Note: Descriptions are shown in the official language in which they were submitted.
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METHOD FOR FORMING SILICON ANODES
BACKGROUND
1. Technical Field
This disclosure relates generally to batteries and in particular to method for
forming silicon anodes.
2. Description of Related Art
Lithium ion batteries are one of the most important energy storage
technologies, for much of the world's energy storage market ranging from the
consumer electronics to electric vehicles or distributed energy storage
systems. The increasing demands from end users have stimulated the
development of lithium ion batteries with higher energy and power density,
better rate capacity, and longer cycling life.
Silicon (Si) has been long considered as the most promising anode alternative
for use in lithium ion batteries due to its high theoretical capacity,
moderate
working voltage and large abundance on the earth. Despite significant
advances, Si-based anodes still face great problems towards practical
applications. One problem is the lack of a scalable and low-cost fabrication
method for nanostructured Si.
Current fabrication methods have not
adequately addressed a cost-effective way to reduce the cost of Si anode,
due to the process complexity and/or high-cost starting materials. For
example, chemical vapor deposition is able to deposit Si nanoparticles with
less than 100 nm, but requires high temperature and expensive precursors
(such as Si2F16, SiH4). On the other hand, top-down approaches are mostly
based on high-cost electronic-grade Si (n-type, p-type, boron-doped, purity >
99.99999%) as feedstock, and involve the use of template or lithography
steps that increases process complexity. Another problem is that most Si
anodes reported so far are based on half cells (Li metal as counter
electrode),
and the use of excess Li metal in half cells "shield" the efficiency and
cycling
problems of Si anode. It is challenging to assess their feasibility in full
cells
which are required in practical applications.
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SUMMARY OF THE DISCLOSURE
According to a first embodiment, there is disclosed a method for producing an
anode for a battery comprising providing a quantity of metallurgical silicon,
etching the metal deposited metallurgical silicon and depositing a ceramic
layer on the etched metal deposited metallurgical silicon. The method may
further comprise depositing a quantity of a metal on the surface of the
quantity
of metallurgical silicon to produce a metal deposited metallurgical silicon
before etching.
The method may further comprise reducing the size of the metallurgical silicon
into particles having a diameter selected to be between 0.5 and 50 mm. The
reducing may comprise mechanically reducing. The mechanical reducing
may comprise crushing.
The metal may be selected from the group comprising silver, gold, platinum,
nickel and iron. The metallurgical silicon may be immersed in a solution
containing a salt of the metal. The metal may be deposited on to the surface
of the metallurgical silicon through a galvanic displacement reaction.
The ceramic layer may comprise a metal oxide. The metal oxide may be
selected from the group consisting of aluminum oxide, titanium oxide,
zirconium oxide and silicon oxide. The ceramic layer may comprise a metal
oxynitride. The metal oxynitride is selected from the group consisting of
aluminium oxynitride, titanium oxynitride and tantalum oxynitride.
The ceramic layer may be selected to have a thickness between 1 and 20 nm.
The ceramic layer may be formed by atomic layer deposition.
According to a further embodiment, there is disclosed an anode for a battery
comprising at least one particle of porous metallurgical silicon having a
ceramic layer formed thereover.
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The particle of porous metallurgical silicon may be formed by metal assisted
chemical etching a quantity of metallurgical silicon. The ceramic layer may be
formed by atomic layer deposition. The ceramic layer may be selected to
have a thickness of between 1 and 20 nm. A plurality of particles of porous
metallurgical silicon may be secured in a binder.
According to a further embodiment, there is disclosed a lithium ion battery
comprising a cathode, an anode comprising at least one particle of porous
metallurgical silicon having a ceramic layer formed thereover and a non-
aqueous lithium containing electrolyte between the cathode and anode.
Other aspects and features of the present disclosure will become apparent to
those ordinarily skilled in the art upon review of the following description
of
specific embodiments in conjunction with the accompanying figures.
