DESC:FIELD OF THE INVENTION
The present invention relates to a dual step chemical leaching process for the synthesis of porous silicon and co-production of high purity hydrogen gas. More particularly, the present invention discloses a scalable process for synthesizing porous silicon anode material using aluminium – silicon alloy as precursor by dual step leaching technique which includes alkali leaching followed by acidic leaching with co – production of high purity hydrogen gas.

BACKGROUND OF THE INVENTION
We all are predominantly dependent on technologies such as electronic gadgets, automobiles, etc. These dependencies have resulted in an increased search for new materials, which foster our ever-changing demands. Silicon-based devices have ruled the world for the past half-century and will continue to do so in the near future. Since the inception of the semiconductor and solar industry, silicon has played a significant role in developing technologies. As the solar panels, as well as semiconductors devices, are made up of pure silicon solid wafers.

In the last two decades, apart from bulk silicon, porous silicon has also been used in various applications due to its high surface-to-volume ratio. Traditionally, porous silicon was used in light-emitting devices. However, recently porous silicon has gained importance in the fabrication of batteries, fuel cells, cosmetics, pharmaceuticals, drug delivery, health care diagnostic devices, sensing devices, gas filtration units, etc. Owing to several applications, different fabrication routes for porous silicon have been devised, such as electrochemical etching, plasma deposition, glancing angle deposition, vapor etching, wet chemical dealloying, etc. Among these techniques, wet chemical dealloying (or leaching) is a low energy intensive method involving low-cost precursor materials.

The raw material sources of silicon available in developing countries like India are metallurgical grade silicon, aluminum-silicon (Al-Si) alloy, and ferrosilicon alloy. Out of these sources, Al-Si alloy is the cheapest and most viable option to produce high purity porous silicon material.

Indian patent application 201631017873 discloses a process for production of porous silicon from metal-Si alloy. Importantly, the simple and cost-effective process for production of porous Silicon provides porous Si powder with high specific area and porosity, wherein the process starts from metal-Si alloy containing more than 30% silicon. Advantageously, the proposed process provides a high throughput industrially scalable route that can produce porous silicon of desired purity grade satisfying specific application requirements.

US8962183B2 discloses a method of forming a silicon anode material for rechargeable cells includes providing a metal matrix that includes no more than 30 wt % of silicon, including silicon structures dispersed therein. The metal matrix is at least partially etched to at least partially isolate the silicon structures.

Advanced Functional Materials, 28(23), 1800855 discloses a scalable approach to fabricate high performance porous Si anode materials. Herein, a microstructure controlled porous Si is developed by the dealloying in conjunction with wet alkaline chemical etching. The resulting 3D networked structure enables enhancement in lithium storage properties when the Si-based material is applied not only as a single active material but also in a graphite blended electrode.

Materials Chemistry and Physics, 276, 125405 discloses fabrication of porous silicon powder from an Al-Si alloy by etching in strong acids, and its hydrogen storage capacity has been studied. A double acid etching in the fabrication, as well as nickel catalyst addition have been proposed to further enhance the hydrogen storage capacity of the powder. The hydrogen uptake after the double-etching and Ni blending was found to increase and achieved a maximum uptake of 18.6 H atomic % (0.81 H wt%), which was 4 times more than the hydrogen sorption capacity of the sample after single acid etching. The results of this study provide strong evidence on the potential of porous silicon powder from Al-Si alloy to be a cost-effective solid-state storage material for hydrogen.

Applied Clay Science, 30(2), 116-124 discloses preparation of porous silicon by selective leaching of chlorite and the influence on the porous properties of the product of the acid type and acid concentration, solution temperature and leaching time were investigated. After leaching, only SiO2 remained, the other components being selectively leached from the sample in the order MgO < Fe2O3 < Al2O3. The resultant structure of the silica product results in a lower specific surface area than found in leached vermiculite and phlogopite.

In view of the above, there is a requirement of developing a highly scalable alternative process for preparing porous silicon with high specific area and porosity that is both simple and cost-effective.
OBJECT OF THE INVENTION
The main object of the present invention is to provide porous silicon obtained from leaching out Aluminium from Al-Si alloy and a process for synthesis of porous Si from Al-Si alloy.

Another object of the present invention is to use low cost and readily available Al-Si alloy as a precursor material to synthesize porous silicon anode materials.

A further object of the present is to provide a process for synthesis of porous silicon from aluminium-silicon alloy using a dual leaching technique.

Another object of the present invention is to provide a process for production of porous Si with increased porosity.

