U.S. patent application number 14/149055 was filed with the patent office on 2014-07-10 for combined electrochemical and chemical etching processes for generation of porous silicon particulates.
This patent application is currently assigned to Lockheed Martin Corporation. The applicant listed for this patent is Sibani Lisa Biswal, Steven L. Sinsabaugh, Madhuri Thakur, Michael S. Wong. Invention is credited to Sibani Lisa Biswal, Steven L. Sinsabaugh, Madhuri Thakur, Michael S. Wong.
Application Number | 20140193711 14/149055 |
Document ID | / |
Family ID | 51061193 |
Filed Date | 2014-07-10 |
United States Patent
Application |
20140193711 |
Kind Code |
A1 |
Biswal; Sibani Lisa ; et
al. |
July 10, 2014 |
COMBINED ELECTROCHEMICAL AND CHEMICAL ETCHING PROCESSES FOR
GENERATION OF POROUS SILICON PARTICULATES
Abstract
Embodiments of the present disclosure pertain to methods of
preparing porous silicon particulates by: (a) electrochemically
etching a silicon substrate, where electrochemical etching
comprises exposure of the silicon substrate to an electric current
density, and where electrochemical etching produces a porous
silicon film over the silicon substrate; (b) separating the porous
silicon film from the silicon substrate, where the separating
comprises a gradual increase of the electric current density in
sequential increments; (c) repeating steps (a) and (b) a plurality
of times; (d) electrochemically etching the silicon substrate in
accordance with step (a) to produce a porous silicon film over the
silicon substrate; (e) chemically etching the porous silicon film
and the silicon substrate; and (f) splitting the porous silicon
film and the silicon substrate to form porous silicon particulates.
Further embodiments of the present disclosure pertain to the formed
porous silicon particulates and anode materials that contain
them.
Inventors: |
Biswal; Sibani Lisa;
(Houston, TX) ; Wong; Michael S.; (Houston,
TX) ; Thakur; Madhuri; (Houston, TX) ;
Sinsabaugh; Steven L.; (Uniontown, OH) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Biswal; Sibani Lisa
Wong; Michael S.
Thakur; Madhuri
Sinsabaugh; Steven L. |
Houston
Houston
Houston
Uniontown |
TX
TX
TX
OH |
US
US
US
US |
|
|
Assignee: |
Lockheed Martin Corporation
Moorestown
NJ
William Marsh Rice University
Houston
TX
|
Family ID: |
51061193 |
Appl. No.: |
14/149055 |
Filed: |
January 7, 2014 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
61749636 |
Jan 7, 2013 |
|
|
|
Current U.S.
Class: |
429/217 ; 205/57;
429/218.1; 429/219; 429/220; 429/225; 429/229 |
Current CPC
Class: |
C01B 33/02 20130101;
H01M 2004/027 20130101; C01P 2006/16 20130101; C25F 3/12 20130101;
H01M 4/134 20130101; H01M 4/386 20130101; H01M 4/621 20130101; H01M
4/366 20130101; H01M 10/052 20130101; H01M 10/0525 20130101; H01M
4/626 20130101; H01M 4/1395 20130101; H01M 2004/021 20130101; Y02P
20/133 20151101; Y02E 60/10 20130101; C01P 2004/61 20130101; H01M
4/622 20130101 |
Class at
Publication: |
429/217 ; 205/57;
429/218.1; 429/219; 429/220; 429/229; 429/225 |
International
Class: |
H01M 4/04 20060101
H01M004/04; H01M 4/38 20060101 H01M004/38; H01M 4/36 20060101
H01M004/36 |
Claims
1. A method of preparing porous silicon particulates, wherein the
method comprises: (a) electrochemically etching a silicon
substrate, wherein the electrochemical etching comprises exposure
of the silicon substrate to an electric current density, and
wherein the electrochemical etching produces a porous silicon film
over the silicon substrate; (b) separating the porous silicon film
from the silicon substrate, wherein the separating comprises a
gradual increase of the electric current density in sequential
increments; (c) repeating steps (a) and (b) a plurality of times;
(d) electrochemically etching the silicon substrate in accordance
with step (a) to produce a porous silicon film over the silicon
substrate; (e) chemically etching the porous silicon film and the
silicon substrate; and (f) splitting the porous silicon film and
the silicon substrate to form porous silicon particulates.
2. The method of claim 1, wherein the silicon substrate comprises a
silicon wafer.
3. The method of claim 1, wherein the electrochemical etching of
the silicon substrate comprises use of an acid.
4. The method of claim 3, wherein the acid comprises hydrofluoric
acid.
5. The method of claim 1, wherein the electrochemical etching
comprises exposure of the silicon substrate to an electric current
density of about 1 mA/cm.sup.2 to about 10 mA/cm.sup.2.
6. The method of claim 1, wherein the gradual increase of the
electric current density during the separating step comprises an
increase of the electric current density by about 1-2 mA/cm.sup.2
per sequential increment.
7. The method of claim 1, wherein steps (a) and (b) are repeated
more than 5 times.
8. The method of claim 1, wherein steps (a) and (b) are repeated
until the porous silicon film becomes inseparable from the silicon
substrate.
9. The method of claim 1, wherein steps (a) and (b) are repeated
until the silicon substrate develops one or more cracks.
10. The method of claim 1, wherein the chemical etching occurs by
exposure of the porous silicon film and the silicon substrate to a
metal.
11. The method of claim 10, wherein the metal is selected from the
group consisting of silver, copper, chromium, gold, aluminum,
tantalum, lead, zinc, silicon, and combinations thereof.
12. The method of claim 10, wherein the metal is silver.
13. The method of claim 10, wherein the exposure results in coating
of the porous silicon film and the silicon substrate with the
metal.
14. The method of claim 1, wherein the splitting occurs by at least
one of physical grinding, crushing, sonication, ultrasonication,
ultrasonic fracture, pulverization, ultrasonic pulverization, and
combinations thereof.
15. The method of claim 1, wherein the splitting occurs by
ultrasonication.
16. The method of claim 1, further comprising a step of associating
the porous silicon particulates with a binding material.
17. The method of claim 16, wherein the binding material is
selected from the group consisting of binders, carbon materials,
polymers, metals, additives, carbohydrates, and combinations
thereof.
18. The method of claim 16, wherein the binding material comprises
a polymer selected from the group consisting of polyacrylonitrile
(PAN), pyrolyzed polyacrylonitrile (PPAN), polyvinyldiene
difluoride (PVDF), polyacrylic acid (PAA), carboxymethyl cellulose
(CMC), and combinations thereof.
19. The method of claim 16, wherein the binding material is
carbonized.
20. The method of claim 19, wherein the binding material comprises
a carbonized polyacrylonitrile.
21. The method of claim 1, wherein the formed porous silicon
particulates comprise a plurality of pores, wherein the plurality
of pores comprise diameters between about 1 nanometer to about 5
micrometers.
22. The method of claim 1, wherein the plurality of pores comprise
diameters of at least about 50 nm, less than about 50 nm, less than
about 2 nm, and combinations thereof.
