Eco-Friendly Fabricated Porous Carbon Nanofibers Decorated with

Research Article pubs.acs.org/journal/ascecg

Eco-Friendly Fabricated Porous Carbon Nanofibers Decorated with Nanosized SnOx as High-Performance Lithium-Ion Battery Anodes Yuan Liu,† Xiaodong Yan,†,§ Yunhua Yu,*,†,‡ and Xiaoping Yang† †

State Key Laboratory of Organic−Inorganic Composites, Beijing University of Chemical Technology, Beijing 100029, China Changzhou Institute of Advanced Materials, Beijing University of Chemical Technology, Beijing 100029, China

‡

S Supporting Information *

ABSTRACT: In this work, one-dimensional polyvinylpyrrolidone-derived porous carbon nanofibers decorated with SnOx nanoparticles (denoted as SnOx@PCNFs) were prepared by an electrospinning technique, followed by a simple one-step heat treatment and a postetching process. The structural evolution of SnOx and the morphological change of the carbon nanofiber webs during the heat treatment are investigated by varying the content of the SnOx precursor in the electrospinning solutions. The highly interconnected pores, created by etching off the in situ generated SiO2 template in the carbon nanofibers, are beneficial for the easy penetration of Li+-carrying electrolyte into the nanocomposites and thus enable the direct contact between embedded SnOx nanoparticles and electrolyte. When tested as anode materials for lithium-ion batteries, SnOx@PCNFs with optimal SnOx component show outstanding initial reversible capacity of 1057 mA h g−1 at 0.2 A g−1, long cycling capability (511 mA h g−1 at 1 A g−1 after 900 cycles), and good rate performance (323 mA h g−1 at 2 A g−1). The remarkable electrochemical properties of the nanocomposites can be attributed to the highly interconnected pores, high surface area, and well-controlled SnOx nanoparticles. KEYWORDS: Polyvinylpyrrolidone, Porous carbon nanofibers, Tin oxides nanoparticles, Anode, Lithium-ion battery

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INTRODUCTION Lithium-ion batteries (LIBs) have been widely used in portable electronic devices for several decades, and their applications are expanding to the emerging markets, such as electric vehicles (EVs) and smart grids.1−4 To date, graphite is still the most successful commercial anode material for LIBs. Restricted by its low theoretical capacity of 372 mA h g−1, a graphite anode cannot satisfy the ever-increasing demands of next-generation LIBs. Thus, it is urgently necessary to develop new anode materials to improve storage capacity. Metallic tin and tin oxides (SnOx) have attracted considerable attention due to their high theoretical capacities (Sn, 994 mAh g−1; SnO, 1273 mAh g−1; and SnO2, 1494 mAh g−1).5,6 However, during lithium insertion and extraction processes, large volume expansion generally cannot be avoided in these Sn-based active materials, which causes the cracking and crumbling of the electrodes, and the continual formation of an unstable and insulating solid electrolyte interphase (SEI) layer, thus resulting in the loss of electrical contact between the active materials and current collector and consequently a fast capacity fading during cycling.5−10 Tremendous efforts have been devoted to improving the cycling stability of SnOx-based materials by maintaining their structural stability and integrity. One of the most effective approaches is to confine the nanostructured SnOx in a carbon matrix, which not only buffers the strain induced by the volume © XXXX American Chemical Society

change but also highly improves the electrical conductivity of the SnOx/carbon composite as a whole.11−14 Over the past years, SnOx nanoparticles embedded in electrospun carbon nanofibers (CNFs) have attracted much attention due to their unique one-dimensional structure, easy preparation, and scalability.5,6,15−25 A variety of polymer media,26 greatly affecting the physiochemical properties of as-synthesized CNFs,27,28 have been tried as the carbon sources. Among various polymers, polyacrylonitrile (PAN) is the most commonly used polymer medium for supporting SnO x nanoparticles,5,6,15−23,29 owing to its thermal stability or conformal transformation,27 in situ nitrogen doping,5,30 and excellent mechanical properties.31 However, PAN is more expensive, and its commonly used solvent, dimethylformamide (DMF), is harmful to human beings,27 making the PAN/DMF electrospun system less attractive for industrial mass production. A water or ethanol/polymer system is a better choice to avoid potential risk and lower the production cost.24,25,27 For example, Zou and co-workers used poly(vinyl alcohol) (PVA) as the polymer medium and SnCl2·H2O as the SnOx precursor to prepare well-controlled SnOx/C composite nanofibers as Received: October 6, 2015 Revised: April 8, 2016

