Method for stabilizing copper-rich silicide phases and use of the same in lithium-ion batteries

Abstract

The present invention relates to a method for stabilizing copper-rich silicide phases, in which a silicon layer structure is applied to a carrier substrate. In particular, the problem addressed by the present invention, which is to identify a method by which the characteristics of phase separation and microstructure formation can be varied in a controlled manner while making the process as simple, fast and efficient as possible, is a method for stabilizing copper-rich silicide phases, in which a silicon layer structure is applied to a carrier substrate, a layer of the silicon layer structure is applied from a mixture of at least one metal and silicon, this mixture is subsequently subjected to a short tempering, the short tempering having a pulse duration in the range of 0.01-100 ms and / or a pulse width of 0.1-100 J / cm2. 2 The solution is achieved by a method in which the phase separation of the applied layers is controlled by setting process parameters such as the amount of pulse energy in the range of 0.5 to 100 °C, pre-heating or cooling of the carrier substrate to a range of 4 °C to 200 °C, as well as by material selection of the applied mixture of layers of the silicon layer structure.

Description

[Technical field]

[0001] The invention relates to a method for stabilizing a copper-rich silicide phase in a silicon layer structure applied to a carrier substrate.

[0002] The invention further relates to the use of the method of the invention for producing high capacitance electrode materials in lithium ion batteries, more particularly for silicon anodes, as well as to anode materials and their uses in battery cells and lithium ion batteries, and to anodes produced by the method of the invention. [Background technology]

[0003] When an unspecified copper (Cu)-silicon (Si) mixed lamina is exposed to high temperatures, the respective concentrations of copper and silicon determine the formation of single or multiple phases of copper silicide or mixed crystals, called solid solutions.

[0004] Three silicides exist in the copper-silicon mixed thin layer at low temperature in equilibrium. 3 Si, Cu 15 S 4 , Cu 5 The most stable intermetallic phase with the highest silicon concentration is Cu. 3 As the proportion of silicon increases (hypereutectic phase, Cu3-αSi(α>0)), the corresponding (Si) and Cu 3 In hypoeutectic Si-Cu mixtures, i.e., mixtures with more copper, Cu is formed when heated above 170°C. 3 The Si phase is formed first (Non-Patent Document 2), and at higher temperatures this phase changes to the copper phase Cu 15 S 4 and Cu 5Si. A microstructure develops in which the phases are formed according to their ratio. The morphology and distribution of this microstructure is determined by the cooling rate of the annealing treatment applied to the thin layer. A slow cooling rate and / or a small temperature gradient favors a grown phase separation with a large microstructure, whereas a fast cooling rate and / or a high temperature gradient results in a phase separation with a small grain microstructure. The phase separation depends on the constituent elements of the thin layer and the intermetallic phases formed. Besides binary systems such as Cu-Si with three intermetallic phases and Ni-Si with five intermetallic phases, there are also systems such as Al-Si where eutectics exist but no intermetallic phases are formed. See the phase diagrams for Cu-Si (Figure 1), Ni-Si (Figure 2), Al-Si (Figure 3) and Ti-Si (Figure 4).

[0005] A number of different metal and silicon combinations can be used to further customize the phase separation and microstructure formation properties. [Prior art documents] [Non-patent literature]

[0006] [Non-Patent Document 1] Chromik, RR, Neils, WK & Cotts, EJ Thermodynamic and kinetic study of solid state reactions in the Cu-Si system. Journal of Applied Physics 86, 4273 (1999) [Non-Patent Document 2] Russell, SW, Li, J. & Mayer, JW In situ observation of fractal growth during a‐Si crystallization in a Cu3Si matrix. Journal of Applied Physics 70, 5153–5155 (1991) Summary of the Invention [Problem to be solved by the invention]

[0007] It is therefore an object of the present invention to identify in particular a method by which the characteristics of the phase separation and the corresponding microstructure formation can be customized, with the intention of making it possible to carry out this process with maximum simplicity, rapidity and efficiency. [Means for solving the problem]

