High-yield synthesis of single-crystal silicon nanoparticles as anode materials of lithium ion batteries via photosensitizer-assisted laser pyrolysis

Article abstract of DOI:10.1039/c4ta03358b

Single crystal silicon nanoparticles (Si-NPs) of 20 nm were produced via laser pyrolysis with a virtually complete conversion from SiH₄ to Si-NPs. SF₆ was used as the photosensitizer to transfer laser beam energy to silicon precursor

Full text of DOI:10.1039/c4ta03358b

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2014, DOI: 10.1039/C4TA03358B.
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†
‡
‡
Si-NPs have been introduced into market, the cost-effective
production of Si-NPs is still emphasized. Compared with other
Introduction
Silicon nanoparticles (Si-NPs) have been broadening
application boundaries of elemental silicon since they can be
dispersed in various solvents via proper surface chemical
treatments.1,2 Physicochemical properties inherited from bulk
silicon as well as photoluminescence characteristics due to
quantum confinement effects of Si-NPs facilitate their uses in
optoelectronic devices, energy devices and biomedical
imaging.3-16 As a practically foreseeable application, Si-NPs
can be used as an anode material of Li-ion batteries (LIBs) with
the highest gravimetric capacity (4200 mAhg-1) among anode
candidates developed until now. A number of attempts using
nanostructured silicon for LIB applications have been made to
overcome the demerits of silicon such as volume expansion,
pulverization and evolving electrolyte decomposition which are
practically observed with macro-sized silicon materials under
repeated charge/discharge cycles.17-20 Realizing full capacity of
the silicon anodes with stable operations might shed light on
ubiquitous environments and environment-friendly societies
based on electric vehicles and smart grids.
methods,
laser pyrolysis of silicon precursor gases is
considered relatively more promising for production of Si-NPs
in terms that high-purity Si-NPs can be produced without being
exposed to contaminants in a continuous manner.22,23,25-29 High
conversion efficiency of precursors to Si-NPs is practically
important in the laser pyrolysis because of the high cost of the
precursors. To save energy required for operating laser at high
power, also, thermal loss should be reduced by directing the
laser beam energy to the chemical conversion of precursors to
Si-NPs in an efficient way. By using a photosensitizer, in this
work, we successfully demonstrate a virtually complete
conversion of silicon precursors (SiH4) to Si-NPs by the laser
pyrolysis. SF6 as the photosensitizer absorb laser beam
efficiently and release thermal energy that can be utilized for
the conversion. We also demonstrate the stable operation of
LIB using the Si-NPs as an anode material.
Experimental
Various methods are available for Si-NPs production. Si-NPs
can be prepared from bulk silicon by using mechanical ball
milling or chemical/electrochemical etching with distributed
dimensions.21,22 By using plasma, Si-NPs are produced from
gaseous precursors with high yield.23 However, deposition of
silicon is inevitable and large space is necessary for mass
production system. Solution-phase synthesis of Si-NPs is also
available but high temperature process is necessary.24 Even if
Laser pyrolysis for Si-NPs synthesis
All the gas flows were controlled by mass flow controllers. CO2
laser at 57 W (95% of maximum power) was used with the
chamber pressure at 400 Torr. The produced Si-NPs were
collected through a membrane filter installed in a tightly sealed
collector. The Si-NPs were separated by a membrane filter and
treated after being transferred to a N2-filled glove box.
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Emission spectroscopy
Electrochemical tests
Emission of reaction flames was measured through an optical All the cells were galvanostatically lithiated and delithiated at
fiber by a linear diode array (with a total length of 24.6 mm and indicated C-rates (WBCS3000S, WonATech). 1C was
a height of 3.0 mm) with a triple grating monochromator (focal determined experimentally to be 1762 mA g-1, based on the
length = 0.3 m, 1200 grooves/mm grating; SpectraPro-300i, available capacity measured at low current discharge condition
Acton Research corporation). The spectra obtained were (0.05C based on the above definition).
calibrated in wavelength (not intensity).
Electrochemical impedance spectra were obtained from 200
kHz to 100 mHz (VMP3, BioLogic).
