U.S. patent number 10,403,889 [Application Number 15/520,526] was granted by the patent office on 2019-09-03 for high-capacity silicon nanowire based anode for lithium-ion batteries.
This patent grant is currently assigned to RAMOT AT TEL-AVIV UNLVERSITY LTD.. The grantee listed for this patent is RAMOT AT TEL-AVIV UNIVERSITY LTD.. Invention is credited to Guy Davidi, Kathrin Freedman, Diana Golodnitsky, Fernando Patolsky, Emanuel Peled, Dan Schneier.
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United States Patent |
10,403,889 |
Peled , et al. |
September 3, 2019 |
High-capacity silicon nanowire based anode for lithium-ion
batteries
Abstract
The present invention provides anodes comprising an electrically
conductive substrate, comprising at least one non-uniform surface;
and a random network of silicon nanowires (Si NWs) chemically grown
on said at least one non-uniform surface of the substrate, wherein
the Si NWs have at least about 30% amorphous morphology, and
methods of manufacturing of the anodes. Further provided are
lithium ion batteries comprising said anodes.
Inventors: |
Peled; Emanuel (Even Yehuda,
IL), Patolsky; Fernando (Rehovot, IL),
Golodnitsky; Diana (Rishon Letzion, IL), Freedman;
Kathrin (Tsoran, IL), Davidi; Guy (Even Yehuda,
IL), Schneier; Dan (Haifa, IL) |
Applicant: |
Name |
City |
State |
Country |
Type |
RAMOT AT TEL-AVIV UNIVERSITY LTD. |
Tel Aviv |
N/A |
IL |
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Assignee: |
RAMOT AT TEL-AVIV UNLVERSITY
LTD. (Tel Aviv, IL)
|
Family
ID: |
55760380 |
Appl.
No.: |
15/520,526 |
Filed: |
October 21, 2015 |
PCT
Filed: |
October 21, 2015 |
PCT No.: |
PCT/IL2015/051037 |
371(c)(1),(2),(4) Date: |
April 20, 2017 |
PCT
Pub. No.: |
WO2016/063281 |
PCT
Pub. Date: |
April 28, 2016 |
Prior Publication Data
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Document
Identifier |
Publication Date |
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US 20170309903 A1 |
Oct 26, 2017 |
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Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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62066379 |
Oct 21, 2015 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H01M
4/70 (20130101); H01M 4/0428 (20130101); H01M
4/366 (20130101); H01M 4/667 (20130101); H01M
4/134 (20130101); H01M 4/386 (20130101); H01M
4/663 (20130101); C23C 18/44 (20130101); H01M
10/052 (20130101); H01M 4/80 (20130101); H01M
4/1395 (20130101); H01M 4/661 (20130101); B82Y
30/00 (20130101); Y02T 10/7011 (20130101); Y02T
10/70 (20130101) |
Current International
Class: |
H01M
4/38 (20060101); H01M 4/04 (20060101); H01M
4/134 (20100101); H01M 4/1395 (20100101); C23C
18/44 (20060101); H01M 10/052 (20100101); H01M
4/36 (20060101); H01M 4/66 (20060101); H01M
4/70 (20060101); H01M 4/80 (20060101); B82Y
30/00 (20110101) |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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103000865 |
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Mar 2013 |
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CN |
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2579365 |
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Apr 2013 |
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EP |
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201248976 |
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Jun 2009 |
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TW |
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2010052704 |
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May 2010 |
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WO |
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2010138617 |
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Dec 2010 |
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WO |
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2012080252 |
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Jun 2012 |
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WO |
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2013052456 |
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Apr 2013 |
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WO |
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2013098657 |
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Jul 2013 |
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WO |
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|
Primary Examiner: Slifka; Sarah A.
Attorney, Agent or Firm: Browdy and Neimark, PLLC
Claims
The invention claimed is:
1. An anode comprising: an electrically conductive substrate,
comprising at least one continuous non-uniform surface; and a
random network of silicon nanowires (SiNWs) chemically grown on the
at least one non-uniform surface of the substrate, wherein the
SiNWs have at least about 30% amorphous morphology.
2. The anode according to claim 1, wherein said anode is a lithium
ion battery anode.
3. The anode according to claim 1, wherein from about 30% to about
95% of the mass of each SiNW is amorphous.
4. The anode according to claim 1, wherein the SiNWs have a
core-shell structure, wherein the shell is amorphous.
5. The anode according to claim 1, wherein the SiNWs have a
thickness of from about 10 nm to about 500 nm and a length of from
about 1 .mu.m to about 200 .mu.m.
6. The anode according to claim 1, having a silicon loading on the
substrate of from about 0.5 mg/cm.sup.2 to about 20
mg/cm.sup.2.
7. The anode according to claim 1, wherein the substrate comprises
a non-uniform bulk portion, comprising a plurality of non-uniform
surfaces, wherein the SiNWs are chemically grown in the non-uniform
bulk portion.
8. The anode according to claim 1, wherein the at least one
non-uniform surface comprises elongated structures, selected from
the group consisting of fibers, trenches and combinations
thereof.
9. The anode according to claim 8, wherein the elongated structures
have a thickness of from about 0.1 .mu.m to about 100 .mu.m and a
length of from about 1 mm to about 10000 mm.
10. The anode according to claim 1, wherein the substrate comprises
a material selected from the group consisting of carbon, graphite,
metal, metal alloy and combinations thereof.
11. The anode according to claim 10, wherein the metal or metal
alloy comprises at least one element selected from the group
consisting of copper (Cu), nickel (Ni), iron (Fe) and chromium
(Cr).
12. The anode according to claim 1, wherein the substrate is
selected from the group consisting of paper, woven cloth, non-woven
cloth, film and foil.
13. The anode according to claim 12, wherein the surface of the
film or foil is selected from the group consisting of an etched,
carved, scratched, engraved surface and combinations thereof.
14. The anode according to claim 13, wherein the substrate further
comprises a carbon or graphite coating.
15. The anode according to claim 1, wherein the substrate has a
thickness of from about 5 .mu.m to about 500 .mu.m.
16. The anode according to claim 1, wherein the SiNWs comprise a
conducting coating selected from the group consisting of an
electron conducting coating, a Li cation conducting coating and a
combination thereof.
17. The anode according to claim 16, wherein the electron
conducting coating comprises a material selected from the group
consisting of carbon, graphite, reduced graphene oxide and
combinations thereof.
18. The anode according to claim 16, wherein the Li cation
conducting coating comprises a solid lithium electrolyte, selected
from the group consisting of lithium imide (Li.sub.3N), lithium
oxide (Li.sub.2O), lithium sulfate (Li.sub.2SO.sub.4), lithium
fluoride (LiF), lithium phosphate (Li.sub.3PO.sub.4), lithium
carbonate (Li.sub.2CO.sub.3) and combinations thereof; or a ceramic
coating selected from the group consisting of aluminum oxide
(Al.sub.2O.sub.3), zinc oxide (ZnO), titanium oxide (TiO), hafnium
oxide (HfO) and combinations thereof.
19. A lithium ion battery comprising the anode according to claim 2
and further comprising an electrolyte, comprising a Li salt
selected from the group consisting of lithium hexafluorophosphate
(LiPF.sub.6), lithium trifluoromethanesulfonate (lithium triflate,
CF.sub.3SO.sub.3Li), lithium bis(perfluoroethylsulfonyl)imide and
combinations thereof and at least one cathode selected from the
group consisting of lithium iron phosphate (LiFePO.sub.4),
sulfur-based cathode, lithium metal oxide-based cathode, air
cathode and oxygen cathode.
20. The lithium ion battery according to claim 19, wherein the
lithium metal oxide-based cathode comprises a metal selected from
the group consisting of nickel (Ni), manganese (Mn), cobalt (Co)
and aluminum (Al).
21. The lithium ion battery according to claim 19, wherein the
anode comprises a substrate comprising two opposed non-uniform
surfaces, wherein the random network of SiNWs is chemically grown
on said two opposed non-uniform surfaces and the anode is disposed
between two cathodes.
22. The lithium ion battery according to claim 19 packed in a cell
configuration selected from the group consisting of a prismatic
cell, pouch cell, cylinder cell and coin cell.
23. A method of manufacturing the anode according to claim 1, the
method comprising: a. providing an electrically conductive
substrate comprising at least one continuous non-uniform surface;
and b. chemically growing a random network of silicon nanowires
(SiNWs) on the at least one non-uniform surface of the
substrate.
24. The method according to claim 23, wherein the step of providing
an electrically conductive substrate comprises forming a
non-uniform surface on an essentially flat substrate.
25. The method according to claim 23, wherein the substrate
comprises a non-uniform bulk portion, comprising a plurality of
non-uniform surfaces, and wherein the step of chemically growing a
random network of SiNWs further comprises growing said SiNWs in the
non-uniform bulk portion of the substrate.
26. The method according to claim 23, wherein the process of
chemically growing a random network of SiNWs comprises a Chemical
Vapor Deposition (CVD) process.
27. The method according to claim 25, wherein the process of
chemically growing a random network of SiNWs comprises a step of
depositing catalytic nanoparticles on the at least one non-uniform
surface of the substrate and in the non-uniform bulk portion of the
substrate.
28. The method according to claim 27, wherein the deposition of the
catalytic nanoparticles is performed by immersing the substrate
into an aqueous colloidal solution of the catalytic nanoparticles
or by electroless deposition of metal nanoparticles.
29. The method according to claim 28, wherein the substrate is
immersed in the aqueous colloidal solution for at least about 5
min.
30. The method according to claim 23, further comprising a step of
coating the SiNWs with a conducting coating by using a technique
selected from the group consisting of CVD, Physical Vapor
Deposition (PVD), pyrolysis of a precursor, salt precipitation
combined with thermal treatment, and combinations thereof.
Description
FIELD OF THE INVENTION
The present invention is directed to silicon nanowire-based anodes
and methods of fabrication thereof.
BACKGROUND OF THE INVENTION
The increasing demand in energy storage has stimulated a
significant interest in lithium-battery research. Most commercially
available lithium-ion batteries include graphite based anodes,
wherein the theoretical capacity of graphite is 372 mAh/g. In order
to increase the energy density of the lithium batteries, higher
capacity anodes are required. Silicon has attracted considerable
attention in the field of Li-batteries due to its theoretical
capacity of 4200 mAh/g, which is an order of magnitude greater than
that of graphite. Additionally, Si exhibits a low de-lithiation
potential against Li/Li.sup.+, such that high battery voltages can
be reached. Furthermore, silicon is a low-cost and
environmentally-friendly material, and is the second most abundant
material on Earth.
The main disadvantage of high-capacity anode materials, such as Si,
is their particularly large volume expansion and contraction during
Li insertion/de-insertion, followed by cracking and pulverization
of the anode material. For instance, silicon exhibits up to about
320% volume expansion upon complete alloying with lithium, thus
inducing a rapid degradation of Si-based anodes. One plausible way
to deal with the detrimental pulverization is to reduce the size,
and/or thickness of the anode down to the nanoscale. Several
approaches have been reported, including the use of nanospheres,
nanotubes, nanowire arrays and porous structures (Y. Yao, M. T.
McDowell, I. Ryu, H. Wu, N. A. Liu, L. B. Hu, W. D. Nix, Y. Cui
Nano Lett., 11 (2011), pp. 2949B. Hu; H. Ma, F. Y. Cheng, J. Chen,
J. Z. Zhao, C. S. Li, Z. L. Tao, J. Liang Adv. Mater., 19 (2007),
p. 4067; T. Song, J. L. Xia, J. H. Lee, D. H. Lee, M. S. Kwon, J.
M. Choi, J. Wu, S. K. Doo, H. Chang, W. Il Park, D. S. Zang, H.
Kim, Y. G. Huang, K. C. Hwang, J. A. Rogers, U. Paik Nano Lett., 10
(2010), pp. 1710; M. H. Park, M. G. Kim, J. Joo, K. Kim, J. Kim, S.
Ahn, Y. Cui, J. Cho, Nano Lett., 9 (2009), pp. 3844).
Si nanostructures have the advantage of a shorter diffusion
distance for lithium species, which can improve the power
performance of the battery. It has been shown that the high
surface-to-volume ratio of nanoparticles helps to better withstand
stress, and substantially limit the cracking extent. The existence
of a strong particle size-dependent fracture behavior of Si
nanoparticles during the first lithiation cycle was shown
experimentally; that is, there exists a critical particle size of
.about.150 nm below which cracking does not occur, and above which
surface cracking and particle fracture is observed. Silicon
nanowires (SiNWs) provide a highly porous medium, which allows easy
expansion of silicon during lithium insertion.
There are two main approaches for the preparation of silicon
nanowires: growth methods and etching methods. The
vapor-liquid-solid (VLS) mechanism, discovered about 50 years ago
by Wagner and Ellis is the most popular of the growth methods (R.
S. Wagner and W. C. Ellis, Appl. Phys. Lett., 1964, 4, 89-91). VLS
growth is usually performed in a chemical-vapor-deposition (CVD)
reactor, by decomposition of silicon-bearing gases, like silane
(SiH.sub.4) or silicon tetrachloride (SiCl.sub.4), over a
temperature range of about 300-1000.degree. C., depending on the
gas precursor and the type of metal catalysts employed. Silicon NWs
can be grown on different types of metal catalysts, like Au, Cu,
Ag, In, Ga, Zn and others.
SiNWs for rechargeable Li battery applications, grown on the
surface of a substrate, for example on stainless steel, generally
have a "forest" structure (C. K. Chan, H. L. Peng, G. Liu, K.
McIlwrath, X. F. Zhang, R. A. Huggins and Y. Cui, Nat.
Nanotechnol., 2008, 3, 31-35; B. Laik, L. Eude, J.-P.
Pereira-Ramos, C. S. Cojocaru, D. Pribat and E. Rouviere,
Electrochim. Acta, 2008, 53, 5528-5532). The main drawbacks of this
approach are: low surface capacity (typically less than 1
mAh/cm.sup.2), very high irreversible capacity (about 30%), which
is required for the formation of the SEI (solid electrolyte
interphase), and insufficient current efficiency (typically 95 to
99.5%). In contrast, in the state-of art lithium-ion-battery
technology, employing graphite-based anodes, the irreversible
capacity is about 10% or less, the surface capacity is about 3-4
mAh/cm.sup.2, and the current efficiency is over 99.9%.
Furthermore, most publications on SiNWs-based anodes demonstrate a
single desired property (low Q.sub.ir, high surface capacity, high
electrode capacity (mAh/gSi), high current efficiency or high cycle
number) but not all of said properties are achieved for the same
electrode. In most cases, good performances were demonstrated for
very low and impractical surface capacity only. Furthermore, it was
shown that SiNW forest agglomerates to a thick, solid mass of Si
near the substrate, during the lithiation and de-lithiation
processes, leading to the delamination of the SiNWs from the
substrate (A. Kohandehghan, P. Kalisvaart, M. Kupsta, B. Zahiri, B.
Shalchi Amirkhiz, Zh. Li, E. L. Memarzadeh, L. A. Benderskyc and D.
Mitlin, J. Mater. Chem. A, 2013, 1, 1600-1612).
U.S. Pat. No. 8,637,185 is directed to conductive substrates having
open structures and fractional void volumes of at least about 25%
or, more specifically, or at least about 50% for use in lithium ion
batteries. Nanostructured active materials are deposited over such
substrates to form battery electrodes. In specific embodiments, a
nanoscale layer of silicon is deposited over a metallic mesh to
form a negative electrode. In another embodiment, a conductive
substrate is a perforated sheet with multiple openings, such that a
nanostructured active material is deposited into the openings but
not on the external surfaces of the sheet.
Silicon nanowires or whiskers supported on carbonaceous materials
or on silicon substrates, for use as Li-ion battery anode have also
been reported.
U.S. Pat. No. 8,791,449 is directed to a process for etching a
silicon-containing substrate to form nanowire arrays, which can be
used for manufacturing an anode material for lithium ion batteries
comprising nanostructured silicon.
US Patent Application No. 2011/0117436 is directed to carbon
nanofibers having a surface and including at least one crystalline
whisker extending from the surface of the carbon nanofiber, and to
battery anode compositions that can be formed from a plurality of
carbon nanofibers each including a plurality of crystalline
whiskers.
International Patent Application No. 2013/052456 discloses
nanostructured materials including silicon-based nanostructures
such as silicon nanowires and coated silicon nanowires,
nanostructures disposed on substrates comprising active materials
or current collectors such as silicon nanowires disposed on
graphite particles or copper electrode plates, and lithium-ion
battery anode composites comprising high-capacity active material
nanostructures formed on a porous copper and/or graphite powder
substrate.
A recent study used Si nanowires grown on a conducting carbon-fiber
support to provide a robust model battery system that can be
studied by .sup.7Li in situ NMR spectroscopy (K. Ogata, E. Salager,
C. J. Kerr, A. E. Fraser, C. Ducati, A. J. Morris, S. Hofmann &
C. P. Grey, Nature Communications 5, 3217 (2014)).
Additional problem related to depositing or growing high loading
SiNWs on a substrate surface or applying SiNWs to a conductive
substrate, in order to incorporate said SiNWs into an anode
structure, is a poor electric contact between the nanowires and the
substrate and lack of direct electron conduction path to the
substrate. Furthermore, in all lithium batteries the anode is
covered by a thin solid electrolyte interphase (SEI), which is
formed during the first charging cycle. Ideally, this SEI is
permeable to lithium ions, while being an electronic insulator,
thus preventing or slowing down further electrolyte decomposition
during the cycles that follow. However, in the case of the
silicon-based anodes, "breathing" of the anode material during
insertion/de-insertion of lithium causes cracks, exposing the bare
silicon surface to the electrolyte, and this is followed by the
creation of a fresh SEI, thus losing battery capacity and
increasing battery impedance (H. Wu, G. Chan, J. W. Choi, I. Ryu,
Y. Yao, M. T. McDowell, S. W. Lee, A. Jackson, Y. Yang, L. Hu and
Y. Cui, Nat. Nanotechnol., 2012, 7, 310-315). Reduction in silicon
nanowire diameter with number of cycles due to SEI formation was
also reported, while significantly greater Si loss was near the
nanowire base, which was in contact with the current collector
(J.-H. Cho and S. T. Picraux, Nano Lett. 2014, 14, 3088-3095).
