A method of manufacturing porous silicon

Abstract

A method of manufacturing porous silicon comprising providing magnesium silicide with silica nanoparticles and silica microparticles, and heating the mixture at up to 500°C. The magnesium silicide may

Description

TECHNICAL FIELD

[0001] The invention relates to a method of manufacturing porous silicon. BACKGROUND

[0002] Silicon has enormous potential for use as a lithium-ion battery anode material due to having a high specific capacity and, being able to store, for example, almost lOx more charge than graphite, which is currently a common choice for battery anodes.

[0003] One problem with silicon is that it exhibits a large volumetric expansion, which can be problematic in the confined space of a battery, potentially leading to cell swelling and damage. This can be mitigated by using a porous silicon, which provides a void volume into which the silicon can expand.

[0004] Producing porous silicon in bulk is currently achieved by magnesio-thermic reduction (reduction in the presence of magnesium). This method, however, requires high temperatures in excess of 650°C in order to attain meaningful yields.

[0005] Furthermore, performing magnesio-thermic reduction at such temperatures causes loss of the required porosity due to sintering of the silicon in the process. BRIEF SUMMARY

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

[0007] Optionally, the method further comprises forming the magnesium silicide by: providing magnesium with silica particles; and heating the mixture at up to 500 °C. In this instance, the manufacturing process can be considered a “one-pot” method, negating the need for multiple additions of reactants, thereby shortening and simplifying manufacture.

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

[0009] Suitably, the silica nanoparticles have a dimension in the range 1 nm - 75 nm, and preferably less than 20 nm. The silica nanoparticles act as a facilitator for the reaction of the larger particles, such as, but limited to, by acting as a trigger for the reaction.

[0010] Preferably, the silica microparticles have a dimension in the range of 1 micron - 1000 microns. The inclusion of larger particles ensures the mixture exhibits improved handling properties, as the larger particles are less prone to dispersion or scattering and hold the smaller silica nanoparticles in place prior to reaction. Larger particles also ensure the presence of void spaces ensuring a porous end product.

[0011] Suitably, the ratio of silica nanoparticles to silica microparticles is in the range 10:90 to 80:20, preferably 20:80 to 50:50, and still more preferably wherein the ratio is 25:75. These ranges provide a mixture that exhibits the ease of handling of the larger particles, while maintaining sufficient nanoparticles for the reaction to be successful at less than 500°C.

[0012] Optionally, forming the magnesium silicide by providing magnesium with silica particles, and heating the mixture at up to 500 °C, may be preformed immediately prior to, or simultaneously with, the method of manufacturing porous silicon. The method may be performed as a continuous production, where the magnesium silicide is formed prior to manufacturing the porous silicon, or the method may be performed in discrete batches, where the magnesium silicide is formed simultaneously with the manufacturing of the porous silicon.

[0013] Alternatively, forming the magnesium silicide by providing magnesium with silica particles, and heating the mixture at up to 500 °C, may be preformed remote from the method of manufacturing porous silicon. Magnesium silicide may therefore be purchased, or manufactured remotely from the method of manufacturing the porous silicon. The manufacture of magnesium silicide may be independently optimized or isolated from the manufacturing of the porous silicon.

[0014] Suitably, the mixture is heated to up to 450°C or preferably up to 380°C. Further reducing the reaction temperature from 500°C enhances the benefits of energy reduction, thereby making the method more cost efficient.

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

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

[0017] Suitably, the method further comprises forming a battery anode from the porous silicon. The large volumetric expansion of silicon in batteries, potentially leading to cell swelling and damage is therefore mitigated by using the porous silicon, which provides a void volume into which the silicon can expand.

[0018] Preferably, the silica nanoparticles and silica microparticles are sourced from any of the following, alone or in combination: precipitated silica, silica gel, microsilica, custom produced silicas including organosilicas and / or bioinspired silicas, sand, rice husks, crushed glass, silicates or mixtures thereof. The present method is ambivalent to silica feedstock, no particular purity or special grade of feedstock is required to obtain acceptable results.

[0019] Suitably, the porous silicon has a specific surface area in the range 100 to 235 m2 / g.

[0020] Preferably, the porous silicon has a total pore volume in the range 0.2 to 0.6 cm3 / g.

