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carbon nanocomposite for high-performance lithium-ion battery anode
Applied Surface Science 481 (2019) 1377–1384
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Fabricating thin two-dimensional hollow tin dioxide/carbon nanocomposite for high-performance lithium-ion battery anode ⁎
Qinghua Tiana, , Feng Zhanga, Li Yangb, a b
T
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Department of Chemistry, School of Sciences, Zhejiang Sci-Tech University, Hangzhou 310018, PR China School of Chemistry and Chemical Engineering, Shanghai Jiao Tong University, Shanghai 200240, PR China
A R T I C LE I N FO
A B S T R A C T
Keywords: Thin two-dimensional hollow nanostructure Synergistic effect SnO2/C composite Anode Lithium-ion batteries
Both thin two-dimensional (2D) and hollow nanostructural materials are of great potential for high-performance anodes of lithium-ion batteries owing to their unique structural properties and surface characteristics. In this context, tin dioxide (SnO2) materials possessing well-designed thin 2D or hollow nanostructures have attracted immense attention and been considered as promising candidates to serve as anodes for advanced lithium-ion batteries. It is believed that thin 2D hollow nanostructures of SnO2 materials will have the advantages of both thin 2D and hollow nanostructures but they are difficult to be prepared due to the increased complexity of structures. Herein, a peculiar thin 2D hollow nanostructure of SnO2/carbon (SnO2/C) composite, composed of carbon in-situ coating thin SnO2 hollow nanosheets, has been prepared through a well-designed strategy. It is thus clear that this composite possesses the advantages of both thin 2D and hollow nanostructures as an anode for lithium-ion batteries. Expectedly, it is demonstrated that thus peculiar architecture offers the as-prepared SnO2/C composite superior electrochemical kinetics and outstanding structural stability. As a result, the asprepared SnO2/C composite exhibits outstanding electrochemical lithium storage performance, delivering 707.8 and 483.2 mAh g−1 after 100 cycles at 200 and 1000 mA g−1, respectively, as well as superior rate capabilities.
1. Introduction Rechargeable lithium ion batteries (LIBs) have gained great attentions owing to their significant successes in applications of portable electronics, electric vehicles, electric grids, and so on [1–3]. However, the pressing demands of popular electric vehicles for longer running distance within once charge, and high safety have presented LIBs with a higher challenge such as higher capacity and safety. The performances of the LIBs depend intimately on the properties of their materials. In the case of anode material, the dominantly commercial graphite anode cannot meet the need of LIBs for higher capacity and safety due to its limited theoretical specific capacity and possibility of generating dendritic lithium, this makes great efforts have been continuously devoted to search and develop alternatively admired anode materials to replace the graphite anode [4–9]. In view of its high theoretical specific capacity (1494 mAh g−1) and safer operation potential over graphite, SnO2 has been thought to be one of the most promising substitutes for graphite anode for next-generation LIBs, and hence received wide interests [10]. Unfortunately, it was found that the practical performance of SnO2 is far more less than the expectations due to the fast capacity fading which is known to be majorly resulted from the huge volume
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changes (> 200%) of the electrode materials during repeated lithium insertion/extraction process, which ultimately lead to disintegration/ pulverization of the electrode materials and hence a loss of electrical contacts between adjacent particles [11]. Besides, the poor conductivity of SnO2 severely limits its rate capability which determines the power density of the battery. Therefore, many efforts have been made to find effective solutions for solving above problems. Because nanosized materials have decrease absolute volume change and shortened ions diffusion distance compared to their bulk counterparts, development and preparation of nanosized SnO2 have been considered as an effective strategy for enhancing the lithium storage performance of SnO2 anodes such as lifespan and rate capability [12–14]. Moreover, it is suggested that constructing surface-dominated ultrathin 2D nanostructure is a more suitable strategy to improve electrochemical lithium storage performance of the SnO2 materials. Therefore, 2D nanostructure of SnO2 materials have drew extensive attentions for great potential in lithium storage owing to their exceptional architectural and surface properties [15]. The molecular-scale thick 2D SnO2 materials with an ultrahigh surface atom percentage could not only offer more lithium-insertion channels owing to the great efficient surface active sites, but also allow ultrafast surface lithium
Corresponding authors. E-mail addresses: [email protected] (Q. Tian), [email protected] (L. Yang).
https://doi.org/10.1016/j.apsusc.2019.03.252 Received 11 December 2018; Accepted 23 March 2019 Available online 23 March 2019 0169-4332/ © 2019 Elsevier B.V. All rights reserved.
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storage owing to the shortened transfer pathway of lithium ions and electrons. Thus, the fabrication of 2D nanostructures would be beneficial to the use of SnO2 materials as advanced anode materials for LIBs [16–18]. Besides, nanostructures of SnO2 materials with hollow interiors have also gained increasing attention in LIBs anode materials field, because their inside and outside two surfaces could provide larger interface between active materials and electrolyte as well as their hollow interiors could offer sufficient extra free space for relieving the structural strain and mitigating the large volume variation [11,18]. Hence, the hollow nanostructures could provide the SnO2 with extraordinarily high activated surface and robust stability, namely high capacity, good rate capability, and enhanced cycling performance. Although these exceptional nanostructures have successfully improved the lithium storage performance of SnO2 to a certain extent, their lifespan usually maintains at about 50 cycles, which is still far more less than the expectations [15–17,19]. Encouragingly, it is found that incorporation of the nanostructured SnO2 in carbon matrix to fabricate SnO2/C composites inheriting the morphologies of nanostructured SnO2 could greatly improve the lithium storage performance such as the rate capability and cycling lifespan (could be extended to 100 cycles and even more cycles), which is attributed to the flexible and good conductive carbon could not only buffer the volume variation of SnO2 but also improve the conductivity of whole electrode [20–23]. Hence, the development and fabrication of versatile nanostructures of SnO2/C composites would be highly desirable for promoting the practical use of SnO2 anode materials in advanced LIBs. Herein, taking above into consideration, we have successfully fabricated a characteristic thin 2D hollow nanostructured SnO2/C composite (SnO2@C HNSs) by an elaborate ultrathin 2D template-assisted hydrothermal method, as shown in Fig. 1. As we can see from Fig. 1 that the as-prepared SnO2@C HNSs is composed of carbon in-situ coating thin SnO2 hollow nanosheets. So, the SnO2@C HNSs is capable of the advantages of thin 2D nanostructures, hollow nanostructures and nanostructured SnO2/C composites simultaneously. As a result, the SnO2@C HNSs exhibits outstanding electrochemical lithium storage performance owing to the combined effect of thin SnO2 hollow nanosheets and carbon coating, delivering 707.8 and 483.2 mAh g−1 after 100 cycles at 200 and 1000 mA g−1, respectively, as well as superior rate capabilities. Thus SnO2@C HNSs could be expected to serve as a promisingly alternative anode for high-performance LIBs.
