METHOD FOR THE PRODUCTION OF SILICON ELECTRODES AS ANODES FOR LITHIUM BATTERIES

Description

[0001] The invention relates to a method for producing a silicon electrode as an anode for lithium batteries, in which a first silicon layer is deposited on a substrate, preferably copper, which is then subjected to short-term annealing.

[0002] Electrochemical energy storage is a crucial pillar of a globally pursued energy transition, enabling the temporary storage of fluctuating, renewable energy generation and its provision for stationary and mobile applications. To counteract raw material shortages and the resulting cost increases, particularly for secondary batteries, diversification of energy storage concepts and the development of new materials are essential. These materials should improve the technical performance of such energy storage concepts (including capacity, energy density, and lifespan) while simultaneously minimizing manufacturing costs. The latter can be achieved, in particular, through the use of readily available chemical elements, such as silicon, for which a broad technological base already exists.

[0003] Batteries are electrochemical energy storage devices and are divided into primary and secondary batteries.

[0004] Primary batteries are electrochemical power sources in which chemical energy is irreversibly converted into electrical energy. A primary battery is therefore not rechargeable. Secondary batteries, also called accumulators, on the other hand, are rechargeable electrochemical energy storage devices in which the chemical reaction is reversible, allowing for multiple uses. Electrical energy is converted into chemical energy during charging, and back again during discharging.

[0005] Battery is the generic term for interconnected cells. Cells are galvanic units consisting of two electrodes, electrolyte, separator, and cell casing. Figure 1 This shows an example of the structure and function of a lithium-ion cell during the discharge process. The components of a cell are briefly explained below.

[0006] Each lithium-ion cell consists of two different electrodes 7, 9: one negatively charged when charged and one positively charged when charged. Since ions migrate from the negatively charged electrode to the positively charged electrode during energy release, i.e., during discharge, the positively charged electrode is called the cathode 7 and the negatively charged electrode the anode 9. Each electrode comprises a current collector 2, 8 and an active material applied to it. Between the electrodes are the ion-conducting electrolyte 4, which enables the necessary charge exchange, and the separator 5, which ensures the electrical separation of the electrodes.

[0007] The cathode, for example, consists of mixed oxides applied to an aluminum collector.

[0008] Transition metal oxides containing cobalt (Co), manganese (Mn), and nickel (Ni), or aluminum oxide (Al₂O₃), are the most common compounds. During cell discharge, the applied metal oxide layer serves to trap the lithium ions.

[0009] The anode of a lithium-ion cell can consist of a copper foil as the collector and a layer of carbon as the active material. Natural or synthetic graphite is typically used as the carbon compound because it has a low electrode potential and exhibits low volume expansion during charging and discharging. During charging, lithium ions are reduced and incorporated into the graphite layers.

[0010] In lithium-ion battery designs, the cathode typically supplies the lithium atoms for charging and discharging in the anode; therefore, the battery capacity is limited by the cathode capacity. As mentioned previously, typical cathode materials used include Li(Ni,Co,Mn)O₂ and LiFePO₄. Due to the cathode's composition of lithium metal oxides, increasing the capacity is only marginally possible.

[0011] It is also known to use silicon instead of carbon in lithium-ion battery anodes. Silicon as an anode material exhibits a high storage capacity of 3579 mAh / g at room temperature compared to conventional carbon-based materials such as graphite, which has a storage capacity of 372 mAh / g. However, challenges arise when using silicon as an anode material due to the sometimes considerable volume changes (volume contraction and expansion) of the host matrix during the insertion and removal of mobile ion species during charging and discharging of the corresponding energy storage devices. The volume change is approximately 10% for graphite, but around 400% for silicon. The volume change of the anode material when using silicon leads to internal stresses, cracking, pulverization of the active material of the host matrix (silicon), and ultimately to the complete destruction of the anode.

