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Facile preparation of graphene/SnO2 xerogel hybrids as the anode material in Li-ion batteries Zhefei Li, Qi Liu, Yadong Liu, Le Xin, Fan Yang, Yun Zhou, Hangyu Zhang, Lia A. Stanciu, and Jian Xie ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b05819 • Publication Date (Web): 30 Sep 2015 Downloaded from http://pubs.acs.org on October 1, 2015

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ACS Applied Materials & Interfaces

Facile Preparation of Graphene/SnO2 Xerogel Hybrids as the Anode Material in Li-ion Batteries

Zhe-Fei Li,a Qi Liu,a Yadong Liu,a Fan Yang,a Le Xin,a Yun Zhou,a,b Hangyu Zhang,c Lia Stanciu,b,c and Jian Xiea,*

a

Department of Mechanical Engineering, Indiana University-Purdue University

Indianapolis, Indianapolis, Indiana, 46202 (USA). b

School of Materials Engineering, Purdue University, West Lafayette, Indiana,

47907 (USA). c

Weldon School of Biomedical Engineering, Purdue University, West Lafayette,

Indiana, 47907 (USA).

Abstract: SnO2 has been considered as one of the most promising anode materials for Li-ion batteries due to its theoretical ability to store up to 4 Li+. However, it suffers from poor rate performance and short cycle life due to the low intrinsic electrical conductivity and particle pulverization caused by the large volume change upon lithiation/delithiation. Here, we report a facile synthesis of graphene/SnO2 xerogel hybrids as anode materials using epoxide-initiated gelation method. The synthesized hybrid materials (19% graphene/SnO2 xerogel) exhibit excellent electrochemical performance: high specific capacity, stable cyclability and good rate capability. Even cycled at a high current density of 1 A/g for 300 cycles, the hybrid electrode can still

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deliver a specific capacity of about 380 mAh/g, corresponding to more than 60% capacity retention. The incorporation of graphene sheets provides fast electron transfer between the interfaces of the graphene nanosheets and the SnO2 and a short lithium ion diffusion path. The porous structure of graphene/xerogel and the strong interaction between SnO2 and graphene can effectively accommodate the volume change and tightly confine the formed Li2O and Sn nanoparticles, thus preventing the irreversible capacity degradation.

Keyword: graphene, SnO2, xerogel, hybrids, Li-ion battery, anode *

Corresponding author: [email protected]


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1. Introduction Over the past decades, lithium-ion batteries (LIBs) have attracted much attention in both scientific and industrial fields due to their high energy density.1 However, the wide use of LIBs in electric and hybrid vehicles is limited by the low energy and power density of commercial LIBs. The widely used anode material, graphite, has a theoretical specific capacity of only 372 mAh/g.2 Therefore, it is of great interest to develop high-capacity anode materials for the next-generation LIBs.3 Conversion anode materials, such as oxides, nitrides, phosphides, and sulfides, have been widely studied due to their much higher theoretical capacities through reactions generalized by:4 Li + MaXb ↔ a LinM + b LimX

(1)

where M is a transition metal and X is an anion (O, N, P, S, etc). Among these candidate materials, SnO2 has been considered as one of the most promising anode materials to replace the present graphite anode due to its low cost, facile synthesis, and large specific and volumetric capacity.5-6 However, the practical use of SnO2 in LIBs is greatly hindered by its poor rate performance and short cycling life. The poor rate performance is caused by the low electrical conductivity of SnO2.7-8 The short cycle life mainly results from the large volume change (> 200%) during Li insertion/deinsertion, which can cause the pulverization of SnO2 particles, an unstable solid-electrolyte interface (SEI) layer, and the loss of a conductive path.9 In addition, the formation of the Li2O phase during the Li insertion process can lead to an irreversible capacity.10



