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 ytqian@ustc.edu.cn; ychzhu@ustc.edu.cn

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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Nano Res.

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