Nano Res
1
Hydrothermal Synthesis of Nano-silicon from Silica Sol
and its Lithium Ion Batteries Property
Jianwen Liang, Xiaona Li, Yongchun Zhu(), Cong Guo, Yitai Qian()
Nano Res., Just Accepted Manuscript • DOI: 10.1007/s12274-014-0633-6
http://www.thenanoresearch.com on November 7 2014
© Tsinghua University Press 2014
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Nano Research
DOI 10.1007/s12274-014-0633-6
TABLE OF CONTENTS (TOC)
Hydrothermal Synthesis of Nano-silicon from
Silica Sol and its Lithium Ion Batteries Property
Jianwen Liang, Xiaona Li, Yongchun Zhu,* Cong
Guo, Yitai Qian*
Hefei National Laboratory for Physical Science at
Microscale and Department of Chemistry, University of
Science and Technology of China, Hefei, Anhui 230026,
PR China
A large-scale hydrothermal synthetic strategy for synthesis of
porous silicon nanospheres from industrial silica sol at 180 °C.
The hydrothermal synthetic route reported here is relatively
simple and low-cost, and the as-prepared porous silicon
nanospheres anode delivers a high reversible specific capacity
and significantly cycling stability.
Hydrothermal Synthesis of Nano-silicon from Silica Sol
and its Lithium Ion Batteries Property
Jianwen Liang, Xiaona Li, Yongchun Zhu(), Cong Guo, Yitai Qian()
Received: day month year
Revised: day month year
Accepted: day month year
(automatically inserted by
the publisher)
© Tsinghua University Press
and Springer-Verlag Berlin
Heidelberg 2014
KEYWORDS
silicon, hydrothermal
synthesis, nanomaterials,
silicon sol, energy storage
ABSTRACT
There are rare reports concerning hydrothermal synthesis of silicon anode
materials. In this manuscript, starting from the very cheap silica sol, we
hydrothermally prepared porous silicon nanospheres in autoclave at 180 °C. As
anode materials for lithium-ion batteries (LIBs), the as-prepared nano-silicon
anode without carbon coating delivers a high reversible specific capacity of
2650 mA h g−1 at 0.36 A g-1 and significantly cycling stability about 950 mA h g-1
at 3.6 A g-1 during 500 cycles.
1 Introduction
Silicon has been considered as a promising anode
candidate material for advanced lithium-ion batteries
(LIBs) due to their high theoretical capacity (3579
mAh g-1), relatively low discharge potential (<0.5 V
versus Li/Li+) [1]. However, Si exhibits serious
volume change (>270 %) during
lithiation-delithiation, which leads to a rapid
reduction in capacity [2,3]. Similar with other
electrode materials, using Si materials with
nanostructure is one of means to relieve this problem
[4-9].
For this purpose, various methods have been
developed to produce nano-silicon anode materials
improving LIBs performance. One of these methods
was chemical vapour deposition (CVD) of silanes, by
which silicon nanotubes were prepared. Then after
SiO2 surface-coating, the Si/SiO2 nanotubes shown
the long cycle life (6,000 cycles with 88% capacity
retention), high specific charge capacity (~2971/1780
mAh g-1 at 0.4 A g-1, ~940/600 mAh g-1 at 24 A g-1) [10].
Nano Research
DOI (automatically inserted by the publisher)
Address correspondence to [email protected]; [email protected]
Research Article
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2 Nano Res.
Nano-Silicon anode materials are also prepared by
the typical magnesiothermic reduction reaction
[11-16]. For example, magnesiothermic reduction of
SiO2 at 650°C was used to synthesize Si nanotubes,
which obtain a capacity of about 1900 mAh g−1 at 0.4
A g-1 and retention as ~50% after 90 cycles after
carbon coating [11].
