Nanoporous silicon prepared through air-oxidation demagnesiation of Mg2Si and properties of its lithium ion batteries

Article abstract of DOI:10.1039/c5cc01659b

Nanoporous silicon has been prepared through the air-oxidation demagnesiation of Mg₂Si at 600 °C for 10 hours (Mg₂Si + O₂ → Si + MgO), followed by HCl washing. Mg₂Si was prepared from 200 mesh commercial Si at 500 °C for 5 h in an autoclave. The as-prepared Si exhibits a reversible capacity of 1000 mA h g^-1 at 36 A g^-1 and ～1200 mA h g^-1 at 1.8 A g^-1 over 400 cycles. This journal is

Full text of DOI:10.1039/c5cc01659b

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Nanoporous silicon prepared through
air-oxidation demagnesiation of Mg2Si and
Cite this:DOI: 10.1039/c5cc01659b
properties of its lithium ion batteries†
Received 25th February 2015,
Accepted 19th March 2015
Jianwen Liang, Xiaona Li, Zhiguo Hou, Cong Guo, Yongchun Zhu* and Yitai Qian*
DOI: 10.1039/c5cc01659b
www.rsc.org/chemcomm
Nanoporous silicon has been prepared through the air-oxidation capacity of B2200 mA h gÀ1 at 400 mA gÀ1 after 50 cycles.14
demagnesiation of Mg2Si at 600 8C for 10 hours (Mg2Si + O2 Porous Si was synthesized through acid etching the Al–Si alloy
-
Si + MgO), followed by HCl washing. Mg2Si was prepared from 200 (Al-80%, Si-20%) powder, which exhibited a discharge capacity of
mesh commercial Si at 500 8C for 5 h in an autoclave. The as-prepared 281 mA h gÀ1 at 100 mA gÀ1 after 100 cycles.15
Si exhibits a reversible capacity of 1000 mA h gÀ1 at 36 A gÀ1 and
B1200 mA h gÀ1 at 1.8 A gÀ1 over 400 cycles.
With regard to the conversion of Mg2Si into Si, the previous
work was carried out by metathesis reaction,16–20 such as the
reaction between Mg2Si and SiCl4 in organic solvents,16 Mg2Si
Silicon is a candidate anode material of lithium ion batteries (LIBs) reacted with amorphous silica or silicon monoxide through a high
owing to its high theoretical specific capacity of 3600 mA h gÀ1 and energy mechanical milling process.21,22 Si nanoparticle anodes
relatively low discharge potential (o0.5 V versus Li/Li+).1 However, obtained by the reaction of Mg2Si with ZnCl2 (Mg2Si + ZnCl2 -
Si exhibits rapid capacity fading over galvanostatic cycling caused MgCl2 + Zn + Si) showed a reversible capacity of 795 mA h gÀ1 at
by serious volume change (4300%) during lithiation–delithiation. 3.6 A gÀ1 over 250 cycles. Recently, nanoporous silicon anodes
Choosing nano-silicon or porous silicon materials as anodes is an synthesized through the reaction of Mg2Si and molten bismuth
effective strategy to address this issue.2
Various methods have been applied to prepare Si anode 1500 mA h gÀ1 at 1.8 A gÀ1 after 500 cycles.23
(Bi) in high-purity He gas presented a capacity retention of
materials. Si anode materials can be synthesized from silica, such as
In this study, we report the preparation of nanoporous
magnesiothermic reduction,3–6 electrochemical reduction in molten silicon through air-oxidation demagnesiation of Mg2Si at
salts7 and hydrothermal reduction with Mg.8 Also, Si anode 600 1C for 10 h, following by HCl washing. Mg2Si is prepared
materials can be obtained by reduction of SiCl4 in organic from 200 mesh commercial Si at 500 1C for 5 h in an autoclave.
solvents9,10 or in molten salts.11
The as-obtained Si materials show a porous structure with a
Converting the commercial bulk Si powders into nano-Si or Brunauer–Emmett–Teller (BET) surface area of 86.6 m2 gÀ1 and
porous Si is an alternative route to obtain silicon anode materials a pore size of B4 nm. As an anode for rechargeable LIBs,
of LIBs. Si nanoparticles obtained by high-energy ball-milling macro- nanoporous Si exhibits a reversible capacity of 1000 mA h gÀ1 at
sized Si particles displayed a performance of 1600 mA h gÀ1 at 36 A gÀ1 and B1200 mA h gÀ1 at 1.8 A gÀ1 over 400 cycles. The
240 mA gÀ1 after 600 cycles.12 Porous Si/C composites were related reaction in the air-oxidation demagnesiation of Mg2Si
prepared based on the etching reaction of bulk Si and gas CH3Cl can be expressed as follows: Mg2Si + O2 (air) - Si + MgO.