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Tremendous work has been done to solve these problems. Various SnO2 materials with

different

nanostructures

including

flower-like

SnO2,11

nanoparticles,12

nanowires,13 and hollow spheres14-15 have been reported to show improved cycling stability. One of the approaches is to fabricate nanocomposites by embedding SnO2 in a carbon matrix.6, 16-18 Recently, graphene, the single-atomic-thick graphite layer, has attracted tremendous attention due to its exceptional properties, such as excellent electrical conductivity, surface area, mechanical strength, and chemical stability.19 Graphene/SnO2 composite materials have been extensively studied by encapsulating SnO2 nanoparticles within graphene nanosheets to create an empty space to accommodate the large volume change and improve the electrical conductivity.20-25 However, most of this work focused on the nanocomposites of SnO2 nanoparticles and graphene. Our group recently discovered that graphene-modified V2O5 xerogel hybrids exhibited extraordinary electrochemical performance as cathode materials.26 To our best knowledge, very little work has been done on developing SnO2 xerogel as a LIB anode material. Here, we report a facile synthesis of graphene/SnO2 xerogels as LIB anodes. SnO2 xerogel exhibits a high surface area and large porosity, which can facilitate Li-ion diffusion and provide empty space to accommodate the large volume change during charge/discharge cycles. The incorporation of graphene sheets is expected to provide a conductive path as well as excellent mechanical flexibility to further accommodate the volume change, potentially leading to significantly enhanced electrochemical performance and stability.


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2. Experimental 2.1 Synthesis of graphene oxide (GO). GO was prepared following a previously published procedure.27-28 Typically, natural graphite flakes are pre-oxidized and then subjected to oxidation by the Hummer's method. Two grams of pre-oxidized graphite, 1 g sodium nitrate, and 46 mL of sulfuric acid were mixed and stirred for 15 min in a 500 mL round-bottom flask immersed in an ice bath. Potassium permanganate (6 g) was slowly added to the above suspension solution. After 15 min, the temperature was heated to about 35 ºC and maintained at that temperature for 2 h. 92 mL of water was then added dropwise to the suspension, causing a violent effervescence. The temperature was maintained above 98 ºC for 30 min. Finally, the suspension was diluted by 280 mL of water and treated with 10 mL of 30% H2O2 to react with the potassium permanganate. The precipitated GO was washed successively with 1 M HCl solution and DI water by centrifugation at 12000 rpm several times to remove residual salts and acid. The obtained GO was ultrasonicated to achieve a stable GO dispersion in water (at a concentration of about 10 mg/mL). Then, the GO dispersion was subjected to another centrifugation at 5000 rpm for 5 min to remove the large unexfoliated GO sheets. 2.2 Synthesis of graphene/SnO2 xerogel hybrids. GO (varying amounts) and SnCl4·5H2O (0.56 g) were dispersed in 10 mL DI H2O. The mixture was sonicated to achieve a homogeneous dispersion. Propylene oxide was rapidly added to the dispersion, which was vigorously stirred for 1 min, and then left undisturbed for 24 h. The resulting hydrogel was washed by successive immersion in water and then freeze



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dried. Finally, the hybrids were heat treated at 550 ºC for 2 h under an Argon atmosphere at a heating rate of 5 ⁰C/min. For comparison, pure SnO2 xerogel was prepared following the same procedure without the addition of GO. Graphene was prepared by heat treatment of freeze-dried GO at 550 ºC for 2 h under an Argon atmosphere. The resulting samples were denoted as Gx-SnO2, where x represents the weight percentage of graphene in the hybrids determined by thermal gravimetric analysis (TGA). 2.3. Characterization. TGA curves were obtained using the TA Instrument SDT Q600. The morphology was characterized by a Philips CM 200 transmission electron microscope (TEM) and a JOEL-7800 scanning electron microscope (SEM). X-ray diffraction patterns were obtained by a Bruker APEX II diffractometer. The N2 adsorption/desorption isotherms were determined by a Quantachrome Autosorb-iQ gas sorption analyzer at 77 K. The Brunauer-Emmett-Teller (BET) specific surface area was calculated using adsorption data at the relative pressure range of 0.05-0.3. The total pore volumes were estimated from the amount adsorbed at a relative pressure (P/P0) of 0.98. The non-local density functional theory (NLDFT) pore size distribution was calculated based on the adsorption branch of the isotherm.29 2.4. Electrode fabrication and electrochemical test. The electrochemical tests were measured in a CR2016-type coin cell. A lithium foil was used as the counter electrode. The working electrode was fabricated by pasting the slurry containing 75% active

