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S composite and application for lithium-sulfur battery cathode material
Accepted Manuscript Full Length Article Preparation of SnO2@rGO/CNTs/S composite and application for lithium-sulfur battery cathode material Qingqing Liu, Qi Jiang, Li Jiang, Junqi Peng, Yike Gao, Zhihong Duan, Xiaoying Lu PII: DOI: Reference:
S0169-4332(18)32157-3 https://doi.org/10.1016/j.apsusc.2018.08.038 APSUSC 40075
To appear in:
Applied Surface Science
Received Date: Revised Date: Accepted Date:
2 May 2018 16 July 2018 4 August 2018
Please cite this article as: Q. Liu, Q. Jiang, L. Jiang, J. Peng, Y. Gao, Z. Duan, X. Lu, Preparation of SnO2@rGO/ CNTs/S composite and application for lithium-sulfur battery cathode material, Applied Surface Science (2018), doi: https://doi.org/10.1016/j.apsusc.2018.08.038
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Preparation of SnO2@rGO/CNTs/S composite and application for lithium-sulfur battery cathode material Qingqing Liu, Qi Jiang*, Li Jiang, Junqi Peng, Yike Gao, Zhihong Duan and Xiaoying Lu Key Laboratory of Advanced Technologies of Materials (Ministry of Education of China), School of Materials Science and Engineering, Superconductivity and New Energy R&D Centre, Southwest Jiaotong University, Chengdu 610031, P. R. China
Abstract: In this paper, SnO2 was introduced in to suppress the "shuttle effect" of lithium-sulfur battery for its efficient adsorption for lithium polysulfides, and a three-dimensional conductive network constructed by reduced graphene oxide (rGO) and carbon nanotubes (CNTs) was used to improve the composite conductivity and mechanical properties. Thus, a SnO2@rGO/CNTs/S composite was prepared to use as the lithium-sulfur battery cathode material. The obtained samples were characterized by X-ray diffraction, scanning electron microscopy, energy dispersive X-ray spectroscopy, high-resolution transmission electron microscopy and thermogravimetric analysis. The electrochemical performance was characterized by cyclic voltammogram, constant current charge-discharge, rate performance, cycle life and electrochemical impedance spectroscopy after being assembled into lithium-sulfur battery. The results show that the obtained composite has a promising electrochemical performance: the initial discharge capacity is 1205.4 mAh g-1 at 0.1 C and there is a reversible capacity of 958.6 mAh g-1 after 50 cycles. Keywords: Lithium-sulfur battery; cathode material; SnO2 ; rGO; CNTs
1.
Introduction Lithium-sulfur (Li-S) battery has recently attracted considerable attention as a promising energy
storage device due to its low cost, environmental friendliness, high theoretical capacity (1675 mAh g-1) and energy density (2600 Wh kg-1)[1, 2], which is much higher than those of conventional lithium-ion batteries. However, the practical electrochemical performance of Li-S battery have been restricted by the low conductivity of sulfur and Li2S/Li2S2 (discharge final products), and the shuttle effect of the lithium polysulfides (LiPSs)
†
[3]
. To address these problems, carbon materials and metal
*Corresponding authors. E-mail address: [email protected] or [email protected] (Qi Jiang)
oxides are used to enhance the conductivity and prevent the shuttle effect, respectively carbon
[6]
, reduced graphene oxide (rGO)
[7]
and carbon nanotubes (CNTs)
[8]
[4, 5]
. Porous
were usually used to
increase the conductivity of sulfur and Li2S/Li2S2 during the charge-discharge process owing to their high electrical conductivity
[9]
. TiO2
[10]
, SiO2
[11]
and MnO2
[12]
were used to weaken the shuttle
effect of the LiPSs by forming strong chemical bonds with them [13]. However, carbon materials have poor weakening effect to the shuttle effect and metal oxides also have poor conductivity
[14]
. So in
order to integrate their merits, carbon materials and metal oxides are used together to enhance the Li-S battery electrochemical performance. For example, Seyyedin etc. composite showing a high capacity and long cycle life. Hwang etc.
[16]
[15]
reported a CuO@GO/S
prepared a good cycle life of
TiO2@CNTs/S. SnO2, as a chemical material, is currently used as an anode material in lithium ion batteries and shows a promising application in this field
[17, 18]
. Considering the possible adsorption to LiPSs, SnO2
has been tried to use in Li-S batteries in recent years [19-21]. But the obtained results are not satisfied, which is maybe the result of the SnO2 poor conductivity or pressed adsorption effect to LiPSs. So in order to research the contribution of SnO2 in Li-S batteries, we design and prepare four materials (S, SnO2/S, SnO2@rGO/S and SnO2@rGO/CNTs/S) by a melting-diffusion strategy in this paper. Their structure and electrochemical performance were researched including SnO2, rGO and CNTs. Amongst which, rGO and CNTs are not only used as the conductive agent, but also the stabilizing agent by forming a three-dimensional network structure. At the same time, rGO is also a template for SnO2. The results show that SnO2 can be used as the adsorbent to efficient adsorb the LiPSs to hinder or reduce the shuttle effect, but its conductivity is poor. rGO can be used as the SnO2 template to avoid its agglomeration and increase the SnO2 reaction sites for its large specific surface area and good conductivity. CNTs work as the conductive network of the composite with the help of rGO, which accelerates the electron and ions’ transfer and improves the utilization of active materials. The SnO2 @rGO/CNTs/S composite exhibits a promising electrochemical performance: the initial discharge capacity is 1205.4 mAh g -1 at 0.1 C (1 C =1675 mAh g-1), and there is a reversible capacity of 958.6 mAh g-1 after 50 cycles.
