Method for producing porous silicon

Abstract

A method of making porous silicon includes providing magnesium silicide with silica nanoparticles and silica microparticles, or by providing magnesium with silica nanoparticles and silica microparticles. Any of the mixtures is then heated to a maximum of 500 DEG C.

Description

Technical Field

[0001] This invention relates to a method for manufacturing porous silicon. Background Technology

[0002] Silicon has great potential as a negative electrode material for lithium-ion batteries because of its high specific capacity and, for example, its ability to store nearly 10 times more charge than graphite, which is currently the conventional choice for battery negative electrodes.

[0003] One problem with silicon is that it exhibits large volume expansion, which can cause problems within the limited space of a battery, potentially leading to cell bulging and damage. This can be mitigated by using porous silicon, which provides the void volume into which silicon can expand.

[0004] Currently, the mass production of porous silicon is achieved through magnesothermic reduction (reduction in the presence of magnesium). However, this method requires temperatures exceeding 650°C to obtain meaningful yields.

[0005] Furthermore, magnesium thermal reduction at such temperatures results in a loss of the required porosity due to silicon sintering during the process. Summary of the Invention

[0006] According to a first aspect of this disclosure, a method for manufacturing porous silicon is provided, comprising: providing magnesium silicide with silica nanoparticles and silica micron-sized particles; and then heating the mixture at a maximum temperature of 500°C. Heating the mixture to a maximum temperature of 500°C is a less energy-intensive method than known methods, thereby reducing the cost and severity of the manufacturing process.

[0007] Optionally, the method further includes forming magnesium silicide by providing magnesium with silica particles and heating the mixture at a maximum temperature of 500°C. In this case, the manufacturing process can be considered a "one-pot" process, eliminating the need for multiple additions of reactants, thus shortening and simplifying the manufacturing process.

[0008] Alternatively, according to a second aspect of this disclosure, a method for manufacturing porous silicon includes providing magnesium with silica nanoparticles and silica microparticles; and heating the mixture at a maximum temperature of 500°C.

[0009] Suitablely, the size of the silica nanoparticles is in the range of 1 nm to 75 nm, and preferably less than 20 nm. The silica nanoparticles act as promoters for reactions involving larger particles, for example, but not limited to, acting as initiators of the reaction.

[0010] Preferably, the size of the silica micron particles is in the range of 1 micrometer to 1000 micrometers. The inclusion of larger particles ensures that the mixture exhibits improved handling properties because the larger particles are less prone to dispersion or scattering and hold the smaller silica nanoparticles in place before the reaction. The larger particles also ensure the presence of void spaces, thus guaranteeing a porous final product.

[0011] Suitablely, the ratio of silica nanoparticles to silica microparticles is in the range of 10:90 to 80:20, preferably 20:80 to 50:50, and even more preferably, wherein the ratio is 25:75. These ranges provide a mixture that has the workability of larger particles while retaining enough nanoparticles to allow the reaction to proceed successfully at temperatures below 500°C.

[0012] Optionally, magnesium silicide can be formed by providing a mixture of magnesium and silica particles and heating it at a maximum temperature of 500°C, either prior to or simultaneously with the process for manufacturing porous silicon. This process can be carried out as a continuous production process, in which magnesium silicide is formed prior to the manufacture of porous silicon, or it can be carried out in discrete batches, in which magnesium silicide is formed simultaneously with the manufacture of porous silicon.

[0013] Alternatively, magnesium silicide can be formed by providing a mixture of magnesium and silica particles and heating it at a maximum temperature of 500°C, which can be done independently of methods for manufacturing porous silicon. Therefore, magnesium silicide can be purchased or manufactured independently of methods for manufacturing porous silicon. The manufacture of magnesium silicide can be optimized independently of or isolated from the manufacture of porous silicon.

[0014] Suitable, the mixture is heated to a maximum of 450°C or preferably a maximum of 380°C. Further reducing the reaction temperature from 500°C enhances the energy-saving benefits, making the method more cost-effective.

[0015] Optionally, the mixture is heated to the target temperature at a rate of 1 °C / min.

[0016] Preferably, the mixture is kept at the target temperature for up to 6 hours.

[0017] Suitablely, the method further includes forming the battery negative electrode from porous silicon. Thus, the large volume expansion of silicon in the battery (which could lead to cell bulging and damage) is mitigated by using porous silicon, which provides the void volume into which silicon can expand.