nanosheets. Six pots of ultrathin MnO2 nanosheets were re-dispersed into 60 ml deionized water under ultrasonication for 40 min, and then tin (II) chloride dehydrate (0.2 g) and glucose (1.2 g) were dissolved into above black suspension under magnetic stirring for 0.5 h. Then, ammonium fluoride (0.05 g) was dissolved into above suspension under magnetic stirring for another 30 min. After that, the black suspension was sealed into a Teflon-lined stainless steel autoclave and maintained at 180 °C for 24 h in an oven. When reaction time was up and let the stainless steel autoclave cool naturally, and then the collection approach of generated black brown precipitate was the same with ultrathin MnO2 nanosheets. The collected precipitate was further dried at 70 °C overnight. Finally, to obtain the SnO2@C HNSs the as-prepared precipitate was carbonized at 500 °C for 3 h under an argon atmosphere. For comparison, the pure SnO2 hollow nanosheets were prepared via calcination of as-prepared SnO2@C HNSs at 500 °C for 3 h under an air atmosphere. The pure carbon hollow nanosheets were prepared under the same conditions with SnO2@C HNSs but without tin (II) chloride dehydrate. 2.2. Materials characterizations Scanning electron microscopy (SEM, Hitachi S-4800) coupled with an energy dispersive X-ray spectrometer (EDX) and transmission electron microscopy (TEM, JEOL JEM-2010) were used to study the morphology and microstructure of SnO2@C HNSs. The examination of crystalline phase and element valence of SnO2@C HNSs was performed on an X-ray diffraction measurement (XRD, Rigaku, D/max-Rbusing Cu Kα X-ray radiation) and X-ray photoelectron spectroscopy (XPS, using Al Kα X-ray radiation). The record of Raman spectra of SnO2@C HNSs was achieved on a Raman spectroscopy (Bruker, Senterra R200-L dispersive Raman microscope) with 532 nm excitation wavelength. The carbon content and specific surface area of SnO2@C HNSs was respectively measured with thermogravimetric analysis (TGA, SDT Q600 V8.2 Build 100) and Brunauer-Emmett-Teller (BET) nitrogen adsorption-desorption instrument (Micromeritics ASAP 2010 instrument). 2.3. Electrochemical characterization The electrochemical lithium storage properties of SnO2@C HNSs was studied with 2016-type coin cells which were assembled in a glove box crowded with argon (German, M. Braun Co., [O2] < 1 ppm, [H2O] < 1 ppm). The working electrodes were composed of 70 wt% active material (SnO2@C NSs), 20 wt% conductive material (acetylene black, AB), and 10 wt% binder (sodium carboxymethyl cellulose, CMC) and pasted on a pure Cu foil. The counter electrode and the separator respectively chose pure lithium foil and Cellgard 2400 membrane. And the electrolyte chose 1 M LiPF6 solution made up of LiPF6 and mixture of ethylene carbonate and dimethyl carbonate (1:1 volume ratio). Then, the galvanostatic discharge/charge cycle tests of SnO2@C HNSs electrode were carried out on CT2001a cell test instrument (LAND Electronic Co.) with a potential range of 0.01 to 3.0 V under room temperature. Cyclic voltammetry (CV) was performed on CHI660D electrochemical workstation with a scan rate of 0.3 mV s−1 between 0.01 and 3.0 V to study the lithium storage behavior of SnO2@C HNSs electrodes. Electrochemical impedance spectroscopy (EIS) of SnO2@C HNSs electrodes was also obtained from CHI660E electrochemical workstation with an oscillation amplitude of 5 mV in the frequency
2. Experimental section 2.1. Synthesis of SnO2@C HNSs The thin 2D hollow SnO2@C HNSs had been facilely synthesized from ultrathin 2D template-assisted hydrothermal and subsequently carbonized method. The ultrathin 2D template MnO2 nanosheets were prepared via referring to a previous literature [24]. Manganese acetate acid (0.50 g) and ethylenediaminetetraacetic acid disodium salt (EDTA, 1.5 g) were added into 50 ml deionized water and fully dissolved by magnetic stirring at 30 °C. Subsequently, 50 ml 0.25 M NaOH aqueous solution was dropwise added into above solution. Then, 50 ml 0.12 M K2S2O8 aqueous solution was added dropwise to initiate the chemical reaction. The obtained mixture was maintained at 40 °C for 12 h, and then the generated black precipitates were centrifuged, washed with deionized water several times to obtain the black ultrathin MnO2
Fig. 1. Preparation schematic of SnO2@C HNSs (the Pa is short for polysaccharides). 1378
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Fig. 2. (a) XRD patterns and (b) high-resolution XPS spectrum of Sn element of SnO2@C HNSs.
Au elements are respectively derived from the objective table Cu foil and conductive gold plating during SEM operating process. It is indicated that the SnO2@C HNSs is composed of SnO2 and carbon, which further identified by the elemental mappings in Fig. 3(c–e). Obviously, these elemental mappings well match with the SEM image marked by red square in Fig. 3a, demonstrating the well distribution of Sn, O and C elements in SnO2@C HNSs. However, the diffraction peaks for carbon are not observed in XRD patterns, which may be ascribed to the carbon crystallinity either is amorphous or too weak to be detected, or covered up by SnO2 diffraction peaks. Subsequently, TEM was carried out to characterize the microstructures of SnO2@C HNSs further, as presented in Fig. 4. By comparing TEM images of MnO2 template (Fig. 4(a, b)) and SnO2@C HNSs (Fig. 4(a, b)), it can be clearly found that the SnO2@C HNSs well preserve the thin 2D nanostructure of MnO2 nanosheets. Both they have an unambiguous and well-defined outline with large lateral size. The magnified regions selected on edges of the sheets (Fig. 4(b, d)) indicate that these nanosheets spontaneously bend and fold to become weaved together leaving a porous architecture. More interestingly, the SnO2@C HNSs exhibits an extra hollow structure, determined by the edges of the sheets, marked by red arrows in Fig. 4d, with an obvious contrast between the dark edge and the pale center. The average widths of hollow interiors are about 5–12 nm, are good agreement with the average
range from 100 kHz to 0.01 Hz.
3. Results and discussion The distinctive SnO2@C HNSs can be facilely prepared via a facile hydrothermal method using the ultrathin MnO2 nanosheets as templates, as briefly illustrated in Fig. 1. XRD characterization was firstly performed on SnO2@C HNSs for detection of its crystal structure. As we can see from the as-acquired XRD patterns (Fig. 2a) that all the identifiably dominated diffraction peaks can be indexed to a tetragonal rutile SnO2 (JCPDF card no. 41-1445; space group: P42/mnm; a0 = 4.738 Å, c0 = 3.1865 Å) [25]. And no evident impurity diffraction peaks are observed, testifying the SnO2@C HNSs has a high crystallinity. Moreover, the electronic valence of Sn element in SnO2@C HNSs was further carefully studied by XPS analysis, as shown in Fig. 2b. Two strong peaks are observed at 487.2 and 495.8 eV, respectively, with a splitting energy of 8.4 eV, attributed to Sn 3d5/2 and Sn3d3/2, indicating the Sn4+ oxidation state [26]. It is in good agreement with the XRD result. Then, Fig. 3a presents the SEM image of SnO2@C HNSs. It is clearly identified that the SnO2@C HNSs consists of thin nanosheets. Fig. 3b further depicts the EDX spectrum of SnO2@C HNSs. The peaks for Sn, O and C elements are perspicuously viewed whereas the peaks for Cu and
Fig. 3. (a) SEM image and (b) EDX spectrum of SnO2@C HNSs; (c-e) Elemental mappings based on the red square in (a). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.) 1379
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Fig. 4. (a, b) TEM images of MnO2 NSs; (c–e) TEM and (f) HRTEM images of SnO2@C HNSs.
