METHOD FOR PRODUCING A CYCLE-STABLE SILICON ANODE FOR SECONDARY BATTERIES AND SILICON ANODE FOR SECONDARY BATTERIES

Description

[0001] The invention relates to a method for producing silicon anodes for secondary batteries - also: accumulators - in particular for lithium-ion batteries, and to a silicon anode that can be produced using the method.

[0002] Silicon-based anodes are known to be advantageous in lithium-ion batteries. The performance of a rechargeable lithium-ion battery is measured in stored energy per unit mass (unit: mAh / g). A crucial factor is how much lithium can be stored in the two electrodes—anode and cathode—per gram of electrode mass. Other quality-determining factors for secondary batteries, which depend significantly on the electrodes, include storage stability (i.e., the possible storage time of an unloaded battery without damage), the degree of self-discharge, the recharging speed, and cycle stability. Cycling refers to the repeated complete charging and discharging of the battery (charge cycle), and a cycle-stable battery exhibits very little—ideally no—capacity loss over a large number of charge cycles.

[0003] Despite the technological excellence of established battery concepts, manufacturing costs, and thus the price-performance ratio, must remain within a competitive range. Particularly for batteries with very high storage capacity, such as those used in electromobility or home energy storage, there are still shortcomings in this regard. These shortcomings stem from safety requirements on the one hand and the manufacturing effort involved in producing large and robust electrodes on the other.

[0004] In terms of safety, silicon battery electrodes offer advantages because they could replace the carbon (graphite) electrodes used previously, significantly reducing the battery's fire risk. It has long been known that silicon, through the formation of silicon-lithium compounds, can intercalate approximately 11 times more lithium per gram of silicon than a conventional graphite anode. With a theoretical capacity of over 4000 mAh / g, it even surpasses that of metallic lithium. However, early attempts to use silicon anodes failed because virtually no cycle stability was achieved. The reason for silicon's extremely poor cycle stability lies in the fourfold volume expansion associated with lithium intercalation. The resulting mechanical stresses are so great that the material is pulverized.

[0005] For the anode of a lithium-ion battery, the work by Chan et al. ("High-performance lithium battery anodes using silicon nanowires", Nature Nanotechnology 3, 31 (2008)) proposes arranging silicon nanowires vertically on a metal film (charge collector). Chan et al.'s work involves growing silicon nanowires on, for example, a steel substrate using established techniques (here: liquid-vapor-solid, LVS). The nanowires are flexible and can double in diameter without breaking. The nanostructuring of the silicon increases the surface area for lithium ion uptake and also creates space to avoid the aforementioned mechanical stresses. However, the fabrication process of Chan et al. is cumbersome and expensive. Growing the silicon nanowires using LVS requires gold particles as nucleation seeds, which remain at the tips of the nanowires.The nanowires themselves are saturated with gold, which makes the production of thicker wires or larger areas very expensive. Moreover, the resulting nanowires are not homogeneous. There are thick and thin, long and short, upright and bent nanowires, nanowires that are both fixed to the substrate and detached. Silicon nanowires that are not in contact with the metal film are particularly undesirable in commercial production. They contribute nothing to the battery's capacity but nevertheless absorb lithium ions during the initial charge, which are then irreversible. And, of course, in the process according to Chan et al., these nanowires are also saturated with gold.

[0006] The publications EP 2 460 214 B1 and US 2016 / 118643 A1 aim to prevent the detachment of silicon columns from a metal film during cycling as completely as possible by having sections at the base of the columns enclosed by the metal film, which is at least one micrometer thick. The monocrystalline silicon columns thus embedded are held in place by the metal film—in this case, copper—and are also prevented from absorbing lithium ions in the enclosed sections. They remain firmly anchored during cycling and thus in low-resistance contact with the metallic conductor. Furthermore, in EP 2 460 214 B1, all columns are of the same thickness, height, crystallographic orientation, and even regularly arranged, which is due to the manufacturing process described therein.The nanopillars are formed as a regular array on a silicon wafer by selectively etching macropores. The metal film is then electroplated onto the bases of the upright pillars and subsequently detached from the wafer using thermal or mechanical force. The embedded silicon pillars remain within the metal film and are torn away from the wafer. It is noteworthy that a silicon anode, according to EP 2 460 214 B1, shows no loss of capacity even after hundreds of charging cycles. However, the manufacturing process is hardly suitable for large-scale production, partly because both the etching of relatively deep macropores and the electroplating are time-consuming, and moreover, large portions of the expensive wafer material cannot be used efficiently.