2.
Experimental The GO and CNTs were synthesized by our group with the modified Hummer’s method and 2
chemical vapor deposition. SnO2 @rGO precursor was prepared by SnCl4.5H2O and graphene oxide (GO) at 180 ℃ for 12 h by in situ hydrothermal method. The obtained precipitation was treated at 450 ℃ for 2 h under Argon condition, the obtained sample was SnO2 @rGO. SnO2 was obtained without adding GO as the similar operations. SnO2@rGO/CNTs sample was prepared via mixing SnO2 @rGO and CNTs powder by a planetary ball mill with a speed of 200 rpm for 2 h. The mass ratio of ball-to-powder was 30:1. SnO2 @rGO/CNTs/S composite was prepared with SnO2 @rGO/CNTs and S by a melting-diffusion treatment at 155 ℃ for 12 h in Argon atmosphere. SnO2/S and SnO2@rGO/S were prepared by the similar operations. The obtained samples were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), high-resolution transmission electron microscopy (HRTEM) and thermogravimetric analysis (TG, N2 atmosphere with 10 ℃ min-1 heating rate). The obtained samples were assembled into CR2032 button batteries with the lithium metal as the anode. 1.0 mol l-1 lithium bis-trifluor-omethanesulphonyphonylimide (LiTFSI, in 1, 3-dioxolane and 1, 2 - dimethoxyethane solution with 1:1 volume ratio) with 0.5 wt% LiNO3 additive was used as the electrolyte. The cathodes were fabricated by pressing the mixture of 80 wt% sample powders, 10 wt% conductive acetylene black and 10 wt% polyvinylidene fluoride onto the aluminum foil. A microporous separator (Celgard 2400) was sandwiched between the anode and cathode. The Li-S batteries were assembled in an argon-filled glove box without water and oxygen. The electrochemical performance was characterized by automatic batteries tester and CHI660 electrochemical workstation at 25 ℃, including cyclic voltammogram (CV) curves with a scan rate of 0.1 mV s-1, initial charge - discharge curves, rate performance curves, cycle life curves at 0.1 C and electrochemical impedance spectroscopy (EIS) curves from 100 kHz to 0.01 Hz with the amplitude of 5 mV.
3.
Results and discussion The schematic diagram of the composite preparation process is shown in Fig. 1a. GO is used as
the template to grow SnO2. During the hydrothermal process, SnO2 is obtained and GO is reduced to rGO. CNTs are added by ball milling to form a three-dimensional conductive network with rGO. S is 3
introduced by a melting-diffusion method at Argon atmosphere. The sulfur contents in SnO2 @rGO/CNTs/S, SnO2/S and SnO2 @rGO/S are about 65.5 %, 68.9% and 63.2%, respectively, which is shown by the TG data in Fig. 1b. Fig. 1c is the XRD patterns of the samples. As shown in Fig. 1c, the S obtained by the melting-diffusion treatment belongs to orthorhombic system, which also appears on the composites’ XRD patterns. The characteristic peak (10.99°) of GO is disappeared in the composites, indicating the GO is reduced to rGO during the hydrothermal treatment process. The three main peaks of the SnO2 @rGO sample are listed at 26.6°, 33.8°and 51.8°, indicating the obtained SnO2 belongs to tetragonal system referred to JCDPS NO. 01-077-0449
[22]
. Calculated by the Scherrer equation
[23]
,
the grain size of SnO2 is about 25 nm. So, the SnO2@rGO/CNTs/S has been prepared successfully. Fig. 2 is the SEM and HRTEM images of the samples. As shown in Fig. 2c, the SnO2 @rGO material exhibits an obvious regular layered structure, which benefits from the GO (as the template) structure. The GO layered structure can be clearly seen in Fig. 2a. The SnO2 particles with a diameter of about 20-35 nm are uniformly dispersed on the rGO surface in Fig. 2d, which is consistent with the result of Fig. 1c. After adding S, the lamellae becomes thicker and the particles also get larger (Fig. 2e). With the addition of CNTs (marked as the black arrow in Fig. 2f), a porous three-dimensional conductive network is formed. The CNTs become thicker in Fig. 2f and the surface of SnO2 @rGO/CNTs/S becomes smoother than that of SnO2@rGO, indicating that sulfur has been uniformly distributed in the composite. Fig. 2g shows the EDS element mapping images of S, Sn, O, and C in the left selected regions. They are well overlapped, indicating that S, Sn, O, and C are all homogeneous distribution within the SnO2@rGO/CNTs host. As shown from the HRTEM in Fig. 2h and i (Fig. 2i is an enlarged picture of the red box area in Fig. 2h), tubular CNTs, sheet-like rGO and granular SnO2 can be clearly seen. The network structure composed of rGO and CNTs also can be clearly seen in Fig. 2h. So the SnO2@rGO/CNTs/S composite has been successfully prepared. Comparing Fig. 2h and i, it can be sure that the sample with the lattice spacing of 0.33 nm is SnO2 and the 0.33 nm is its (110) crystal plane spacing which is consistent with the discussion from the XRD data.