A further object of the present is to provide an easily industrial scalable process for the synthesis of porous silicon without requirement of energy-intensive equipment.

Another object of the present invention is to provide a process that is environment friendly and does not produce fumes as compared to other high temperature reduction processes.

Another object of the present invention is to provide a process to achieve repeatability and particle size control.

SUMMARY OF THE INVENTION
The present invention provides a process for synthesizing porous silicon anode material using Al – Si alloy as precursor by dual step leaching technique, wherein the process comprises alkali leaching followed by acidic leaching with co – production of high purity hydrogen gas. A low cost and readily available Al-Si alloy is used as a precursor material to synthesize porous silicon anode materials.

In an aspect, the present invention provides a process for producing porous silicon material comprising steps of:
a. providing Al-Si alloy;
b. crushing of Al-Si alloy to obtain 90 to 110 µm particles;
c. sieving of crushed Al-Si alloy of step (b) to obtain 25 to 75 µm particles;
d. alkaline leaching of sieved alloy particles of step (c) is carried out in presence of an alkaline leaching agent for 8 to 12 minutes followed by washing in water till pH in the range 10 to 12 is attained; and
e. carrying out acidic leaching of solution of step (d) in presence of an acidic leaching agent to obtain a dual leached porous silicon material.

BRIEF DESCRIPTION OF THE DRAWINGS
These and other features, aspects, and advantages of the present invention will become better understood when the following detailed description is read with reference to the accompanying drawings in which like characters represent like parts throughout the drawings, wherein:

Figure 1 illustrates XRD patterns of (a) Porous Si after 3 wt% KOH leaching, (b) Porous Si after both 3 wt% KOH leaching and 3M H2SO4 leaching.
Figure 2 illustrates FESEM micrograph of (a) Porous Si after 3 wt% KOH leaching, (b) Porous Si after both 3 wt% KOH leaching and 3M H2SO4 leaching.
Figure 3 illustrates EDX spectra of (a) Porous Si after 3 wt% KOH leaching, (b) Porous Si after both 3 wt% KOH leaching and 3 M H2SO4 leaching.
Figure 4 illustrates Nitrogen adsorption/desorption isotherms of (a) Porous Si after 3 wt% KOH leaching. (b) Porous Si after both 3 wt% KOH leaching and 3 M H2SO4 leaching.
Figure 5 illustrates Pore size distribution of (a) Porous Si after 3 wt% KOH leaching. (b) Porous Si after both 3 wt% KOH leaching and 3 M H2SO4 leaching.
Figure 6 illustrates Particle size distribution of (a) Porous Si after 3 wt% KOH leaching. (b) Porous Si after both 3 wt% KOH leaching and 3 M H2SO4 leaching.
Figure 7 illustrates Initial Columbic efficiency of a) KOH only leached sample. b) dual leached sample.
Figure 8 illustrates Capacity vs. Cycle index curve for coin cell 2032 at test current of 700 mAh.

DETAILED DESCRIPTION OF THE INVENTION
The invention will now be described in detail in connection with certain preferred and optional embodiments, so that various aspects thereof may be more fully interpreted and comprehended. However, any skilled person or artisan will appreciate the extent to which such embodiments could be generalized in practice.

It is further to be understood that all terminology used herein is for the purpose of describing embodiments only and is not intended to be limiting in any manner the scope.

Unless defined otherwise, all technical and scientific expressions used herein have the same meaning as commonly understood by one of ordinary skill in the art to which embodiments of the invention pertains.

In describing and claiming the embodiments of the present invention, the following terminology will be used in accordance with the definitions set out below which are known in the state of art.

The singular forms “a,” “an,” and “the” include plural reference unless the context clearly dictates otherwise. Also, the term ‘composition’ does not limit the scope of the invention for multiple compositions that can be illustrated for best mode of the invention.

All modifications and substitutions that come within the meaning of the description and the range of their legal equivalents are to be embraced within their scope. A description using the transition “comprising” allows the inclusion of other elements to be within the scope of the invention.

In an aspect, the present invention provides a process for synthesizing porous silicon using Al– Si alloy as precursor by dual step leaching technique, wherein the process comprises alkali leaching followed by acidic leaching with co – production of high purity hydrogen gas.