[0008] This object is achieved by a method according to independent claim 1. A method for stabilizing copper-rich silicide phases in microstructures, in which a silicon layer structure is applied to a carrier substrate, is provided, in which a layer of said silicon layer structure consisting of a mixture of at least one metal and silicon is applied, followed by rapid annealing, in which the rapid annealing pulse duration is in the range of 0.01-100 ms and / or the pulse current is 0.1-100 J / cm 2 By setting the operating parameters such as the pulse energy in the range of , pre-heating or cooling of the carrier substrate in the range of 4 °C to 200 °C as well as the choice of materials in the applied mixture of the layers of the silicon layer structure, the phase separation in the applied layers is controlled and the microstructures are developed.

[0009] Rapid annealing specifically refers to flash lamp annealing and / or laser annealing. Flash lamp annealing is performed at 0.3 to 100 J / cm2 with pulse durations or annealing times in the range of 0.3 to 20 ms. 2 For laser annealing, the annealing time is set by the scanning speed of the local heating area, and the pulse energy is in the range of 0.1 to 100 J / cm. 2 This produces an energy density of 10^4-10^7 K / s. The heating gradients achieved in rapid annealing are in the range of 10^4-10^7 K / s required for this method. Flash lamp annealing for this purpose uses a spectrum within the visible wavelength range, while laser annealing uses discrete wavelengths within the infrared (IR) to ultraviolet (UV) spectrum.

[0010] Silicon layer structure refers to the various thin layers of a thin layer structure or thin layer stack of Si electrodes. A thin layer stack comprises at least one layer or thin layer, which is made of at least one material or a mixture of two or more materials. Layer and thin layer are terms used interchangeably.

[0011] The method of the present invention allows any desired layer structure or thin-layer stack to be built on multiple substrates without vacuum interruption as is usual in sputtering processes. Rapid annealing allows the annealing step to be performed quickly and efficiently. Rapid annealing allows for a wide variety of operating settings, such as flash lamp energy or laser energy, pulse duration, and preheating or cooling of the substrate.

[0012] These basic operational steps result in a vast spectrum of parameters that can be custom tuned to the desired application. Rapid annealing in particular offers a crucial advantage due to the targeted energy input. Rapid annealing can be used to stabilize high temperature phases that cannot tolerate equilibrium conditions.

[0013] In one embodiment of the method of the present invention, phase separation results in the formation of a conductive matrix of metal or silicide in which nanoscale silicon is embedded.

[0014] Nanoscale silicon refers to amorphous silicon or nanocrystalline silicon with at least one dimension less than 100 nm in size. In the case of crystalline silicon, 100 nm is considered to be the limit of volume expansion due to lithium intercalation, and below this value, stress can be dissipated without destroying the morphology, i.e. without causing cracks, fractures, etc.

[0015] In another embodiment of the method of the present invention, the carrier substrate is formed primarily of copper.

[0016] The use of a copper substrate and the influence of selected operating parameters on the phase separation leads to the formation of a copper silicide matrix containing amorphous or nanocrystalline regions of pure silicon, resulting in a heterogeneous structure with conductive regions that have little or no capacity for lithium intercalation and further regions that contain substantially pure Si (ideally amorphous) with high storage capacity.

[0017] In the raw state, copper-rich silicide phases are more conductive than less copper-rich silicide phases. This is advantageous for use in batteries and their performance if Si-rich and Si-poor Si-Cu regions are formed in a non-specific mixture. Rapid annealing results in the formation of Cu 3 If a higher concentration of copper silicide is formed instead of Si, then correspondingly more areas of pure silicon are left. As a result, the silicide matrix achieves a high electrical conductivity, while the remaining (amorphous or nanocrystalline) silicon present is highly utilized. In extreme cases, it has been shown that phase separation can even result in the formation of a pure copper matrix in the silicon layer, which represents the ideal case for the corresponding nanostructures.

[0018] In one embodiment of the method of the present invention, phase separation results in the formation of a copper silicide matrix in a layer of the silicon layer structure.