4
[14]
,
110
b
100
SnO2/S SnO2@rGO/S SnO2@rGO/CNTs/S
Weight percentage / %
90 80
63.2 wt%
70
65.5 wt% 68.9%
60 50 40 30 20 10 0 0
100
200
300
400
500
600
Temperature / ℃
c
Intensity (a.u.)
SnO2@rGO/CNTs/S SnO2@rGO/S SnO2@rGO CNTs GO pure S SnO2 JCDPS 01-077-0449 10
20
30
40
50
60
70
80
2 theta
Fig. 1 Schematic diagram of the composite preparation process (a), TG curves (b) and XRD patterns (c) of the samples
5
Fig. 2 SEM images of the samples (a, GO; b, CNTs; c-d, SnO2@rGO; e, SnO2@rGO/S; f SnO2 @rGO/CNTs/S; g, EDS elemental mapping images of Sn, O, S and C in the left selected regions) and HRTEM images (h, i) of SnO2 @rGO/CNTs/S. a
2.5
S SnO2/S SnO2@rGO/S SnO2@rGO/CNTs/S
2.8 +
Potential (V vs.Li/Li )
1.5
Current (mA)
b
3.0 S SnO2/S SnO2@rGO/S SnO2@rGO/CNTs/S
2.0
1.0 0.5 0.0 -0.5
2.6
2.4
2.2
2.0
-1.0
1.8 1.6
1.8
2.0
2.2
2.4
2.6
2.8
3.0
0
200
c
0.1 C 0.5 C
1000
600
800
1000
1200
Capacity (mAh.g )
S SnO2/S SnO2@rGO/S SnO2@rGO/CNTs/S
0.2 C
d
1200
0.1 C
1000 -1
Capacity (mAh.g )
1C
-1
Capacity (mAh.g )
1200
400
-1
+
Potential (V vs.Li/Li )
2C 800
3C 600
400
800
600
400 S SnO2/S SnO2@rGO/S SnO2@rGO/CNTs/S
200
200
0
0 0
5
10
15
20
25
30
35
0
Cycle number
10
20
30
40
50
Cycle number
Fig. 3 Electrochemical performance of the Li-S batteries based the samples (a, CV curves; b, initial 6
charge - discharge curves; c, rate performance curves; d, cycle life curves).
Fig. 3 shows the electrochemical performance of the Li-S batteries based the samples. As shown in Fig. 3a, the redox peak currents gradually increase with gradually adding SnO2, rGO and CNTs, indicating better electrochemical performance. Moreover, they are the typical Li-S battery CV curves: two reduction peaks and one oxidation peak. The reduction peak around 2.2 V is attributed to the reduction of the S8 ring to S82-. Another reduction peak around 2.0 V is associated with the reduction of the soluble high-order polysulfides (Li2Sn, 4≤ n ≤8) and insoluble low-order lithium sulfides (such as Li2S2, Li2S). The oxidization peak around 2.5 V is assigned to the conversion of the low-order and high-order polysulfides into S8
[24]
. With the addition of SnO2, rGO and CNTs, the peak differences
between the oxidation peak and the reduction peak are getting smaller and smaller, indicating the reversibility is getting better and better
[25]
. As shown in Fig. 3b, there are obvious two discharge
platforms and one charge platform, which is consistent with CV results in Fig. 3a. Moreover, the charge - discharge platforms are lengthening with adding SnO2, rGO and CNTs, indicating the enhancement of electrochemical capacity. Rate performance curves in Fig. 3c show that with the addition of SnO2, rGO, and CNTs, the initial discharge capacities increase gradually, which are 591.1, 735.7, 859.1 and 1205.4 mAh g-1 at 0.1C, respectively. The SnO2 @rGO/CNTs/S sample has the highest initial discharge capacity, which is about twice than those of the SnO2/S and S samples. At high rate (3C), the SnO2@rGO/CNTs/S composite also has the highest capacity. Moreover, the capacities of the SnO 2@rGO/CNTs/S are always the highest at all rates, indicating that the adding of CNTs is important to the composite electrochemical capacity. The cycle life curves in Fig. 3d show that the discharge capacity of the SnO2 @rGO/CNTs/S falls to 958.6 mAh g-1 after 50 cycles, and its average capacity decay rate is about 0.4 %. The discharge capacities of the other three samples are 671.6, 523.6 and 175.2 mAh.g-1 after 50 cycles, respectively. Thus, the introduction of SnO 2, rGO and CNTs at the same time can greatly enhance the electrochemical performance of the Li-S battery, indicating that there is a synergistic effect between the SnO2, rGO and CNTs. SnO2 can be used as the adsorbent to efficient adsorb the LiPSs to hinder or reduce the shuttle effect, which is benefit to improve the cycle performance of the batteries