23. The method of claim 1, wherein the plurality of pores comprise
hierarchical pores.
24. The method of claim 23, wherein the hierarchical pores comprise
micropores and mesopores within macropores.
25. The method of claim 1, wherein the formed porous silicon
particulates have thicknesses ranging from about 10 micrometers to
about 200 micrometers.
26. The method of claim 1, wherein the formed porous silicon
particulates comprise pores that span at least 50% of a thickness
of the porous silicon particulates.
27. The method of claim 1, wherein the formed porous silicon
particulates comprise pores that span an entire thickness of the
porous silicon particulates.
28. The method of claim 1, wherein the porous silicon particulates
comprise diameters from about 1 .mu.m to about 50 .mu.m.
29. The method of claim 1, further comprising a step of controlling
a thickness of the porous silicon film.
30. The method of claim 29, wherein the thickness of the porous
silicon film is controlled by adjusting one or more parameters
selected from the group consisting of electric current density
during electrochemical etching, resistivity of the silicon
substrate during electrochemical etching, concentration of
electrolyte etchants used during electrochemical or chemical
etching, temperature during electrochemical or chemical etching,
and combinations thereof.
31. An anode material comprising: porous silicon particulates,
wherein the porous silicon particulates comprise a plurality of
pores, wherein the plurality of pores comprise diameters between
about 1 nanometer to about 5 micrometers; a coating associated with
the porous silicon particulates; and a binding material associated
with the porous silicon particulates, wherein the binding material
is selected from the group consisting of binders, carbon materials,
polymers, metals, additives, carbohydrates, and combinations
thereof.
32. The anode material of claim 31, wherein the coating comprises a
metal coating.
33. The anode material of claim 32, wherein the metal is selected
from the group consisting of silver, copper, chromium, gold,
aluminum, tantalum, lead, zinc, silicon, and combinations
thereof.
34. The anode material of claim 32, wherein the metal is
silver.
35. The anode material of claim 31, wherein the binding material
comprises a polymer selected from the group consisting of
polyacrylonitrile (PAN), pyrolyzed polyacrylonitrile (PPAN),
polyvinylidene difluoride (PVDF), polyacrylic acid (PAA),
carboxymethyl cellulose (CMC), and combinations thereof.
36. The anode material of claim 31, wherein the binding material
comprises carbonized polyacrylonitrile.
37. The anode material of claim 31, wherein the plurality of pores
comprise diameters of at least about 50 nm, less than about 50 nm,
less than about 2 nm, and combinations thereof.
38. The anode material of claim 31, wherein the plurality of pores
comprise hierarchical pores.
39. The anode material of claim 31, wherein the hierarchical pores
comprise micropores and mesopores within macropores.
40. The anode material of claim 31, wherein the porous silicon
particulates have thicknesses ranging from about 10 micrometers to
about 200 micrometers.
41. The anode material of claim 31, wherein the porous silicon
particulates comprise pores that span at least 50% of a thickness
of the porous silicon particulates.
42. The anode material of claim 31, wherein the porous silicon
particulates comprise pores that span an entire thickness of the
porous silicon particulates.
43. The anode material of claim 31, wherein the porous silicon
particulates comprise diameters from about 1 .mu.m to about 50
.mu.m.
44. The anode material of claim 31, wherein the anode material has
a discharge capacity of at least about 600 mAh/g over at least 50
cycles.
45. The anode material of claim 31, wherein the anode material has
a discharge capacity of at least about 1000 mAh/g over at least 50
cycles.
46. The anode material of claim 31, wherein the anode material has
a Coulombic efficiency of at least about 90% over at least 50
cycles.
47. The anode material of claim 31, wherein the anode material is
utilized as part of a lithium ion battery.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application claims priority to U.S. Provisional Patent
Application No. 61/749,636, filed on Jan. 7, 2013. This application
is also related to U.S. patent application Ser. No. 13/589,588,
filed on Aug. 20, 2012, and International Application No.
PCT/US2010/054577, filed on Oct. 28, 2010. The entirety of each of
the aforementioned applications is incorporated herein by
reference.
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH
[0002] Not applicable
BACKGROUND
[0003] Current methods of making porous silicon particles suffer
from numerous limitations, including efficiency, quality,
electrochemical efficacy, and cost-effectiveness. Therefore, there
is currently a need for new methods to produce porous silicon
particles that address the aforementioned limitations.
SUMMARY
[0004] In some embodiments, the present disclosure pertains to
methods of preparing porous silicon particulates. In some
embodiments, the methods comprise: (a) electrochemically etching a
silicon substrate, where the electrochemical etching comprises
exposure of the silicon substrate to an electric current density,
and where the electrochemical etching produces a porous silicon
film over the silicon substrate; (b) separating the porous silicon
film from the silicon substrate, where the separating comprises a
gradual increase of the electric current density in sequential
increments; (c) repeating steps (a) and (b) a plurality of times;
(d) electrochemically etching the silicon substrate in accordance
with step (a) to produce a porous silicon film over the silicon
substrate; (e) chemically etching the porous silicon film and the
silicon substrate; and (f) splitting the porous silicon film and
the silicon substrate to form porous silicon particulates.
[0005] In some embodiments, the electrochemical etching comprises
the use of an acid, such as hydrofluoric acid. In some embodiments,
the electrochemical etching comprises exposure of the silicon
substrate to an electric current density of about 1 mA/cm.sup.2 to
about 10 mA/cm.sup.2. In some embodiments, the gradual increase of
the electric current density during the separating step comprises
an increase of the electric current density by about 1-2
mA/cm.sup.2 per sequential increment.
[0006] In some embodiments, the chemical etching occurs by exposure
of the porous silicon film and the silicon substrate to a metal
(including transition metals and metalloids). In some embodiments,
the metal is selected from the group consisting of silver, copper,
chromium, gold, aluminum, tantalum, lead, zinc, silicon, and
combinations thereof. In some embodiments, the exposure results in
coating of the porous silicon film and the silicon substrate with
the metal.
[0007] In some embodiments, the splitting occurs by at least one of
physical grinding, crushing, sonication, ultrasonication,
ultrasonic fracture, pulverization, ultrasonic pulverization, and
combinations thereof. In some embodiments, the splitting occurs by
ultrasonication.
[0008] In some embodiments, the methods of the present disclosure
further comprise a step of associating the formed porous silicon
particulates with a binding material. In some embodiments, the
binding material is selected from the group consisting of binders,
carbon materials, polymers, metals, additives, carbohydrates, and
combinations thereof. In some embodiments, the binding material
comprises a carbonized polyacrylonitrile.
[0009] In some embodiments, the methods of the present disclosure
also include a step of controlling a thickness of the porous
silicon film used to form the porous silicon particulates. In some
embodiments, the thickness of the porous silicon film is controlled
by adjusting one or more parameters selected from the group
consisting of electric current density during electrochemical
etching, resistivity of the silicon substrate during
electrochemical etching, concentration of electrolyte etchants used
during electrochemical or chemical etching, temperature during
electrochemical or chemical etching, and combinations thereof.