[0012] To reduce volume changes, existing battery manufacturing processes utilize nanostructured carbon- or silicon-based nanoparticles or nanowires as anode materials in rechargeable lithium batteries. Besides increasing the rate of lithium insertion and removal, the greatest advantage of such nanomaterials is their surface effect. This means that a larger surface area increases the contact area for the electrolyte and the associated flow of Li+ ions (vacancies) across the interface, as described in the publication by MR Zamfir, HT Nguyen, E. Moyen, YH Leeac and D. Pribat: Silicon nanowires for Li-based battery anodes: a review, Journal of Materials Chemistry A (a review), 1, 9566 (2013).Silicon-based nanoparticles and nanowires, in particular, exhibit more stable silicon structures with respect to the volume change of the silicon after lithium incorporation up to a certain silicon structure size, as described in the publication by M. Green, E. Fielder, B. Scrosati, M. Wachtier and J.S. Moreno: Structured Silicon anodes for lithium battery applications, Electrochem. Solid-State Letter, 6, A75-A79 (2003). The structural limit is considered to be 1 µm for amorphous silicon and 100 nm for crystalline silicon, ensuring a uniform volume change.

[0013] Thus, the volume expansion in the anode material can be accommodated by the free space between the nanostructures. Furthermore, the miniaturization of the structures facilitates phase transitions during alloy formation, leading to an increase in the performance of the anode material. In CHANG-MOOK HWANG ET AL: "Electrochemical characterizations of multi-layer and composite silicongermanium anodes for Li-ion batteries using magnetron sputtering", JOURNAL OF POWER SOURCES, ELSEVIER, AMSTERDAM, NL, Vol. 196, No. 16, October 20, 2010 (2010-10-20), pages 6772-6780, a multilayer Si / Ge structure with Si layers up to 25 nm thick is investigated as an anode for lithium-ion batteries. In ANTONIA REYES JIMeNEZ ET AL: "A Step toward High-Energy Silicon-Based Thin Film Lithium Ion Batteries", ACS NANO, Vol. 11, No. 5, April 24, 2017 (2017-04-24), pages 4731-4744, a Si / C multilayer anode with Si layers up to 140 nm thick is described.

[0014] However, the use of silicon-based nanoparticles and nanowires is very complex. The Si nanostructures are produced using both physical and chemical processes, including ball milling, sputtering, PVD / CVD processes, chemical or electrochemical etching, and reduction of SiO₂ (Feng, K. et al. Silicon-Based Anodes for Lithium-Ion Batteries: From Fundamentals to Practical Applications. Small 14, 1702737 (2018)). According to current technology, the produced nanostructures are then mixed with conductive carbon and a binder and, on an industrial scale, deposited onto a copper current collector by calendering and drying to form the anode. The disadvantage of these processes is that the nanostructures separate from each other during battery operation, thus reducing the anode's capacity.Another disadvantage is the large surface area of ​​the nanostructures, which leads to high electrolyte consumption through the formation of a SEI (solid electrolyte interphase) and the drying out of the battery.

[0015] WO 2017 / 140581 A1 therefore describes a process for manufacturing silicon-based anodes for secondary batteries that does not have these disadvantages. In this process, a silicon layer is deposited on a metal substrate that serves as an integrated current collector and then subjected to flash lamp annealing. Flash lamp processes are typically used to quickly and locally melt and crystallize silicon, for example, in the semiconductor industry and for solar cells. However, this is not the purpose of the process described in WO 2017 / 140581 A1. Rather, flash lamp annealing is used as follows: Generally, silicon crystallization can only be achieved at approximately 700 °C. This temperature is reached briefly at the surface using flash lamp annealing, while the rest of the substrate remains significantly cooler. Even at considerably lower temperatures, from approximately...At 200°C, diffusion occurs on the surface and along the grain boundaries of the metal substrate because the covalent bond between the silicon atoms is weakened at the interface with the metal. This has already been demonstrated in several metal / semiconductor systems (e.g., Au / a-Si and Ag / a-Si) and has proven to be energetically favored, as described in the publication by ZM Wang, JY Wang, LPH Jeurgens, and EJ Mittemeijer: Thermodynamics and mechanism of metal-induced crystallization in immiscible alloy systems: Experiments and calculations on Al / a-Ge and Al / a-Si bilayers, Physical Review B 77, 045424 (2008). Furthermore, silicon crystallization can be achieved by introducing metal at a comparatively low temperature. This is referred to as metal-induced crystallization.In a highly simplified manner, crystalline growth can begin once the melting temperature is reached; this can be used as a criterion for phase transitions. The process described in WO 2017 / 140581 A1 allows the fabrication of multiphase silicon-metal structures that mitigate the volume changes caused by delithiation and lithiation and stabilize the entire material composite. Lithiation refers to the incorporation of lithium ions into the host material, such as silicon or graphite.

