U.S. patent application number 13/773105 was filed with the patent office on 2014-05-29 for method for recovering elemental silicon from silicon sludge by electrolysis in non-aqueous electrolyte.
This patent application is currently assigned to Kumoh National Institute of Technology Industry-Academic Cooperation Foundation. The applicant listed for this patent is Kumoh National Institute of Technology Industry-Academic Cooperation Foundation. Invention is credited to Churl Kyoung Lee, Jae Jun Park, Je sik Park.
Application Number | 20140144784 13/773105 |
Document ID | / |
Family ID | 49988103 |
Filed Date | 2014-05-29 |
United States Patent
Application |
20140144784 |
Kind Code |
A1 |
Lee; Churl Kyoung ; et
al. |
May 29, 2014 |
METHOD FOR RECOVERING ELEMENTAL SILICON FROM SILICON SLUDGE BY
ELECTROLYSIS IN NON-AQUEOUS ELECTROLYTE
Abstract
The present invention relates to a method for recovering
elemental silicon from silicon sludge by electrolysis in a
non-aqueous electrolyte. The recovery method of silicon according
to the present invention can achieve direct reduction of silicon by
electrolysis at a low temperature (below 200.degree. C.), control
the structure of silicon by a simple process and a change in
electrolysis conditions, and perform a continuous process by adding
a silicon salt.
Inventors: |
Lee; Churl Kyoung; (Seoul,
KR) ; Park; Je sik; (Gyeongsangbuk-do, KR) ;
Park; Jae Jun; (Gyeongsangnam-do, KR) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Cooperation Foundation; Kumoh National Institute of Technology
Industry-Academic |
|
|
US |
|
|
Assignee: |
Kumoh National Institute of
Technology Industry-Academic Cooperation Foundation
Gyeongsangbuk-do
KR
|
Family ID: |
49988103 |
Appl. No.: |
13/773105 |
Filed: |
February 21, 2013 |
Current U.S.
Class: |
205/339 |
Current CPC
Class: |
C25B 1/006 20130101 |
Class at
Publication: |
205/339 |
International
Class: |
C25B 1/00 20060101
C25B001/00 |
Foreign Application Data
Date |
Code |
Application Number |
Nov 23, 2012 |
KR |
10-2012-0133845 |
Claims
1. A method for recovering elemental silicon, the method comprising
the steps of: (a) mixing waste silicon sludge and an organic
solvent to separate cutting oil from the silicon sludge; (b)
separating iron (Fe) from the silicon sludge, from which the
cutting oil is removed, using a magnetic separator; (c) adding
chlorine to the silicon sludge, from which the iron is removed, and
heating the resulting silicon sludge to prepare silicon
tetrachloride (SiCl4); (d) placing an electrolytic cell provided
with conductive electrodes in a conductive non-aqueous solvent, in
which the silicon tetrachloride is dissolved, in a high-purity
inert gas atmosphere; and (e) applying an electric power to the
electrolytic cell in step (d) such that a reduction of silicon
occurs in a negative electrode and a silicon thin film is formed on
the surface of the electrode.
2. The method of claim 1, wherein in step (a), the organic solvent
comprises chloroform, ethyl acetate, tetrahydrofuran (THF), or
dichloromethane (CH.sub.2Cl.sub.2).
3. The method of claim 1, wherein in step (b), the magnetic
separator has a magnetic flux density of 500 gauss.
4. The method of claim 1, wherein in step (c), the heating is
performed at 800 to 1,200.degree. C. for 30 to 90 minutes.
5. The method of claim 1, wherein in step (d), the electrodes
comprises at least two selected from the group consisting of gold,
platinum, and copper.
6. The method of claim 1, wherein in step (d) the inert gas
comprises at least one selected from the group consisting of
nitrogen, helium, argon, neon, and xenon.
7. The method of claim 1, wherein in step (d), the conductive
non-aqueous solvent is a conductive non-aqueous solvent containing
a bis(trifluoromethylsulfonyl)imide (TFSI) anion and comprises at
least one selected from the group consisting of
(1-Butyl-3-methyl-pyridinium bis(trifluoromethylsulfonyl)imide)
[BMPy]TFSI, (1-methyl-propylpiperidinium
bis(trifluoromethylsulfonyl)imide) PP13TFSI, and
(1-Ethyl-3-methyl-imidazolium bis(trifluoromethylsulfonyl)imide)
[EMIM]TFSI.