materials,

15%

SuperP,

and

10%

poly(vinylene

difluoride)

in

N-methyl-2-pyrrolidinone onto an Cu foil using the doctor-blade method. The


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electrode was dried in a vacuum oven at 80 ºC overnight and then assembled into the coin cell in an Ar-filled glove box (O2 and H2O < 0.1 ppm). The mass loading of the active material on each electrode was about 2.5-3 mg/cm2. The electrolyte used was 1.2 M LiPF6 in EC/EMC (3:7) and a Celgard 2400 polypropylene membrane was used as the separator. Cyclic Voltammetry (CV) and Electrochemical impedance spectroscopy (EIS) were performed using the Solartron system (1287/1260). The galvanostatic charge/discharge tests were measured on an Arbin battery testing station.

3. Results and discussion Graphene/SnO2 xerogels with different graphene loadings were prepared based on epoxide-initiated gelation method.30 When SnCl4 was dissolved in water, Sn4+ formed leads to a mixture of tin chloride monomeric complexes. The positively charged complexes could have a strong interaction with negatively-charged GO due to the electrostatic force.31 Upon the addition of propylene oxide, an acid scavenger, the hydrolysis and condensation of hydrated tin complexes occurred, resulting in the formation of SnO2 sol. The presence of GO sheets served as the substrate to anchor SnO2, which grew into interconnected SnO2 gel anchored on the surface of the GO sheets. After the gelation, the hydrogel was freeze dried to maintain the porous structure and finally heat treated to reduce GO. The graphene loading was determined by TGA, as shown in Figure 1a. XRD was used to characterize SnO2 xerogel and graphene/SnO2 xerogel, and the results are shown in Figure 1. The graphene/SnO2



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xerogel exhibited a typical tetragonal rutile structure.32 However, the broad diffraction peak of graphene around 25º (usually associated with the graphite 002 face, indicating the restacking of the graphene sheets in the graphene-containing hybrids) was not observed in the diffraction pattern of the graphene/SnO2 xerogel; this is probably due to its overlapping with the strong (110) peak of the SnO2. It is also possible that these graphene sheets are uniformly dispersed within the SnO2 matrix in a single or double sheet form, rather than restacking together. We further compared the XRD patterns of different graphene/SnO2 xerogels and calculated the crystallize size of SnO2 nanocrystals using Scherrer equation (Figure S1). It was discovered that the crystallize size of pure SnO2 was about 15.1 nm. As the GO was incorporated, the crystallize size increased and peaked at about 32 nm for G5-SnO2 and then decrease to 8.9 nm for G19-SnO2. This might be due to the presence of GO, which can significantly affect the nucleation of SnO2. As a small amount of GO was added, these GO nanosheets have high surface energy and strong electrostatic interaction with hydrated Sn4+ complex, causing rapid heterogeneous nucleation and thus large crystallite size. When more GO was added, there exist more available nucleation sites, consequently, the nucleation of SnO2 was more uniform and its crystallite size was smaller.


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110

a

100

b

Graphene/SnO2

90 80

Intensity (a.u.)

Weight percentage (%)

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SnO2 SnO2 G2-SnO2 G2-SnO2

70

G5-SnO2 G5-SnO2

SnO2

G9-SnO2 G9-SnO2

60

Graphene

G19-SnO2 G19-SnO

2

50 0

200

400

600

800

1000

10

20

30

Temperature (ºC)

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50 60 2 Theta (º)