[0016] The silicon anodes produced using the method known from WO 2017 / 140581 A1 are a mixture of silicon, pure metals, and silicides, provided only a copper foil is used as the substrate and a silicon layer is deposited on it. This results in a microstructure of copper, copper silicide, and silicon. The advantages of silicon anodes produced in this way compared to silicon anodes made from nanoparticles or wires are their high electrical conductivity compared to pure silicon and also conventional graphite, since silicides exhibit approximately two orders of magnitude better electrical conductivity than graphite. Furthermore, very good adhesion is achieved between the silicon layer as the active material and the copper substrate, with the copper from the copper foil diffusing into the deposited silicon layer during flash lamp annealing.Due to the active areas for lithium storage, formed by the pure silicon, and the inactive areas formed by the silicides / metals in the matrix, the known adverse volume expansion during charging is compensated. Another advantage is that, due to the layered structure, only a small area forms a boundary layer with the electrolyte, resulting in less electrolyte degradation than with nanostructured active materials. The silicon layer is referred to as the active material.

[0017] A disadvantage of the process described in WO 2017 / 140581 A1 is that, as a result of flash lamp annealing, the silicon layer reacts uncontrollably to form copper silicide, with the conversion reaction always starting at the Cu-Si layer interface. As a result of the reaction, either no silicon remains as the active material for lithium storage, or, if the energy input is so low, the reaction is insufficient and the layer is not sufficiently stable during battery operation, thus leading to a loss of battery capacity. Sufficiently thick silicon layers (10 µm corresponds to approximately 4 mAh / cm²) are necessary for achieving a sufficient target capacity in the production of the lithium battery. If the conversion reaction Cu + Si to copper silicide is initiated uncontrollably by an annealing process, including flash lamp annealing, the entire copper substrate, e.g.,a copper foil, which reacts completely with the silicon to form copper silicide and is associated with the loss of the current collector of the lithium battery.

[0018] It is therefore an object of the present invention to provide a method that allows control of the proportions of silicon to silicide and metal. A compromise should be found between the maximum proportion of pure silicon, ideally amorphous or nanocrystalline, which must be available as active material for lithium insertion, while simultaneously ensuring a sufficient number of inactive areas to achieve stability and good electrical conductivity, and a sufficient anode layer thickness with a high silicon content for high capacity.

[0019] Furthermore, the process should allow control of the respective metal phase formed, e.g. Cu 5 Si instead of Cu 3 Si.

[0020] The problem is solved by a method according to independent claim 1. In the method for producing a silicon electrode as an anode for lithium batteries, in which a first silicon layer is deposited on a substrate, preferably copper, which is then subjected to short-term annealing, the deposition of a further silicon layer and the subsequent short-term annealing are repeated at least once.

[0021] Short-term annealing refers specifically to flash lamp annealing and / or laser annealing. Flash lamp annealing is performed with a pulse duration or annealing time in the range of 0.3 to 20 ms and a pulse energy in the range of 0.3 to 100 J / cm². In laser annealing, the annealing time of 0.01 to 100 ms is set by the scanning speed of the local heating point to generate an energy density of 0.1 to 100 J / cm². The heating ramps achieved in short-term annealing are in the range of 10⁴–10⁷ K / s required for the process. Flash lamp annealing utilizes a spectrum in the visible wavelength range, whereas laser annealing employs discrete wavelengths in the infrared (IR) to ultraviolet (UV) spectrum.

[0022] In a preferred embodiment of the inventive method, the deposition of a further silicon layer and the subsequent short-term annealing are repeated two or more times. This allows layer thicknesses of several micrometers, preferably more than 5 µm, to be built up without the entire layer structure reacting uncontrollably to form copper silicide, which would result in the loss of the current collector.

[0023] The advantage of repeated silicon deposition and subsequent short-term annealing is that each sequence forms a stable ("reacted") layer with a closed interface, which acts as an interface for the subsequent layers. This reduces the copper content in the layer, so that after only two to three sequences, the copper-silicon reaction to copper silicide is incomplete, resulting in a copper gradient and leaving pure silicon for high capacitance. This is achieved because each layer in the layer structure with its interface acts as a diffusion barrier for further layer build-up. Additionally, short-term annealing allows each layer to be individually annealed. The layer structure can also be described and understood as a layer stack.