[0019] By varying the pulse length, a targeted influence on the nanostructuring can be achieved. A dendrite-based conductive matrix is ​​developed, which also provides good electrical conductivity for a relatively thick layer of low-conductivity silicon. Dendrites are tree-like or bush-like crystalline structures. The structure is produced to a greater extent by incorporating aluminum into the thin silicon-metal layer.

[0020] In another embodiment of the method of the present invention, the copper silicide matrix is ​​formed from a stable intermetallic phase (copper silicide phase) Cu, which exists in thermodynamic equilibrium at room temperature. 3 Si, Cu 15 S 4 , and Cu 5 Not only by Si but also by Cu as a result of rapid annealing 7 Si and Cu 9 These copper-rich intermetallic phases are also formed in silicon-rich Si-Cu mixtures, which grow despite the high ratio of silicon to copper, i.e., in hypoeutectic concentrations.

[0021] In a further embodiment of the method of the invention, the layers of the silicon layer structure are admixed with one or more of the elements nickel (Ni), aluminium (Al), tin (Sn) or titanium (Ti).

[0022] In the nickel-silicon binary system, five intermetallic phases exist, whereas in systems such as Al-Si, only eutectic phases exist and no intermetallic phases are formed. Aluminum acts, for example, to promote the formation of dendrites in the Cu-Si system, enhancing the electrical conductivity of silicon. In contrast to copper, lithium-active phases exist in Sn and Ti. In silicon, these phases can cushion the volume expansion without creating hard interfaces during volume expansion.

[0023] In another further embodiment of the method of the invention the nanostructuring of the silicide matrix is ​​established by rapid annealing.

[0024] For example, by varying the pulse length of the flash in flash lamp annealing or laser annealing it becomes possible to tune the nanostructuring.

[0025] In one embodiment of the method of the present invention, the morphology and distribution of the growing phases within the copper silicide matrix is ​​established by the cooling rate.

[0026] Advantageously, slow cooling rates can establish phase separation with a large microstructure, while fast cooling rates can establish phase separation with a small grain microstructure.

[0027] In another embodiment of the method of the present invention, in the copper silicide matrix formed, Cu 5 Si, Cu 7 Si, Cu 9 Si or Cu where x and y are natural numbers x S y The proportion of copper-rich silicides, such as copper silicide, reaches more than 50% of the total silicide proportion. Advantageously, the amount of silicide in the thin layer is adjusted so that the total capacity of the copper silicide matrix / silicon layer is 2000 mAh / cm 2 in order to ensure sufficient battery capacity. 2 should be set not lower than

[0028] Characteristically, the copper silicide matrix is ​​grown in the multilayer structure produced according to the invention in the silicon thin layer. 3 Si, Cu 15 S 4 , and Cu 5 The extension of the formed phases, such as Si, is visible in a thin silicon layer of 1 μm thickness, reaching 200 nm. By further dividing the individual layers finer, this can be adapted as required so that the battery operation is sufficiently stabilized. The aim is a copper silicide matrix that stabilizes the silicon. The proportion of copper silicide (CuSi) must be so large that it does not exceed the stability limits of pure silicon.

[0029] Many results can be obtained by varying the parameters of the rapid annealing, such as pulse duration, pulse energy, and / or preheating / cooling. For stable cell operation over large volume expansions, amorphous silicon regions on the order of 100 nm are generally ideal. A good electrical contact of the relatively thick layer of low-conductivity silicon is also achieved by the conductive matrix of dendrites. These dendrites can be formed to a larger extent by incorporating aluminum into the Si metal layer.

[0030] The layers of the silicon layer structure are advantageously applied by dry deposition techniques, such as physical vapor deposition (PVD), typically sputtering, and / or chemical vapor deposition (CVD).