[0021] Suitably, the magnesium silicide is in a 1:1 weight ratio with the combined weight of the silica nano particles and the silica micro particles. BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

[0022] To easily identify the discussion of any particular element or act, the most significant digit or digits in a reference number refer to the figure number in which that element is first introduced.

[0023] FIG. 1 is a scanning electron micrograph of silicon produced using the methods disclosed herein.

[0024] FIG. 2 is an X-ray diffractogram of multiple samples processed using the methods disclosed herein.

[0025] FIG. 3 is a graph plotting the measured yield of porous silicon vs the maximum expected and maximum practical yields of porous silicon for the methods disclosed herein.

[0026] FIG. 4A to 4E are transmission electron micrographs of silica heated with magnesium according to the methods disclosed herein. DETAILED DESCRIPTION

[0027] Precursors for magneisothermic reduction (henceforth MgTR) are prepared using magnesium powder (such as, but not limited to 325 mesh, 99.8%) and silica in a 1:1 weight ratio, giving the stoichiometric mole ratio of 2.5:1. In mixed size silica precursors, the total mass of silica used in each reaction remained the same.

[0028] Any suitable source of silica may used, commercial examples of silicas include Syloid™ AL-1 FP (as described or based on US patent US 10,660,856B2), and fumed silica. The method is therefore agnostic to the source of silica. Any of the following alone or in combination may be suitable: precipitated silica, silica gel, microsilica, custom produced silicas including organosilicas and / or bioinspired silicas, sand, rice husks, crushed glass, silicates or mixtures thereof

[0029] Once formed the magnesium silicide is mixed in a 1:1 weight ratio with the silica particles. A temperature ramp rate of l°C / min is applied to perform the reduction to porous silicon, although it will be apparent that other temperature ramp rates may be suitable. Samples were heated at maximum temperatures for a duration of 6 hours then cooled naturally to room temperature, noting that artificial cooling and controlled cooling rates may be implemented, particularly where the method is conducted in large scale batch processes. Likewise, the duration of application of the maximum temperature may be varied according to the scale of the production method to ensure the required yield is achieved.

[0030] Reduced samples washed in acid (such as HC1, at, for example IM) and agitated for a period of time (around 24 hours, although this may vary depending on the scale of the production method used, and whether the method is a batch process or continuous). The solution is then filtered (using e.g. a mesh of pore size 450 nm), and then further washed with de-ionised water to remove NaCl salts. The silicon / silica powder is then dried by any suitable method.

[0031] Nanoparticles can exhibit enhanced reactivity compared to larger particles due to their large surface area compared to particle volume.

[0032] The MgTR reaction is highly exothermic and produces sufficient heat to become self-sustaining. Once the reduction has begun, the large release of heat is absorbed by the surrounding particles which, in turn, undergo reduction themselves. By mixing small particles (<20 nm) that can be reduced at 400°C and large particles (»20 nm), the reduction of the large particles can be triggered at low temperatures (i.e. sub-6000C) by small particles. This is demonstrated herein using silica particles of 20 nm (henceforth denoted “S20”) and 500 nm (henceforth denoted “S500”), both were mixed together at different ratios, and reduced in the presence of Mg at 450°C. Note that S500 does not undergo any reduction at temperatures less than 500°C.

[0033] FIG. 1 is a scanning electron micrograph (SEM) of the product of the reduction of 50 mol% of S20 mixed with S500 and reduced at 450°C. Scale bar represents 1 pm. The SEM in FIG. 1 shows that the S500 particles maintain their spherical shape upon MgTR at 450°C, with nodules on their surfaces.

[0034] FIG. 2 shows diffractograms of mixed silica having peaks corresponding to crystalline silicon after reduction at 450°C, in all samples except for 100 mol% S500. Samples containing mixed sizes are given in mol% of S20, with the remainder made up to 100 mol% with S500. The products of the mixed silica reductions all contained crystalline silicon, confirmed by X-ray diffraction.

[0035] FIG. 3 shows the measured yield alongside the maximum expected and maximum practical yield for different mol% mixtures of S20 and S500. Samples containing mixed sizes are given in mol% of S20, with the remainder made up to 100 mol% with S500.

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

[0037] The increased yield observed beyond the maximum expected yield demonstrates that larger particles were also reduced due to the triggering effect from the smaller particles.