Additionally, the low electrical conductivity of Si sometimes
requires the use of conductive additives in the anode film.
There remains an unmet need for the improved silicon
nanostructures-based anodes, in particular for Li-ion batteries,
which would meet the requirements of said batteries for portable
and electric-vehicle applications. The silicon nanostructures-based
anodes should be capable of providing high capacity, low
irreversible capacity, high current efficiency and a stable cycle
life.
SUMMARY OF THE INVENTION
The present invention provides a high-capacity silicon
nanowires-based anode for lithium ion batteries. In order to
overcome the disadvantages of the "SiNW forest" concept, inventors
of the present invention developed a novel approach based on the
growth of SiNWs on non-uniform surfaces of conductive substrates.
The present invention is based in part on the unexpected finding
that growth of a random network of highly dense SiNWs on carbon
fiber-based conductive networks, having non-uniform surfaces,
enables production of Li ion battery anodes having remarkably high
loadings of up to about 20 mg Si/cm.sup.2. It was further
surprisingly found that said high-loading anodes were capable of
providing very high capacity of up to about 16 mAh/cm.sup.2,
negligible irreversible capacity of not more than 10% for 3-4
mAh/cm.sup.2 capacity anodes, and current efficiency greater than
99.5%. The random-network SiNWs-based anodes of the present
invention not only meet the capacity and current efficiency
requirements of lithium batteries for portable and electric-vehicle
applications, but also exhibit a stable cycle life. The SiNWs-based
anodes of the present invention were cycled for over 300 cycles,
with only a minor capacity degradation of 10-20%. The chemical
bonds formed between the SiNWs and the non-uniform surfaces of the
conductive substrates are therefore sufficiently stable and the
SiNWs remain chemically connected to the substrate for more than
300 charge/discharge cycles. Importantly, the SiNWs grown on the
conductive substrates having at least one non-uniform surface do
not require addition of a binder to apply the SiNWs to the
substrate and/or to a current collector, thus increasing mechanic
strength of the anode and enhancing electric contact between the
components thereof. The SiNWs grown on the non-uniform surfaces of
the conductive substrate according to the principles of the present
invention are at least partially amorphous. Without wishing to
being bound by theory or mechanism of action, the stable cycle life
of the SiNWs-based anodes of the present invention can be
attributed, inter alia, to the random network structure of the
SiNWs grown on the non-uniform surfaces, to the good contact of the
SiNWs to the substrate and to the at least partially amorphous
nature of the SiNWs.
According to a first aspect, the present invention provides an
anode comprising an electrically conductive substrate, comprising
at least one non-uniform surface; and a random network of silicon
nanowires (SiNWs) chemically grown on the at least one non-uniform
surface of the substrate, wherein the SiNWs have at least about 30%
amorphous morphology. According to some embodiments, said anode is
a Li-ion battery anode. According to some embodiments, said
non-uniform surface is continuous.
According to some embodiments, at least about 30% of the SiNWs mass
is amorphous. According to further embodiments, at least about 30%
of the mass of each SiNW is amorphous.
According to some embodiments, from about 30% to about 95% of the
SiNWs mass is amorphous. According to further embodiments, from
about 30% to about 95% of the mass of each SiNW is amorphous.
In some embodiments, the SiNWs have a core-shell structure. In some
embodiments, the SiNW core is crystalline. In some embodiments, the
SiNW shell is amorphous. In some embodiments, the SiNW shell
constitutes at least about 30% of the mass of the SiNW. In further
embodiments, the SiNW shell constitutes from about 30% to about 95%
of the mass of the SiNW.
According to some embodiments, the SiNWs have a thickness of from
about 10 nm to about 500 nm. According to further embodiments, the
SiNWs have a thickness of from about 20 nm to about 300 nm.
According to some embodiments, the length of the SiNWs is from
about 1 .mu.m to about 200 .mu.m. In particular embodiments, the
SiNWs have an aspect ratio of from about 20:1 to about
20,000:1.
According to some embodiments, the SiNW core has a thickness of
from about 5 nm to about 40 nm. According to some embodiments, the
SiNW shell has a thickness of from about 10 nm to about 120 nm. In
certain embodiments, said shell is amorphous.
According to some embodiments, the substrate comprises a plurality
of non-uniform surfaces. According to further embodiments, the
plurality of non-uniform surfaces is interconnected. According to
further embodiments, the SiNWs are chemically grown on the
plurality of non-uniform surfaces of the substrate.
According to some embodiments, the substrate comprises a
non-uniform bulk portion, comprising a plurality of non-uniform
surfaces. According to further embodiments, the plurality of
non-uniform surfaces is interconnected. In some embodiments, the
non-uniform bulk portion comprises an open structure. According to
further embodiments, the SiNWs are chemically grown in the
non-uniform bulk portion of the substrate.
According to some embodiments, the SiNWs are connected to the
substrate via a chemical bond. According to some embodiments, the
SiNWs are connected to the substrate without a binder.
According to some embodiments, the anode has a silicon loading on
the substrate of from about 0.5 mg/cm.sup.2 to about 20
mg/cm.sup.2. In further embodiments, the loading is from about 2
mg/cm.sup.2 to about 6 mg/cm.sup.2. In some embodiments, the weight
ratio between the carbon substrate and the silicon nanowires ranges
from about 1:0.1 to about 1:6.
According to some embodiments, the at least one non-uniform surface
constitutes from about 10% to about 80% of the total substrate
volume. In further embodiments, the at least one non-uniform
surface constitutes from about 30% to about 50% of the total
substrate volume.
The term `non-uniform surface", as used herein, refers in some
embodiments, to a surface having topographical irregularities, such
irregularities rendering the surface non-smooth. In some
embodiments, the non-uniform surface comprises a random network of
said irregularities. In other embodiments, the irregularities are
ordered.
According to some embodiments, the at least one non-uniform surface
comprises elongated structures. The elongated structures can be
selected from fibers, trenches or combinations thereof. Each
possibility represents a separate embodiment of the invention.
According to some embodiments, the elongated structures have a
thickness of from about 0.1 .mu.m to about 300 .mu.m. According to
some embodiments, the elongated structures have a length of from
about 1 mm to about 10000 mm. According to some embodiments, the
non-uniform surface comprises elongated structures organized in a
random network. According to other embodiments, the non-uniform
surface comprises elongated structures organized in an ordered
network.
According to some exemplary embodiments, the at least one
non-uniform surface comprises fibers. According to further
embodiments, the fibers have a thickness of from about 0.1 .mu.m to
about 300 .mu.m. According to yet further embodiments, the fibers
have a thickness of from about 0.5 .mu.m to about 150 .mu.m.
According to still further embodiments, the fibers have a thickness
of from about 1 .mu.m to about 30 .mu.m.
According to some embodiments, the non-uniform surface comprises
fibers organized in a random network. According to other
embodiments, the non-uniform surface comprises fibers organized in
an ordered network. The fibers can comprise hollow fibers,
non-hollow fibers or a combination thereof. Each possibility
represents a separate embodiment of the invention. According to
some embodiments, the fibers form the at least one continuous
non-uniform surface of the substrate.
According to some embodiments, the at least one non-uniform surface
comprises trenches. According to some embodiments, the trenches
have a thickness of from about 0.1 .mu.m to about 300 .mu.m.
According to yet further embodiments, the trenches have a thickness
of from about 0.5 .mu.m to about 150 .mu.m. According to still
further embodiments, the trenches have a thickness of from about 1
.mu.m to about 30 .mu.m.
In some embodiments, the substrate comprises a material selected
from the group consisting of carbon, graphite, metal, metal alloy
and combinations thereof. Each possibility represents a separate
embodiment of the invention. In some embodiments, the substrate is
made of carbon, graphite or a combination thereof.
The substrate can be in a form of a paper, woven or non-woven
cloth, foil or film. Each possibility represents a separate
embodiment of the invention.
According to some embodiments, the substrate comprises a paper made
of carbon or graphite. In some exemplary embodiments, the substrate
comprises a carbon paper. In some exemplary embodiments, the
substrate comprises a woven or non-woven cloth made of carbon. Each
possibility represents a separate embodiment of the invention. In
further embodiments, the at least one non-uniform surface of the
carbon paper or cloth comprises fibers. In yet further embodiments,
the bulk non-uniform portion of the carbon paper or cloth comprises
fibers.
In some embodiments, the substrate is made of a metal or metal
alloy. The metal or metal alloy can comprise at least one element
selected from the group consisting of copper (Cu), nickel (Ni),
iron (Fe) and chromium (Cr). Each possibility represents a separate
embodiment of the invention. In certain embodiments, the substrate
comprises a film or foil made of a metal or metal alloy. Each
possibility represents a separate embodiment of the invention.
In some embodiments, the at least one non-uniform surface of the
foil or film is rough. In other embodiments, the at least one
non-uniform surface comprises trenches. The trenches can be etched,
carved, scratched or engraved on the foil of film to obtain the
non-uniform surface. Thus, in some embodiments, the substrate
comprises a foil or a film having an etched, carved, scratched,
engraved surface or a combination thereof. Each possibility
represents a separate embodiment of the invention.
In some embodiments, the substrate further comprises a layer of
carbon or graphite. In further embodiments, said layer is deposited
onto the metal or metal alloy foil or film.
According to some embodiments, the substrate has a thickness of
from about 5 .mu.m to about 500 .mu.m. According to further
embodiments, the substrate has a thickness of from about 10 .mu.m
to about 300 .mu.m. According to other embodiments, the substrate
has a thickness of from about 5 .mu.m to about 10 .mu.m. According
to some embodiments, the substrate is a macro substrate.
In some particular embodiments, the substrate fibers have a
thickness of from about 1 to about 30 .mu.m and the SiNWs have a
thickness of from about 20 nm to about 300 nm.
According to some embodiments, the SiNWs are chemically grown on
the substrate by a chemical vapor deposition (CVD) technique.
The SiNWs can be doped with at least one element selected from the
group consisting of boron, arsenic, phosphorus, iron, chromium,
aluminum and a combination thereof. Each possibility represents a
separate embodiment of the invention. The atomic ratio between the
doping element and silicon can range from about 1:500 to about
1:10,000.
According to some embodiments, the SiNWs comprise a conducting
coating. The conducting coating can cover the entire surface of the
SiNW. Without wishing to being bound by theory or mechanism of
action, the conductive coating prevents or diminishes formation of
silicon oxide layer on the SiNWs. Thus, in some embodiments, the
SiNWs according to the principles of the present invention do not
include an oxide layer. The conducting coating can be selected from
an electron conducting coating, a Li cation conducting coating or a
combination thereof. Each possibility represents a separate
embodiment of the invention. The electron conducting coating can
comprise a material selected from carbon, graphite, reduced
graphene oxide or combinations thereof. Each possibility represents
a separate embodiment of the invention. The Li cation conducting
coating can comprise a solid lithium electrolyte, selected from the
group consisting of lithium imide (Li.sub.3N), lithium oxide
(Li.sub.2O), lithium sulfate (Li.sub.2SO.sub.4), lithium fluoride
(LiF), lithium carbonate (Li.sub.2CO.sub.3), lithium phosphate
(Li.sub.3PO.sub.4) and combinations thereof. Each possibility
represents a separate embodiment of the invention. In some
embodiments, the Li cation conducting coating comprises a ceramic
material, including aluminum oxide (Al.sub.2O.sub.3, alumina), zinc
oxide (ZnO), titanium oxide (TiO), or hafnium oxide (HfO). In
certain embodiments, the Li cation conducting coating forms a
Solid-Electrolyte Interphase (SEI) on the SiNWs.
In some embodiments, the conducting coating thickness is from about
1 nm to about 10 nm. In further embodiments, the conducting coating
thickness is from about 3 nm to about 7 nm. The conducting coating
can be applied to the SiNWs by a technique selected from the group
consisting of CVD, Physical Vapor Deposition (PVD), salt
precipitation combined with thermal treatment, pyrolysis and any
combination thereof. Each possibility represents a separate
embodiment of the invention.
According to some embodiments, the anode has a geometric capacity
of from about 0.5 mAh/cm.sup.2 to about 20 mAh/cm.sup.2. According
to further embodiments, the anode has a geometric capacity of from
about 1 mAh/cm.sup.2 to about 10 mAh/cm.sup.2. According to some
embodiments, the anode has an irreversible capacity of up to about
15% of the total geometric capacity. According to further
embodiments, a decrease in the geometric capacity of the anode is
below about 10% for at least about 100 cycles at a charge/discharge
rate of 0.1 mA/cm.sup.2. In yet further embodiments, the anode is
configured to be charged and or discharged at C rate of at least
about 0.4C. In still further embodiments, the anode is configured
to be charged and or discharged at C rate of at least about 1.0C.
In yet further embodiments, the anode is configured to be charged
at C rate of at least about 1.3C. In some embodiments, the anode
provides at least about 92% Faradaic efficiency. In further
embodiments, the anode provides at least about 98% Faradaic
efficiency. In yet further embodiments, the anode provides at least
about 99% Faradaic efficiency.
In another aspect, there is provided a lithium ion battery
comprising the anode according to the principles of the present
invention.
According to some embodiments, the lithium ion battery further
comprises an electrolyte. The electrolyte can comprise a Li salt
selected from the group consisting of lithium hexafluorophosphate
(LiPF.sub.6), lithium trifluoromethanesulfonate (lithium triflate,
CF.sub.3SO.sub.3Li), and lithium bis-perfluoroethylsulfonylimide.
Each possibility represents a separate embodiment of the invention.
In some currently preferred embodiments, the substrate material is
compatible with the electrolyte of the battery. According to
further embodiments, the non-uniform surface of the substrate
contacts the electrolyte in the lithium ion battery.
According to some embodiments, the lithium ion battery further
comprises at least one cathode. The anode of the present invention
can be disposed in the battery, such that at least one non-uniform
surface comprising SiNWs faces said cathode. In further
embodiments, the SiNWs are grown in the non-uniform bulk portion of
the substrate.
The at least one cathode can be selected from the group consisting
of lithium iron phosphate (LiFePO.sub.4), sulfur-based cathode,
lithium metal oxide-based cathode, and air or oxygen cathode. Each
possibility represents a separate embodiment of the invention.
According to some embodiments, the lithium metal oxide-based
cathode comprises a metal selected from the group consisting of
nickel (Ni), manganese (Mn), cobalt (Co) and aluminum (Al). Each
possibility represents a separate embodiment of the invention.
According to further embodiments, the lithium ion battery comprises
two cathodes. In further embodiments, the anode is disposed between
two cathodes. In still further embodiments, the substrate comprises
at least two opposed non-uniform surfaces. In certain embodiments,
the SiNWs are grown on said two opposed non-uniform surfaces of the
substrate facing said cathodes. In further embodiments, the SiNWs
are further grown in the non-uniform bulk portion of the
substrate.
According to some embodiments, the lithium ion battery further
comprises at least one current collector. In some embodiments, a
surface of the substrate facing said current collector is coated
with an electrode ink. In some embodiments, said surface is a
non-uniform surface. In further embodiments, said non-uniform
surface comprises SiNWs, grown thereon. In other embodiments, the
lithium ion battery does not include an anode current collector. In
certain such embodiments, the anode is disposed between two
cathodes in the lithium ion battery.
According to some embodiments, the lithium ion battery is packed in
a cell configuration selected from a prismatic cell, pouch cell,
cylinder cell or coin cell configuration. Each possibility
represents a separate embodiment of the invention.
According to further embodiments, there is provided a device,
comprising the lithium ion battery according to the principles of
the present invention.
In another aspect the invention provides a method of manufacturing
the anode according to the principles of the present invention, the
method comprising providing an electrically conductive substrate,
comprising at least one non-uniform surface; and chemically growing
a random network of silicon nanowires (SiNWs) on the at least one
non-uniform surface of the substrate. In some embodiments, said
anode is a lithium ion battery anode. In some embodiments, the
non-uniform surface of the electrically conductive surface is
continuous.
According to some embodiments, the substrate comprises a plurality
of non-uniform surfaces. According to some embodiments, the step of
chemically growing a random network of SiNWs further comprises
growing the SiNW on the plurality of non-uniform surfaces.
According to some embodiments, the substrate comprises a
non-uniform bulk portion, comprising a plurality of non-uniform
surfaces. In some exemplary embodiments, the SiNW are grown in the
non-uniform bulk portion of the substrate.
In some embodiments, the step of providing an electrically
conductive substrate comprising at least one non-uniform surface,
comprises formation of the non-uniform surface on an essentially
flat substrate. In certain embodiments, said step includes etching,
carving, engraving or scratching at least one surface of a metal or
metal alloy foil or film. Each possibility represents a separate
embodiment of the invention. In further embodiments, said step
comprises depositing a conformal carbon or graphite coating on the
at least one surface of a metal or metal alloy foil or film. In
other embodiments, said step includes deposition of fibers on the
at least one surface of a metal or metal alloy foil or film.
According to some embodiments, the process of chemically growing a
random network of SiNWs comprises a Chemical Vapor Deposition (CVD)
process.