[0024] The inventive method represents a very simple process with which very good adhesion at the interface between the copper collector and silicon, as well as very good electrical contact, is achieved. Apart from the deposited silicon and the copper originating from the copper collector, no additional material is required.

[0025] The first embodiment of the process according to the invention described above can be used as a pre-process in combination with the other claimed process steps, since roughening the surface, as an example of a positive effect of a layer-by-layer reaction, results in good adhesion for subsequent layers, promotes the growth of columnar structures, thus enabling better ionic conductivity and allowing for good control of the copper content for subsequent processes. For example, the first layer can consist of 1 µm silicon, which reacts completely with the copper substrate to form CuSiₓ, usually Cu₃Si. This results in surface roughening and very good adhesion to the copper substrate.Although some of the copper substrate is consumed in the process, the reaction proceeds in a controlled manner through the selection of the short-term annealing process parameters in the inventive method, thus preventing loss of the current collector. The strong adhesion of the subsequent layer structure to the copper substrate can also be achieved by mechanical roughening or by using a material other than silicon.

[0026] The excellent adhesion to the substrate is crucial to prevent the subsequently applied multilayers from tearing when the lithium is absorbed and expands during battery operation. The inventive method is what makes stable battery operation possible in the first place.

[0027] This is not feasible with the method described in WO 2017 / 140581 A1. WO 2017 / 140581 A1 only involves depositing a silicon layer that is then subjected to flash lamp annealing. The necessary silicon layer thickness of up to 10 µm for battery applications is deposited on the substrate in a single process step and annealed by the flash lamp. However, the layers produced in this way are brittle and easily flake off or react completely with the copper of the substrate, leaving no usable silicon for energy storage.

[0028] According to the invention, a thin layer of a further layer of silicon-different material is applied before the deposition of the further silicon layer. This thin layer of the further layer can be, for example, carbon, a metal, a metal oxide, or a metal nitride; in particular, the thin layer of the further layer can be formed from one of the following materials: titanium (Ti), nickel (Ni), aluminum (Al), tin (Sn), gold (Au), silver (Ag), tungsten (W), and / or copper (Cu). A layer thickness of 5 to 200 nm is considered thin. The thickness is defined as the thickness at which the desired function of the added layer is considered sufficient for the manufacturing process, in particular for short-term annealing.

[0029] The insertion of an additional material into the layers of the silicon layer structure has the advantage that the copper of the copper substrate reacts only to a very small extent with the applied layers, which ensures particularly good adhesion, and on the other hand, the copper of the current collector is not degraded by preventing a reaction with the other deposited layers.

[0030] It has been shown that the effect of short-term annealing differs when depositing an additional layer other than the copper substrate material, particularly when copper is deposited directly onto the copper substrate. This is attributed to a variety of chemical processes involving silicon. As a result, different structures can form in the resulting anode layer, such as columnar structures in the case of nickel or dendrites in the case of copper. Dendrites are tree- or shrub-like crystal structures. Furthermore, other silicides can form, which, unlike copper silicides (which can only incorporate little or no lithium), can also incorporate lithium. Utilizing these differences offers the advantage of controlling the volume expansion of silicon during lithium incorporation. This significantly increases the stability of battery operation.

[0031] In the case of nickel as a thin layer of a further layer prior to the deposition of the next silicon layer, nickel silicide is initially formed during short-term annealing. If flash lamp annealing is used for short-term annealing, the nickel is replaced by copper by means of a flash lamp, which can have a flash duration between 0.2 ms and 20 ms and an energy density of 0.6 J / cm² and 160 J / cm², leaving nickel as pure metal in the layer. Since nickel silicide has a lower density, copper silicide with a lower density is formed. This improves the compensation during lithium incorporation. If short-term tempering is carried out using a laser (laser tempering), this is achieved with a tempering time in the range of 0.01 ms to 100 ms by setting a scanning speed of a local heating point and an energy density in the range of 0.1 to 100 J / cm², as well as preheating or cooling in the range of 4°C to 200°C.Since this is a non-equilibrium process that can only be realized in the millisecond range, the use of a flash lamp or a laser is required.

