C hybrid hollow spheres for superior lithium-sulfur batteries

C hybrid hollow spheres for superior lithium-sulfur batteries

Materials Chemistry and Physics 239 (2020) 122070 Contents lists available at ScienceDirect Materials Chemistry and Physics journal homepage: www.el...

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Materials Chemistry and Physics 239 (2020) 122070

Contents lists available at ScienceDirect

Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys

Materials science communication

Solvent-free template synthesis of SnO2/C hybrid hollow spheres for superior lithium-sulfur batteries Qi Xiao a, *, Kun Wang a, Xinxin Wang a, Suping Huang b, Nana Cai a, Neng Li a a b

School of Minerals Processing and Bioengineering, Central South University, Changsha, 410083, China State Key Lab of Powder Metallurgy, Central South University, Changsha, 410083, China



� A novel solvent-free template (SFT) strategy was developed towards SnO2/C hybrid hollow spheres. � SnO2/C hybrid hollow spheres were synthesized via a facile one-step low temperature diffusion with postannealing in air. � There is a synergistic action between SnO2 and carbon. � Sulfur cathode containing SnO2/C hybrid hollow spheres exhibited excel­ lent cycle/rate performances.



Keywords: Hollow spheres SnO2/C hybrid composites Solvent-free template (SFT) method Lithium-sulfur batteries

Hollow structures could be promising for various applications such as lithium-sulfur batteries (LSBs). The existing two types of synthesis routes to fabricate hollow structures include vapor processing route and solution processing route. Herein, we report a new and facile solvent-free template (SFT) synthetic strategy for the synthesis of SnO2/C hybrid hollow spheres by mixing and heating solid raw materials, in which 3-aminophenol/ formaldehyde (APF) resin spheres and Tin salt have been used as hard template and precursor, respectively. During heating, Tin salt is melted and the ions can diffuse into the insides of APF spheres easily, leading to effective loading ions, thus to successfully shape replication of template. Sulfur cathode containing the asprepared porous SnO2/C hybrid hollow spheres exhibited superior electrochemical performances for lithiumsulfur batteries. The present SFT strategy will pave a new avenue for the synthesis of mixed carbon/metal ox­ ides hollow spheres and their promising application in different areas.

1. Introduction In recent years, hollow structures have attracted great attention due to their excellent features of large surface area, low density, high loading capacity, multifunctional and short charge transport [1–3]. These unique characteristics of the hollow structure have opened up a new

world of opportunities for the potential applications in the fields of energy storage [4–7], catalysis [8–10], sensors [11–13], bioapplicatio [14–16], and so on. In most cases, hollow nanostructures are synthesized by straight­ forward hard-template methods [1]. In general, the hard-template method involves three major steps: i) template preparation; ii) target

* Corresponding author. E-mail address: [email protected] (Q. Xiao). https://doi.org/10.1016/j.matchemphys.2019.122070 Received 27 April 2019; Received in revised form 18 July 2019; Accepted 27 August 2019 Available online 28 August 2019 0254-0584/© 2019 Elsevier B.V. All rights reserved.