In some embodiments, a one-step CVD process is performed to obtain
the random network of SiNWs. In certain embodiments, the SiNWs
having a crystalline core and an amorphous shell are grown in a
one-step CVD process.
According to some embodiments, the process of chemically growing a
random network of SiNWs comprises a step of depositing catalytic
nanoparticles on the at least one non-uniform surface of the
substrate. According to further embodiments, the catalytic
nanoparticles are deposited in the non-uniform bulk portion of the
substrate.
According to some embodiments, the deposition of the catalytic
nanoparticles is performed by immersing the substrate into an
aqueous colloidal solution of the catalytic nanoparticles or by an
electroless deposition of the catalytic nanoparticles. Each
possibility represents a separate embodiment of the invention.
According to further embodiments, the substrate is immersed in the
aqueous colloidal solution for at least about 5 min.
According to some embodiments, the method of manufacturing the
lithium ion battery anode according to the principles of the
present invention further comprises a step of coating the SiNWs
with a conducting coating. The step of coating the SiNWs with a
conducting coating can be performed by a technique selected from
the group consisting of CVD, Physical Vapor Deposition (PVD),
pyrolysis of a precursor, salt precipitation combined with thermal
treatment, and any combination thereof. Each possibility represents
a separate embodiment of the invention.
In some exemplary embodiments, SiNWs coating by a solid lithium
electrolyte is performed by the salt precipitation combined with
thermal treatment. In further embodiments, the salt precipitation
combined with thermal treatment comprises applying to the SiNWs a
composition comprising a Li salt selected from the group consisting
of lithium nitrate (LiNO.sub.3), lithium carbonate (LiCO.sub.3),
lithium triflate (CF.sub.3SO.sub.3Li), lithium imide (lithium
bis-perfluoroethylsulfonylimide) and combinations thereof. Each
possibility represents a separate embodiment of the invention. In
further embodiments, the composition comprises a non-aqueous
solvent. In yet further embodiments, the composition comprises a
binder.
In further exemplary embodiments, SiNWs coating by a ceramic
material is performed by CVD. In additional exemplary embodiments,
SiNWs coating by an electron conducting coating is performed by
pyrolysis. Pyrolysis can be performed by applying to the SiNWs a
composition comprising a carbonaceous material and heating the
SiNWs to a temperature of from about 500.degree. C. to about
1000.degree. C. in the presence of oxygen. The carbonaceous
material can include a saccharide, such as sucrose, a
polysaccharide, or a polymer, such as, poly(methyl methacrylate)
(PMMA) or polynitrile (PN).
Further embodiments and the full scope of applicability of the
present invention will become apparent from the detailed
description given hereinafter. However, it should be understood
that the detailed description and specific examples, while
indicating preferred embodiments of the invention, are given by way
of illustration only, since various changes and modifications
within the spirit and scope of the invention will become apparent
to those skilled in the art from this detailed description.
BRIEF DESCRIPTION OF THE DRAWINGS
FIGS. 1A-1D: Various types of substrates: (FIG. 1A) photographic
image of a copper substrate with a non-uniform surface including
parallel trenches; (FIG. 1B) photographic image of a copper
substrate with a non-uniform surface including randomly-oriented
trenches; (FIG. 1C) Scanning electron microscopy (SEM) micrograph
of SGL AA25 carbon paper substrate; (FIG. 1D) SEM micrograph of
Freudenberg carbon non-woven cloth substrate.
FIGS. 2A-2J: SEM and Transmission electron microscopy (TEM)
micrographs of the SiNWs anode: (FIG. 2A) SEM micrograph of SiNWs
anode on SGL AA25, 0.36 mg/cm.sup.2 SiNWs loading; (FIG. 2B) SEM
micrograph of SiNWs anode on SGL AA25, 0.36 mg/cm.sup.2 SiNWs
loading; (FIG. 2C) SEM micrograph of SiNWs anode on SGL AA25, 4.7
mg/cm.sup.2 SiNWs loading; (FIG. 2D) SEM micrograph of the cross
section of SiNWs anode on SGL AA25 (loading, 4.7 mg/cm.sup.2
SiNWs); (FIG. 2E) Low magnification TEM micrograph of SiNWs with
crystalline core and amorphous shell, grown at 25 Torr for 30 min;
(FIG. 2F) TEM micrograph of a representative single nanowire of
FIG. 2F, displaying a total thickness of about 70 nm and a core
diameter of about 36 nm; (FIG. 2G) High-magnification (HRTEM)
micrograph of a single nanowire with crystalline core (in the
middle) having a thickness of about 17 nm and amorphous shell
(bottom-left and upper-right corners of the micrograph); (FIG. 2H)
TEM micrograph of a representative single nanowire with a
crystalline core and an amorphous shell grown at 25 Torr for 30
min, displaying a total diameter of about 80 nm and a core diameter
of about 34 nm; (FIG. 2I) TEM micrograph of a representative single
nanowire with a crystalline core and an amorphous shell, grown at
25 Torr for 60 min, displaying a total thickness of about 120 nm
and a core diameter of about 34 nm; (FIG. 2J) Low-magnification TEM
micrograph of nanowires with a crystalline core and an amorphous
shell, grown at 25 Torr for 120 min, displaying a total thickness
of about 200 nm and a core thickness of about 32 nm.
FIGS. 3A-3D: Schematic representation of growth of SiNWs on the
substrate comprising at least one non-uniform surface or a
plurality of non-uniform surfaces: (FIG. 3A) A schematic
representation of the substrate having one non-uniform surface;
(FIG. 3B) A schematic representation of the anode including the
substrate having one non-uniform surface and a random network of
SiNWs chemically grown on said substrate; (FIG. 3C) A schematic
representation of the substrate having a plurality of non-uniform
surfaces; and (FIG. 3D) A schematic representation of the anode
including the substrate having the plurality of non-uniform
surfaces and a random network of SiNWs chemically grown on said
substrate.
FIGS. 4A-4C: Schematic representation of growth of SiNWs on the
substrate comprising a non-uniform bulk portion: (FIG. 4A) A
schematic representation of the substrate comprising a non-uniform
bulk portion; (FIG. 4B) A schematic representation of the anode
including said substrate and a random network of SiNWs chemically
grown on said substrate; (FIG. 4C) A schematic representation of
the cross section of the anode depicted in FIG. 4B along line
AA'.
FIG. 5: An XRD spectra of SiNWs anode on SGL AA25, 1.92 mg/cm.sup.2
SiNWs loading.
FIGS. 6A-6C: (FIG. 6A) Cycle life of 1.06 mg Si/cm.sup.2 anode
(cell name "SGL2"), cycled at 0.1-1 mA/cm.sup.2 and voltage range
0.01-1V, with an average efficiency of 99.5%; (FIG. 6B) Separate
capacity contribution of the carbon paper (SGL) and of the SiNWs at
the initial cycles, 1.06 mg Si/cm.sup.2 anode, 0.1-1 mA/cm.sup.2,
voltage range 0.01-1V, anode mass is 5.5 mg (20% Si); (FIG. 6C)
Charge-Discharge Profile-SGL2 (1.05 cm.sup.2, 1.1 mgSi; all
SiNWs/Li cells cycled with 1M LiPF.sub.6 in 1:1 EC:DEC+15% w/w FEC
at 30.degree. C.
FIGS. 7A-7C: (FIG. 7A) Effect of cycling on dQ/dV curves; 1 mg
SiNWs/cm.sup.2, current--0.1 mA/cm.sup.2, peaks a3, a4, c2, c3
belong to graphite peaks of SGL, peaks a1, a2 and c1 belong to
SiNWs; (FIG. 7B) Effect of cycling on dQ/dV curves--comparison
between SiNWs on SGL anode with pristine SGL, 0.1 mA/cm.sup.2;
(FIG. 7C) Effect of cycling on dQ/dV curves of pristine SGL (no
Si), 0.1 mA/cm.sup.2.
FIGS. 8A-8C: (FIG. 8A) a first lithiation/de-lithiation cycle for a
1.92 mg/cm.sup.2 SiNWs anode (in a SiNWs/Li cell) 1 mA, 0.01-1V,
anode mass: 6.5 mg; (FIG. 8B) a first lithiation/de-lithiation
cycle for 0.95 mg/cm.sup.2 SiNWs anode, 1 mA. 0.01-1V, Anode mass:
6 mg; (FIG. 8C) a first lithiation/de-lithiation cycle for 4.46
mg/cm.sup.2 SiNWs anode, 1-st cycle at 1 mA.
FIG. 9: Polarization curve for anode made of 1.92 mg/cm.sup.2
SiNWs, 1 minute pulse de-lithiation and lithiation at different
currents.
FIG. 10: Capacity--cycle life plot for a battery including: 1.6
mg/cm.sup.2 SiNWs anode and LiFePO4 cathode (loading 6 mg/cm.sup.2,
of which 84% LiFePO4+8% PVDF+8% Carbon C-65), applied current--0.1
mA/cm.sup.2 (about C/4), cycling at 30.degree. C.
FIGS. 11A-11B: Effect of boron doping of the SiNWs on
(.box-solid.)--charge capacity, (.circle-solid.)--discharge
capacity: (FIG. 11A) Cycle life of 2 mg SiNWs, 1.01 cm.sup.2 anode
with B:Si ratio of 1:8000 (cell name "FRDS11"), cycled at 0.1-1
mA/cm.sup.2 and voltage range of 1-0.05V, with an average
efficiency of 99.26%; (FIG. 11B) Cycle life of 1.9 mg SiNWs, 1.01
cm.sup.2 anode with B:Si ratio of 1:1000 (cell name "FRDS14"),
cycled at 0.1-1 mA/cm.sup.2 and voltage range of 1-0.05V, with an
average efficiency of 99.48%.
FIGS. 12A-12C: Effect of carbon coating of the SiNWs on
(.box-solid.)--charge capacity, (.circle-solid.)--discharge
capacity: (FIG. 12A) Cycle life of 1.4 mg SiNWs, 1.01 cm.sup.2
anode with carbon coated SiNWs and B:Si ratio of 1:8000 (cell name
"FRDC3"), cycled at 0.1-1 mA/cm.sup.2 and voltage range of 1-0.05V,
with an average efficiency of 99.87%; (FIG. 12B) Cycle life of 1.4
mg Si, 1.01 cm.sup.2 anode with carbon coated SiNWs and B:Si ratio
of 1:8000 (cell name "FRDC4"), cycled at 0.1-1 mA/cm.sup.2 and
voltage range of 1-0.05V, with an average efficiency of 100.03%;
(FIG. 12C) Cycle life of 1.7 mg SiNWs, 0.97 cm.sup.2 anode with
carbon coated SiNWs and B:Si ratio of 1:1000 (cell name "FRDC5"),
cycled at 0.1-1 mA/cm.sup.2 and voltage range of 1-0.05V, with an
average efficiency of 99.77%.
FIG. 13: Effect of LiF coating of the SiNWs on
(.box-solid.)--charge capacity, (.circle-solid.)--discharge
capacity: Cycle life of about 0.5 mg SiNWs, 1.01 cm.sup.2 anode
with LiF coated SiNWs (cell name "FRDL1"), cycled at 0.1-1
mA/cm.sup.2 and voltage range of 1-0.05V, with an average
efficiency of 99.898%.
FIGS. 14A-14D: Effect of alumina coating of the SiNWs on
(.box-solid.)--charge capacity, (.circle-solid.)--discharge
capacity: (FIG. 14A) Cycle life of 0.44 mg SiNWs, 1.04 cm.sup.2
anode with alumina coated SiNWs (cell name "SGLA2"), cycled at
0.1-1 mA/cm.sup.2 and voltage range of 1-0.05V, with an average
efficiency of 99.68%; (FIG. 14B) Cycle life of 0.99 mg Si, 1.02
cm.sup.2 anode with alumina coated SiNWs (cell name "SGLA3"),
cycled at 0.1-1 mA/cm.sup.2, with an average efficiency of 99.51%;
(FIG. 14C) Cycle life of 0.99 mg SiNWs, 1.02 cm.sup.2 anode with
alumina coated SiNWs (cell name "SGLA4"), cycled at 0.1-1
mA/cm.sup.2 and voltage range of 1-0.05V, with an average
efficiency of 99.45%; (FIG. 14D) Cycle life of a reference 0.98 mg
SiNWs, 0.99 cm.sup.2 anode without alumina coating (cell name
"SGLA14"), cycled at 0.1-1 mA/cm.sup.2 and voltage range of
1-0.05V, with an average efficiency of 99.47%.
DETAILED DESCRIPTION OF THE INVENTION
The present invention provides a silicon nanostructures-based
anode, which is suitable for use, for example, in lithium ion
batteries, characterized by a high capacity, low irreversible
capacity and stable cycle life. In particular, provided herein is
an anode, comprising Si nanowires chemically grown on at least one
non-uniform surface of an electrically conductive substrate, which
is capable of providing geometric capacity of from about 1 to about
15 mAh/cm.sup.2, irreversible capacity of only up to about 20% of
the total geometric capacity and current efficiency greater than
98%. Additionally, the lithium battery anodes of the present
invention have cycle life stability for over 200 cycles. Said
capacity, current efficiency and cycle-life properties meet the
current requirements of lithium batteries for portable and
electric-vehicle applications, making the SiNWs-based anodes of the
present invention excellent candidates for practical Li battery
applications and providing significant advantages over the prior
art SiNWs-based anodes.
The present invention is based in part on the unexpected finding
that SiNWs chemically grown on a substrate having at least one
non-uniform surface are superior to the existing "SiNWs forest"
technology anodes, wherein SiNWs are epitaxially grown on a flat
surface of a metal substrate. SiNWs of the present invention are
grown on the non-uniform surface of the substrate in a random
network configuration, thus reducing loss of Si surface area during
battery cycling due to agglomeration of the adjacent nanowires. The
substrates suitable for chemically growing thereon a random network
of silicon nanowires have at least one or a plurality of
non-uniform surfaces, providing a continuous structure from which
SiNWs can protrude in various directions.
Additionally, SiNWs grown on the non-uniform surface of the
substrate in a random network configuration provide a significantly
higher surface area for the silicon lithiation and de-lithiation
reactions, and enough space for SEI formation, thus reducing
irreversible capacity due to the formation of the SEI during the
first battery cycle. While the existing Si-based anodes display
very high irreversible capacity values of over 30%, the SiNWs-based
cathodes of the present invention are characterized by a very low
irreversible capacity loss of up to about 10%, which is
characteristic of state-of-the-art graphite anodes. Substrates
including a plurality of the non-uniform surfaces provide
exceptionally high Si loadings and geometric capacity of the
electrode, which are achieved by growing the SiNWs on said
plurality of the non-uniform surfaces. Such anodes provide high
charge-discharge rate without increasing the risk of agglomeration
of adjacent SiNWs and further delamination thereof during battery
cycling. Additional advantage of the high-loading anode including a
plurality of non-uniform surfaces with SiNWs grown thereon is that
it can be used without an anode current collector, being disposed
between two cathodes in a lithium ion battery.
Furthermore, the SiNWs according to the principles of the present
invention have at least about 30% amorphous morphology. Without
wishing to being bound by theory or mechanism of action, the
amorphous morphology of the SiNW eliminates or diminishes the
change from crystalline state to amorphous state at the first
charge-discharge cycle of the state of the art SiNWs anode. It is
assumed that said change from crystalline to the amorphous state
leads to a fast anode degradation. Without further wishing to being
bound by theory, it is contemplated that the amorphous morphology
of the SiNWs according to the principles of the present invention,
promotes high-rate charge-discharge rate or the anode.
The SiNWs according to the principles of the present invention are
chemically grown on the electrically-conducting substrate and,
thus, do not require a binder for the attachment to an electrode,
to be incorporated in a lithium ion battery. The anodes of the
present invention thus provide an enhanced electric contact between
the conducting substrate and lithium ions, during silicon
lithiation and de-lithiation reactions, as compared to the
electrodes comprising SiNWs mechanically attached to a conducting
substrate, for example by means of a binder. Furthermore, the
chemically-grown SiNWs maintain their chemical bonding to the
substrate throughout battery cycling, demonstrating a stable cycle
life for hundreds of lithiation and de-lithiation cycles. Without
wishing to being bound by theory or mechanism of action, it can be
assumed that the crystalline core prevents or diminishes detachment
of the nanowires during cycling and the amorphous shell reduces
degradation resulting from changes from crystalline to amorphous
state.
Thus, according to a first aspect there is provided an anode
comprising: an electrically conductive substrate, comprising at
least one non-uniform surface; and a random network of silicon
elongated nanostructures chemically grown on the at least one
non-uniform surface of the substrate. According to another aspect,
there is provided a method for manufacturing an anode comprising
providing an electrically conductive substrate, comprising at least
one non-uniform surface; and chemically growing a random network of
silicon elongated nanostructures on the at least one non-uniform
surface of the substrate. In some embodiments, said anode is for
use in an alkaline metal ion-based battery. In some embodiments,
said anode is for use in a lithium ion battery.
According to some embodiments, the non-uniform surface is
continuous. In some currently preferred embodiments, the silicon
elongated structures have at least about 30% amorphous
morphology.
The term "silicon (Si) elongated nanostructure", as used herein,
refers to a silicon nanostructure having one dimension (referred to
as the length of the nanostructure) elongated with respect to the
other two dimensions (referred to as the thickness and the width of
the nanostructure), such as a cylindrical or tubular configuration.
It is to be understood that the term "silicon elongated
nanostructure" as used herein refers to structures having length in
the nanometer as well as micrometer range.
According to some exemplary embodiments, said silicon elongated
nanostructures comprise nanowires. According to some embodiments,
said silicon elongated nanostructures comprise tubular
nanostructures. In certain embodiments, the silicon elongated
structures do not include crystalline whiskers. In further
embodiments, the nanowires do not include crystalline whiskers.