Fig. 5. (a) Raman spectrum, (b) TG curve and (c) N2 adsorption and desorption isotherms of SnO2@C HNSs.
Fig. 6. TEM images of (a) SnO2 HNSs and (b) CHNSs.
displayed in Fig. 4(e, f). The unambiguous contrast between the dark inside edge and bright outside edge testifies that the dark SnO2 is in-situ coated by a carbon layer during hydrothermal process which with a thickness of 2–6 nm indicated by black arrows in Fig. 4f. Additionally, a
thicknesses of MnO2 nanosheets, indicating the hollow interiors are derived from the leaving space after the MnO2 nanosheets are completely etched by NH4F under certain hydrothermal condition (as seen in Fig. 7). Furthermore, the HRTEM images of SnO2@C HNSs are 1380
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Fig. 7. The possible formation schematic of 2D hollow nanostructure.
Fig. 8. (a) CV curves and (b) galvanostatic discharge/charge profiles (at 200 mA g−1) of SnO2@C HNSs electrode; (c) Cycling performances of SnO2@C NSs, CHNSs and SnO2 NPs electrodes; (d) Nyquist plots of different electrodes; (e) Rate capability of SnO2@C HNSs electrode.
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which may be ascribed to the deposition rate of polysaccharide is slower than the remove rate of MnO2 template, hence the well carbon layer cannot form on the surface of MnO2 template before the MnO2 template is completely etched by NH4F under certain hydrothermal condition, finally resulting in the structure of CHNSs is poor. For the preparation of SnO2@C HNSs, the deposition rate of SnO2 nanocrystals is faster than the remove rate of MnO2 template, hence the well coating layer composed of SnO2 nanocrystals can form on the surface of MnO2 template before the MnO2 template is completely etched by NH4F under certain hydrothermal condition, and subsequent the polysaccharide continues to deposit on the surface of SnO2 layer, finally resulting in good structure of SnO2@C HNSs, as shown in the possible formation schematic (Fig. 7). The as-prepared SnO2@C HNSs is of great potential for high-performance lithium storage because of its unique structural properties and surface characteristics. Therefore, the lithium storage performance of SnO2@C HNSs has been investigated as an anode material for LIBs. In order to better insight into the scheme of the electrochemical process, the CV of the battery from the first to three cycles were performed, as displayed in Fig. 8a. During the first cycle an almost irreversibly small cathodic peak centered at 0.9 V is viewed, corresponding to irreversible generation of solid electrolyte interphase (SEI) film and the only partially reversible reduction of SnO2 to Li2O and metallic Sn [28]. Subsequently, a reversibly sharp cathodic peak between 0.68 and 0.01 V is observed, corresponding to the formation of a series of LixSn alloys, and lithium insertion into carbon further [29,30]. Then, a reversibly definite anodic peak located at 0.52 V, ascribing to the dealloying reaction of LixSn alloys [31]. After first cycle the peak values between 0.68 and 0.01 V reduce, which may be ascribed to the weak structural variation of electrodes, formation of SEI film and enhanced electrolyte immersion on the surface [32]. Furthermore, after the first cycle the following two CV curves become stable and overlap, showing an outstanding reversibility of the electrochemical reaction and good cycling stability. Fig. 8b reveals several representative galvanostatic discharge/charge profiles of SnO2@C HNSs electrodes at 200 mA g−1 between 0.01 and 3.0 V. In agreement with CV results, it presents an extended voltage plateau at about 1.02 V (corresponding to the irreversible generation of SEI film and the partially reversible reduction of SnO2 to Li2O and metallic Sn) during the first discharge profile, subsequent a sloping curve down to the cutoff voltage of 0.01 V (corresponding to the reversible formation of a series of LixSn alloys, and lithium insertion into carbon further). Also it presents a sloping curve from about 0.8 V down to the cutoff voltage of 0.01 V during the first charge profile, corresponding to the reversible dealloying reaction of LixSn alloys. After the first cycle the following cycles show overlapping trends, further indicating superior cycling stability. The first discharge and charge capacities are 1348.6 and 794.5 mAh g−1, respectively, coupled with a coulombic efficiency of 58.9%. The capacity loss in the first cycle is commonly ascribed to the irreversible reactions such as formation of SEI film and just partially reversible reduction of SnO2 to Li2O and metallic Sn [33]. Fig. 8c shows the cycling performances of SnO2@C HNSs electrodes at 200 and 1000 mA g−1 between 0.01 and 3.0 V. It is
Table 1 The comparison of SnO2@C HNSs with the reported SnO2/C-based anode materials. SnO2/C-based anode materials
SnO2@C HNSs SnO2@C HNSs SnO2/C SnO2 NSs-graphene SnO2 NPs in CNTs SnO2/mesoporous carbon SnO2@C yolk–shell SnO2@C@MnO2 SnO2@C@VO2-3 GF@SnO2 NRAs@ PANI-40 SnO2–C SnO2/C
Potential cutoff (V)
Current density (mA g−1)
Cycles
Capacity (mAh g−1)
Reference
3.0–0.01 3.0–0.01 2.5–0.05 1.2–0.01 2.5–0.005 2–0.01
200 1000 300 400 50 100
100 100 350 50 50 50
707.8 483.2 653 518 560 473
This work This work [34] [35] [36] [37]
3.0–0.0 3.0–0.01 3.0–0.01 3.0–0.05
100 100 100 500
100 200 100 50
630 644.5 765.1 540
[38] [39] [40] [41]
3.0–0.005 2.0–0.05
500 500
100 75
ca. 610 464
[42] [43]
clear interplanar spacing of 0.33 nm is viewed in the HRTEM image (Fig. 4f) and, indexed to the d110-spacing of SnO2 [27], confirming the good crystallization of SnO2@C HNSs, which well matches with the XRD characterization results. Thus, it is testified that the peculiar SnO2@C HNSs is composed of carbon in-situ coating thin SnO2 hollow nanosheets. Therefore, to robustly determine the presence of carbon, the Raman characterization was carried out on SnO2@C HNSs, as shown in Fig. 5a. Obviously, the as-obtained Raman spectrum of SnO2@C HNSs presents two characteristic peaks of carbon, consisting of a D band of 1357 cm−1 and a G band of 1583 cm−1. The integral area ratio, ID/IG, is 1.38, indicating low crystallization, which is consistent with the result of XRD pattern [22]. The mass content, carbon in SnO2@C HNSs, is evaluated to be about 48 wt%, based on the TG analysis as displayed in Fig. 5b. To evaluate the specific surface area of SnO2@C HNSs, BET N2 adsorptiondesorption analysis was achieved. As seen from the as-recorded nitrogen adsorption and desorption isotherms in Fig. 5c that the SnO2@C HNSs has a large BET surface area of 228.9 m2 g−1, thus large BET surface area should be attributed to its individual thin 2D hollow nanostructure. Additionally, to highlight the synergistic effect of SnO2 nanosheets and carbon coating on the electrochemical performance of SnO2@C HNSs, control samples of pure SnO2 hollow nanosheets (SnO2 HNSs) and pure carbon hollow nanosheets (CHNSs) were prepared. Fig. 6a gives the TEM image of SnO2 HNSs. It can be viewed that the SnO2 HNSs well retains the 2D nanostructure of SnO2@C HNSs after removed the carbon via calcination of SnO2@C HNSs in air atmosphere. Interestingly, some of hollow structures are also maintained, as indicated by red arrows in Fig. 6a. In case the tin (II) chloride dehydrate is absent during the preparation process of SnO2@C HNSs, the CHNSs can be obtained. Fig. 6b depicts the TEM image of as-obtained CHNSs. It also has a 2D hollow nanostructure, as indicated by red arrows. It is found that the structure of CHNSs is not as good as SnO2@C HNSs,