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Figure 1. (a) TGA curves and (b) XRD patterns of SnO2 xerogel and G19-SnO2 xerogel. The morphology and structure of the as-prepared SnO2 xerogel and graphene/SnO2 xerogel with different graphene loadings were characterized by SEM. As shown in Figure 2a, a monolith structure of the SnO2 xerogel can be observed from the SEM image. Close examination of Figure 2b reveals that the SnO2 xerogel showed a nanoporous structure. Upon the addition of 2 wt.% graphene, the morphology of the graphene-SnO2 xerogel became a layered structure, as indicated in Figure 2c. In addition, a wrinkled structure can be observed with the increase in graphene content, which corresponds to the wrinkles of the graphene (Figure 2c-2f). When the graphene content was increased to about 19 wt.%, the morphology of graphene-SnO2 xerogel (Figure 2f) looked very similar to pure graphene (Figure S2),33 which suggests that the SnO2 xerogel was uniformly formed over the graphene sheets. From these SEM images (Figure 2a-f), it is clear that the higher the graphene content is in the hybrid, the thinner the SnO2 xerogel layer over the graphene sheets is, and the better the dispersion of these SnO2 xerogel hybrid sheets is in the material. Such a thin layer



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structure should facilitate fast Li+ transport (i.e. short diffusion path within the SnO2 xerogel) and provide more of a surface for Li+ access. These features will likely lead to an increased rate performance and higher specific capacity, which will be shown later. The composition of the hybrids can be obtained by Energy Dispersive X-Ray Spectroscopy (EDS) mapping images of carbon, tin, and oxygen in the graphene-SnO2 xerogel hybrids. Figure 3 shows that the distribution of carbon, tin, and oxygen was very uniform, indicating homogeneous dispersion of the graphene in the SnO2 matrix. The morphologies of both the SnO2 xerogel and the graphene/SnO2 xerogel were also characterized by TEM. The SnO2 xerogel had a morphology of aggregated nanoparticles (Figure 4a). The graphene/SnO2 xerogel showed that the SnO2 xerogel was uniformly mixed with the graphene sheets over a large area in the hybrids (Figure 4b).


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Figure 2. SEM images of (a) and (b) SnO2 xerogel, and graphene/SnO2 xerogel with (c) 2% graphene, (d) 5% graphene, (e) 9% graphene, and (f) 19% graphene.



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Figure 3. EDS mapping of carbon, oxygen, and tin elements in G19-SnO2 xerogel hybrids.


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Figure 4. TEM images of (a) SnO2 and (b) G19-SnO2 hybrids. Red arrows point to the SnO2, and black arrows point to the wrinkles/edges of the graphene sheets.

The surface area, pore volume, and pore size of the samples were characterized by N2 adsorption and desorption measurements and the NLDFT pore size distribution is shown in Figure 5. The surface area is calculated by the Brunauer–Emmett–Teller (BET) method. Due to the incorporation of graphene sheets, the G19-SnO2 xerogel hybrids had a much higher surface area (166 m2/g) than that of the SnO2 xerogel (69 m2/g). The SnO2 xerogel showed typical mesoporous structure with average pore size around 9 nm. Interestingly, there also exist a large amount of pores larger than 15 nm in the G19-SnO2 xerogel hybrids, in addition to the small pores around 4 nm. The small pores around 4 nm may result from the slit-shape pores in the hybrids, similar to graphene.33 The presence of pores larger than 15 nm suggests that SnO2 act as spacers and insert between graphene sheets, partially preventing the graphene restacking.34 The hybrids also exhibited a larger pore volume (0.28 cm3/g) than the pure SnO2



xerogel (0.19 cm3/g). The large specific surface area and pore volume is expected to benefit the electrolyte ion transport, provide more access for Li+ over the SnO2 surface, and accommodate the huge volume expansion during lithiation.28

0.02

Derivative pore volume dV/dr

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0.015 SnO2 SnO2

G19-SnO2 G19-SnO2

0.01

0.005

0 0

10 20 Pore Size (nm)

30

Figure 5. NLDFT pore size distribution of SnO2 xerogel and G19-SnO2 hybrids.

