Silicon anode for lithium ion battery and method for producing same

Abstract

The present invention relates to a method for the manufacture of silicon anodes for lithium batteries, in which an active layer made of silicon is deposited on a substrate, preferably copper, and which is then subjected to a rapid annealing process. The object of the present invention is to provide a method for the manufacture of silicon anodes for lithium batteries, which allows to minimize the stress in the deposited layer and which can be easily integrated into existing manufacturing processes. This is achieved either by the substrate surface being structured by a process prior to the application of the active layer, or by the substrate surface being modified in an unstructured state by a process, or by the active layer being structured during its manufacture by photolithography and a subsequent physical deposition process (preferably sputtering or evaporation) and an annealing process (preferably a rapid annealing process) to form segments.

Description

[Technical field]

[0001] The present invention relates to methods for producing silicon anodes for lithium batteries, in which an active layer made of silicon is deposited on a substrate, preferably copper, after which the active layer is subjected to rapid thermal annealing.

[0002] The invention also relates to the use of the method according to the invention for the production of silicon anodes for lithium-ion batteries, to the silicon anodes themselves and to the use of silicon anodes in battery cells and lithium-ion batteries. [Background technology]

[0003] Batteries are electrochemical energy storage devices, and a distinction is made between primary and secondary batteries.

[0004] A primary battery is an electrochemical power source in which chemical energy is irreversibly converted into electrical energy. Therefore, primary batteries cannot be recharged. In contrast, secondary batteries, also called accumulators, are rechargeable electrochemical energy stores that can be used repeatedly because the chemical reactions that occur can be reversed. When charging, electrical energy is converted into chemical energy, and when discharging, chemical energy is converted into electrical energy.

[0005] The term "battery" is a collective term for interconnected cells. A cell is a galvanic unit consisting of two electrodes, an electrolyte, a separator, and a cell housing. Figure 1 shows an exemplary structure and function of a lithium-ion cell during the discharge process. A brief description of the cell components follows:

[0006] Each Li-ion cell consists of two different electrodes: one that is negatively charged in the charged state, and one that is positively charged in the charged state. During the release of energy, i.e. discharging, ions move from the negatively charged electrode to the positively charged electrode, so the positively charged electrode is called the cathode (positive electrode) and the negatively charged electrode the anode (negative electrode). Each electrode consists of a charge collector (also called collector) and an active material attached to it. Between the electrodes is an ionically conductive electrolyte that allows the necessary charge exchange, and a separator that ensures the electrical insulation of the electrodes.

[0007] The cathode may, for example, consist of a mixed oxide material attached to an aluminum collector.

[0008] The anode of a Li-ion battery can consist of a copper foil as a collector and a layer of carbon or silicon as the active material. During the charging process, lithium ions are reduced and intercalated into the graphite or silicon layers.

[0009] Silicon as the anode active material is Li 15 The Si4 phase has a storage capacity of approximately 3579 mAh / g, which is higher than that of conventional carbon-based materials (e.g., graphite, which has a storage capacity of 372 mAh / g).

[0010] The capacity of the battery is determined by the thickness of the active layer, more precisely the Si layer. In a battery, the electrical conductivity of the active material must be as high as possible. Silicon, as a semiconductor, simply has a low electrical conductivity, in contrast to conductive graphite. Therefore, silicon needs a high doping or structure that enhances its electrical conductivity.

[0011] When the electrode material used is silicon, the challenge is that the volume of the host matrix sometimes changes considerably (volume contraction and volume expansion) upon intercalation and deintercalation of mobile ionic species (lithium) during charging and discharging of the corresponding energy store. The volume change is about 10% for graphite, but about 400% for silicon. When silicon is used, the volume change of the electrode material leads to internal stresses, crack formation, crushing of the active material of the host matrix (silicon) and ultimately to the complete destruction of the electrode.

[0012] Conventional active materials are usually applied to copper foil in the form of particles in a binder layer and dried. This process does not create stress because the active material is fixed on the current collector by a flexible adhesive. The flexible adhesive compensates for the stress on the current collector when the active material expands in volume during the operation of the battery, ensuring a permanent electrical contact. If flexibility is not ensured, the active material will shatter, losing power contact and reducing the capacity of the battery. Energy density is reduced as a result of using the adhesive as an inactive component of the active material.