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material coating/deposition or adsorption of target metal ions, iii) template removal. However, the existing conventional hard-templating method normally requires the presence of solvents [8,13,14,17], which caused a large amount consumption of solvent and environmental pollution. Recently, solvent-free method has attracted considerable attention. The illustrative examples are solvent-free synthesis of microporous zeolites [18] and mesoporous carbon materials [19]. Therefore, the development of a straightforward solvent-free template approach could significantly promote the synthesis of hollow structured materials. Compared with solvent template method, solvent-free template method has lots of obvious advantages: i) significantly reduced envi­ ronment pollutants. Normally, the dissolution process is avoided to prepare the precursors of hollow spheres by solvent-free template method. Thereby, the metal ions in wastewater were significantly reduced; ii) Better energy efficiency and saving energy. The space occupied by solvents in the autoclave was obviously reduced, and the heat can be absorbed directly by the reactants; iii) The security has been greatly improved. The pressure was remarkably decreased in the auto­ clave due to the lack of solvent evaporation by the solvent-free template method, so the risk of explosion of autoclave can be greatly reduced; iv) Ions and small molecules can enter into the inside of the template. Metal salts are melted and strongly interact with the template, leading to effective ions loading and diffusion. Ions can diffuse into the inside of 3aminophenol/formaldehyde (APF) resin spheres from high concentra­ tion to low concentration; v) The shapes of template can be well repli­ cated. The ions of metal salts between the interspace of template grains can in situ become stable oxide nanoparticles after calcination process. Considering these excellent features, it is believable that the solvent-free template method will be great potential application in the synthesis of hollow nanostructures in the future. Rechargeable batteries with high performance have attracted tremendous attention [20,21]. The lithium-sulfur batteries by virtue of considerable advantages have been considered as one of the most promising devices for next-generation energy storage systems [22,23]. For lithium-sulfur batteries, hollow spheres are increasing attention as a result of the following advantages [1,3]: i) hollow spheres can provide the lots of loading sites for sulfur both the interior void space and porous shell; ii) The interior void space of hollow spheres can accommodate sulfur volumetric expansion to avoid pulverization during both the lithiation and delithiation process; iii) The shell wall can form a physical barrier to minimize the lithium polysulfide dissolution in the electrolyte; iv) The porous shells on the shell of the hollow spheres can provide high surface area, which reduce transport pathway for both electrons and Li ions. Moreover, the M O groups from hydrophilic metal-oxides can combine with polysulfide anions, which relieve the dissolution process of polysulfide [23–26]. For example, some researchers found that SnO2 can form strong chemisorptions with polysulfides [25,26]. Carbon ma­ terials not only can provide effective physical confinement for poly­ sulfides, but also the good electronic conductivity [27,28]. Based on the above opinions, metal-oxides/C hollow spheres can provide better per­ formance for lithium-sulfur batteries due to the combination of the ad­ vantages of metal-oxides, carbon and hollow spheres. Recently, nanostructured phenol formaldehyde resins, such as resorcinol–formaldehyde (RF) resin and 3-aminophenol/formaldehyde (APF) resin nanospheres, have been used for the preparation of hollow nanostructures [14–16]. In this work, we attempt to propose a new and facile strategy of solvent-free template (SFT) method to synthesize the SnO2/C hybrid hollow spheres structure by mixing and heating solid raw materials, in which 3-aminophenol/formaldehyde (APF) resin spheres and Tin salt have been used as hard template and precursor, respec­ tively. This strategy achieves the hybrid composites of carbon and SnO2 by one-step, low-temperature solid-solid diffusion process for lithium-sulfur batteries. Furthermore, favorable handleability is very important for its practical use in high-performance Li–S batteries. Herein, by simply adding the as-prepared SnO2/C hollow spheres in the

cathode, sulfur cathode containing SnO2/C hollow spheres exhibited largely enhanced electrochemical performance for lithium-sulfur bat­ teries. This simple fabrication process can be seamlessly coupled to the current industrial battery manufacturing processes. 2. Experimental section 2.1. Preparation of 3-aminophenol/formaldehyde (APF) resin spheres 3-aminophenol/formaldehyde resin spheres were synthesized ac­ cording to literature [29]. Briefly, 3-aminophenol (3 g) was mixed with ammonia aqueous solution (0.75 mL), absolute ethyl alcohol (28 mL) and deionized water (140 mL). Then, the mixture was magnetic stirred for 10 min at 30 � C. After that, formaldehyde solution was added, and continued to be stirred at 30 � C for 24 h. Finally, the product was separated by centrifugation, washed several times with ethanol, and dried in an oven at 50 � C for 6 h. 2.2. Preparation of SnO2/C hybrid hollow spheres Firstly, 0.3 g of 3-aminophenol/formaldehyde (APF) resins spheres and 0.3 g of SnCl2�2H2O salts were mixed together. Secondly, the ob­ tained mixture from Tin salt and APF spheres were heated in a quasisealed container at 80 � C for 12 h, and the APF-SnO2 composite spheres could be obtained. Finally, the resultant APF-SnO2 composite spheres were annealed in a quasi-sealed container at 400 � C for 1 h in air with a heating rate of 5 � C min 1, and the SnO2/C hollow spheres were obtained, and marked as SnO2/C-HS. 2.3. Preparation of SnO2 powders 0.3 g of SnCl2�2H2O were annealed in an open container at 400 � C for 1 h in air with a heating rate of 5 � C min 1, and the SnO2 powders were obtained 2.4. Preparation of S/conductive acetylene black (S/Super P) composites Conductive acetylene black (Super P) and sulfur (1:3 in mass) were mixed and co-heated at 155 � C for 6 h, then the S/conductive acetylene black (S/Super P) composites were obtained. 2.5. Material characterization The XRD patterns of the samples were recorded with a DX-2700 Xray powder diffractometer (XRD) with Cu Kα radiation (λ ¼ 1.54056 Å). The scanning electron microscope (SEM) images were taken on a JEOL JSM-6490LV SEM. The transmission electron microscopy (TEM) images and EDX mapping were taken on a FEI Tecnai G2 20S-Twin. The EDAX mappings were taken on a FEI Tecnai G2 F20. The X-ray photoelectron spectra (XPS) measurements were performed on a VG Scientific ESCA­ LAB Mark II spectrometer equipped with two ultrahigh vacuum (UHV) chambers using Al Ka radiation (1486.6 eV) to investigate the surface properties. The binding energy of the XPS spectra is calibrated with the reference to the C 1s peak (284.8 eV) arising from adventitious carbon. Ar ions sputtering are applied to clean the surface of the samples. The thermogravimetric (TG) analysis was performed with NETZSCH STA 449C instrument. The nitrogen adsorption and desorption isotherms were collected at 77 K on a Micromeritic ASAP 2010 instrument. The specific surface areas were calculated using the Brunnauer–Emmett–­ Teller (BET) equation and the pore size distributions were calculated by applying the Barrett–Joyner–Halenda (BJH) method using the desorp­ tion branch of the isotherms. The Raman spectrum analysis was con­ ducted on a Labram HR800 Laser Raman Spectroscopy made by Jobin Yvon, France, using the 632.8 nm He Ne ion laser as an excitation source. The laser power on the sample was 10 mW. 2