The Substrate Including a Non-Uniform Surface
The substrates suitable for the anodes of the present invention
have at least one non-uniform surface, configured to enable growth
of the Si elongated nanostructures, including, inter alia, SiNWs,
in a random network configuration. The term "non-uniform surface",
as used therein, refers in some embodiments, to a surface having
topographical irregularities, said irregularities rendering the
surface non-smooth.
According to some embodiments, the height of said irregularities is
from about 1 to about 1000 .mu.m. In further embodiments, the
height of said irregularities is from about 1 to about 100 .mu.m,
from about 1 to about 50 .mu.m, from about 5 to about 40 .mu.m, or
from about 10 to about 30 .mu.m. Each possibility represents a
separate embodiment of the invention. The term "height of the
irregularities", as used herein, refers in some embodiments to the
dimension of the irregularity, which is perpendicular to a portion
of the non-uniform surface of the substrate, devoid of said
irregularities.
According to some embodiments, the thickness of said irregularities
is from about 0.1 .mu.m to about 300 .mu.m. In further embodiments,
the thickness of said irregularities is from about 0.1 .mu.m to
about 150 .mu.m, from about 0.1 .mu.m to about 100 .mu.m, from
about 0.5 .mu.m to about 150 .mu.m or from about 1 .mu.m to about
30 .mu.m. Each possibility represents a separate embodiment of the
invention. The term "thickness of the irregularities", as used
herein, refers in some embodiments to the smallest dimension of the
irregularity, which is parallel to a portion of the non-uniform
surface of the substrate, devoid of said irregularities. In some
embodiments, the height of said irregularities is essentially the
same as the thickness thereof.
According to some embodiments, the length of said irregularities is
from about 0.1 .mu.m to about 10000 mm. According to further
embodiments, the length of said irregularities is from about 1 mm
to about 10000 mm. According to still further embodiments, the
length of said irregularities is from 10 mm to about 1000 mm. The
term "length of the irregularities", as used herein, refers in some
embodiments to the largest dimension of the irregularity, which is
parallel to a portion of the non-uniform surface of the substrate,
devoid of said irregularities.
In some embodiments, the non-uniform surface comprises a random
network of said irregularities. In other embodiments, the
irregularities are ordered. In further embodiments, the
irregularities are organized in an ordered network.
According to some embodiments, the non-uniform surface comprises a
curve, a slope or combinations thereof. Each possibility represents
a separate embodiment of the invention. According to further
embodiments, the non-uniform surface comprises a plurality of
curves and/or slopes. The non-limiting example of the non-uniform
surface comprising a plurality of curves and/or slopes includes a
wave-like surface. Said wave can be, for example, a sine,
triangular or sawtooth wave. According to some embodiments, the
non-uniform surface comprises a rough or leveled surface. In
certain embodiments, said surface is a micro-leveled surface. The
term "micro-leveled", as used herein, refers in some embodiments to
the non-uniform surface having topographical irregularities which
height is below about 1 mm. In some embodiments, said surface is an
etched surface. In some embodiments, a substrate having an
essentially smooth surface is chemically or mechanically modified
to include at least one non-uniform surface.
According to some embodiments, the substrate comprises a plurality
of non-uniform surfaces. According to further embodiments, the
plurality of non-uniform surfaces is interconnected. The plurality
of non-uniform surfaces can be electronically and/or mechanically
interconnected. Each possibility represents a separate embodiment
of the invention. According to further embodiments, the Si
elongated nanostructures, including, inter alia, SiNWs, are
chemically grown on the plurality of non-uniform surfaces of the
substrate.
According to some embodiments, the substrate comprises a
non-uniform bulk portion, comprising a plurality of non-uniform
surfaces. According to further embodiments, the plurality of
non-uniform surfaces is interconnected. The plurality of
non-uniform surfaces can be electronically and/or mechanically
interconnected. Each possibility represents a separate embodiment
of the invention. According to further embodiments, the Si
elongated nanostructures, including, inter alia, SiNWs, are
chemically grown in the non-uniform bulk portion of the
substrate.
According to some embodiments, the non-uniform bulk portion of the
substrate is permeable to lithium ions. According to other
embodiments, said non-uniform bulk portion is permeable to an
electrolyte of the battery. According to further embodiments, said
non-uniform bulk portion is permeable to catalytic metal particles.
According to still further embodiments, said non-uniform bulk
portion is permeable to gases, such as, but not limited to silane.
According to some embodiments, the non-uniform bulk portion of the
substrate is continuous.
According to some embodiments, the substrate comprising at least
one non-uniform surface is an open-structure substrate. According
to further embodiments, the substrate comprising the non-uniform
bulk portion is an open-structure substrate. The term
"open-structure", as used herein, refers in some embodiments to a
structure having a continuous solid medium comprising pores, open
space, open non-uniform surfaces or openings of variety of forms.
In some embodiments, said pores or opening are permeable to lithium
ions. According to other embodiments, said pores or openings are
permeable to an electrolyte of the battery. According to further
embodiments, said pores or openings are permeable to catalytic
metal particles. According to still further embodiments, said pores
or openings are permeable to gases, such as, but not limited to
silane. Without wishing to being bound by theory or mechanism of
action, it is contemplated that the open-structure of the
substrate, in combination with the plurality of the non-uniform
surfaces thereof provides open space for the electrolyte to
penetrate into the anode and allows room for the growth of the SEI
without blocking the ion path in the electrolyte.
According to some embodiments, the non-uniform surfaces constitute
from about 5% to about 80% of the total substrate volume. In
further embodiments, the non-uniform surfaces constitute from about
10% to about 70% of the total substrate volume, from about 15% to
about 60%, or from about 20% to about 50% of the total substrate
volume. Each possibility represents a separate embodiment of the
invention.
According to other embodiments, the non-uniform surface comprises
elongated structures. The term "elongated structure", as used
herein, refers to a structure having one dimension (referred to as
the length of the structure) elongated with respect to the other
two dimensions (referred to as the thickness and the width of the
structure). The elongated structures can have an a circle-like
cross section, or other cross sectional shapes including, but not
limited to, trapezoidal, triangular, square, strips or rectangular.
According to some embodiments, the elongated structures have a
circle-like cross section. In certain such embodiments, the
thickness of the elongated structure is essentially the same as the
diameter thereof.
According to some embodiments, the elongated structures have a
thickness of from about 0.1 .mu.m to about 300 .mu.m. According to
further embodiments, the elongated structures have a thickness of
from about 0.1 .mu.m to about 150 .mu.m. According to still further
embodiments, the elongated structures have a thickness of from
about 0.1 .mu.m to about 100 .mu.m. According to yet further
embodiments, the elongated structures have a thickness of from
about 0.5 .mu.m to about 50 .mu.m. According to still further
embodiments, the elongated structures have a thickness of from
about 1 .mu.m to about 30 .mu.m.
According to some embodiments, the elongated structures have a
width of from about 0.1 .mu.m to about 300 .mu.m. According to
further embodiments, the elongated structures have a width of from
about 0.1 .mu.m to about 150 .mu.m. According to still further
embodiments, the elongated structures have a width of from about
0.1 .mu.m to about 100 .mu.m. According to yet further embodiments,
the elongated structures have a width of from about 0.5 .mu.m to
about 50 .mu.m. According to still further embodiments, the
elongated structures have a width of from about 1 .mu.m to about 30
.mu.m.
According to some embodiments, the elongated structures have a
length of from about 1 mm to about 10000 mm. According to further
embodiments, the elongated structures have a length of from about
10 mm to about 1000 mm.
According to some embodiments, the non-uniform surface comprises
elongated surfaces present in a random network. According to other
embodiments, the non-uniform surface comprises elongated structures
organized in an ordered network.
According to some exemplary embodiments, the elongated structures
comprise fibers. Thus, in some embodiments, the substrate is
fibrous. In certain such embodiments, the substrate comprises
interconnected fibers, which form the non-uniform surfaces of said
substrate. In further embodiments, the interconnected fibers form
the continuous non-uniform surfaces of said substrate.
The fibers can have an elongated cylinder-like shape with a
circle-like cross section, or equivalent dimensions, wherein the
fibers have other cross sectional shapes including, but not limited
to, trapezoidal, triangular, square, strips or rectangular. The
fibers can have a thickness, a diameter, or equivalent dimensions
for shapes other than cylinder in the range from about 0.1 .mu.m to
about 300 .mu.m. According to some embodiments, the fibers have a
thickness of from about 0.3 .mu.m to about 200 .mu.m, from about
0.5 .mu.m to about 100 .mu.m, or from about 1 .mu.m to about 30
.mu.m. Each possibility represents a separate embodiment of the
invention.
According to some embodiments, the fibers have an amorphous
morphology. According to further embodiments, at least a portion of
said fibers has an amorphous morphology. According to still further
embodiments, at least 30% of said fibers have an amorphous
morphology, or at least about 40%, or even at least about 50% of
said fibers have an amorphous morphology. Each possibility
represents a separate embodiment of the invention. According to
other embodiments, said fibers are crystalline.
According to some embodiments, the non-uniform surface comprises
fibers present in a random network. According to other embodiments,
the non-uniform surface comprises fibers organized in an ordered
network. The fibers can comprise hollow fibers, non-hollow fibers
or a combination thereof. Each possibility represents a separate
embodiment of the invention.
According to some embodiments, the elongated structures comprise
trenches. The trenches can have an a circle-like cross section, or
other cross sectional shapes including, but not limited to,
trapezoidal, triangular, square, strips or rectangular. According
to some embodiments, the trenches have a circle-like cross section.
In certain such embodiments, the thickness of the trench is
essentially the same as the diameter thereof.
According to some embodiments, the trenches have a thickness of
from about 0.1 .mu.m to about 300 .mu.m. According to further
embodiments, the trenches have a thickness of from about 0.1 .mu.m
to about 150 .mu.m. According to still further embodiments, the
trenches have a thickness of from about 0.1 .mu.m to about 100
.mu.m. According to yet further embodiments, the trenches have a
thickness of from about 0.5 .mu.m to about 50 .mu.m. According to
still further embodiments, the trenches have a thickness of from
about 1 .mu.m to about 30 .mu.m.
According to some embodiments, the trenches have a width of from
about 0.1 .mu.m to about 300 .mu.m. According to further
embodiments, the trenches have a width of from about 0.1 .mu.m to
about 150 .mu.m. According to still further embodiments, the
trenches have a width of from about 0.1 .mu.m to about 100 .mu.m.
According to yet further embodiments, the trenches have a width of
from about 0.5 .mu.m to about 50 .mu.m. According to still further
embodiments, the trenches have a width of from about 1 .mu.m to
about 30 .mu.m.
According to some embodiments, the trenches have a length of from
about 1 mm to about 10000 mm. According to further embodiments, the
trenches have a length of from about 10 mm to about 1000 mm.
According to some embodiments, the non-uniform surface comprises
trenches present in a random network. According to other
embodiments, the non-uniform surface comprises trenches organized
in an ordered network. In further embodiments, the trenches form
the continuous non-uniform surfaces of the substrate.
According to some embodiments, the at least one non-uniform surface
constitutes at least about 80% of the total geometrical surface
area of the substrate, at least about 85% of the total geometrical
surface area, at least about 90% of the total geometrical surface,
or at least about 99.9% of the total geometrical surface area of
the substrate. Each possibility represents a separate embodiment of
the invention.
According to some embodiments, the at least two non-uniform
surfaces constitute at least about 80% of the total geometrical
surface area of the substrate, at least about 85% of the total
geometrical surface area, at least about 90% of the total
geometrical surface, or at least about 99.9% of the total
geometrical surface area of the substrate. Each possibility
represents a separate embodiment of the invention.
In some embodiments, the substrate includes at least one or at
least two non-uniform surfaces. The non-limiting examples of such
substrate include a foil or a film.
In some embodiments, the substrate comprises one non-uniform
surface. In some embodiments, the substrate comprises two
non-uniform surfaces. In some embodiments, the substrate comprises
at least two opposed non-uniform surfaces. In some embodiments,
said at least two non-uniform surfaces are substantially parallel.
In some embodiments, the substrate comprises a first non-uniform
surface and a second non-uniform surface.
In some embodiments, the at least one or at least two non-uniform
surfaces of the foil or film are rough. In other embodiments, the
foil or film has one or two essentially flat surfaces. In further
embodiments, at least one surface of said foil or film is modified
to provide a non-uniform surface. The surface of the foil or film
can be etched, carved, scratched, engraved or any combination
thereof. The etching can be performed by a technique selected from
dry etching, wet chemical etching or a combination thereof. The
engraving can be performed, inter alia, by photolithography. In
certain embodiments, the at least one or at least two non-uniform
surfaces of the foil or film comprise trenches. FIGS. 1A and 1B
show a photographic image of a foil substrate comprising one
non-uniform surface, according to some embodiments of the
invention. The essentially flat surface of the foil substrate was
scratched in order to obtain the non-uniform surface comprising
trenches, as shown in FIGS. 1A and 1B.
In some embodiments, the substrate comprises a plurality of
non-uniform surfaces, such as but not limited to 2, 3, 4, 5, 6 10,
20, 50, 200, 500 or more non-uniform surfaces. In some embodiments,
the substrate comprises a non-uniform bulk portion. The non-uniform
bulk portion can comprise a plurality of non-uniform surfaces. In
some embodiments, the substrate comprises a first non-uniform
surface, a second non-uniform surface and a non-uniform bulk
portion between said first and said second non-uniform surfaces.
The non-limiting examples of said substrate include a paper, a
cloth or a mesh. The cloth can be selected from a woven or a
non-woven cloth. Each possibility represents a separate embodiment
of the invention. Such substrates can be fabricated using, for
example, sintering fibers, molding fibers, electrospinning fibers,
or weaving fibers. FIG. 1C shows a SEM micrograph of a fibrous
substrate comprising a carbon paper, comprising a plurality of
non-uniform surfaces, according to some embodiments of the
invention. FIG. 1D shows a SEM micrograph of a fibrous substrate
comprising a non-woven carbon cloth, comprising a plurality of
non-uniform surfaces, according to some embodiments of the
invention.
The substrate can be of any geometric shape, suitable for
incorporation as an anode into battery, such as, for example, a
lithium ion battery. According to some embodiments, the substrate
is configured to be rolled and/or folded in order to be
incorporated into a cylinder, prismatic or pouch cell
configuration.
The substrate is typically a macro substrate, wherein the at least
one non-uniform surface have a geometrical surface area of from
about 0.1 to about 10,000 cm.sup.2. The geometric area of the
substrate can be in the range from about 0.1 to about 10,000
cm.sup.2, from about 1 to about 1000 cm.sup.2, or from about 10 to
about 100 cm.sup.2. Each possibility represents a separate
embodiment of the invention.
The thickness of the substrate is typically from about 1 .mu.m to
about 500 .mu.m. In some embodiments, the thickness of the
substrate is from about 5 .mu.m to about 500 .mu.m, such as from
about 10 .mu.m to about 400 .mu.m or from about 20 .mu.m to about
300 .mu.m. Each possibility represents a separate embodiment of the
invention. In some exemplary embodiments, the thickness of the
substrate is from about 50 .mu.m to about 200 .mu.m. In certain
exemplary embodiments, the thickness of the substrate is about 160
.mu.m. In other exemplary embodiments, the thickness of the
substrate is about 7 .mu.m. According to certain embodiments, the
thickness of the fiber is about 7 .mu.m. In some embodiments, the
thickness of the substrate is defined as a thickness of the bulk
portion of the substrate. In other embodiments, the thickness is
defined as a distance between the two opposed non-uniform surfaces
of the substrate.
The Substrate Material
The substrate can be made of any electronically conducting
material, suitable for providing at least one non-uniform surface.
According to some embodiments, the substrate is compatible with the
electrolyte of the lithium ion battery. The non-limiting examples
of such material include carbon, graphite, metal, metal alloy and
combinations thereof. Each possibility represents a separate
embodiment of the invention. The metals or metal alloys, suitable
for the formation of the substrate of the Li ion battery anode
include, inter alia, copper (Cu), nickel (Ni), iron (Fe), chromium
(Cr) or combinations thereof. In some exemplary embodiments, the
substrate material comprises copper. In further exemplary
embodiments, the substrate material comprises carbon, graphite, or
a combination thereof. Each possibility represents a separate
embodiment of the invention.
In some embodiments, the substrate material comprises carbon,
graphite, or a combination thereof, coated with a metal or a
metal-alloy coating. Each possibility represents a separate
embodiment of the invention.
According to some embodiments, the substrate comprises a metal or a
metal alloy covered by a conductive coating. The non-limiting
examples of the conducting coating of the metal substrate include
carbon, graphite and graphene. In further embodiments, said
conductive coating is deposited onto the metal or metal alloy foil
or film.
In some embodiments, the foil or film comprises an essentially flat
surface and the carbon or graphite conductive coating comprises
fibers. In certain such embodiments, the silicon nanowires are
chemically grown on said fibers. In further embodiments, the fibers
are applied to the film or foil by using a binder.
In some embodiments, the foil or film surface comprises trenches
and the carbon or graphite conductive coating comprises a conformal
coating. The conformal carbon coating may be formed by CVD or by
pyrolysis of a carbonaceous precursor. The non-limiting examples of
such precursors include saccharide, oligomer or polymer precursors.
Each possibility represents a separate embodiment of the invention.
One non-limiting example of suitable saccharide is sucrose. The
non-limiting examples of the polymers include poly(methyl
methacrylate) (PMMA) or polynitrile (PN).