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DOI: 10.1021/acssuschemeng.5b01236 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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ACS Sustainable Chemistry & Engineering

Figure 1. Schematic illustration of the synthesis process of SnOx@PCNFs. ethylhexanoate/TEOS fiber webs were carbonized at 600 °C for 1 h at a heating rate of 3 °C min−1 under nitrogen atmosphere to prepare SnOx/SiO2@CNFs nanocomposites. Finally, the SnOx@PCNF nanocomposites were obtained by using NaOH solution to remove the SiO2 template. According to the content of tin(II) 2-ethylhexanoate added to the precursor solutions, the as-prepared samples were denoted as PCNFs, SnOx@PCNFs-1, SnOx@PCNFs-2, and SnOx@ PCNFs-3. Characterization. The crystal structure and composition of the sample were investigated by wide-angle X-ray diffraction (XRD) (WAXD, D8 Advance, Bruker, Cu Kα, λ = 0.154 nm). Thermogravimetric analysis (TGA) was conducted on a TGA instrument (TA-Q50, America) at a heating rate of 10 °C min−1 from 25 to 800 °C in air. The morphology and mircrostructure of these samples were observed using a field emission scanning electron microscope (FE-SEM, Supra55, Carl Zeiss) and a high resolution transmission electron microscope (HR-TEM, Tecnai G2 F30 S-TWIN), respectively. Nitrogen adsorption and desorption isotherms were carried out by a Quantachrome SI instrument at 77 K, and the pore-size distribution was calculated using the nonlocal density functional theory (NLDFT) method. Electrochemical Measurements. Electrochemical measurements were investigated using 2025-type coin cells which were assembled in an Ar-filled glovebox (OMNI-LAB, [O2] < 0.02 ppm, [H2O] < 0.02 ppm). The SnOx@PCNFs samples were first mixed with carbon black and poly(vinylidene difluoride) (PVDF) to form a slurry at a weight ratio of 7:2:1 in N-methyl pyrrolidone (NMP). Subsequently, the working electrodes were prepared by coating the slurry onto nickel foams through a doctor blade method and then dried in a vacuum oven at 120 °C overnight. Lithium metal foil was used as the counter electrode and a Celgard 2300 membrane was used as a separator. The electrolyte for the tests was 1 M LiPF6 in ethylene carbonate (EC)/ dimethyl carbonate (DMC) (1:1 v/v). Cyclic voltammetry (CV) measurements were performed between 0.005 and 3 V using an Autolab PGSTAT 302 N (Metrohm) workstation with a scan rate of 0.1 mV s−1. The charge/discharge curves were obtained between 0.005−3.0 V on a Land CT2001A (China). The mass loading of the active material (1.2 mg), including both SnOx and PCNFs, was taken into account when the capacities were calculated. Electrochemical impedance spectra (EIS) measurements were also conducted at the same electrochemical workstation with an amplitude of 10 mV and a frequency in the range of 10 kHz to 0.1 Hz.