8. The method of claim 7, wherein the conductive non-aqueous
solvent further comprises propylene carbonate (PC), dichloromethane
(DCM), tetrahydrofuran (THF), dicyanamide (DCA), or
N-methylpyrrolidone (NMP).
9. The method of claim 1, further comprising, before step (d), the
steps of: (d') cleaning the electrodes and the cell with a mixture
solution of sulfuric acid and hydrogen peroxide; and (d'') drying
the non-aqueous solvent at 80 to 120.degree. C. for 20 to 30
hours.
10. The method of claim 1, further comprising, after step (e), the
step of performing heat treatment in an inert gas atmosphere at 800
to 900.degree. C. for 30 to 90 minutes.
11. The method of claim 10, wherein the inert gas comprises at
least one selected from the group consisting of nitrogen, helium,
argon, neon, and xenon.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] This application claims priority to and the benefit of
Korean Patent Application No. 10-2012-133845, filed on Nov. 23,
2012, the disclosure of which is incorporated herein by reference
in its entirety.
BACKGROUND
[0002] 1. Field of the Invention
[0003] The present invention relates to a method for recovering
elemental silicon from silicon sludge by electrolysis in a
non-aqueous electrolyte.
[0004] 2. Discussion of Related Art
[0005] Cutting slurry containing silicon carbide of 20 .mu.m in
size and the like is used in a process of cutting wafers from a
silicon ingot. In this process, sludge containing silicon, silicon
carbide, metal powder, cutting oil, etc. is discharged and
effective separation and recovery of waste sludge for reuse or
recycling as a useful resource has a significant meaning in terms
of efficient use of resources as well as in terms of environmental
protection.
[0006] In general, the refinement of silicon can be obtained by
reacting concentrated silicon dioxide (SiO.sub.2) with a reductant
such as carbon at high temperature, and the refined silicon has a
purity of about 98% and thus has limitations on its use. Moreover,
in the refinement process, a lot of energy is used, a large amount
of carbon dioxide is emitted, and impurities are contained, which
are very problematic. A purification process of silicon is
necessary to use silicon in higher value-added industries such as
semiconductor, photovoltaic, secondary battery industries. The
purification of silicon yields a purity of 99.9% or higher by
conversion into halogen compounds, fractional distillation, and
reduction, but the process is complicated, energy-consuming, and
uses toxic substances. However, no alternative process has yet been
suggested.
[0007] Moreover, a thinning process is necessary to use silicon as
a semiconductor or a solar cell, which requires the formation of
wafers by an ingot cutting process after melting and solidification
or a deposition method such as chemical vapor deposition or
sputtering. This process is performed under high vacuum and is
difficult to be carried out as a continuous process, which involves
a lot of expenses.
[0008] Over the past few decades, extensive research has been
conducted on processes for easily manufacturing silicon thin films,
but there is no alternative yet. Electrolysis is a simple process
that yields materials of desired properties by simple manipulation
of voltage, current, etc. and is suitable for continuous mass
production. However, the electrolytic reduction of silicon is very
difficult to achieve in a typical aqueous electrolyte due to low
oxidation-reduction potential and high reduction overpotential.
Moreover, even a very small amount of reduced silicon is reoxidized
with oxygen in an aqueous solution, and thus it is impossible to
achieve the electrolytic reduction of elemental silicon.
[0009] According to literature search, a method for electrolytic
reduction of silicon by electrolysis in a non-aqueous electrolyte
has been studied. Cohen and Huggins prepared SiF.sub.6 by
electrolysis in a LiH-KF process (U. Cohen and R. A. Huggins, 1976:
Silicon Epitaxial Growth by Electrodeposition from Molten
Fluorides, J. Electrochem. Soc., 123, pp. 381-383), and research on
monocrystalline and polycrystalline silicon started since then.
Elwell et al. examined the possibility of continuous deposition of
silicon thin films in LiF--KF--NaF--K.sub.2SiF.sub.6 in a molten
state (D. Elwell and G. M. Rao, 1988: Electrolytic production of
silicon, J. Appl. Electrochem., 18, pp. 15-22). They studied
electrodeposition using a molten salt at high temperatures of 500
to 1,400.degree. C., which requires a complicated process and a
high temperature process, resulting in significant cost and energy.
