Synthesis of Double-Shell SnO2@C Hollow Nanospheres as Sulfur

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Synthesis of double-shell SnO2@C hollow nanospheres as sulfur/sulfide cages for lithium-sulfur batteries Bokai Cao, De Li, Bo Hou, Yan Mo, Lihong Yin, and Yong Chen ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b09918 • Publication Date (Web): 27 Sep 2016 Downloaded from http://pubs.acs.org on September 29, 2016

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ACS Applied Materials & Interfaces

Synthesis of Double-Shell SnO2@C Hollow Nanospheres as Sulfur/Sulfide Cages for Lithium-Sulfur Batteries Bokai Cao, De Li*, Bo Hou, Yan Mo, Lihong Yin, Yong Chen*

State Key Laboratory of Marine Resource Utilization in South China Sea, Hainan Provincial Key Laboratory of Research on Utilization of Si-Zr-Ti Resources, Hainan University, 58 Renmin

Road,

Haikou

570228,

China.

Corresponding

author’s

email

address:

[email protected]; [email protected]

ABSTRACT: Double-shell SnO2@C hollow nanospheres were synthesized by a template method, and then the sulfur was loaded to form a cathode material of S/SnO2@C composite. In Li-S batteries, it delivered a high initial specific capacity of 1473.1 mAh/g at a current density of 200 mA/g, and the capacity retention was even up to 95.7% over 100 cycles at 3200 mA/g, i.e., a capacity fade rate of only 0.043% per cycle. These good electrochemical performances should be attributed to the SnO2@C hollow nanospheres. They can enhance the electronic conductivity by the outside carbon shell, and confine the lithium polysulfides by S-Sn-O and S-C chemical bonds to suppress the shuttle effect. Besides, the hollow nanospheres can readily accommodate the sulfur/sulfides to prevent the electrical/mechanical failure of the cathode, instead of their agglomeration on the external surface of SnO2@C.

KEYWORDS: Hollow nanospheres; Chemical adsorption; SnO2; Li-S batteries; Reaction kinetics

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1. INTRODUCTION Due to the rapidly increasing demand of rechargeable batteries for portable electronic devices and various electrical vehicles, besides their amount, it is of the great importance to improve the battery capacity and life, while the energy density and cycle life of current lithium-ion batteries (LIBs) remain insufficient for many applications.1,2 Over this decade, lithium-sulfur (Li-S) batteries have received much attention as one of the most promising next-generation rechargeable batteries, due to their high theoretical specific capacity (1675 mAh/g) and energy density (2600 Wh/kg).3,4 As a cathode material, the sulfur possesses the advantages of low cost, environmentally benign and naturally abundant,5,6 but there are still a few challenges for the practical application and commercialization of Li-S batteries, including the poor electrical conductivity of sulfur, the high solubility of intermediate polysulfide, and the large volume expansion during the discharge.7-9 The shuttle effect is a most severe problem, in which the soluble lithium polysulfides penetrate through the separator, diffuse to the lithium anode, convert into the insoluble Li2S or Li2S2, and deposit on the metal surface, leading to active material loss, low coulombic efficiency, poor cycle life and self-discharge.10-14 To address these problems, significant efforts have been made to design and prepare new matrix materials for immobilizing sulfur, including porous carbon,15-21 graphene22-27 and conductive polymers.28,29 As to the microstructure, the porous structure is effective in storing sulfur, trapping polysulfides and accommodating the large volumetric expansion,30-33 moreover, the particle morphology of cathode materials have been proved with better electrical connectivity.17 For instance, tunable porous spherical carbons (PSCs) and


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hierarchical porous graphene have been fabricated to improve the conductivity of the cathode and restrain the diffusion of the dissoluble polysulfides.17,34 Furthermore, by infusing sulfur into porous hollow spheres, as hollow graphene nanoshells35 and nitrogen-doped double-shelled hollow carbon spheres36, the sulfur-carbon composites show excellent cycle stability and rate performance. Recently, heteroatom doping and polar inorganics additives are investigated as two promising