anode materials for LIBs.24 Further, they introduced porous structure in their SnOx/C composite nanofibers using an icetemplating method,25 as the pore in the CNFs is versatile in enhancing the overall electrochemical performance. 32−34 Polyvinylpyrrolidone (PVP) is another polymer that is aqueous soluble and has been widely used in industry due to its merits of low cost, nontoxicity, and good compatibility with metallic precursors.35,36 Recently, the preparation of PVP-derived CNFs has been comprehensively investigated and used as lithiumstorage materials; however, the pure PVP-derived CNF electrode showed a small reversible capacity of ∼210 mAh g−1.27 Therefore, confining SnOx nanoparticles into PVPderived CNFs may be a promising strategy to obtain environmentally friendly, low-cost composite anode materials, and further higher-energy and higher-power density could be achieved by introducing interconnected pores into the CNFs to create large surface area that can offer more charge transfer.37 In this work, we report a simple route to synthesize porous carbon nanofibers containing ultrafine SnOx nanoparticles (denoted as SnOx/PCNFs) with a diameter of 3−5 nm by using PVP as the carbon precursor and tin(II) 2-ethylhexanoate as the SnOx precursor. The interconnected and channeled pores are introduced by etching off the SiO2 template that arises from the added tetraethyl orthosilicate (TEOS) in precursor solution. In addition, the structural evolution of SnOx with the increase of tin(II) 2-ethylhexanoate in PVP-derived CNFs during carbonization process was studied and was demonstrated to affect the electrochemical performance of asprepared SnOx/PCNF nanocomposites. When tested as the anodes for LIBs, the optimum SnOx@PCNFs electrode exhibited outstanding reversible capacity, long-time cycling capacity, and good rate performance, making PVP-derived porous carbon nanofibers a promising carbon matrix to support nanosized SnOx.

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EXPERIMENTAL SECTION

Synthesis of SnOx@PCNF Nanocomposites. In a typical synthesis, 1 g of polyvinylpylrrolidone (PVP, Mw = 1 300 000 g mol−1) was dissolved in 10 mL of ethanol and 3 mL of acetic acid at room temperature for 2 h under magnetic stirring. Eight millimoles of TEOS was added stepwise to the PVP solution. Then, a certain amount of tin(II) 2-ethylhexanoate (0, 1, 2, or 3 mmol) was added to the mixed solution and stirring conitnued for 0.5 h to obtain the precursor solution for electrospinning. During electrospinning, the precursor solution was delivered to the stainless steel needle by a syringe pump with a flow rate of 0.5 mL h−1 and electrospun at a voltage of 14 kV by a high voltage supply. The electrospun fibers were collected as a thin web on a roller wrapped by an aluminum foil. The rotation speed of the roller was 500 rpm, and the distance between the needle and the roller was 15 cm. The electrospun PVP/tin(II) 2-

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RESULTS AND DISCUSSION The SnOx@PCNFs nanocomposites were fabricated by removing the SiO2 template from the SnOx/SiO2@CNFs hybrid nanofibers, which were first prepared via an electrospinning technique and subsequent carbonization process in N2, as shown in Figure 1. The PCNFs substrate not only provides highly efficient channels for the penetration of electrolyte and thus shortens the lithium ion diffusion pathway by direct contact between the electrolyte and the embedded active SnOx nanoparticles32−34 but also facilitates the electron B


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Figure 2. (a) XRD patterns and (b) TGA curves of SnOx@PCNF nanocomposites.