[0010] Further embodiments of the present disclosure pertain to
porous silicon particulates formed by the methods of the present
disclosure. Additional embodiments of the present disclosure
pertain to anode materials that contain the porous silicon
particulates of the present disclosure. In some embodiments, the
anode materials of the present disclosure have discharge capacities
of at least about 600 mAh/g over at least 50 cycles. In some
embodiments, the anode materials of the present disclosure have
discharge capacities of at least about 1000 mAh/g over at least 50
cycles. In some embodiments, the anode materials of the present
disclosure have Coulombic efficiencies of at least about 90% over
at least 50 cycles.
[0011] In some embodiments, the anode materials of the present
disclosure are utilized as components of energy storage devices,
such as batteries. In more specific embodiments, the anode
materials of the present disclosure are utilized as components of
lithium ion batteries.
BRIEF DESCRIPTION OF FIGURES
[0012] FIG. 1 provides a scheme of a method for making porous
silicon particulates (PSPs).
[0013] FIG. 2 provides illustrations of methods of making porous
silicon particulates. FIG. 2A provides an illustration of porous
silicon film formation from a silicon wafer at a current density of
1-10 mA/cm.sup.2 for 1-4 hours. FIGS. 2B-2C provide scanning
electron microscope (SEM) images for the top view (FIG. 2B) and the
side view of porous silicon films (FIG. 2C).
[0014] FIG. 3 provides SEM images of electrochemically and
chemically etched porous silicon particulates (FIG. 3A) and
chemically etched porous silicon particulates (FIG. 3B). Additional
images of electrochemically and chemically etched porous silicon
particulates are shown in FIGS. 3C-E.
[0015] FIG. 4 shows discharge capacity and efficiency vs. cycle
number of the porous silicon particulates of FIG. 3 during
galvanostatic charge/discharge studies. Discharge capacity (red
square, A) and coulombic efficiency (blue square, C) for
electrochemically and chemically etched porous silicon particulates
and discharge capacity (red triangle, B) and coulombic efficiency
(blue triangle, D) for chemically etched porous silicon
particulates are shown.
[0016] FIG. 5 provides discharge capacity and efficiency vs. cycle
number of porous silicon particulates when used as anodes along
with cathode materials (i.e., lithium cobalt oxide (LiCoO.sub.2))
during galvanostatic charge/discharge between 2.8-4V at a constant
charge capacity of 1000 mAhg.sup.-1.
DETAILED DESCRIPTION
[0017] It is to be understood that both the foregoing general
description and the following detailed description are illustrative
and explanatory, and are not restrictive of the subject matter, as
claimed. In this application, the use of the singular includes the
plural, the word "a" or "an" means "at least one", and the use of
"or" means "and/or", unless specifically stated otherwise.
Furthermore, the use of the term "including", as well as other
forms, such as "includes" and "included", is not limiting. Also,
terms such as "element" or "component" encompass both elements or
components comprising one unit and elements or components that
include more than one unit unless specifically stated
otherwise.
[0018] The section headings used herein are for organizational
purposes and are not to be construed as limiting the subject matter
described. All documents, or portions of documents, cited in this
application, including, but not limited to, patents, patent
applications, articles, books, and treatises, are hereby expressly
incorporated herein by reference in their entirety for any purpose.
In the event that one or more of the incorporated literature and
similar materials defines a term in a manner that contradicts the
definition of that term in this application, this application
controls.
[0019] Rechargeable batteries continue to draw attention because
energy storage devices with higher energy storage capabilities are
required for numerous applications. Researchers continue to focus
on the development of new electrode materials with higher
capacities and longer lifetimes for the major components of Li-ion
batteries: cathode and anode. Therefore, developing new electrode
materials with higher energy capacities can lead to significant
improvements in the performance and lifetimes of the rechargeable
batteries.
[0020] There are a number of new approaches that can be undertaken
to create rechargeable batteries (e.g., lithium ion batteries) with
higher energy capacities and longer life cycles. For instance, the
capacity of lithium ion batteries generally depends on the amount
of lithium (Li) ion an anode material can hold. A material that
reacts with lithium at low potential is silicon. Presently,
carbon-based materials (e.g. graphite) are utilized as anode
materials in most rechargeable batteries.
[0021] At room temperature, the highest achievable specific
capacity for silicon is 3579 mAhg.sup.-1, far greater than the
theoretical capacity of graphite (372 mAhg.sup.-1). However, when
silicon is lithiated, it undergoes a large volume expansion
(.about.300%). This in turn causes severe cracking of the silicon
and leads to electrode failure.
[0022] Many research groups have focused on exploring a variety of
silicon-based nanostructures, such as nanosized particles, thin
film, silicon nanowires, silicon nanotubes, core-cell nanowires,
porous silicon (PSi), and interconnected silicon hollow
nanospheres. Many of these structures have shown success in
addressing the mechanical breaking issues associated with
silicon.
[0023] Recently, composite materials of porous silicon and carbon
have also shown promising results. For instance, Bang et al. have
synthesized a macroporous silicon anode using silver (Ag)
nanoparticles as a template to chemically etch silicon particles,
and a thermal decomposition method to coat the anode with a carbon
layer (Advanced Energy Materials, 2012, 2:878-883). The material
demonstrated a capacity of 2050 mAhg.sup.-1 for fifty cycles.
Likewise, Kim et al. have synthesized mesoporous Si/carbon
core-shell nanowires as well as three dimensional (3-D) porous
silicon (c-Si) particles (Nano Letters, 2008, 8:3688-3691 and
Angewandte Chemie-International Edition, 2008, 47:10151-10154). Ge
et al. have also shown that silicon nanowires grown and then
scraped off from a substrate can be combined with an alginate
binder (Nano Letters, 2012, 12:2318-2323). They showed that this
form of silicon with high porosity and large pore sizes results in
materials with capacities over 1000 mAhg-.sup.1 for hundreds of
cycles.
[0024] Applicants have at least two pending patent applications on
electrochemically etched porous silicon materials. The first patent
application describes in some embodiments an electrochemically
etched porous silicon with metal coatings and a freestanding
macroporous silicon with pyrolyzed polyacrylonitrile (PPAN)
infiltration (International Application No. PCT/US2010/054577,
filed on Oct. 28, 2010). The second patent application describes in
some embodiments a macroporous silicon micro-particulate with PPAN
composite as an anode material for lithium ion batteries (U.S.
patent application Ser. No. 13/589,588, filed on Aug. 20,
2012).
[0025] Applicants have also found that binder free metal-coated
porous silicon with bulk silicon exhibits a higher capacity and
good cycle life than other forms of binder free silicon materials,
such as silicon nanowires. However, metal coatings may add to the
cost of the materials.
[0026] Another limitation of porous silicon films with bulk silicon
is that bulk silicon adds to the overall weight of the materials
without adding to the specific capacity. The bulk silicon substrate
can be removed by backside chemical etching processes. However,
such processes usually result in waste of useful silicon
materials.