[0007] In their study "Accelerated growth from amorphous clusters to metallic nanoparticles observed in electrochemical deposition of platinum within nanopores of porous silicon" (Electrochemistry Communications 71, 9-12 (2016), XP029723135, ISSN: 1388-2481, DOI: 10.1016 / J.ELECOM.2016.07.013), Muñoz-Noval Älvara et al. investigate the formation of amorphous platinum (Pt) clusters in nanopores of porous silicon during an initial stage of pore filling. The time dependence of the chemical state and local structure of Pt in nanoporous silicon is characterized by X-ray absorption fine structure spectroscopy (XAFS). Initially, the Pt deposits contain non-negligible amounts of PtO2, which is formed from atmospheric surface oxidation, indicating that the particle size is quite small. Detailed analysis of the extended XAFS (EXAFS) strongly suggests that the Pt is amorphous in the early stages of deposition.The mechanism of amorphous Pt formation is discussed on the basis of the confinement effect for Pt complex anions in nanopores.

[0008] Furthermore, G. Korotcenkov et al., in "Silicon Porosification: State of the Art" (Critical Reviews in Solid State and Material Sciences, 35 (3), 153-260 (2010), XP055710747, ISSN: 1040-8436, DOI: 10.1080 / 10408436.2010.495446), present the specific characteristics of silicon porosification and the principles of producing porous layers. Various methods for porosity of silicon are analyzed, such as chemical etching, chemical vapor etching, laser-induced etching, metal-assisted etching, spark processing, and reactive ion (plasma) etching. However, the main focus is on the electrochemical porosity of Si. The publication discusses in detail the influence of parameters such as electrolyte composition and pH value, current density, etching time, temperature, doping and wafer orientation, illumination, magnetic field and ultrasonic motion on the Si porosification process.It is shown that the structure of porous silicon strongly depends on both the technological parameters of the electrochemical etching process and the parameters of the semiconductor being treated. The publication also addresses the main properties of porous silicon, including the formation of porous multilayers and 3D structures, the oxidation of porous silicon, pore release, drying, storage, etching, filling, and surface functionalization of porous silicon. The characteristics of the porosity of III-V compounds are also briefly analyzed.

[0009] Also known from the publication DE 10 2009 056530 A1 is a structure consisting of parallel nanowires formed from a semiconductor material, with at least one flat side having freestanding, regularly arranged nanowire ends, and with at least one macroporous stabilizing layer formed from the same semiconductor material and perpendicularly penetrated by the nanowires, wherein the freestanding nanowire ends protrude less than 100 µm above the stabilizing layer.

[0010] Other approaches to creating silicon-based electrodes for lithium-ion batteries involve various silicon crystalline powders, such as shattered silicon particles from a wafer, mixed with electrically conductive particles, such as carbon black, which are further mixed with an organic binder and applied as a thick film to metal sheets. The isolated silicon particles can intercalate lithium and thereby increase their volume without necessarily causing significant mechanical stresses in the layers, for example, because the matrix of a layer can absorb the mechanical forces through deformation. Nevertheless, these mechanical stresses do have long-term consequences, leading, for example, to the interruption of the electrical contact between the silicon and the discharge electrode, possibly even to the delamination of the coating, and thus to a loss of capacity.

[0011] German patent application DE 10 2015 120 879 A1 proposes a silicon anode for lithium-ion batteries consisting of a copper film coated with silicon, deposited from the gas or liquid phase, followed by the etching of mesoporous pores into the silicon. According to the IUPAC definition, mesoporous pores are those with diameters between 2 and 50 nm. The intended porosity (proportion of pore volume to layer volume) is 60–90%, providing the silicon coating with internal void space for volume expansion under lithium loading. Since silicon adheres poorly to copper, the application includes an additional adhesion promoter film containing titanium, nickel, or vanadium, positioned between the copper film and the deposited silicon film. The silicon is p-doped during deposition to facilitate the one-sided electrochemical etching of mesopores.The authors discuss cycle stability only in paragraph 48 of the publication: "Compared to industrially established carbon-based materials, the capacity is increased by a factor of 4, but the cycle stability is still lower." It should be noted that mesopores can also be electrochemically etched into a silicon wafer using suitable process parameters. Doping the silicon can be helpful, but is not crucial for generating mesopores in Si single crystals. In particular, different pore morphologies can be selectively generated at various depths of a wafer by changing the etching parameters during the etching process. This is a complex but technically mastered technique.