[0038] The maximum practical yield is based on real-world experimental data for the reduction of S20 alone (i.e. real-world observed yield), which is a measured yield of 39 mol% at 450°C as opposed to 100% assumed in the maximum expected yield. Hence, for a mixture containing only 25 wt% S20, a yield of 39% of 25 wt% sample should be practically achieved (i.e. 9.75 mol%).

[0039] The measured yield from the “mixed” sample (line denoted as “Yield” in FIG. 3) exceeded both the maximum expected and maximum practical yield when S20 was <75% in the mixture. For example, a sample with 25 wt% of S20, the maximum practical yield is 9.75 mol% and maximum expected yield is 25%. The measured yield, however, was found to be 62 mol%, showing that the synergistic effect of mixing the S20 and S500 particles increased the yield significantly. It is thus clear that S500 particles have been reduced.

[0040] FIG. 4 shows transmission electron micrograph ("TEM") images of Stober silica heated with Mg to 450°C. In A) and B) S500 was the only silica source. In C) and D), a 50-50 wt% mixture of S500 and S20 was used. A higher magnification of the marked area in image in C) is shown in E) with crystalline phases marked by arrows. Scale bars are 200 nm (A, C), 100 nm (B, D) and 20 nm (E).

[0041] S500 silica remained unreacted at450°C (FIG. 4 A and B). The particles in these images were intact and non-porous spheres, which confirms the XRD and yield results (shown in FIG. 3) that no reaction has occurred. However, a dramatic change in the S500 particle microstructure was observed when this sample was mixed with S20 before MgTR (see FIG. 4 C and D). The mild nature of this low-temperature reduction method retains the spherical structure, while the formation of smaller crystals is evident, which is in stark contrast to those shown in FIG. 4 A and B. The appearance of crystalline phases, in an otherwise amorphous precursor silica, can be seen throughout the particles (FIG. 4 E, arrows), which confirms that S500 particles have been reduced to crystalline silicon.

[0042] The presence of both small and large particles provides thermal regulation, while also triggering the reduction of larger particles. The MgTR reaction is able to proceed at 450°C for particles less than or equal to 20 nm, the exothermic heat released from the reduction of small particles is likely to provide the activation energy needed to trigger the reduction of large particles. There is therefore a synergy between the larger and smaller particles.

[0043] S20 particles (alone) are able to undergo reduction at 450°C, however, their electrochemical performance is poor (<250 mAh / g in each cycle, Table 1). Thus it is clear that although 100% S20 can be reduced at ultra-low temperatures, the product silicon is not suitable for use in lithium-ion battery cells.

[0044] 100% S500 will not reduce at 450°C. The reduction is only possible at 650°C (with a yield of 55%). The reduced S500 product showed a capacity of 523 mAh / g after 100 cycles (Table 1). This performance only slightly exceeds the expected maximum capacity of graphite. Further, the need for a high temperature its reduction (650°C) means that S500, when used alone is not an attractive feedstock for producing Si for anodes. However, when a mixture of S20 and S500 is used, a yield of 64% is achievable when the mixture is reduced at 450°C. This higher yield translates to a high initial capacity of 1020 mAh / g, maintaining 853 mAh / g after 100 cycles (Table 1). This performance of the mixed system surpasses that of each part of the mixture, which highlights synergistic and beneficial effects of this method of using mixed particle sizes. Firstly, it provides a sustainable process by lowering the energy cost required for the reduction reaction. Secondly, it enables higher yields despite reducing the materials at lower temperatures. Finally, the mixed systems exhibit improved electrochemical performance. Capacity (mAh / g) Specific surface area (m2 / g) Total pore volume (cm 3 / g) Sample Yield (mol%) Initial After 100 cycles S20 450°C 35 ± 1.5 217 204 143 0.24 S500 650°C 55 ± 1.6 955 523 179 0.29 S20:S500 450°C 64 ± 1.4 1020 853 219 0.55 F7:SY 380°C 81 ± 1.2 1822 861 112 0.24

[0045] TABLE 1: The properties and electrochemical performance of samples post-MgTR using mixed (S20:S500 ratio of 25:75) and unmixed silicas. “SY” is silica gel with 6pm particle size. “F7” is fumed silica with 7nm particle size. The ratio of F7:SY is 25:75.