[0027] To overcome the above limitations, Applicants have developed
a method of producing porous silicon films from a silicon substrate
by etching the silicon substrate through the application of current
densities (U.S. patent application Ser. No. 13/589,588). This
results in the formation of a porous silicon film over the silicon
substrate. The porous silicon film can then be separated from the
silicon substrate through a multi-step lift-off process that
applies higher current densities during etching. As such, multiple
films can be removed from a single wafer, thereby leading to less
silicon waste.
[0028] A limitation of porous silicon films produced by Applicants'
lift-off processes is that they may have limited processability in
various circumstances. To design a more processable material,
Applicants changed from a film structure to a particulate structure
that can be combined with PAN (or any other binders) to form
slurries that can be processed with standard coating technologies.
Though the aforementioned materials promise much higher specific
capacities and longer life cycles, the lift-off processes can lead
to cracking of the silicon substrate due to its brittle nature
before the lift-off of the layer. Therefore, the cracked silicon
substrate cannot be reused. This in turn leads to waste of the
silicon materials.
[0029] As such, current methods of making porous silicon
particulates have limitations that need to be addressed. Various
embodiments of the present disclosure address the aforementioned
limitations.
[0030] In some embodiments, the present disclosure pertains to
novel methods of preparing porous silicon particulates. In some
embodiments, the present disclosure pertains to anode materials
that include such porous silicon particulates.
Methods of Preparing Porous Silicon Particulates
[0031] In some embodiments, the present disclosure pertains to
methods of preparing porous silicon particulates. In some
embodiments illustrated in FIG. 1, the methods of the present
disclosure include: electrochemically etching a silicon substrate
to produce a porous silicon film over the silicon substrate (step
10); separating the porous silicon film from the silicon substrate
(step 12); repeating steps 10 and 12 a plurality of times;
electrochemically etching the silicon substrate in accordance with
step 10 to produce a porous silicon film over the silicon substrate
(step 14); chemically etching the porous silicon film and the
silicon substrate (step 16); and splitting the porous silicon film
and the silicon substrate to form porous silicon particulates (step
18). In some embodiments, the methods of the present disclosure
also include a step of associating the porous silicon particulates
with a binding material (step 20). In some embodiments, the methods
of the present disclosure also include a step of controlling the
thickness of the porous silicon films that are used to make the
porous silicon particulates.
[0032] As set forth in more detail herein, the methods of the
present dislcosure can have numerous embodiments. For instance,
various silicon substrates, binding materials, electrochemical
etching techniques, porous film separation techniques, chemical
etching techniques, and splitting techniques may be utilized to
form various types of porous silicon particulates.
[0033] Silicon Substrates
[0034] The methods of the present disclosure may utilize various
types of silicon substrates. For instance, in some embodiments, the
silicon substrates may include bulk silicon substrates. In some
embodiments, the silicon substrates include crystalline silicon,
semicrystalline silicon, amorphous silicon, doped silicon, coated
silicon, silicon pre-coated with silicon nanoparticles, and
combinations thereof.
[0035] In some embodiments, the silicon substrate is a silicon
wafer. In some embodiments, the silicon substrate is a crystalline
silicon wafer. In some embodiments, the silicon substrate is a
doped silicon wafer. In some embodiments, the silicon substrate is
a silicon wafer doped with boron, phosphorous, arsenic, antimony,
other dopants, and combinations thereof. In some embodiments, the
silicon substrate is a p-type silicon wafer, an n-type silicon
wafer, and combinations thereof. In some embodiments, the silicon
substrate may be an n-doped or a boron doped silicon wafer. The use
of additional silicon substrates can also be envisioned.
[0036] Electrochemical Etching of Silicon Substrate
[0037] Various methods may also be utilized to electrochemically
etch silicon substrates. In some embodiments, the electrochemical
etching produces a porous silicon film over the silicon substrate.
In some embodiments, the electrochemical etching may include the
use of one or more strong acids, such as nitric acid (HNO.sub.3),
hydrofluoric acid (HF), sulfuric acid (H.sub.2SO.sub.4),
hydrochloric acid (HCl), and combinations thereof. In more specific
embodiments, the electrochemical etching of the silicon substrate
occurs in the presence of hydrofluoric acid. In some embodiments,
the electrochemical etching of the silicon substrate occurs in the
presence of hydrofluoric acid in dimethylformamide (DMF).
[0038] In some embodiments, the electrochemical etching occurs in
the presence of an applied electric field, such as an electric
field with a constant electric current density. In some
embodiments, electrochemical etching includes exposure of the
silicon substrate to an electric current density. In some
embodiments, the etching occurs by the use of a strong acid (e.g.,
HF) in the presence of an applied electric field.
[0039] In some embodiments, the applied electric field may contain
various levels of electric current densities. In some embodiments,
the electric current density is from about 0.5 mA/cm.sup.2 to about
50 mA/cm.sup.2. In some embodiments, the electric current density
is from about 1 mA/cm.sup.2 to about 10 mA/cm.sup.2. In some
embodiments, the maximum electric current density is about 20
mA/cm.sup.2. In some embodiments, the electric current density is
applied to a silicon substrate in an electrochemical cell.
[0040] During electrochemical etching, an electric current density
may be applied to silicon substrates in one or more increments. In
some embodiments, the etching process may include from 1 increment
to about 10 increments. In some embodiments, the electric current
density may be from about 1 mA/cm.sup.2 to about 20 mA/cm.sup.2 per
increment. In some embodiments, each increment may last from about
30 seconds to about 60 minutes. In some embodiments, each increment
may last for about 10 minutes. In some embodiments, the increments
may be separated by intervals. In some embodiments, the intervals
may be from about 30 seconds to about 30 minutes.
[0041] In addition, silicon substrates may be exposed to various
current densities for various periods of time. For instance, in
some embodiments, electrochemical etching occurs for about 3 hours
to about 5 hours. In more specific embodiments, electrochemical
etching occurs by exposure of silicon substrates to electric
current densities of 1 mA/cm.sup.2 to 10 mA/cm.sup.2 for about 1
hour to about 4 hours.
[0042] Separating Porous Silicon Film from Silicon Substrate
[0043] Various methods may also be utilized to separate the formed
porous silicon films from silicon substrates (also referred to as a
"lift-off" procedure). In various embodiments, such separation
steps can occur during or after electrochemical etching.
[0044] In some embodiments, the separating includes a gradual
increase of the electric current density in sequential increments
until the porous silicon film has been separated from the silicon
substrate. As used herein, a gradual increase in electric current
density generally refers to a stepwise increase in electric current
density over several sequential increments. For instance, in some
embodiments, the electric current density may increase gradually in
at least 5-10 sequential increments that may last from about 30
seconds to 60 minutes per increment. In some embodiments, the
gradual increase in electrical current density may occur through at
least 5 to 10 sequential increments that may be separated by
intervals of about 30 seconds to 60 minutes per increment.