[0012] The object of the invention is to propose a cost-effective, industrially applicable method for the production of cycle-stable silicon anodes for secondary batteries.

[0013] The problem is solved by a process for manufacturing a silicon anode for secondary batteries characterized by the following steps: a. Providing a monocrystalline silicon wafer with (100)-oriented flat faces; b. Contacting the back face of the wafer with a planar first electrode; c. Immersing the front face of the wafer in an etching bath comprising a hydrofluoric acid-containing electrolyte and a second electrode; d. Electrochemically etching mesopores of at least 4 micrometers pore depth into the front face of the silicon wafer by setting a predetermined etching current density while e. generating a porosity between 40% and 80% in the mesoporous layer; f. Generating a microporous delamination layer below the mesoporous layer by increasing the etching current density; g. Transferring the etched wafer into an electroplating bath; h. Electroplating an elemental metal into the etched mesopores to a predetermined pore depth of less than 2 micrometers; i.Deposition of a metal layer at least a few micrometers thick onto the etched front face of the wafer, creating electrically conductive and mechanically adhesive contacts between the metal layer and the elemental metal in the mesopores; j. Removal of the metal layer and the mesoporous, monocrystalline, (100)-oriented silicon layer, partially filled with elemental metal in the mesopores, mechanically destroying the microporous removal layer.

[0014] Furthermore, the present application comprises a silicon anode according to claim 8 and its use according to claim 9. The dependent claims specify advantageous embodiments.

[0015] The invention adopts the concept of DE 10 2015 120 879 A1, which uses mesoporous silicon for the anode to provide space for volume expansion during the intercalation, particularly of lithium ions. However, the approach used there, to coat a metal film with silicon, is replaced here by the opposite: metal deposition onto a monocrystalline, etched silicon wafer. It is essential according to the invention that the silicon is monocrystalline and that the two flat sides of the wafer are (100)-oriented, i.e., perpendicular to the (100) direction of the bulk crystal.

[0016] It should be noted that the silicon deposited on a metal film according to DE 10 2015 120 879 A1 is anything but monocrystalline. The publication therefore points in a completely different direction.

[0017] Particularly rapid growth of the silicon crystal is possible along the (100) direction. It can therefore be assumed that a (100)-oriented surface has a strong ordering effect on mobile silicon atoms. However, in order to exploit this effect, the crystal structure of the silicon anode must never be completely lost, despite the significant restructuring of the silicon during charging cycles. According to the invention, the crystal structure is therefore "frozen" in a portion of the mesoporous silicon by electroplating an elemental metal down to a predetermined pore depth. The deposited elemental metal, preferably copper or nickel, is mechanically rigid and incompressible, thus preventing any movement or volume expansion of the silicon down to the depth of the metal deposition and preventing intercalation in the layer region filled with the elemental metal.This ensures that the crystallinity and (100) orientation of the silicon in this layer region are maintained throughout the charging cycles of a secondary battery. While the portions of the mesoporous silicon not filled with elemental metal are largely restructured by ion uptake, they self-organize during ion discharge in a manner that results in uniform, predictable, and cycle-stable behavior of the silicon anode. It is assumed that this self-organization is only made possible—or at least greatly facilitated—by the presence of the monocrystalline (100) surface.

[0018] A side effect of the electroplating of elemental metal into the etched mesopores of the wafer and the subsequent deposition of a metal film onto the same flat side of the wafer is the robust mechanical coupling between the metal film and silicon via the lock-and-key principle. While the metallurgical adhesion of the elemental metal to the metal film within the pores is known to be very good, the adhesion of silicon to some metals, such as copper, is rather poor. However, the mesopores filled with elemental metal are usually not smooth and channel-like, but rather exhibit diameters that vary with the pore depth and laterally directed protrusions or serrations—that is, perpendicular to the pore orientation. When such pores are filled with solid material, the material cannot be pulled out; instead, it is mechanically anchored by positive locking. This makes it virtually impossible to detach the elemental metal-filled area of ​​the silicon layer from the metal film.