Fig. 9. (a, b) TEM images of SnO2@C HNSs electrode at 200 mA g−1 after 100 cycles; (c) The lithium storage schematic of SnO2@C HNSs electrode. 1382
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Acknowledgments
thus evident that the SnO2@C HNSs exhibits outstanding cycling performance, delivering high capacities of 707.8 and 483.2 mAh g−1 after 100 cycles at 200 and 1000 mA g−1, respectively. Whereas, the two control samples show worse lithium storage performances than that of SnO2@C HNSs at 200 mA g−1. The CHNSs reveals good cycling stability, but the capacities are low. For SnO2 HNSs, the cycling stability is terrible and the capacities quickly decay from 1724.7 mAh g−1 in first cycle down to 199.4 mAh g−1 in 100th cycles due to the well-known structural damage resulted from the larger volume variation during long-term discharge/charge process. The comparative result of these three samples identifies that the well-designed synergistic effect of thin SnO2 hollow nanosheets and carbon coating plays an important role in achievement of outstanding electrochemical performances for SnO2@C HNSs. Followed by Fig. 8d shows the Nyquist plots of these three samples. It is thus obvious that the SnO2@C HNSs and CHNSs possess higher conductivity than that of SnO2 HNSs due to the charge transfer impedance is proportional to the diameter of the depressed semicircle in high frequency. It is testified that the carbon coating is able to improve the conductivity of SnO2@C HNSs. Hence, Fig. 8e depicts the rate capacity of SnO2@C HNSs, giving 757.4, 636.3, 606.9, 564.4, 512.3 and 476.8 mAh g−1 in 5th cycle at 0.1, 0.3, 0.5, 1.0, 2.0, and 3.0 A g−1, respectively, as well as a capacity of 731.8 mAh g−1 can be restored after current is set back to 100 mA g−1. The good rate capabilities should be attributed to the improved conductivity of SnO2@C HNSs electrodes. Besides, the lithium storage properties of SnO2@C NSs are comparable to and even better than the previously state-of-the-art SnO2/C-based composites, as seen in Table 1. To further insight into the structural stability of SnO2@C HNSs during cycling process, TEM characterization was achieved to observe the structure variation of SnO2@C HNSs electrode after 100 cycles at 200 mA g−1, as seen in Fig. 9(a, b). It is thus clear that the bent 2D nanostructure is well retained but the hollow interior becomes ambiguous which resulted from the large volume change of SnO2 hollow nanosheets during long-term cycling process, determining superior structural stability. Thus, the outstanding performance of SnO2@C HNSs should be attributed to the well-designed synergistic effect of thin SnO2 hollow nanosheets and carbon coating (Fig. 9c), which offers the SnO2@C HNSs improved electrochemical kinetics and robust structural stability owing to two key aspects: one hand, the good conductive and flexible carbon coating can not only improve the conductivity of whole electrode but also buffer the large volume change; on the other hand, the thin 2D hollow nanosheet architecture can not only offer more lithium-insertion channels owing to the great efficient surface active sites, but also allow ultrafast surface lithium storage owing to the shortened transfer pathway of lithium ions and electrons, as well as enhanced structural stability owing to the sufficient extra free space for relieving the structural strain and mitigating the large volume variation.
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4. Conclusions In view of the unique structural properties and surface characteristics of thin 2D hollow nanostructures, a peculiar SnO2/C composite composed of carbon in-situ coating thin SnO2 hollow nanosheets has been prepared through a well-designed strategy. It is demonstrated that thus peculiar architecture offers the as-prepared SnO2/C composite superior electrochemical kinetics and outstanding structural stability owing to the well-designed synergistic effect of thin SnO2 hollow nanosheets and carbon coating. Consequently, the as-prepared SnO2/C composite presents outstanding electrochemical lithium storage performance, delivering 707.8 and 483.2 mAh g−1 after 100 cycles at 200 and 1000 mA g−1, respectively, as well as superior rate capabilities. Therefore, this unique SnO2/C composite could be expected to use as an advanced anode for next generation of lithium-ion batteries. Moreover, this work may open up a route to facile preparation of complex thin 2D hollow nanostructures of SnO2/C composites. 1383
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[25] J.H. Sun, L.H. Xiao, S.D. Jiang, G.X. Li, Y. Huang, J.X. Geng, Fluorine-doped SnO2@ graphene porous composite for high capacity lithium-ion batteries, Chem. Mater. 27 (2015) 4594–4603. [26] Z.T. Li, G.L. Wu, S.Z. Deng, S.J. Wang, Y.K. Wang, J.Y. Zhou, S.P. Liu, W.T. Wu, M.B. Wu, Combination of uniform SnO2 nanocrystals with nitrogen doped graphene for high-performance lithium-ion batteries anode, Chem. Eng. J. 283 (2016) 1435–1442. [27] W.L. Wei, P.C. Du, D. Liu, H.X. Wang, P. Liu, Facile mass production of nanoporous SnO2 nanosheets as anode materials for high performance lithium-ion batteries, J. Colloid. Interf. Sci. 503 (2017) 205–213. [28] I.A. Courtney, J.R. Dahn, Electrochemical and in situ x-ray diffraction studies of the reaction of lithium with tin oxide composites, J. Electrochem. Soc. 144 (1997) 2045–2052. [29] M. Winter, J.O. Besenhard, Electrochemical lithiation of tin and tin-based intermetallics and composites, Electrochim. Acta 45 (1999) 31–50. [30] D. Pan, S. Wang, B. Zhao, M. Wu, H. Zhang, Y. Wang, Z. Jiao, Li storage properties of disordered graphene nanosheets, Chem. Mater. 21 (2009) 3136–3142. [31] D.A. Stevens, J.R. Dahn, The mechanisms of lithium and sodium insertion in carbon materials, J. Electrochem. Soc. 148 (2001) A803–A811. [32] B. Zhao, Z.X. Wang, S.S. Wang, J.L. Jiang, J. Si, S.S. Huang, Z.W. Chen, W.R. Li, Y. Jiang, Sandwiched spherical tin dioxide/graphene with a three-dimensional interconnected closed pore structure for lithium storage, Nanoscale 10 (2018) 16116–16126. [33] A. Birrozzi, R. Raccichini, F. Nobili, M. Marinaro, R. Tossici, R. Marassi, High-stability graphene nano sheets/SnO2 composite anode for lithium ion batteries, Electrochim. Acta 137 (2014) 228–234. [34] H. Hu, H.Y. Cheng, G.J. Li, J.P. Liu, Y. Yu, Design of SnO2/C hybrid triple-layer nanospheres as Li-ion battery anodes with high stability and rate capability, J. Mater. Chem. A 3 (2015) 2748–2755.