To evaluate the electrochemical lithium storage properties of the graphene/SnO2 xerogel hybrids as anode materials for LIBs, a series of electrochemical measurements were carried out. Figure 6 shows cyclic voltammograms (CV) of the SnO2 xerogel and graphene/SnO2 xerogel hybrids at a scan rate of 0.5 mV/s in the voltage range of 0.01-3.00 V (vs. Li/Li+). The major cathodic peaks that appear in the


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CV curves are consistent with the lithiation behavior of pure SnO2.7, 35 In the first discharge curve, the small peak around 2.5 V observed in the graphene-SnO2 hybrids may be due to the reaction with the functional groups and the defects on graphene.36 The peak at around 0.7-1.3 V results from the formation of solid electrolyte interface (SEI), reduction of SnO2 to Sn, and the formation of Li2O. The peak between 0.5-0.01 V corresponds to further lithiation into the Sn. It can also be seen in Figure 6 that the graphene/SnO2 electrode showed more reversible delithiation behavior than the pure SnO2 electrode. This is likely because the LixSn and Li2O phases formed during lithiation can be tightly confined within the graphene matrix to prevent the particle pulverization and coalescence.37

1 0.5

Current (A/g)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0 -0.5

SnO2 SnO2 Graphene Graphene Graphene/SnO2 Graphene/SnO2

-1 -1.5 -2 0

0.5

1

1.5 2 2.5 Voltage (V) vs Li/Li+

3

3.5

Figure 6. Cyclic voltammograms of SnO2 xerogel and graphene/SnO2 xerogel hybrids at a scan rate of 0.5 mV/s.



3.5

a

3

SnO2-1st discharge SnO2-1st SnO SnO2-1st 2-1st charge G2-SnO2-1st discharge G2-SnO2-1st G2-SnO2-1st charge G2-SnO2-1st

Voltage (V)

2.5

G19-SnO2-1st G19-SnO2-1st discharge G19-SnO2-1st charge G19-SnO2-1st

2 1.5 1 0.5 0 0

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Specific Capacity (mAh/g)

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b Specific Capacity (mAh/g)

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1000 SnO2 SnO2 G19-SnO2 G19-SnO2

800 600 400 200 0 0

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Current Density (A/g)


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800


SnO2 SnO2

c

700

G2-SnO2 G2%-SnO2

600

G5%-SnO2 G5-SnO2

500

G19%-SnO2 G19-SnO2

G9%-SnO2 G9-SnO2

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Coulombic Efficiency (%)


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20

100 0

0 0

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150 200 Cycle number

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300

Figure 7. (a) Charge/discharge curves of SnO2 and graphene/SnO2 hybrids at a current density of 50 mA/g; (b) comparison of the capacity retention of SnO2 and graphene/SnO2 hybrids with different graphene content at 1 A/g; (c) rate performance of SnO2 and G19-SnO2 hybrids; and (d) capacity retention and coulombic efficiency of G19-SnO2 hybrids at 1 A/g for 300 cycles.



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Figure 7a presents the charge/discharge curves of the SnO2 and the graphene/SnO2 hybrids at a current density of 50 mA/g. The typical charge/discharge plateau of SnO2 can be observed and is consistent with the CV results. At 50 mA/g, the initial discharge and charge capacity of the SnO2 xerogel electrode was about 1326 and 741 mAh/g, respectively. The introduction of a small amount of graphene (i.e. 2 wt.%) into the SnO2 had an extraordinary effect on its electrochemical performance. Based on the total weight of the graphene and the SnO2, for G2-SnO2, the initial discharge and charge capacity was about 1495 and 810 mAh/g, respectively. The reversible specific capacity was found to further increase with higher graphene content. When graphene content was about 19 wt.%, the initial discharge and charge capacity was about 1581 and 903 mAh/g, respectively. To estimate the capacity contributed by SnO2, graphene was tested as the anode in LIB. The initial discharge capacity of graphene is about 600 mAh/g, but quickly decays to below 300 mAh/g after several cycles (Figure S4). Considering the wt.% of graphene in the G19- SnO2 hybrids is only 19%, the contribution of graphene is not very significant. The actual specific charge capacity of SnO2 in the hybrids was about estimated to be about 1100 mAh/g, close to the theoretical specific capacity of SnO2 (1491 mAh/g assuming 8.4 Li+ insertion). The incorporation of graphene also enhanced the rate performance of the SnO2 xerogel, as shown in Figure 7b. Pure SnO2 can deliver a specific capacity of only 315 mAh/g at a current density of 1 A/g (~1C rate). The specific capacity increased to 607 mAh/g with 19 wt.% graphene loading, which is almost two times of that of pure SnO2. At a high rate of 5 A/g, the SnO2 electrode can hardly be cycled,


