures 11 each define a first-level branch of the tree, and several third-generation substructures 12 each define a second-level branch of the tree (and so forth if any further generations are present). In the present example, all generations of substructures are formed as nanowires, but other types of nanostructures may be used instead, depending on the desired average shape of the nanowires. For example, one or more of nanowires may each be replaced by a nanotube, a nanosphere, a nanorod, or a nanowhisker.

[0097] Typically, the first-generation substructure 10 will be stronger than the higher-generation substructures 11 and 12. Particularly, the first-generation substructure 10 may have a stronger (maximum) diameter d. Specifically, selecting a maximum diameter d with d > 140 nm has provided good, particularly mechanically robust, nanostructures. Each of the substrate and the nanostructure template are made from a respective electrically conductive material. For example, the substrate may comprise a (typically doped) semiconductorbased material and the nanostructure 9 may comprise metal or a (typically doped) semiconductor-based material. Specifically, the nanostructure 9 may comprise or consists of one or more of the following materials: a (typically doped) semiconductor material (e.g. GaAs or InP), gold, copper, silver.

[0098] Fig. 3 schematically illustrates an exemplary nanostructure template 13 provided on a substrate 8 and having multiple (tree-shaped) multi-generation nanostructures 9 of Fig. 2 arranged next to each other in both dimensions (only one dimension drawn) of the top surface of the substrate, thus forming a so-called “nanostructure forest”. The distance between neighboring nanostructures 9 may vary, but on average, the distance is preferably selected such that at least some branches of neighboring nanostructures 9 extend close to each other or even mutually extend into the space provided between different branches of respective neighboring nanostructures 9.

[0099] Specifically, for each tree-shaped nanostructure in at least a subset of two or more of the nanostructures, the average spatial extension of its first branches, when measured for each such branch between its proximal end at the stem portion and its distal end, is L = D z, wherein z > 0.25 and D is the average distance between the stem portion of such nanostructure and the stem portion of its nearest neighboring nanostructure.. Particularly, values for z with z > 1 , z > 2, or even z > 3 have yielded good results, i.e. a template that led to an advantageous shape of the coating (as will be further discussed below) and good mechanical and electrical properties of the resulting electrode.

[0100] The manufacturing of the nanostructure template 13 may be controlled so that in the resulting nanostructure template 13, for at least one generation of the substructures 10, 11 , 12, the respective longest spatial extensions of such substructures 10, 11 , 12 are distributed in such a way, that lengths of these extensions can be binned, in the sense of a statistical data binning, in two or more bins, each bin defining a limited range for the lengths, so that the smallest difference between two lengths from any two different neighboring bins is greater by at least a factor y > 1 ,2 than the largest difference between any two lengths from a same one of these neighboring bins. For example, in the present example, the lengths (heights) of the first-generation substructures (nanowires) 10 may be defined so that at least a vast majority (e.g. > 75%) of these nanowires can be binned into three mutually separated bins “short”, “medium”, “long”. As another example, the lengths of the nanowires of the various generations may be defined so that a first bin defines the lengths of the first- generation substructures, a second bin defines the lengths of the second-generation substructures 11 , and a third bin defines the lengths of the third-generation substructures 12, and so on (if further generations are present).

[0101] While in the example of Fig. 3, for the sake of simplifying the drawing, all nanostructures have been drawn to have an equal size and shape, there will be many variations of size and shape in a real nanostructure template 13. An example of such a real nanostructure template 13 is shown in Fig. 4.

[0102] Fig. 5 schematically illustrates a view 14 of an exemplary single (tree-shaped) multigeneration nanostructure of Fig. 2 and a coating thereof with a first and a second layer. The nanostructure template 13 may be regarded as a scaffold of the electrode, but in addition, an electrochemically active material is needed to turn the structure into an effective electrode configured to use a reversible intercalation of ions into the electrode’s active material to store energy.

[0103] To that purpose, a first layer 15 comprising a suitable active material is disposed as a first coating on the nanostructure template 13. Specifically, the active material may be a semiconductor-based material, such as a silicon-based material. Silicon (Si) or silicon- based materials such as SiNx, are particularly preferable choices, because of their capability to enable a reversible intercalation of ions, specifically small metal ions such as lithium ions or sodium ions.