[26]
. rGO can be used as the SnO2 template to avoid its agglomeration
and increase the SnO2 reaction site for its large specific surface area and good conductivity. CNTs 7
work as the conductive network of the composite with the help of rGO, which accelerates the electron and ions’ transfer and improves the utilization of active materials [27]. Fig. 4 shows the EIS curves (a) and Z'-ω-1/2 curves (b) of the samples. As shown in Fig. 4a, the EIS curves are all composed of one semicircle at high frequency region and one inclined line at low frequency region. The inset in Fig. 4a is the equivalent circuit diagram, which is used to match the data. The intersection of the EIS curve and the real axis at high frequency region stands for the system ohmic resistance (Rs). The semicircle is assigned to the charge transfer resistance (Rct) on the solid / electrolyte interface. The inclined line is ascribed to the Warburg impedance of Li+ diffusion in bulk electrode. The Warburg factor (σω) can be calculated by the slopes of the plots in Fig. 4b. Moreover, the Li+ ions diffusion coefficient (DLi+) of different samples can be obtained according to equation DLi+=R2T2/2A2F4n4C2σω2
[28, 29]
. The related data is listed in Table 1. As shown in Table 1,
the Rs data of the samples increases slightly from 3.7 to 4.4, 5.8 and 6.6 Ω after adding the SnO2, rGO and CNTs. However, the Rct data decreases sharply from 105.8 to 80.1, 49.4 and 38.7 Ω after adding the SnO2, rGO and CNTs. All these indicate that in spite of a little increase in Rs, the adding of SnO2, rGO and CNTs can effectively reduce the Rct of the composites, which is benefit for the migration of Li+. From the data of the DLi+ of the samples, it can be seen that the DLi+ data of the samples increases from 1.2×10-15 to 2.6×10-15, 3.8×10-15 and 8.6×10-15 after adding the SnO2, rGO and CNTs. So, the obtained SnO2 @rGO/CNTs/S composite has the least the Rct data and the largest DLi+ data in the obtained samples, indicating the best electrochemical energy performance, which is consistent with the previous electrochemical test results in Fig. 3.
a
140
b
S SnO2/S SnO2@rGO/S SnO2@rGO/CNTs/S
100 120 80
Z'
Z '' / Ω
100 60 80 40 60 S SnO2/S SnO2@rGO/S SnO2@rGO/CNTs/S
20
40
0 0
50
100
150
200
0.10
Z'/Ω
0.12
0.14
0.16
0.18
0.20
0.22
0.24
0.26
0.28
-1/2
W
Fig. 4 EIS curves (a, the inset is the equivalent circuit diagram) and Z'-ω-1/2 curves (b) of the samples. 8
Table 1 EIS data of the samples. Sample
Rs(Ω)
Rct(Ω)
σω
DLi+(cm2 s-1×10-15)
S
3.7
105.8
71.9
1.2
SnO2/S
4.4
80.1
48.2
2.6
SnO2@rGO/S
5.8
49.4
40.0
3.8
SnO2@rGO/CNTs/S
6.6
38.7
26.6
8.6
4.
Conclusions A SnO2@rGO/CNTs/S composite is successfully prepared by the experimental methods and
routes mentioned above and used as the Li-S battery cathode material. The composite has shown good electrochemical performance: an initial discharge capacity of 1205.4 mAh g-1 and a reversible capacity of 958.6 mAh g-1 after 50 cycles at 0.1 C. The introduction of SnO2, rGO and CNTs can greatly enhance the electrochemical performance of the Li-S battery. There is a synergistic effect between the three materials: SnO2 is used as the adsorbent to efficient adsorb the LiPSs to hinder or reduce the shuttle effect, but its conductivity is poor. rGO is used as the SnO2 template to avoid its agglomeration and increase the SnO2 reaction sites for its large specific surface area and good conductivity. CNTs work as the conductive network of the composite with the help of rGO, which accelerates the electron and ions’ transfer and improves the utilization of active materials.
Acknowledgments The work was supported by the National Natural Science Foundation of China (50907056, 51602266), Sichuan Key Research and Development Program (2017GZ0109) and Sichuan Science and Technology Support Projects (2016GZ0273, 2016GZ0275).