[0045] In some embodiments, the applied electric current density
may be from about 0.5 mA/cm.sup.2 to about 50 mA/cm.sup.2. In some
embodiments, the electric current density may gradually increase
from about 1 mA/cm.sup.2 to about 2 mA/cm.sup.2 per increment. In
some embodiments, the maximum electric current density may be about
15 mA/cm.sup.2. In some embodiments, the electric current density
may gradually increase in small increments of 1 mA/cm.sup.2 at
10-60 minutes per increment for up to 15 mA/cm.sup.2. In some
embodiments, the electric current density may gradually increase in
13 sequential increments by at least about 1 mA/cm.sup.2 per
increment for up to 15 mA/cm.sup.2. In some embodiments, the
electric current density may gradually increase in small increments
of 0.5 mA/cm.sup.2 at 1-2 hours per increment for up to 20
mA/cm.sup.2.
[0046] Without being bound by theory, it is envisioned that the
aforementioned "lift off" procedures may occur through various
mechanisms. For instance, in some embodiments that utilize HF as an
electrochemical etchant, it is envisioned that, as the depth of the
pores in the formed porous silicon films increases, the
availability of fluoride ions at the pore tip decreases. Such a
decrease may in turn lead to isotropic etching at the tip of the
pores, thereby resulting in a layer of silicon that is more porous
at the point of contact with the silicon substrate. See, e.g., FIG.
2A. In this embodiment, it is also envisioned that the hydrogen
byproduct accumulates and starts to exert a hydrodynamic pressure
onto the walls of the pores. At some point, the pore walls may not
be able to withstand this hydrodynamic pressure. This in turn may
lead to separation of the porous silicon film from the silicon
substrate.
[0047] The separation or "lift off" procedures of the present
disclosure may also include additional steps. For instance, in some
embodiments, separation steps may also include a step of physically
removing the formed porous silicon film from the silicon substrate.
In some embodiments, the physical removal may occur by the use of a
razor blade, a tweezer, or other objects. In some embodiments, the
physical removal may occur by a rinsing step or a washing step.
[0048] Repetition of Electrochemical Etching and Separation
Steps
[0049] In some embodiments, the electrochemical etching and
separation steps are repeated a plurality of times. For instance,
in some embodiments, the electrochemical etching and separation
steps are repeated more than 5 times. In some embodiments, the
electrochemical etching and separation steps are repeated more than
10 times. In some embodiments, the electrochemical etching and
separation steps are repeated until the porous silicon film becomes
inseparable from the silicon substrate. In some embodiments, the
electrochemical etching and separation steps are repeated until the
silicon substrate develops one or more cracks.
[0050] Chemical Etching of the Porous Silicon Film and Silicon
Substrate
[0051] After repeating the steps of electrochemical etching and
porous silicon film separation for a desired number of times, the
methods of the present disclosure can include a final step of
electrochemically etching the silicon substrate to produce a porous
silicon film over the silicon substrate. Thereafter, the porous
silicon film and the silicon substrate may be chemically
etched.
[0052] Various methods may also be utilized to chemically etch the
porous silicon films and the silicon substrates of the present
disclosure. For instance, in some embodiments, the chemical etching
occurs by exposure of the porous silicon film and the silicon
substrate to a metal (including transition metals and metalloids).
In some embodiments, the metal includes at least one of silver,
copper, chromium, gold, aluminum, tantalum, lead, zinc, silicon and
combinations thereof. In some embodiments, the metal is silver. In
more specific embodiments, the metal includes silicon, such as
silicon nitride, silicon oxide, and combinations thereof.
[0053] The exposure of the porous silicon films and the silicon
substrates of the present disclosure to a metal can have various
effects. For instance, in some embodiments, the exposure results in
coating of the porous silicon film and the silicon substrate with
metals. In some embodiments, the coating may be uniform and
homogenous. In some embodiments, the exposure may result in the
partial coating of the porous silicon film and the silicon
substrate with metals. In some embodiments, the exposure may result
in the full coating of the porous silicon film and the silicon
substrate with metals. In some embodiments, the porous silicon film
and the silicon substrate may become infiltrated with or embedded
with the metals.
[0054] Splitting of the Porous Silicon Film and the Silicon
Substrate
[0055] In some embodiments, a chemical etching step is followed by
splitting the porous silicon film and the silicon substrate to form
porous silicon particulates. Various splitting methods may be
utilized for such purposes. For instance, in some embodiments, the
splitting occurs by at least one of physical grinding, crushing,
sonication, ultrasonication, ultrasonic fracture, pulverization,
ultrasonic pulverization, and combinations thereof. In more
specific embodiments, the splitting occurs by ultrasonication.
[0056] Association of Porous Silicon Particulates with Binding
Materials
[0057] In some embodiments, the porous silicon particulates may
also be associated with one or more binding materials. In various
embodiments, the association may occur prior to, during, or after
porous silicon particulate formation.
[0058] Binding materials generally refer to materials that may
enhance the electric conductivity or stability of porous silicon
films. In some embodiments, the binding materials may include at
least one of binders, carbon materials, polymers, metals,
additives, carbohydrates, and combinations thereof.
[0059] In some embodiments, the binding materials may include a
polymer. In some embodiments, the polymer may include at least one
of polyacrylonitrile (PAN), pyrolyzed polyacrylonitrile (PPAN),
polyvinylidene difluoride (PVDF), polyacrylic acid (PAA),
carboxymethyl cellulose (CMC), and combinations thereof. In some
embodiments, the polymers may be in polymerized form prior to
association with porous silicon particulates. In some embodiments,
the polymers may polymerize during or after association with porous
silicon particulates.
[0060] In some embodiments, the binding material is an additive. In
some embodiments, the additive is sodium alginate.
[0061] In some embodiments, the binding materials may include one
or more metals. In some embodiments, the metals may include,
without limitation, gold, copper, silver, titanium, iron, and
combinations thereof.
[0062] In some embodiments, the binding materials may include one
or more carbon materials. Non-limiting examples of suitable carbon
materials include carbon nanotubes, carbon black, graphite, carbon
fibers, carbon nanofibers, graphene sheets, fullerenes, graphene
platelets, sodium alginate binders associated with carbon black,
carbohydrates, and combinations thereof. In some embodiments, the
binding material includes a carbohydrate. In some embodiments, the
carbohydrate is glucose.
[0063] In addition, various methods may be used to associate
binding materials with porous silicon particulates. In some
embodiments, the association may occur by sputtering, spraying, or
physically applying the one or more binding materials onto the
porous silicon particulates. In some embodiments, the association
may occur by dipping the porous silicon particulates into a
solution containing one or more binding materials.
[0064] In some embodiments, the association may result in the
partial coating of the porous silicon particulates with a binding
material. In some embodiments, the association may result in the
full coating of the porous silicon particulates with a binding
material. In some embodiments, the porous silicon particulates may
become infiltrated with, embedded with or dispersed in the binding
materials.
[0065] In some embodiments, the binding materials that are
associated with porous silicon particulates may be in carbonized
form. In some embodiments, the binding materials may become
carbonized before, during, or after association with the porous
silicon particulates. In some embodiments, the binding materials
may become carbonized by pyrolysis before, during or after
association with porous silicon particulates. In more specific
embodiments, the binding materials may include PAN that has been
carbonized by pyrolysis after association with porous silicon
particulates. In some embodiments, pyrolysis may occur by heating
porous silicon particulates at high temperatures (e.g., 550.degree.