The graphene layer enables the growth of nanowires directly on the
metal surface, without affecting the direct electrical contract
between the SiNWs and the current collector. Graphene layers can be
grown on metal surfaces, such as, but not limited to, copper or
nickel matrixes, by well-developed CVD procedures.
In some embodiments, the substrate material is amorphous. In other
embodiments, the substrate material is crystalline. In additional
embodiments, the substrate material comprises both amorphous and
crystalline fractions.
In certain embodiments, the substrate comprises a metal or a
metal-alloy foil. In particular embodiments, the substrate
comprises a copper foil. In some embodiments, the copper foil
comprises trenches. In further embodiments, the copper foil
comprises a carbon conformal coating.
In certain embodiments, the substrate comprises a carbon paper or
woven or non-woven carbon cloth. Each possibility represents a
separate embodiment of the invention. In some exemplary
embodiments, the substrate comprises a carbon paper. In certain
such embodiments, the carbon paper is graphitized. In some
exemplary embodiments, about 50% of the carbon paper is graphitized
and about 50% is amorphous. In some embodiments, the carbon paper
thickness is from about 5 .mu.m to about 500 .mu.m, from about 10
.mu.m to about 400 .mu.m, from about 20 .mu.m to about 300 .mu.m,
or from about 50 .mu.m to about 200 .mu.m. Each possibility
represents a separate embodiment of the invention. In further
embodiments, the carbon paper comprises carbon fibers having a
thickness of about 0.3 .mu.m to about 200 .mu.m, from about 0.5
.mu.m to about 100 .mu.m, or from about 1 .mu.m to about 30 .mu.m.
Each possibility represents a separate embodiment of the invention.
In yet further embodiments, the carbon paper porosity is from about
20% to about 95%, from about 30% to about 90%, from about 40% to
about 85%, or from about 50% to about 70%. Each possibility
represents a separate embodiment of the invention.
In further exemplary embodiments, the substrate comprises a carbon
cloth. In some embodiments, the carbon cloth is a non-woven cloth.
In further embodiments, the carbon cloth has a thickness of from
about 5 .mu.m to about 500 .mu.m, from about 10 .mu.m to about 400
.mu.m, from about 20 .mu.m to about 300 .mu.m, or from about 50
.mu.m to about 200 .mu.m. Each possibility represents a separate
embodiment of the invention. In further embodiments, the carbon
cloth comprises carbon fibers having a thickness of about 0.3 .mu.m
to about 200 .mu.m, from about 0.5 .mu.m to about 100 .mu.m, or
from about 1 .mu.m to about 30 .mu.m. Each possibility represents a
separate embodiment of the invention. In some embodiments at least
about 80% of the carbon cloth is amorphous. In further embodiments,
at least about 90% of the carbon cloth is amorphous. In still
further embodiments, about 100% of the carbon cloth is
amorphous.
Silicon Nanowires (SiNWs)
According to some embodiments, there is provided an anode
comprising: an electrically conductive substrate, comprising at
least one non-uniform surface; and a random network of SiNWs
chemically grown on the at least one non-uniform surface of the
substrate.
The SiNWs are present in the anode in a random network
configuration. The term "random network", as used herein, refers in
some embodiments, to SiNWs, which are substantially non-parallel to
each other. The random network of SiNWs is shown, for example, on
SEM micrographs in FIGS. 2A-2D. Without wishing to being bound by
theory or mechanism of action, the random network configuration of
the SiNWs provides improved cycle life stability and increased
capacity of the anode throughout battery cycling, as compared to
the prior art Si "forest" anodes.
The silicon nanowires have an elongated cylinder-like shape with a
circle-like cross section, or equivalent dimensions wherein the
nanowires have other cross sectional shapes including, but not
limited to, trapezoidal, triangular, square, strips or rectangular.
In some exemplary embodiments, the nanowire has a circle-like cross
section. According to some embodiments, the trenches have a
circle-like cross section. In certain such embodiments, the
thickness of the SiNW is essentially the same as the diameter
thereof. In some embodiments, the nanowire has a strip cross
sectional shape.
SiNWs can have a thickness, a diameter, or equivalent dimensions
for shapes other than cylinder in the range from about 10 nm to
about 500 nm. According to some embodiments, the SiNWs have a
thickness of from about 20 nm to about 400 nm, from about 50 nm to
about 300 nm, or from about 100 nm to about 200 nm. Each
possibility represents a separate embodiment of the invention.
According to some embodiments, the length of the SiNWs is from
about 1 .mu.m to about 200 .mu.m, from about 5 .mu.m to about 100
.mu.m, or from about 20 .mu.m to about 60 .mu.m. Each possibility
represents a separate embodiment of the invention. In some
embodiments, the length of the SiNWs is above about 1 .mu.m. In
further embodiments, the length of the SiNWs is above about 5
.mu.m. In yet further embodiments, the length of the SiNWs is above
about 10 .mu.m. In still further embodiments, the length of the
SiNWs is above about 20 .mu.m.
The SiNWs can have an aspect ratio of from about 2:1 to about
20,000:1. In some embodiments, the SiNWs have an aspect ratio of
from about 10:1 to about 20,000:1. In particular embodiments, the
SiNWs have an aspect ratio of from about 20:1 to about 20,000:1. In
further embodiments, the SiNWs have an aspect ratio of from about
20:1 to about 10,000:1, from about 20:1 to about 5,000 or from
about 20:1 to about 1,000. Each possibility represents a separate
embodiment of the invention.
According to some embodiments, the thickness and length of the
SiNWs throughout the at least one non-uniform surface of the
substrate is essentially uniform. The term "essentially uniform",
as used herein, refers to the variation of the thickness and/or or
length of less than about 30%. In further embodiments, the term
"essentially uniform" refers to the variation of less than about
20%, less than about 10% or less than about 5%. Each possibility
represents a separate embodiment of the invention.
The SiNWs according to the principles of the present invention have
at least about 30% amorphous morphology. The term "amorphous
morphology" refers in some embodiments to the structure (for
example, volume) of a single SiNW, which is amorphous. In further
embodiments, the term "amorphous morphology" refers to the mass of
a single SiNW, which is amorphous. In further embodiments, the term
"amorphous morphology" refers to the total number of the SiNWs of
the anode, which are amorphous. In yet further embodiments, the
term "amorphous morphology" refers to the total mass of the SiNWs
of the anode, which is amorphous.
According to some embodiments, the SiNWs have at least about 40%
amorphous morphology. According to further embodiments, the SiNWs
have at least about 50% amorphous morphology. According to yet
further embodiments, the SiNWs have at least about 60% amorphous
morphology. According to still further embodiments, the SiNWs have
at least about 70% amorphous morphology. According to yet further
embodiments, the SiNWs have at least about 80% amorphous
morphology. According to some embodiments, the SiNWs have from
about 30% to about 95% amorphous morphology. According to further
embodiments, the SiNWs have from about 40% to about 70% amorphous
morphology.
According to some embodiments, at least about 30% of the SiNWs have
an amorphous morphology. According to further embodiments, at least
about 50% of the SiNWs have an amorphous morphology. According to
still further embodiments, at least about 70% of the SiNWs have an
amorphous morphology. According to some embodiments, from about 30%
to about 95% of the SiNWs have an amorphous morphology. According
to further embodiments, from about 40% to about 70% of the SiNWs
have an amorphous morphology.
According to some embodiments, at least about 50% of the SiNWs
structure is amorphous. According to further embodiments, at least
about 70% of the SiNWs structure is amorphous. According to some
embodiments, from about 30% to about 95% of the SiNWs structure is
amorphous. According to some embodiments, from about 30% to about
95% of the SiNWs structure is amorphous. According to further
embodiments, from about 40% to about 70% of the SiNWs structure is
amorphous.
According to some embodiments, at least about 50% of the SiNWs mass
is amorphous. According to further embodiments, at least about 70%
of the SiNWs mass is amorphous. According to some embodiments, from
about 30% to about 95% of the SiNWs mass is amorphous. According to
some embodiments, from about 30% to about 95% of the SiNWs mass is
amorphous. According to further embodiments, from about 40% to
about 70% of the SiNWs mass is amorphous.
According to some embodiments, at least about 50% of the mass of
each SiNW is amorphous. According to further embodiments, at least
about 70% of the mass of each SiNW is amorphous. According to some
embodiments, from about 30% to about 95% of the mass of each SiNW
is amorphous. According to some embodiments, from about 40% to
about 70% of the mass of each SiNW is amorphous.
In some embodiments, the SiNWs have a core-shell structure. In some
embodiments, the inner part of the SiNW (which can also be referred
to as "SiNW core") is crystalline. In some embodiments, the outer
part of the SiNW (which can also be referred to as "SiNW shell") is
amorphous. In some embodiments, the amorphous shell constitutes a
conformal coating on the crystalline core. In further embodiments,
the core-sell structure of the SiNWs in obtained by a single step
CVD process. Without wishing to being bound by theory or mechanism
of action, it is contemplated that the crystalline core-amorphous
shell structure of the SiNWs according to some embodiments of the
invention, affords for the high loadings of Si on the substrate
without compromising the anode stability.
According to some embodiments, at least about 30% of the SiNWs have
a core-shell structure. According to further embodiments, at least
about 50% of the SiNWs have a core-shell structure. According to
still further embodiments, at least about 70% of the SiNWs have a
core-shell structure. According to some embodiments, from about 30%
to about 95% of the SiNWs have a core-shell structure. According to
further embodiments, from about 40% to about 70% of the SiNWs have
a core-shell structure. In certain embodiments, said shell is
amorphous.
In some embodiments, the SiNW shell constitutes at least about 30%
of the mass of the SiNW, at least about 40%, at least about 50%, at
least about 60%, at least about 70% or at least about 80% of the
mass of the SiNW. Each possibility represents a separate embodiment
of the invention. In further embodiments, the SiNW shell
constitutes from about 30% to about 95% of the mass of the SiNW. In
yet further embodiments, the SiNW shell constitutes from about 40%
to about 70% of the mass of the SiNW. In certain embodiments, said
shell is amorphous.
In some embodiments, the SiNW shell constitutes at least about 30%
of the SiNW structure, at least about 40%, at least about 50%, at
least about 60%, at least about 70% or at least about 80% of the
SiNW structure. Each possibility represents a separate embodiment
of the invention. In further embodiments, the SiNW shell
constitutes from about 30% to about 95% of the SiNW structure. In
yet further embodiments, the SiNW shell constitutes from about 40%
to about 70% of the SiNW structure. In certain embodiments, said
shell is amorphous.
The core-shell shape of the SiNWs depends on the shape of the
SiNWs. In some exemplary embodiments, the SiNWs have a radial
heterostructure, wherein the core is crystalline and the shell is
amorphous. In certain such embodiments, the thickness of the core
is essentially the same as the diameter thereof. The thickness (or
diameter) of the SiNW core can be measured, inter alia, by
Transmission Electron Microscopy (TEM) or by TEM coupled with
Fourier transform analyzer. The thickness of the shell can be
calculated by subtracting the diameter of the core from the
diameter of the SiNW, and dividing the obtained value by the factor
of 2, wherein the diameters of the SiNW and of the core are
extracted from the TEM micrograph.
According to some embodiments, the SiNW core has a thickness of
from about 5 nm to about 40 nm. According to some embodiments, the
SiNW shell has a thickness of from about 5 nm to about 250 nm. In
further embodiments, the SiNW shell has a thickness of from about
10 nm to about 120 nm. In still further embodiments, the SiNW shell
has a thickness of from about 20 nm to about 100 nm. In certain
embodiments, the shell has a thickness of about 20 nm, 25 nm, 30
nm, 40 nm, 50 nm, 60 nm, 70 nm, 80 nm or 85 nm. Each possibility
represents a separate embodiment of the invention. In certain
embodiments, said shell is amorphous.
The length and thickness of the SiNWs according to the principles
of the present invention can be controlled by the CVD growth time.
In particular, the thickness of the amorphous shell can be defined
by the CVD growth time.
FIGS. 2E-2J show low and high-magnification TEM micrographs of
SiNWs according to some embodiments of the present invention,
wherein the SiNWs have a crystalline core and amorphous shell.
The SiNWs can be doped with at least one element selected from the
group consisting of boron, arsenic, phosphorus, iron, chromium,
aluminum and a combination thereof. In certain embodiments, the
doping element is boron. The atomic ratio between the doping
element and silicon can range from about 1:500 to about 1:10,000.
In certain embodiments, the atomic ratio is between about 1:1000
and 1:8000. Composite nanowires, such as, but not limited to,
silicon/germanium alloy nanowires or core-shell nanowires, wherein
the core and the shell comprise distinct materials, are also within
the scope of the present invention.
Si Tubular Nanostructures
According to some embodiments, there is provided an anode
comprising: an electrically conductive substrate, comprising at
least one non-uniform surface; and a random network of silicon
tubular nanostructures, chemically grown on the at least one
non-uniform surface of the substrate.
The term "tubular nanostructure", as used herein, relates in some
embodiments to nanostructures, which are shaped as hollow tubes,
preferably entirely hollow along their longitudinal axis. The terms
"tubular nanostructure" and "nanotubes" can be used
interchangeably.
The tubular nanostructures may include single walled nanotubes or
multi walled nanotubes. The term "multi walled nanostructure", as
used herein, refers to a nanostructure which comprises two or more
walls, each wall being a hollow tubular nanostructure, as defined
herein, wherein each tubular nanostructure is interposed within a
consecutive larger tubular nanostructure. The multi walled
nanotubes may further include hybrid tubular nanostructures such
as, for example, "tube-in-tube" and "wire-in-tube" nanostructures.
In some embodiments of the present invention, the multi-walled
nanostructures can be tubular, conical or both ("funnel-like").
Si nanotubes can have a diameter in the range from about 5 nm to
about 500 nm. According to some embodiments, the length of the Si
nanotubes is from about 0.5 .mu.m to about 500 .mu.m.
According to the preferred embodiments, at least about 30% of the
Si nanotubes have an amorphous morphology. The Si nanotubes can be
doped with at least one element selected from the group consisting
of boron, arsenic, phosphorus, iron, chromium, aluminum and a
combination thereof. Composite nanotubes, such as, but not limited
to, silicon/germanium alloy nanotubes or core-shell nanotubes,
wherein the core and the shell comprise distinct materials, are
also within the scope of the present invention.
Growth of the Si Elongated Nanostructures on the Substrate
The SiNWs and/or Si nanotubes can be grown on the at least one
non-uniform surface of the substrate or, optionally, on the
plurality of non-uniform surfaces of the substrate. Thus, in some
embodiments, the SiNWs are disposed on the first non-uniform
surface of the substrate or, optionally, on the plurality of
non-uniform surfaces of the substrate. According to some
embodiments, the SiNWs are concurrently grown on the plurality of
non-uniform surfaces of the substrate.
Reference is now made to FIGS. 3A-3B, which schematically
illustrate growth of SiNWs on the substrate comprising at least one
non-uniform surface, according to some embodiments of the
invention. FIG. 3A shows bare substrate 101, including non-uniform
surface 103a having a plurality of topographical irregularities
107. FIG. 3B shows anode 201, comprising a plurality of SiNWs 203
chemically grown on non-uniform surface 103a of substrate 101.
Without wishing to being bound by theory or mechanism of action,
the plurality of irregularities 107 afford for the growth of SiNWs
203 in a random network configuration on non-uniform surface
103a.
Reference is now made to FIGS. 3C-3D, which schematically
illustrate growth of SiNWs on the substrate comprising a plurality
of non-uniform surfaces, according to some embodiments of the
invention. FIG. 3C shows bare substrate 111, including a plurality
of non-uniform surfaces 103a, 103b, 103c and 103d having a
plurality of topographical irregularities 107. FIG. 3C shows anode
211, comprising a plurality of SiNWs 203 chemically grown on the
plurality of non-uniform surfaces 103a, 103b, 103c and 103d of
substrate 111. Without wishing to being bound by theory or
mechanism of action, the plurality of irregularities 107 afford for
the growth of SiNWs 203 in a random network configuration on the
plurality of non-uniform surfaces 103a, 103b, 103c and 103d of
substrate 111.
Alternatively, the SiNWs can be grown on the first non-uniform
surface of the substrate and optionally, in the non-uniform bulk
portion of the substrate and the second non-uniform surface of the
substrate. Thus, in some embodiments, the SiNWs are disposed on the
first non-uniform surface of the substrate and optionally in the
non-uniform bulk portion of the substrate and on the second
non-uniform surface of the substrate. In some embodiments, the
SiNWs are grown in the non-uniform bulk surface of the substrate
and on the first non-uniform surface of the substrate. In other
embodiments, the SiNWs are grown in the non-uniform bulk portion of
the substrate, on the first non-uniform surface of the substrate
and on the second non-uniform surface of the substrate. According
to additional embodiments, the SiNWs are concurrently grown on the
first non-uniform surface of the substrate, in the non-uniform bulk
portion of the substrate, and optionally, on the non-uniform second
surface of the substrate.
Reference is now made to FIGS. 4A-4C, which schematically
illustrate growth of SiNWs on the substrate having a non-uniform
bulk portion, according to some embodiments of the invention. FIG.
4A shows bare substrate 121, including first non-uniform surface
123a, second non-uniform surface 123b and non-uniform bulk portion
125. Substrate 121 is made of a plurality of fibers 127. FIG. 4B
shows anode 221, comprising a plurality of SiNWs 203 chemically
grown on non-uniform surface 123a, non-uniform surface 123b and
additional non-uniform surfaces of substrate 121. FIG. 4C shows a
cross section of anode 221 along the line AA' in FIG. 4B. It can be
seen, that a plurality of SiNWs 203 are grown also in non-uniform
bulk portion 125 of substrate 121, comprising a plurality of
non-uniform surfaces, such as 123c, 123d, 123e and 123f.