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Highlights: SnO2 @rGO/CNTs/S composite is prepared and used as Li-S battery cathode material. SnO2 works as the adsorbent to weaken the shuttle effect of lithium polysulfides. CNTs and rGO work as the conductive network to enhance the Li-S battery electrochemical performance.
13
S0169-4332(18)32157-3 https://doi.org/10.1016/j.apsusc.2018.08.038 APSUSC 40075
To appear in:
Applied Surface Science
Received Date: Revised Date: Accepted Date:
2 May 2018 16 July 2018 4 August 2018
Please cite this article as: Q. Liu, Q. Jiang, L. Jiang, J. Peng, Y. Gao, Z. Duan, X. Lu, Preparation of SnO2@rGO/ CNTs/S composite and application for lithium-sulfur battery cathode material, Applied Surface Science (2018), doi: https://doi.org/10.1016/j.apsusc.2018.08.038
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Preparation of SnO2@rGO/CNTs/S composite and application for lithium-sulfur battery cathode material Qingqing Liu, Qi Jiang*, Li Jiang, Junqi Peng, Yike Gao, Zhihong Duan and Xiaoying Lu Key Laboratory of Advanced Technologies of Materials (Ministry of Education of China), School of Materials Science and Engineering, Superconductivity and New Energy R&D Centre, Southwest Jiaotong University, Chengdu 610031, P. R. China
Abstract: In this paper, SnO2 was introduced in to suppress the "shuttle effect" of lithium-sulfur battery for its efficient adsorption for lithium polysulfides, and a three-dimensional conductive network constructed by reduced graphene oxide (rGO) and carbon nanotubes (CNTs) was used to improve the composite conductivity and mechanical properties. Thus, a SnO2@rGO/CNTs/S composite was prepared to use as the lithium-sulfur battery cathode material. The obtained samples were characterized by X-ray diffraction, scanning electron microscopy, energy dispersive X-ray spectroscopy, high-resolution transmission electron microscopy and thermogravimetric analysis. The electrochemical performance was characterized by cyclic voltammogram, constant current charge-discharge, rate performance, cycle life and electrochemical impedance spectroscopy after being assembled into lithium-sulfur battery. The results show that the obtained composite has a promising electrochemical performance: the initial discharge capacity is 1205.4 mAh g-1 at 0.1 C and there is a reversible capacity of 958.6 mAh g-1 after 50 cycles. Keywords: Lithium-sulfur battery; cathode material; SnO2 ; rGO; CNTs
1.
Introduction Lithium-sulfur (Li-S) battery has recently attracted considerable attention as a promising energy
storage device due to its low cost, environmental friendliness, high theoretical capacity (1675 mAh g-1) and energy density (2600 Wh kg-1)[1, 2], which is much higher than those of conventional lithium-ion batteries. However, the practical electrochemical performance of Li-S battery have been restricted by the low conductivity of sulfur and Li2S/Li2S2 (discharge final products), and the shuttle effect of the lithium polysulfides (LiPSs)
†
[3]
. To address these problems, carbon materials and metal
*Corresponding authors. E-mail address: [email protected] or [email protected] (Qi Jiang)
oxides are used to enhance the conductivity and prevent the shuttle effect, respectively carbon
[6]
, reduced graphene oxide (rGO)
[7]
and carbon nanotubes (CNTs)
[8]
[4, 5]
. Porous
were usually used to
increase the conductivity of sulfur and Li2S/Li2S2 during the charge-discharge process owing to their high electrical conductivity
[9]
. TiO2
[10]
, SiO2
[11]
and MnO2
[12]
were used to weaken the shuttle
effect of the LiPSs by forming strong chemical bonds with them [13]. However, carbon materials have poor weakening effect to the shuttle effect and metal oxides also have poor conductivity
[14]
. So in
order to integrate their merits, carbon materials and metal oxides are used together to enhance the Li-S battery electrochemical performance. For example, Seyyedin etc. composite showing a high capacity and long cycle life. Hwang etc.
[16]
[15]
reported a CuO@GO/S
prepared a good cycle life of
TiO2@CNTs/S. SnO2, as a chemical material, is currently used as an anode material in lithium ion batteries and shows a promising application in this field
[17, 18]
. Considering the possible adsorption to LiPSs, SnO2
has been tried to use in Li-S batteries in recent years [19-21]. But the obtained results are not satisfied, which is maybe the result of the SnO2 poor conductivity or pressed adsorption effect to LiPSs. So in order to research the contribution of SnO2 in Li-S batteries, we design and prepare four materials (S, SnO2/S, SnO2@rGO/S and SnO2@rGO/CNTs/S) by a melting-diffusion strategy in this paper. Their structure and electrochemical performance were researched including SnO2, rGO and CNTs. Amongst which, rGO and CNTs are not only used as the conductive agent, but also the stabilizing agent by forming a three-dimensional network structure. At the same time, rGO is also a template for SnO2. The results show that SnO2 can be used as the adsorbent to efficient adsorb the LiPSs to hinder or reduce the shuttle effect, but its conductivity is poor. rGO can be used as the SnO2 template to avoid its agglomeration and increase the SnO2 reaction sites for its large specific surface area and good conductivity. CNTs work as the conductive network of the composite with the help of rGO, which accelerates the electron and ions’ transfer and improves the utilization of active materials. The SnO2 @rGO/CNTs/S composite exhibits a promising electrochemical performance: the initial discharge capacity is 1205.4 mAh g -1 at 0.1 C (1 C =1675 mAh g-1), and there is a reversible capacity of 958.6 mAh g-1 after 50 cycles.