[0018] Preferably, the silica nanoparticles and silica microparticles are derived from any single source or any combination thereof: precipitated silica, silica gel, microparticle silica, custom-produced silica (including organic silica and / or bio-inspired silica), sand, rice husks, crushed glass, silicates, or mixtures thereof. This method has no particular preference for silica raw materials and does not require specific purity or grade of raw materials to obtain acceptable results.

[0019] Suitable for porous silicon, the specific surface area is between 100 and 235 m². 2 Within the range of / g.

[0020] Preferably, the total pore volume of the porous silicon is between 0.2 and 0.6 cm³. 3 Within the range of / g.

[0021] Suitablely, the weight ratio of magnesium silicide to the total weight of silica nanoparticles and silica microparticles is 1:1. Attached Figure Description

[0022] To facilitate the identification of any particular element or action in the discussion, the most significant digit in the reference number refers to the figure number in which that element was first introduced.

[0023] Figure 1 This is a scanning electron microscope image of silicon produced using the method disclosed herein.

[0024] Figure 2 These are X-ray diffraction patterns of multiple samples processed using the methods disclosed herein.

[0025] Figure 3 The figure shows the measured yield of porous silicon versus the maximum expected yield and the maximum actual yield of porous silicon for the method disclosed in this paper.

[0026] Figure 4 A through 4E are transmission electron micrographs of silicon dioxide heated with magnesium according to the method disclosed herein. Detailed Implementation

[0027] The precursor used for magnesothermic reduction (hereinafter referred to as MgTR) is prepared using magnesium powder (e.g., but not limited to 325 mesh, 99.8%) and silica in a 1:1 weight ratio, which gives a stoichiometric molar ratio of 2.5:1. In mixed-size silica precursors, the total mass of silica used in each reaction remains constant.

[0028] Any suitable source of silica can be used; examples of commercial silica include Syloid. TMAL-1 FP (as described in or based on US Patent 10,660,856B2) and fumed silica. Therefore, the method does not limit the source of the silica. Any single one or any combination of the following may be suitable: precipitated silica, silica gel, particulate silica, custom-produced silica (including organic silica and / or bio-inspired silica), sand, rice husks, crushed glass, silicates, or mixtures thereof.

[0029] Once formed, magnesium silicide and silica particles are mixed in a 1:1 weight ratio. Reduction is performed at a heating rate of 1 °C / min to obtain porous silica, although it will be apparent that other heating rates may also be suitable. The sample is heated at the maximum temperature for 6 hours and then allowed to cool naturally to room temperature. Note that artificial cooling and controlled cooling rates can be implemented, especially when this method is carried out in large-scale batch processes. Similarly, the duration of application of the maximum temperature can be varied depending on the scale of the production method to ensure the desired yield is achieved.

[0030] The reduced sample is washed with an acid (such as HCl, e.g., 1M HCl) and stirred for a period of time (approximately 24 hours, although this may vary depending on the scale of the production method used and whether it is a batch or continuous process). The solution is then filtered (e.g., using a mesh with a pore size of 450 nm) and further washed with deionized water to remove NaCl salts. The silica / silica powder is then dried by any suitable method.

[0031] Compared to larger particles, nanoparticles can exhibit enhanced reactivity due to their large surface area to particle volume ratio.

[0032] The MgTR reaction is highly exothermic, generating enough heat to sustain itself. Once reduction begins, the large amount of heat released is absorbed by the surrounding particles, which in turn causes them to undergo reduction. By mixing large particles (>>20 nm) with small particles (≤20 nm) that can be reduced at 400 °C, the reduction of large particles can be triggered by small particles at low temperatures (i.e., below 600 °C). This was demonstrated in this paper using 20 nm (hereinafter “S20”) and 500 nm (hereinafter “S500”) silica particles mixed in different proportions and reduced at 450 °C in the presence of Mg. Note that S500 does not undergo any reduction at temperatures below 500 °C.

[0033] Figure 1 This is a scanning electron microscope (SEM) image of the reduction product obtained by mixing 50 mol% S20 and S500 and reducing them at 450 °C. The scale bar represents 1 µm. Figure 1SEM images showed that the S500 particles retained their spherical shape after MgTR at 450°C, and their surface had nodules.

[0034] Figure 2 Diffraction patterns of the mixed silica are shown. Except for the 100 mol% S500 sample, all samples exhibited peaks corresponding to crystalline silicon after reduction at 450 °C. Samples containing the mixed size are given as mol% of S20, with the remainder rounded up to 100 mol% by S500. The products of the reduction of the mixed silica all contained crystalline silicon, which was confirmed by X-ray diffraction.