With regard to wet chemical synthesis of
nano-silicon anode materials, much attention has
been paid to the preparation in organic solvent
[17-19]. For example, nest-like silicon nanospheres
were prepared via reaction of NaSi and NH4Br in the
mixed solvent of pyridine and dimethoxyethane in
autoclave at 80 °C for 24h, which exhibiting
reversible specific capacity of 1095 mAh g-1 at 2 A g-1
after 50 cycles. Si nanoparticles were produced by
reduction of SiCl4 with naphthalene sodium in
anhydrous tetrahydrofuran at 380 °C in Hastelloy
Parr reactor, which showing specific charge capacity
of 3535 mAh g-1 at 0.9 A g-1 and retention as 96 %
after 40 cycles after carbon coating [17]. Si
nanoparticle were synthesized via reduction
of anhydrous SiCl4 with sodium potassium alloy
(NaK) in toluene solution under reflux for 4 h,
followed by oxidation, the resulting sample as
Si/SiOx/SiO2 shows an initial charge capacity of 610
mA h g−1 at 0.2 A g-1, good cycling stability
(maintained at ~600 mA h g−1 after 350 cycles) [20].
Recently, it is reported aqueous synthesis of
hydrophilic silicon nanoparticles (~2.2 nm) based on
the reaction of (3-Aminopropyl) trimethoxysilane
(C6H17NO3Si) and trisodium citrate dihydrate under
the microwave irradiation at 140 °C [21]. Silicon
nanowires were also grown in an aqueous solution
[22]. Previously, we have fabricated silicon
micromaterials by reducing crystalline Na2SiO3·9H2O
with Mg in autoclave at 200 °C [23]. After combined
with graphene, the as-prepared anode shows
reversible capacity of ~600 mA h g-1 at the current
density of 3.6 A g-1 after 360 cycles.
In this study, we hydrothermally prepared porous
silicon nanospheres by reducing silica sol with
metallic magnesium in autoclave at 180 °C. Moreover,
besides silica sol, this hydrothermal reduction
reaction can be extended to the reduction of solid
silica powders such as silica aerogel and silicic acid
(hydrated silica). The as-prepared nano-silicon anode
delivers a high reversible specific capacity of 2650
mA h g−1 at 0.36 A g-1 and cycling stability about 950
mA h g-1 at 3.6 A g-1 after 500 cycles.
2 Experimental
2.1 Materials
30% alkaline silica sol (industrial, pH = 13) was
purchased from GuangZhou Sui Ze Environmental
Protection Technology Co., Ltd (China,
http://www.yuancailiao.net/company/shop138367076
4546/index.aspx). Mg (99%, 100-200 mesh powder),
HCl (37%) and Hydrofloric acid (≥ 40%) were
purchased from Sinopharm Chemical Reagent Co.,
Ltd (China).
2.2 Hydrothermal synthesis of nano-silicon
The porous silicon nanospheres were prepared by
reduction of industrial silica sol with magnesium
according to hydrothermal reduction reaction
under the stainless autoclave. 2.2g silica sol and 1.5g
Mg powder were mixed and added into a 20 mL
stainless autoclave before sealed. Subsequently, the
autoclave was maintained at 180 °C for 10 h and
cooled to room temperature. Here, the temperature
as 180 0C is the lowest temperature in such reaction
of silica sol and Mg. The samples were immersed in
hydrochloric acid (1 mol L-1) for several hours to
remove MgO. The resultant solution was washed
with distilled water and centrifuged (5000 rpm, 5
min) to collect silicon. The obtained silicon was
dissolved in 10 wt% dilute HF solution for 10 min to
remove unreacted silica and other impurities
during reduction. The brown-black precipitate was
collected by centrifuge, washed with deionized
water and ethanol and dried for overnight at 60 °C
in vacuum oven to evaporate rest solvent. We also
tested the synthesis of silicon materials by using
other kinds of silicon precursor such as silica
aerogel and silicic acid (hydrated silica).
Experimental detail is shown in supporting
information.