2.
Experimental The GO and CNTs were synthesized by our group with the modified Hummer’s method and 2
chemical vapor deposition. SnO2 @rGO precursor was prepared by SnCl4.5H2O and graphene oxide (GO) at 180 ℃ for 12 h by in situ hydrothermal method. The obtained precipitation was treated at 450 ℃ for 2 h under Argon condition, the obtained sample was SnO2 @rGO. SnO2 was obtained without adding GO as the similar operations. SnO2@rGO/CNTs sample was prepared via mixing SnO2 @rGO and CNTs powder by a planetary ball mill with a speed of 200 rpm for 2 h. The mass ratio of ball-to-powder was 30:1. SnO2 @rGO/CNTs/S composite was prepared with SnO2 @rGO/CNTs and S by a melting-diffusion treatment at 155 ℃ for 12 h in Argon atmosphere. SnO2/S and SnO2@rGO/S were prepared by the similar operations. The obtained samples were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), high-resolution transmission electron microscopy (HRTEM) and thermogravimetric analysis (TG, N2 atmosphere with 10 ℃ min-1 heating rate). The obtained samples were assembled into CR2032 button batteries with the lithium metal as the anode. 1.0 mol l-1 lithium bis-trifluor-omethanesulphonyphonylimide (LiTFSI, in 1, 3-dioxolane and 1, 2 - dimethoxyethane solution with 1:1 volume ratio) with 0.5 wt% LiNO3 additive was used as the electrolyte. The cathodes were fabricated by pressing the mixture of 80 wt% sample powders, 10 wt% conductive acetylene black and 10 wt% polyvinylidene fluoride onto the aluminum foil. A microporous separator (Celgard 2400) was sandwiched between the anode and cathode. The Li-S batteries were assembled in an argon-filled glove box without water and oxygen. The electrochemical performance was characterized by automatic batteries tester and CHI660 electrochemical workstation at 25 ℃, including cyclic voltammogram (CV) curves with a scan rate of 0.1 mV s-1, initial charge - discharge curves, rate performance curves, cycle life curves at 0.1 C and electrochemical impedance spectroscopy (EIS) curves from 100 kHz to 0.01 Hz with the amplitude of 5 mV.
3.
Results and discussion The schematic diagram of the composite preparation process is shown in Fig. 1a. GO is used as
the template to grow SnO2. During the hydrothermal process, SnO2 is obtained and GO is reduced to rGO. CNTs are added by ball milling to form a three-dimensional conductive network with rGO. S is 3
introduced by a melting-diffusion method at Argon atmosphere. The sulfur contents in SnO2 @rGO/CNTs/S, SnO2/S and SnO2 @rGO/S are about 65.5 %, 68.9% and 63.2%, respectively, which is shown by the TG data in Fig. 1b. Fig. 1c is the XRD patterns of the samples. As shown in Fig. 1c, the S obtained by the melting-diffusion treatment belongs to orthorhombic system, which also appears on the composites’ XRD patterns. The characteristic peak (10.99°) of GO is disappeared in the composites, indicating the GO is reduced to rGO during the hydrothermal treatment process. The three main peaks of the SnO2 @rGO sample are listed at 26.6°, 33.8°and 51.8°, indicating the obtained SnO2 belongs to tetragonal system referred to JCDPS NO. 01-077-0449
[22]
. Calculated by the Scherrer equation
[23]
,
the grain size of SnO2 is about 25 nm. So, the SnO2@rGO/CNTs/S has been prepared successfully. Fig. 2 is the SEM and HRTEM images of the samples. As shown in Fig. 2c, the SnO2 @rGO material exhibits an obvious regular layered structure, which benefits from the GO (as the template) structure. The GO layered structure can be clearly seen in Fig. 2a. The SnO2 particles with a diameter of about 20-35 nm are uniformly dispersed on the rGO surface in Fig. 2d, which is consistent with the result of Fig. 1c. After adding S, the lamellae becomes thicker and the particles also get larger (Fig. 2e). With the addition of CNTs (marked as the black arrow in Fig. 2f), a porous three-dimensional conductive network is formed. The CNTs become thicker in Fig. 2f and the surface of SnO2 @rGO/CNTs/S becomes smoother than that of SnO2@rGO, indicating that sulfur has been uniformly distributed in the composite. Fig. 2g shows the EDS element mapping images of S, Sn, O, and C in the left selected regions. They are well overlapped, indicating that S, Sn, O, and C are all homogeneous distribution within the SnO2@rGO/CNTs host. As shown from the HRTEM in Fig. 2h and i (Fig. 2i is an enlarged picture of the red box area in Fig. 2h), tubular CNTs, sheet-like rGO and granular SnO2 can be clearly seen. The network structure composed of rGO and CNTs also can be clearly seen in Fig. 2h. So the SnO2@rGO/CNTs/S composite has been successfully prepared. Comparing Fig. 2h and i, it can be sure that the sample with the lattice spacing of 0.33 nm is SnO2 and the 0.33 nm is its (110) crystal plane spacing which is consistent with the discussion from the XRD data.