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Full length article
Fabricating thin two-dimensional hollow tin dioxide/carbon nanocomposite for high-performance lithium-ion battery anode ⁎
Qinghua Tiana, , Feng Zhanga, Li Yangb, a b
T
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Department of Chemistry, School of Sciences, Zhejiang Sci-Tech University, Hangzhou 310018, PR China School of Chemistry and Chemical Engineering, Shanghai Jiao Tong University, Shanghai 200240, PR China
A R T I C LE I N FO
A B S T R A C T
Keywords: Thin two-dimensional hollow nanostructure Synergistic effect SnO2/C composite Anode Lithium-ion batteries
Both thin two-dimensional (2D) and hollow nanostructural materials are of great potential for high-performance anodes of lithium-ion batteries owing to their unique structural properties and surface characteristics. In this context, tin dioxide (SnO2) materials possessing well-designed thin 2D or hollow nanostructures have attracted immense attention and been considered as promising candidates to serve as anodes for advanced lithium-ion batteries. It is believed that thin 2D hollow nanostructures of SnO2 materials will have the advantages of both thin 2D and hollow nanostructures but they are difficult to be prepared due to the increased complexity of structures. Herein, a peculiar thin 2D hollow nanostructure of SnO2/carbon (SnO2/C) composite, composed of carbon in-situ coating thin SnO2 hollow nanosheets, has been prepared through a well-designed strategy. It is thus clear that this composite possesses the advantages of both thin 2D and hollow nanostructures as an anode for lithium-ion batteries. Expectedly, it is demonstrated that thus peculiar architecture offers the as-prepared SnO2/C composite superior electrochemical kinetics and outstanding structural stability. As a result, the asprepared SnO2/C composite exhibits outstanding electrochemical lithium storage performance, delivering 707.8 and 483.2 mAh g−1 after 100 cycles at 200 and 1000 mA g−1, respectively, as well as superior rate capabilities.
1. Introduction Rechargeable lithium ion batteries (LIBs) have gained great attentions owing to their significant successes in applications of portable electronics, electric vehicles, electric grids, and so on [1–3]. However, the pressing demands of popular electric vehicles for longer running distance within once charge, and high safety have presented LIBs with a higher challenge such as higher capacity and safety. The performances of the LIBs depend intimately on the properties of their materials. In the case of anode material, the dominantly commercial graphite anode cannot meet the need of LIBs for higher capacity and safety due to its limited theoretical specific capacity and possibility of generating dendritic lithium, this makes great efforts have been continuously devoted to search and develop alternatively admired anode materials to replace the graphite anode [4–9]. In view of its high theoretical specific capacity (1494 mAh g−1) and safer operation potential over graphite, SnO2 has been thought to be one of the most promising substitutes for graphite anode for next-generation LIBs, and hence received wide interests [10]. Unfortunately, it was found that the practical performance of SnO2 is far more less than the expectations due to the fast capacity fading which is known to be majorly resulted from the huge volume
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changes (> 200%) of the electrode materials during repeated lithium insertion/extraction process, which ultimately lead to disintegration/ pulverization of the electrode materials and hence a loss of electrical contacts between adjacent particles [11]. Besides, the poor conductivity of SnO2 severely limits its rate capability which determines the power density of the battery. Therefore, many efforts have been made to find effective solutions for solving above problems. Because nanosized materials have decrease absolute volume change and shortened ions diffusion distance compared to their bulk counterparts, development and preparation of nanosized SnO2 have been considered as an effective strategy for enhancing the lithium storage performance of SnO2 anodes such as lifespan and rate capability [12–14]. Moreover, it is suggested that constructing surface-dominated ultrathin 2D nanostructure is a more suitable strategy to improve electrochemical lithium storage performance of the SnO2 materials. Therefore, 2D nanostructure of SnO2 materials have drew extensive attentions for great potential in lithium storage owing to their exceptional architectural and surface properties [15]. The molecular-scale thick 2D SnO2 materials with an ultrahigh surface atom percentage could not only offer more lithium-insertion channels owing to the great efficient surface active sites, but also allow ultrafast surface lithium
Corresponding authors. E-mail addresses: [email protected] (Q. Tian), [email protected] (L. Yang).
https://doi.org/10.1016/j.apsusc.2019.03.252 Received 11 December 2018; Accepted 23 March 2019 Available online 23 March 2019 0169-4332/ © 2019 Elsevier B.V. All rights reserved.
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storage owing to the shortened transfer pathway of lithium ions and electrons. Thus, the fabrication of 2D nanostructures would be beneficial to the use of SnO2 materials as advanced anode materials for LIBs [16–18]. Besides, nanostructures of SnO2 materials with hollow interiors have also gained increasing attention in LIBs anode materials field, because their inside and outside two surfaces could provide larger interface between active materials and electrolyte as well as their hollow interiors could offer sufficient extra free space for relieving the structural strain and mitigating the large volume variation [11,18]. Hence, the hollow nanostructures could provide the SnO2 with extraordinarily high activated surface and robust stability, namely high capacity, good rate capability, and enhanced cycling performance. Although these exceptional nanostructures have successfully improved the lithium storage performance of SnO2 to a certain extent, their lifespan usually maintains at about 50 cycles, which is still far more less than the expectations [15–17,19]. Encouragingly, it is found that incorporation of the nanostructured SnO2 in carbon matrix to fabricate SnO2/C composites inheriting the morphologies of nanostructured SnO2 could greatly improve the lithium storage performance such as the rate capability and cycling lifespan (could be extended to 100 cycles and even more cycles), which is attributed to the flexible and good conductive carbon could not only buffer the volume variation of SnO2 but also improve the conductivity of whole electrode [20–23]. Hence, the development and fabrication of versatile nanostructures of SnO2/C composites would be highly desirable for promoting the practical use of SnO2 anode materials in advanced LIBs. Herein, taking above into consideration, we have successfully fabricated a characteristic thin 2D hollow nanostructured SnO2/C composite (SnO2@C HNSs) by an elaborate ultrathin 2D template-assisted hydrothermal method, as shown in Fig. 1. As we can see from Fig. 1 that the as-prepared SnO2@C HNSs is composed of carbon in-situ coating thin SnO2 hollow nanosheets. So, the SnO2@C HNSs is capable of the advantages of thin 2D nanostructures, hollow nanostructures and nanostructured SnO2/C composites simultaneously. As a result, the SnO2@C HNSs exhibits outstanding electrochemical lithium storage performance owing to the combined effect of thin SnO2 hollow nanosheets and carbon coating, delivering 707.8 and 483.2 mAh g−1 after 100 cycles at 200 and 1000 mA g−1, respectively, as well as superior rate capabilities. Thus SnO2@C HNSs could be expected to serve as a promisingly alternative anode for high-performance LIBs.