Cell preparation
The Si-NPs were carbon-coated at 10 wt. % of the active
material by vapor deposition. Toluene gases were introduced
Results and Discussion
into a tube furnace at which the samples were located and
A CO2 laser beam (10.6 μm wavelength; continuous wave
carbonized at 900 oC for10 min. Coin-type half cells were
(CW)) crossed gas streams jetted from coaxially aligned inner
configured with Si-NP-containing electrodes as a working
and outer tubes (Fig. 1a). The feedstock gases containing SiH4
electrode and lithium metal as a reference electrode. A
with or without SF6 and H2 are supplied from the inner tube
separator (NH 716, Asahi) was sandwiched between the two
while helium gas is supplied by the outer tube to confine the
electrodes. The working electrodes were prepared by coating
gas stream lines. The main reaction to produce Si-NPs is:
mixture slurries on copper foils. The mixture slurries were
made of the carbon coated Si-NPs with a mixed polysaccharide
binder and carbon black at 60:20:20 in weight. Loading amount
of the composite was 1.7 mg cm-2 with 20 μm thickness.
SiH4  Si + 2H2
(1)
Fig. 1 (a) A schematic of the laser pyrolysis converting silicon precursors to Si-NPs. (b) The gas-to-solid conversion ratio as a function of gas compositions (SF6 and H2).
(c) Photos of reaction flames during the pyrolysis. Gas flows of SF6 in presence of 25 sccm of SiH4 and 100 sccm of H2 were used at 0, 10, 15 and 60 sccm as indicated.
As a control, the most right photo was obtained at the same condition of 60 sccm of SF6 and 100 sccm of H2 except SiH4. (d) The emission spectra of the reaction
flames. Deep slump at ~950 nm is due to absorption by optical fibers while sharp peaks at 675 nm come from a guide laser beam. The spectra were c alibrated in
wavelength but not in intensity.
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The reference Si-NP sample have been prepared by the above following energy transfer to SiH4 or the detonation reactions
simple reactions in absence of photosensitizers because SiH4 between SF6 and SiH4. The colors of flames were blue-shifted
molecules as a precursor is known to absorb the CO2 laser with increasing concentration of SF6 (Fig. 1d).36 With 10 sccm
beam at the 10.6 μm wavelength.30 In other cases such as Ge SF6 at which the virtually complete conversion was obtained,
nanoparticle production, however, the use of photosensitizers is the emission spectrum was only intensified without spectral
required because typical Ge-containing precursors (GeH4) changes (or no color change) when compared with the SF6-
cannot absorb the laser beam efficiently.31,32 In this work, a absent but SiH4-present control. It indicates the formation
photosensitizer (SF6 here) was first used for Si-NP production reaction of Si-NPs are dominant over the detonation reaction at
to enhance the production yield, considering high absorption the condition with producing more amount of Si-NPs.
cross section of SF6 molecules upon the CO2 laser beam. The However, with 15 sccm and 60 sccm SF6, the detonation
absorbed energy excites vibrational and rotational molecular reaction would be competitively or dominantly involved. Blue
motions of SF6 and then is transferred to SiH4 molecules via spectral region became more emphasized so that the spectra at
collisions33 leading to an enhancement in SiH4 molecule 60 sccm SF6 finally reached the spectral shape similar to the
,
dissociation and subsequent Si-NPs synthesis.
SiH4-absent but SF6-present control (far right photo in Fig. 1c).
The gas-to-solid conversion during the laser pyrolysis was One more characteristic of the spectra that should be notified is
calculated by the mass ratio of produced Si-NPs to SiH4 gas the periodic fluctuation of spectra only with the detonation-
passing through laser beam path (Fig. S1 to S5, the equation in dominant situations (15 to 60 sccm SF6 in presence or absence
Supplementary Information). By the help of SF6, the gas-to- of SiH4). The periodic peaks observed in 350 to 450 nm and
solid conversion increased significantly (Fig. 1b, Table S1). 850 to 950 nm are attributed to the emission from molecular
Without H2 gas, the conversion increased more than six folds vibration modes of HF as a product of the detonation
from 12.4 % to 75.0 % by addition of 5 standard cubic reaction.34,37 Monitoring the spectra of the reaction flames
centimeters per minute (sccm) of SF6. More amounts of SF6 makes it possible to distinguish detonation reaction from the Si-
were not tested in absence of H2 because uncontrollable NPs production process.