C.) in the presence of an inert gas (e.g., Argon).
[0066] In some embodiments, the binding material includes a
carbonized polyacrylonitrile. An advantage of using carbonized PAN
as a binding material is that it forms conjugated carbon chains
upon carbonization. This in turn can enhance the electrical
properties of the porous silicon particulates.
[0067] Control of Thickness of Porous Silicon Films
[0068] In some embodiments, the methods of the present disclosure
also include a step of controlling a thickness of the porous
silicon films that are used to form the porous silicon
particulates. Various methods may also be utilized to control the
thickness of the porous silicon films. For instance, in some
embodiments, the thickness of the porous silicon films is
controlled by adjusting one or more parameters. In some
embodiments, the controllable parameters include at least one of
electric current density during electrochemical etching,
resistivity of the silicon substrate during electrochemical
etching, concentration of electrolyte etchants used during
electrochemical or chemical etching, placement of the electrode,
process temperature, temperature during electrochemical or chemical
etching, and combinations thereof.
[0069] Formed Porous Silicon Particulates
[0070] The methods of the present disclosure may be utilized to
form various types of porous silicon particulates. For instance, in
some embodiments, the formed porous silicon particulates include a
plurality of pores. In some embodiments, the pores include various
diameters. In some embodiments, the pores of the porous silicon
particulates include diameters between about 1 nanometer to about 5
micrometers. In some embodiments, the pores include macropores with
diameters of at least about 50 nm. In some embodiments, the pores
include macropores with diameters between about 50 nanometers to
about 3 micrometers. In some embodiments, the pores include
macropores with diameters between about 500 nanometers to about 2
micrometers. In some embodiments, the pores include mesopores with
diameters of less than about 50 nm. In some embodiments, the pores
include micropores with diameters of less than about 2 nm.
[0071] By way of background, porous materials have been classified
according to their pore diameters. For instance, micropores are
those with diameters less than 2 nm. Mesopores have diameters that
range from 2 nm to 50 nm. Macropores have diameters that are
greater than 50 nm. In further embodiments, the pores in the formed
porous silicon particulates may include various combinations of
micropores, mesopores and macropores. For instance, in some
embodiments, the porous silicon particulates include hierarchical
pores. In some embodiments, the hierarchical pores include
micropores and mesopores within macropores.
[0072] The pores in the formed porous silicon particulates can also
have various arrangements. For instance, in some embodiments, the
formed porous silicon particulates include pores that span at least
50% of a thickness of the porous silicon particulates. In some
embodiments, the formed porous silicon particulates include pores
that span an entire thickness of the porous silicon
particulates.
[0073] The formed porous silicon particulates can also have various
thicknesses. For instance, in some embodiments, the formed porous
silicon particulates have thicknesses ranging from about 10
micrometers to about 200 micrometers. In more specific embodiments,
the formed porous silicon particulates have thicknesses ranging
from about 10 micrometers to about 50 micrometers.
[0074] The formed porous silicon particulates can also have various
diameters. For instance, in some embodiments, the porous silicon
particulates include diameters from about 1 .mu.m to about 50
.mu.m. In some embodiments, the porous silicon particulates include
diameters from about 10 .mu.m to about 20 .mu.m.
[0075] The porous silicon particulates of the present disclosure
can also have various electrical properties. For instance, in some
embodiments, the porous silicon particulates of the present
disclosure have discharge capacities of at least about 600 mAh/g
over numerous cycles, such as at least 20 cycles, at least 40
cycles, at least 50 cycles, at least 60 cycles, at least 80 cycles,
at least 100 cycles, at least 120 cycles, at least 140 cycles, at
least 160 cycles, at least 180 cycles, at least 200 cycles, or at
least 220 cycles. In more specific embodiments, the porous silicon
particulates of the present disclosure have discharge capacities of
at least about 1,000 mAh/g over numerous cycles, such as at least
20 cycles, at least 40 cycles, at least 50 cycles, at least 60
cycles, at least 80 cycles, at least 100 cycles, at least 120
cycles, at least 140 cycles, at least 160 cycles, at least 180
cycles, at least 200 cycles, or at least 220 cycles. In some
embodiments, the porous silicon particulates of the present
disclosure have Coulombic efficiencies of at least about 90% over
numerous cycles, such as at least 20 cycles, at least 40 cycles, at
least 50 cycles, at least 60 cycles, at least 80 cycles, at least
100 cycles, at least 120 cycles, at least 140 cycles, at least 160
cycles, at least 180 cycles, at least 200 cycles, or at least 220
cycles.
Anode Materials
[0076] Further embodiments of the present disclosure pertain to
anode materials. In some embodiments, the anode materials include
the porous silicon particulates of the present disclosure. In more
specific embodiments, the anode materials of the present disclosure
include: (1) porous silicon particulates with a plurality of pores;
(2) a coating associated with the porous silicon particulates; and
(3) a binding material associated with the porous silicon
particulates.
[0077] The porous silicon particulates in the anode materials of
the present disclosure can have various types of pores. For
instance, in some embodiments, the pores include diameters between
about 1 nanometer to about 5 micrometers. In some embodiments, the
pores include diameters of at least about 50 nm. In some
embodiments, the pores include diameters of less than about 50 nm.
In some embodiments, the pores include diameters of less than about
2 nm. In some embodiments, the porous silicon particulates in the
anode materials include hierarchical pores. In some embodiments,
the hierarchical pores include micropores and mesopores within
macropores.
[0078] In some embodiments, the porous silicon particulates include
pores that span at least 50% of a thickness of the porous silicon
particulates. In some embodiments, the porous silicon particulates
include pores that span an entire thickness of the porous silicon
particulates. In some embodiments, the porous silicon particulates
have thicknesses ranging from about 10 micrometers to about 200
micrometers.
[0079] The porous silicon particulates in the anode materials of
the present disclosure may also be associated with various types of
coatings. For instance, in some embodiments, the porous silicon
particulates may be associated with metal coatings. In some
embodiments, the metal coatings may include, without limitation,
silver, copper, chromium, gold, aluminum, tantalum, lead, zinc,
silicon, and combinations thereof. In more specific embodiments,
the metal coating is silver.
[0080] The porous silicon particulates in the anode materials of
the present disclosure may also be associated with various types of
binding materials. For instance, in some embodiments, the binding
materials may include at least one of binders, carbon materials,
polymers, metals, additives, carbohydrates, and combinations
thereof. In some embodiments, the binding materials may include
polymers. In some embodiments, the polymers may include at least
one of polyacrylonitrile (PAN), pyrolyzed polyacrylonitrile (PPAN),
polyvinylidene difluoride (PVDF), polyacrylic acid (PAA),
carboxymethyl cellulose (CMC), and combinations thereof. In more
specific embodiments, the binding materials may include carbonized
polyacrylonitriles, carbohydrate (e.g., glucose), additives (e.g.,
sodium alignate), and combinations thereof.