[0013] The adhesion of the active layer can be improved by using rapid thermal annealing. The term "rapid thermal annealing" is understood to mean in particular flash lamp annealing and / or laser annealing. Flash lamp annealing has a pulse duration or annealing time of 0.3 to 20 ms and a laser power of 0.3 to 100 J / cm. 2 Laser annealing is performed with a pulse energy of 0.1 to 100 J / cm. 2 To generate a 100-nm thick film, the annealing time is set to 0.01-100 ms depending on the scanning speed of the local heating point. The heating gradient achieved in rapid thermal annealing is 10 4 ~10 7K / s method. Flash lamp annealing uses the visible wavelength range of the spectrum for this purpose, while laser annealing uses discrete wavelengths in the infrared (IR) to ultraviolet (UV) spectral region. If an active layer of silicon is deposited on a substrate, preferably copper, and then the active layer is subjected to rapid thermal annealing, this produces a very strong bond between the silicon and the copper foil, not observed so far in the case of conventionally produced anodes. Normally, as a result of volume expansion / contraction during the operation of the battery, the active material is held on the current collector by a soft adhesive or simply crushed, but this is not the case in the case of anodes produced by rapid thermal annealing. However, the strong adhesive forces cause the substrate, i.e. the current collector, to warp during the operation of the battery, which is shown by a clear serpentine structure (wavy nature of the ply structure) in the side view of the layer stack (see Figure 2).

[0014] Another challenge is therefore that when the anode is manufactured on a planar foil substrate, such a one-sided coated foil will show a curvature after manufacturing, either towards the foil or towards the coated side, depending on the stack structure. The cause is the internal stress of the layers formed after deposition and annealing. This makes the manufacturing of the battery more difficult, since the battery usually needs to be built up from various plies of stacked foils / layers. The term "ply" is understood to mean the different layers of the layer structure or layer stack of the Si electrode. In this application, the terms "ply" and "layer" are used synonymously. The curvature is an obstacle to manufacturing, since the planarization requires mechanical forces.

[0015] Applying the layers by vacuum or temperature processes can cause internal stress build-up as a result of the different expansion coefficients and densities of the layers and the substrate. Especially with the use of rapid thermal annealing, strong adhesion of the applied layers to the substrate or current collector can result in warping of the copper foil, which can prevent it from being processed and functioning as an anode.

[0016] These stresses can be tuned by the process, for example by changing process parameters such as pressure, gas, substrate temperature, power, layer material, reaction, etc. In this way, compressive and tensile stresses in the layers can be combined to produce a finally relaxed layer. This requires precise tuning of the stress management for exactly one manufacturing process, and repeating this for small variations. Coating both sides of the foil is possible, but this would put even more stress on the entire foil, which may tear during the operation of the battery. Summary of the Invention [Problem to be solved by the invention]

[0017] It is therefore an object of the present invention to provide methods for manufacturing silicon anodes for lithium batteries that can minimize stress in the deposited layers. These methods should be easy to integrate into existing manufacturing processes and should allow precise control of stress management in the deposited layers of the anode structure. [Means for solving the problem]

[0018] This object is achieved by a first method as defined in independent claim 1. A method for producing a silicon anode for a lithium battery, comprising depositing an active layer of silicon on a substrate, preferably copper, after which the active layer is subjected to a rapid thermal annealing, the substrate surface of the substrate being structured by a process prior to application of the active layer.

[0019] In one embodiment of the method according to the invention, the substrate surface is structured by means of a laser.

[0020] In another embodiment of the method according to the invention, the substrate surface is structured by embossing, rolling or stamping, which can produce height variations in the range of up to 20 μm. The structuring is carried out before additional layers are deposited or applied to the substrate surface.

[0021] In a further embodiment of the method according to the invention, the substrate surface is structured by photolithography followed by physical deposition, preferably sputtering or evaporation.

[0022] The structuring of the surface of the substrate is a technological means to simplify the stress management and thereby generate isolated segments that are not connected to each other in two dimensions. By fine structuring, the stresses of the layer are interrupted in the individual segments. As a result, the processing of the foil / substrate after manufacture is significantly simplified. Furthermore, this structuring allows a controlled reduction of the meandering structure during the operation of the battery, which significantly reduces the microscopic delamination of the active material. If the structuring is sufficiently fine, i.e. less than 10 μm, preferably between 1 μm and 5 μm, the segmentation caused by the structuring can further compensate the volume expansion of the active material during lithium intercalation without crushing it. In the manufactured ply structures described in the literature, the size of the cracks in the destroyed layers is of the order of micrometers when amorphous silicon is used. The method according to the invention allows a precise control of the cracks.

[0023] Regular structuring by laser, embossing, rolling or stamping processes is particularly suitable for large-scale production.