methods

in

trapping

lithium

polysulfides.37-39

Remarkably,

the

oxygen-functionalized carbon, heteroatom-doped carbon,37,38 transition metal oxides39 were adopted to stabilize the lithium polysulfides via chemical binding and facilitate the electrochemical kinetics of the Li-S redox. For example, the N-doped aligned carbon nanotube/graphene sandwiches37 and B-doped porous carbon38 could exhibit greatly quite better cycle and rate performance in Li-S batteries. Besides carbon or polymer based counterparts, metal oxides such as TiO2, SnO2, SiO2 and Al2O3 have been also investigated as promising cathode frameworks,40-46 of which the hydrophilic metal-oxides (M-O) groups can combine with polysulfide anions and prevent the polysulfide from dissolution.40,44,45 e.g., the ultrafine TiO2 nanoparticles were used as anchoring points on carbon nanofibers to trap the polusulfides and thus the Li-S battery delivered a capacity retention of 74.2% after 500 cycles.39 Meanwhile, the poor electronic conductivity of metal oxides, could be overcome by the carbon coating.41,47 In this work, the double-shell SnO2@C hollow nanospheres (SnO2@C) were designed as an electrode matrix for stable and long life Li-S batteries. The SnO2 and carbon shells can not only improve the electrical conductivity of sulfur electrode, but also absorb the lithium



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polysulfides by chemical bonding to suppress the shuttle effect. As a result, the S/SnO2@C cathode shows excellent rate performance and superior cycle stability.

2. EXPERIMENTAL 2.1 Sample preparation Firstly, about 1 g SiO2 nanospheres prepared by Stober’s method48 were dispersed in a 100 ml solution of deionized water/ethanol (50 vol% water) under ultrasound for 30 min. Then 2 mmol SnCl2·2H2O, 9.0 g urea and 9.3 g poly(vinylpyrrolidone) (PVP) were sequentially added to the above solution under stirring. After the powder dissolved thoroughly, the solution was evaporated at 80 °C to coat SnO2 onto the SiO2 surface. In order to obtain the carbon coated SnO2 hollow nanospheres (SnO2@C), in a typical carbon coating synthesis, 240 mg SiO2/SnO2 composite nanospheres were dispersed in a mixture of 6.4 g glucose, 64 ml deionized water and 160 ml ethanol under ultrasound. Then the suspension was transferred to some 25 ml Teflon-lined stainless steel autoclaves and heated at 180 °C for 15 h. After washing with ethanol and drying, the product was carbonized at 600 °C for 2 h in argon atmosphere. The obtained black powder was then heat-treated at 300 °C for 1 h in the air atmosphere. Finally, The SiO2 cores were etched off in 1% HF aqueous solution. The amount of SnO2 in the SnO2@C hollow nanospheres is 19 wt% according to the quality change obtained from heating at 700 °C for 2 h in the atmosphere to remove the carbon coatings.


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The as-prepared SnO2@C nanospheres were mixed with sublimed sulfur in the proportion of 4 : 6 by milling sufficiently, follow by heating to 155 ºC held for 24 h in a Teflon-lined stainless steel autoclave, finally result in sulfur-loaded SnO2@C (S/SnO2@C). Then the cathode for lithium-sulfur batteries was produced by mixing S/SnO2@C composite with carbon black and polytetrafluoroethene (PTFE) with a weight ratio of 8:1:1. The obtained slurry was rolled into a film, punches into wafer with a diameter of 7 mm and pressed on Al foil, then dried at 55 ºC overnight in a vacuum oven. The areal sulfur loading of 1.0 mg/cm2 was obtained. The CR2016 coin cells were then assembled in an argon-filled glove box with S/SnO2@C composite as cathode and Lithium metal foil as anode. The electrolyte was 1 M lithium bis(trifluoromethane)sulfonimide (LiTFSI) and 0.1 M lithium nitrate (LiNO3, Alfa Aesar, 99.99%) dissolved into a mixture of 1,3-dioxolane (DOL, Sigma Aldrich) and 1,2-dimethoxyethane (DME, Sigma Aldrich) (DOL:DME, 1:1 volume ratio).49 The electrolyte used for each coin cell is around 18 µL. Additionally, 2 mmol/L Li2S6 solution was prepared by mixing 2.3 mg Li2S and 4.8mg sulfur in 25ml mixture electrolyte of 1,2-dimethoxyethane (DME) and 1,3-dioxalane (DOL) (1:1 volume ratio) and then heated at 80 °C under vigorous stirring in glove box for 18 h, followed by the absorption test of Li2S6 in the SnO2@C nanospheres.

