Nano Research
Nano Res
DOI
10.1007/s12274-014-0609-6
Ice-templated preparation and sodium storage of
ultrasmall SnO2 nanoparticles embedded in three
dimensional graphene
Longkai Pei, Qi Jin, Zhiqiang Zhu, Qing Zhao, Jing Liang and Jun Chen ()
Nano Res., Just Accepted Manuscript • DOI: 10.1007/s12274-014-0609-6
http://www.thenanoresearch.com on October 16 2014
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1
1
Nano
Research
Award
Issue:
Ice-templated preparation and sodium
storage
of
ultrasmall
SnO2
nanoparticles embedded in three
dimensional graphene
Longkai Pei, Qi Jin, Zhiqiang Zhu, Qing
Zhao, Jing Liang and Jun Chen()
Key
Laboratory
of
Advanced
Energy
Materials Chemistry (Ministry of Education),
Collaborative Innovation Center of Chemical
Science
and
Chemistry,
Engineering,
Nankai
300071, China
of
SnO2@3DG was explored as the anode material of rechargeable Na-ion
Tianjin
batteries, showing the discharge capacity of 1155 mAh g-1 in the first cycle at
College
University,
100 mA g-1. Meanwhile, it exhibits capacity retention of 85.7% from the
second cycle (504 mAh g-1) to the 200th cycle (432 mAh g-1).
Nano Research
DOI (automatically inserted by the publisher)
Review Article/Research Article Please choose one
Ice-templated preparation and sodium storage of
ultrasmall SnO2 nanoparticles embedded in three
dimensional graphene
Longkai Pei, Qi Jin, Zhiqiang Zhu, Qing Zhao, Jing Liang and Jun Chen ()
Received: day month year
ABSTRACT
Revised: day month year
In this paper, we reported on the ice-templated preparation and sodium storage
of ultrasmall SnO2 nanoparticles (34 nm) embedded in three dimensional (3D)
graphene (SnO2@3DG). SnO2@3DG was fabricated by a hydrothermal assembly
with ice-templated 3DG and tin source. The structure and morphology analyses
showed that 3DG is character of an interconnected porous architecture with a
large pore volume of 0.578 cm3 g-1 and a high surface area of 470.5m2 g-1. In
comparison, SnO2@3DG exhibited a pore volume of 0.321 cm3 g-1 and a surface
area of 237.7 m2 g-1 with homogeneous distribution of ultrasmall SnO2
nanoparticles in 3DG network. SnO2@3DG showed a discharge capacity of 1155
mA h g-1 in the initial cycle, a reversible capacity of 432 mAh g-1 after 200 cycles
at 100 mA g-1 (with capacity retention of 85.7% to that in the second cycle), and
a discharge capacity of 210 mAh g-1 at high-rate 800 mA g-1. This is due to
highly distribution of SnO2 nanoparticles in 3DG network and the enhanced
facilitation of electron/ion transport in the electrode.
Accepted: day month year
(automatically inserted by
the publisher)
© Tsinghua University Press
and Springer-Verlag Berlin
Heidelberg 2014
KEYWORDS
graphene,
three-dimensional,
ice-template, metal oxide,
sodium-ion battery
1. 1 Introduction
1
15 graphene
2 much
Three dimensional graphene (3DG) has attracted
attention for its desirable physical and
3 chemical properties such as low weight density, rich
4 macroporosity and high electronic conductivity [1, 2].