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while the G19-SnO2 was still able to deliver a capacity of around 369 mAh/g. Over all, the rate performance was improved by 22%, 36%, 62%, and 93% at 50 mA/g, 200 mA/g, 500 mA/g, and 1 A/g, respectively. Such huge increases can be attributed to the enhanced conductivity and ion diffusion due to the uniform distribution of graphene sheets between SnO2 particles in the hybrids. The cycling life performance was also found to be better in the graphene/SnO2 hybrids. For example, the capacity of the pure SnO2 xerogel decreased to less than 100 mAh/g after only 33 cycles at a current density of 1 A/g, while the cycle life of the graphene-SnO2 hybrids increased with the graphene loading. The capacity decayed at a slower rate with the addition of only 2 wt.% graphene in the hybrids. Furthermore, with 19 wt.% graphene, the capacity retention of the hybrids was more than 450 mAh/g after 100 cycles. The longer cycling test was performed for the graphene/SnO2 hybrids (19 wt.% graphene). It was observed that after 300 cycles at 1 A/g, the hybrid electrode still showed a specific capacity of about 370 mAh/g, corresponding to about 60% of the initial charge capacity and indicating the excellent structural stability of the graphene/SnO2 xerogel. The decay rate of the graphene/SnO2 hybrids (19 wt.% graphene) was approximately 0.13% capacity loss per cycle which is much lower than 0.9% capacity loss per cycle of the pristine SnO2. It should also be noted that the coulombic efficiency reached about 99-100% after several cycles, indicating the stability of the hybrid electrode.



250

a 200

Z" (ohm)

SnO2 SnO2

G2-SnO2 G2-SnO2

150

G9-SnO2 G9-SnO2 G19-SnO2 G19-SnO2

100

50

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Z" (ohm)

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30

G19-SnO2 100 cycles G19-SnOafter 100 cycles 2 after

20

SnO2 cycles SnO2after after100 100 cycles

10 0 0

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Z' (ohm)

Figure 8. (a) EIS spectra of the SnO2 xerogel and the graphene-SnO2 hybrids with different graphene percentages; (b) comparison of EIS spectra of the SnO2 xerogel and the G19-SnO2 hybrids after 100 cycles.


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Figure 9. SEM images of the graphene-SnO2 electrode (a) before and (b) after cycling.

To investigate the origin of the enhanced electrochemical performance of the graphene/SnO2 xerogel, EIS measurements were carried out and are shown in Figure 8. The equivalent circuit used to fit the EIS data is shown in Figure 8b, and the fitting parameters are summarized and shown in Table 1. Figure 8a and Table 1 both show that the electrical resistance (Re) of the G19-SnO2 xerogel electrode (51.8 ohm) is much lower than that of the SnO2 electrode (133.6 ohm). This increase in the electrical conductivity can be attributed to the high conductivity of graphene sheets. Similarly, the G19-SnO2 xerogel (2.3 ohm) exhibited a lower charge transfer resistance (Rct) than that of the SnO2 (5.4 ohm), indicating improved Li+ transference from electrolyte to the graphene/SnO2 xerogel electrode which consequently improves the electrochemical reaction kinetics. The Li+ diffusion coefficient can be calculated from the Warburg coefficient (W). It was found that the Li+ diffusion coefficient of the G19-SnO2 xerogel electrode (8.7E-08 cm2/s) was higher than that of the SnO2 electrode (3.1E-08 cm2/s), which is consistent with the more porous structure of the