with Cu-based catalyst assistance, which showed a discharge
The XRD pattern of the sample after washing with HCl is
capacity of about 1036 mA h gÀ1 at 50 mA gÀ1 after 100 cycles.13 shown in Fig. 1a, all the peaks can be indexed to cubic Si with a
Macroporous Si was produced via Ag deposition and chemical calculated lattice constant of a = 5.415 Å, close to the reported
etching with HF and H2O2, after carbon coating, which showed a value of a = 5.430 Å (JPCDS 27-1402). Based on the Scherrer
equation, the size of the Si particles is estimated to be about
15 nm. Scanning electron microscopy (SEM) images in Fig. 1b
and Fig. S1 (ESI†) exhibit that the obtained Si sample appears as
micrograins comprising of many nanoparticles. In the transmis-
sion electron microscopy (TEM) image of Fig. 1c, the disordered
nanometer pores and nanoparticles of several nanometers can
be clearly observed. The high-resolution transmission electron
microscopy (HRTEM) image, shown in Fig. 1d, reveals lattice
Hefei National Laboratory for Physical Science at Micro-scale and Department of
Chemistry, University of Science and Technology of China, Hefei, Anhui 230026,
P. R. China. E-mail: ytqian@ustc.edu.cn, ychzhu@ustc.edu.cn
† Electronic supplementary information (ESI) available: Experimental details;
SEM image and nitrogen adsorption curves of as-prepared silicon; characteriza-
tion of previous commercial silicon, Mg2Si/Mg and MgO/Si compounds during
the experimental process; and table presenting several repeated experiments. See
DOI: 10.1039/c5cc01659b
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Fig. 2 XRD patterns of (a) Mg2Si/Mg powder, and the as-prepared sam-
ples after the air-oxidation process at (b) 550 1C for 10 h, (c) 650 1C for 1 h.
Fig. 1 (a) XRD pattern, (b) SEM image, (c) TEM image, (d) HRTEM image,
(e) XPS spectrum, and (f) BJH pore diameter distribution of the as-prepared
porous nano-Si.
fringes of the Si nanocrystals. The interplanar distances are
measured to be about 0.33 nm, corresponding to the (111) crystal
planes of the cubic Si. Besides, amorphous layers above 2 nm
can be observed. The surface contents of the nanoporous Si are
investigated using the Si 2p X-ray photoelectron spectroscopy
(XPS) spectrum, as shown in Fig. 1e. A peak at B103 eV indicates
the existence of SiO2.24 The surface density of oxygen is further
Fig. 3 XRD patterns of the raw product during oxidizing Mg2Si/Mg in air at
temperatures of 600 1C for (a) 2 h, (b) 5 h, (c) 10 h.
quantified to be as high as 61.2 at%, which is close to the heating of as-prepared Mg2Si/Mg in air was carried out at
theoretical content of pure SiO2 (66.7 at%). The results of XPS temperatures of 400 1C, 550 1C and 650 1C. When Mg2Si/Mg
and HRTEM give a hint that a small amount of amorphous SiO2 was heated at 400 1C, the XRD pattern of the resulting products
is present on the surface of Si nanoparticles. Barrett–Joyner– shows that there are only Mg2Si and Mg (Fig. S5, ESI†) even
Halenda (BJH, Fig. 1f) analysis of the nitrogen adsorption curves after 10 h. As the temperature was increased to 550 1C, at 10 h,
(Fig. S2, ESI†) indicates that the pore size of the Si sample is mainly crystalline Si and MgO appear, however Mg2Si remained
B4 nm. The BET surface area of the porous Si is 86.6 m2 gÀ1
.
(Fig. 2b). As the temperature was kept at 650 1C for 1 h, it is
Since the oxidized behaviors of magnesium and silicon are found that SiOx (labeled as ‘‘#’’, JCPDS No. 81-0066, Fig. 2c) is
different, it is possible to control the oxidation process of Mg2Si to observed in the products. So we set the temperature of air-
obtain Si. For this purpose, we prepared Mg2Si containing a small oxidation of as-prepared Mg2Si/Mg at 600 1C for 2 h, 5 h and
excess of Mg. A small excess of Mg should be helpful for the air- 10 h. Fig. 3 shows the XRD patterns of the products obtained at
oxidation of Mg2Si to Si at relatively lower temperatures. Using 600 1C for 2 h, 5 h and 10 h, indicating that Mg2Si can be completely
commercial silicon (200 mesh, SEM images in Fig. S3, ESI†) and transformed to Si after 10 h. The raw product of oxidizing Mg2Si/Mg
metallic Mg with a Mg/Si ratio of 2.1, Mg2Si with Mg in excess of in air at 600 1C after 10 h is presented in Fig. 3c (the SEM image of
5 at% (Mg2Si/Mg) was prepared at 500 1C for 5 h in an autoclave. The the raw product is shown in Fig. S6, ESI†). The diffraction peaks
XRD pattern of the as-prepared Mg2Si (Fig. 2a) confirms that the (labeled as ‘‘E’’) are characterized as 111, 220, 311, 400, and
bulk Si converted to Mg2Si (labeled as ‘‘’’’ JCPDS No. 35-0773) and 331 planes of cubic Si (JCPDS No. 27-1402) and the peaks (labeled
the excess of Mg existing (labeled as ‘‘m’’ JCPDS No. 04-0770). The as ‘‘K’’ JCPDS No. 45-0946) are indexed to 111, 200, 220, 311 and
SEM image of the Mg2Si/Mg powder is shown in Fig. S4 (ESI†).