5 To date, various methods including template-guided
6 process, chemical vapor deposition （ CVD ） and
7 solvothermal reactions have been devoted to the
8 fabrication of 3DG [3, 4]. Ice-templated process,
9 which is also known as freeze drying, is a
10 template-guided technique [5, 6]. So far, various
11 types of 3D architecture graphene have been
12 successfully synthesized by freeze drying. Mann and
13 co-workers firstly prepared high-order 3D scaffolds
14 by freeze casting of polystyrene sulfonate-stabilized
16 liquid
sheets and poly(vinyl alcohol) solution in
nitrogen [7]. Recently, 3D porous structure of
17 graphene/cellulose composites were obtained on a
18 large scale by ball milling and ice-templated process
19 [8]. It is found that assistant agents play an important
20 role of linker to obtain 3D structure, but it meanwhile
21 causes troublesome puriﬁcation. Thus, it is desirable
22 to develop an additive-free approach. One of the
23 critical issues to fabricate 3DG via ice-templated
24 process is the precursor which should be highly
25 stable [9]. Recently, graphene colloid was synthesized
26 by simply employing reactants of graphene oxide
27 (GO) and N2H4·H2O in ammonia solution [10]. As the
28 residual volatile NH3·H2O or N2H4·H2O can be easily
2
Nano Res.
by evaporation, the colloid graphene
is a promising candidate to synthesize 3D
3 graphene.
4
To date, graphene-based composites have gained
5 tremendous interest for rechargeable batteries. For
6 example, SnO2/graphene showed interest in lithium
7 ion batteries (LIBs) due to high theoretical capacity
8 (782 mAh g-1 for SnO2) and alleviation of particle
9 aggregation as well as volume change during
10 charge/discharge
[11-14]. Moreover, SnO2@3DG
11 showed better lithium storage performance than that
12 of SnO2@2DG , which was due to the high surface
13 area and 3D porous architecture [15]. Recently,
14 SnO2@2DG composites have been successfully
15 applied in sodium-ion batteries (SIBs). Wang’s group
16 proposed SnO2/graphene composites with a stable
17 capacity of 302 mAh g-1 over 100 cycles at a current of
18 160 mA g-1 [16]. Kim’s group reported that the
19 composites showed a reversible capacity of 330 mAh
20 g-1 over 150 cycles at 100 mA g-1 [17]. Thus, one
21 would
ask how about the electrochemical
22 performance of SnO2@3DG for SIBs?
23
In this work, SnO2@3DG was successfully prepared
24 and applied in SIBs. The preparation of 3DG
25 involved a novel method by using the graphene
26 colloid and ice-templated process. Furthermore,
27 ice-templated 3DG turned out to be a robust
28 substrate to anchor SnO2 via a simple hydrothermal
29 treatment. The SnO2@3DG displayed the character of
30 mesopores with a size distribution from 2 to 100 nm
31 (centered at 3 nm and 30 nm), macropores (3-10 μm),
32 large pore volume (0.321 cm3 g-1) and high surface
33 area (237.7 m2 g-1). The SnO2@3DG composites
34 delivered a reversible capacity of 432 mA h g-1 after
35 200 cycles at 100 mA g-1 with a capacity retention of
36 85.7% to that in the second cycle. Meanwhile, a
37 reversible capacity of 210 mAh g-1 was obtained at a
38 high current density of 800 mA g-1. This benefits the
39 highly distribution of SnO2 nanoparticles in 3DG and
40 the enhanced facilitation of electron/ion transport in
41 the electrode.
C for 1h. Second, the extremely dispersed
solution was frozen under -18C to form an
51 ice template with a following lyophilization to obtain
52 the reduced graphene oxide (rGO). Third, the 3DG
53 were obtained by a calcination treatment at 350C for
54 2h in an Ar atmosphere. During the thermal process,
55 most of residual water was removed, but the
56 oxygenated groups were still kept [18]. This results in
57 an improvement of conductivity (Figure S1). Finally,
58 3DG
(80
mg),
SnCl2·2H2O
(400
mg),
59 N,N-dimethylformamide (80 mL) were added into
60 teflon-lined autoclave (100 mL in capacity) followed
61 by a hydrothermal treatment of 120C for 12 hours to
62 obtain SnO2@3DG. For comparison, SnO2@2DG was
63 obtained in a similar way, except for the absence of
64 freeze-drying process.