transport between the adjacent nanoparticles. More importantly, PCNFs play a role in accommodating the volume change and preventing the aggregation of nanosized SnOx. The structural evolution of SnOx was investigated by the XRD measurement. As shown in Figure 2a, the XRD patterns of the SnOx@PCNFs series show a wide peak at about 24°, which demonstrates the amorphous nature of the PVP-derived PCNFs. No any characteristic diffraction peaks assigned to tin or its oxides was found in SnOx@PCNFs-1 due to their low content. Upon the addition of the SnOx precursor, the characteristic peaks for rutile SnO2 (JCPDS card no. 411445) appear in the pattern of SnOx@PCNFs-2, and the intensity of the corresponding peaks further grow as shown by the pattern of SnOx@PCNFs-3. Meanwhile, several peaks indexed to those of tetragonal Sn (JCPDS card no. 4-673) are also observed in the two XRD patterns and grow with the increase of tin content, indicating that some SnO2 nanoparticles have been reduced into large and well-crystallized Sn particles by carbon at 600 °C during carbonization.5 Interestingly, some high peaks indexed to Sn2O3 (JCPDS card no. 25-1259) can be also seen in the pattern of SnOx@PCNFs-3. Figure 2b shows the TGA curves of the three samples. From Figure 2b, it is seen that all of the samples experience two typical stages of weight loss: one is for the desorption of water vapor and the evacuation of the gaseous content of the samples in the temperature range of 25−200 °C; the other is for the removal of the carbon matrix from 200 to 550 °C.16 Assuming complete oxidation of the PCNFs and further conversion from SnOx to SnO2, based on the final weight ratios of SnO2 and carbon after TGA, the weight percentages of SnOx in SnOx@PCNFs-1, SnOx@PCNFs-2, and SnOx@PCNFs-3 are calculated to be a little less than 21.5, 35.1, and 40.9 wt %, respectively. Figure 3 shows the SEM images of the SnOx@PCNFs series. All of the samples show fibrous morphology with diameters of 200−300 nm. Meanwhile, it should be noted that the SnOx@ PCNFs-1 webs are severely cross-linked with nanofibers adhered to each other (Figure 3a). This is because the thermoplastic PVP with poor thermal stability will easily melt during the carbonization process, which is unfavorable for maintaining the original fibrous structure.27,28 Fortunately, due to the addition of a metallic precursor that can efficiently stabilize the PVP substrate, the fibrous structure of SnOx@ PCNFs-1 did not completely collapse and aggregate into large bulk after carbonization.28 As demonstrated by Figure 3b and c,

Figure 3. Low and high magnification SEM images of (a,d) SnOx@ PCNFs-1, (b,e) SnOx@PCNFs-2, and (c,f) SnOx@PCNFs-3.

with the increase of SnOx precursor content, both SnOx@ PCNFs-2 and SnOx@PCNFs-3 show a well-separated fibrous morphology without any clear cross-linking. To observe the internal structure of the samples, the fibrous webs were ground and then observed using SEM under high magnification. As shown in Figure 3d, e, and f, a porous structure was observed in the cross-section of a single fiber for all of the samples. The porous structure was further demonstrated by the HRTEM images of SnOx@PCNFs-2, as shown in Figure 4a and b. It can be seen that these pores are all channel-like, elongating along the axial direction of the fiber with a length over hundreds of nanometers. In addition, many SnOx nanoparticles with diameters of 3−5 nm can be found to disperse uniformly within the PCNF substrate (Figure 4c). The lattice fringes of some SnOx nanoparticles are clearly observed, with adjacent plane distance of 0.334 nm, corresponding to the (110) plane C


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Figure 4. (a−c) HR-TEM images of SnOx@PCNFs-2 and (d) STEM image of SnOx@PCNFs-2 and the corresponding element mapping images of (e) carbon, (f) oxygen, and (g) tin.

the SnOx nanoparticles sparsely distribute in the PCNF substrate, indicating the limited lithium-storage capacity of SnOx@PCNFs-1 due to the low content of high-capacity SnOx. As for SnOx@PCNFs-3 with high SnOx content shown in Figure 5c and d, some large aggregated SnOx particles are clearly found to locate on the surface of PCNFs, indicating an inhomogeneous phase evolution during carbonization. Figure 5e demonstrates the polycrystalline nature of the large SnOx particle. The magnified HR-TEM image (Figure 5f) indicates a lattice distance of 0.279 nm, which is assigned to the (101) planes of metal Sn; many SnO2 nanocrystals can be easily found near the metal Sn (Figure 5g). These facts combined with the XRD results suggest that once the pure amorphous Sn phase forms in the composite nanofibers during the carbothermal reduction process, it will melt out of the composite nanofibers to form large crystalline Sn particles and that the Sn on the surface of the exposed large crystal is easily oxidized to Sn2O3 and SnO2. Generally, the large particles on the fiber surface will easily crack and collapse without the protection of the carbon matrix during cycling, probably leading to inferior cycle performance.5,28 Detailed information on the porous structure in the SnOx@ PCNFs-2 sample was analyzed by nitrogen adsorption/ desorption measurements (Figure 6). The specific surface area of the sample is 324.4 m2 g−1 derived by the Brunauer− Emmett−Teller (BET) method. As expressed in Figure 6a, the mesopores make a much greater contribution to the total pore volume with a majority of the nitrogen uptake occurring at a relative pressure above 0.2, and especially the H1-type hysteresis loops, which is a typical characteristic of large-pore mesoporous materials. Figure 6b shows the pore size distribution of the SnOx@PCNFs-2 sample calculated by the DFT method. As shown in Figure 6b, the pores with diameters of greater than 2 nm dominate the pore structure, which agrees well with the nitrogen sorption/desorption isotherm. Most of the pores range from ∼3 to 150 nm, which is important for the easy permeation of the electrolyte across the relatively large