2.2 Sample characterization The phase of SnO2@C nanospheres and S/SnO2@C composite were characterized by X-ray diffraction using Bruker D8 Advance (XRD, Germany) with Cu Kα radiation at 40 V and 40



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mA with a scanning rate of 2°/min from 10° to 80°. The surface morphology of the samples was surveyed by field-emission scanning electron microscopy using Hitachi S-4800 (FESEM, Japan). The hollow structure of SnO2@C nanospheres was characterized by transmission electron microscopy using JEOL 200CX (TEM, Japan). The sulfur content of the composite was measured through thermogravimetric analysis by using NETZSCH STA449C (TGA, Germany) at a heating rate of 10 K/min in nitrogen. The specific surface area and pore size analysis of the SnO2@C nanospheres were carried out by N2 adsorption method using JW-BK132F (China). The adsorption property of the SnO2@C was measured by the UV-vis spectra using SHIMADZU UV-1800 (UV-vis, Japan). The Fourier transform infrared spectroscopy analysis was performed on PerkinElmer Frontier (FTIR, America). The Raman spectrum was obtained from DXRxi Raman imaging Microscope (Thermo Scientific, America). The galvanostatic charge/discharge measurement was conducted to evaluate the cycling performance of the batteries over the potential range from 1.7 to 2.8 V (vs. Li/Li+) by using LAND CT2001A battery-testing instruments. The electrochemical impedance spectroscopy (EIS) was also carried out by the electrochemical workstation over the frequency range from 100 kHz to 10 mHz with the voltage amplitude of 5 mV. The current density and the specific capacity were all based on the weight of sulfur obtained from TGA.

3. RESULTS AND DISCUSSION The synthesis process of SnO2@C nanospheres is schematically illustrated in Figure 1. First, silica nanospheres were coated with a uniform SnO2 shell through the hydrolysis of


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SnCl2·2H2O in urea solution at 80 °C. Then, the core-shell SiO2@SnO2 nanospheres were coated with glucose-derived carbon-rich polysaccharide (GCP) through a simple hydrothermal process. It is well known that such GCP contains abundant hydroxyl groups,47 which bind favorably with polysulfide anions.40 Finally, the SiO2 cores were removed by 1% aqueous HF solution to produce SnO2@C nanospheres. The outside carbon-shell is significant because it not only protects the thin SnO2 shell during the preparation, but also enhances the electron conductivity in the electrochemical measurements.

Figure 1. Schematic illustration of the fabrication of hollow SnO2@C nanospheres.

The prepared SiO2 nanospheres with a diameter of 220 nm are well-dispersed with smooth surface as shown in Figure 2a. After coating SnO2 (Figure 2b), the change of surface roughness confirms the existence of SnO2. The core-shells SiO2@SnO2@C spheres from hydrothermal reactions were obtained by adsorbing glucose on SiO2@SnO2 surface and then carbonization. The difference of SEM images (Figure 2b and Figure 2c) indicates that a thin carbon shell was formed outside. By HF etching, the SiO2 cores were removed and the SnO2@C hollow nanospheres were left. Some SnO2@C nanospheres are broken into pieces, which just confirming the hollow structure. More detailed structure of the SnO2@C double-shells can be revealed by the TEM image (Figure 3a). It shows that the SnO2@C



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hollow nanospheres have an average diameter of 250 nm with uniform coating. The thickness of the SnO2 and carbon shell is about 15 nm, respectively.

Figure 2. SEM images of the SiO2 spheres (a), core-shell SiO2@SnO2 nanospheres (b), SiO2@SnO2@C structures (c) and double-shell SnO2@C hollow nanospheres (d)

The S/SnO2@C composite was prepared through the melting-infusion process. As shown in Figure 3b, there is no conspicuous morphological difference between the SnO2@C nanospheres (Figure 2d) and the S-SnO2@C composite (Figure 3b), and the TEM image (inset in Figure 3b) clearly shows sulfur is mainly distributed in the SnO2@C hollow structure.


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Figure 3. TEM image of SnO2@C double-shells (a) and SEM image of the S/SnO2@C (b) and its TEM image (inset).

The pore structures of SnO2@C nanospheres were characterized by N2 adsorption method. The type IV isotherm showed in Figure 4a indicates that both micropores and mesopores existed in SnO2@C. The specific surface area of SnO2@C hollow spheres was calculated to be 608.19 m2/g, corresponding to a hierarchical porous structure with a narrow size distribution near 0.64 and 2.5 nm, respectively. The large surface is beneficial to the sulfur diffusion into the SnO2@C hollow nanospheres. The mesoporous structure can provide the access for the electrolyte into the SnO2@C nanospheres. And the micropores can prevent the sulfur from dissolving in the organic electrolyte outside due to the strong adsorption.14,16 Additionally, the hollow spheres with small pore size (

Synthesis of Double-Shell SnO2@C Hollow Nanospheres as Sulfur

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