[0019] The invention will be explained in more detail below with the aid of figures. These show: Fig. 1 A sketch of the silicon anode along a section perpendicular to the wafer or metal film; Fig. 2 A scanning electron microscope (SEM) image of the mesoporous Si wafer; Fig. 3 Measured specific capacitances for the Si anode in a test cell with a lithium-ion electrolyte during the first 100 charge cycles; Fig. 4a) and b) SEM images of a Si anode with a 6-micrometer-thick silicon layer after several charge cycles in the lithium test cell, shown at two magnifications; Fig. 5 Measured charge capacitances for a Si anode with a layer thickness of 26 µm compared to a nickel-manganese-cobalt cathode; Fig. 6 Measured specific capacitances for the Si anode in a test cell with a sodium-ion electrolyte during the first 17 charge cycles; Fig. 7a) and b) SEM images of the Si anode after several charging cycles in the sodium test cell at two magnifications; Fig.8 Measured specific capacities for the Si anode in a test cell with a potassium ion-containing electrolyte during the first 15 charge cycles.

[0020] A monocrystalline silicon wafer can be electrochemically etched using known methods to create various types of pores in the silicon. Those skilled in electrochemistry are familiar with the process steps a) to f) and know how to determine the precise etching parameters (current density, etching time, electrolyte temperature and flow) when mesoporously etching a predetermined batch of wafers with a predetermined doping concentration and an electrolyte composition of their choice. They can, for example, refer to German patent DE 103 18 995 B4 and the sources cited therein. As already mentioned, a specific silicon doping concentration is not strictly necessary. However, using doped silicon is advantageous for mesoporous etching. Such silicon is also generally less expensive than high-purity silicon.Experiments show that a p-doped silicon wafer – for example with boron – which has a specific resistance of less than 10 mΩcm, preferably around 8 mΩcm, is an advantageous choice.

[0021] From US patent 7,208,069 B2, an etching device for large silicon wafers is known that is also suitable for producing mesopores. For example, an aqueous solution containing 20 wt% hydrogen fluoride (HF), 5 wt% ethanol, and 1 wt% polyethylene glycol (PEG) can be used as the electrolyte; optionally, 1 wt% sulfuric acid (H₂SO₄) can be added in addition to or instead of PEG. The bath temperature is typically a constant 20 °C, and the current density can be set within a typical range of 50 to 100 mA / cm², with higher values ​​resulting in greater porosity.

[0022] According to the invention, the total volume of the mesopores should be between 40% and 80% of the original silicon volume in the mesoporously etched layer. The reason for the width of the interval is explained further below. Particularly in the case of silicon anodes for lithium-ion batteries, it is advantageous to set the porosity to a value between 70% and 75%.

[0023] The expert also knows—or can easily determine through preliminary tests—how far and for how long the etching current density needs to be increased in their setup to create a microporous release layer with a sponge-like pore structure beneath the mesoporous layer. The release layer can be designed so that it has only very thin, brittle pore walls that are easily broken mechanically. As long as the pore walls remain intact, however, the mesoporous layer remains bonded to the wafer and can be moved along with it.

[0024] According to the invention, the etched wafer is then placed in an electroplating bath containing an electrolyte with elemental metal ions. Preferably, copper or nickel ions are suitable for this purpose.

[0025] Possible electrolytes include aqueous copper / nickel sulfate solutions (with a molarity of 0.25 M to 1.25 M) with additions of sulfuric acid (H₂SO₄) and 1 wt.% PEG (M ~3360), preferably at a pH between 1 and 2.7. The best results for nickel deposition are achieved with Watts solution: 200 g / L nickel sulfate (NiSO₄), 45 g / L nickel chloride (NiCl₂), 45 g / L boric acid (H₃BO₃). Good results can also be achieved using nickel sulfamate (Ni(SO₃NH₂)) at a concentration of 30 g / L, which is added instead of the NiSO₄.

[0026] The electrolyte penetrates the mesopores, at least in a near-surface region, and the mesopores are then partially filled with elemental metal by energizing the electrolyte and the resulting reduction of ions. "Partially" here means that not the entire pore depth—which should be at least 4 micrometers—is filled with elemental metal. This is because deposition in the mesopores becomes difficult at pore depths beyond a few hundred nanometers, as the electrolyte flow is significantly impeded. For the purposes of the invention, metal deposition over a pore depth of between a few tens and several hundred nanometers is perfectly adequate. Those skilled in the art know that the penetration depth of the electrolyte can be influenced, among other things, by controlling its viscosity.