[0031] The described phase separation takes place, in some cases simultaneously and in some cases consecutively, with the formation of various intermetallic phases. These intermetallic phases have different densities and / or lattice constants. Thus, before the final state or stage is reached, the formation of interfaces with a relatively low density or occupying a larger spatial volume is possible. As a result, at the end of the process, a foam structure is obtained, in which void structures are distributed in a heterogeneous silicide matrix in which amorphous silicon is embedded. These void structures can further compensate for the volume expansion of silicon due to lithium intercalation. The method of the invention has been demonstrated to increase the layer thickness of the material system by a factor of five, while a factor of two or three is realistic with typical lattice expansion and oxide formation. The remaining thickness or volume increase is therefore due to the void structures formed.

[0032] The inventive method for stabilizing the copper-rich silicide phase as defined in the method claims is therefore advantageously used for producing high-capacity electrode materials in lithium-ion batteries, more particularly for silicon anodes.

[0033] It would further be advantageous to produce an anode material for electrochemical cells, and more particularly for lithium-ion batteries.

[0034] The anode material can be employed in a battery cell, and the battery cell can be installed in a battery having at least one battery cell.

[0035] The advantage of the method of the present invention is that the described properties are not obtained by complex manipulations, but instead they are naturally brought about by the targeted use of rapid annealing.This is achieved in a single manipulation step, and is highly scalable, therefore extremely cost-effective.Other methods are much more complex, require much more energy than rapid annealing, and cannot be applied in a scalable manner.

[0036] The object on which the invention is based is also achieved by an anode according to claim 16. The anode according to the invention is suitable for use in lithium-ion batteries and comprises a current collector, preferably made of copper, and a multilayer structure deposited on the current collector, produced by the method according to claims 1 to 11. The multilayer structure is formed of at least two layers, one layer being formed of a mixture of at least one metal and silicon, which form a copper silicide matrix containing (intermetallic) phases depending on the metal used.

[0037] In one embodiment of the anode of the present invention, the copper silicide matrix exhibits a lateral extension of 50% to 90%, normalized to the final thin layer thickness of the multilayer structure.

[0038] In another embodiment of the anode of the present invention, a microstructure is grown in the multilayer structure, the microstructure comprising Cu 3 Si, Cu 15 S 4 , and Cu 5 Si or high percentage of copper-rich silicides, e.g. Cu 5 Si, Cu 7 Si and Cu9 The expansion of the different intermetallic phases, including metal-rich phases such as Si, formed phases should reach at least 50% normalized to the final thin layer thickness in the microstructure, with pure silicon having a maximum thickness of 1 μm per layer. For example, for a 1.5 μm thick Cu-Si layer, the expansion of the copper silicide matrix should reach at least 0.5 μm. A maximum expansion of 300 nm for crystalline silicon and 1 μm for amorphous silicon are considered as the upper limit to achieve a stable uniform volume expansion by lithium intercalation without the silicon structure being shattered. This is called the stability criterion for pure silicon.

[0039] In a further embodiment of the anode of the present invention, the total volume percentage of nanoscale silicon intercalated in the copper silicide matrix, calculated relative to the total Si content of the multilayer structure, is between 40% and 95%, so that / in which case the stability criterion is not reached.

[0040] The invention is explained in more detail below by means of exemplary examples. [Brief description of the drawings]