nanosheets. Six pots of ultrathin MnO2 nanosheets were re-dispersed into 60 ml deionized water under ultrasonication for 40 min, and then tin (II) chloride dehydrate (0.2 g) and glucose (1.2 g) were dissolved into above black suspension under magnetic stirring for 0.5 h. Then, ammonium fluoride (0.05 g) was dissolved into above suspension under magnetic stirring for another 30 min. After that, the black suspension was sealed into a Teflon-lined stainless steel autoclave and maintained at 180 °C for 24 h in an oven. When reaction time was up and let the stainless steel autoclave cool naturally, and then the collection approach of generated black brown precipitate was the same with ultrathin MnO2 nanosheets. The collected precipitate was further dried at 70 °C overnight. Finally, to obtain the SnO2@C HNSs the as-prepared precipitate was carbonized at 500 °C for 3 h under an argon atmosphere. For comparison, the pure SnO2 hollow nanosheets were prepared via calcination of as-prepared SnO2@C HNSs at 500 °C for 3 h under an air atmosphere. The pure carbon hollow nanosheets were prepared under the same conditions with SnO2@C HNSs but without tin (II) chloride dehydrate. 2.2. Materials characterizations Scanning electron microscopy (SEM, Hitachi S-4800) coupled with an energy dispersive X-ray spectrometer (EDX) and transmission electron microscopy (TEM, JEOL JEM-2010) were used to study the morphology and microstructure of SnO2@C HNSs. The examination of crystalline phase and element valence of SnO2@C HNSs was performed on an X-ray diffraction measurement (XRD, Rigaku, D/max-Rbusing Cu Kα X-ray radiation) and X-ray photoelectron spectroscopy (XPS, using Al Kα X-ray radiation). The record of Raman spectra of SnO2@C HNSs was achieved on a Raman spectroscopy (Bruker, Senterra R200-L dispersive Raman microscope) with 532 nm excitation wavelength. The carbon content and specific surface area of SnO2@C HNSs was respectively measured with thermogravimetric analysis (TGA, SDT Q600 V8.2 Build 100) and Brunauer-Emmett-Teller (BET) nitrogen adsorption-desorption instrument (Micromeritics ASAP 2010 instrument). 2.3. Electrochemical characterization The electrochemical lithium storage properties of SnO2@C HNSs was studied with 2016-type coin cells which were assembled in a glove box crowded with argon (German, M. Braun Co., [O2] < 1 ppm, [H2O] < 1 ppm). The working electrodes were composed of 70 wt% active material (SnO2@C NSs), 20 wt% conductive material (acetylene black, AB), and 10 wt% binder (sodium carboxymethyl cellulose, CMC) and pasted on a pure Cu foil. The counter electrode and the separator respectively chose pure lithium foil and Cellgard 2400 membrane. And the electrolyte chose 1 M LiPF6 solution made up of LiPF6 and mixture of ethylene carbonate and dimethyl carbonate (1:1 volume ratio). Then, the galvanostatic discharge/charge cycle tests of SnO2@C HNSs electrode were carried out on CT2001a cell test instrument (LAND Electronic Co.) with a potential range of 0.01 to 3.0 V under room temperature. Cyclic voltammetry (CV) was performed on CHI660D electrochemical workstation with a scan rate of 0.3 mV s−1 between 0.01 and 3.0 V to study the lithium storage behavior of SnO2@C HNSs electrodes. Electrochemical impedance spectroscopy (EIS) of SnO2@C HNSs electrodes was also obtained from CHI660E electrochemical workstation with an oscillation amplitude of 5 mV in the frequency
2. Experimental section 2.1. Synthesis of SnO2@C HNSs The thin 2D hollow SnO2@C HNSs had been facilely synthesized from ultrathin 2D template-assisted hydrothermal and subsequently carbonized method. The ultrathin 2D template MnO2 nanosheets were prepared via referring to a previous literature [24]. Manganese acetate acid (0.50 g) and ethylenediaminetetraacetic acid disodium salt (EDTA, 1.5 g) were added into 50 ml deionized water and fully dissolved by magnetic stirring at 30 °C. Subsequently, 50 ml 0.25 M NaOH aqueous solution was dropwise added into above solution. Then, 50 ml 0.12 M K2S2O8 aqueous solution was added dropwise to initiate the chemical reaction. The obtained mixture was maintained at 40 °C for 12 h, and then the generated black precipitates were centrifuged, washed with deionized water several times to obtain the black ultrathin MnO2
Fig. 1. Preparation schematic of SnO2@C HNSs (the Pa is short for polysaccharides). 1378
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Fig. 2. (a) XRD patterns and (b) high-resolution XPS spectrum of Sn element of SnO2@C HNSs.
Au elements are respectively derived from the objective table Cu foil and conductive gold plating during SEM operating process. It is indicated that the SnO2@C HNSs is composed of SnO2 and carbon, which further identified by the elemental mappings in Fig. 3(c–e). Obviously, these elemental mappings well match with the SEM image marked by red square in Fig. 3a, demonstrating the well distribution of Sn, O and C elements in SnO2@C HNSs. However, the diffraction peaks for carbon are not observed in XRD patterns, which may be ascribed to the carbon crystallinity either is amorphous or too weak to be detected, or covered up by SnO2 diffraction peaks. Subsequently, TEM was carried out to characterize the microstructures of SnO2@C HNSs further, as presented in Fig. 4. By comparing TEM images of MnO2 template (Fig. 4(a, b)) and SnO2@C HNSs (Fig. 4(a, b)), it can be clearly found that the SnO2@C HNSs well preserve the thin 2D nanostructure of MnO2 nanosheets. Both they have an unambiguous and well-defined outline with large lateral size. The magnified regions selected on edges of the sheets (Fig. 4(b, d)) indicate that these nanosheets spontaneously bend and fold to become weaved together leaving a porous architecture. More interestingly, the SnO2@C HNSs exhibits an extra hollow structure, determined by the edges of the sheets, marked by red arrows in Fig. 4d, with an obvious contrast between the dark edge and the pale center. The average widths of hollow interiors are about 5–12 nm, are good agreement with the average
range from 100 kHz to 0.01 Hz.