It was reported that silicon was reduced in high temperature molten
salts using K.sub.2SiF.sub.6, Na.sub.2SiF.sub.6, SiO.sub.2, etc. as
a source of silicon for the electrolytic reduction of silicon in an
aprotic solvent (A. K. Agrawal and A. E. Austin, 1981:
Electrodeposition of silicon from solutions of silicon halides in
aprotic solvents, J. Electrochem. Soc., 128, pp. 2292-296) or in a
molten salt (K. L. Carleton, J. M. Olson, and A. Kibbler, 1983:
Electrochemical nucleation and growth of silicon in molten
fluorides, J. Electrochem. Soc., 130, pp. 782-786) and it was
possible to form a uniform thin film up to 0.25 .mu.m in thickness
(J. Gobet and H. Tannenberger, 1986: Electrodeposition of silicon
from a nonaqueous solvent, J. Electrochem. Soc., 133, pp. C322).
Moreover, recently, Nicholson et al. reported that silicon could be
electrodeposited in a low temperature molten salt using a
non-aqueous solvent but the electrodeposited silicon had strong
oxidative properties, resulting in the formation of silica (J. P.
Nicholson, 2005: Electrodeposition of silicon from nonaqueous
solvents, J. Electrochem. Soc., 152, pp. C795-802).
[0010] It was reported that the electrolytic reduction of highly
active metals such as aluminum, titanium, magnesium, etc., which
was impossible in an aqueous electrolyte, was possible using an
ionic liquid having wide electrochemical window and excellent
electrical conductivity as an electrolyte (F. Endres, 2002: Ionic
Liquids: Solvents for the electrodeposition of metals and
Semiconductors, Phys. Chem. Chem. Phys., 3, pp. 144-154). A highly
conductive electrolyte for minimizing hydrogen generation using an
organic solvent was studied by Austin (A. E. Austin, 1976: U.S.
Pat. No. 3,990,953 and November 9 (1976) CA 86: 10098c). In
general, the electrolyte was prepared using an aprotic solvent such
as silicon halide, propylene carbonate (PC), or tetrahydrofuran
(THF), and this electrolyte has no conductivity and thus requires a
supporting electrolyte. In the study of Austin, silicon halide and
tetrabutylammonium perchlorate (Bu.sub.4NClO.sub.4) as the
supporting electrolyte were added to prepare various organic
electrolytes, which was described by Bucker and Amick (E. R. Bucker
and J. A. Amick, 1980: U.S. Pat. No. 4,192,720. October 16 (1980)
CA 92: 10098). Recently, a study on electrolytic reduction of
silicon using PC was reported by Fukunaka et al. (Y. Nishimura and
Y. Fukunaka, 2007: Electrochemical reduction of silicon chloride in
a non-aqueous solvent, Electrochimca Acta, 53, pp. 111-116), but as
a result of XPS analysis, the electrodeposited silicon was exposed
to the air and oxidized, and Munisamy et al. performed a study on
the initial growth of silicon in acetonitrile by electrolysis (T.
Munisamy and A. J. Bard, 2010: Electrodeposition of Si from organic
solvents and studies related to initial stages of Si growth,
Electrochimca Acta, 55(11, 15), pp. 3797-3803). Abedin et al.
reported that silicon was electrodeposited using a room temperature
ionic liquid (S. Z. El Abedin, N. Borissenko, and F. Endres, 2004:
Electrodeposition of nanoscale silicon in a room temperature ionic
liquid, Electrochem. Comm., 6, pp. 510-514), and Mallet et al.
reported that silicon nanowires were first synthesized by the same
method (J. Mallet, M. Molinari, F. Martineau, F. Delavoie, P.
Fricoteaux, and M. Troyon, 2008: Growth of silicon nanowires of
controlled diameters by electrodeposition in ionic liquid at room
temperature, Nano Lett. 8(10), pp. 3468-3474).
[0011] They all reported a study on the electrolytic reduction of
silicon at room temperature but did not solve the problem that the
reduced silicon is oxidized. Moreover, it is not yet determined
whether the electrodeposited silicon oxide is reduced along with
dissolved oxygen in the electrolyte or whether the electrodeposited
porous silicon is exposed to the air and oxidized. Furthermore, a
study aimed at stabilizing the electrodeposited silicon by
annealing before being exposed to the air has been partially
performed, but this problem has not yet been solved.
[0012] Thus, there is an urgent need to develop electrolysis
conditions in the recovery of elemental silicon from silicon sludge
by electrolysis that is simpler than the conventional
processes.