detonation reactions with higher concentration of SF6 was
expected:
3SiH4 + 2SF6 2S + 3SiF4 + 2HF + 5H2
(2)
The detonation reaction during the photosensitizer-assisted
laser pyrolysis might provoke fatal accidents as well as failures
in Si-NPs production (Fig. S6).34 Also, by-products of the
detonation such as HF could make troubles to the facilities. To
suppress the detonation reaction, 100 sccm of H2 gas was
additionally incorporated. The backward reaction of the eqn 2 is
encouraged with the higher H2 concentration. Also, the reaction
zone is maintained below the critical temperature of SF6
dissociation by dilution with additional H2. Surprisingly, the
conversion increased up to 97.1 % in a condition of 10 sccm of
SF6 and 100 sccm of H2. The small loss of Si-NPs is
unavoidable due to the Si-NPs leftover on a membrane filter
during weighing. Therefore, the most of SiH4 molecules is
considered to convert to Si-NPs. Further increase of SF6 flow
rate up to 15 sccm in presence of the same contents of H2 led to
a dramatic decrease in conversion to 16.3 %. The detonation
reaction was not controlled at the high concentration of SF6.
The competition between the silicon production reaction and
the detonation reaction during the photosensitizer-assisted laser
pyrolysis was investigated by emission spectra of reaction
flames (Fig. 1c and d). Si-NPs emitted light in red to orange
color (far left photo with SiH4 in absence of the photosensitizer
in Fig. 1c)35 while flames from SiH4-absent gas were in blue
(far right photo with SF6 in absence of the silicon precursor).
Intensities of reaction flames increased significantly with SF6
contents when compared with both the SF6-absent and SiH4-
absent controls, indicating thermal activation of SF6 and the
Fig. 2 a-d) HR-TEM images of Si-NPs produced with different contents of SF6 in
presence of SiH4:25 sccm and H2:100 sccm. Scale bar = 50 nm for (a), (c) and 10
nm for (b), (d). The flow rates of SF6 were indicated as numbers in sccm.
Si-NPs of 15 to 20 nm in average (Fig. S7) were obtained
successfully by the laser pyrolysis of SiH4 (Fig. 2).The most
distinguished feature between Si-NPs prepared in absence and
presence of SF6 (Si-NP(-SF6) and Si-NP(+SF6), respectively) is
their crystallographic difference. Single crystalline Si-NPs were
obtained in presence of SF6 with their lattice fringes clear (Fig.
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2c and d, Fig. S8) while polycrystalline Si-NPs were developed
Electrochemical performances of alloying and dealloying of
in absence of SF6 (Fig. 2a and b). The lattice distance measured Si-NPs with lithium were investigated between Si-NP(-SF6) and
at 0.317 nm (Fig. 2d) matches with the spacing value of (111) Si-NP(+SF6) (Fig. S11). Both of them showed the same capacity
plane of the silicon. The size distributions were fairly narrow retention during cycling charge and discharge at slow rates (0.2C
for all samples: average size (standard deviation) = 18.6 nm (6 and 0.5C, respectively) (Fig. 4a and b). Also, Si-NP(+SF6)
nm) for Si-NP(-SF6) and 18.7 nm (4.3 nm) for Si-NP(+SF6). X- showed the rate-dependency of capacities similar to or a little bit
ray diffraction patterns and Raman spectra confirmed the higher superior to Si-NP(-SF6) (Fig. 4c and d). More interestingly,
crystallinity of the Si-NP(+SF6) (Fig. 3) than that of the Si-NP(- however, the cyclability at fast charge and discharge rates
SF6). Crystallite size of the Si-NP(+SF6) was estimated at 20.1 revealed their intrinsic difference (Fig. 4e). The Si-NP(-SF6) did
nm by Scherer equation, which is approximates the particle not deliver electrochemically meaningful capacities at 3C for
sizes (Fig. 3a). Crystallinity of the Si-NP(+SF6) was estimated both lithiation and delithiation after the 600th cycle. On the other
more than five times as high as that of the Si-NP(-SF6) in terms hand, Si-NP(+SF6) delivered its capacities at 36 % of the initial
of the area fraction of crystalline phase in Raman spectra: 10 % capacity at the 1000th cycle.