4
[14]
,
110
b
100
SnO2/S SnO2@rGO/S SnO2@rGO/CNTs/S
Weight percentage / %
90 80
63.2 wt%
70
65.5 wt% 68.9%
60 50 40 30 20 10 0 0
100
200
300
400
500
600
Temperature / ℃
c
Intensity (a.u.)
SnO2@rGO/CNTs/S SnO2@rGO/S SnO2@rGO CNTs GO pure S SnO2 JCDPS 01-077-0449 10
20
30
40
50
60
70
80
2 theta
Fig. 1 Schematic diagram of the composite preparation process (a), TG curves (b) and XRD patterns (c) of the samples
5
Fig. 2 SEM images of the samples (a, GO; b, CNTs; c-d, SnO2@rGO; e, SnO2@rGO/S; f SnO2 @rGO/CNTs/S; g, EDS elemental mapping images of Sn, O, S and C in the left selected regions) and HRTEM images (h, i) of SnO2 @rGO/CNTs/S. a
2.5
S SnO2/S SnO2@rGO/S SnO2@rGO/CNTs/S
2.8 +
Potential (V vs.Li/Li )
1.5
Current (mA)
b
3.0 S SnO2/S SnO2@rGO/S SnO2@rGO/CNTs/S
2.0
1.0 0.5 0.0 -0.5
2.6
2.4
2.2
2.0
-1.0
1.8 1.6
1.8
2.0
2.2
2.4
2.6
2.8
3.0
0
200
c
0.1 C 0.5 C
1000
600
800
1000
1200
Capacity (mAh.g )
S SnO2/S SnO2@rGO/S SnO2@rGO/CNTs/S
0.2 C
d
1200
0.1 C
1000 -1
Capacity (mAh.g )
1C
-1
Capacity (mAh.g )
1200
400
-1
+
Potential (V vs.Li/Li )
2C 800
3C 600
400
800
600
400 S SnO2/S SnO2@rGO/S SnO2@rGO/CNTs/S
200
200
0
0 0
5
10
15
20
25
30
35
0
Cycle number
10
20
30
40
50
Cycle number
Fig. 3 Electrochemical performance of the Li-S batteries based the samples (a, CV curves; b, initial 6
charge - discharge curves; c, rate performance curves; d, cycle life curves).
Fig. 3 shows the electrochemical performance of the Li-S batteries based the samples. As shown in Fig. 3a, the redox peak currents gradually increase with gradually adding SnO2, rGO and CNTs, indicating better electrochemical performance. Moreover, they are the typical Li-S battery CV curves: two reduction peaks and one oxidation peak. The reduction peak around 2.2 V is attributed to the reduction of the S8 ring to S82-. Another reduction peak around 2.0 V is associated with the reduction of the soluble high-order polysulfides (Li2Sn, 4≤ n ≤8) and insoluble low-order lithium sulfides (such as Li2S2, Li2S). The oxidization peak around 2.5 V is assigned to the conversion of the low-order and high-order polysulfides into S8
[24]
. With the addition of SnO2, rGO and CNTs, the peak differences
between the oxidation peak and the reduction peak are getting smaller and smaller, indicating the reversibility is getting better and better
[25]
. As shown in Fig. 3b, there are obvious two discharge
platforms and one charge platform, which is consistent with CV results in Fig. 3a. Moreover, the charge - discharge platforms are lengthening with adding SnO2, rGO and CNTs, indicating the enhancement of electrochemical capacity. Rate performance curves in Fig. 3c show that with the addition of SnO2, rGO, and CNTs, the initial discharge capacities increase gradually, which are 591.1, 735.7, 859.1 and 1205.4 mAh g-1 at 0.1C, respectively. The SnO2 @rGO/CNTs/S sample has the highest initial discharge capacity, which is about twice than those of the SnO2/S and S samples. At high rate (3C), the SnO2@rGO/CNTs/S composite also has the highest capacity. Moreover, the capacities of the SnO 2@rGO/CNTs/S are always the highest at all rates, indicating that the adding of CNTs is important to the composite electrochemical capacity. The cycle life curves in Fig. 3d show that the discharge capacity of the SnO2 @rGO/CNTs/S falls to 958.6 mAh g-1 after 50 cycles, and its average capacity decay rate is about 0.4 %. The discharge capacities of the other three samples are 671.6, 523.6 and 175.2 mAh.g-1 after 50 cycles, respectively. Thus, the introduction of SnO 2, rGO and CNTs at the same time can greatly enhance the electrochemical performance of the Li-S battery, indicating that there is a synergistic effect between the SnO2, rGO and CNTs. SnO2 can be used as the adsorbent to efficient adsorb the LiPSs to hinder or reduce the shuttle effect, which is benefit to improve the cycle performance of the batteries
[26]
. rGO can be used as the SnO2 template to avoid its agglomeration
and increase the SnO2 reaction site for its large specific surface area and good conductivity. CNTs 7