[0013] In this study, cutting oil, metal impurities, etc. were
removed from silicon sludge by a separation process such as
mechanical separation to separate silicon, and a mixture of the
separated silicon and silicon carbide was subjected to chloridizing
roasting to prepare silicon tetrachloride. The silicon
tetrachloride was dissolved in a conductive non-aqueous solvent,
and elemental silicon was directly reduced by electrolysis. The
crystallinity and composition of electrodeposits and the presence
of impurities were determined from the analysis of the finally
produced electrodeposits.
SUMMARY OF THE INVENTION
[0014] The present inventors have studied on a method for
recovering silicon from silicon sludge by electrolysis and found
that elemental silicon could be directly reduced in a conductive
non-aqueous electrolyte prepared from silicon tetrachloride and,
and through subsequent heat treatment, the efficiency of
electrolysis of silicon and the stabilizing efficiency of silicon
could be significantly improved, thus completing the present
invention.
[0015] Accordingly, an object of the present invention is to
provide a method for recovering elemental silicon from silicon
sludge by electrolysis in a non-aqueous electrolyte, in which
silicon tetrachloride simply prepared from silicon sludge is
dissolved, which can easily control electrolysis parameters to
control the structure of silicon and easily recover the elemental
silicon using the non-aqueous electrolyte.
[0016] To achieve the above object, the present invention provides
a method for recovering elemental silicon, the method comprising
the steps of: (a) mixing waste silicon sludge and an organic
solvent to separate cutting oil from the silicon sludge; (b)
separating iron (Fe) from the silicon sludge, from which the
cutting oil is removed, using a magnetic separator; (c) adding
chlorine to the silicon sludge, from which the iron is removed, and
heating the resulting silicon sludge to prepare silicon
tetrachloride (SiCl.sub.4); (d) placing an electrolytic cell
provided with conductive electrodes in a conductive non-aqueous
solvent, in which the silicon tetrachloride is dissolved, in a
high-purity inert gas atmosphere; and (e) applying an electric
power to the electrolytic cell such that a reduction of silicon
occurs in a negative electrode and a silicon thin film is formed on
the surface of the electrode.
BRIEF DESCRIPTION OF THE DRAWINGS
[0017] The above and other objects, features and advantages of the
present invention will become more apparent to those of ordinary
skill in the art by describing in detail exemplary embodiments
thereof with reference to the accompanying drawings, in which:
[0018] FIG. 1 is a schematic diagram showing a method for preparing
silicon tetrachloride from silicon sludge;
[0019] FIG. 2 is a diagram showing the analysis results of solid
components, obtained by separate cutting oil and iron powder from
silicon sludge, using X-ray diffraction and electron
microscopy;
[0020] FIG. 3 is a diagram showing electrochemical behaviors of
silicon in an [EMIM]TFSI electrolyte, in which 0.1 M silicon
tetrachloride is dissolved, measured by cyclic voltammetry using
gold as an electrode;
[0021] FIG. 4 is a diagram showing the results of SEM-EDS of
silicon reduced in a gold electrode;
[0022] FIG. 5 is a diagram showing the results of XPS measurement
of silicon recovered by the present invention; and
[0023] FIG. 6 is a diagram showing the results of XRD measurement
of silicon recovered by the present invention.
DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS
[0024] Hereinafter, the present invention will be described in
detail.
[0025] The present invention provides a method for recovering
elemental silicon, the method comprising the steps of: (a) mixing
waste silicon sludge and an organic solvent to separate cutting oil
from the silicon sludge; (b) separating iron (Fe) from the silicon
sludge, from which the cutting oil is removed, using a magnetic
separator; (c) adding chlorine to the silicon sludge, from which
the iron is removed, and heating the resulting silicon sludge to
prepare silicon tetrachloride (SiCl.sub.4); (d) placing an
electrolytic cell provided with conductive electrodes in a
conductive non-aqueous solvent, in which the silicon tetrachloride
is dissolved, in a high-purity inert gas atmosphere; and (e)
applying an electric power to the electrolytic cell such that a
reduction of silicon occurs in a negative electrode and a silicon
thin film is formed on the surface of the electrode.
[0026] In the recovery method of silicon according to the present
invention, step (a) is to separate cutting oil from silicon sludge,
in which an organic solvent miscible with the cutting oil is mixed
with waste sludge to selectively dissolve the cutting oil, thus
separating the cutting oil from the silicon sludge.