for Si-NP(-SF6) and 54 % for Si-NP(+SF6) (Fig. 3b). Impurities
such as sulfur or fluorine were not detected in the Si-NP(+SF6)
by energy dispersive spectrometer under the TEM (Fig. S9 and
S10) while 0.4 wt. % sulfur was found in Si-NP(+SF6) (Table
S2). The higher crystallinity of Si-NPs which is produced by
photosensitizer-assisted pyrolysis is due to the efficient
absorption of energy from the laser beam by SF6 and the
following energy transfer to the conversion reaction to Si-NPs.
The use of 10 sccm SF6 would be an optimized condition in this
work, achieving the virtually complete conversion from SiH4 to
Si-NPs while preventing detonation reactions. At this condition,
the temperature of reaction zone would be controlled between
~900 K (dissociation temperature of SiH4) and ~1500 K
(dissociation temperature of SF6)38.
Fig. 4 (a and b) Potential profiles and capacities (Q) during repeated cycles of
lithiation at 0.2C and delithiation at 0.5C. (c and d) Potential profiles and
capacities at various C-rates. Lithiation and delithiation rates were used at the
same values as indicated. (e) Capacity retention with cycles at rates of 3C. (f and
Fig. 3 (a) X-ray diffraction patterns of Si-NPs. (b) Raman spectra (Si-NP(-SF6) at
g) Electrochemical impedance spectra measured at 0.4 V after the 1st and 500th
the top; Si-NP(+SF6) at the bottom). The spectra were deconvoluted by using
cycle. The possible reason for the capacity increase during the initial cycles (b)
Gaussian and Lorentzian models for amorphous (~ 480 cm-1) and crystalline (~
and after rate variation experiments (d) was discussed in Supplementary
520 cm-1) phases. The area fractions of crystalline phase were estimated at 10 %
Information.
for Si-NP(-SF6) and at 54 % for Si-NP(+SF6).
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The electrochemically kinetic superiority of Si-NP(+SF6) to single crystal Si-NP(+SF6) showed anisotropic or directional
Si-NP(-SF6) was supported by impedances (Fig. 4f and g). The volume expansion following specific crystallographic
a
single crystalline Si-NP(+SF6) showed smaller resistance direction. The preferential growth of volume expansion consists
responsible for charge transfer through the interfaces between with an experimental evidence that the <110> direction of
electrolyte and the solid-electrolyte interphase (SEI) layer silicon is favored for the expansion because lithium ions
(RSEI) and/or between the SEI layer and silicon mass (RCT), preferentially diffuse along the channels.40 Also, a coarse
compared with their amorphous counterparts Si-NP(-SF6). Due structure was developed with the Si-NP(+SF6) while a dense
to serious merging between RSEI and RCT, the spectra were sphere was formed with Si-NP(-SF6). We consider that the
described by a single parallel circuit of resistance and kinetic facility of the single crystal Si-NP(+SF6) comes from a
capacitance: RSEI+CT = 14 ohm for Si-NP(+SF6) versus 37 ohm channel-footprinted morphology formed during amorphization.
for Si-NP(-SF6). After cycling, the semicircles responsible for That is to say, the channels of lithium ions are memorized in
charge transfer resistances were decoupled to two different ones the originally single-crystal Si-NPs during amorphization even
as the SEI layer developed more. The higher-frequency parts if we cannot say there are aligned routes for lithium ions. A
(left ones) are related to RSEI while the lower-frequency ones lithium diffusion pathway would follow a shortcut for alloying.
are for that of RCT. Both of RSEI and RCT of Si-NP(+SF6) were In the synthetically-born amorphous Si-NP(-SF6), however,
measured at still smaller values, compared with those of Si- there would be less possibilities that the footprints are left
NP(-SF6): RSEI = 6 ohm for Si-NP(+SF6) versus 13 ohm for Si- because the Si-NPs were initially amorphous-phase-dominant.
