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二氧化硅水热合成纳米硅

Hydrothermal synthesis of nano-silicon from a silica sol and its use in lithium ion batteries

Jianwen Liang, Xiaona Li, Yongchun Zhu (?), Cong Guo, and Yitai Qian (?)

Hefei National Laboratory for Physical Science at Microscale and Department of Chemistry, University of Science and Technology of China, Hefei 230026, China

Received: 11 September 2014 Revised: 31 October 2014 Accepted: 3 November 2014

? Tsinghua University Press and Springer-Verlag Berlin Heidelberg 2014

KEYWORDS silicon,

hydrothermal synthesis, nanomaterials,

silicon sol,

energy storage ABSTRACT

There have been few reports concerning the hydrothermal synthesis of silicon anode materials. In this manuscript, starting from the very cheap silica sol, we hydrothermally prepared porous silicon nanospheres in an autoclave at 180 °C.

As anode materials for lithium-ion batteries (LIBs), the as-prepared nano-silicon anode without any carbon coating delivers a high reversible specific capacity of 2,650 mAh·g–1 at 0.36 A·g–1 and a significant cycling stability of about 950 mAh·g–1 at 3.6 A·g–1 during 500 cycles.

1Introduction

Silicon has been considered as a promising anode candidate material for advanced lithium-ion batteries (LIBs) due to its high theoretical capacity (3,579 mAh·g–1) and relatively low discharge potential (<0.5 V versus Li/Li+) [1]. However, Si exhibits serious volume changes (>270%) during lithiation–delithiation, which leads to a rapid reduction in capacity [2, 3]. Similar to other

二氧化硅水热合成纳米硅

electrode materials, using Si materials with a nano-structure is one of means to relieve this problem [4–9].

二氧化硅水热合成纳米硅

Various methods have been developed to produce nano-silicon anode materials in order to improve LIBs performance. One of these methods is chemical vapour deposition (CVD) of silanes, by which silicon nanotubes were prepared. After subsequent SiO2 surface-coating, the Si/SiO2 nanotubes were shown to have long cycle life (6,000 cycles with 88% capacity retention), high specific charge capacity (~2,971/1,780 mAh·g–1 at 0.4 A·g–1, and ~940/600 mAh·g–1 at 24 A·g–1) [10]. 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 showed a capacity of about 1,900 mAh·g–1 at 0.4 A·g–1, with a reten-tion of ~50% after 90 cycles after carbon coating [11].

Nano Research 2015, 8(5): 1497–1504

DOI 10.1007/s12274-014-0633-6

Address correspondence to Yitai Qian, ytqian@http://www.doczj.com/doc/77f784042379168884868762caaedd3383c4b51d.html ; Yongchun Zhu, ychzhu@http://www.doczj.com/doc/77f784042379168884868762caaedd3383c4b51d.html

二氧化硅水热合成纳米硅

With regard to wet chemical synthesis of nano-silicon anode materials, much attention has been paid to the preparation in organic solvents [17–19]. For example, nest-like silicon nanospheres were prepared via reaction of sodium silicide and NH 4Br in a mixed solvent of pyridine and dimethoxyethane in an autoclave at 80 °C for 24 h, and exhibited a reversible specific capacity of 1,095 mAh ·g –1 at 2 A·g –1 after 50 cycles. Si nanoparticles were produced by reduction of SiCl 4 with naphthalene sodium in anhydrous tetrahydrofuran at 380 °C in a Hastelloy Parr reactor, and showed a specific charge capacity of 3,535 mAh ·g –1 at 0.9 A·g –1 and retention as high as 96% after 40 cycles after carbon coating [17]. Si nanoparticles were synthesized via reduction of anhydrous SiCl 4 with sodium–potassium alloy (NaK) in toluene solution under reflux for 4 h, followed by oxidation; the resulting sample of Si/SiO x /SiO 2 showed an initial charge capacity of 610 mAh·g –1 at 0.2 A·g –1 and good cycling stability (maintained at ~600 mAh ·g –1 after 350 cycles) [20]. Recently, an aqueous synthesis of hydrophilic silicon nanoparticles (~2.2 nm) based on the reaction of (3-aminopropyl)trimethoxysilane (C 6H 17NO 3Si) and trisodium citrate dihydrate under