[0027] The organic solvent is preferably, but not limited to,
chloroform, ethyl acetate, tetrahydrofuran (THF), or
dichloromethane (CH.sub.2Cl.sub.2).
[0028] Step (b) is to remove iron (Fe) powder contained in the
silicon sludge from which the cutting oil is removed. The iron
powder is generated by friction between wire saw and silicon and is
separated using a magnetic separator (Eric manufacturing). The
magnetic separator preferably has a magnetic flux density of 500
gauss.
[0029] Step (c) is to prepare silicon tetrachloride (SiCl.sub.4)
from the silicon sludge after the iron is separated therefrom. The
silicon sludge from which the iron is removed is a mixture of
silicon and silicon carbide, and chlorine is added to the silicon
sludge and heated to recover the silicon tetrachloride. The heating
is preferably performed at 800 to 1,200.degree. C. for 30 to 90
minutes.
[0030] In step (d), the basic structure of the electrolytic cell
for electrolytic reduction of silicon is a three-electrode system
in which a reference electrode is additionally used in an
electrolytic cell comprising a working electrode and a counter
electrode.
[0031] The electrodes may be made of any conductive material and
preferably comprise at least two selected from the group consisting
of gold, platinum, and copper, but not limited thereto.
[0032] In step (d), the high-purity inert gas may comprise at least
one selected from the group consisting of nitrogen, helium, argon,
neon, and xenon, and preferably argon. When the silicon is
recovered in a high-purity inert gas atmosphere, it is possible to
prevent oxidation of silicon.
[0033] In step (d), a conductive non-aqueous solvent capable of
dissolving the silicon tetrachloride may be used as an electrolyte.
The conductive non-aqueous solvent is a conductive non-aqueous
solvent containing a bis(trifluoromethylsulfonyl)imide (TFSI) anion
and comprises, but not limited to, at least one selected from the
group consisting of (1-Butyl-3-methyl-pyridinium
bis(trifluoromethylsulfonyl)imide) [BMPy]TFSI,
(1-methyl-propylpiperidinium bis(trifluoromethylsulfonyl)imide)
PP13TFSI, and (1-Ethyl-3-methyl-imidazolium
bis(trifluoromethylsulfonyl)imide) [EMIM]TFSI.
[0034] Moreover, the conductive non-aqueous solvent may further
comprise propylene carbonate (PC), dichloromethane (DCM),
tetrahydrofuran (THF), dicyanamide (DCA), or N-methylpyrrolidone
(NMP). These materials have very low viscosity and have no effect
on the dissociation of silicon, but improve the electrical
conductivity of the electrolyte.
[0035] The method of the present invention may further comprise,
before step (d), the steps of (d') cleaning the electrodes and the
cell with a mixture solution of sulfuric acid (H.sub.2SO.sub.4) and
hydrogen peroxide (H.sub.2O.sub.2); and (d'') drying the
non-aqueous solvent at 80 to 120.degree. C. for 20 to 30 hours. In
step (d'), the sulfuric acid and the hydrogen peroxide may
preferably be mixed in a volume ratio of 50:50, and through this
step, the impurities can be removed from the electrodes and the
cell. Moreover, through step (d''), a small amount of water
contained in the non-aqueous solvent can be removed.
[0036] Step (e) is to apply an electric power to the electrolytic
cell such that a reduction of silicon occurs in a negative
electrode and a silicon thin film is formed on the surface of the
electrode. As used in typical electroplating, when two metal
electrodes are placed in an electrolyte in which silicon is
dissolved and an electric power is applied thereto, a reduction of
silicon occurs in the negative electrode, and silicon
electrodeposits are formed on the surface of the electrode.
[0037] The method of the present invention may further comprise,
after step (e), the step of performing heat treatment in an inert
gas atmosphere at 800 to 900.degree. C. for 30 to 90 minutes. The
inert gas may comprise, but not limited to, nitrogen, helium,
argon, neon, and xenon. With the heat treatment, it is possible to
remove impurities from the silicon and stabilize the
electrodeposited silicon.
[0038] As mentioned above, when the silicon is recovered by the
recovery method according to the present invention, it is possible
to directly obtain elemental silicon by the electrolysis, which is
easier than the conventional processes. Moreover, the silicon can
be recovered at low temperature, and thus it is possible to allow
mass production and reduce the production cost. Furthermore, the
recovery method of silicon according to the present invention can
prepare the silicon tetrachloride by a simple process and
continuously recover the silicon by repeatedly dissolving the
silicon tetrachloride.