222 planes of MgO. The related reaction in the air-oxidation
To find the appropriate temperature for the oxidation of demagnesiation of Mg2Si can be expressed as: Mg2Si + O2 (air) -
Mg2Si to Si, a series of related experiments concerning the Si + 2MgO. After washing with HCl, nanoporous Si can be obtained.
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Fig. 4 The scheme and photo of the 5 L stainless steel reactor.
The air-oxidation demagnesiation of Mg2Si was performed
using a 5 L stainless steel reactor (Fig. 4). The as-prepared
Mg2Si/Mg was oxidized at a heating rate of 2 1C minÀ1 to 600 1C
and then kept for 10 h under air flow. The final nano-silicon
production is over 10 g for each experiment, and the yield of
nano-silicon is above 90%. At the same time, the tap density of
Fig. 5 Electrochemical performances of nanoporous silicon anodes.
(a) Typical galvanostatic discharge–charge curves for the first five cycles
of the nanoporous silicon sample in the potential region of 0.005–1.50 V
versus Li/Li+ at 360 mA gÀ1. (b) Cycling properties of the cell with the
nanoporous silicon and commercial bulk silicon sample at 360 mA gÀ1
.
the nano-porous Si powder is estimated to be B0.597 g cmÀ3
.
The solid symbol represents discharge capacity and the hollow symbol
represents charge capacity. (c) The typical galvanostatic discharge–charge
curves at different current densities, and (d) the rate performance of the
nanoporous silicon sample and (e) long term cycling properties of the
The yields and the tap density of this nano-silicon material are
calculated based on five repeated experiments which are pro-
vided in the ESI† (Table S1).
nanoporous silicon electrode at 1.8 A gÀ1
.
Next, the electrochemical performance of the nanoporous
silicon anodes was investigated in CR2016 coin cells with
lithium foil as a counter electrode. Fig. 5a shows the typical 3335, 3187, 2941, 2508, 2003 and 1446 mA h gÀ1, and the CE is
voltage profiles for the first five cycles of the as-synthesized Si almost 100% at current densities of 0.36, 0.72, 1.8, 3.6, 7.2 and
electrode at a current density of 360 mA gÀ1. In the first 18 A gÀ1, respectively. Even at a higher current density of
discharge curve, a plateau at around 0.8 V is assigned to the 36 A gÀ1, the electrode is able to maintain a capacity of
formation of a solid electrolyte interface (SEI) layer, which can B1000 mA h gÀ1. Moreover, the capacity could recover to
result in an initial irreversible capacity loss and disappears in the approximately 2850 mA h gÀ1 when the current density goes
subsequent cycles.25 The discharge plateau located at around 0.2 V back to 0.36 A gÀ1. In addition, long cycling performance at a
is related to the alloy formation process between Li and crystal Si. rate of 1.8 A gÀ1 was also tested. Gradual capacity fading in the
Subsequent discharge and charge cycle curves exhibit the voltage initial cycles is observed for such high rate cycling, which is
profiles characteristic of amorphous Si.25–27 The porous Si electrode probably due to the lower electrical conductivity of the amor-
shows a discharge capacity of 3740 mA h gÀ1 and a charge capacity phous SiO2 layer and the Si materials. In order to further
of 3291 mA h gÀ1 for the first cycle, corresponding to the initial improve the electrochemical performance, several strategies such
Coulombic efficiency (CE) of 88%. Meanwhile, after cycling up to as carbon coating, graphene encapsulation and preparation of high
85 cycles, the discharge capacity of the Si electrode is still retained electrical conductivity metal composites can be adopted. After
at 2156 mA h gÀ1, which is about 64% of its initial reversible the initial dozens of cycles, the capacity in the 50th cycle is
capacity (Fig. 5b). Furthermore, the CE remains at about 100% after B2100 mA h gÀ1 and remained stable at B1200 mA h gÀ1 up to
the first cycle, showing a good reversibility. While, a contrast 400 cycles (57% capacity retention compared to the 50th cycle).
electrode with the initial commercial bulk Si as an active material These results indicate that both the porous structure and the
shows poor cycling stability, which is nearly reduced to 10 mA h gÀ1 surface coating of the amorphous SiO2 layer play an important
at 15 cycles, thus further indicating that the good cycling perfor- role in electrochemical performance of silicon anodes in LIBs.
mance of the as-prepared porous silicon electrode arises from the The nanostructure offers a short Li+ diffusion path and large BET
fabrication process.
surface area just as the as-synthesized nanoporous silicon, which
The rate capability for the porous Si anode is evaluated using increased the number of active sites and enhanced the contact
galvanostatic charge–discharge measurements upon increasing surface between the electrode material and the electrolyte.4,28
the current density from 0.36 A gÀ1 to 36 A gÀ1 and then back to Besides, the amorphous SiO2 layer can protect nanoporous silicon
0.36 A gÀ1 (shown in Fig. 5c and d). It is obvious that the material particles from the side reaction with the electrolyte.29,30
nanoporous silicon electrode exhibits reversible capacities of Moreover, the existence of pores may effectively accommodate the
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