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graphene/SnO2 xerogel and smaller SnO2 crystallite size. These values are comparable with other values reported in the literature about Li+ diffusion coefficients in SnO2.15 The EIS spectra of the coin cells were also measured after 100 cycles. The charge transfer resistance (Rct) increased, while the electrode resistance (Re) and Li+ diffusion coefficient decreased significantly. A huge decrease in the Re of SnO2 electrode was observed, possibly due to improved electrical contact with the current collector by activation during the first few cycles38 and the formation of conductive Sn nanoparticles during cycling. The increase in Li+ diffusion coefficient is also probably due to the formation of Sn nanoparticles during cycling.39-40 The increase in charge transfer resistance suggests the poor charge transfer behavior of the electrode reaction, which could result from the increased SEI layer thickness, the generated cracks within the SEI layer, and the agglomeration of the SnO2 and Sn nanoparticles. Compared with the SnO2, the graphene/SnO2 xerogel electrode suffered from less decay, indicating excellent structural integrity/stability. SEM images of the G19-SnO2 electrodes before and after cycling were taken and are shown in Figure 9. Although some aggregation was observed, the major structure of the graphene wrinkles was retained after cycling, indicating the benefit of using graphene to stabilize the structure of the G19-SnO2 hybrid. In short, the superior electrochemical performance of the graphene/SnO2 xerogel is attributed to the unique porous structure of the xerogel prepared by epoxide-initiated gelation method. The incorporation of graphene sheets provides fast electron transfer between the interfaces of the graphene nanosheets and the SnO2 and a short lithium ion diffusion path by changing the


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morphology of SnO2 from a dense, bulk morphology to a thin, nanocrystal layer over the graphene sheets. The porous structure of graphene/xerogel could effectively prevent the structural degradation and delamination of the electrode materials upon cycling. The stability may be further enhanced by stabilizing the SEI layer with electrolyte additives, i.e. vinylene carbonate, fluoroethylene carbonate, etc. It should also be noted that another advantage is that our graphene/SnO2 xerogel hybrids have a relatively low carbon content (about 19 wt.%). As shown in Table S1, many other previous works used large amount of carbon (up to 66 wt.%), which can significantly decrease the practical volumetric capacity of the cell due to low capacity and tapping density of carbonaceous materials.

Table 1. Fitting parameters of EIS results Re (ohm)

Rct (ohm)

Li+ diffusion coefficient (cm2/s)

SnO2

133.6 ± 4.6%

5.4 ± 7.1%

3.1E-08 ± 1.7%

SnO2 after 100 cycles

5.02 ± 6.4%

24.9 ± 2.1%

5.1E-07 ± 1.6%

G19-SnO2

51.7 ± 0.7%

2.3 ± 2.8%

8.7E-08 ± 1.4%

G19-SnO2 after 100 cycles

6.24 ± 3.4%

4.5 ± 2.9%

3.3E-06 ± 2.7%

4. Conclusions In summary, we report a simple epoxide-initiated gelation method to prepare graphene/SnO2 xerogel hybrids. The hybrids with different graphene loadings were prepared and studied as LIB anode materials. The SnO2 xerogel was found to be uniformly distributed within the graphene sheets due to the electrostatic interaction



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between hydrated Sn4+ complex and GO during the hydrolysis. Consequently, the xerogel hybrids showed large surface area and pore volume. When tested as LIB anodes, the graphene/SnO2 xerogel hybrid (19 wt.% graphene) delivered a specific capacity of 607 mAh/g at a current density of 1 A/g, as compared to only 315 mAh/g for pure SnO2. Even when charged/discharged for 300 cycles at 1A/g, the hybrid still maintained about 60% of this initial capacity. The excellent electrochemical performance results from the improved electronic conductive path, fast ion transport, and excellent stability of the graphene sheets. It is expected that this facile synthesis of the graphene/SnO2 /xerogel can be used in various applications including energy storage, gas sensing, and catalysis.

Acknowledgments We would like to acknowledge the Integrated Nanosystems Development Institute (INDI) for the use of their JEOL7800F Field Emission Scanning Electron Microscope, which was awarded through NSF grant MRI-1229514.

Supporting Information Available: XRD patterns of different graphene/SnO2 xerogel composites, SEM and TEM image of graphene, and specific capacity retention of graphene. This material is available free of charge via the Internet at http://pubs.acs.org.


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