The SiNWs are chemically grown on the substrate. Thus, one of the
advantages of the present invention is that SiNWs are directly
connected to the substrate. Accordingly, the SiNWs can be connected
to the substrate without a binder.
According to some embodiments, the SiNWs are chemically grown in
the trenches of the non-uniform surface of the substrate. In
further embodiments, the SiNWs are chemically bonded to the
substrate non-uniform surface.
According to some embodiments, the SiNWs are chemically grown on
the fibers of the substrate. In further embodiments, the SiNWs are
chemically bonded to the substrate fibers. According to some
embodiments, the substrate fibers have a thickness of from about 1
to about 30 .mu.m and the SiNWs have a thickness of from about 20
nm to about 300 nm.
One of the multiple advantages of the anodes of the present
invention is that they provide a wide range of silicon loadings,
which can be adjusted to the desired battery configuration,
including exceptionally high silicon loadings. The anode of the
present invention comprises a random network of SiNWs on the
substrate, wherein a silicon loading on the substrate can be from
about 0.3 mg/cm.sup.2 to about 20 mg/cm.sup.2. In some embodiments,
the silicon loading is from about 0.5 mg/cm.sup.2 to about 20
mg/cm.sup.2. In further embodiments, the loading is from about 2
mg/cm.sup.2 to about 6 mg/cm.sup.2. In some embodiments, the
silicon loading is at least about 0.5 mg/cm.sup.2, or at least
about 1 mg/cm.sup.2, or at least about 2 mg/cm.sup.2, or at least
about 3 mg/cm.sup.2, or at least about 4 mg/cm.sup.2, or even at
least about 5 mg/cm.sup.2. Each possibility represents a separate
embodiment of the invention. The high silicon loadings of the
anodes of the present invention can beneficially be used in a
"sandwich-like" cell configuration, comprising an anode positioned
between two cathodes. Said configuration does not require
depositing the anode on the anode current collector. The high
silicon loading of the anode supplies sufficient mass of silicon
for each cathode.
The weight ratio between the carbon substrate and the silicon
nanowires according to the principles of the present invention can
range from about 1:0.1 to about 1:6. In some embodiments, the range
is from about 1:0.2 to about 1:5, from about 1:0.5 to about 1:4 or
from about 1:1 to about 1:3. Each possibility represents a separate
embodiment of the invention. In certain embodiments, the weight
ratio between the carbon substrate and the silicon nanowires is at
least about 1:0.5. In further embodiments, the weight ratio is at
least about 1:1. In still further embodiments, the weight ratio is
at least about 1:2.
According to some embodiments, at least about 10% of the substrate
non-uniform surface is covered by SiNWs. In further embodiments, at
least about 40% of the substrate non-uniform surface is covered by
SiNWs, or at least about 70%, or even at least about 80% of the
substrate non-uniform surface. According to some embodiments, at
least about 10% of the substrate trenches' surface is covered by
SiNWs. In further embodiments, at least about 40% of the substrate
trenches' surface is covered by SiNWs, or at least about 70%, or
even at least about 80% of the substrate trenches' surface.
According to some embodiments, at least about 10% of the substrate
elongated structures' surface is covered by SiNWs. In further
embodiments, at least about 40% of the substrate elongated
structures' surface is covered by SiNWs, or at least about 70%, or
even at least about 80% of the substrate elongated structures'
surface. According to some embodiments, at least about 10% of the
substrate fibers' surface is covered by SiNWs. In further
embodiments, at least about 40% of the substrate fibers' surface is
covered by SiNWs, or at least about 70%, or even at least about 80%
of the substrate fibers' surface. Each possibility represents a
separate embodiment of the invention.
According to some embodiments, at least about 30%, 60%, or 90% of
the SiNWs of the anode is disposed within the non-uniform bulk
portion of the substrate. In some preferred embodiments, the SiNWs
are homogeneously dispersed throughout the substrate.
The thickness of the anode of the present invention can be defined
as a total thickness of the substrate and a layer of the SiNWs
chemically grown on the at least one non-uniform surface thereof.
In some embodiments, the anode has a thickness of from about 5
.mu.m to about 500 .mu.m, such as from about 10 .mu.m to about 400
.mu.m, or from about 20 .mu.m to about 300 .mu.m, or even from
about 50 .mu.m to about 200 .mu.m. Each possibility represents a
separate embodiment of the invention.
SiNWs Chemical Vapor Deposition
According to some embodiments, there is provided a method for
manufacturing an anode comprising providing an electrically
conductive substrate, comprising at least one non-uniform surface;
and chemically growing a random network of silicon nanowires on the
at least one non-uniform surface of the substrate. In certain
embodiments, the SiNWs have at least about 30% amorphous
morphology. In some embodiments, the anode is a lithium ion battery
anode.
The random network of SiNWs can be chemically grown on the
electrically-conductive substrate comprising at least one
non-uniform surface by any suitable technique, configured to
provide growth of the SiNWs on said non-uniform surface, and,
optionally, throughout the total volume of the substrate. The
non-limiting examples of such technique include chemical vapor
deposition (CVD) and solution growth. Each possibility represents a
separate embodiment of the invention. CVD is a chemical process
used to produce high-purity, high-performance solid materials. In
typical CVD, the substrate is exposed to one or more volatile
precursors, which react and/or decompose on the substrate surface
to produce the desired deposit.
According to some embodiments, the SiNWs are grown on the substrate
via a catalytic CVD technique. Catalytic CVD employs a noble metal
catalyst, such as, but not limited to gold and aluminum, in order
to assist the growth of SiNWs. The most common noble metal in the
SiNWs growth process from silicon-containing precursor gases
(silane, disilane) or silicon liquid precursors (trisilane) is gold
(Au). The growth of SiNWs is explained by the vapor-liquid-solid
(VLS) mechanism. Silane is decomposed to form a liquid Au--Si
eutectic on Au particles at low temperatures. According to the
phase diagram of Au--Si, the solubility of Si in solid Au is
negligibly small Pure silicon then precipitates at the liquid-solid
interface.
According to some embodiments, the process of chemically growing a
random network of SiNWs on the substrate includes a step of
pretreating the substrate. Without wishing to being bound by theory
or mechanism of action, the substrate is pretreated in order to
improve the surface wetting properties thereof. According to some
embodiments, the process of chemically growing a random network of
SiNWs on the substrate comprises a step of treating the substrate
by oxygen plasma. The substrate can be pre-treated by oxygen-plasma
gas treatment to increase hydrophilicity of the substrate and
improve its wetting properties. According to some embodiments, the
process further includes a step of treating the substrate by
adsorption of an electrostatic adhesion agent. Said electrostatic
adhesion agent can be poly-L-lysine. Poly-Lysine deposits on the
substrate non-uniform surface, making them positively charged.
Without wishing to being bound by theory or mechanism of action,
deposition of poly-L-lysine on the substrate surface allows the
further deposition of negatively charged gold nanoparticles at a
controlled density on the plurality of substrate non-uniform
surfaces and/or in the non-uniform bulk portion thereof.
The process of chemically growing a random network of SiNWs
according to some embodiments further comprises a step of
deposition of catalytic nanoparticles on the substrate. In some
embodiments, the catalytic nanoparticles are deposited on the at
least one non-uniform surface of the substrate. In some
embodiments, the catalytic nanoparticles are deposited on the
plurality of non-uniform surfaces of the substrate. In some
embodiments, the catalytic nanoparticles are deposited in the
non-uniform bulk portion of the substrate. In further embodiments,
the catalytic nanoparticles are deposited on the first and/or on
the second non-uniform surface of the substrate. The non-limiting
examples of said catalytic nanoparticles include gold, aluminum or
copper nanoparticles. Each possibility represents a separate
embodiment of the invention. Typically, the catalytic nanoparticles
have a diameter in the range from about 5 nm to 500 nm, preferably
from about 20 nm to about 80 nm.
According to some embodiments, the deposition of catalytic
nanoparticles is performed by immersing the substrate into an
aqueous colloidal solution of the catalytic nanoparticles.
According to the principles of the present invention, the substrate
is immersed in the aqueous colloidal solution for at least about 5
min, for at least about 10 min, or for at least about 15 min Each
possibility represents a separate embodiment of the invention.
According to some embodiments, the deposition of catalytic
nanoparticles is performed by electroless deposition. According to
some embodiments, the electroless deposition includes immersing the
substrate into an aqueous/ethanolic solution of a gold salt
precursor and a reducing agent, at different incubation times.
Control over the water/ethanol ratios, gold salt precursor
concentration, reducing agent nature and concentration, and time of
incubation, provide the control over density and dimensions of
deposited gold nanoparticles.
The CVD process is typically performed in a CVD reactor. In some
embodiments, the CVD process is performed at a temperature of about
280-600.degree. C. In some embodiments, the CVD process is
performed at a pressure of from about 400.degree. C. to about
500.degree. C. In some embodiments, the CVD process is performed at
a temperature of at least about 480.degree. C. In some embodiments,
the CVD process is performed in a gaseous atmosphere, comprising
silane (SiH.sub.4), diborane (B.sub.2H.sub.6) and argon (Ar).
According to some embodiments, the CVD process is performed at a
pressure of above about 30 torr. In further embodiments, the CVD
process is performed at a pressure of above about 50 torr.
Without wishing to being bound by theory or mechanism of action,
said CVD process conditions enable formation of SiNWs having at
least about 30% amorphous morphology. It is further contemplated
that said CVD process conditions enable formation of the SiNWs
core-shell structure, wherein the shell is amorphous. In further
embodiments, the core is crystalline.
The CVD process, according to the principles of the present
invention, is typically performed for about 30 to about 120 min. As
mentioned hereinabove, the CVD growth period affects the length and
thickness of the SiNWs, according to some embodiments of the
invention. Without wishing to being bound by theory or mechanism of
action, said correlation between the growth time and the length
and/or thickness of the SiNWs provides effective control over the
Si loading of the anode.
According to some exemplary embodiments, the CVD is performed in a
one-step process. In further embodiments, the one-step CVD process
provides core formation in combination with pyrolytic deposition of
the amorphous silicon shell at the same CVD process conditions.
Accordingly, the one step CVD process is fast and convenient, since
it does not require sequential core formation followed by pyrolytic
deposition of amorphous silicon at different growth conditions. As
mentioned hereinabove, the CVD growth period can define the
thickness of the amorphous shell, thereby providing effective
control over the Si loading of the anode. Without further wishing
to being bound by theory, it is assumed that the crystalline core
thickness is defined by the mean diameter of the catalytic
nanoparticle.
According to other embodiments, the CVD is performed in a
multi-step process.
Si Nanotubes Chemical Vapor Deposition
According to some embodiments, there is provided a method for
manufacturing an anode comprising providing an electrically
conductive substrate, comprising at least one non-uniform surface;
and chemically growing a random network of silicon nanotubes on the
at least one non-uniform surface of the substrate. In some
embodiments, the anode is a lithium ion battery anode.
According to some embodiments, silicon nanotubes are grown on the
substrate through a template assisted method. The template assisted
method generally includes a step of growing sacrificial nanowires
of a given diameter, such as, but not limited to, germanium
nanowires. Further, the nanowires are covered conformally by a
layer of silicon, of a given thickness. Finally the sacrificial
nanowire cores are removed by oxidation and/or sublimation of cores
in a CVD system. In some embodiments, the CVD operating parameters
include oxygen pressure of under about 1 Atm and temperature of
above 350.degree. C. More details about a template assisted method
for growing Si nanotubes may be found in International Patent
Application WO 2010/052704, which is hereby incorporated by
reference in its entirety.
Si Elongated Nanostructures Conducting Coating
Typically, when a Si-based sample is removed from the CVD chamber,
it immediately reacts with air to form a thin silicon oxide
(SiO.sub.2) layer on the surface of Si. The oxide film may increase
impedance of the SiNWs and slow down the silicon lithiation and
de-lithiation processes. In order to avoid or reduce the formation
of the oxide layer on the freshly-prepared SiNWs, the nanowires can
be coated by a protective layer. In some embodiments, the SiNWs are
coated by a conducting coating. The SiNWs comprising the conducting
coating may have a core-shell structure, wherein the core comprises
the SiNW (which can have the crystalline core-amorphous shell
structure) and the shell comprises the conducting coating. In some
embodiments, the Si nanotubes are coated by said conducting
coating.
The conductive coating can be selected to provide additional
desired properties or enhance the existing properties of the SiNWs
grown on the electrically-conductive substrate. For example, the
SiNWs can be coated by an electron-conducting coating, thus
improving electrical contact between the anode, and particularly
the substrate thereof, which can be connected to a current
collector, and Li ions. In that way, the battery impedance is
decreased, providing improved efficiency and a more stable cycle
life. Alternatively, or additionally, the SiNWs can be coated by a
Li ion conducting coating. In some embodiments, the Li conducting
coating forms a solid electrolyte film. Without wishing to being
bound by theory or mechanism of action, formation of the solid
electrolyte film prior to the cycling of the electrode reduces
formation of the SEI during cycling, thus allowing to significantly
decrease the capacity loss and/or the irreversible capacity of the
anode. Thus, according to some embodiments, the conducting coating
comprises an electron conducting coating, a Li cation conducting
coating or a combination thereof. In certain embodiments, the anode
comprises a random network of SiNWs chemically grown on the
electrically conductive substrate, wherein the SiNWs are coated
with the electron conducting coating. In other certain embodiments,
the anode comprises a random network of SiNWs chemically grown on
the electrically conductive substrate, wherein the SiNWs are coated
with the Li cation conducting coating. In further certain
embodiments, the anode comprises a random network of SiNWs
chemically grown on the electrically conductive substrate, wherein
the SiNWs are coated with the electron conducting coating and with
the Li cation conducting coating.
The non-limiting examples of the electron conducting coatings
include carbon, graphite, reduced graphene oxide and combinations
thereof.
The Li cation conducting coating can comprise a solid lithium
electrolyte, including, but not limited to, lithium imide
(Li.sub.3N), lithium oxide (Li.sub.2O), lithium sulfate
(Li.sub.2SO.sub.4), lithium fluoride (LiF), lithium carbonate
(Li.sub.2CO.sub.3), lithium phosphate (Li.sub.3PO.sub.4) or
combinations thereof. In some embodiments, the Li cation conducting
coating on the SiNWs forms a Solid-Electrolyte Interface (SEI). In
some embodiments, the Li cation conducting coating further
comprises a polymer. In some embodiments, the polymer is present in
the Li cation conducting coating in a weight percent of up to about
50% of the total weight of the Li cation conducting coating. The
polymer may be selected from the group consisting of polyolefins,
polycarbonate, poly(methyl methacrylate) and combinations
thereof.
In further embodiments, the Li cation conducting cation comprises a
ceramic material, such as, but not limited to Al.sub.2O.sub.3, ZnO,
TiO, or HfO.
The conducting coating can comprise one or more layers of the
conducting material. In some embodiments, the thickness of the
conducting coating is from about 1 nm to about 10 nm. In further
embodiments, the thickness is from about 2 nm to about 8 nm, or
from about 4 nm to about 6 nm. Each possibility represents a
separate embodiment of the invention.
Conducting Coating Preparation
The conducting coating can be applied to the SiNWs by any suitable
technique, such as but not limited to, CVD, Physical Vapor
Deposition (PVD), pyrolysis of a precursor or salt precipitation
combined with thermal treatment. Each possibility represents a
separate embodiment of the invention. According to some
embodiments, the coating is applied to the SiNWs prior to the
formation of the SiO.sub.2 layer.
In some embodiments, the application of the electron conducting
coating is performed by CVD, PVD, pyrolysis or a combination
thereof. The CVD process for forming a carbon conducting coating
can be performed using methane gas precursor and hydrogen gas as a
carrier. In further embodiments said CVD process is performed at a
temperature of from about 800.degree. C. to about 900.degree. C.
The pressure in the CVD chamber can be about 1-50 torr. The
pyrolysis process can be performed by heating a solution of a
suitable precursor to a temperature of from about at 500 to
1000.degree. C. in the presence of oxygen. The non-limiting
examples of sitable precursors include saccharide, oligomer or
polymer precursors. One non-limiting example of suitable saccharide
is sucrose. The non-limiting examples of the polymers include
poly(methyl methacrylate) (PMMA) or polynitrile (PN).
In some embodiments, the application of the Li cation conducting
coating is performed by PVD, salt precipitation combined with
thermal treatment or a combination thereof. The method of salt
precipitation combined with thermal treatment comprises applying to
the SiNWs a composition comprising a Li salt. The non-limiting
examples of suitable Li salts include lithium nitrate (LiNO.sub.3),
lithium carbonate (Li.sub.2CO.sub.3), lithium triflate
(CF.sub.3SO.sub.3), lithium imide (lithium
bis(perfluoroethylsulfonyl)imide or combinations thereof. The
salt-solvent composition, which is applied to the SiNWs before the
heat treatment, includes a non-aqueous solvent. The non-limiting
examples of suitable non-aqueous solvents include acetonitrile;
propylene carbonate; diethyl carbonate; dimethyl carbonate; glyme,
including without a limitation monoglyme, diglyme, triglyme or
tetraglyme; ether or combinations thereof. The composition can
further comprise a polymer. The non-limiting examples of suitable
polymers include polyolefins, polycarbonate and poly(methyl
metacrylate).
In some embodiments, the method of salt precipitation combined with
thermal treatment comprises evaporating the non-aqueous solvent.
The method can further include heat treating the Li salt at a
temperature of from about 100 to about 1000.degree. C. preferably
from about 300 to about 700.degree. C.