2.3 Material Characterization
The morphology of the reaction product were
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3 Nano Res.
characterized by scanning electron microscopy (SEM,
JEOL-JSM-6700F), Transmission electron
microsocopy (TEM, Hitachi H7650 and HRTEM,
JEOL 2010). X-ray diffractometer (XRD) was
performed on a Philips X’ Pert Super diffract meter
with Cu Kα radiation (λ=1.54178 Å). The Brunauer-
Emmett-Teller (BET) surface area and
Barrett-Joyner-Halenda (BJH) pore distribution plots
were measured on a Micromeritics ASAP 2020
accelerated surface area and porosimetry system.
Before analysis, the samples were allowed to dry
under vacuum and degas at 300 °C for 0.5h under
vacuum (10−5 bar).
2.4 Electrochemical Measurement
The electrochemical properties of nano-silicon
electrodes were measured with coin-type half cells
(2016 R-type) which assemble under an argon-filled
glove box (H2O, O2 < 1 ppm). Working electrode was
prepared by mixing the nano-silicon material, super
P carbon black and sodium alginate (SA) binder in a
weight ratio of 60:20:20 in water solvent. The slurry
was pasted onto a Cu foil and then dried in a vacuum
oven at 80 0C for 12 h. The active material density of
each cell was determined to be 0.5-1.0 mg cm-2.
Metallic Li sheet was used as counter electrode, and 1
M LiPF6 in a mixture of ethylene
carbonate/dimethylcarbonate (EC/DMC; 1:1 by
Volume) and 10 wt% fluoroethylene carbonate (FEC)
was used as the electrolyte (Zhuhai Smoothway
Electronic Materials Co., Ltd (china)). Galvanostatic
measurements were made using a LAND-CT2001A
instrument at room temperature that was cycled
between 0.005 V and 1.50 V versus Li+/Li at a rate of
0.36–18 A g-1.
3 Results and discussion
Silica sol was converted into H2/Si as-prepared
products by the hydrothermal reduction process with
Mg under the stainless steel autoclave. X-ray powder
diffraction (XRD) analysis (Figure S1) confirmed the
main component of MgO and Si in the reacted
specimens. A trace amount of Mg(OH)2 and Mg2SiO4
was also detected from the XRD pattern. The
composite reacted specimens were then immersed in
a 1 M HCl solution for several hours to remove MgO
and Mg(OH)2 (Fig. S2) and further dissolved in HF
solution to remove unreacted silica and other
impurities during reduction. The XRD pattern of the
resulting silicon-based product is shown in Figure 1a.
All the peaks can be indexed to the cubic silicon
(JPCDS 27-1402) with calculated lattice constants of
a=5.408 Å , which is close to the reported value of
5.430 Å . The yield of this Si material is above 25%.
Moreover, 2p Si XPS (X-ray photoelectron
spectroscopy) spectrum of the as-prepared sample
(Figure 1b) exhibits that besides the strong peak of
silicon at 99 eV, a weak peak at 103.5 eV appears,
indicating the existence of small amount of
amorphous SiO2 [24].
Figure 2 is the scanning electron microscopy
(SEM) and transmission electron microscopy (TEM)
images of the as-prepared nano-silicon. From the
SEM image (Figure 2a), one can see that the
as-prepared silicon is composed of uniform
sphere-like nanoparticles with average diameter of
80 nm. TEM image in Figure 2b shows the presence
of pores in the spherical nanostructures, which is
further confirmed by the enlarged TEM image of
Figure 2c. It is obvious that the primary pores with
the size of several nanometers are homogeneously
distributed. A HRTEM image in Figure 2d taken on
the edge of an individual nanosphere presents that
the porous nanosphere is composed of many
nanocrystallites and porous size is 2-5 nm. The
interplanar spacing is about 3.1 Å , corresponding to
the (111) planes of the crystalline Si. Nitrogen
adsorption measurements (Figure S3) indicated
that the specific surface areas of nano-silicon were
about 11 m2 g-1. BJH analyses (Figure S4) of the
nitrogen desorption curves indicated that the
porous Si nanosphere possessed a large amount of
nanoporous (~4nm and ~10nm) and a small amount
of macroporous.