[0104] To protect the surface of the resulting nanostructure 9, a supplementary second layer 16 is disposed on the first layer 15 as a second coating. The second layer 16 may further coat exposed surface portions of the nanostructure template 13 which are not covered by the first layer 15, e.g. near the roots of the nanostructures 9, and / or the exposed surface portions of the substrate 8, as illustrated in Fig. 5. The material of the second layer 16 is preferably harder than the material of the first layer below in order to provide an optimal protection thereof. Particularly, the second layer 16 may be made of SiNx, with 0.01 < x < 0.85. The second layer 16 can thus be used to improve the protection of the first layer 15 against damages, such as pulverization. Particularly, it limits the capability of the first layer 15 to expand and protect it from direct contact to the electrolyte of a battery in which the electrod is used, such as to improve the formation of a stable Solid Electrolyte Interface (SEI) layer at the electrode.

[0105] Fig. 6 schematically illustrates an exemplary view 17 of an electrode (e.g. anode) comprising the nanostructure template (forest) of Fig. 3 and a coating thereof with the first and a second layers 15 and 16. Due to the tree-shapes and the proximity of neighboring nanostructures 9, the coating is not established separately per nanostructure (as shown in Fig. 5), but instead collectively over the scaffold defined by the plurality of nanostructures 9 in the template 13, i.e. the nanostructure forest. The second layer 16 typically also covers the surface of the substrate 8, in the spaces between the nanostructures of the nanostructure template.

[0106] While the coating may fill the interstitial spaces between the neighboring nanostructures 9 to a large degree, e.g. a degree of > 30% of their volume, it does not fill these spaces completely but leaves pores 18 open, into which the active material (coating) can expand during operation of the battery. Such expansion may particularly be caused by temperature changes, esp. when the electrode’s temperature increases under electrical load, or be a side effect of the intercalation of ions into the active material. Particularly, in the case of Si as active material, expansions by a factor of around 400% can occur, depending on the number of ions being intercalated. Accordingly, the pores play a significant role for the mechanical robustness of the electrode and can help to avoid cracks or other damages in the active material or even the underlying nanostructure template. Fig. 6 further illustrates that due to the multiple generations of substructures 10, 11 , 12 in being present in the nanostructure template 13, the nanostructures penetrate the active material with a high penetration degree which leads not only to a high mechanical robustness but also to a ubiquitous supply and / or collection of electrical charges within the electrode and hence a strong capability of an energy storage device 1 equipped with such an electrode to supply or receive high currents.

[0107] Figs. 8A-8D schematically jointly illustrate an exemplary embodiment of a method of making an electrode, e.g. the electrode of Fig. 6 or Fig. 7.

[0108] Fig. 8A illustrates a process (a) at the beginning of the method, where a substrate 8 is provided, optionally cleaned (again) and a buffer layer 9a of the material, e.g. a semiconductor-based material, rooted in which the nanostructure template 13 will later be formed is deposed, e.g. by molecular beam epitaxy (MBE) or chemical vapor deposition (CVD or PECVD), particularly in the case of a semiconductor material, or by sputtering, particularly in the case of a metal.

[0109] Then, an iterative processing of the structure resulting from process (a) begins, wherein in a process (b), nanoparticles, such as gold nanoparticles, are distributed as seeds 19 for growing first-generation substructures of the nanostructure template across a surface of the buffer layer 9a. Distributing the gold nanoparticles may particularly involve evaporating, in a chamber, gold on the surface of the substrate so that gold droplets (nanoparticles) are formed in self-assembled manner.

[0110] Then, in a further process (c), the substrate 8 with the buffer layer 9a is subjected to environmental conditions that support a growth of first-generation substructures 10, such as nanowires, on the buffer layer 9a. Particularly, such environmental conditions may comprise the exclusion of air and a provision of a suitable controlled temperature range. Specifically, growing the substructures of the first (or any further generation) substructures of the multi-generation nanostructures 9 at the location of the seeds 19 may comprise application of one or more of the following processes: a molecular beam epitaxy, MBE; applying hydrothermal environmental conditions to the substrate; Electrodeposition, Chemical Vapor Deposition, CVD; Plasma Enhanced Chemical Vapor Deposition, PECVD, excluding air or another oxidant at the location of growth.