of rutile SnO2. The scanning transmission electron microscopy (STEM) image and the corresponding element mapping images of carbon, oxygen, and tin are shown in Figure 4d−g, which shows that SnOx nanoparticles are uniformly distributed within SnOx@PCNFs-2 along the 1D nanostructure. To confirm the excellent electrochemical performances of SnOx@PCNFs-2 with such a homogeneous architecture, we also observe the microstructures of SnOx@PCNFs-1 and SnOx@PCNFs-3 nanocomposites by HR-TEM (Figure 5). As the content of tin(II) 2-ethylhexanoate in precursor solution is only 1 mmol, it is easily shown by Figure 5a and b that a few of

Figure 5. (a,b) HR-TEM images of SnOx@PCNFs-1; (c−g) HR-TEM images of SnOx@PCNFs-3. D


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Figure 6. (a) Nitrogen adsorption−desorption isotherms and (b) the pore size distribution of SnOx@PCNFs-2.

Figure 7. CV curves of the first three cycles of (a) SnOx@PCNFs-1, (b) SnOx@PCNFs-2, and (c) SnOx@PCNFs-3 electrodes at a scan rate of 0.1 mV s−1 between 0.005 and 3.0 V; (d) the first three charge/discharge profiles of SnOx@PCNFs-2 electrode at the rate of 0.2 A g−1.

appearing at 0.55 and 1.20 V (marked by the red circle) correspond to the phase transitions from Li−Sn alloy and Li2O to SnOx, respectively.6,38,40,41 From Figure 7a to c, it is easily seen that the two electrochemical reaction humps increase in height with the increase of SnOx content in the three samples at the first anodic sweep. However, at the second anodic process, the two humps of SnOx@PCNFs-3 with the highest SnOx content weaken obviously, showing the inferior reversible capability of the phase transitions (Figure 7c). This is attributed to the fact that the large Sn or SnOx cluster on the surface of PCNFs will easily pulverize and detach from the conductive matrix during cycling, thus leading to the capacity fade.5,37,39 In

pores to the embedded nanoparticles, thus leading to a good rate performance and expected higher capacity.32−34 Figure 7a−c shows the CV curves of the initial three cycles of the SnOx@PCNFs electrodes at a scan rate of 0.1 mV s−1. For all three samples, the sharp cathodic peak observed in the first cycle at about 0.8 V (marked by blue circle) is mainly attributed to the decomposition of the electrolyte to form the solid electrolyte interphase (SEI) layer, and this peak disappears from the second cycle.5,6 Another cathodic peak from 0.75 to 0.005 V (marked by the green rectangle) is attributed to the alloying of LixSn as well as the intercalation of Li+ storage in the PCNFs.5,6,38 In the following anodic sweep, two humps E


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Figure 8. (a) Cycling performances of the three SnOx@PCNFs and PCNFs electrodes at 0.5 A g−1. (b) Rate performance of the SnOx@PCNFs-2 electrode; (c) long-term cycling performance of the SnOx@PCNFs-2 electrode at 1 A g−1 for 900 cycles.