According to some embodiments, the PVD process for the application
of the Li cation conducting coating includes a step of precursor
evaporation. The precursor can be a Li salt, including, but not
limited to, lithium nitrate (LiNO.sub.3), lithium carbonate
(Li.sub.2CO.sub.3), lithium triflate (CF.sub.3SO.sub.3), lithium
imide (lithium bis(perfluoroethylsulfonyl)imide) or combinations
thereof. According to some embodiments, the PVD process evaporation
is performed at a temperature between 500-850.degree. C., depending
on salt precursor vapor pressure. According to further embodiments,
the precursor is volatilized under the flow of inert carrier gas,
such as, but not limited to argon or helium). The volatilized
precursors are then deposited as a shell on surfaces of silicon
nanowires which are held at a lower temperature of about
350-600.degree. C. Control over the volatilizing temperature of
lithium salt precursor, the flow of inert gas, the time of
deposition and the deposition temperature allows to finely control
the Li salt shell thickness within 1-100 nm.
Ceramic Li conducting coating can be applied to the SiNWs by means
of ALD (atomic layer deposition), CVD (chemical vapor deposition),
solution phase coating, or sputtering.
Anode Electrochemical Properties
The lithium ion battery anodes according to the principles of the
present invention exhibit improved electrochemical properties, as
compared to the prior art SiNWs-based anodes. Specifically, the
geometric capacity, irreversible capacity, cycle life stability and
current efficiency of the anodes of the present invention are close
to those of graphite anodes and are required in the current lithium
ion battery technology.
The very low irreversible capacity of about 10% is close to that of
graphite, such that no change needs to be made in the electrode
capacity balance.
In various embodiments, the anode according to the principles of
the present invention has a geometric capacity of from about 0.5
mAh/cm.sup.2 to about 20 mAh/cm.sup.2, from about 1 mAh/cm.sup.2 to
about 16 mAh/cm.sup.2, from about 1 mAh/cm.sup.2 to about 15
mAh/cm.sup.2, from about 1 mAh/cm.sup.2 to about 10 mAh/cm.sup.2,
from about 2 mAh/cm.sup.2 to about 8 mAh/cm.sup.2, or from about 4
mAh/cm.sup.2 to about 6 mAh/cm.sup.2. Each possibility represents a
separate embodiment of the invention. In some embodiments, the
anode has a geometric capacity of at least about 2 mAh/cm.sup.2, or
about 3 mAh/cm.sup.2, or even about 4 mAh/cm.sup.2. Each
possibility represents a separate embodiment of the invention. In
certain embodiments, the anode maintains said geometric capacity
for at least about 20 cycles.
Presently known lithium ion batteries are assembled with 3-4
mAh/cm.sup.2 anode capacity. Thus, the high SiNWs-based anode
capacity of about 4 mAh/cm.sup.2 of the anodes, according to the
principles of the present invention, allows the application of
these anodes in practical, pouch or prismatic, lithium ion
batteries for portable and electric vehicles applications.
Moreover, SiNWs-based anodes of the present invention, having
extremely high capacity of about 7-16 mAh/cm.sup.2 can be used in a
sandwich-like configuration, between two cathodes with no current
collector, in pouch and in prismatic lithium ion batteries. In some
exemplary embodiments, the thickness of said anode is about 160
microns, and divided by a factor of 2 when facing two cathodes,
gives a thickness of about 80 microns which is similar to the
thickness of the graphite anode in common, high-energy lithium ion
batteries.
According to some embodiments, a decrease in the geometric capacity
is below about 10% for at least about 100 cycles at a
charge/discharge rate of 0.1 mA/cm.sup.2. According to further
embodiments the decrease is below about 7%, or below about 5%, or
even below about 3%. According to some exemplary embodiments, the
anode maintains from about 90% to about 97% of the geometric
capacity thereof for at least about 100 cycles at a
charge/discharge rate of 0.1 mA/cm.sup.2.
According to some embodiments, a decrease in the geometric capacity
is below about 27% for at least about 200 cycles at a
charge/discharge rate of 0.1 mA/cm.sup.2. According to further
embodiments the decrease is below about 25%, or below about 20%, or
even below about 16%. Each possibility represents a separate
embodiment of the invention. According to some exemplary
embodiments, the anode maintains from about 73% to about 84% of the
geometric capacity thereof for at least about 200 cycles at a
charge/discharge rate of 0.1 mA/cm.sup.2. According to further
exemplary embodiments, the decrease in the geometric capacity of
the anode is about 0.1% per cycle a charge/discharge rate of 0.1
mA/cm.sup.2. According to further embodiments, the initial
geometric capacity of said anode is at least about 2 mAh/cm.sup.2,
at least about 3 mAh/cm.sup.2, or at least about 4 mAh/cm.sup.2.
Each possibility represents a separate embodiment of the
invention.
It has been shown by the inventors of the present invention that
the anodes according to the principles of the present invention can
be taken out of the exploited cell, washed and reassembled in said
cell, providing at least about 70% of the initial geometric
capacity. In further embodiments, the anode provides at least about
80% of the initial capacity or at least about 90% of the initial
geometric capacity, following washing and cell reassembly. It can
be, therefore, assumed that since washing of the anode enables
substantial rehabilitation thereof, the decrease in the geometric
capacity of the cell during cycling is essentially not caused by
the irreversible pulverization or detachment of the SiNWs from the
substrate. Accordingly, in some embodiments, the SiNWs remain
chemically connected to the at least one non-uniform surface of the
substrate for at least about 100 cycles. In further embodiments,
the SiNWs remain chemically connected to the plurality of
non-uniform surfaces, to the non-uniform bulk portion or to a
combination thereof. Each possibility represents a separate
embodiment of the invention. In some embodiments, at least about
70% of the SiNWs remain chemically connected for at least about 100
cycles, at least about 80% of the SiNWs or even at least about 90%
of the SiNWs. Each possibility represents a separate embodiment of
the invention. In further embodiments, the SiNWs remain chemically
connected to the at least one non-uniform surface, to the
non-uniform bulk portion or a combination thereof for at least
about 200 cycles. In further embodiments, the SiNWs remain
connected for at least about 300 cycles, for at least about 400
cycles or for at least about 500 cycles. Each possibility
represents a separate embodiment of the invention.
According to some embodiments, the irreversible capacity of the
anode is below about 15% of the total geometric capacity. According
to further embodiments, the irreversible capacity of the anode is
below about 12.5% of the total geometric capacity. According to
still further embodiments, the irreversible capacity of the anode
is below about 10% of the total geometric capacity. The very low
irreversible capacity of about 10% is close to that of graphite,
thus, advantageously, no change needs to be made in the electrode
capacity balance.
According to some embodiments, the anode is configured to be
charged and discharged at C rate of at least about 0.4C. According
to some embodiments, the anode is configured to be charged and
discharged at C rate of at least about 1.3C. In some exemplary
embodiments, the decrease in the geometric capacity is below about
10% for cycling at C rate of about C/4 for at least about 70
cycles. In these embodiments, the C rate is calculated for a
LiFePO.sub.4 cathode. The high power demonstrated by the
SiNWs-based anodes of the present invention (less than one hour
discharge and charge time, as disclosed in the experimental section
hereinbelow), is very appealing for various lithium ion batteries
applications.
According to some embodiments, the anode of the present invention
provides at least about 92% Faradaic efficiency. According to some
preferred embodiments, the anode provides at least about 98%
Faradaic efficiency, or at least about 99%, or even at least about
99.5% Faradaic efficiency. Each possibility represents a separate
embodiment of the invention. In said preferred embodiments, the
geometric capacity of the anode is at least about 2 mAh/cm.sup.2,
at least about 3 mAh/cm.sup.2, or at least about 4 mAh/cm.sup.2.
Each possibility represents a separate embodiment of the
invention.
Lithium Ion Battery
The anodes according to the principles of the present invention can
be incorporated into lithium ion batteries. Thus, according to
another aspect, there is provided a lithium ion battery comprising
an anode comprising a three-dimensional electrically conductive
substrate, comprising at least one non-uniform surface; and a
random network of silicon elongated nanostructures chemically grown
on the at least one non-uniform surface of the substrate. In some
embodiments, the silicon elongated nanostructures comprise Si
nanowires. In some embodiments, the silicon elongated
nanostructures comprise Si nanotubes.
The lithium ion battery according to the principles of the present
invention can further comprise an electrolyte. Typically, the
lithium ion battery electrolyte includes a Li salt in a non-aqueous
solvent. The non-limiting examples of suitable electrolytes include
lithium hexafluorophosphate (LiPF.sub.6), lithium
trifluoromethanesulfonate (lithium triflate, CF.sub.3SO.sub.3Li),
or lithium imide (lithium bis(perfluoroethylsulfonyl)imide.
In some embodiments, the anode in the battery faces the
electrolyte. In particular embodiments, the SiNWs grown on the
non-uniform surface of the substrate faces the electrolyte. In
certain such embodiments, the substrate comprises at least one
non-uniform surface. In further certain such embodiments, the
substrate comprises a first non-uniform surface and a second
uniform surface, wherein the first non-uniform surface faces the
electrolyte. The substrate material, according to the principles of
the present invention, should be compatible with the
electrolyte.
The lithium ion battery according to the principles of the present
invention can further comprise at least one cathode. In some
embodiments, the lithium ion battery comprises two cathodes. The
non-limiting examples of suitable electrolytes include lithium iron
phosphate (LiFePO.sub.4), sulfur-based cathode, lithium metal
oxide-based cathode, and air or oxygen cathode. The lithium metal
oxide-based cathode can comprise a metal selected from Ni, Mn, Co
or Al.
The lithium ion battery according to the principles of the present
invention can further comprise at least one current collector. In
certain such embodiments, the substrate comprises at least one
non-uniform surface and a flat surface, wherein the flat surface
faces the current collector. In other embodiments, the substrate
comprises a first non-uniform surface and a second uniform surface,
wherein the second non-uniform surface faces the current collector.
In order to improve electric contact between the anode and the
current collector, an electronically-conducting material can be
applied to the flat surface or the second non-uniform surface of
the anode substrate. Thus, according to some embodiments, the
second non-uniform surface of the substrate is coated with an
electrode ink. According to further embodiments, the electrode ink
comprises carbon powder and a binder. The non-limiting examples of
suitable binders include poly(styrene-co-butadiene), polyvinylidene
fluoride (PVDF), poly(methyl methacrylate) (PMMA) or combinations
thereof. According to yet further embodiments, the electrode ink
further comprises a non-aqueous solvent, such as, but not limited
to, acetonitrile; propylene carbonate; diethyl carbonate; dimethyl
carbonate; glyme, including without a limitation monoglyme,
diglyme, triglyme or tetraglyme; ether or combinations thereof.
Typically, about 0.03 ml to about 0.3 ml, preferably about 0.05 to
about 0.1 ml of the electrode ink is applied to 1 cm.sup.2 of the
substrate second surface.
The anode according to the principles of the present invention,
comprising a random network of SiNWs chemically grown on two
opposed non-uniform surfaces of the substrate can be advantageously
incorporated into a Li ion battery in a "sandwich-like"
configuration. Thus, according to some embodiments, the anode is
disposed in the battery between two cathodes. In certain such
embodiments, the anode is not placed on current collector but
connected to it at one or more of its edges. The anode suitable for
use in said "sandwich-like" configuration can have a geometric
capacity of above about 5 mAh/cm.sup.2, such as, for example, from
about 7 to about 10 mAh/cm.sup.2. In certain embodiments, the anode
further includes SiNWs chemically grown in the non-uniform bulk
portion of the substrate.
According to some embodiments, the lithium ion battery according to
the principles of the present invention is packed in a prismatic
cell, pouch cell, cylinder cell or coin cell configuration.
As used herein and in the appended claims the singular forms "a",
"an," and "the" include plural references unless the content
clearly dictates otherwise. Thus, for example, reference to "an
organic coating" includes a plurality of such organic coatings and
equivalents thereof known to those skilled in the art, and so
forth. It should be noted that the term "and" or the term "or" is
generally employed in its sense including "and/or" unless the
content clearly dictates otherwise.
As used herein, the term "about", when referring to a measurable
value such as an amount, a temporal duration, and the like, is
meant to encompass variations of +/-20%, more preferably +/-5%,
even more preferably +/-1%, and still more preferably +/-0.1% from
the specified value, as such variations are appropriate to perform
the disclosed methods.
The term "plurality," as used herein, means two or more.
The following examples are presented in order to more fully
illustrate some embodiments of the invention. They should, in no
way be construed, however, as limiting the broad scope of the
invention. One skilled in the art can readily devise many
variations and modifications of the principles disclosed herein
without departing from the scope of the invention.
EXAMPLES
Example 1--Anode and Battery Fabrication
SiNWs Growth on the Carbon-Based Substrate:
TABLE-US-00001 TABLE 1 SiNWs anodes having various loadings and
different substrates. Si loading cells built substrate
(mg/cm.sup.2) SiNWs modification of samples cell name Freudenberg
1.08 2 FRD1, FRD2 SGL 1.056 2 SGL1, SGL2 Freudenberg 0.11 2 FRDN1,
FRDN2 SGL 0.46 2 SGLN1, SGLN2 Freudenberg 0.24 2 FRDS1, FRDS2 SGL
0.17 2 SGLS1, SGLS2 SGL 0.23 -- -- SGL 0.36 -- -- Freudenberg 0.59
3 FRDS3, FRDS4, FRDS5 Freudenberg 0.62 -- -- SGL 1.63 -- -- SGL
0.95 3 SGLS3, SGLS4, SGLS5 SGL 1.92 3 SGLS6, SGLS7, SGLS8
Freudenberg 3.46 -- -- SGL 5.16 -- -- Freudenberg 7.88 2 FRDS6,
FRDS7 SGL 4.7 2 SGLS9, SGLS10 Freudenberg 2.63 -- -- SGL 2.87 -- --
Freudenberg 1.88 1 vs. Li, FRDS8, FRDF1 1 vs. LFP SGL 1.55 1 vs.
Li, SGLS11, SGLF1 1 vs. LFP Freudenberg 0.96 -- SGL 0.94 --
Freudenberg 0.48 -- SGL 2.30 -- Freudenberg 2.28 -- Freudenberg
1.98 Boron doping 1:8000 2 FRDS11, FRDS12 Freudenberg 1.88 Boron
doping 1:1000 2 FRDS13, FRDS14 Freudenberg 1.39 Boron doping
1:8000, 1 FRDC3 carbon coating Freudenberg 1.39 Boron doping
1:8000, 1 FRDC4 carbon coating Freudenberg 1.75 Boron doping
1:1000, 1 FRDC5 carbon coating SGL 0.42 Alumina coating 1 SGLA2 SGL
0.97 Alumina coating 1 SGLA3 SGL 0.97 Alumina coating 1 SGLA4 SGL
0.99 -- 1 SGLS14 Freudenberg 0.5 LiF coating 1 FRDL1
Silicon nanowires were grown by the CVD-VLS method on the carbon
fibers-based three-dimensional conductive networks substrates. The
substrates included 160 .mu.m thick SGL25AA carbon paper substrate
(FIG. 1C) and 150 .mu.m thick Freudenberg carbon non-woven cloth
substrate (FIG. 1D). The fibers thickness of the SGL25A carbon
substrate was about 7 microns. The fibers thickness of the
Freudenberg carbon non-woven cloth was about 10 microns. The SiNWs
were grown in a three dimensional configuration, on the surface and
inside the substrate, throughout the whole volume thereof. First,
the carbon fiber conductive networks substrate was treated by
oxygen plasma (400 mTorr, 100 watts) for 10 minutes, in order to
modify and improve its surface wetting properties, followed by the
adsorption of poly-L-lysine at room temperature for 60 min. The
positively charged polylysine layer serves as an electrostatic
adhesion agent for the subsequent deposition of gold nanoparticles.
Following the substrate pretreatment step, the substrate was
immersed in an aqueous solution of Au nanoparticles (AuNPs) having
a diameter of 20-80 nm, for 15 minutes.
SiNWs growth was carried out in CVD reactor via the VLS mechanism
at 460-550.degree. C. and 25-50 Torr, using SiH.sub.4 gas as
precursor (flow rate 5 sccm), B.sub.2H.sub.6 (flow rate 6.25 sccm)
and diluted with Ar gas carrier (flow rate 10 sccm) for a period of
30-120 minutes. CVD was carried out for different periods of time
in order to obtain anodes having various mass loadings of Si, which
are summarized in Table 1.
SiNWs Coating by a Conducting Coating:
Some of the manufactured anodes were subjected to the formation of
a Li ion conducting coating or a carbon coating on the SiNWs, as
presented in FIG. 1.
Solid electrolyte film was formed by first coating the SiNWs by a
thin layer of a non-aqueous solution (NAS) of LiNO.sub.3 or
Li.sub.2CO.sub.3. Following the coating, the non-aqueous solvent
was evaporated at a temperature of 100 to 200.degree. C. to
precipitate the lithium salt. Further, the coated anode was heat
treated at a temperature of 200 to 700.degree. C. to decompose the
LiNO.sub.3, or the Li.sub.2CO.sub.3 salt into its decomposition
product Li.sub.2O or possibly Li.sub.3N.
Alumina coating was formed by an ALD (atomic layer deposition)
process as follows: 10-200 cycles of TMA (trimethyl
aluminum)/Water. deposition temperature 80-150 degrees Celsius.
Trimethyl Alumminum precursor. Water precursor. TMA pulse length
0.015-0.3 seconds. Water pulse length 0.015-0.3 seconds. Purging
duration between pulses 30 seconds-2 minutes.
Carbon coating was prepared by: 1. CVD using methane gas precursor
and hydrogen gas as carrier at a temperature of 850.degree. C. at a
total pressure of 1-50 torr, and 2. Pyrolysis at 500 to
1000.degree. C. of a saccharide or polymer precursor. The sucrose
precursor was sucrose, which was dissolved in a solution of water
and ethanol and pyrolized at 750.degree. C. The polymers included
PMMA and PN.