[0032] The flash lamps preferably consist of gas discharge lamps that emit a major portion of the radiation in the wavelength range between the visible and infrared regions (400 nm - 800 nm) and operate with a total power of approximately 12 MW in less than 20 ms, thus enabling the sample surfaces to be heated to temperatures of up to 2000°C. Flash lamp curing serves to promote the metal-induced layer exchange process, also known as metal-induced crystallization.

[0033] In the case of titanium, Ti silicide is formed, which, in the correct phase, can be capable of lithium embedding (see: Xu, J. et al. Preparation of TiSi₂ Powders with Enhanced Lithium-Ion Storage via Chemical Oven Self-Propagating High-Temperature Synthesis. Nanomaterials 11, 2279 (2021)). This has the advantage that no clear active-inactive lithium interface exists, thus ensuring good electrical contact even during cycling. Other metals, such as aluminum, do not form a compound with silicon, and therefore no silicides. Consequently, these metals mix with silicon, increasing its electrical conductivity. In the short-term annealing step, the morphology and hardness of the silicon-metal layer can also improve compared to hard, pure silicon.

[0034] The carbon acts as a diffusion barrier to prevent further uncontrolled reaction of the copper from the substrate / current collector with the subsequent active layer. The active layer is the subsequent silicon layer into which lithium can be incorporated.

[0035] The use of carbon has the advantage that carbon, in the form of graphite, is already used in lithium-ion battery production and can therefore be easily and seamlessly integrated into the manufacturing process. Another advantage is that the sputtered carbon can be used as a copper diffusion barrier, thus reducing silicide formation. Carbon also has the advantage of being very light and electrically conductive, allowing lithium to diffuse through it readily. Its weight and good electrical and ionic conductivity are advantages over all other metals for use in the interlayer. Carbon can also serve as a protective layer at the silicon / electrolyte interface because it is chemically inert and, as mentioned, possesses good Li+ ion conductivity.

[0036] In a further embodiment of the process according to the invention, a metal layer is deposited onto the additional silicon layer before short-term annealing. In addition to the properties mentioned so far, this variant of the invention offers the possibility that the reaction with the metal roughens the surface of the active layer. This can improve the ionic conductivity or the electrolyte-active material interface.

[0037] Deposition of an additional metal layer on the silicon layer has the advantage that the silicide formation reaction takes place at the interface with the copper foil and also at the interface of the top-applied metal layer. This ensures good adhesion between the copper foil and the silicon layer, while simultaneously controlling the reaction of Cu and Si to copper silicide by the metal layer on the top surface. A metal layer can improve surface conductivity. High electrical conductivity of the battery is crucial for fast charging / discharging processes, as is the ability of Li+ ions to penetrate the layers effectively. Therefore, all layers and interfaces must be as conductive as possible. For example, doped Si can be highly conductive, but it can react with the electrolyte or with degradation products, forming SiO₂ (a very good insulator) or even Li₄SiO₄ (Li-silicate).Lithium silicate is a very good insulator, and losses of lithium occur due to strong bonding. Therefore, this must be prevented in the overall design of the anode, for example, by using open / porous surfaces, conductive interfaces, and soluble / soft protective layers. An additional metal layer at the interface can prevent precisely this.

[0038] In an advantageous embodiment of the inventive method, the metal layer is formed from one of the materials titanium (Ti), nickel (Ni), aluminum (Al), tin (Sn), gold (Au), silver (Ag) and / or tungsten (W). The advantages are those mentioned above.

[0039] According to the invention, a layer other than silicon, preferably nickel (Ni) and / or copper (Cu), is deposited on the copper substrate before the first silicon layer is deposited. This has the advantage that the reaction rate of the layer can be controlled by means of short-term annealing, and that, on the other hand, no copper from the current collector is used to form the copper silicide; instead, the metal silicide formation occurs in a controlled manner between the deposited metal layer, such as nickel or copper, and the subsequently deposited first silicon layer.

[0040] The invention will be explained in more detail below using exemplary embodiments.

[0041] The drawings show Fig. 1 Exemplary structure and function of a lithium-ion cell during the discharge process; Fig. 2 Schematic representation of the inventive method for producing a silicon anode for a lithium-ion battery: a) repeated coating of the Cu substrate with silicon and subsequent short-term annealing; b) deposition of a thin metal layer between the Cu substrate and the Si layer; Fig. 3 A multilayer structure produced according to the inventive method for stabilizing the Si layer system: a) SEM-SE image, b) SEM-BSE image; Fig. 4 Image of a multilayer structure according to the inventive method a) before battery operation, b) after 200 battery operation cycles; Fig. 5 Further image of a multilayer structure according to the inventive method.