contrast, the two humps of the first three cathodic/anodic cycles shown by Figure 7b are almost overlapped together, indicating a stable electrochemical reversibility of SnOx@ PCNFs-2 with Li+. In addition, the galvanostatic charge− discharge profiles of the SnOx@PCNFs-2 electrode (Figure 7d) present sloping lines during charge/discharge processes, which correspond to the broad peaks showed by the CV curves.5 The first charge capacity and discharge capacity of SnOx@PCNFs-2 are 1057 and 1529 mAh g−1, respectively, with a first Coulombic efficiency (CE) around 69.1%. Generally, such large initial capacity loss (23.1%) is mainly attributed to the irreversible formation of the SEI layer on the surface of the nanocomposites during the first discharge process. Figure 8a compares the cycling performances of the three SnOx@PCNF and PCNF electrodes at the rate of 0.5 A g−1. It is evident that the reversible capacities and cycling stabilities of the three electrodes are closely related to the content of the SnOx precursor. The SnOx@PCNFs-2 electrode with moderate SnOx content shows the highest reversible capacity (discharge capacity) of 684 mA h g−1 among the three electrodes after 100 cycles, which is 57.7% of the first discharge capacity (1185 mA h g−1). In sharp contrast, SnOx@PCNFs-3 shows a relatively inferior cycling performance. Despite its high initial discharge capacity (1298 mA h g−1), the discharge capacity of SnOx@ PCNFs-3 has decreased to 601 mA h g−1 after 100 cycles, with a low capacity retention of only 46.3%. The inferior cycling stability is ascribed to the fact that a large amount of SnOx particles in SnOx@PCNFs-3 separate from the PCNFs substrate to form aggregates external to the fibers, which easily pulverize and lose contact with the current collector during

lithiation/delithiation processes.5,6,39 For the SnOx@PCNFs-1 electrode with a low SnOx content, it exhibits the lowest initial discharge capacity of 1040 mA h g−1, maintaining 580 mA h g−1 after 100 cycles, which is still higher than 470 mA h g−1 of the as-prepared PCNFs electrode. Thus, the capacity retention of the SnOx@PCNFs-1 electrode is calculated to be 55.8%, which is close to the value of SnOx@PCNFs-2, demonstrating its relatively stable cycling performance as compared with the SnOx@PCNFs-3 electrode. Therefore, by tuning the adding content of the SnOx precursor, the SnOx@PCNFs-2 electrode with an optimum reversible capacity and stable cycling performance has been obtained. Moreover, because the 1D porous nanostructure can provide efficient channels for the easy penetration of the electrolyte into the nanocomposite electrodes and thus shorten the transport length of Li+ from the electrolyte to the internal active nanoparticles, the SnOx@ PCNF-2 electrode also presents a good rate performance. In Figure 8b, it is seen that SnOx@PCNF-2 shows the slightly decreased reversible capacities of 819, 639, 468, and 323 mA h g−1 at the various rates of 0.2, 0.5, 1, and 2 A g−1, respectively. Remarkably, when the current rate returns to 0.2 A g−1 after 40 cycles, a high stable capacity of 856 mA h g−1 can be still obtained after subsequent 10 cycles. The superior long cycling performance of the electrode was further demonstrated by testing the SnOx@PCNFs-2 electrode under the rate of 1 A g−1 for 900 cycles As shown in Figure 8c, the SnOx@PCNF-2 electrode still maintains the discharge capacity of 511 mA h g−1 even after 900 cycles. To the best of our knowledge, such a stable capacity after 900 cycles has not been reported by previous works on SnOx-based nanomaterials using PANF


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the embedded active nanoparticles, leading to the further improved Li storage capability of the overall electrode.

derived CNFs as the matrix. Table 1 shows the comparison of the capacities between our study and other SnOx-containing

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CONCLUSIONS In summary, a series of SnOx@PCNF nanocomposites have been successfully prepared by an eco-friendly electrospinning of PVP/ethanol solution with various amounts of the SnOx precursor, followed by the cost-effective one-step heat treatment and postetching process. By controlling the content of tin(II) 2-ethylhexanoate in the precursor solution, the optimum SnOx@PCNFs nanocomposite has been obtained. When used as anodes for LIBs, the optimum electrode shows a high reversible capacity of 684 mA h g−1 at the rate of 0.5 A g−1 after 100 cycles and an outstanding long cycling capability with a capacity of 511 mA h g−1 at the rate of 1 A g−1 even after 900 cycles as well as good rate performance. These excellent electrochemical performances are benefited from the synergetic effect of the conductive CNFs, abundant porous channels, and ultrasmall SnOx nanoparticles. Meanwhile, our works highlight the advantages of PVP-derived PCNFs as the substrate to support nanosized SnOx and thus make the SnOx@PCNFs nanocomposites potential materials for next-generation LIBs.