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Scheme 1. Schematic illustration of the synthesis of SnO2/C-HS by SFT method.

Fig. 1. XRD pattern (a), SEM image (b), TEM image (c) and EDX spectrum (d) of SnO2/C-HS.

was about 0.96 mg cm 2 (based on the total mass of sulfur in electrodes). Lithium chips were used as both the counter electrode and reference electrode. The electrolyte was made up of 1 M LITFSI, 0.1 M lithium nitrate (LiNO3) in 1, 2-dimethoxyethane (DME) and 1, 3-dioxolane (DOL) (volume ratio 1:1). The electrochemical measurements were carried out using CR2025 coin cells. Cells were fabricated in an Ar-filled glovebox with moisture and oxygen concentrations below 0.1 ppm. The

2.6. Electrochemical measurement The S/Super P composites (70 wt%) were mixed with SnO2/C-HS (or SnO2 powders) (10 wt%), Super P (10 wt%) and polyvinylidene fluoride (PVDF) (10 wt%) in N-methyl-2-pyrrolidone (NMP) solvent to form the mixed slurry. This slurry was then coated onto Al foil and dried at 60 � C for 12 h. The areal mass loading of the active material on electrode film 3

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Fig. 2. Raman spectra (a), TG (b), N2 adsorption-desorption isotherms (c) and the corresponding pore size distribution curves (d) of the as-prepared SnO2/C-HS.

cyclic voltammetry (CV) with a scan rate of 0.05 mV s 1 (1.7–2.8 V versus Liþ/Li) and electrochemical impedance spectroscopy (EIS) ranged from 100 kHz to 0.01 Hz was carried out on a Gamry Interface 1000T electrochemical work station. Galvanostatic charge–discharge measurements were conducted on a Neware BTS4000 battery tester (Shenzhen, China) between 1.7 and 2.8 V. As comparison, the sulfur cathodes without SnO2/C-HS (or SnO2 powders) (S/Super P composites: Super P: PVDF ¼ 7:2:1) were prepared in the same method and tested under the same condition.