BRIEF DESCRIPTION OF THE DRAWINGS
The accompanying drawings constitute part of the disclosure. Each drawing
illustrates exemplary aspects wherein similar characters of reference denote
corresponding parts in each view,
Figure 1 is a flowchart illustrating one exemplary process of the present
disclosure.
Figure 2 depicts a quantity of metallurgical silicon for use in the
exemplary
process of the present disclosure.
Figure 3 depicts a plurality of particles of mechanically reduced
metallurgical silicon in accordance with the exemplary process of
the present disclosure.
Figure 4 depicts a cleaned plurality of particles of mechanically
reduced
metallurgical silicon in accordance with the exemplary process of
the present disclosure.
Figure 5 is an image of the surface of a particle deposited with a metal in
accordance with the exemplary process of the present disclosure.
Figure 6 is an image of the etched surface of the particle of Figure
5 in
accordance with the exemplary process of the present disclosure.
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Figures 7A-F are images of the surface of the during the exemplary process of
the present disclosure.
Figure 8A-8D are graphs illustrating the cycling performance of batteries
utilizing
an anode produced using exemplary processes of the present
disclosure.
DETAILED DESCRIPTION
Aspects of the present disclosure are now described with reference to
exemplary apparatuses, methods and systems. Referring to Figure 1, an
exemplary process for forming nano-porous silicon (Si) according to a first
embodiment is shown generally at 10. In particular, the present method
provides a cost-effective route to obtain nano-porous Si particles by scalable
metal-assisted chemical etching (MCE) procedure using inexpensive
metallurgical silicon and further introducing of an ultrathin ceramic or other
protective layer deposited by atomic layer deposition (ALD) to stabilize the
Si
anode surface. The combination of the nano-porous structure and nanoscale
ALD coating layer can not only benefit for the electrolyte filtration but also
accommodate large mechanical strains during the lithium insertion and
extraction processes, thus leading to a significant improvement in
electrochemical performance.
As illustrated in Figure 1, the present method 10 utilizes a quantity of
metallurgical silicon 8 as an initial material. It
will be appreciated that
metallurgical silicon is commonly formed by reducing naturally occurring
silica
to silicon however not to a purity level typically required for electronics
production. The metallurgical silicon 8 is then mechanically reduced in size
to
a particle size between 0.5 and 50 mm in step 12. In practice it has been
found for use in battery anode production a size of approximately 5 mm has
been useful. The
size reduction of the metallurgical silicon may be
accomplished by any known means and in particular may be performed by
mechanical reduction including crushing. Thereafter in step 14, the size
reduced metallurgical silicon is cleaned so as to provide a clean surface for
subsequent steps. In particular, the size-reduced metallurgical silicon may be
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degreased in known degreasing compositions including acetone and
isopropyl, water sulphuric acid, hydrogen peroxide and the like.
The cleaned metallurgical silicon particles then have nanoparticles of a metal
deposited on the surface thereof in step 16. In particular, the metallic
deposition may be accomplished by known methods including immersion in a
solution of the metallic salt and deposition on the surface by a galvanic
displacement reaction although it will be appreciated that other methods, such
as, by way of non-limiting example, physical vapour deposition could also be
utilized. In the present disclosure, the metal utilized is silver (Ag)
although it
will be appreciated that other metals may also be utilized, such as, by way of
non-limiting example, gold, platinum, nickel or iron. After the application of
metal particles, the surface of the particles may again be optionally cleaned
or
rinsed in step 18 to remove any residual metal or other materials.