1 removed
49 95
2 solution
50 graphene
65
66
67 Scheme
1. Schematic illustration for the preparation of
68 SnO2@3DG:
(a) graphene oxide aqueous solution, (b) reduced
69 graphene
oxide colloid, (c) ice-templated solution, (d) reduced
70 graphene
oxide, (e) 3DG frameworks, (f) SnO2@3DG
71 composite.
1a-b shows the scanning electron
(SEM) of 3DG. From Figure 1a, it can be
74 seen that a highly interconnected 3D network is
75 formed with various macropores. From Figure 1b,
76 the cross-linked spots between 3DG can be seen,
77 which stems from the partial stacked flexible
42
78 graphene sheets. Figure 1c shows the transmission
43 2 Results and discussion
79 electron microscopy (TEM) image of graphene with
44
The fabrication process of SnO2@3DG is illustrated 80 few layers. Figure 1d shows the high-resolution TEM
45 in Scheme 1 with four steps. First, graphene colloid 81 (HRTEM) image of the margin of the crumpled
46 was obtained by adding 750 μL NH3·H2O (35 wt.%) 82 regions in 3DG. Both the above SEM and TEM
47 and 60 μL N2H4·H2O (80 wt.%) into 100 mL GO 83 analyses demonstrate the successful construction of
48 aqueous suspension (1 mg/mL) under the oil bath at 84 3DG. This is owing to the following points: (1) after
72
Figure
73 microscopy
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3
Nano Res.
36 potential
(Figure S3b). After further adding 60 μL
into the above mixed solution, the
38 graphene colloids shows a more negative charge
39 density. As a consequence, 3DG can be obtained with
40 750 μL NH3·H2O and 60 μL N2H4·H2O added into GO
41 solution. The success for the formation of 3DG is
42 ascribed to the moderately residual hydrophilic
43 functionalities on the graphene, which can not only
44 provide optimal electrostatic repulsion between
45 sheets, but also grasp ample water molecules to form
46 the macro-porous. However, it fails to obtain the 3D
47 framework when N2H4·H2O is added with the
48 volumes of 30 μL or 90 μL (Figure S3c-d).
49
The 3DG has been further used as a support to
50 embed SnO2 nanoparticles owing to that: (1) the 3DG
51 delivers a higher conductivity (1.7*10-1 S/cm) than
52 that of graphene oxide (3.4*10-5 S/cm), which is
53 attributed to the less presence of functional groups.
54 (2) The nucleation sites for SnO2 on the GO are more
55 than that on rGO, which leads to more severe
56 aggregation of SnO2 particles (Figure S4). Figure 2
57 shows the SEM and TEM images of SnO2@3DG.
58 Compared with 3DG (Figure 1a-b), a 3D structure
59 without significantly breakdown is still kept for
60 SnO2@3DG (Figure 2a). Uniform nanocrystals are
61 homogeneously distributed on the graphene sheets
62 (Figure 2b). TEM image of Figure 2c further
63 illustrates that SnO2 nanoparticles with an average
64 particle size of 3.4 nm (inset of Figure 2c) are
65 homogeneously dispersed on graphene sheets.
66 Moreover, tremendous nano-lacunas are formed
37 N2H4·H2O
1
2 Figure 1. (a) SEM image of 3DG. (b) The magnified image of the
3 selected region
4 the
in (a). (c) TEM and (d) HRTEM images of 3DG.
reduction process with partially removing
groups such as hydroxyl and phenolic
6 hydroxyl groups, the carboxylic acid groups are still
7 remained [19]. This leads to optimal electrostatic
8 repulsion and thus forming well-dispersed graphene
9 colloid (Figure S2). (2) In the colloidal dispersion, the
10 large conjugated structure of graphene sheets can
11 provide tremendous - stacking sites to form strong
12 bindings between each other. This induces the
13 formation of cross links through partial coalescing or
14 overlapping of graphene sheets. Simultaneously, the
15 residual hydrophilic functionalities on the graphene
16 sheets can entrap ample water molecules. (3) During
17 the freezing process under -18C, the formed ice
18 crystals act as the templates, which lead to the
19 formation of 3D networks after a sublimation
20 process.