[0035] Figure 3 The measured yields, maximum expected yields, and maximum actual yields for mixtures of S20 and S500 at different mol% are shown. Samples including the mixture size are given as mol% of S20, with the remainder rounded to 100 mol% by S500.

[0036] The maximum expected yield is a theoretical estimate that assumes only that all small particles react at 450°C and achieve 100% maximum yield. In mixed samples, the maximum yield equals the percentage of small silica particles in the original sample. For example, a 75:25 wt% mixture of large and small particles should only give a yield of 25 mol%.

[0037] The observed yield increase exceeding the maximum expected yield suggests that larger particles were also reduced due to the triggering effect of small particles.

[0038] The maximum actual yield is based on actual experimental data of S20 reduction alone (i.e., the actual observed yield), with a measured yield of 39 mol% of S20 at 450 °C, which differs from the 100% assumed in the maximum expected yield. Therefore, for a mixture containing only 25 wt% S20, the actual yield of the 25 wt% sample should be 39% (i.e., 9.75 mol%).

[0039] When S20 in the mixture is <75%, the measured yield of the "mixed" sample is ( Figure 3 The line marked "Yield" exceeds both the maximum expected yield and the maximum actual yield. For example, for a sample with an S20 content of 25 wt%, the maximum actual yield was 9.75 mol%, and the maximum expected yield was 25%. However, the measured yield was 62 mol%, indicating that the synergistic effect of mixing S20 and S500 particles significantly improved the yield. Therefore, it is clear that the S500 particles were reduced.

[0040] Figure 4Transmission electron microscopy (TEM) images of Stöber silica heated with Mg to 450 °C are shown. In A) and B), S500 was the sole source of silica. In C) and D), a 50–50 wt% mixture of S500 and S20 was used. High-magnification magnification of the marked areas in the image of C) is shown in E), where the crystalline phases are marked with arrows. Scale bars are 200 nm (A, C), 100 nm (B, D), and 20 nm (E).

[0041] S500 silica remained unreacted at 450°C. Figure 4 A and B). The particles in these images are complete and non-porous spheres, which confirms the XRD and yield results ( Figure 3 As shown), no reaction occurred. However, when the sample was mixed with S20 before MgTR, a significant change in the microstructure of the S500 particles was observed (see Figure 1). Figure 4 (C and D). The mild nature of this low-temperature reduction method preserves the spherical structure while significantly forming smaller crystals, which is consistent with... Figure 4 The formations shown in A and B present a stark contrast. In the originally amorphous precursor silica, a crystalline phase can be observed throughout the entire particle (…). Figure 4 E (indicated by the arrow), which confirms that the S500 particles have been reduced to crystalline silicon.

[0042] The coexistence of small and large particles provides thermal regulation and also triggers the reduction of larger particles. For particles smaller than or equal to 20 nm, the MgTR reaction can proceed at 450 °C, and the exothermic heat released from the reduction of small particles may provide the activation energy required to trigger the reduction of large particles. Therefore, there is a synergistic effect between large and small particles.

[0043] S20 particles (individually) can be reduced at 450°C; however, their electrochemical performance is poor (<250 mAh / g per cycle, Table 1). Therefore, it is clear that although 100% S20 can be reduced at ultra-low temperatures, the resulting silicon is not suitable for use in lithium-ion battery cells.

[0044] 100% S500 does not reduce at 450°C. Reduction can only occur at 650°C (with a yield of 55%). The reduced S500 product exhibits a capacity of 523 mAh / g after 100 cycles (Table 1). This performance only slightly exceeds the expected maximum capacity of graphite. Furthermore, the high temperature (650°C) required for its reduction means that S500, used alone, is not an attractive feedstock for producing Si for anodes. However, when using a mixture of S20 and S500, reducing the mixture at 450°C achieves a yield of 64%. This higher yield translates into a high initial capacity of 1020 mAh / g, which is maintained at 853 mAh / g after 100 cycles (Table 1). This performance of the mixed system surpasses that of each individual component of the mixture, highlighting the synergistic and beneficial effects of this approach using mixed particle sizes. First, it provides a sustainable method by reducing the energy costs required for the reduction reaction. Second, it achieves a high yield despite reducing the feedstock at a lower temperature. Finally, the hybrid system exhibited improved electrochemical performance.

[0045]

[0046] Table 1: Properties and electrochemical performance of samples after MgTR using mixed (S20:S500 ratio of 25:75) and unmixed silica. “SY” is silica with a particle size of 6 μm. “F7” is fumed silica with a particle size of 7 nm. The ratio of F7:SY is 25:75.