[0111] In Figures 8A - 8D, the nanoparticles 19, 20 and 21 are initially drawn at their initial positions. However, during the processes, they do typically not remain there, e.g., in the case of nanoparticles 19 they do typically not remain on the surface 9a during the growth of nanowires 10 (and also 11 and 12). Rather, nanowires 10 typically grow from the bottom of nanoparticles 19, so that nanoparticles 19 are lifted up by the nanowires 10 to the tip of the nanowires as illustrated in Fig. 8A / process (c). In the other drawings, for the sake of simplicity of the figures, the seeds are no longer drawn, after they have moved along the substructure for which they serve as a seed, independent of the generation of such substructure.

[0112] Processes (b) and (c) define a first iteration cycle of the method.

[0113] In a yet further process (d), further nanoparticles, such as gold nanoparticles, are distributed as seeds 20 for growing second-generation substructures 11 extending from the first-generation substructures 10.

[0114] Then, in a yet further process (e) which is similar to process (c), the structure resulting from process (d) is subjected to environmental conditions that support a growth of second- generation substructures 11 , such as nanowires, nanotubes, or the like, at the location of the seeds 20 and from material sourced from the first-generation substructures 10.

[0115] Thus, processes (d) and (e) define a second iteration cycle of the method.

[0116] In a yet further process (f), further nanoparticles, such as gold nanoparticles, are distributed as seeds 21 for growing third-generation substructures 12 extending from the second- generation substructures 11.

[0117] Then, in a yet further process (g) which is similar to processes (e), the structure resulting from process (f) is subjected to environmental conditions that support a growth of third- generation substructures 11 , such as nanowires, nanotubes, nanowhiskers, or the like, at the location of the seeds 20 and from material sourced from the second-generation substructures 11 .

[0118] Accordingly, processes (f) and (g) define a third iteration cycle of the method.

[0119] The method may even be extended to further iteration cycles and corresponding higher- generation substructures (no drawn). When the iteration is completed, a multi-generation nanostructure template 13 has been produced, as illustrated in the image of process (g) in Fig. 8C. Optionally, a supplementary layer (not drawn) of an electrically highly-conductive material, such as Ti, Ni, or (doped) Si may be deposited on the nanostructure template 13 to improve its electrical conductivity at its surface and / or to improve the interface between template 13 and the first layer 15 (see process (h) below)..

[0120] Now, in a yet further process (h), a first layer 15 of electrochemically active material, such as silicon, e.g. a layer of amorphous silicon, is disposed on the nanostructure template 13 as a first coating. Due to the shape and proximity of the neighboring nanostructures 9 in the nanostructure template 13, the first layer 15 extends between multiple, preferably all of the nanostructures 9, and forms interconnections (bridges) therebetween. Within the first coating, interstitial spaces remain between the interconnections (and / or elsewhere within the coating of the individual nanostructures 9).

[0121] Finally, in a yet further process (i), a supplementary second layer 16 is disposed on the nanostructure template 13 as a second coating. The second layer may particularly have an average mass density being higher than that of the first layer.

[0122] Even after this second coating process, many interstitial spaces remain and form pores in the overall produced structure which corresponds to a finished electrode, e.g. an anode 4.

[0123] While above at least one exemplary embodiment of the present solution has been described, it has to be noted that a great number of variations thereto exists. Furthermore, it is appreciated that the described exemplary embodiments only illustrate non-limiting examples of how the present solution can be implemented and that it is not intended to limit the scope, the application or the configuration of the herein-described apparatuses and methods. Rather, the preceding description will provide the person skilled in the art with constructions for implementing at least one exemplary embodiment of the present solution, wherein it must be understood that various changes of functionality and the arrangement of the elements of the exemplary embodiment can be made, without deviating from the subject-matter defined by the appended claims and their legal equivalents. LIST OF REFERENCE SIGNS

[0124] 1 electrochemical energy storage device

[0125] 2 battery housing

[0126] 3 positive electrode, cathode

[0127] 4 negative electrode, anode

[0128] 5 separator

[0129] 6 electrolyte

[0130] 7 (tree-shaped) multi-generation nanostructure rooted to a substrate

[0131] 8 substrate

[0132] 9 (tree-shaped) multi-generation nanostructure

[0133] 9a buffer layer

[0134] 10 first-generation substructure (stem in tree-shaped nanostructure)