21
To gain a better understanding of the correlation
22 between graphene colloid and 3DG, a series of
23 control experiments were performed by zeta
24 potential analysis (Figure S3). According to the
25 standard for colloid stabilization, zeta potential
26 values with more negative than -30 mV are generally
27 considered to present adequate repulsion for
28 ensuring the stability of colloids. Ammonia is found
29 to be one of the key points to obtain the highly stable
30 graphene colloid, which can lead to the ionization of
31 residual carboxylic acid on the graphene. As the
32 content of NH3·H2O reaches to 750 μL (added into
33 100 mL GO solution, 1 mg/ mL), it gains the most
34 negative charge density on graphene sheets (Figure
35 S3a). Besides, N2H4·H2O also has an effect on the zeta
5 oxygenated
67
68
69 Figure
2. (a), (b) SEM image of SnO2@3DG. (c) TEM image of
70 SnO2@3DG.
71 diagram
72 of
Inset: The corresponding particle size distribution
with the calculation of 100 particles. (d) HRTEM image
SnO2@3DG. Inset: The corresponding SAED pattern.
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Nano Res.
1 among
49 3D
2 tolerating
the nanoparticles. This is favoring for
the large volume expansion during the
3 charge-discharge process [20]. The HRTEM image in
4 Figure 2d reveals a clear crystal lattice of 0.33nm,
5 corresponding to the interspacing of the (110) plane
6 of SnO2. The selected-area-electron diffraction (SAED)
7 pattern (inset of Figure 2d) confirms the presence of
8 polycrystalline SnO2 nanoparticles. In comparison,
9 SnO2@2DG shows a typical sheet-like 2D structure, in
10 which ultrasmall SnO2 nanocrystals (35 nm) are
11 anchored on the surface of graphene (Figure S5a-c).
12
The crystal structures of 3DG, SnO2@2DG, and
13 SnO2@3DG were analyzed by X-ray diffraction (XRD),
14 as shown in Figure 3a. The XRD of 3DG clearly
15 reveals typical characteristic of graphene with a
16 hump at 24°-28°; While, the XRD of both SnO2@3DG
17 and SnO2@2DG exhibits similar diffraction peaks that
18 can be readily indexed to tetragonal rutile SnO2
19 (JCPDS No.41-1445). However, no diffraction peaks
20 of graphene could be found in XRD patterns of both
21 composites, which could be explained by the fact that
22 the strong signals of SnO2 shelter the characteristic of
23 graphene sheets. Figure 3b shows the Raman spectra
24 of the three samples, in which two broad peaks at
25 1335 cm-1 (D band) and 1593 cm-1 (G band) can be
26 assigned to the typical characteristic of graphene.
27 Moreover, the X-ray photoelectron spectroscopy (XPS)
28 was investigated to explore the nature of interaction
29 among SnO2 and graphene (Figure S6). The scan
30 spectra up to binding energy of 1200 eV (Figure S6a)
31 display the presence of C, O and Sn elements. The
32 spectrum in binding energy range of 480 to 500 eV
33 (Figure S6b) shows the peaks of Sn 3d3/2 and Sn 3d5/2,
34 indicating
the formation of SnO2-containing
35 composites. The C 1s spectra of the composites are
36 shown in Figure S6c. The peak at 284.8 eV is assigned
37 to C-C, while the peak at 286.6 eV corresponds to
38 C-O-Sn. This reflects the formation of C-O-Sn bond
39 between the SnO2 and graphene [21].