microwave irradiation at 140

°C was reported [21]. Silicon nanowires have also been grown in an aqueous solution [22]. Previously, we have fabricated silicon micromaterials by reducing crystalline Na 2SiO 3·9H 2O with Mg in an autoclave at 200 °C [23]. After being combined with graphene, the as-prepared anode showed a reversible capacity of ~600 mAh ·g –1 at a 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 an 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 2,650 mAh ·g –1 at 0.36 A·g –1 and cycling stability about 950 mAh ·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:// http://www.doczj.com/doc/77f784042379168884868762caaedd3383c4b51d.html /company/shop1383670764546/ index.aspx). Mg (99%, 100–200 mesh powder), HCl (37%) and hydrofluoric 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 using a hydrothermal reduction reaction in a stainless autoclave. Silica sol (2.2 g) and Mg (1.5 g) powder were mixed and added into a 20 mL stainless autoclave which was then sealed. Subsequently, the autoclave was maintained at 180 °C for 10 h and then cooled to room temperature. Here, the temperature as 180 °C 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 resulting solution was washed with distilled water and centrifuged (5,000 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 formed during reduction. The brown-black precipitate was collected by centrifugation, washed with deionized water and ethanol and dried for overnight at 60 °C in a vacuum oven to evaporate residual solvent. We also tested the synthesis of silicon materials by using other kinds of silicon precursors such as silica aerogel and silicic acid (hydrated silica). Experimental details are shown in the Electronic Supplementary Material (ESM). 2.3 Materials characterization

The morphologies of the reaction products were characterized by scanning electron microscopy (SEM, JEOL-JSM-6700F), transmission electron microscopy (TEM, Hitachi H7650 and HRTEM, JEOL 2010). X-ray diffraction (XRD) was performed on a Philips X’ Pert Super diffractometer with Cu K α radiation (λ = 1.54178 ?). The Brunauer–Emmett–Teller (BET) surface area and Barrett–Joyner–Halenda (BJH) pore distri-bution plots were measured on a Micromeritics ASAP 2020 accelerated surface area and porosimetry system. Before analysis, the samples were allowed to dry

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under vacuum and degas at 300 °C for 0.5 h under

vacuum (10–5 bar).

2.4 Electrochemical measurements The electrochemical properties of nano-silicon electrodes

were measured with coin-type half cells (2016 R-type)

which were assembled in an argon-filled glove box (H 2O, O 2 < 1 ppm). The 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 as solvent. The slurry was

pasted onto a Cu foil and then dried in a vacuum oven

at 80 °C 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 the counter electrode, and 1 M LiPF 6

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 H 2/Si by the hydrothermal

reduction process with Mg in a stainless steel autoclave.

X-ray powder diffraction (XRD) analysis (Fig. S1, in the ESM) confirmed the main components to be MgO

and Si. Trace amounts of Mg(OH)2 and Mg 2SiO 4 were

also detected in 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, in the ESM) and further treated

with HF solution to remove unreacted silica and other impurities formed during reduction.

The XRD pattern of the resulting silicon-based

product is shown in Fig. 1(a). All the peaks can be indexed to the standard pattern of cubic silicon (JPCDS 27-1402) with calculated lattice constants of a = 5.408 ?,

which is close to the reported value of 5.430 ?. The yield of this Si material is above 25%. Moreover, the 2p Si XPS (X-ray photoelectron spectroscopy) spectrum

of the as-prepared sample (Fig. 1(b)) shows—in addition to the strong peak of silicon at 99 eV—a weak peak at 103.5 eV , indicating the existence of small amount of amorphous SiO 2 [24]. Figure 2 shows the scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images of the as-prepared nano-silicon. From the SEM image (Fig. 2(a)), one can see that the as-prepared silicon is composed of uniform sphere-like nano-particles with average diameter of 80 nm. The TEM

image in Fig. 2(b) shows the presence of pores in the spherical nanostructures, which is further confirmed by the enlarged TEM image of Fig. 2(c). It is obvious that the primary pores with a size of several nano-meters are homogeneously distributed. A HRTEM

image (Fig. 2(d)) taken on the edge of an individual nanosphere shows that the porous nanospheres are composed of many nanocrystallites and the pore size is 2–5

nm. The interplanar spacing is about 3.1 ?, corresponding to the (111) planes of the crystalline Si. Nitrogen adsorption measurements (Fig. S3, in the ESM) indicated that the specific surface area of nano- silicon was about 11 m 2·g –1. BJH analyses (Fig. S4, in

the ESM) of the nitrogen desorption curves indicated

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Figure 1 (a) XRD pattern and (b) XPS spectrum of the porous Si nanospheres after hydrothermal reduction silica sol process.

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Figure 2 The morphology, size, and structure of the resulting porous Si nanospheres. (a) SEM and (b) and (c) TEM image of the spherical porous silicon; (d) HRTEM image taken on the edge of an individual nanosphere.

that the porous Si nanosphere possessed a large amount of nanopores (~4 nm and ~10 nm) and a small amount of macropores.

Moreover, we also synthesized silicon materials by using other kinds of silicon precursor, such as silica aerogel and silicic acid (hydrated silica). Experimental details are shown in the ESM (Extension experiments 1 and 2). XRD, SEM and TEM analysis (Figs. S5–S8) clearly reveals that the crystalline phase of nano-silicon can be hydrothermally synthesized by using other kinds of silicon precursor at relatively low temperature (~200 °C).