3. Results and discussion The distinctive SnO2@C HNSs can be facilely prepared via a facile hydrothermal method using the ultrathin MnO2 nanosheets as templates, as briefly illustrated in Fig. 1. XRD characterization was firstly performed on SnO2@C HNSs for detection of its crystal structure. As we can see from the as-acquired XRD patterns (Fig. 2a) that all the identifiably dominated diffraction peaks can be indexed to a tetragonal rutile SnO2 (JCPDF card no. 41-1445; space group: P42/mnm; a0 = 4.738 Å, c0 = 3.1865 Å) [25]. And no evident impurity diffraction peaks are observed, testifying the SnO2@C HNSs has a high crystallinity. Moreover, the electronic valence of Sn element in SnO2@C HNSs was further carefully studied by XPS analysis, as shown in Fig. 2b. Two strong peaks are observed at 487.2 and 495.8 eV, respectively, with a splitting energy of 8.4 eV, attributed to Sn 3d5/2 and Sn3d3/2, indicating the Sn4+ oxidation state [26]. It is in good agreement with the XRD result. Then, Fig. 3a presents the SEM image of SnO2@C HNSs. It is clearly identified that the SnO2@C HNSs consists of thin nanosheets. Fig. 3b further depicts the EDX spectrum of SnO2@C HNSs. The peaks for Sn, O and C elements are perspicuously viewed whereas the peaks for Cu and
Fig. 3. (a) SEM image and (b) EDX spectrum of SnO2@C HNSs; (c-e) Elemental mappings based on the red square in (a). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.) 1379
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Fig. 4. (a, b) TEM images of MnO2 NSs; (c–e) TEM and (f) HRTEM images of SnO2@C HNSs.
Fig. 5. (a) Raman spectrum, (b) TG curve and (c) N2 adsorption and desorption isotherms of SnO2@C HNSs.
Fig. 6. TEM images of (a) SnO2 HNSs and (b) CHNSs.
displayed in Fig. 4(e, f). The unambiguous contrast between the dark inside edge and bright outside edge testifies that the dark SnO2 is in-situ coated by a carbon layer during hydrothermal process which with a thickness of 2–6 nm indicated by black arrows in Fig. 4f. Additionally, a
thicknesses of MnO2 nanosheets, indicating the hollow interiors are derived from the leaving space after the MnO2 nanosheets are completely etched by NH4F under certain hydrothermal condition (as seen in Fig. 7). Furthermore, the HRTEM images of SnO2@C HNSs are 1380
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Fig. 7. The possible formation schematic of 2D hollow nanostructure.
Fig. 8. (a) CV curves and (b) galvanostatic discharge/charge profiles (at 200 mA g−1) of SnO2@C HNSs electrode; (c) Cycling performances of SnO2@C NSs, CHNSs and SnO2 NPs electrodes; (d) Nyquist plots of different electrodes; (e) Rate capability of SnO2@C HNSs electrode.
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which may be ascribed to the deposition rate of polysaccharide is slower than the remove rate of MnO2 template, hence the well carbon layer cannot form on the surface of MnO2 template before the MnO2 template is completely etched by NH4F under certain hydrothermal condition, finally resulting in the structure of CHNSs is poor. For the preparation of SnO2@C HNSs, the deposition rate of SnO2 nanocrystals is faster than the remove rate of MnO2 template, hence the well coating layer composed of SnO2 nanocrystals can form on the surface of MnO2 template before the MnO2 template is completely etched by NH4F under certain hydrothermal condition, and subsequent the polysaccharide continues to deposit on the surface of SnO2 layer, finally resulting in good structure of SnO2@C HNSs, as shown in the possible formation schematic (Fig. 7). The as-prepared SnO2@C HNSs is of great potential for high-performance lithium storage because of its unique structural properties and surface characteristics. Therefore, the lithium storage performance of SnO2@C HNSs has been investigated as an anode material for LIBs. In order to better insight into the scheme of the electrochemical process, the CV of the battery from the first to three cycles were performed, as displayed in Fig. 8a. During the first cycle an almost irreversibly small cathodic peak centered at 0.9 V is viewed, corresponding to irreversible generation of solid electrolyte interphase (SEI) film and the only partially reversible reduction of SnO2 to Li2O and metallic Sn [28]. Subsequently, a reversibly sharp cathodic peak between 0.68 and 0.01 V is observed, corresponding to the formation of a series of LixSn alloys, and lithium insertion into carbon further [29,30]. Then, a reversibly definite anodic peak located at 0.52 V, ascribing to the dealloying reaction of LixSn alloys [31]. After first cycle the peak values between 0.68 and 0.01 V reduce, which may be ascribed to the weak structural variation of electrodes, formation of SEI film and enhanced electrolyte immersion on the surface [32]. Furthermore, after the first cycle the following two CV curves become stable and overlap, showing an outstanding reversibility of the electrochemical reaction and good cycling stability. Fig. 8b reveals several representative galvanostatic discharge/charge profiles of SnO2@C HNSs electrodes at 200 mA g−1 between 0.01 and 3.0 V. In agreement with CV results, it presents an extended voltage plateau at about 1.02 V (corresponding to the irreversible generation of SEI film and the partially reversible reduction of SnO2 to Li2O and metallic Sn) during the first discharge profile, subsequent a sloping curve down to the cutoff voltage of 0.01 V (corresponding to the reversible formation of a series of LixSn alloys, and lithium insertion into carbon further). Also it presents a sloping curve from about 0.8 V down to the cutoff voltage of 0.01 V during the first charge profile, corresponding to the reversible dealloying reaction of LixSn alloys. After the first cycle the following cycles show overlapping trends, further indicating superior cycling stability. The first discharge and charge capacities are 1348.6 and 794.5 mAh g−1, respectively, coupled with a coulombic efficiency of 58.9%. The capacity loss in the first cycle is commonly ascribed to the irreversible reactions such as formation of SEI film and just partially reversible reduction of SnO2 to Li2O and metallic Sn [33]. Fig. 8c shows the cycling performances of SnO2@C HNSs electrodes at 200 and 1000 mA g−1 between 0.01 and 3.0 V. It is
Table 1 The comparison of SnO2@C HNSs with the reported SnO2/C-based anode materials. SnO2/C-based anode materials
SnO2@C HNSs SnO2@C HNSs SnO2/C SnO2 NSs-graphene SnO2 NPs in CNTs SnO2/mesoporous carbon SnO2@C yolk–shell SnO2@C@MnO2 SnO2@C@VO2-3 GF@SnO2 NRAs@ PANI-40 SnO2–C SnO2/C
Potential cutoff (V)
Current density (mA g−1)
Cycles
Capacity (mAh g−1)
Reference
3.0–0.01 3.0–0.01 2.5–0.05 1.2–0.01 2.5–0.005 2–0.01
200 1000 300 400 50 100
100 100 350 50 50 50
707.8 483.2 653 518 560 473
This work This work [34] [35] [36] [37]