[0027] Preferably, the metal deposition process is continued until a metal layer at least a few micrometers thick has formed on the wafer surface. The metal layer and the elemental metal deposits in the mesopores are bonded together, ensuring optimal electrical conductivity and mechanical adhesion. Alternatively, the metal layer can be formed from a different metal than the one used to fill the mesopores. For example, the electrolyte can be changed, or a different metallization method can be used to deposit the metal layer. At the end of the process, the metal layer must be able to freely support the silicon single crystal containing the mesopores, which, after the aforementioned detachment layer is destroyed, adheres to the metal layer separately from the wafer.

[0028] The release layer is very brittle and can be mechanically damaged, for example, by peeling the finished metal film and the mesoporous silicon single crystal from the wafer. Another method of mechanical damage involves irradiating the release layer with pulsed and / or focused ultrasound.

[0029] The result of the procedure is in Fig. 1 The diagram is sketched. In this cross-sectional sketch, shown perpendicular to the metal film (Met) and the Si wafer, the jagged mesopore walls (Si) are depicted, and the pores are shown as spaces between them. Between the dashed lines, the metal is deposited in the pores; the pores are therefore partially filled. In the case shown, the electroplating of an elemental metal in the pores was continued until a metal film at least a few micrometers thick was formed, i.e., the metal inside and outside the pores is the same.

[0030] The thickness of the silicon layer adhering to the metal film is determined by the choice of the mesopore depth during the etching process and, according to the invention, should be at least 4 micrometers. Preferably, the mesoporous silicon layer is 4 to 16 micrometers thick, and particularly preferably between 6 and 12 micrometers thick. The etching rate in the embodiment described above is slightly more than 2 micrometers per minute.

[0031] At first glance it reminds you Fig. 1 to a sketch of the metal-encased silicon nanowires from publication EP 2 460 214 B1. However, this association would be misleading, because the in Fig. 1 The depicted silicon is completely interconnected outside the image plane. In reality, the silicon here encloses metal protrusions from the metal film.

[0032] The Fig. 1 The silicon anode according to the invention for secondary batteries comprises a metal layer and a mesoporous, monocrystalline, (100)-oriented silicon layer, the mesopores of which are partially filled with elemental metal, wherein the elemental metal in the mesopores is in electrically conductive and mechanically adhesive contact with the metal layer. The anode can be directly connected to electrical leads on the metal film and inserted into a lithium secondary battery, wherein an electrode made of metallic lithium can serve as the cathode. The silicon side of the anode is arranged facing an anhydrous electrolyte containing lithium ions. When the delamination layer is completely removed, the silicon side looks exactly like the mesoporously etched front side of the Si wafer; this is shown as an SEM image in Fig. 2 to see.

[0033] For example, the following two electrolyte types are suitable for use in a secondary battery: a) A carbonate-based electrolyte. This consists of 1 M lithium hexafluorophosphate (LiPF₆) dissolved in solvents ethylene carbonate (EC) and dimethyl carbonate (DMC) in a 1:1 ratio (commercially available). b) An ether-based electrolyte. This can be prepared as a solution of the salt lithium bis(trifluoromethanesulfonyl)imide (LiTFSi) in dioxolane-1,3 (DOL) and dimethyl ether (DME) in a 1:2 ratio.

[0034] Fig. 3 The graph shows the measured specific charge capacities of a lithium-ion test cell, comprising a silicon anode with a 6-micrometer-thick silicon layer and a lithium metal cathode. As is typical in battery technology, the lithium charge is only applied up to approximately 75% of the theoretical maximum. The discharge curve shows that the available charge capacity stabilizes at a high level after just a few cycles. From about the 20th cycle onward, no further changes are detectable. Charging and discharging the test cell each takes two hours in the test shown.