Moreover, we also synthesized silicon materials by
using other kinds of silicon precursor such as silica
aerogel and silicic acid (hydrated silica).
Experimental detail is shown in supporting
information (Extension experiment 1, 2). XRD, SEM
and TEM analysis (Figure S5-8) clearly reveals that
the crystalline phase of nano-silicon can be
analogously hydrothermal synthesized by using
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4 Nano Res.
other kinds of silicon precursor under the low
temperature (~200 0C).
Different with the previous reports, silica sol
hydrothermal reduction proceeds through the
formation of active H intermediates. First, the
reaction of H2O and Mg by the reaction equation as
H2O + Mg = MgO + H2 (△Hθ
= -316 KJ mol-1)
could produce highly reaction heat in instant to
stimulate the following reaction for the production of
silicon. On the other hand, based on the chemical
potential evaluation E0(H/H2O) = -0.93, E0(Mg/Mg2+)
= -2.36 and E0(Si/SiO2) = -0.86 (where E0 is the
standard potential), reaction as (1) Mg + H2O = MgO
+ 2H, (2) H +SiO2 = Si +H2O and (3) Mg +SiO2 = MgO
+ Si are thermodynamically spontaneous. It is
proposed that the generating of the active H
intermediates will promote the reaction, which is
similar to the Clemmensen reaction in traditional
organic synthesis [25]. In Clemmensen reaction,
active H intermediates generated from Zn and HCl
to transform methylene from carbonyl. Moreover,
such silica sol hydrothermal reduction reaction can
be carried out even lower than 180 °C through
activated the reaction of active H intermediates
formation by adding acid, which provide the direct
proof that the formation of active H intermediates
should facilitate the reaction to silicon. An Extension
experiment by adjusting the pH of reaction system has
been provided in Supporting Information (Extension
experiment 3 and Figure S9).
The electrochemical performance of spherical
porous Si nanostructures as anode was investigated
in CR2016 coin cells with lithium foil as a counter
electrode. Figure 3a shows typical voltage curves
during the first 5 cycles in the voltage window of
0.005–1.50 V versus Li/Li+ at a current density of 0.36
A g-1. In the first discharge curve, a discharge plateau
at around 0.8 V was assigned to the formation of a
solid electrolyte interface (SEI) layer, which
disappear in the following cycle and relate to an
initial irreversible capacity loss. [12, 26, 27] The
discharge plateau located at around 0.2 V is related
to the alloy formation process between Li and crystal
Si. [28, 29] Subsequent discharge and charge cycles
curves had the voltage profiles characteristic of
amorphous Si. [30, 31] The porous silicon
nanospheres anode delivers initial discharge and
charge capacities of 4055 and 3015 mAh g-1 at 0.36 A
g-1, respectively, corresponding to a first cycle
coulombic efficiency (CE) of 74%. The irreversible
capacity loss can be mainly attributed to the
formation of SEI layer on the electrode surface and
presumably arises partly from electrolyte
decomposition, partly from electrically disconnected
particles due to the large volume changes, [10] and
perhaps also from Li atoms “trapped” in the
electrically connected particles. [32, 33] Meanwhile,
after cycling up to 40 cycles at 0.36 A g-1, the porous
silicon nanospheres anode retained 2650 mAh g-1
(Figure 3b), corresponding to about 89 % of its initial
charge capacity (3015 mAh g-1). Furthermore, the
coulombic efficiency retains near 100 % after the first
cycle, which means that the porous silicon
nanospheres structure can enhance the coulombic
efficiency of the electrode due to the increasing of the
active sites for reversible electrochemical Li storage
[14]. A capacity of 950 mAh g-1 after 500 cycles at a
higher current density of 3.6 A g-1 and good capacity
retention were also attained, with the first three
cycles activated at 0.72 A g-1 (Figure 3e). Some
oscillations in the specific capacity values over
several cycle periods could sometimes be observed
(Figure 3e), while neither the average value of the
specific capacities, nor the ability for long cycles were
affected. Such oscillations have sometime been
observed in the previous reports for not only Si
anodes [34-36] but other kind of electrode materials
[37, 38]. The slight capacity oscillations might result
from the temperature variation (the inside of the cell
and the measurement environment, Figure S10-11
for temperature variation measure), the surface
condition and the active site of the sample and the
SEI layer thickness [39].