(SEM) and transmission electron microscopy (TEM). The SEM image (Fig. 1b) showed the uniform particle diameter distribution of SnO2/C-HS with an average diameter of 600 nm. The broken outer layer shell of the hollow spheres occasionally appeared, revealing the for­ mation of hollow spheres. In agreement with the above SEM observa­ tions, the hollow structure with a diameter of 600 nm could be observed in the TEM image (Fig. 1c). And the shell thickness of SnO2/C-HS was determined to be about 33 nm. Also, nanoparticles could be clearly observed on the shell wall, suggesting the hollow spheres were assem­ bled by numerous nanocrystals with a diameter of 20–30 nm. And the EDX spectrum (Fig. 1d) provided the typical signal of C, O and Sn in the SnO2/C hybrid hollow spheres. Fig. 2a shows the Raman spectra of SnO2/C-HS. Two typical Raman peaks located at around 1364 cm 1 and 1555 cm 1 could be attributed to D band and G band of carbon vibrational modes [30], respectively, indicating the carbonization of APF, and confirming the existence of carbon in the SnO2/C-HS, which is helpful for enhancing the electronic conductivity of electrode materials. Moreover, the carbon content in the SnO2/C hybrid hollow spheres was determined to be 41.13% by ther­ mogravimetric analysis (TG) (Fig. 2b). Fig. 2c shows the nitrogen adsorption-desorption isotherms of SnO2/C-HS, which reveals the highly porous structure. According to the Brunauer-Emmett-Teller (BET) computational method (Fig. 2d), the specific surface area of SnO2/C-HS was 350 m2/g, and the data of pore structure is provided in Table S1 (Supporting Information). The element chemical states of SnO2/C hollow spheres were inves­ tigated by the X-ray photoelectron spectrometry (XPS), and the results are shown in Fig. 3. Fig. 3a exhibits the survey scan XPS spectra of SnO2/

3. Results and discussion As schematically illustrated in Scheme 1, the SFT method starts with solid 3-aminophenol/formaldehyde (APF) resin spheres (shown in Fig. S1, Supporting Information) and solid metal (Tin) salt. Firstly, APF resin spheres were mixed with SnCl2�2H2O by direct solid-solid mixing. Secondly, the ions diffusion process was realized in a quasi-sealed container by keeping diffusion temperature higher than the melting point of metal salt. During this process, the metal salt was melted, and the ions in liquid molten salt can easily diffuse into the inside of APF spheres. Finally, SnO2/carbon hybrid hollow spheres (SnO2/C-HS) were obtained after calcination in the quasi-sealed container in air. The powder X-ray diffraction (XRD) pattern of SnO2/C-HS is pro­ vided in Fig. 1a. The sharp peaks in the range of 20–80� could be indexed to tetragonal cassiterite SnO2 (JCPDS No. 41–1445), and the wide diffraction peaks located at around 20� of the samples could be attrib­ uted to amorphous carbon in the XRD pattern [29]. The morphology and structure of SnO2/C-HS were examined by scanning electron microscope 4

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Fig. 3. XPS spectra of SnO2/C-HS: the full survey scan spectrum (a), the spectra of Sn 3d (b), the spectra of O 1s (c), the spectra of C 1s (d), the spectra of N 1s (e).

C-HS, and only peaks related to Sn, O, N, and C elements were observed [30–32]. Fig. 3b exhibits the high resolution XPS spectrum of Sn 3d revealed two peaks with good symmetry at 487.1 eV and 495.5 eV, corresponding to Sn 3d5/2 located and Sn 3d3/2, respectively. The splitting binding energy between the two peaks was 8.4 eV, indicating the single oxidation valence state of Sn4þ [30]. The O 1s spectrum (Fig. 3c) is deconvoluted into two peaks at 530.9 (Olatt), 532.1 (Ox-), which could ascribed to lattice oxygen (Olatt) and the adsorbed oxygen (O-x) species on the sureface of SnO2, respectively [31,32]. The high resolution XPS spectrum of C 1s (Fig. 3d) exhibits that there are three peaks at about 284.8 eV, 286.7 eV, and 288.8 eV, which could be – C–OH group, assigned to C–C bond, the remnant C–OH group, and O– respectively, indicating the existence of carbon [32]. The high resolu­ tion XPS spectrum of N 1s (Fig. 3e) exhibits a graphitic-type peak [33]. The accordant XRD, SEM, TEM, BET, XPS, and Raman results confirm the successful synthesis of porous SnO2/C hybrid hollow spheres. In order to investigate the ions diffusion process, HAADF-STEM element mapping images of the APF-SnO2 composite spheres gotten after the mixtures of APF spheres and SnCl2 were heated at 80 � C for 12 h was shown in Fig. 4. As shown in Fig. 4a, the typical solid spherical structure of the APF-SnO2 composite spheres are presented, indicating that ions diffusion process had no effect on the morphology of APF spheres compared with APF spheres (shown in Fig. S1). Fig. 4b, c, and d display a uniform distribution of C, O, and Sn in the solid APF-SnO2 composite spheres, suggesting that Tin ions could effectively diffuse into the solid APF spheres when the mixtures of APF spheres and SnCl2 are heated at 80 � C for 12 h. In the final step, the APF template was removed to obtain SnO2/C hollow spheres (SnO2/C-HS) after calcination. Fig. 4e showed the scanning transmission electron microscopy (STEM) image of the SnO2/C-HS. It is shown that SnO2/C-HS with a diameter of 600 nm were successful fabricated. And Fig. 2f–h exhibited the corresponding energy dispersive X-ray spectroscopy (EDXS) elemental mapping images for Sn, O, and C elements, indicating the uniform distribution of amor­ phous carbon and ultrafine SnO2 nanoparticles throughout the whole hollow spheres.