[0035] The anode, stabilized and discharged after cycling, can be removed and inspected. Under a scanning electron microscope, the silicon side shows a restructuring as depicted in the Fig. 4 a) und b) The image shows two magnifications. Tower-like structures have formed across the entire surface, separated by cracks. These cracks form a largely continuous network and are approximately 1 micrometer wide. The surface resembles a dried-up riverbed, where desiccation cracks appear as the water evaporates from the sediments. Closer examination reveals that the tower-like structures are coated on their surface with a solid, silicon-lithium mixed-crystal phase known as a "solid-electrolyte interface" (SEI). This SEI is known to expand with the volume of silicon during lithium intercalation and contract again during discharge (S Hansen, S Shree, G Neubüser, J Carstensen, L Kienle, R Adelung, "Corsetlike solid electrolyte interface for fast charging of silicon wire anodes", Journal of Power Sources 381, 8-17, 2018).The SEI remains intact even when the battery is completely discharged and permanently binds a certain proportion of the lithium ions in the electrolyte. It does not crack or grow permanently, as is the case in conventional lithium-ion batteries with graphite. It perfectly encapsulates these cracks and provides them with additional mechanical support and stability.

[0036] It is important to note here that the in Fig. 4 The structure shown does not depend significantly on the number of charge cycles after which the anode is examined. Despite the considerable restructuring of the silicon layer during each individual charge cycle, the described flowbed structure reforms repeatedly. This is considered the main reason for the measurable cycle stability of the new silicon anode. At the same time, its storage capacity is also significantly superior to the best available carbon-based anodes. According to the inventors' estimates, a mass-produced lithium-ion battery with the anode according to the invention and the best cathodes currently available commercially can be expected to at least double the charge capacity of such batteries.

[0037] In Fig. 5 A plot of measured charging capacities for a silicon anode according to the invention, with a silicon layer 26 micrometers thick and a porosity of 65% at a pore diameter of 30 nanometers, is also shown compared to a nickel-manganese-cobalt cathode. Fig. 3 The capacity level is considerably lower, primarily due to the limited cathode, which cannot fully utilize the silicon anode's performance. More important, however, is the anode's stability over 40 charge cycles (charge rate C / 4), with both charge and discharge capacities exceeding 300 mAh / g in all cycles. Although these measurements indicate that only a relatively small portion of the silicon layer actively participates in lithium storage, anchoring the monocrystalline, (100)-oriented, mesoporous silicon layer to the metal film is essential. Without the electroplating deposits in the mesopores to establish the lock-and-key principle, the 26-micrometer layer crumbles after approximately 10 charge cycles, losing its electrical contact.

[0038] Investigations at various silicon layer thicknesses indicate that a cycle-stable silicon anode for secondary batteries can be realized with silicon layer thicknesses of at least 4 micrometers. The measurements further show that layer thicknesses between 4 and 16 micrometers, and especially between 6 and 12 micrometers, currently offer the best results—particularly efficient lithium-ion storage—and are therefore preferred.

[0039] Finally, it should be noted that silicon can also intercalate other alkali metals, especially sodium and potassium. Therefore, the silicon anode can also be used and cycled in a test cell with a sodium or potassium ion electrolyte and a metallic sodium or potassium cathode. Suitable electrolytes for this purpose include, for example, the aforementioned ether-based electrolytes, in which either sodium trifluoromethanesulfonimide (NaTFSi) for sodium batteries or potassium trifluoromethanesulfonimide (KTFSi) for potassium batteries are dissolved.

[0040] In fact, the silicon anode according to the invention also proves suitable for use in sodium or potassium ion batteries. Surprisingly stable charging cycles can be achieved, although these sometimes take longer than with lithium batteries.

[0041] However, because sodium or potassium cause a smaller increase in the volume of silicon during intercalation than lithium, it may be advantageous to design the silicon anode to accommodate this by providing a smaller porosity, in particular to a value from the interval of 40% to 60%.

[0042] For example, in Fig. 6 A plot showing the measured charging capacities of a sodium test cell is shown. The anode is charged with sodium ions up to the theoretically maximum charging capacity, which, according to various sources, is around 1000 mAh / g. The measurement data covers 11 charging cycles, the first 6 of which are performed at a high charging rate of C / 4. Initially, the sodium cathode is not yet passivated, i.e., coated with a protective layer, which explains the initially high charging capacity. Once this SEI (sodium electrolyte intercalation) has formed on both the sodium cathode and the silicon anode, the charging capacity drops significantly, while the discharging capacity remains low. From cycle 7 onward, the charging rate is significantly reduced to C / 10, meaning a charging time of 10 hours for a full charge.This has the clearly noticeable effect that the maximum capacity can be utilized during charging, while the discharge capacity increases with each subsequent cycle – until a final level is reached, which has not yet been achieved. The reason for the significantly improved battery performance lies in the fact that the sodium ions require sufficient time to intercalate into the silicon and overcome the activation energy. Thus, it is indeed possible to create a sodium-ion secondary battery with the silicon anode, although it currently stores and releases electrical charge relatively slowly. However, the advantage of being able to use sodium ions instead of lithium ions, for example to store energy from photovoltaic cells, is certainly worth mentioning here.