The metal deposited metallurgical silicon may then be etched by any suitable
means in step 20. In particular, the etching may be performed in a suitable
acid, such as, by way of non-limiting example, hydrofluoric acid and hydrogen
peroxide. Alternatively the etching may be performed using other means
including without limitation electrochemical etching without the addition of
the
metal particles. The working principle of chemical etching is based on a local
oxidation of the Si surface by a metal catalyst and H202, and the subsequent
dissolution of silicon oxide in an HF solution. In this galvanic reaction,
metal
Ag nanoparticles act as a local cathode and a catalyst to promote the
reduction of H202 and produce free holes at the interface of Ag and Si, while
the Si surface serves as anode. On the anode, the Si is dissolved
continuously by transferring electrons to the Ag particles at the interface of
Ag
and Si in order to reduce H202 to H20. At the cathode, Ag particles are
oxidized into Ag+ ions by H202, and the Ag+ ions are reduced to Ag by
accepting electrons from Si. With this repeated procedure, the Si underneath
the Ag particles is continuously etched down to make porous structures. The
cathode, anode and total chemical reactions can be listed as the following:
Cathode:
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H202 + 2H+ +2e- 2H20 E = 1.78 V
Anode:
Si+ 2H20 ¨. Si02 + 4H+ + 4e- E = - 0.91 V
Si02 + 6HF ¨. [SiF6]2-+ 2H20 + 2H+
Totals:
Si + 2H202 + 6F- + 4H+ [SiF6]2-+ 4H20
For the total reaction, the standard potential is 2.69 V, indicating that the
etching process is highly thermodynamically favored.
In step 22, the residual nano particles may then be removed from the etched
metallurgical silicon surface through the use of a suitable rinsing agent,
such
as water nitric acid or the like depending upon the metal used and optional
mechanical agitation with ultrasonics or the like. The etched and cleaned
metallurgical silicon particles then have a layer of a ceramic applied thereto
through the use of atomic layer deposition or the like in step 24. In
particular
the protective coating layer deposited in step 24 may be selected to be a
metal oxide, such as, by way of non-limiting example, Aluminum Oxide
(A1203), titanium dioxide (TiO), zirconium oxide (ZrO2) or silicon oxide
(5i02)
or a metal oxynitride, such as, by way of non-limiting example, aluminium
oxynitride (AION), titanium oxynitride (TiOxNy) or tantalum oxynitride (Ta0N).
It will be appreciated that the thickness of this layer may be varied
depending
on the size of the particles and the intended use of the coated particles and
in
particular for use in a lithium ion battery anode may be selected to be
between 1 and 20 nm thick.
It will be appreciated that the process of the present disclosure provides a
cost-effective route to obtain nano-porous Si particles using a scalable metal-
assisted chemical etching procedure from inexpensive metallurgical silicon
and further introducing of an ultrathin ceramic coating to stabilize the Si
anode
surface. The combination of the nano-porous structure and nanoscale
ceramic coating layer may provide benefits for the electrolyte filtration but
also
accommodates large mechanical strains during the lithium insertion and
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extraction processes, thus leading to a significant improvement in
electrochemical performance. The etched and coated particles may
subsequently be mixed into a binder and applied to a current collector to form
an anode for use in a battery.
Example 1
A massive metallurgical silicon (-5 cm) was firstly mechanical crushed into
small particles (-5 mm) for later treatment.
Surface clean: the surface of metallurgical silicon particles was thoroughly
washed by the following procedure. Firstly, the metallurgical silicon was
degreased in acetone and isopropanol for 30 min, rinsed with deionized (DI)
water. Secondly, they were cleaned in a piranha solution (3:1 concentrated
H2504/30% H202) for 15 min at room temperature followed by rinsing by DI
water.
Ag deposition: the metallurgical silicon particle with clean surface was
immersed into the solution of 10 mM AgNO3 and 5 M HF for the Ag
nanoparticles deposition. Ag nanoparticles were deposited onto the surface
of Si via a galvanic displacement reaction. Subsequently, excess silver
precursors were completely removed by rinsing several times with DI water.
Metal-assisted chemical etching: the Ag-deposited Si were immersed into an
etchant consisting of 10M HF and 0.5M H202 at 50 C for 3 h for the etching
reaction. After the etching process, the Si particles were rinsed with
concentrated HNO3 and large amount of DI water to remove any residual Ag
nanoparticles. The nano-porous Si samples (Etched metallurgical silicon)
were obtained by intensive ultrasonication.