Some of the SiNWs were doped by boron during the CVD growth
step
Coin Cell Fabrication:
The fabricated anodes were cut into about 1 cm.sup.2 square pieces,
and a drop of an ink made of Shawinigan Black.RTM. carbon and poly
(styrene-co-butadiene) (Sigma-Aldrich) mixture in toluene (9:1 w/w)
was applied to one side of the sheet in order to improve the
electric contact to the stainless steel current collector. The
carbon loading was about 0.5 mg/cm.sup.2. The electrodes were dried
in vacuum for 24 h at 50.degree. C., and 2 h at 100.degree. C.
CR2032 coin cells were assembled inside a glove-box (O.sub.2 and
H.sub.2O<0.1 ppm) filled with ultra-high purified argon. The
coin cells were comprised of the SiNWs-based anode, a 2400 Celgard
separator and a lithium foil. A about 2 to 3 drops of electrolyte
were applied to the separator and to the SiNWs-based anode. In all
tests the electrolyte was 1M LiPF.sub.6EC:DEC (1:1) with 2%
vinylene carbonate (VC) to which, 15% by volume, fluoroethylene
carbonate (FEC) (Solvay-Fluor) was added. The battery-grade
electrolyte was purchased from Solvionic. Lithium metal foil was
purchased from Rockwood Lithium Inc.
Example 2--Morphology Characterization of the Anodes
The morphology of the samples was examined by using several
techniques, including Scanning Electron Microscopy (SEM) on Quanta
200FEG ESEM equipped with an energy dispersive X-ray spectrometer),
Transmission Electron Microscopy (TEM) on FEG-HRTEM, Fei Tecnai
F20, X-ray diffraction (XRD) on Bruker, D8 Discovery, Transmission
Electron Microscopy (TEM) and Environmental Scanning Electron
Microscopy (ESEM).
FIGS. 2A-2C show SEM micrographs of the one of the surfaces of the
anodes, including SiNWs chemically grown on the fibrous carbon
substrate (FIG. 1C), manufactured according to the procedure
described in Example 1, having various Si loadings (0.36, 1.92 and
4.7 mgSi/cm.sup.2, respectively), while FIG. 2D represents a SEM
micrograph of the 4.7 mgSi/cm.sup.2 anode cross-section. It can be
seen that SiNWs are present both on the surface of the anode and in
the bulk portion thereof, thus significantly increasing Si loading
and geometric capacity of the anode.
FIGS. 2E-2J show low and high-magnification TEM micrographs of
SiNWs, prepared according to the procedure described in Example 1,
wherein the SiNWs have a crystalline core and amorphous shell.
FIGS. 4E-4H show the SiNWs grown at 25 Torr for 30 min. The
thickness of the SiNWs ranges from about 60 nm to about 80 nm,
wherein the average core diameter is about 30 nm.
FIG. 2I shows a TEM image of a single nanowire with a crystalline
core and an amorphous shell displaying a total thickness of about
120 nm and a core diameter of about 34 nm Said SiNW was grown at 25
Torr for 60 min. FIG. 2J shows low-magnification TEM image of
nanowires with a crystalline core and an amorphous shell,
displaying a total thickness of about 200 nm and a core thickness
of about 32 nm. Said SiNWs were grown at 25 Torr for 120 min.
A representative high-resolution TEM micrograph shows that the
nanowires consist of a highly crystalline core with visible Si(111)
lattice fringes with a diameter of 30-42 nm surrounded by an thick
amorphous silicon shell, resulting in a total nanowire thickness of
about 60-250 nm, depending on the growth time. Notably, this
crystalline-amorphous core-shell radial heterostructures, formed on
the carbon matrix, were grown in a single CVD step, as described in
Example 1. In can be seen from the TEM images that the thickness of
the amorphous silicon shell, formed by the pyrolitic deposition of
silicon on the growing crystalline core, depends on the growth
time. Longer growth times lead to longer wires, as well as thicker
amorphous shells. Importantly, the diameter of the crystalline
cores remained between 30 and 40 nm (being close to the average
diameter of AuNPs used as catalysts). This single-step approach for
the synthesis of core-shell nanowires of controlled dimensions and
morphology allows for the simple fine-tuning over the silicon
loadings of the resulting anodes.
The higher loading anode (4.7 mgSi/cm.sup.2) was further tested by
Energy-dispersive X-ray spectroscopy (EDS), revealing that both the
crystalline core and amorphous shell of the SiNWs are composed of
100% Si, as presented in Table 2.
TABLE-US-00002 TABLE 2 EDS atomic composition evaluation of the
SiNWs Element % atomic C K 0 O K 0 F K 0 Si K 100 P K 0
FIG. 5 shows an XRD spectrum of the anode having a 1.92 mg/cm.sup.2
Si loading on the SGL carbon paper. According to the XRD spectrum,
1.2% of the entire sample is comprised of crystalline silicon,
whereas silicon makes up about 30% of the total weight of the
sample. It can be therefore concluded that more than 90% of the
silicon in the sample is amorphous.
Example 3--Lithiation and De-Lithiation Capacity and Faradaic
Efficiency of the Anodes
Several SiNW-SGL anodes, with Si loadings in the range of 0.36 to
4.7 mgSi/cm.sup.2 were tested in SiNW/Li coin cells. The cells were
tested at 30.degree. C., with the use of an Arbin BT-2000 battery
tester. For aging tests, the cells were cycled at a constant
current of 1 mA, in the range of 0.01-1V. At the beginning of the
tests, and after 100 and/or 200 cycles, the cells were cycled for a
few cycles at 0.1 mA (FIG. 6A). This was done in order to determine
the maximum (full) capacity of the SiNWs-based anode. The anodes
contain both carbon (in the SGL paper) and SiNW. In order to
calculate the specific capacity of each component, parallel cycle
life tests of only the SGL paper were run. FIG. 6B depicts both the
SiNWs-based anode capacity, and the SGL electrode capacity versus
the cycle number and FIG. 6C shows the SiNWs-based anode capacity.
For anodes having 1.06 mgSi/cm.sup.2 loading, at high current
density of 1 mA/cm.sup.2, the SGL paper capacity is one order of
magnitude lower while, at 0.1 mA/cm.sup.2 the SGL paper provides
about 20% of the anode capacity. The initial de-lithiation capacity
for a 1.06 mgSi/cm.sup.2 anode is 3.7 mAh/cm.sup.2 (FIG. 6B), and
drops to about 3 mAh/cm.sup.2 after 7 cycles, stabilizing at this
level for tens of subsequent cycles. At cycle 27 the current
density was reduced to .about.0.1 mA/cm.sup.2, causing a capacity
rise to about 4 mAh/cm.sup.2. At this point it is important to note
that this value, 4 mAh/cm.sup.2, is one of the highest values
reported for silicon-based anodes of any kind, and in particular
for SiNWs-based anodes.
FIG. 6A depicts a plot of the net silicon de-lithiation capacity
(excluding the carbon paper capacity) as a function of cycle
number. The initial capacity, measured at 1 mA/cm.sup.2, is 3100
mAh/gSi. It drops quickly, after 7 cycles, to 2650 mAh/gSi, and
further decreases slowly to 1350 mAh/gSi after 200 cycles. In order
to find out whether the major reason for capacity loss is due to
disintegration, pulverization (or contact loss) of the SiNWs, or an
increase of the cell impedance, the cells were cycled for a few
cycles at a low rate of 0.1 mA, both at the beginning and at the
end of the tests (FIG. 6A). The capacity loss of three SiNW-SGL
anodes, two having capacity of about 4 mAh/cm.sup.2 and one with
extremely high capacity of 8.63 mAh/cm.sup.2, are summarized in
Table 3. The capacity loss of the SiNW-SGL anodes (measured at 0.1
mA) is between 3 to 10% after 100 cycles, and between 16 to 27%
after 200 cycles, i.e. about 0.1% per cycle and close to that of
the common graphite anode. The loss of the heavily-loaded anode
(8.63 mAh/cm.sup.2) is larger, 39% after 100 cycles and 63% after
200 cycles. These results clearly show that, at least for the 4
mAh/cm.sup.2 anodes, the major reason for the capacity loss is an
increase of cell impedance and not disintegration or pulverization
of the SiNWs.
So far over 300 cycles were demonstrated and some cells are still
running. The average faradaic efficiency, measured for 200 cycles,
for three 4 mAh/cm.sup.2 samples, is 100, 99.2 and 98%, and that
measured for the very heavily loaded 8.63 Ah/cm.sup.2 sample is
92.3% (Table 3). Faradaic efficiencies close to 100%, for the 4
mAh/cm.sup.2 samples, are the highest values measured for such
anodes, as well as of extremely large applicative importance. The
capacity loss for the 4 mAh/cm.sup.2 anodes, after 200 cycles, was
on average 21% (Table 3). Thus, this is the maximum amount of SiNWs
that physically degraded (pulverized) or lost contact to the
current collector during 200 cycles.
TABLE-US-00003 TABLE 3 Capacity and Faradaic efficiency of the
anodes capacity capacity loss Capacity at the initial loss up up to
final Faradaic cycles to mid-life cycles efficiency Cell cycle
[mAh/ [mAh/ cycles cycle (%) for all code no. cm.sup.2] grSi] cycle
no. % no. % cycles SGL2 1-2 4.19 3088 28-32 3 200-204 21 100 SGLS3
11-13 3.78 3088 101-103 10 201-203 27 98 SGLS4 11-13 4.19 3481
101-103 8.7 201-203 16 99.2 SGLS7 11 8.63 4368 101-103 39 201-203
63 92.3
Example 4--SiNWs Morphology Change During Cycling
In order to investigate the effect of cycling on the morphology of
the SiNWs, dQ/dV plots were extracted at several low rate cycles
(0.1 mA/cm.sup.2). A typical behavior is seen in FIG. 7A, where
four anodic peaks (a1, a2, a3, a4) and three cathodic peaks (c1,
c2, c3) are displayed by a 1 mgSi/cm.sup.2 anode at cycle 28. A
dQ/dV plot for the SGL carbon substrate (free of SiNWs) is seen in
FIG. 7B. The sharp a1 peaks and the broad c1 peak are assigned to
the lithiation reaction of SiNWs (c1) and de-lithiation of SiNWs
(a1, a2). The as-prepared SiNWs are mostly amorphous (determined
from XRD data), and during several cycles, crystalline phase is
formed in equilibrium with the amorphous phase as shown by the
sharp de-lithiation peak (a1) at 0.425V (FIG. 7A). Peaks a3, a4, c2
and c3 (FIG. 7A) are associated with the de-intercalation and
intercalation of lithium into the SGL carbon substrate, as seen in
FIG. 7B. After 200 cycles, the carbon SGL peaks become sharper
indicating a higher degree of crystallinity (or a faster
equilibrium between phases). After 200 cycles, the de-lithiation
peak of the SiNWs (a1) almost disappears, reflecting an amorphous
state, while the lithiation peak of SiNWs (c1), remains almost
unchanged, possibly reflecting a change from amorphous to a
partially crystalline state.
Example 5--Irreversible Capacity of the Anodes
Another very important property of silicon anodes is the value of
the irreversible capacity loss at the first intercalation, which is
consumed for the formation of the SEI on the anode (E. Peled, J.
Electrochem. Soc. 126, 2047-2051 (1979)). Most publications on
SiNWs-based anodes display very high irreversible capacity values,
over 30%. One of the significant achievements of the present
invention is a very low irreversible capacity loss of the
SiNWs-based anode, lower than 10%, as can be seen in FIGS. 8A and
8B. These figures depict the first lithiation and de-lithiation
curves for 0.95 and 1.92 mgSi/cm.sup.2 SiNW-based anodes, showing a
8.6% 10.4% loss, respectively. These values are close to the
irreversible capacity loss of common lithium ion batteries using a
graphite anode. FIG. 8C depicts the first lithiation and
de-lithiation curves for a 4.46 mgSi/cm.sup.2 SiNW-based anode,
having the irreversible capacity of about 38%.
Example 6--Charge and Discharge Rate Capability of the Anodes
Additionally, the three-dimensional SiNWs-based anodes of the
present invention provide very high rate capabilities, uncommon for
Si anodes. In FIG. 9, the de-lithiation over-potential was measured
as a function of current density and C rate values for a 1.92
mgSi/cm.sup.2 anode (using the coin cell). At 5 mA/cm.sup.2 (about
0.7C), at 10 mA/cm.sup.2 (about 1.3C) and at 20 mA/cm.sup.2 (about
2.7C) the over-voltage was 0.2, 0.55 and 0.72V, respectively. A
2.7C value means that the battery, assuming a high rate cathode,
can be discharged at 22 minutes only. These overvoltage values are
acceptable for lithium ion battery applications. The lithiation
overvoltage value is, up to 10 mA/cm.sup.2, similar to that of the
de-lithiation process (FIG. 9) indicating a potential for high rate
(about 1.3C) charge. A full cell consisting of a SiNWs-based anode
and a LiFePO.sub.4 cathode was assembled and tested (FIG. 10),
where no efforts were made to balance the anode and cathode
capacity. The cell was cycled at C/4 for over 70 cycles, displaying
only minor capacity loss. It delivered 90 mAh/g of LiFePO.sub.4,
which is about 70% of its theoretical capacity. Further balancing
of the anode and cathode capacity should yield even better
performances.
Example 7--SiNWs Modification
Effect of boron doping and application of conducting coating to the
SiNWs was investigated.
Cells FRDS11 and FRDS14 included SiNWs doped with boron, wherein
the B:Si ratio in FRDS11 cell was 1:8000 and in FRDS14 was 1:1000.
FIGS. 11A and 11B show geometric capacities of said cells cycled at
0.1 mA-1 mA in the voltage range of 1-0.05V. The average Faradaic
efficiency of the anodes of cells FRDS11 and FRDS14 was 99.26% and
99.48% respectively. Irreversible capacity of the anodes of cells
FRDS11 and FRDS14 was 15.8% and 18%, respectively. The lower doping
of SiNWs provided higher charge and discharge capacities and
Faradaic efficiency and lower irreversible capacity.
Cells FRDC3, FRDC4 and FRDS14 included SiNWs doped with boron and
coated with a carbon coating, applied to the SiNWs by pyrolysis of
sucrose. Cells FRDC3 and FRDC4 included doped SiNWs with the B:Si
ratio of 1:8000 and cell FRDS14 included SiNWs with the B:Si ratio
1:1000. FIGS. 12A-12C show geometric capacities of said cells
cycled at 0.1 mA-1 mA in the voltage range of 1-0.05V. The average
Faradaic efficiency of the anodes of cells FRDC3, FRDC4 and FRDS14
was 99.87% (calculated starting from cycle 11), 100.03% (calculated
starting from cycle 4) and 99.77% (calculated starting from cycle
11), respectively. It can be clearly seen that the carbon coating
has a positive effect on the cycle-life stability of the cell,
regardless of the doping level.
Li ion conducting coating including LiF, applied to the SiNWs by a
method described in Example 1, provided an anode having an
irreversible capacity of 39.3% and the average Faradaic efficiency
of 99.898%. FIG. 13 shows geometric capacities of cell FRDL1,
including LiF-coated SiNWs-based anode, cycled at 0.1 mA-1 mA in
the voltage range of 1-0.05V. Despite the relatively low capacity,
the LiF coating allowed cycling of the cell at 1 mA until cycle 300
in contrast to the cells, which were cycled at 1 mA only up to 200
cycles. LiF therefore imparts stability to the SiNWs anode and to
the cell including said anode.
Cells SGLA2, SGLA3 and SGLA4 included SiNWs coated with a Li ion
conducting coating including alumina. Si loading of the anodes of
cells SGLA2, SGLA3 and SGLA4 was 0.42 mg/cm.sup.2, 0.97
mg/cm.sup.2, and 0.97 mg/cm.sup.2, respectively and the weight of
alumina on the SiNWs was negligible. Reference cell SGLA14 included
an anode having Si loading of 0.99 mg/cm2, without alumina coating.
FIGS. 14A-14D show geometric capacities of said cells cycled at 0.5
mA-1 mA in the voltage range of 1-0.05V. The average Faradaic
efficiency of the anodes of cells SGLA2, SGLA3, SGLA4 and SGLA14
was 99.68% (calculated starting from cycle 5), 99.51%, 99.45% and
99.47%, respectively. The irreversible capacity of cells SGLA3,
SGLA4 and SGLA14 was 20.3%, 21.9% and 14.7%, respectively.
Accordingly, anodes including alumina coating on the SiNWs show a
noticeable improvement in the cycle-life stability and the
stability of current efficiency.
Example 8--SiNWs Bonding Stability
Cells FRDS11 and FRDS14 were disassembled after 300 cycles of
operation and the anodes were washed with DMC, ethanol and water.
The anodes were assembled in the cells again and ran for additional
200 cycles. Washing and reassembly of the cells provided charge and
discharge capacities, which were only about 20% lower than the
initial capacities, wherein the cells were ran at the same
currents. It can therefore be contemplated that the decrease in the
geometric capacity of the anode during the cell cycling mainly
results from the adsorption of decomposition products on the
substrate and not from the detachment or pulverization of the
SiNWs.
It is appreciated by persons skilled in the art that the present
invention is not limited by what has been particularly shown and
described hereinabove. Rather the scope of the present invention
includes both combinations and sub-combinations of various features
described hereinabove as well as variations and modifications.
Therefore, the invention is not to be constructed as restricted to
the particularly described embodiments, and the scope and concept
of the invention will be more readily understood by references to
the claims, which follow.
* * * * *