[0043] In Fig. 7 a) und b) SEM images of the silicon anode cycled in the sodium test cell are shown in two magnifications. A silicon surface wetted with thin lamellae is visible. However, due to the reduced volume expansion of the silicon during sodilation, a phenomenon occurs—particularly visible in the Fig. 7 b) - a significantly smaller crack pattern. It is noteworthy that the crack pattern re-establishes itself even after repeated cycling. This suggests that high cycle stability of the silicon anode according to the invention can also be expected in a sodium-ion battery.

[0044] Fig. 8Figure 1 shows a plot of measured charge capacities for a potassium-ion test cell comprising a silicon anode and a potassium cathode according to the invention. The cell's behavior during the first 15 charge cycles is shown, with each charge and discharge cycle taking 4 hours (charge rate C / 4). Interestingly, the electrode matching problems seen with sodium cells are not observed here, and the final level of usable charge capacity can be accurately estimated at approximately 550 mAh / g. Note that the left-hand scale only begins at 350 mAh / g. The ratio of discharge capacity to charge capacity is therefore roughly 75%.

Claims

1. A method for producing a silicon anode for secondary batteries, characterised by the steps of: a. providing a monocrystalline silicon wafer with (100)-oriented flat faces; b. contacting the rear side of the wafer with a flat first electrode; c. introducing the front side of the wafer into an etching bath having a hydrofluoric acid-containing electrolyte and a second electrode; d. electrochemically etching mesopores with a pore depth of at least 4 micrometres into the front side of the silicon wafer by establishing a predetermined etching current density while e. generating a porosity of between 40% and 80% in the mesoporous layer; f. generating a microporous release layer below the mesoporous layer by increasing the etching current density; g. placing the etched wafer in an electroplating bath; h. electrodepositing an elemental metal into the etched mesopores to a predetermined pore depth of less than 2 micrometres; i. depositing a metal layer at least several micrometres thick onto the etched front side of the wafer while producing electrically conductive and mechanically adherent contacts of the metal layer with the elemental metal in the mesopores; j. lifting off the metal layer and the mesoporous, monocrystalline, (100)-oriented silicon layer partially filled with elemental metal in the mesopores while mechanically destroying the microporous release layer.

2. A method according to claim 1, wherein the monocrystalline silicon wafer is p-doped and has a resistivity of less than 10 mΩcm, preferably around 8 mΩcm.

3. A method according to one of the preceding claims, wherein the mesopores are etched with pore depths of between 4 and 16, preferably between 6 and 12 micrometres.

4. A method according to any of the preceding claims, wherein the porosity of the mesoporous layer is between 70% and 75%.

5. A method according to one of the preceding claims, wherein one of the elemental metals copper or nickel is deposited into the mesopores.

6. A method according to claim 5, wherein the deposition of the elemental metal into the mesopores takes place to a pore depth of between a few tens of nanometres and several hundred nanometres.

7. A method according to any one of the preceding claims, wherein the metal layer, which is at least a few micrometres thick, is formed from a metal other than the elemental metal deposited in the mesopores.

8. Silicon anode for secondary batteries, characterised by a metal layer and a mesoporous, monocrystalline, (100)-oriented silicon layer partially filled with elemental metal in the mesopores, wherein - the elemental metal in the mesopores is in electrically conduction and mechanically adhering contact with the metal layer - the mesopores have a pore depth of at least 4 micrometres and - the mesopores are filled with elemental metal up to a pore depth of less than 2 micrometres.

9. Use of the silicon anode according to claim 8 as an anode in a secondary battery having an electrolyte containing alkali metal ions, wherein the alkali metal is at least one from the group consisting of lithium, sodium or potassium.