40
The SnO2 contents in both composites were
41 measured by thermogravimetric analysis (TGA). As
42 shown in Figure 3c, the weight percentages of SnO2
43 in SnO2@3DG and SnO2@2DG are 71.5% and 70.4%,
44 respectively. In addition, the content of SnO2 particles
45 in SnO2@3DG has an obviously effect on the 3D
46 structure. The 3D structure with many large pores is
47 kept with a lower loading mass of SnO2 (40.2%) in
48 composite (Figure S7a). Figure S7b-d shows that the
50 SnO2
structure gradually changes as the content of
increases. When the mass loading of SnO2 is
51 increased to 90.5%, the 3D structure in the composite
52 completely collapses. Figure 3d shows that the
53 Brunauer-Emmett-Teller (BET) surface area of the
54 3DG is as high as 470.5 m2 g-1. Moreover, the specific
55 surface area of SnO2@3DG (237.7m2 g-1) is two times
56 higher than that of SnO2@2DG (113.5 m2 g-1). This
57 result highlights that the ice-templated strategy is an
58 effective way to obtain a high surface area. Based on
59 the Barrett-Joyner-Halenda method, the pores size
60 distribution of SnO2@3DG mostly centers at around 3
61 nm and 30 nm (Figure S8). The broad pore size
62 distribution spanning from 2 to 100 nm implies that
63 SnO2@3DG is rich in hierarchical pores. Consequently,
64 the high surface area and rich porosity of SnO2@3DG
65 are closely related to the presence of meso/macro
66 pores.
67
68
69 Figure
70 (d)
3. (a) XRD profiles, (b) Raman spectra, (c) TGA curves,
N2 adsorption-desorption isotherms of SnO2@3DG (71.5%
71 SnO2),
SnO2@2DG (70.4% SnO2) and 3DG.
The sodium storage performance of SnO2@3DG
73 and SnO2@2DG was further investigated. Figure 4a-b
74 show the charge-discharge curves of SnO2@3DG and
75 SnO2@2DG tested at a current density of 100 mA g-1.
76 The electrode of SnO2@3DG exhibits a high discharge
77 capacity of 1155 mAh g-1 and a charge capacity of 480
78 mAh g-1 in the initial cycle (inset of Figure 4a). The
79 irreversible capacity loss (675 mAh g-1) is mainly
80 originated from the formation of the solid electrolyte
81 interphase (SEI) film [16, 17, 22], while the side
82 reaction of oxygen-containing functional groups on
83 graphene also leads to partial capacity loss (ca.70
72
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5
Nano Res.
36 SnO2
are compared (Figure S11). The composites
low mass loadings of SnO2 (40.2% and 57.4%)
38 deliver relative low capacity but with good cycling
39 performance. In contrast, the composite with much
40 higher mass loading of SnO2 (90.5%) shows an initial
41 high capacity but with inferior cycling stability,
42 which can be ascribed to the severe aggregation and
43 pulverization of nanoparticles (Figure S12). The 3D
44 frameworks
limit
the
agglomeration
and
45 pulverization
of SnO2 nanoparticles during
46 sodiation/desodiation process, which is mostly
47 responsible for the enhanced capacity and high-rate
48 cycling performance. Thus, the SnO2@3DG composite
49 with 71.5% of SnO2 mass loading is optimized to
50 show a combination of high capacity and stable
51 cycling.