[0047] By utilizing the triggering effect of nanoscale silica particles, larger particles can be reduced at temperatures below 500°C, even as low as 380°C, with yields comparable to those obtained by known methods at 650°C or higher. Furthermore, silicon produced by this method exhibits superior electrochemical performance compared to silicon manufactured at higher temperatures. The highest yield can be achieved using a mixture containing 25 wt% small particles.

[0048] A synergistic effect was observed between small and large particles through triggering, which minimized the impact of low-performance small particles while reducing energy requirements. It is estimated that reducing the MgTR temperature from 650°C to 380°C reduced the total energy cost by 45%. The method disclosed herein is further applicable to commercially available silica and silicates manufactured or derived from various synthetic routes. Therefore, readily available and inexpensive raw materials can be converted into high-performance porous silicon anodes at low temperatures.

[0049] In the description and claims of this specification, the words “comprising” and “including” and variations thereof mean “including, but not limited to”, and they are not intended to (and do not) exclude other parts, additives, components, integers, or steps. In the description and claims of this specification, the singular encompasses the plural unless the context otherwise requires. Specifically, when using indefinite articles, this specification should be understood to consider both the plural and the singular unless the context otherwise requires.

[0050] Features, integers, properties, compounds, chemical parts, or groups described in connection with a particular aspect, embodiment, or example of the invention should be understood to be applicable to any other aspect, embodiment, or example described herein, unless incompatible therewith. All features disclosed in this specification (including any appended claims, abstract, and drawings), and / or all steps of any method or process so disclosed, may be combined in any combination, except where at least some of such features and / or steps are mutually exclusive. The invention is not limited to the details of any of the foregoing embodiments. The invention extends to any novel feature or any novel combination of features disclosed in this specification (including any appended claims, abstract, and drawings), or to any novel step or any novel combination of steps of any method or process so disclosed.

[0051] Readers are drawn to all papers and documents submitted concurrently with or prior to this specification that are related to this application and are publicly available together with this specification, the contents of which are incorporated herein by reference.

Claims

1. A method for manufacturing porous silicon, comprising: Magnesium silicide and silica nanoparticles and silica micron particles are available; as well as The mixture is heated at a maximum temperature of 500°C.

2. The method of claim 1, further comprising forming magnesium silicide by: Provides magnesium and silicon dioxide particles; and The mixture is heated at a maximum temperature of 500°C.

3. A method for manufacturing porous silicon, comprising: Provides magnesium and silica nanoparticles and silica micron-sized particles; as well as The mixture is heated at a maximum temperature of 500°C.

4. The method according to any one of the preceding claims, wherein, The size of the silica nanoparticles is in the range of 1 nm to 75 nm, and preferably less than 20 nm.

5. The method according to any one of the preceding claims, wherein, The size of the silica micron particles ranges from 1 micrometer to 1000 micrometers.

6. The method according to any one of the preceding claims, wherein, The ratio of silica nanoparticles to silica microparticles is in the range of 10:90 to 80:20, preferably 20:80 to 50:50, and even more preferably, the ratio is 25:

75.

7. The method according to claim 2 and any one of claims 4 to 6 when referring to claim 2, wherein, The method of claim 2 is performed before or simultaneously with the method of claim 1.

8. The method according to claim 1 or any one of claims 4 to 6 when claim 2 is not cited, wherein, The method of claim 2 is performed in a manner different from the method of claim 1.

9. The method according to any one of the preceding claims, wherein, The mixture is heated to a maximum of 450°C or preferably a maximum of 380°C.

10. The method according to claim 1 or claim 9, wherein, The mixture is heated to the target temperature at a rate of 1 °C / min, and optionally, the mixture is held at the target temperature for up to 6 hours.

11. The method according to any one of the preceding claims, wherein, The method further includes forming a battery negative electrode from the porous silicon.

12. The method according to any one of the preceding claims, wherein, Silica nanoparticles and silica microparticles are derived from any one or any combination of the following: precipitated silica, silica gel, microparticle silica, custom-produced silica including organic silica and / or bio-inspired silica, sand, rice husks, crushed glass, silicates or mixtures thereof.

13. The method according to any one of the preceding claims, wherein, The specific surface area of ​​the porous silicon is between 100 and 235 m². 2 Within the range of / g.

14. The method according to any one of the preceding claims, wherein, The total pore volume of the porous silicon is between 0.2 and 0.6 cm³. 3 Within the range of / g.

15. The method according to any one of the preceding claims, wherein, The weight ratio of magnesium silicide to the total weight of silica nanoparticles and silica microparticles is 1:1.

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