ALD A1203 deposition: an ultrathin A1203 coating was obtained at 100 C by
alternatively supplying trimethylamine (TMA) and H20 into a commercial ALD
reactor (GEMStarTm XT Atomic Layer Deposition System) with 30 ALD cycles.
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Example 2
PEALD metal nitride deposition: In an alternative method, metal nitrides (AIN,
TiN) coating on the as-prepared etched metallic silicon are performed in a
plasma-enhanced ALD system (PLEAD). ALD-AIN is deposited at
temperatures of 100 C - 150 C by using trimethylamine (TMA) and plasma
N2 gas (99.999%) as precursors. ALD-TiN is deposited at temperatures of
100 C - 150 C by using tetrakis(dimethylamido)titanium (TDMAT) and
plasma N2 gas (99.999%) as precursors. TiN has a higher electronic
conductivity than ceramics (such as A1203). The use of TD MAT, rather than
TiCI4, as Ti precursor was found to be advantageous for achieving uniform
and high-quality TiN thin film.
Example 3
PEALD metal oxynitride deposition: In an alternative method, Titanium
oxynitride (TiON) is deposited on the as-prepared etched metallic silicon at
150 C by combining ALD TiO2 and TiN deposition cycles in the PEALD
system. TiO2 is deposited by alternatively introducing TD MAT and plasma 02,
while TiN is deposited by using TDMAT and plasma N2. The composition of
TiON is controlled by adjusting the subcycle ratio of TiO2 and TiN (2:1, 1:1,
1:2) in order to tailor the electronic conductivity of titanium oxynitride
thin film.
The coating of TiON is directly applied on the as-prepared etched metallic
silicon.
The morphology and element mapping were observed by using scanning
electron microscope (SEM) equipped with energy-dispersive X-ray
spectroscopy (EDX). Images of the scanned surface are illustrated in Figure
7A through 71. In particular Figure 7A illustrates the resultant particles
after
Ag deposition, Figure 7B illustrates the particles after etching and Figure 7C
illustrates the particles after sonication.
As can be seen from Figure 7a, there shows a homogenous Ag nanoparticles
deposition on the surface of Si particles via a galvanic displacement
reaction.
The deposited Ag nanoparticles will act as an important role in the following
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chemical reaction process in HF-H202 system. As illustrated in Figures 7b and
7c, an obvious porous structure was observed at the surface of the Si. Figures
7d, 7d-1 and 7d-2 show the elements mapping after etching and the
corresponding mapping of Si and Ag, indicating a certain amount of residual
Ag existing on the surface. After being treated in concentrated HNO3, as
shown in Figures 7e, 7e-1 and 7f there is no Ag signal mapping can be
detected any more, meaning that the successful removal of residual Ag.
Finally, the nano porous Si with high purity was obtained as displayed in
Figure 71.
Electrochemical measurements of the etched and coated porous metallurgical
silicon were performed using two-electrode coin cells assembled in an argon
filled glove box. For preparing the working electrode, 60 wt% Etched
metallurgical silicon ALD-A1203 active material, 30 wt% Super P conductive
agent, and 10 wt% poly(vinylidene fluoride) binder were mixed in N-methy1-2-
pyrrolidone to form a slurry which was applied onto a copper foil current
collector using doctor-blade technique and then dried under vacuum at 80 C
overnight. After that, the electrode was cut into round disks (<1>=12 mm) for
the testing electrode in coin cells. The mass loading was around 2.0 mg on
each disk. Electrochemical tests were carried out using CR2032 coin cells
assembled in a glove box filled Ar gas with high purity. Li metal foil was
used
as the counter electrode and polypropylene (PP) (Celgard 2500) as the
separator. 1.3 M LiPF6 in a 3:7 ethylene carbonate: diethyl carbonate (EC:
DEC) with 10% fluoroethylene carbonate (FEC) additive were used as
electrolyte and each cell was filled with 80 pL electrolyte. The cells were
galvanostatically charged and discharged in a voltage window of 0.05-1.0 V
(vs. Li+/Li) at different current densities on a Neware BTS 4000 battery
tester.