In contrast to previous reports, hydrothermal reduction of the silica sol proceeds through the formation of active H intermediates. First, the reaction of H 2O and Mg according to the equation H 2O + Mg = MgO + H 2 (ΔH θ = –316 kJ· mol –1) produces a large amount of heat in a short time which promotes the subsequent reaction leading to the production of silicon. On the other hand, based on the standard electrode potentials E θ(H/H 2O) = –0.93 V , E θ(Mg/Mg 2+) = –2.36 V and E θ(Si/SiO 2) = –0.86 V, reactions such as (1) Mg + H 2O = MgO + 2H, (2) H + SiO 2 = Si + H 2O and (3) Mg + SiO 2 = MgO + Si are thermodynamically spontaneous. It is proposed that the generation of active H intermediates will promote the reaction, which is similar to the Clemmensen reaction in traditional organic synthesis [25]. In the Clemmensen reaction, active H intermediates generated from Zn and HCl

transform carbonyl groups into methylene groups. Moreover, such a hydrothermal reduction of silica sol can be carried out at even lower temperatures than 180 °C through activating the formation of active H intermediates by adding acid, which provides direct proof that the formation of active H intermediates should facilitate the formation of silicon. An extension experiment involving adjusting the pH of the reaction system is given in the ESM (Extension experiment 3 and Fig. S9).

The electrochemical performance of the spherical porous Si nanostructures as an anode was investigated in CR2016 coin cells with lithium foil as a counter electrode. Figure 3(a) shows typical voltage curves during the first five 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 can be assigned to the for-mation of a solid electrolyte interface (SEI) layer, which disappears in the following cycle and is related 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 4,055 and 3,015 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 the 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 electri-cally connected particles [32, 33]. Meanwhile, after cycling up to 40 cycles at 0.36 A·g –1, the porous silicon nanospheres anode retained a charge capacity of 2,650 mAh·g –1 (Fig. 3(b)), corresponding to about 89% of its initial charge capacity (3,015 mAh·g –1). Further-more, the coulombic efficiency remains 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 increased number of active sites for reversible electrochemical Li storage

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[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 (Fig. 3(e)). Some oscillations in the specific capacity values over several cycle periods could sometimes be observed (Fig. 3(e)), although neither the average value of the specific capacities, nor the ability for long cycling were affected. Such oscillations have sometime been observed in previous reports for not only Si anodes [34–36] but also for other kinds of electrode materials [37, 38]. The slight capacity oscillations might result from the temperature difference (between the inside of the cell and the

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measurement environment—see Figs. S10 and S11 (in

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 (Q d ), ● as charge capacity (Q c ) and ◆ as coulombic efficiency (Q c /Q d ). (c) The typical galvanastatic discharge–charge curves of spherical porous silicon at different current densities, and (d) the rate performance of a spherical porous silicon anode. (e) Cycling property at 3.6 A·g –1 for 500 cycles.

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the ESM) for temperature difference measurements), the surface conditions, the active sites of the sample and the SEI layer thickness [39]. The rate capability of the spherical porous Si anode was evaluated using galvanostatic charge–discharge measurements by increasing the current density from

a low value of 0.36 A·g –1 to a high value of 18 A·g –1 and then back to 1.8 A·g –1 and 0.72 A·g –1 (shown in Figs. 3(c) and 3(d)). The discharge specific capacities were ~2,900, 2,600, 2,000, 1,500, 800 and 350 mAh·g –1 and the coulombic efficiencies were almost 100% at current densities of 0.36, 0.72, 1.8, 3.6, 7.2, and 18 A·g –1, respectively. It is worthwhile to note that the specific capacity reversibly recovers to approximately 1,800 mAh·g –1 once the current density goes back to 1.8 A·g –1 and then go back to about 2,450 mAh·g –1

when the current density back to 0.72 A·g –1. The specific capacity is almost recovered, indicating an excellent rate performance of the silicon anode. The high electrochemical performance of the silicon may be

attributed to its porous nanostructure [12, 40, 41] as

well as the presence of amorphous SiO 2 [10, 42, 43].

The porous nanostructure offers a pathway for the

efficient access of electrons and electrolytes, increased

number of active sites and enhances the contact surface

between the electrode material and electrolyte. Fur-thermore, the presence of pores and amorphous SiO 2

may effectively accommodate the volume changes of

silicon and allow for facile strain relaxation without large mechanical stress during cycling. This should provide a new avenue for large-scale production of anode materials with high electrochemical performance. 4 Conclusions

Porous Si nanospheres have been hydrothermally synthesized based on the reduction of silica sol by magnesium metal in an autoclave at 180 °C. Moreover, besides silica sol, this hydrothermal process can be extended to the reduction of solid silica powders such as silica aerogel and silicic acid (hydrated silica). The porous Si nanospheres without any 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 SiO 2. Since the reactant is cheap, and the reaction conditions are relatively mild, this study should provide a new avenue for the large- scale production of silicon anode materials. Acknowledgements

This work was supported by the 973 Project of China (No. 2011CB935901), and the National Natural Science Fund of China (Nos. 91022033, 21201158). Electronic Supplementary Material: Supplementary material is available in the online version of this article at http://www.doczj.com/doc/77f784042379168884868762caaedd3383c4b51d.html /10.1007/s12274-014-0633-6. References

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