[0046] By taking advantage of the triggering effect of nano-sized silica particles, larger particles can be reduced at temperatures below 500°C. and as low 380°C. giving yields comparable to known methods conducted at 650°C or above. Furthermore, The silicon produced through this method exhibits superior electrochemical performance when compared with silicon made at higher temperatures. The highest yields can be achieved with a mixture containing 25 wt% small particles.

[0047] Through triggering, a synergistic effect is observed between small and large particles, which minimizes the impact of low performing small particles while also reducing the energy requirements. It is estimated that by lowering the MgTR temperature from 650°C to 380°C leads to a reduction by 45% in the total energy cost. The methods disclosed herein are further transferrable to commercial silicas and silicates made or sourced via different synthesis routes. As such, cheap feedstock which is available in large volumes can be converted into high performance porous silicon anodes at low temperatures.

[0048] Throughout the description and claims of this specification, the words “comprise” and “contain” and variations of them mean “including but not limited to”, and they are not intended to (and do not) exclude other moieties, additives, components, integers or steps. Throughout the description and claims of this specification, the singular encompasses the plural unless the context otherwise requires. In particular, where the indefinite article is used, the specification is to be understood as contemplating plurality as well as singularity, unless the context requires otherwise.

[0049] Features, integers, characteristics, compounds, chemical moieties or groups described in conjunction with a particular aspect, embodiment or example of the invention are to be understood to be applicable to any other aspect, embodiment or example described herein unless incompatible therewith. All of the features disclosed in this specification (including any accompanying claims, abstract and drawings), and / or all of the steps of any method or process so disclosed, may be combined in any combination, except combinations where at least some of such features and / or steps are mutually exclusive. The invention is not restricted to the details of any foregoing embodiments. The invention extends to any novel one, or any novel combination, of the features disclosed in this specification (including any accompanying claims, abstract and drawings), or to any novel one, or any novel combination, of the steps of any method or process so disclosed.

[0050] The reader's attention is directed to all papers and documents which are filed concurrently with or previous to this specification in connection with this application and which are open to public inspection with this specification, and the contents of all such papers and documents are incorporated herein by reference.

Claims

1. A method of manufacturing porous silicon comprising: providing magnesium silicide with silica nanoparticles and silica microparticles; and heating the mixture at up to 500°C.

2. The method of claim 1, further comprising forming the magnesium silicide by: providing magnesium with silica particles; and heating the mixture at up to 500 °C.

3. A method of manufacturing porous silicon comprising: providing magnesium with silica nanoparticles and silica microparticles; and heating the mixture at up to 500°C.

4. The method of any preceding claim, wherein the silica nanoparticles have a dimension in the range 1 nm - 75 nm, and preferably less than 20 nm.

5. The method of any previous claim, wherein the silica microparticles have a dimension in the range of 1 micron - 1000 microns.

6. The method of any previous claim, wherein the ratio of silica nanoparticles to silica microparticles is in the range 10:90 to 80:20, preferably 20:80 to 50:50, and still more preferably wherein the ratio is 25:75.

7. The method as claimed in any one of claim 2 and claims 4 to claim 6, when dependent upon claim 2, wherein the method of claim 2 is preformed immediately prior to, or simultaneously with, the method of claim 1.

8. The method as claimed in any one of claim 1 or 4 to claim 6 when not dependent upon claim 2, wherein the method of claim 2 is preformed remote from the method of claim 1.

9. The method of any previous claim, wherein the mixture is heated to up to 450°C or preferably up to 380°C.

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

11. The method of any previous claim, wherein the method further comprises forming a battery anode from the porous silicon.

12. The method of any previous claim, wherein the silica nanoparticles and silica microparticles are sourced from any of the following, alone or in combination: precipitated silica, silica gel, microsilica, custom produced silicas including organosilicas and / or bioinspired silicas, sand, rice husks, crushed glass, silicates or mixtures thereof.

13. The method of any previous claim, wherein the porous silicon has a specific surface area in the range 100 to 235 m2 / g.

14. The method of any previous claim, wherein the porous silicon has a total pore volume in the range 0.2 to 0.6 cm3 / g.

15. The method of any previous claim, wherein the magnesium silicide is in a 1:1 weight ratio with the combined weight of the silica nano particles and the silica micro particles.

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