Table 1. Comparison of Our SnOx@PCNFs-2 with Other SnOx-Containing PAN/PVA-Derived CNFs Nanocomposites Reported by Previous Related Works 1D SnOx/C-based anode materials our work our work SnOx@PAN-derived CNFs Sn@PAN-derived CNFs Ti-doped SnOx@PANderived CNFs P-doped SnOx@PANderived CNFs Sn QDs@PAN-derived CNFs Sn@PAN-derived PMCTs SnOx@PAN-derived CNFs SnOx@PVA-derived PCNFs

current density (A g−1)

cycles

capacity (mA h g−1)

ref

0.5 1 0.5

100 900 200

684 511 608

5

0.025 0.2

30 60

450 670

15 16

0.2

100

676

17

0.4

200

508

19

0.1 0.5

140 100

648 430

20 21

0.03

40

510

25

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ASSOCIATED CONTENT

S Supporting Information *

PAN/PVA-derived CNFs nanocomposites. It can be seen that our SnOx@PCNF-2 presents the improved Li-storage capability and cycling performance compared with them.5,15−17,19−21,25 Figure S1 shows the Nyquist plots of the SnOx@PCNF-2 electrode after different cycles. It can be seen that the diameters of the semicircles in the middle-high frequency corresponding to the interfacial resistance of SnOx@PCNF-2 reduce during cycling. On the one hand, this is probably attributed to the delay permeation of the electrolyte into the sample.5,19 On the other hand, it suggests that the SEI layer formed during cycling should be stable and that the integrity of the overall electrode should be well maintained because the collapse of active materials will highly add to the resistance. 19,28,42 To demonstrate our speculation, the tested cell with the SnOx@ PCNF-2 electrode in the fully delithiated and lithiated states was disassembled, and the morphology of the nanocomposites was further investigated by using SEM (Figure S2). It easily can be observed from Figure S2 that both the fully delithiated and lithiated composite nanofibers, after the long-term cycling test, still have a well-maintained fibrous morphology and that the SEI layer on the fiber surface is uniform and thin. All of the results demonstrate that the PVP-derived PCNF substrate can effectively buffer the volume change of SnOx during cycling, thus maintaining the integrity of the overall electrode. On the basis of the aforementioned results, the excellent electrochemical performance of the as-prepared SnOx@PCNF2 electrode can be attributed to the following reasons. First, the ultrasmall size of SnOx can provide a large number of active sites for lithium storage and simultaneously a short pathway for the Li+ transport, thus leading to the high reversible capacity and rate performance. Second, the PCNF substrate not only offers a sufficient 1D conductive pathway for the electron transport but also buffers the volume expansion and prevents the aggregation of nanosized SnOx during cycling. More importantly, the porous channels along the nanofibers play an important role in facilitating the penetration of electrolyte into the PCNFs and therefore shortening the Li+-diffusion length to

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.5b01236. Electrochemical impedance spectra of the SnOx@ PCNFs-2 electrode after different cycles; and SEM images of the SnOx@PCNFs-2 electrode in fully delithiated and lithiated states after cycling (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Tel/Fax: +8610-6442-7698/2084. E-mail: [email protected]. edu.cn. Present Address

§ X.Y.: Department of Chemistry, University of Missouri Kansas City, Kansas City, Missouri 64110, United States.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by National Natural Science Foundation of China (Nos. 51072013, 51272021, 51142004, and 51402010) and Natural Science Foundation of Jiangsu Province (Nos. BK20131147 and BK20140270).

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REFERENCES

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