[0024] This object is also achieved by a second method as claimed in independent claim 5. A method for producing a silicon anode for a lithium battery, comprising depositing an active layer of silicon on a substrate, preferably copper, which active layer is subsequently subjected to rapid thermal annealing, the substrate surface and / or the active layer being modified in a non-structuring manner by the process.

[0025] An unstructured modified surface is likewise understood to be a rough surface on which there are separated surface segments that are not connected to one another in two dimensions.

[0026] In one embodiment of the method according to the invention, the non-structuring modification is carried out by rapid thermal annealing, or by etching, or by chemical deposition, or by physical deposition of a highly cohesive material followed by rapid thermal annealing for cohesion, resulting in a height variation of up to 20 μm. The height variation is in the range of the thickness of the active layer. The expression "highly cohesive material" is understood to mean a substance in which the interatomic or intermolecular bonding forces are strong enough that clusters or aggregates are formed.

[0027] For example, in one embodiment of this method, a thin layer of silver is deposited on the surface of a copper substrate. Rapid thermal annealing, particularly flash lamp annealing, results in the silver layer agglomerating to form particles / droplets / clusters, resulting in a non-structured roughened surface of the copper foil. The foil can then be further processed as usual.

[0028] The height variations thus produced allow the subsequently deposited active layer to be divided into regions of different size on the thus modified substrate, thus significantly reducing the stress in the active layer while maintaining good adhesion properties of the active layer to the substrate.

[0029] This object is also achieved by a third method as claimed in independent claim 7. A method for producing a silicon anode for a lithium battery, comprising depositing an active layer of silicon on a substrate, preferably copper, and subsequently subjecting said active layer to rapid thermal annealing, the active layer being structured into segments during production of the active layer by photolithography followed by physical deposition, preferably sputtering or evaporation, and annealing, preferably rapid thermal annealing.

[0030] To this end, in this embodiment the substrate surface is not structured or roughened, rather the active layer itself is structured.

[0031] In one embodiment of the method according to the invention, the active layer is structured into segments having a size between 10 μm and 5 mm, which has the advantage that the anode formed from the active layer thus produced is only stressed locally and not over the entire area of ​​the anode as a whole.

[0032] In another embodiment of the method according to the invention, the active layer is structured into segments, the distance between the segments being between 2 μm and 10 μm. The advantage is that in addition to reducing the stress in the active layer, local expansion of the active layer due to lithium intercalation during or before the operation of the battery can be controlled and a widespread effect of stress on the foil is avoided.

[0033] A silicon anode produced by one of the methods according to the invention comprises a substrate on which an active layer of silicon and / or silicon-based components is arranged, the active layer having a layer thickness of at least 1 μm to a maximum of 20 μm, preferably at least 2 μm to 15 μm, particularly preferably at least 4 μm to 10 μm, an area coverage of more than 85%, and thus a low porosity of 15% or less.

[0034] For sufficient battery capacity, with an assumed storage density of 2000 mAh / g for silicon, the thickness of the layers used in battery fabrication should be at least in the range of 4 μm to 10 μm.

[0035] In the prior art, silicon particles surrounded by carbon and an adhesive, the so-called binder, are typically applied to a smooth surface, so that the layer has a defined porosity, allowing the silicon to expand without stress when lithiation occurs. Without the use of a binder, in a pure silicon layer, there is only adhesion due to the rough surface. This provides sufficient cavities in the layer structure to compensate for stress when volume expansion occurs. According to the prior art, porosities between 15% and 80% are used. By the method according to the invention, pure silicon anodes can be modified and prepared in such a way that the silicon can expand on the substrate without stress, without losing electrical contact with the substrate during the lithiation and / or delithiation process, even when the areal coverage is more than 85%.

[0036] In the silicon anode according to the invention, the active layer is substantially formed of a part of amorphous or semicrystalline silicon and / or a part of silicide and / or a part of a solid solution of one or more metals in silicon and / or a mixture of these parts. The different morphologically formed parts of the active layer have the advantage that there is both nanostructured silicon that can expand isotropically without structural collapse, and a stable conductive framework structure that is permanently adjacent to the amorphous silicon and ensures a stable electrical contact.

[0037] In one embodiment of a silicon anode produced according to the present invention, the substrate is formed from copper, copper-bearing alloys, nickel, aluminum, carbon, and / or steel.

[0038] The method according to the invention is advantageously used for producing silicon anodes for lithium-ion batteries as defined in the method claims.