SnO2/C hollow spheres (SnO2/C-HS) were used as adsorbent to subsequently investigate the electrochemical properties for lithiumsulfur batteries compared to the densely structured SnO2 powders and conductive acetylene black, respectively. Fig. 5a shows the cyclic vol­ tammogram (CV) curves of different sulfur cathodes with a voltage window of 1.7 V–2.8 V (vs. Li/Liþ) at a scan rate of 0.05 mV s 1. On the cathodic side, all the samples exhibit two reductive peaks as the sulfur electrode. The first peak is related to the reduction of cyclo-octasulfur (S8) to long chain lithium polysulfides (Li2Sx, 4 < x < 8) [34–38]. The second peak is related to the further reduction of lithium polysulfides (Li2Sx, 4 < x < 8) to Li2S2 and Li2S [34–37]. Compared with S/conduc­ tive acetylene black (2.05 V and 2.33 V), S/SnO2 powders (2.06 V and 2.31 V) and S/SnO2/C-HS (2.07 V and 2.32 V) show slightly positive shifts. On the anodic side, two oxidation peaks of the samples are observed, corresponding to the oxidation of Li2S2 and Li2S to Li2S8 [34–37]. Compared with S/conductive acetylene black (2.33 V and 2.37 V) electrode materials, S/SnO2 powders (2.32 V and 2.37 V) and S/SnO2/C-HS (2.31 V and 2.35 V) show slightly negative shifts, indi­ cating lower polarization of the electrodes during the charge-discharge process after the addition of SnO2 powders and SnO2/C-HS. As shown in Fig. 5b, the charge-discharge curves of different sulfur cathodes (the 1st and 2nd) at 0.2C have a good agreement with the CV analysis. Espe­ cially, S/SnO2/C-HS shows both highest first-discharge plateau at ~2.33 V (corresponding to the reduction of sulfur to long-chain poly­ sulfides) and longest second-discharge plateau at ~2.1 V (the formation of short-chain polysulfides), suggesting much better redox reaction ki­ netics and more efficient utilization of the active sulfur material in S/SnO2/C-HS [26]. Furthermore, the charge-discharge curves of S/SnO2/C-HS cathode form the 2nd to 50th cycles at 0.2C (shown Fig. S3, Supporting Information) show that a good retention with high columbic efficiency is demonstrated due to the good utilization of the active mass and prevention of the Li-polysulfide shuttle mechanism. However, it should be noted that the low initial coulombic efficiency (CE) of S/SnO2/C-HS in the first cycle may be caused by the polysulfide dissolution dissolved into the organic electrolyte, causing the loss of S 5

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Fig. 4. HAADF-STEM element mapping images of the APF-SnO2 composite spheres gotten after the mixtures of APF spheres and SnCl2 were heated at 80 � C for 12 h: a) detector, b) carbon element, c) oxygen element, d) Sn element; HAADF-STEM element mapping images of the SnO2/C-HS gotten after the APF-SnO2 composite spheres were heated at 400 � C for 1 h: e) detector, f) carbon element, g) oxygen element, h) Sn element.

materials and shuttle effect [24,25]. That is to say, although adding SnO2/C-HS can effectively improve the electrochemical performance of LSBs, the “shuttle effect” cannot totally be restrained during the initial charge/discharge because of simply mechanically mixing SnO2/C-HS with S/Super P. Therefore, in the future we should investigate more effective strategy to further restrain the “shuttle effect” in order to improve the electrochemical performance of LSBs. Fig. 5c exhibits the cycle performance of the three cathodes at 0.2C. They demonstrate very different initial discharge capacities of 523, 549 and 1057 mA h g 1 for S/conductive acetylene black, S/SnO2 powders and S/SnO2/C-HS, respectively. Obviously, S/SnO2/C-HS cathode re­ veals the highest initial discharge capacity. In addition, S/SnO2/C-HS suggests distinguishingly higher stable discharge specific capacity and higher capacity retention capability than the other two cathodes (S/ conductive acetylene black, SnO2 powder/S). S/SnO2/C-HS cathode remains a high stable discharge specific capacity (766 mA h g 1) after 50 cycles and its capacity retention is 72.5%. In addition, pure SnO2/C-HS and SnO2 powders (shown in Fig. S2, Supporting Information) can only deliver 34 mA h g 1 and 13 mA h g 1 of discharge specific capacity in the voltage window of 1.7–2.8 V, respectively, which made little contribution to total capacity. As shown in Fig. 5d, the rate capabilities of the three sulfur cathodes were evaluated by cycling at various current