In another aspect, the present invention provides a process for producing porous silicon material comprising steps of:
a. providing Al-Si alloy;
b. crushing of Al-Si alloy to obtain 90 to 110 µm particles;
c. sieving of crushed Al-Si alloy of step (b) to obtain 25 to 75 µm particles;
d. alkaline leaching of sieved alloy particles of step (c) is carried out in presence of an alkaline leaching agent for 8 to 12 minutes followed by washing in water till pH in the range 10 to 12 is attained; and
e. carrying out acidic leaching of solution of step (d) in presence of an acidic leaching agent to obtain a dual leached porous silicon material.

In an embodiment of the present invention, the Al-Si alloy in step (b) is crushed to obtain 100 µm particles.

In an embodiment of the present invention, the alkaline leaching of sieved alloy particles of step (c) is done for 10 minutes followed by washing in water till pH of 11.0 is attained.

In an embodiment of the present invention, the alkaline leaching is followed by acidic leaching.

In another embodiment of the present invention, the alkaline leaching agent is potassium hydroxide.

In another embodiment of the present invention, the acidic leaching agent is sulphuric acid.

In another embodiment of the present invention, the dual leached porous silicon material has specific surface area in a range of 100 to 110 m2/gm.

In another embodiment of the present invention, the dual leached porous silicon material has specific surface area in the range of 106.252 m2/gm.

In another embodiment of the present invention, the obtained dual leached porous silicon material works as anode electrode in batteries.

In another embodiment of the present invention, the hydrogen gas is produced in the alkaline leaching step.

In one embodiment, the present invention follows approach of using low cost and readily available Al-Si alloy as a precursor material to synthesize porous silicon.

In another embodiment, the present invention provides a process for synthesis of porous silicon from aluminium-silicon alloy using a dual leaching technique.

In another embodiment of the present invention, the incorporation of acidic leaching results in a reduced amount of water due to minimal pH neutralization steps.

In another embodiment of the present invention, the porosity of the silicon particles is increased when the alkaline leaching is followed by the acidic leaching step. Thus, this sequence plays a major role in obtaining the porous silicon.

In another embodiment of the present invention, the initial particle size of the Al-Si alloy powder affects the final silicon morphology and performance matrix in the lithium-ion battery application.

In another embodiment of the present invention, the generation of highly porous microstructure is mainly due to galvanic coupling between silicon and aluminium present in the alloy during alkaline leaching.

In another embodiment of the present invention, the process can be scaled-up easily without requirement of energy-intensive equipment.

In another embodiment of the present invention, the process is environment friendly and does not produce fumes as compared to other high temperature reduction processes.

In another embodiment of the present invention, the repeatability and particle size control of porous silicon can be achieved.

In another embodiment of the present invention, the co-production of high purity hydrogen gas by (KOH solution water splitting in presence of Al) can be used to mix in the natural gas to make blended hydrogen / natural gas fuel.

Examples:
The present disclosure with reference to the accompanying examples describes the present invention. However, those of skill in the art should, in light of the present disclosure, appreciate that many changes can be made in the specific embodiments which are disclosed and still obtain a like or similar result without departing from the spirit and scope of the invention. It is understood that the examples are provided for the purpose of illustrating the invention only and are not intended to limit the scope of the invention in any way.

Example 1: Synthesis of porous silicon.
The acquired Al-Si alloy is crushed to ~ 100 µm size particle. Then, for proper leaching purposes and homogeneous particle size distribution of silicon powder, the as-crushed Al-Si alloy powder is sieved to obtain particles in 25 – 75 µm size. The obtained alloy particles are leached in 100 ml of 1M KOH solution for 10 minutes. After leaching, 150 ml of de-ionized water is added to the solution. The leached particles are washed in water till pH 11.0 is attained. The pH of solution is further reduced with the addition of 0.5 M H2SO4 solution, thereby reducing the number of steps to neutralize the pH. At the same time, the earlier step helps to remove the residual Al and Al(OH)3 impurities from the system in two stages. The process is scaled to produce 250 gm of powder in a single batch (the current limitation is scalable production of silicon powders by any other known methods). Hydrogen is produced during the alkaline leaching reaction step. For every 50 kg silicon production batch, 5 kg of hydrogen is released, which can be blended with natural gas to develop blended hydrogen fuel.

Example 2: Comparison of powders after basic (alkaline) leaching and after both basic and acidic leaching.
Example 2a: XRD analysis of KOH leached and dual leached sample for detection of phase purity.
Purity of the silicon powder affects the lithium battery performance and the degree of side reaction which are responsible for the columbic efficiency and cycle life of the battery. Lower the impurity, better the silicon powder for lithium battery application.

With single stage KOH leaching there is always a possibility of aluminium hydroxide phase presence which will hamper the performance of the battery. Moreover, these phases are crystalline as well as amorphous which are hard to detect.