[0081] The porous silicon particulates in the anode materials of
the present disclosure may also have various diameters. For
instance, in some embodiments, the porous silicon particulates
include diameters from about 1 .mu.m to about 50 .mu.m.
[0082] The anode materials of the present disclosure can also have
various electrical properties. For instance, in some embodiments,
the anode materials of the present disclosure have discharge
capacities of at least about 600 mAh/g over numerous cycles, such
as at least 20 cycles, at least 40 cycles, at least 50 cycles, at
least 60 cycles, at least 80 cycles, at least 100 cycles, at least
120 cycles, at least 140 cycles, at least 160 cycles, at least 180
cycles, at least 200 cycles, or at least 220 cycles. In more
specific embodiments, the anode materials of the present disclosure
have discharge capacities of at least about 1,000 mAh/g over
numerous cycles, such as at least 20 cycles, at least 40 cycles, at
least 50 cycles, at least 60 cycles, at least 80 cycles, at least
100 cycles, at least 120 cycles, at least 140 cycles, at least 160
cycles, at least 180 cycles, at least 200 cycles, or at least 220
cycles. In some embodiments, the anode materials of the present
disclosure have Coulombic efficiencies of at least about 90% over
numerous cycles, such as at least 20 cycles, at least 40 cycles, at
least 50 cycles, at least 60 cycles, at least 80 cycles, at least
100 cycles, at least 120 cycles, at least 140 cycles, at least 160
cycles, at least 180 cycles, at least 200 cycles, or at least 220
cycles.
[0083] The anode materials of the present disclosure may also be
associated with various types of energy storage devices. For
instance, in some embodiments, the anode materials of the present
disclosure may be associated with batteries. In more specific
embodiments, the anode materials of the present disclosure may be
associated with lithium ion batteries.
Applications and Advantages
[0084] In the present disclosure, Applicants have developed novel
processes that can be utilized to generate large quantities of
porous silicon particulates in a cost effective and efficient
manner. Furthermore, the porous silicon particulates of the present
disclosure have various advantageous properties, such as enhanced
discharge capacities and enhanced Coulombic efficiencies over
numerous cycles. As such, the methods and porous silicon
particulates of the present disclosure can find numerous
applications.
[0085] For instance, in some embodiments, the porous silicon
particulates of the present disclosure can be utilized as anode
materials for various types of energy storage devices in numerous
fields, including the defense industry, the automotive industry,
the renewable energy industry, the aerospace industry, the
telecommunication industry, information technology, consumer
electronics, implantable devices, and electric vehicles. In more
specific embodiments, the porous silicon particulates of the
present disclosure can be utilized as anode materials in batteries,
such as lithium ion batteries.
[0086] In fact, Applicants envision that the methods and porous
silicon particulates of the present disclosure can improve the
performance and lower the cost of high performance anode materials
in many energy storage devices, such as lithium ion batteries. For
instance, batteries that contain the porous silicon particulates of
the present disclosure have potential discharge capacities up to an
order of magnitude higher than today's lithium ion batteries. As
such, Applicants envision that batteries containing the porous
silicon particulates of the present disclosure can provide optimal
cycleability and capacities of 1000 mAhg.sup.-1 for hundreds of
cycles.
[0087] In more specific embodiments, the methods and porous silicon
particulates of the present disclosure can provide additional
advantages and applications, including use as improved anode
materials for lithium ion batteries; use for development of lithium
ion batteries with improved cycling behavior and high capacity,
which can be 1000 mAhg.sup.-1 for more than 200 cycles; use as low
cost methods for manufacturing anodes for lithium ion batteries;
use as reproducible methods for making anode battery materials; and
use for development of lithium ion batteries with substantially
higher discharge capacities than current batteries.
Additional Embodiments
[0088] Reference will now be made to more specific embodiments of
the present disclosure and experimental results that provide
support for such embodiments. However, Applicants note that the
disclosure below is for illustrative purposes only and is not
intended to limit the scope of the claimed subject matter in any
way.
EXAMPLE 1
Generation of Hierarchical Porous Silicon Particulates
[0089] This Example illustrates a combined electrochemical/chemical
etching process to generate porous silicon micron size particulates
as an anode for lithium ion batteries.
[0090] In this Example, the silicon wafer is first
electrochemically etched to a depth of a few hundreds of microns.
Next, the porous film is electrochemically lifted-off. This process
is repeated until the remaining wafer is thinned and begins
cracking. Once the wafer starts cracking, the wafer is chemically
etched and crushed.
[0091] Applicants have tested these electrochemically/chemically
etched porous silicon particulates as anode materials for lithium
ion batteries. To compare the results, Applicants have done
controlled experiments. Initially, a porous silicon film is formed
by electrochemically etching at room temperature with constant
current density of 1-5 mA/cm.sup.2 for 3-5 hour, resulting in a
wafer composed of a porous silicon layer with a thickness of 10-200
.mu.m. Next, chemical etching is performed by placing the
previously electrochemically etched wafer into solution silver
nitrate/hydrofluoric acid solution that is in a 1:10 ratio by
volume for 1-10 minutes. The wafer is transferred to a chemical
etchant solution (10 ml of HF and 0.1 ml of 30% hydrogen peroxide
(H.sub.2O.sub.2)) for 10-120 minutes. The
electrochemically/chemically etched wafers are ultrasonically
crushed into a particulate format. FIGS. 3A-B show the scanning
electron microscopic (SEM) images for electrochemically/chemically
etched (FIG. 3A) and chemically etched (FIG. 3B) porous silicon
particulates. Additional images of the electrochemically/chemically
etched porous silicon particulates are shown in FIGS. 3C-E.
[0092] FIG. 4 shows the cycle performance of the
electrochemically/chemically etched porous silicon particulates in
comparison to the chemically etched porous silicon particulates
(PSP) (controlled). The mass of the anode materials was 1.5
mg/cm.sup.2. The anode materials are mixed with the
Polyacrylonitrile (PAN) in a ratio of 7:3 and coated on the
stainless steel foil. The coated porous silicon particulates/PAN
composite are pyrolyzed at 550.degree. C. at argon atmosphere. Both
the materials are charged/discharged at 500 mAcm.sup.-2 between 0-1
V at a constant charge capacity of 1000 mAhg.sup.-1. As suggested
by Obrovac et al. (Journal of the Electrochemical Society, 2007,
154:A103-A108), the volume expansion of the silicon can be control
by limiting the intercalation of the lithium into the silicon. Cui
et al. (Nano Letters, 2009, 9:491-495) also found that limiting the
intercalation of the silicon between 30-50% of the maximum specific
capacity resulted in extended life cycle, and that charging silicon
microparticles and nanoparticles at constant charge capacity
increased the life cycle of the anode.