work as the conductive network of the composite with the help of rGO, which accelerates the electron and ions’ transfer and improves the utilization of active materials [27]. Fig. 4 shows the EIS curves (a) and Z'-ω-1/2 curves (b) of the samples. As shown in Fig. 4a, the EIS curves are all composed of one semicircle at high frequency region and one inclined line at low frequency region. The inset in Fig. 4a is the equivalent circuit diagram, which is used to match the data. The intersection of the EIS curve and the real axis at high frequency region stands for the system ohmic resistance (Rs). The semicircle is assigned to the charge transfer resistance (Rct) on the solid / electrolyte interface. The inclined line is ascribed to the Warburg impedance of Li+ diffusion in bulk electrode. The Warburg factor (σω) can be calculated by the slopes of the plots in Fig. 4b. Moreover, the Li+ ions diffusion coefficient (DLi+) of different samples can be obtained according to equation DLi+=R2T2/2A2F4n4C2σω2
[28, 29]
. The related data is listed in Table 1. As shown in Table 1,
the Rs data of the samples increases slightly from 3.7 to 4.4, 5.8 and 6.6 Ω after adding the SnO2, rGO and CNTs. However, the Rct data decreases sharply from 105.8 to 80.1, 49.4 and 38.7 Ω after adding the SnO2, rGO and CNTs. All these indicate that in spite of a little increase in Rs, the adding of SnO2, rGO and CNTs can effectively reduce the Rct of the composites, which is benefit for the migration of Li+. From the data of the DLi+ of the samples, it can be seen that the DLi+ data of the samples increases from 1.2×10-15 to 2.6×10-15, 3.8×10-15 and 8.6×10-15 after adding the SnO2, rGO and CNTs. So, the obtained SnO2 @rGO/CNTs/S composite has the least the Rct data and the largest DLi+ data in the obtained samples, indicating the best electrochemical energy performance, which is consistent with the previous electrochemical test results in Fig. 3.
a
140
b
S SnO2/S SnO2@rGO/S SnO2@rGO/CNTs/S
100 120 80
Z'
Z '' / Ω
100 60 80 40 60 S SnO2/S SnO2@rGO/S SnO2@rGO/CNTs/S
20
40
0 0
50
100
150
200
0.10
Z'/Ω
0.12
0.14
0.16
0.18
0.20
0.22
0.24
0.26
0.28
-1/2
W
Fig. 4 EIS curves (a, the inset is the equivalent circuit diagram) and Z'-ω-1/2 curves (b) of the samples. 8
Table 1 EIS data of the samples. Sample
Rs(Ω)
Rct(Ω)
σω
DLi+(cm2 s-1×10-15)
S
3.7
105.8
71.9
1.2
SnO2/S
4.4
80.1
48.2
2.6
SnO2@rGO/S
5.8
49.4
40.0
3.8
SnO2@rGO/CNTs/S
6.6
38.7
26.6
8.6
4.
Conclusions A SnO2@rGO/CNTs/S composite is successfully prepared by the experimental methods and
routes mentioned above and used as the Li-S battery cathode material. The composite has shown good electrochemical performance: an initial discharge capacity of 1205.4 mAh g-1 and a reversible capacity of 958.6 mAh g-1 after 50 cycles at 0.1 C. The introduction of SnO2, rGO and CNTs can greatly enhance the electrochemical performance of the Li-S battery. There is a synergistic effect between the three materials: SnO2 is used as the adsorbent to efficient adsorb the LiPSs to hinder or reduce the shuttle effect, but its conductivity is poor. rGO is used as the SnO2 template to avoid its agglomeration and increase the SnO2 reaction sites for its large specific surface area and good conductivity. CNTs work as the conductive network of the composite with the help of rGO, which accelerates the electron and ions’ transfer and improves the utilization of active materials.
Acknowledgments The work was supported by the National Natural Science Foundation of China (50907056, 51602266), Sichuan Key Research and Development Program (2017GZ0109) and Sichuan Science and Technology Support Projects (2016GZ0273, 2016GZ0275).
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Highlights: SnO2 @rGO/CNTs/S composite is prepared and used as Li-S battery cathode material. SnO2 works as the adsorbent to weaken the shuttle effect of lithium polysulfides. CNTs and rGO work as the conductive network to enhance the Li-S battery electrochemical performance.
13