[0135] 11 second-generation substructure (first-level branch in tree-shaped nanostructure)

[0136] 12 third-generation substructure (second-level branch in tree-shaped nanostructure)

[0137] 13 nanostructure template with multiple (tree-shaped) multi-generation nanostructures

[0138] 14 single, coated (tree-shaped) multi-generation nanostructure

[0139] 15 first layer, comprising active material

[0140] 16 second layer, e.g. metal

[0141] 17 coated nanostructure forest

[0142] 18 interstitial spaces, esp. pores

[0143] 19 seed nanoparticle for first-generation substructure

[0144] 20 seed nanoparticle for second-generation substructure

[0145] 21 seed nanoparticle for third-generation substructure d diameter

Claims

CLAIMS1 . An electrode for an electrochemical energy storage device being configured to use a reversible intercalation of ions into the electrode’s material to store energy, the electrode comprising: a substrate; a nanostructure template comprising a plurality of multigeneration nanostructures, wherein each of the nanostructures is formed on a surface of the substrate and comprises: one or more first-generation substructures, each being rooted to the substrate; and one or more substructures of at least one further generation extending directly from a respective substructure of the next lower generation; wherein each of the substrate and the nanostructure template are made from a respective electrically conductive material.

2. The electrode of claim 1 , wherein each nanostructure in at least a subset of the nanostructures has a hierarchical branched tree shape comprising as a first- generation substructure a stem portion formed as a nanowire and rooted in the substrate and as second-generation substructure multiple first branches extending from the stem portion.

3. The electrode of claim 2, wherein the hierarchical branched tree shape further comprises as a respective further-generation substructures multiple second or higher-level branches, each extending from a branch of a respective next lower generation.

4. The electrode of claim 2 or 3, wherein for each hierarchical branched-tree-shaped nanostructure in at least a subset of two or more of the nanostructures, the average spatial extension L of its first branches, when measured for each such branch between its proximal end at the stem portion and its distal end, is L = D z, wherein z > 0.25 and D is the average distance between the stem portion of such nanostructure and the stem portion of its nearest neighboring nanostructure.

5. The electrode of claim 4, wherein z > 26. The electrode of claim 4, wherein z > 3.

7. The electrode of any one of claims 2 to 6, wherein the respective stem portion of each nanostructure in at least a subset of the hierarchical branched-tree-shaped nanostructures has a maximum diameter d, with d > 50 nm.

8. The electrode of any one of claims 2 to 7, wherein for at least one generation of substructures of the nanostructures, the respective longest spatial extensions of such substructures are distributed in such a way, that lengths of these extensions can be binned, in the sense of a statistical data binning, in two or more bins, each bin defining a limited range for the lengths, so that the smallest difference between two lengths from any two different neighboring bins is greater by at least a factor y > 1 ,2 than the largest difference between any two lengths from a same one of these neighboring bins.

9. The electrode of any one of the preceding claims, wherein one or more of the nanostructures in the plurality of nanostructures comprises a nanowire, a nanotube, a nanoparticle, a nanosphere, a nanorod, a nanowhisker.

10. The electrode of any one of the preceding claims, further comprising a first layer, which coats at least a portion of the respective surface of each nanostructure and serves as an electrochemically active material of the electrode.

11. The electrode of claim 10, wherein each nanostructure in at least a subset of the nanostructures is physically interconnected with one or more neighboring nanostructures through respective interconnections formed by nanostructure template or the first layer and / or any one or more further layers being disposed on the first layer.

12. The electrode of claim 11 , wherein the interconnections define a plurality of pores as intermediate hollow spaces being located between neighboring nanostructures.

13. The electrode of any one of claims 10 to 12, wherein at each nanostructure of at least a subset of the nanostructures, the first layer coats a respective surface of each of at least two generations of the nanostructure, at least in parts.

14. The electrode of any one of the preceding claims, wherein the nanostructure template comprises or consists of one or more of the following materials: a semiconductor material, gold, copper, silver, nickel, zinc, zinc oxide, aluminum.

15. The electrode of claim 14, wherein the semiconductor material comprises lll-V semiconductor material.

16. The electrode of claim 14 or 15, wherein the substrate material is a Gallium- Arsenide-based semiconductor material.

17. The electrode of any one of the preceding claims, wherein the substrate and the nanostructures are made of a same material.