The XRD image as shown in Figure 1a and 1b illustrate the contrast between KOH leached and dual leached sample. In case of KOH leaching only, Aluminium impurity is present in the sample, however in case of dual leaching (KOH leaching followed by acidic leaching) no Aluminium impurity is present.

The result shows that in case of 3 wt.% KOH leaching only, Aluminium impurity is present in the sample (Fig. 1a), however in case of dual leaching (3 wt.% KOH leaching followed by 3M acidic leaching) no Aluminium impurity is present (Fig. 1b).

Example 2b: Morphology analysis of porous silicon.
Porosity and surface area of porous silicon is very crucial to tackle the huge volume change and provide sufficient diffusion channel for high-rate capability.
The result shows that in case of KOH only leached sample the porosity is lower than the dual leached sample. Also, the dual leached samples have connected open porous morphology (Fig. 2b) which is missing in case of KOH only leached samples (Fig 2a). This morphology is beneficial in avoiding pulverisation that results from huge volume change during charging and discharging.

Example 2c: EDX spectra analysis.
The result of EDX spectra analysis shows that KOH only leached sample (Fig.3a) has higher surface oxide fraction than the dual leached sample (Fig.3b). More surface oxygen on the silicon surface increases the chance of solid electrolyte interface formation that is unstable and is responsible for poor cycle life and early capacity fade.

Example 2d: Analysis of specific surface area and pore size distribution of porous silicon. Higher surface area of porous silicon is always beneficial to increase the diffusion rate and improve the rate capability. However, the higher surface area also brings the problem of poor SEI. So, optimising the surface area and surface oxygen on the porous silicon is key to better performing porous silicon anode material. The dual leached samples have lower surface oxygen even with high surface area. The result shows that the dual leached samples have lower surface oxygen even with high surface area. The surface area of dual leached sample is 106.252 m2/gm (Fig.4b) compared to KOH only leached sample having surface area of only 29.973m2/gm (Fig.4a). BET curve shows the presence of meso pores in the porous silicon sample which are good to tackle huge volume expansion during charging and discharging. The cumulative pore volume in case of dual leached sample (Fig.5b) is (approximately double) higher than the KOH only sample (Fig.5a).

Example 2e: Particle size distribution of porous silicon.
Particle size plays a big role in the lithium diffusion and relative performance of the anode.
The result shows that in case of dual leached sample (Fig.6a) the particle size distribution becomes broader and shifted to lower particle size.

Example 2f: Determination of Initial columbic efficiency of porous silicon.
The result shows that the initial columbic efficiency of dual leached sample (Fig.7b) is better than the KOH only leached sample (Fig.7a), which will result in lower lithium loss in early cycle and thus improve the cost of the batteries and less cathode material will be needed.

Example 2g: Determination of cycle stability of porous silicon.
The result shows that the early stage up-to 40 cycle the capacity fading in case of dual leached sample is less than capacity fading in KOH only leached sample. Also, the initial capacity of dual leached sample is slightly more than that of KOH only leached sample (Fig.8).
,CLAIMS:1. A process for producing porous silicon material comprising steps of:
a. providing Al-Si alloy;
b. crushing of Al-Si alloy to obtain 90 to 110 µm particles;
c. sieving of crushed Al-Si alloy of step (b) to obtain 25 to 75 µm particles;
d. alkaline leaching of sieved alloy particles of step (c) is carried out in presence of an alkaline leaching agent for 8 to 12 minutes followed by washing in water till pH in the range 10 to 12 is attained; and
e. carrying out acidic leaching of solution of step (d) in presence of an acidic leaching agent to obtain a dual leached porous silicon material.

2. The process as claimed in claim 1, wherein alkaline leaching is followed by acidic leaching.

3. The process as claimed in claims 1 and 2, wherein the alkaline leaching agent is potassium hydroxide.

4. The process as claimed in claims 1 and 2, wherein the acidic leaching agent is sulphuric acid.

5. The process as claimed in claims 1 to 4, wherein the dual leached porous silicon material has specific surface area in a range of 100 to 110 m2/gm.

6. The process as claimed in claims 1 to 5, wherein the dual leached porous silicon material has specific surface area of 106.252 m2/gm.

7. The process as claimed in claim 1, wherein the obtained dual leached porous silicon material works as anode electrode in batteries.

8. The process as claimed in anyone of the claims 1 to 4, wherein hydrogen gas is produced in the alkaline leaching step.