[0039] Next, preferred examples will be provided for better
understanding of the present invention. However, the following
examples are provided only for the purpose of illustrating the
present invention, and the scope of the present invention is not
limited by the examples.
Example 1
Preparation of Silicon Tetrachloride from Silicon Sludge
[0040] The outline of a process for preparing silicon tetrachloride
as a pre-treatment for recovering silicon from silicon sludge is
shown in FIG. 1.
[0041] As an organic solvent miscible with cutting oil in silicon
sludge, dichloromethane (CH.sub.2Cl.sub.2) was mixed with silicon
sludge and stirred at 300 rpm or higher for 3 hours to selectively
dissolve the cutting oil in the silicon sludge. After dissolving
the cutting oil, silicon and silicon carbide (Si+SiC) as a solid
phase and organic oil as a liquid phase were separated by
filtration and centrifugation. Iron powder contained in the solid
phase mixture, from which the organic oil is separated, was
separated using a magnetic separator (Eric manufacturing) having a
magnetic flux density of 500 gauss. The resulting mixture was
stirred in 1 mol/L of hydrochloric acid solution at a solid-liquid
ratio of 1:2 at room temperature for 2 hours, and the solid phase
was precipitated, washed with distilled water, and then dried. The
solid components from which the iron powder was separated were
analyzed using X-ray diffraction and electron microscopy, and the
results are shown in FIG. 2.
[0042] As shown in FIG. 2, the amount of iron removed was less than
0.1 wt % with respect to the total solid content, and thus a
mixture of silicon and silicon carbide was confirmed.
[0043] Then, 5 wt % of carbon powder was added to the separated and
concentrated mixture of silicon and silicon carbide, and the
resulting mixture was placed in a furnace. Subsequently, chlorine
gas was injected into the furnace, and gas vaporized by
chloridizing roasting at 1,000.degree. C. for 1 hour was condensed
at 25.degree. C., thus preparing silicon tetrachloride
(SiCl.sub.4).
Example 2
Electrolytic Recovery of Silicon from Silicon Tetrachloride
[0044] 1. Experimental Conditions for Electrolytic Recovery of
Silicon
[0045] A three-electrode system was used as an electrolytic cell
for electrolytic reduction of silicon. [EMIM]TFSI in which silicon
tetrachloride was dissolved was used as an electrolyte. To prevent
oxidation of silicon, the experiment was performed in a glove box
in which high-purity argon gas (5 N, oxygen content of no more than
1 ppm, and vapor content of no more than 3 ppm) was filled. The
effects of the stable voltage window of an ionic liquid and the
electrolysis conditions (electrodes, ionic liquid, silicon
concentration, etc.) on the electrochemical properties were
measured by cyclic voltammetry using a Potentiostat/Galvanostat
(Solartron 1287). At this time, the potential range was -4 to 2 V
(vs. OCV), and the scan rate was 10 mV/s.
[0046] The electrochemical oxidation/reduction behaviors of silicon
when gold (Ag) was used as a working electrode were examined. A
platinum (Pt) wire was used as a counter electrode (4 cm.sup.2) and
a quasi reference electrode (QRE), respectively. To remove
impurities, all electrodes and cells were cleaned with a mixture
solution of sulfuric acid (H.sub.2SO.sub.4) and hydrogen peroxide
(H.sub.2O.sub.2) in a volume ratio of 50:50 and then used in the
experiment. Moreover, all ionic liquids were dried in a vacuum oven
at 100.degree. C. for 24 hours prior to the experiment to remove a
small amount of water contained therein and then used in the
electrochemical experiment. To analyze the morphology, composition,
and crystallinity of elemental silicon obtained by electrolysis, an
experiment was performed to reduce elemental silicon directly from
[EMIM]TFSI, in which 0.5 M silicon tetrachloride was dissolved,
under potentiostatic electrolysis (-1.9 V vs. Pt-QRE).
[0047] 2. Electrolytic Recovery of Silicon
[0048] Electrochemical behaviors of silicon in an [EMIM]TFSI
electrolyte with a gold electrode are shown in FIG. 3.