52
Figure 5a-b shows the Nyquist plots of SnO2@3DG
53 and SnO2@2DG. The high frequency semicircle is
54 associated with sodium-ion migration through SEI
55 film and the middle frequency semicircle is linked to
56 charge transfer reaction. The charge transfer
57 resistance (Rct) values of SnO2@3DG reduce from 48.5
58 Ω at 308 K to 32.2 Ω at 328 K, while the SnO2@2DG
59 shows Rct values of 77.3 Ω at 308 K and 35.9 Ω at 328
60 K. This indicates that in the measured temperature
61 range, higher temperature results in better electrode
62 kinetics. Besides, both two composites show similar
63 Re values while the RSEI values of SnO2@3DG are
64 higher than that of SnO2@2DG, which is due to the
65 larger surface area of SnO2@3DG (Table S1). The
66 exchange currents (i0) and the apparent activation
67 energies (Ea) for the intercalation of sodium are
68 calculated by the following equations[23]:
37 with
1
2 Figure
4. Charge-discharge curves of (a) SnO2@3DG (71.5%
3 SnO2)
and (b) SnO2@2DG (70.4% SnO2) at the current density
4 of
100 mA g-1. (c) Rate performance of SnO2@3DG and
5 SnO2@2DG
6 mA
tested at different current densities from 50 to 800
-1
g . (d) Cycling performance of SnO2@3DG and
7 SnO2@2DG
8 mAh
at the current density of 100 mA g-1.
g-1, Figure S9). During the second cycle, it
9 delivers a discharge capacity of 504 mAh g-1,
10 accounting for 94.4% of the theoretical capacity (533.9
11 mA h g-1, Figure S10). This indicates a high utilization
12 of the active material in the composite. Notably, a
13 reversible capacity of 432 mAh g-1 is still maintained
14 after 200 cycles. In contrast, the electrode of
15 SnO2@2DG exhibits a discharge capacity of 1067 mAh
16 g-1 and a charge capacity of 411 mAh g-1 (inset of
17 Figure 4b). The capacity reservation is 78.9% and the
18 maintained capacity after 200 cycles is only 301 mA h
19 g-1. Figure 4c shows the rate performance of the two
20 composites. SnO2@3DG delivers capacities of 551
21 mAh g-1 and 210 mAh g-1 at 50 and 800 mA g-1,
22 respectively. However, SnO2@2DG only shows
23 capacities of 391 mAh g-1 and 127 mAh g-1 at 50 and
24 800 mA g-1, respectively. Figure 4d exhibits the
25 cycling performance of the composites. Obviously,
26 SnO2@3DG electrode shows an enhanced cycling
27 performance compared to that of SnO2@2DG. The
28 electrode of SnO2@3DG retains a reversible capacity
29 of 432 mAh g-1 after 200 cycles (85.7% capacity
30 retention to the second cycle). This result is superior
31 to that of SnO2@2DG (301 mAh g-1 after 200 cycles,
32 71.4% capacity retention) and those reported for
33 SnO2/graphene materials [16, 17]. To investigate the
34 effect of unique structure on the cycling performance,
35 SnO2@3DG composites with different contents of
69
70 Figure
5. EIS plots of (a) SnO2@3DG (71.5% SnO2) and (b)
71 SnO2@2DG
72 of
(70.4% SnO2) from 308K to 328K with 50% depth
discharge in the second cycle. (c) Arrhennius plots of ln(T/Rct)
73 versus
1/T. Inset: the equivalent circuit of SnO2@3DG and
74 SnO2@2DG.
75 resistance
Re is electrolyte resistance. RSEI and CSEI are the
and constant phase element of SEI film. Rct and Cct
76 present
the charge-transfer resistance and constant phase
77 element
.W is Warburg impendence related to the diffusion of
78 sodium ions.
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Nano Res.
1
40 and
2
i0 = RT/nFRct
(1)
i0 = Aexp(-Ea/RT)
(2)
3 The calculated Ea is 20.2 kJ mol-1 for SnO2@3DG and
4 35.8 kJ mol-1 for SnO2@2DG (Figure 5c, and Equations
5 S1). The lower activation energy of SnO2@3DG is
6 indicative of more facile sodium intercalation kinetics
7 [24, 25]. The enhanced kinetics arises from the
8 particular structure of SnO2@3DG composites, in
9 which 3D graphene architecture can not only provide
10 a highly electronic and ionic conductivity but also
11 offer large surfaces that are favoring effective
12 contacts between electrolyte and materials.