The charge/discharge specific capacities were calculated based on the mass
of active materials. All the electrochemical testing was conducted at room
temperature (20 C).
For comparison, a metallurgical silicon powder, which was prepared from
metallurgical particles through ball-milling, was also tested as LIB anodes.
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The metallurgical silicon powder, etched metallurgical silicon and etched and
coated metallurgical silicon were galvanostatically cycling at 0.3C (1C=2000
mA g-1) in the voltage window of 0.05-1.0 V. As shown in Figure 8A, the
metallurgical silicon powder only shows a discharge capacity of less than 20
mAh g-1 for all the cycles, indicating a non-electrochemical activity for the
raw
metallurgical silicon materials. As expected, the etched metallurgical silicon
shows a dramatic capacity increase after the etching which demonstrating the
adopted MCE method can have great effectiveness in producing highly-
electrochemical active Si. The etched metallurgical silicon delivers an
initial
discharge and charge capacities of 3036.1 mAh g-1 and 2120.6 mAh g-1,
respectively, with a Coulombic efficiency of 69.8%. With additional ALD A1203
coating, the etched and coated metallurgical silicon exhibits an initial
discharge capacity of 3099.1 mAh g-1 and a charge capacity of 2265.7 mAh g-
1, indicating a Coulombic efficiency 0f73.1 %. The improved efficiency of the
etched and coated metallurgical silicon (73.1%) in the first cycle compared to
that of the etched metallurgical silicon (69.9%) should be associated with the
ALD A1203 coating layer, which decrease side reactions between the Si and
electrolyte, and expedites the formation of the stable solid/electrolyte
interface
(SEI) layer on the surface of Si. Furthermore, Figures 8A and 8C shows
clearly that the etched and coated metallurgical silicon displays much better
cycling stability and higher capacities than the etched metallurgical silicon.
The discharge capacity of the etched metallurgical silicon changes from
3036.1 mAh g-1 in the initial cycle to 395. 7 mAh g-1 in the 1001h cycle, with
only a capacity retention of 13.0% after 100 cycles. In contrast, the etched
and coated metallurgical silicon exhibits a discharge capacity of 3099.1 mAh
g-1 in the first cycle, and stables at 791.0 mAh g-1, with a higher capacity
retention of 25.5%. The results fully demonstrated that ALD A1203 coating can
greatly enhance the cycling stability upon cycling. Moreover, when the etched
and coated metallurgical silicon was cycling at high rate of 1C (2 A g-1), its
discharge capacity stabilizes at 607.5 mAh g-1 after over 1000 cycles, and the
Coulombic efficiency stills hold above 99.5% during the whole cycling,
indicating a very impressive cycling stability.
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The enhanced electrochemical performance of etched and coated particles
could be ascribed to three reasons. Firstly, the nano size of these particles
not only shorten the diffusion distance of lithium ions and electrons but also
offer higher electrode/electrolyte contact area, benefiting fast kinetic
reaction
inside the bulk. The enlarged contact area will have negative effect on
deteriorating the electrochemical performance if the Si surface is covered
with
uniform and thin coating layer. Secondly, the nano pores insides the particles
can effectively buffer the huge volume changes during the repeated lithiation
and delithiation process, thus leading to superior cycling performance with
high lithium storage capacity. Lastly, the uniform ALD A1203 coating layer
with
only about 3 nm thickness can prevent the direct contact between the
electrode and electrolyte with suppressed side reactions and create a stable
SEI layer at the electrode/electrolyte interface. In particular, The resulting
etched and coated metallurgical silicon particles showed enhanced
electrochemical cycling stability and superior lithium storage capacity.
While specific embodiments have been described and illustrated, such
embodiments should be considered illustrative only and not as limiting the
disclosure as construed in accordance with the accompanying claims.
Date Recue/Date Received 2021-04-30