[0049] As shown in FIG. 3, the stable electrochemical window was
about 3 V, and the cathodic potential limit was -2.5 V (vs.
Pt-QRE). In an electrolyte in which 0.5 M SiCl.sub.a was dissolved,
a strong reduction current, expected as a stable reduction of
silicon in view of the cathodic potential limit, was observed. The
cyclic voltammetric behaviors in an electrolyte in which silicon
tetrachloride (SiCl.sub.4) was dissolved showed a first reduction
zone where the reduction current increased from 0 V and a second
reduction zone where the reduction current density sharply
increased at -2.2 V. It is considered that tetravalent silicon ions
are changed to other compounds in the first reduction zone and that
ionic silicon and intermediate silicon compounds are reduced to
silicon in the second reduction zone starting from -2.2 V. In
particular, it was found that since the peak, expected as the
reduction current of silicon, was at a higher potential than the
reduction potential limit, the possibility that impurities were
contained in the reduction reaction was low.
Experimental Example 1
Analysis of Silicon Electrodeposited Film
[0050] 1. Analysis Method
[0051] The morphology and composition of the electrodeposited
silicon film were analyzed using a field-emission scanning electron
microscope (FE-SEM, JSM-6500F) with an energy-dispersive
spectrometer (EDS) attached and X-ray diffractometer (XRD).
Impurities in the electrodeposited silicon film were analyzed using
X-ray photoelectron spectroscopy (XPS, ULVAC-PHI, Quantera SXM),
and a depth analysis for a depth of about 50 nm from the surface
was performed along with surface analysis. Moreover, the
crystallinity of silicon was analyzed using X-ray diffractometer
(XRD, Rigaku D/MAX-200-, Cu-Ka).
[0052] 2. Analysis Results of Electrodeposited Film
[0053] The results of SEM-EDS of silicon reduced in a gold
electrode are shown in FIG. 4.
[0054] As shown in FIG. 4, the morphology of silicon is in the form
of particles of about 100 nm, and the results of the EDS analysis
show the silicon reduced along with gold as the working
electrode.
[0055] Since the silicon was obtained in the stable region
confirmed from the cyclic voltammetric curve, other impurities than
the working electrode were not observed, but a large amount of
oxygen was detected. Since the experiment for measuring the
reduction of silicon and the properties of ionic liquids was
performed in an argon-filled glove box, the possibility that the
silicon was reduced to oxide was low. Moreover, since the ionic
liquids were dried in a vacuum oven at 100.degree. C. for more than
24 hours prior to the experiment to remove moisture therein, it was
determined that the detected oxygen was generated in the surface
while the prepared samples were exposed to the outside during
transfer to the analysis instruments. The results of XPS
measurement for the analysis of the surface and impurities are
shown in FIG. 5.
[0056] As shown in FIG. 5, the amount of reduced silicon was very
low as 31.3%, and other impurities than oxygen were not observed.
The silicon was detected in the form of SiO.sub.2, and the depth
analysis was performed to determine whether the silicon surface was
changed or whether the silicon was reduced to oxide. From the
results of analysis at a depth of 20 nm from the surface, it was
found that the intensity of SiO.sub.2 peak was reduced from the
surface to the interior and a Si peak was observed. A complete Si
peak was observed at a depth of more than 20 nm, from which it was
confirmed that SiO.sub.2 was not generated during the electrolytic
reduction but only the surface was oxidized when the samples were
exposed to the outside for the analysis. Moreover, the components
of the electrolyte and chloride ions were not detected, from which
it was confirmed that pure silicon was reduced.
[0057] The results of XRD measurement for the analysis of the
crystallinity of the electrolytically reduced silicon are shown in
FIG. 6. The silicon peaks were analyzed based on the peaks of gold
used as the working electrode, and several silicon crystal faces
were observed, from which it was confirmed that silicon was present
in the form of polycrystalline or amorphous silicon.
[0058] As described above, the recovery method of silicon according
to the present invention can achieve direct reduction of silicon by
electrolysis at a low temperature (below 200.degree. C.), control
the structure of silicon by a simple process and a change in
electrolysis conditions, and perform a continuous process by adding
a silicon salt.
[0059] It will be apparent to those skilled in the art that various
modifications can be made to the above-described exemplary
embodiments of the present invention without departing from the
spirit or scope of the invention. Thus, it is intended that the
present invention covers all such modifications provided they come
within the scope of the appended claims and their equivalents.
* * * * *