[0041] [Figure 1] FIG. 1 is a copper-silicon phase diagram. [Diagram 2] Nickel-silicon phase diagram. [Diagram 3] Aluminum-(copper)-silicon phase diagram. [Figure 4] 1 is a titanium-(aluminum)-silicon phase diagram. [Diagram 5] FIG. 1 is a Cu-Si-Ti phase diagram. [Figure 6] Schematic of the layer structure and phase separation progression after flash lamp annealing. [Figure 7] FIG. 2 is a SEM micrograph of an entire Si / Cu / Si layer with grown copper silicide matrix (dendrite structure) produced by the method of the present invention. [Figure 8] 1 is a SEM image and elemental analysis of a Cu-Si-Ni system produced by the method of the present invention. [Figure 9] This is an SEM micrograph of the Cu-Si-Al system. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0042] FIG. 6 is a schematic diagram of the fabricated silicon layer structure 1, with thin silicon layers 2 alternating with thin copper layers 3 and / or thin layers of materials other than copper. Rapid annealing, more specifically flash lamp annealing or laser annealing 4, results in the formation of a heterogeneous mixed layer with phase separation 5. In the depicted example, the fabricated system contains large regions of amorphous silicon 7, which have a high storage capacity for lithium intercalation. Furthermore, rapid annealing leads to the growth of regions where copper intergrows with silicon and grows the desired dendrites, which are then melted down to a pure copper matrix, forming copper silicide (CuSi x ) matrix 6, which therefore shows high electrical conductivity. Figure 6 shows a mixture of Cu / Si from the layer structure. In principle it would also be possible to produce homogeneous SiCu layers by co-sputtering or sputtering directly from a SiCu target. Phase separation would then occur by rapid annealing. It would therefore also be possible to sputter a mixed layer rather than sputtering Si / Cu layers separately. The advantage is that it is not necessary to apply the layers alternately, but instead only one operating step is required.

[0043] Figure 7 shows an SEM micrograph of the heterogeneous mixed layer due to phase separation realized and produced by the method of the present invention. The whole Si / Cu / Si layer presents two layers of Si, each 1 μm thick, with 300 nm thick Cu between them. After rapid annealing, the Cu intergrows with the Si, resulting in the desired dendrite growth. Approximately 50% of each Si layer contains Cu or CuSi. xDendrites of copper are formed. The lighter areas are either copper-rich silicide or copper distributed inhomogeneously in the silicon (darker areas).

[0044] Using the Cu-Si system produced by the method of the present invention, hypereutectic Cu (3-α) It was demonstrated that a copper-rich phase can be formed even in Si(α>0). Based on this, the low-temperature stable Cu 5 Not only Si phase but also Cu 7 Si phase, and also Cu 9 Si phases were also measured. The incorporation of nickel into the system further promotes the formation of these copper-rich silicide phases. The formation of copper-rich silicide phases is induced by limited kinetics and diffusion during the rapid annealing process, where even a hypereutectic system can have localized hypoeutectic concentrations, resulting in the formation of copper-rich phases. The addition of materials that do not allow the growth of intermetallic phases with silicon, such as aluminum, further supports this process.

[0045] Figure 8 shows the Cu-Si-Ni system (SEM image and elemental analysis), where NiSi x Copper and / or copper-rich silicide grow as dendrites in silicon from the layer (starting layer is Si / Ni / CuSi x structure).

[0046] Figure 9 shows a CuSi 2 O 4 Cr alloy with both dendritic and particulate copper inclusions that are aggregated in silicon due to the incorporation of aluminum. x A complex layer-by-layer structure is shown.

[0047] In summary, the method of the present invention allows the formation of nanoscale Si-embedded metal and silicide conductive matrices. In particular, the method of the present invention allows the formation of copper-rich silicide matrices extending into a pure copper matrix, which leads to a significant improvement in the heterogeneity of the mixed layer and therefore in the battery performance. By targeted variation of the operating parameters such as the pulse duration, pulse energy and pre-heating or cooling of the rapid annealing, it is possible to establish the structure of the surrounding conductive matrix. Possible options to select the optimum structure for the application include particulate embeddings, pyramidal, coral or dendritic structures and even columnar pillar structures. The method of the present invention allows the creation of foam structures in laminar layers, which results in improved stress compensation for the intercalation of lithium into silicon and thus improved battery performance. [Explanation of symbols]

[0048] 1 Silicon layer structure 2 Silicon thin layer / layer 3 Copper thin layer / layer 2+3 Mixed Si / Cu layer with adjustable concentration 4 Rapid annealing, more specifically the flash lamp annealing step or the laser annealing step 5 Phase separation, copper or copper silicide dendrites 6 Copper silicide matrix / Cu matrix 7. Amorphous silicon or nano-scale nanocrystalline silicon 5+6+7 Nanoscale silicon embedded in a conductive silicide matrix