[0039] It is also advantageous to use a silicon anode according to claim 10 in a battery cell, in particular a lithium-ion battery.

[0040] The battery cells can then be advantageously mounted in a battery having at least one battery cell.

[0041] The invention will now be described in more detail with reference to exemplary embodiments. [Brief description of the drawings]

[0042] [Figure 1] An exemplary structure and function of a lithium-ion cell during the discharge process. [Figure 2a] Schematic illustration of the meandering formation of the layer stack due to the 3D volume expansion when lithium is intercalated. [Figure 2b] 4 is an image of the warping of the layers without the structuring according to the invention. [Diagram 3] Forcing one-dimensional expansion of layer stacks in lithium-ion batteries. [Figure 4a] Schematic diagram of a pre-structured substrate surface for reducing stress in the layer stack of a lithium-ion battery according to a variant of the manufacturing method according to the invention (before Li intercalation). [Figure 4b] Schematic diagram of a pre-structured substrate surface for reducing stress in the layer stack of a lithium-ion battery according to a variant of the manufacturing method according to the invention (after Li intercalation). [Figure 5a] Schematic diagram of a structured active layer for reducing stress in the layer stack of a lithium-ion battery according to a variant of the manufacturing method according to the invention, illustrating the structuring process of the active layer. [Figure 5b] Schematic diagram of a structured active layer for reducing stress in the layer stack of a lithium-ion battery according to a variant of the manufacturing method according to the invention: left: before Li intercalation; right: after Li intercalation. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0043] When the active layer 11 is applied directly to the current collector 2, 10 and subjected to rapid thermal annealing 12, it has a very high adhesion to the substrate / current collector 2, 10. As a result of the partially graded stack structure of the silicon anode, the active layer 11 does not shatter and the power connection is durably maintained. However, due to the strong adhesion, stresses of the active layer 11 are transferred to the current collector 10, which are manifested in the form of a bulging foil after fabrication. Here, for the first time, a corrugation of the current collector 2, 10, which corresponds to a serpentine structure, occurs after the operation of the battery due to the large volume expansion of the active material (Figure 2).

[0044] The waviness, or meandering, of the layer or layer stack can be countered by forcing the layer structure into a purely one-dimensional expansion by the measures described below (FIG. 3). Suitable measures for this purpose include using a thicker or stiffer copper substrate 10, precharging or prelithiating the silicon active layer 11 during manufacture, providing a rigid framework in close proximity to the copper substrate 10, a loose enough structure to support the rigidity of the copper substrate 10, or applying sufficient pressure to the anode produced during formation. The term "formation" is understood to mean the first charging and discharging of the completed battery cell. A prerequisite for these options is the use of a rapid thermal anneal 12 by flash lamp or laser during the manufacture of the layer stack, so as to ensure a sufficiently strong adhesion of the active layer 11 on the substrate 10. Without this rapid thermal anneal 12, the active layer 11 would easily peel off from the substrate 10.

[0045] Another means to combat the tortuosity of the layer stack of silicon anodes for lithium-ion batteries is to pre-structure the current collector, i.e. the copper substrate 10, to segment the active layer 11 and to distribute and control the stresses in the anode region by this segmentation (Figure 4). Figure 4a shows a schematic diagram of a structured substrate surface 10, whose surface is microscopically roughened / pre-structured. This can be performed in an ordered manner, for example by rolling, embossing, stamping or lithography. In this way, height variations in the range of 400 nm to 10 μm can be realized. The structuring can also be performed in a disordered manner, for example by brushing or etching, or by electrochemically depositing or agglomerating particles on the copper substrate surface 10 before depositing the silicon 11. It is advantageous that the structuring of the substrate surface is directly mapped onto the structuring of the active layer.

[0046] When the thus structured layer structure is intercalated with lithium 14, the Si layer 11 expands in the planes 15 formed as a result of the structuring (FIG. 4b). As a result, in contrast to monolithically constructed layers, warping (wandering) no longer occurs in the layer stack, since the total stress in the layer stack is distributed and dissipated.

[0047] This provides for the first time a possible solution for relieving the stress on the current collectors 2, 10 in silicon anode stack structures processed and fabricated with a rapid thermal annealing process 12.