The rate capability for the spherical porous Si
anode is evaluated using galvanostatic
charge-discharge measurements with increasing the
current density from a low current density of 0.36 A
g-1 to a high current density of 18 A g-1 and then back
to 1.8 A g-1 and 0.72 A g-1 (showing in Figure 3 c-d) .
The discharge specific capacity is ~2900, 2600, 2000,
1500, 800 and 350 mAh g-1 and the coulombic
efficiency is almost 100% at the current density of
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5 Nano Res.
0.36, 0.72, 1.8, 3.6, 7.2, and 18 A g-1, respectively. It is
worthwhile to indicate that the specific capacity
reversibly recovers to approximately 1800 mAh g-1
once the current density goes back to 1.8 A g-1 and
then go back to about 2450 mAh g-1 when the current
density back to 0.72 A g-1. The specific capacity is
almost recovered, indicating a fine rate performance
of such silicon anode. The high elctrochemical
performance of the silicon may be attributed to the
porous nanostructure [12, 40, 41] as well as the
presence of amorphous SiO2 [10, 42, 43]. The porous
nanostructure offers a path way for the efficient
access of electrons and electrolytes, increased
number of active sites and enhanced the contact
surface between the electrode material and
electrolyte. Besides, the existence of pore and
amorphous SiO2 may effectively accommodate the
volume changes of silicon and allow for facile strain
relaxation without strong mechanical stress during
cycling. It would provide a new avenue for
large-scale production of high electrochemical
performance anode materials.
4 Conclusions
In conclusion, porous Si nanospheres were
hydrothermally synthesized based on the reduction
of silica sol by magnesium metal in autoclave at
180 °C. Moreover, besides silica sol, this
hydrothermal reduction reaction can be extended to
the reduction of solid silica powders such as silica
aerogel and silicic acid (hydrated silica). The porous
Si nanospheres without carbon coating exhibit
excellent lithium-storage capacity, high-rate
capability and long cycling performance, which may
be attributed to the porous nanostructure as well as
the presence of amorphous SiO2. Since the reactant is
cheap and the reaction condition is relatively mild,
this study would provide a new avenue for
large-scale production of silicon anode materials.
Acknowledgements
This work was supported by the 973 Project of China
(No. 2011CB935901), the National Natural Science
Fund of China (No. 91022033, 21201158)
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Nano Res.
(a) (b)
Figure 1 a) XRD pattern and b) XPS spectrum of the porous Si nanospheres after hydrothermal reduction silica sol process.
Figure 2 The morphology, size, and structure of the resulting porous Si nanospheries. a) SEM and b-c) TEM image of the spherical
porous silicon. d) HRTEM image taken on the edge of an individual nanosphere.
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Figure 3 Electrochemical performances of spherical porous silicon anode. a) Typical galvanastatic discharge-charge curves of the cell
with spherical porous silicon in the potential region of 0.005–1.5 V versus Li+/Li at a current density of 0.36 A g-1. b) Cycling property
and coulombic efficiency of the cell with spherical porous silicon at the constant current density of 0.36 A g-1. ■ as discharge capacity
(Qd), ● as charge capacity (Qc) and ◆ as coulombic efficiency (Qc/Qd). c) The typical galvanastatic discharge–charge curves of
spherical porous silicon at different current densities, and d) the rate performance of spherical porous silicon anode. e) Cycling property
at 3.6 A g-1 for 500 cycles.
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Address correspondence to First A. Firstauthor, email1; Third C. Thirdauthor, email2
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