3.0–0.0 3.0–0.01 3.0–0.01 3.0–0.05
100 100 100 500
100 200 100 50
630 644.5 765.1 540
[38] [39] [40] [41]
3.0–0.005 2.0–0.05
500 500
100 75
ca. 610 464
[42] [43]
clear interplanar spacing of 0.33 nm is viewed in the HRTEM image (Fig. 4f) and, indexed to the d110-spacing of SnO2 [27], confirming the good crystallization of SnO2@C HNSs, which well matches with the XRD characterization results. Thus, it is testified that the peculiar SnO2@C HNSs is composed of carbon in-situ coating thin SnO2 hollow nanosheets. Therefore, to robustly determine the presence of carbon, the Raman characterization was carried out on SnO2@C HNSs, as shown in Fig. 5a. Obviously, the as-obtained Raman spectrum of SnO2@C HNSs presents two characteristic peaks of carbon, consisting of a D band of 1357 cm−1 and a G band of 1583 cm−1. The integral area ratio, ID/IG, is 1.38, indicating low crystallization, which is consistent with the result of XRD pattern [22]. The mass content, carbon in SnO2@C HNSs, is evaluated to be about 48 wt%, based on the TG analysis as displayed in Fig. 5b. To evaluate the specific surface area of SnO2@C HNSs, BET N2 adsorptiondesorption analysis was achieved. As seen from the as-recorded nitrogen adsorption and desorption isotherms in Fig. 5c that the SnO2@C HNSs has a large BET surface area of 228.9 m2 g−1, thus large BET surface area should be attributed to its individual thin 2D hollow nanostructure. Additionally, to highlight the synergistic effect of SnO2 nanosheets and carbon coating on the electrochemical performance of SnO2@C HNSs, control samples of pure SnO2 hollow nanosheets (SnO2 HNSs) and pure carbon hollow nanosheets (CHNSs) were prepared. Fig. 6a gives the TEM image of SnO2 HNSs. It can be viewed that the SnO2 HNSs well retains the 2D nanostructure of SnO2@C HNSs after removed the carbon via calcination of SnO2@C HNSs in air atmosphere. Interestingly, some of hollow structures are also maintained, as indicated by red arrows in Fig. 6a. In case the tin (II) chloride dehydrate is absent during the preparation process of SnO2@C HNSs, the CHNSs can be obtained. Fig. 6b depicts the TEM image of as-obtained CHNSs. It also has a 2D hollow nanostructure, as indicated by red arrows. It is found that the structure of CHNSs is not as good as SnO2@C HNSs,
Fig. 9. (a, b) TEM images of SnO2@C HNSs electrode at 200 mA g−1 after 100 cycles; (c) The lithium storage schematic of SnO2@C HNSs electrode. 1382
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Acknowledgments
thus evident that the SnO2@C HNSs exhibits outstanding cycling performance, delivering high capacities of 707.8 and 483.2 mAh g−1 after 100 cycles at 200 and 1000 mA g−1, respectively. Whereas, the two control samples show worse lithium storage performances than that of SnO2@C HNSs at 200 mA g−1. The CHNSs reveals good cycling stability, but the capacities are low. For SnO2 HNSs, the cycling stability is terrible and the capacities quickly decay from 1724.7 mAh g−1 in first cycle down to 199.4 mAh g−1 in 100th cycles due to the well-known structural damage resulted from the larger volume variation during long-term discharge/charge process. The comparative result of these three samples identifies that the well-designed synergistic effect of thin SnO2 hollow nanosheets and carbon coating plays an important role in achievement of outstanding electrochemical performances for SnO2@C HNSs. Followed by Fig. 8d shows the Nyquist plots of these three samples. It is thus obvious that the SnO2@C HNSs and CHNSs possess higher conductivity than that of SnO2 HNSs due to the charge transfer impedance is proportional to the diameter of the depressed semicircle in high frequency. It is testified that the carbon coating is able to improve the conductivity of SnO2@C HNSs. Hence, Fig. 8e depicts the rate capacity of SnO2@C HNSs, giving 757.4, 636.3, 606.9, 564.4, 512.3 and 476.8 mAh g−1 in 5th cycle at 0.1, 0.3, 0.5, 1.0, 2.0, and 3.0 A g−1, respectively, as well as a capacity of 731.8 mAh g−1 can be restored after current is set back to 100 mA g−1. The good rate capabilities should be attributed to the improved conductivity of SnO2@C HNSs electrodes. Besides, the lithium storage properties of SnO2@C NSs are comparable to and even better than the previously state-of-the-art SnO2/C-based composites, as seen in Table 1. To further insight into the structural stability of SnO2@C HNSs during cycling process, TEM characterization was achieved to observe the structure variation of SnO2@C HNSs electrode after 100 cycles at 200 mA g−1, as seen in Fig. 9(a, b). It is thus clear that the bent 2D nanostructure is well retained but the hollow interior becomes ambiguous which resulted from the large volume change of SnO2 hollow nanosheets during long-term cycling process, determining superior structural stability. Thus, the outstanding performance of SnO2@C HNSs should be attributed to the well-designed synergistic effect of thin SnO2 hollow nanosheets and carbon coating (Fig. 9c), which offers the SnO2@C HNSs improved electrochemical kinetics and robust structural stability owing to two key aspects: one hand, the good conductive and flexible carbon coating can not only improve the conductivity of whole electrode but also buffer the large volume change; on the other hand, the thin 2D hollow nanosheet architecture can not only offer more lithium-insertion channels owing to the great efficient surface active sites, but also allow ultrafast surface lithium storage owing to the shortened transfer pathway of lithium ions and electrons, as well as enhanced structural stability owing to the sufficient extra free space for relieving the structural strain and mitigating the large volume variation.
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4. Conclusions In view of the unique structural properties and surface characteristics of thin 2D hollow nanostructures, a peculiar SnO2/C composite composed of carbon in-situ coating thin SnO2 hollow nanosheets has been prepared through a well-designed strategy. It is demonstrated that thus peculiar architecture offers the as-prepared SnO2/C composite superior electrochemical kinetics and outstanding structural stability owing to the well-designed synergistic effect of thin SnO2 hollow nanosheets and carbon coating. Consequently, the as-prepared SnO2/C composite presents outstanding electrochemical lithium storage performance, delivering 707.8 and 483.2 mAh g−1 after 100 cycles at 200 and 1000 mA g−1, respectively, as well as superior rate capabilities. Therefore, this unique SnO2/C composite could be expected to use as an advanced anode for next generation of lithium-ion batteries. Moreover, this work may open up a route to facile preparation of complex thin 2D hollow nanostructures of SnO2/C composites. 1383
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