[0048] FIG. 5a shows a schematic diagram of a structured active layer 11 produced / can be produced according to the method according to the invention as defined in claim 7, the surface of which is / is microscopically roughened. According to the invention, this can be achieved, for example, by applying a functional layer 16, which prevents adhesion and / or reaction between silicon 11 and copper 10. A suitable functional layer 16 can consist, for example, of tungsten, carbon or silver (droplets). In areas where there is no reaction between Si and the functional layer 16, the Si 11 is separated, leaving a structured active layer 18 of Si. This layer is deposited in some areas on the substrate 10 (copper) by photolithography followed by physical deposition, after which the active layer 11 of silicon is deposited. The expression "some areas" means that the functional layer 16 is not applied to the entire surface of the copper substrate 10. In areas where the functional layer 16 prevents adhesion between Si and Cu, the Si is separated in a subsequent manufacturing process, preserving the structured active layer 18 in the layer stack of the lithium-ion battery.

[0049] When lithium 14 is intercalated into such a structured layer structure, the Si layer 11 is able to expand (15) both vertically and horizontally (Figure 5b). As a result, in contrast to unstructured layer structures, bowing (wandering) in the layer stack no longer occurs, since the total stress in the layer stack is distributed and dissipated.

[0050] Both the structuring of the substrate surface and the structuring of the active layer advantageously reduce the stress in the layer structure and at the same time also significantly reduce the microscopic delamination of the active material for battery operation. [Explanation of symbols]

[0051] 1 Lithium-ion battery 2 Anode side collector 3 Solid electrolyte interface (SEI) 4 Electrolytes 5. Separator 6. Conductive Mesophase 7 Cathode, positive electrode 8 Cathode side collector 9 Anode, negative electrode 10 Copper substrate 11 Active layer 12 Rapid thermal annealing (e.g. flash lamp annealing) 13 Reaction area between substrate and active layer after rapid thermal annealing 14 Lithium intercalation 15 Expansion direction after lithium intercalation 16 Functional layers applied to some areas 17 Structured Substrate Surface 18 Structured active layer

Claims

1. A method for manufacturing a silicon anode for a lithium battery, comprising depositing an active layer (11) of silicon on a substrate (10), said substrate consisting of copper, after which said active layer is subjected to a rapid thermal anneal (12), A method characterized in that, before applying said active layer (11), the substrate surface is structured (17) by a process.

2. 2. The method of claim 1, wherein the substrate surface is structured by a laser.

3. 2. The method according to claim 1, wherein the substrate surface is structured by embossing, rolling or stamping, thereby producing height variations of up to 20 μm in the region of the thickness of the active layer.

4. 2. The method of claim 1, wherein the substrate surface is structured by photolithography followed by physical deposition.

5. A method for manufacturing a silicon anode for a lithium battery, comprising depositing an active layer (11) of silicon on a substrate (10), said substrate consisting of copper, after which said active layer is subjected to a rapid thermal anneal (12), A method, characterized in that the substrate surface and / or the active layer is modified in an unstructured manner by a process.

6. 6. The method according to claim 5, characterized in that the non-structuring modification is carried out by rapid thermal annealing (12), or by etching, or by chemical deposition, or by physical deposition of a highly cohesive material followed by rapid thermal annealing (12) for cohesion, thereby generating height variations of up to 20 μm in the thickness range of the active layer.

7. A method for manufacturing a silicon anode for a lithium battery, comprising depositing an active layer (11) of silicon on a substrate (10), said substrate consisting of copper, after which said active layer is subjected to a rapid thermal anneal (12), A method characterized in that said active layer (11) is structured (18) into segments during the production of said active layer by photolithography followed by physical deposition and annealing.

8. 8. The method according to claim 7, characterized in that the active layer (11) is structured into segments having a size between 10 μm and 5 mm.

9. 8. The method according to claim 7, characterized in that the active layer (11) is structured into segments, the distance between said segments being between 2 μm and 10 μm.

10. A silicon anode for a lithium ion battery produced according to any one of the methods of claims 1 to 4, 5 to 6, or 7 to 9, comprising: The silicon anode comprises a substrate (10) on which an active layer (11) of silicon is arranged, the active layer having a thickness of at least 1 μm and a maximum of 20 μm and an area coverage of more than 85%.

11. 11. The silicon anode according to claim 10, characterized in that the active layer (11) is formed substantially from a part of amorphous or semi-crystalline silicon and / or a part of a silicide and / or a part of a solid solution of one or more metals in silicon and / or a mixture of said parts.

12. 11. A silicon anode according to claim 10, characterized in that the substrate (10) is made of copper, a copper-containing alloy, nickel, aluminum, carbon and / or steel.

13. A battery cell comprising the silicon anode of claim 10.

14. A battery comprising at least one battery cell according to claim 13.

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