18. The electrode of any one of claims 1 to 16, wherein the substrate is made from a different material than at least one of the nanostructures.

19. The electrode of any one of the preceding claims, wherein each nanostructure of at least a subset of the nanostructures is rooted to the substrate at a location that is defined by a respective seed nanoparticle being provided at such location and comprising a material being different from that of the nanostructure.

20. The electrode of any one of the preceding claims, wherein each nanostructure of at least a subset of the nanostructures is rooted to the substrate in a seedless manner.21 . The electrode of any one of claims 10 to 20, wherein the electrode further comprises a semiconductor-based second layer that coats the first layer, at least in parts,.

22. The electrode of claim 21 , wherein the first layer has a first average mass density and the second layer has a second average mass density being higher than the first average mass density.

23. The electrode of claim 21 or 22, wherein: the nanostructure template comprises a surface portion which is not coated by the first layer; and the second layer further coats this surface portion in whole or in part.

24. The electrode of any one of claims 21 to 23, further comprising a supplementary third layer that coats the nanostructure template, at least in part, so that at least a section of the first layer is sandwiched between the third layer and the second layer; wherein the third layer has at a temperature of 20°C a higher electrical conductivity than the first layer.

25. The electrode of claim 24, wherein: the first layer is a semiconductor-based layer; the second layer is a semiconductor-based layer; and / or the third layer is a metal layer.

26. The electrode of claim 25, wherein: the first layer is a layer of amorphous silicon; the second layer is a SiNx layer with 0.01 < x < 0.8527. An electrochemical energy storage device having an electrode of any one of the preceding claims and being configured to use a reversible intercalation of ions into the electrode’s material to store energy.

28. A method of making an electrode of any one of claims 1 to 26, the method comprising: providing a substrate; building, on the substrate, a nanostructure template comprising a plurality of multigeneration nanostructures, wherein each of the multigeneration nanostructures is formed on a surface of the substrate and comprises a first-generation substructure being rooted to the substrate and respective substructures of one or more further generations being rooted to a respective substructure of the-next lower generation; wherein each of the substrate and the nanostructure template are made from a respective electrically conductive material.

29. The method of claim 28, further comprising coating at least a portion of the surface of the nanostructure template with a first layer serving as an electrochemically active material of the electrode, wherein the coating comprises coating at least a portion of the respective surface of each nanostructure with the first layer.

30. The method of claim 29, wherein: the first layer is formed with a material having a first average mass density; and the method further comprises coating the first layer, at least in parts, with a second electrically conductive layer having a second average mass density being higher than the first average mass density.31 . The method of claims 29 or 30, wherein: providing the substrate comprises cleaning a surface of the substrate; andbuilding the nanostructure template comprises: growing a buffer layer of material on the cleaned surface of the substrate as a base material of the nanostructure template; depositing a plurality of nanoparticles on the buffer layer as seeds for a growth of a first generation of substructures of the nanostructures on the substrate; applying environmental conditions to the substrate to cause a growth of first- generation substructures of the multi-generation nanostructures from the buffer layer at the location of the seeds; and for each further generation of substructures of the multi-generation nanostructures: depositing a plurality of nanoparticles on the substructures of the next lower generation as seeds for a growth of said further generation of substructures of the nanostructures on the substructures of the next lower generation; and applying environmental conditions to the substrate to cause a growth of the substructures of said further generation of the multi-generation nanostructures from the substructures of the next lower generation at the location of the seeds of said further generation.

32. The method of claim 31 , wherein at least a section of a wafer of a GaAs-based material is used as the substrate.

33. The method of claim 31 or 32, wherein the plurality of particles used as seeds comprises nanoparticles made of one or more of gold, silver, and germanium.

34. The method of any one of claims 28 to 33, wherein the substructures of at least one of the generations of the multi-generation nanostructures comprise nanowires.

35. The method of any one of claims 28 to 34, wherein growing the substructures of at least one of the generations of the multi-generation nanostructures comprises application of one or more of the following processes:- a molecular beam epitaxy, MBE;- applying hydrothermal environmental conditions to the substrate;- Electrodeposition- Chemical Vapor Deposition, CVD;- Plasma Enhanced Chemical Vapor Deposition, PECVD- Causing the growth in the absence of air or another oxidant at the location of growth.

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