[0042] Figure 2aFigure 1 shows the steps of a first process variant according to the invention in a flowchart. A substrate, which simultaneously serves as a current collector, undergoes pre-cleaning under vacuum conditions in a plasma atmosphere. Subsequently, silicon layers are built up by physical vapor deposition, e.g., sputtering or evaporation. A silicon layer 10 is sequentially deposited, with the layers being stabilized by short-term annealing 11, e.g., flash lamp annealing. The advantage of repeated silicon deposition 12 and subsequent short-term annealing is that with each sequence a stable ("reacted") layer with a closed interface is formed, which acts as an interface for the subsequent layers.

[0043] Figure 2bFigure 1 shows the steps of a second process variant according to the invention in a flowchart. A substrate, which simultaneously serves as a current collector, undergoes pre-cleaning under vacuum conditions in a plasma atmosphere. Subsequently, silicon layers are built up by physical vapor deposition, e.g., sputtering and / or evaporation, and a thin layer of another layer other than silicon is applied. First, a silicon layer 10 and a thin layer of another material 13, e.g., carbon, a metal, a metal oxide, a metal nitride, or copper, are applied either to the first silicon layer or to another silicon layer, with the layers being stabilized by short-term annealing 11. The advantage of repeated silicon deposition 12 and additional layers, as well as the subsequent short-term annealing, is that, for example,In the case of titanium, Ti silicide forms, which, in the correct phase, can be used to store lithium. This has the advantage that no clear active-inactive lithium interface exists, thus ensuring good electrical contact even during battery operation. Other metals, such as aluminum, do not form compounds with silicon, and therefore do not produce silicides. Consequently, these metals mix with silicon, increasing its electrical conductivity. During the tempering process, the morphology and hardness of the silicon-metal layer can also be improved compared to hard, pure silicon.

[0044] The advantage of repeated silicon deposition and subsequent short-term annealing is that with each sequence, the proportion of copper in the layer decreases, so that after only two to three sequences, the copper-silicon reaction to copper silicide is incomplete, leaving pure silicon. This is achieved because each layer stack with its interface acts as a diffusion barrier for the subsequent layer build-up. Additionally, short-term annealing allows each layer to be annealed individually.

[0045] The inventive process is a very simple process that achieves excellent adhesion at the copper-collector-silicon interface and a very good electrical transition. Apart from the deposited silicon and the copper from the copper collector, no additional material is required. This allows the battery capacity to be further increased compared to lithium batteries manufactured using conventional methods. The deposition of an additional silicon layer followed by short-term annealing can be used as a pre-process in combination with the deposition of a thin layer of another layer for the silicon layer build-up, since roughening the surface creates good adhesion for further layers, promotes the growth of columnar structures, thus achieving better ionic conductivity, and allows for precise control of the copper content for subsequent processes.

[0046] Figure 3 Two scanning electron microscope (SEM) images show a multilayer structure produced according to the inventive method. Figure 3a The image shows a SEM-SE image (SE secondary electrons), in which a silicon layer, for example 1 µm thick, was deposited on the copper substrate 14. This silicon layer, through short-term annealing, particularly flash lamp annealing, formed a thin, rough, inactive layer of fully reacted CuSiX as an adhesive layer 15. An active layer was deposited on this adhesive layer. This active layer consists of a multilayer structure 16, in which silicon 17 and copper 18 were deposited alternately, and the individual layers were stabilized by short-term annealing. The copper layers significantly increase the adhesion of the layer structure. The copper layers are visible in the SEM-BSE image (BSE - backscattered electrons) in Figure 3bvisible as light stripes. The thickness of the active layer (multilayer structure 16) can be, for example, 6 µm. This corresponds to a capacity of 2.4 mAh / cm² at 2000 mAh / g. The function of the adhesive layer 15 can also be achieved by mechanically roughening the surface or by using a material other than silicon.