13
The much better electrochemical performance of
14 SnO2@3DG than that of SnO2@2DG is schematically
15 shown in Scheme 2. An interpretation follows. First,
16 the elastic and specific 3D structures can not only
17 enhance the mechanical strengths of electrode, but
18 also act as a buffer that allows the accommodation of
19 volume expansion of SnO2 nanocrystals during the
20 charge/discharge process. Second, the large surface
21 area and various meso-/macro pores of SnO2@3DG
22 can offer multidimensional channels to facilitate the
23 transport of Na+ in the electrode. This should shorten
24 the diffusion length from the external electrolyte to
25 the interior surface and gain easy electrolyte
26 accessibility.
41 maintained
high
capacity retention of 85.7% were
over 200 cycles at 100 mA g-1. Even at a
42 high current density of 800 mA g-1, the capacity was
43 210 mAh g-1. Those enhanced performance was due
44 to the 3D internet for effective limitation of
45 agglomeration
and
pulverization
of
SnO2
46 nanoparticles during sodiation/desodiation process
47 and highly electronic and ionic conductivity of 3DG.
48
49
4 Experimental methods
4.1.1 Synthesis of graphene solution with
51 different volume of NH3·
H2O.
50
Graphene oxide (GO) was prepared from graphite
53 (325mesh, Alfa Aesar) by a modified Staudenmeier
54 method [26]. The obtained GO (100 mg) was
55 dispersed in distilled water (100mL) and then
56 exfoliated
to generate GO nanosheets by
57 ultrasonication.
The brown suspension was
58 transferred to a round-bottomed flask, in which
59 different amounts of NH3·H2O (0, 250, 500, 750, 1000,
60 1250, 1500, 1750, 2000μL) were added, respectively.
61 Finally, the solution was stirring under the oil bath at
62 95 C for 1 h.
52
4.1.2 Synthesis of graphene colloid with
64 different volume of N2H4·
H2O.
63
100 mg GO was dispersed in distilled water (100
66 mL) and then exfoliated to generate GO nanosheets
67 by ultrasonication. The brown suspension was
68 transferred to a round-bottomed flask, in which
69 750μL NH3·H2O was added in to the mixture.
70 Subsequently, different amounts of N2H4·H2O (80%,
71 Guangfu fine chemical, Tianjin, China) including 0,
27
72 30, 60, 90, 120, 150, 180, 210, 240μL were added,
28 Scheme 2. Schematic illustration of SnO2@3DG for sodium
73 respectively. The solution was stirring under the oil
29 storage during the charge-discharge process.
74 bath at 95 C for 1 h. After cooling naturally, the
30
75 solution was frozen under -18C in the refrigerator
31 3 Conclusions
76 and directly dehydrated via a freeze drying process
32
In summary, SnO2@3DG was successfully prepared 77 to maintain the 3D monolithic architecture. Finally,
33 by assembling ice-templated 3DG with tin source 78 the as-prepared 3DG was treated under calcination at
34 under a hydrothermal process and further applied in 79 350 C for 2h in Ar.
35 SIBs.
The as-prepared SnO2@3DG shows a
80 4.1.3 Synthesis of SnO2@3DG.
36 hierarchical
porous
architecture
with
In a typical procedure, 80 mg 3D graphene was
37 meso/macro-pores and large surface area. Moreover, 81
in
the
solution
of
80
mL
38 it exhibits a high cycling stability and a high rate 82 dispersed
-1
83
N,N-Dimethylformamide
(DMF)
and
10
mL
39 performance. A reversible capacity of 432 mAh g
65
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Nano Res.
1 deionized
46 Na
2 SnCl2·2H2O
water. Subsequently, a precise amount of
(0.4 g, Guangfu fine chemical, Tianjin,
3 China) were added to the solution followed by
4 adding 1 mL HCl (36.0%~38.0%). After stirring for 5
5 hours, the suspension were transferred to a
6 Teflon-lined autoclave (100 mL in capacity), sealed
7 tightly, and heated up to 120 C for 12 hours. The
8 obtained SnO2@3DG were washed with ethanol
9 several times and dried under vacuum at 120C for
10 24 h.