[0093] To control the intercalation of lithium ion to Applicants'
material, Applicants fixed the charge capacity in the
electrochemical tests at 1000 mAhg.sup.-1. Applicants observed that
the maintenance of the charge capacity greatly improved the number
of useful cycles in the cell. For the controlled experiment,
Applicants saw an increase in the capacity for first few cycles,
but it was not able to reach the cutoff charge capacity (1000
mAhg.sup.-1). The increase in the capacity for first few cycle is
similar to Applicants' previous porous silicon studies, where the
increase in the capacity is due to the phase transfer of the
crystalline silicon to the amorphous silicon (Journal of Power
Sources, 2012, 205:426-432). FIG. 4 shows that the electrochemical
performance of electrochemically/chemically etched porous silicon
particulates is much better as compared to the chemically etched
porous silicon particulates. Without being bound by theory, it is
envisioned that the increase in the capacity of the
electrochemically/chemically etched porous silicon particulates in
comparison to the chemically etched porous silicon particulates is
due to more pores on the surfaces and walls of the silicon
particulates. Applicants also envision that the increase in the
capacity of the electrochemically/chemically etched porous silicon
particulates in comparison to the chemically etched porous silicon
particulates is also due to different pore geometries on the
silicon particulates, such as macropores (>50 nm), mesopores
(<50 nm) and micropores (<2 nm). See, e.g., FIG. 2A.
[0094] The electrochemically/chemically etched porous silicon
particulates were also tested in the full cell by using lithium
cobalt oxide (LiCoO.sub.2) as a cathode material. The mass of the
anode is 0.001 g/cm.sup.2, and the mass of the cathode material
(LiCoO.sub.2 with carbon black and Polyvinylidene fluoride (PVDF))
is 0.002 g/cm.sup.2. The capacity of the full cell was calculated
based on the mass of the anode materials. FIG. 5 shows the cycle
performance of the full cell.
Example 1.1
Electrochemical Etching
[0095] The porous silicon was synthesized via electrochemical
etching of silicon wafer using a multistep lift-off process. The
thickness of the porous silicon films can be modified by
controlling the etching parameters such as applied current, wafer
resistivity, concentration of electrolyte and doping of the wafer.
In porous silicon, prime grade, boron doped, p-type (100) silicon
wafers (Siltronix Corp, silicon sense and silicon quest) were used.
The wafer presented has a thickness of 275.+-.25 .mu.m with an
average resistivity between 14-22 ohm-cm and 10-30 ohm-cm. To
fabricate porous silicon, pores are etched into the wafers at a
constant current density delivered by an Agilent power supply
(E3612A) at room temperature. The etching solution is composed of
20-30 mL dimethylformamide (DMF, Sigma Aldrich) and 2-4 mL 49% HF
(Fisher Scientific) solution. The formation of the pores takes
place when the number of fluoride ions was greater than the number
of holes ([F-]>[h+]). The porous silicon etched can have an
average diameter of 500 nm-2 .mu.m and a depth between 10 .mu.m-200
.mu.m depending on etching time. Initially, a porous silicon film
is formed by etching at room temperature with constant current
density of 1-5 mA/cm.sup.2 for 3-5 hours. This results in the
formation of a porous silicon layer with a thickness of 10
.mu.m-200 .mu.m.
[0096] This above etching conditions generated the porous silicon
films shown in FIG. 2A (right panel). The SEM images for the top
and side views of the porous silicon films are shown in FIGS. 2B
and 2C, respectively.
Example 1.2
Electrochemical Liftoff of Porous Silicon Films
[0097] The formed porous silicon films were lifted off from the
silicon substrate multiple times by increasing the current density
during the electrochemical etching process. The silicon wafers used
have a thickness of 275.+-.25 .mu.m with an average resistivity
between 1-20 ohm-cm. The etching solution is composed of
dimethylformamide/49% HF solution in a ratio of 10:1. A porous
silicon film is formed by etching the wafers at room temperature
with constant current density of 1-5 mA/cm.sup.2 for 3-5 hours.
Once the silicon substrate started cracking during the
electrochemical etching, Applicants were not able to lift-off the
porous silicon film layer.
Example 1.3
Chemical Etching and Splitting
[0098] Chemical etching was performed on the cracked silicon
substrate containing porous silicon film by putting the porous
substrate into 1-10 ml of hydrofluoric acid (HF) and 0.1-1 ml of
silver nitrate (AgNO.sub.3) at room temperature for 1-10 minutes.
This resulted in the coating of the silicon substrate and the
porous silicon film on the silicon substrate with silver particles.
After the silver coating, the porous silicon film and the cracked
silicon substrate were kept in a chemical etchant (10 ml of HF and
0.1 ml of 30% hydrogen peroxide (H.sub.2O.sub.2)) for 10-120
minutes.
[0099] Next, the electrochemically/chemically etched porous silicon
film and silicon substrate were placed in a DMF solution and then
put into a commercial Branson Ultrasound sonicator for 30 minutes
and ultrasonically crushed into a powder to form porous silicon
particulates. FIGS. 3C-E shows the SEM images of the
electrochemically/chemically etched porous silicon
particulates.
Example 1.4
Battery Testing of Porous Silicon Particulates
[0100] Two electrodes and three electrode cells (Hosen Test cell,
Hohsen Corp. Japan) were used for all electrochemical measurements.
A working electrode was prepared by drop casting PAN and
electrochemically/chemically etched porous silicon particulates on
stainless steel. The composition was pyrolyzed at 550.degree. C. in
an Argon atmosphere. Lithium foil (0.75 mm thick, Alfa Aesar) was
used as a counter-electrode in half cell configurations. Lithium
cobalt oxide (LiCoO.sub.2) was used in full cell configurations. A
trilayer polypropylene membrane (Celgard 2325) wetted with an
electrolyte was used as a separator. The electrolyte used was 1 M
LiPF.sub.6 in a 1:1 ratio w/w ethylene carbonate: diethyl carbonate
(Ferro Corporation) or a 1:1 ratio w/w FEC (Ferro Corporation):
dimethyl carbonate (Sigma Aldrich). The anode material was not
exposed to air before assembling into the cell. All the cells were
assembled in an argon-filled glove box (<5 ppm of oxygen and
water, Vacuum Atmospheres Co.). The electrochemical testing is
performed using an Arbin Instruments BT2000. Applicants' anode
material is charged and discharged between 0-1 V versus Li/Li+ at
C/3 and C/2 rates for constant charge capacity (CCC) of 1000
mAhg.sup.-1. The Coulombic efficiency (delithiation
capacity/lithiation capacity) was calculated to be nearly 100%.
[0101] Without further elaboration, it is believed that one skilled
in the art can, using the description herein, utilize the present
disclosure to its fullest extent. The embodiments described herein
are to be construed as illustrative and not as constraining the
remainder of the disclosure in any way whatsoever. While the
embodiments have been shown and described, many variations and
modifications thereof can be made by one skilled in the art without
departing from the spirit and teachings of the invention.
Accordingly, the scope of protection is not limited by the
description set out above, but is only limited by the claims,
including all equivalents of the subject matter of the claims. The
disclosures of all patents, patent applications and publications
cited herein are hereby incorporated herein by reference, to the
extent that they provide procedural or other details consistent
with and supplementary to those set forth herein.
* * * * *