[0047] Figure 4a shows the recording of a multilayer structure 16 according to the inventive method before battery operation and Figure 4bThe same multilayer structure 16 after 200 battery operation cycles. A copper silicide, formed from a deposited silicon layer and the copper of the copper substrate by short-term annealing, in particular flash lamp annealing, which roughens the substrate, was applied as an adhesive layer 15 to a copper substrate 14. A multilayer structure 16 consisting of three layers of silicon 17 and two layers of copper 18 was deposited thereon, with each intermediate layer (Si and Cu) being stabilized by short-term annealing. If necessary, short-term annealing is only required every two (or more) silicon layers. For example, in a first step, a multilayer structure of Si-Cu-Si is deposited and subjected to short-term annealing; subsequently, further layers of Cu and Si are deposited and again subjected to short-term annealing. The process according to the invention is highly adaptable.The less frequently flashing / lasering occurs, and consequently the more energy is saved, the more cost-effective the overall process becomes. Even after 200 battery operation cycles, the multilayer structure 16 remains intact and recognizable. The surface is smooth and stable. Using the inventive method, a stable active layer 10 µm thick can be produced for the anode, corresponding to a battery cell capacity of 4 µm / cm² at 2000 mAh / g.

[0048] Figure 5Figure 1 shows another multilayer structure 16 produced using the inventive method. Clearly visible are the copper substrate 14, the CuSiX adhesive layer 15, an invisible 5 nm thick carbon layer, and five layers of silicon 17 alternating with layers of copper 18. The thickness of each newly applied copper layer 18 increases towards the top. Short-term annealing was performed with a pulse duration of 1 ms, with a total of three pulses of variable energy applied after the 0th, 3rd, and 5th layers. In the bottom layer, the energy input from the short-term annealing resulted in a complete reaction of Cu and Si to CuSiX; in the upper layers, the energy input was selected to ensure a sufficient partial reaction of Cu with Si to CuSiX. Method for manufacturing silicon electrodes as anodes for lithium batteries Reference symbol list

[0049] 1 Lithium-ion battery 2 Collector on anode side 3 SEI solid electrolyte interphase 4 Electrolyte 5 Separator 6 Conductive interphase, CEI cathode-electrolyte interphase 7 Cathode, positive electrode 8 Collector on cathode side 9 Anode, negative electrode 10 Sputtering of the Si layer 11 Short-term annealing, e.g., flash lamp annealing or laser annealing 12 Repetition of process steps 13 Sputtering of an additional material 14 Copper substrate 15 Adhesive layer 16 Multilayer structure 17 Silicon layer, further silicon layer 18 Copper layer 19 Further layer 20 Metal layer

Claims

1. A method for producing a silicon electrode as anode for lithium batteries, wherein a first silicon layer (17) is deposited on a substrate, preferably copper (14), and subsequently subjected to short-term annealing (11), characterized in that the deposition of a further silicon layer (17) and the subsequent short-term annealing (11) are repeated at least once, wherein before the deposition of the first silicon layer (17) on the copper substrate (14), a layer other than silicon made of nickel (Ni and / or copper (Cu) is deposited, or wherein before the deposition of the further silicon layer (17), a thin stratum of a further layer (19) is applied, the thin stratum of the further layer (19) being carbon, a metal, a metal oxide or a metal nitride, wherein the thin stratum of the further layer (19) is formed from one of the materials titanium (Ti), nickel (Ni), aluminum (Al), tin (Sn), gold (Au), silver (Ag), tungsten (W) and / or copper (Cu), wherein the short-term annealing (11) is a flash-lamp annealing and is carried out by means of a flash lamp having a flash light duration of between 0.2 ms to 20 ms and an energy density of 0.6 J / cm2 to 160 J / cm2 and also of preheating or cooling in the range from 4°C to 200°C, or wherein the short-term annealing (11) is a laser annealing and is carried out by means of a laser with an annealing time in the range from 0.01 ms to 100 ms by the establishment of a rate of scanning of a local heating site and an energy density in the range from 0.1 to 100 J / cm2 and also of preheating or cooling in the range from 4°C to 200°C.

2. The method as claimed in claim 1, characterized in that the deposition of a further silicon layer (17) and the subsequent short-term annealing (11) are repeated twice or more than twice.

3. The method as claimed in claim 1, characterized in that a metal layer (20) is deposited onto the further silicon layer (17) before the short-term annealing (11), wherein the metal layer (20) is formed from one of the materials titanium (Ti), nickel (Ni), aluminum (Al), tin (Sn), gold (Au), silver (Ag) and / or tungsten (W).