NP(-SF6); RCT = 10 ohm for Si-NP(+SF6) versus 25 ohm for Si- Lithium ions would travel via detours to find
NP(-SF6).
thermodynamically favored alloying sites. Therefore, the
single-crystal Si-NPs are electrochemically advantageous over
their amorphous counterparts due to in situ generation of Li+
diffusion channels during lithiation.
Conclusions
In summary, we presented a high-yield production of single-
crystal Si-NPs by using
a photosensitizer-assisted laser
pyrolysis. The unique point of this process is a third molecule
(called photosensitizer), different from reactants and inert
guiding gases, transfers energy of the laser beam to the silicon
precursors in a more efficient way. Use of SF6 as the
photosensitizer enhanced the production yield up to virtually
complete conversion from SiH4 to Si-NPs (97.1%) and degree
of Si-NP’s crystallinity. We could investigate the directional
volume expansion of Si-NPs in a clear manner after lithiation
because our particles had well-defined crystallographic planes
developed throughout a whole particle dimension. We believe
this work would provide an impact to industrial as well as
academic societies in terms of enabling mass production of
single-crystal nanoparticles.
Fig. 5 HR-TEM images of the amorphous Si-NP(-SF6; a and c in the left column)
and the single crystalline Si-NP(+SF6; b and d in the right column) before (a and b
in the first row) and after lithiation (c and d in the second row).
Acknowledgements
This work was supported by MOTIE (Star : 20135020900030),
The next question would be why the single crystal Si-NP(+SF6)
showed better kinetics of lithiation/delithiation than their
amorphous counterparts from the viewpoint of materials.
Silicon experiences amorphization during dealloying process
following the first alloying or lithiation.39 There is no
significant difference of crystallography between the single
KETEP (2012T100100740), MOE (BK21Plus
10Z20130011057) and KIER (B4-2424), Korea.
: META,
Notes
a Department of Energy Engineering and School of Energy and Chemical
UNIST, Ulsan 698-798, (Korea) E-mail:
philiphobi@hotmail.com, jykim@unist.ac.kr
crystal Si-NP(+SF6) and their amorphous or polycrystalline Engineering,
counterparts Si-NP(-SF6) after the second cycle because both of
them are amorphized. However, different morphologies were
evolved after volume expansion caused by lithiation (Fig. 5).
The amorphous Si-NP(-SF6) experienced isotropic or non-
directional dimensional changes, leaving hollow cores caused
by outward growth of the silicon mass. On the contrary, the
b
Korea Institute of Machinery and Materials (KIMM), Yuseong-gu,
Daejeon 305-343, (Korea)
School of Advanced Materials and System Engineering, Kumho
c
National Institute of Technology, Gumi 730-701 (Korea)
d
KIER-UNIST Advanced Center for Energy, Korea Institute of Energy
Research, Ulsan 689-798, (Korea)
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†
Footnotes should appear here. These might include comments 22 L. T. Canham, Appl. Phys. Lett. 1990, 57, 1046-1048.
relevant to but not central to the matter under discussion, limited 23 L. Mangolini, E. Thimsen, U. Kortshagen, Nano Lett. 2005,
experimental and spectral data, and crystallographic data. 659.
Electronic Supplementary Information (ESI) available: [details of any 24 C. M. Hessel, D. Reid, M. G. Panthani, M. R. Rasch, B. W.
5
, 655-
supplementary information available should be included here]. See
DOI: 10.1039/b000000x/
Goodfellow, J. W. Wei, H. Fujii, V. Akhavan, B. A. Korgel, Chem.
Mater. 2012, 24, 393-401.
‡Seongbeom Kim and Chihyun Hwang contributed equally to this work.
25 C. M. Hessel, D. Reid, M. G. Panthani, M. R. Rasch, B. W.
Goodfellow, J. Wei, H. Fujii, V. Akhavan, B. A. Korgel, Chem.
Mater. 2011, 24, 393-401.
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