Claims

1. A method for stabilizing copper-rich silicide phases in microstructures, in which a silicon layer structure (1) is applied to a carrier substrate, wherein a layer of said silicon layer structure (1) consisting of a mixture of at least one metal (3) and silicon (2) is applied, followed by rapid annealing (4), in which the rapid annealing pulse duration is in the range of 0.01 to 100 ms and / or 0.1 to 100 J / cm ２ , pre-heating or cooling of the carrier substrate in the range of 4°C to 200°C, and selection of materials in the applied mixture of the layers of the silicon layer structure (1), thereby controlling phase separation (5) in the applied layer and growing the microstructure.

2. 2. The method of claim 1, wherein the phase separation (5) results in the formation of a conductive matrix of metal or silicide embedded with nanoscale silicon (7).

3. The method of claim 1 wherein said carrier substrate is formed primarily of copper.

4. 2. A method according to claim 1, characterized in that said phase separation (5) results in the formation of a copper silicide matrix (6) in said layer of said silicon layer structure (1).

5. The copper silicide matrix is an intermetallic phase, Cu, which exists at thermal equilibrium. ３ Si, Cu １５ Si ４ , and Cu ５ Not only is it made of Si, but also Cu ７ Si and Cu ９ 5. The method of claim 4, wherein the high temperature stabilized copper-rich intermetallic phase, such as Si, is also formed, said copper-rich intermetallic phase also growing in the silicon-rich Si-Cu mixture.

6. 2. The method of claim 1, characterized in that the layers of the silicon layer structure (1) are admixed with one or more of the elements nickel (Ni), aluminum (Al), tin (Sn) or titanium (Ti).

7. 2. The method of claim 1, wherein the nanostructuring of the silicide matrix is established by the rapid annealing.

8. 2. The method of claim 1, wherein the morphology and distribution of the phases growing within the copper silicide matrix is established by the cooling rate.

9. 9. The method of claim 8, wherein a slow cooling rate establishes a phase separation with a large microstructure and a fast cooling rate establishes a phase separation with a small particle microstructure.

10. Cu in the formed copper silicide matrix ５ Si, Cu ７ Si, Cu ９ Si or Cu where x and y are natural numbers ｘ Si ｙ 5. The method of claim 4, wherein the proportion of such copper-rich silicide reaches more than 50% of the total proportion of silicide.

11. 2. The method according to claim 1, characterized in that the layers of the silicon layer structure (1) are applied by dry deposition processes, such as physical vapor deposition, PVD, and / or chemical vapor deposition, CVD.

12. Use of the method for stabilizing a copper-rich silicide phase according to claims 1 to 11 for producing a high-capacity electrode material in lithium-ion batteries, more particularly for silicon anodes.

13. 1. An anode suitable for use in a lithium-ion battery, comprising a current collector, preferably made of copper, and a multilayer structure deposited on said current collector, wherein a layer of said multilayer structure comprises a mixture of at least one metal (3) and silicon (2), said layer of said multilayer structure being a rapidly annealed layer, said multilayer structure being formed from at least two layers, one layer being formed from a mixture of at least one metal (3) and silicon (2) forming a copper silicide matrix, said copper silicide matrix comprising intermetallic phases depending on the metals used.

14. 14. The anode according to claim 13, wherein the copper silicide matrix exhibits a lateral extension normalized to the final thin layer thickness of the multilayer structure of 50% to 90%.

15. A microstructure is grown in the multilayer structure, and the microstructure is a Cu ５ Si, Cu ７ Si and Cu ９ 14. The anode of claim 13, comprising different intermetallic metal-rich phases such as Si, wherein the expansion of the phases formed amounts to at least 50% normalized to the final thin layer thickness of the multilayer structure in the microstructure, and wherein pure silicon has a maximum thickness of 1 μm per layer.

16. 14. The anode of claim 13, wherein the total volume percentage of nanoscale silicon intercalated in the copper silicide matrix, calculated relative to the total Si content of the multilayer structure, is between 40% and 95%.

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