densities. S/SnO2/C-HS exhibits the highest capacity among the three composites at the same discharge rate. When cycled at different current density of 0.2C, 0.5C, 1C, 3C, and 5C (1C ¼ 1672 mA h g 1), the discharge capacities of S/SnO2/C-HS were about 1100, 900, 800, 620 and 570 mA h g 1, respectively. When the current density switched back to 0.2C, a high discharge capacity of 980 mA h g 1 was still maintained, indicating the excellent stability and sulfur utilization efficiency. Fig. S4 (Supporting Information) exhibits the charge-discharge curves vs. voltage at different rates (from 0.2C to 5C). Furthermore, S/SnO2/C-HS exhibits the stable cycling performance over 600 charge/discharge cy­ cles at 1C as displayed in Fig. 4e. After initial discharge capacity of 713 mA h g 1, S/SnO2/C-HS achieves capacity retentions of 81.8%, 77.8%, and 69.1% at the end of 100th, 200th, and 400th cycle, respectively. Most importantly, after prolonged cycling over 600 cycles, the capacity retention was found to be 67%, which corresponds to a very small capacity decay of 0.066% per cycle (6.6% per 100 cycles), and the stable specific capacity exhibits 478 mA h g 1 after 600 cycles at 1C. In addition, S/SnO2/C-HS also exhibited highly stable Coulombic effi­ ciency, indicating the efficient utilization of sulfur. The current S/SnO2/ C-HS prepared by the SFT method revealed comparable rate perfor­ mance and cycling performances as sulfur cathodes compared with the previously reported SnO2 for lithium-sulfur batteries (Table S2, 6

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Fig. 5. CV curves (a), charge-discharge curves (b), cycling performances at 0.2C (c) and Rate capability (d) of tested materials (S/SnO2/C-HS, S/SnO2 powders and S/conductive acetylene black), long-term cycling stability of S/SnO2/C-HS at 1C (e), and EIS curves of tested materials (S/SnO2/C-HS, S/SnO2 powders and S/ conductive acetylene black) (f).

Supporting Information), indicating that the present SFT strategy can be used to prepare metal oxides/carbon hybrid hollow spheres for highperformance LSBs. The EIS results of the electrode materials (S/ conductive acetylene black, S/SnO2 powders and S/SnO2/C-HS) at the initial (Fig. 5f) were analyzed using the equivalent circuits and the calculated data was shown in Table S3 (Supporting Information) [5, 39–42]. It can be observed that S/SnO2/C-HS cathode had the smallest interfacial charge transfer resistance (Rct) and solid electrolyte interface (SEI) film (Rf) compared with S/conductive acetylene black and S/SnO2 powders, which can be attributed to the restrict of unique porous SnO2/C-HS hollow structure with high specific surface area for poly­ sulfides and its high conductivity facilitating the charge transfer.

Acknowledgments The authors are grateful for financial support from the National Natural Science Foundation of China (No.51102285) and the Funda­ mental Research Funds for the Central Universities of Central South University (No.2019zzts684). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.matchemphys.2019.122070. References

4. Conclusions

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In summary, the solvent-free template (SFT) method was success­ fully applied in the field of the synthesis route of SnO2/carbon hybrid hollow spheres using 3-aminophenol/formaldehyde (APF) resin spheres as hard template. The use of solvent was avoided due to the application of solvent-free template (SFT) method. Furthermore, melted Tin ions can diffuse into the inside of the template easily and the shapes of template are successful structure replicated. Sulfur cathode containing the porous SnO2/C hybrid hollow spheres exhibited excellent rate and cycle per­ formances for lithium-sulfur batteries due to the fast transport of elec­ trons and ions, strong chemical adsorption for polysulfides and low volume expansion. Moreover, due to the favorable handleability, which can be seamlessly coupled to current industrial battery manufacturing processes, this approach shows great potential for high-performance Li–S battery applications. Conflicts of interest The authors declare no conflict of interest.


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