47 microfiber
11
4.1.4 Synthesis of SnO2@2DG.
The preparation of SnO2@2DG is similar with the
13 process of SnO2@3DG, except for the replacement of
14 3DG with 2DG. The 2DG was synthesized by adding
15 NH3·H2O (750 μL) and N2H4·H2O (60 μL) into GO
16 (100 mg, 1mg/mL) solution without freeze drying,
17 but a direct filtration and drying process in a vacuum
18 oven.
12
19
4.2 Material characterization
Structural analysis was carried out by powder
21 X-ray diffraction (XRD, Rigaku MiniFlex600, Cu Kα
22 radiation) with a scanning speed of 4° min-1 from
23 10°-80°. Raman spectra were obtained using a
24 confocal Raman microscope (DXR, Thermo-Fisher
25 Scientific) at 532 nm excitation from an argon-ion
26 laser.
Morphologies
and
structures
were
27 characterized by scanning electron microscopy (SEM,
28 FEI Nanosem 430, 10 kV) and transmission electron
29 microscopy (TEM, Philips Tecnai G2F20, 200 kV). The
30 specific surface area and pore size distribution were
31 analyzed
using Brunauer-Emmett-Teller (BET)
32 nitrogen adsorption/desorption isotherm at 77 K on a
33 BELSORP-mini instrument. The thermogravimetric
34 analysis (TGA) measurements were carried out in air
35 flow at a heating rate of 5 C min−1 using Netzsch
36 STA 449 F3 Jupiter analyzer. The zeta potentials were
37 measured with Zetasizer (Malvern, Nano ZS90). The
38 fourier transform infrared spectroscopy (FT-IR)
39 spectra were recorded on FTIR-650 spectrometer
40 (Gangdong Co., Ltd., Tianjin, China).
20
The working electrodes were prepared by
42 dispersing the as-prepared material, carbon black,
43 poly (Vinylidene fluoride) binder at a weight ratio of
44 8:1:1. The resultant slurry was pasted onto copper foil
45 and dried at 110 C in vacuum for 8 hours. By using
41
as counter and reference electrodes, glass
(whatman) as the separator to assemble a
48 CR2032-type coin cell in the argon-filled glove box
49 (Mikrouna Universal 2440/750). The electrolyte was
50 1M NaClO4 in tetraethylene glycol dimethyl ether
51 (TEGDME) with 5 wt.% ﬂuoroethylene carbonate
52 (FEC). The assembled cells were tested at different
53 rates in the voltage range of 0.01 to 2.5 V using a
54 LAND-CT2001A
battery-testing
instrument.
55 Electrochemical impedance spectra (EIS) were
56 collected with an AC voltage of 5 mV amplitude in
57 the frequency range from 100 kHz to 10 m Hz.
58
59
Acknowledgements
60 This
work was supported by the Programs of
973 (2011CB935900), NSFC (51231003),
62 MOE (IRT13R30 and B12015), and Tianjin High-Tech
63 (13JCQNJC06400 and 12ZCZDJC35300).
64 Electronic Supplementary Material: Supplementary
65 material (Colloid test, FT-IR spectra, SEM image,
66 Zeta potential of 3DG with different NH3·H2O and
67 N2H4·H2O. SEM images, TEM images of SnO2@2DG.
68 SEM images, electrochemical performances of
69 SnO2@3DG with different SnO2 contents. Pore size
70 distribution of SnO2@3DG. Rct values of SnO2@3DG
71 and SnO2@2DG.) is available in the online version of
72 this
article
at
73 http://dx.doi.org/10.1007/s12274-***-****-*
74 (automatically inserted by the publisher).
61 National
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