SnO2 Heterostructures Wrapping on CNFs as an

Oct 21, 2016 - *(L.C.) E-mail: [email protected]., *(C.L.) E-mail: [email protected]. ... with Carbon Coating: A High-Rate and Cycle-Stable Anode Mater...
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Carbon Coated SnS/SnO2 Heterostructures Wrapping on CNFs as an Improved-Performance Anode for Li-Ion Batteries: Lithiation-Induced Structural Optimization upon Cycling Qingwang Lian,† Gang Zhou,† Xiaohui Zeng,† Chen Wu,† Yuehua Wei,† Chao Cui,† Weifeng Wei,† Libao Chen,*,† and Chengchao Li*,‡ †

State Key Laboratory of Powder Metallurgy, Central South University, Changsha 410083, China School of Chemical Engineering and Light Industry, Guangdong University of Technology, Guangzhou 523000, China

‡

S Supporting Information *

ABSTRACT: Carbon coated SnS/SnO2 heterostructures wrapping on carbon nanofibers (C@SnS/SnO2@CNFs) was demonstrated to have excellent performance as an anode material for Li-ion batteries. C@SnS/SnO2@CNFs electrode delivers high reversible capacity of 826.8 mA h g−1 (500th cycle) at the current density of 1.0 A g−1. However, an interesting phenomenon of increasing capacity on cycling can be observed. According to the analysis of the evolution of structure and electrochemical property, C@SnS/SnO2@CNFs is demonstrated to experience the progress of conversion from nanowalls containing polycrystals into amorphous nanosheets with high porosity and larger surface upon cycling. The above lithiation-induced structural optimization provides larger effective surface areas and encourages the conversion reactions, which can promote charge transfer and also enhance the reversibility of the conversion reactions of SnS and SnO2 inducing the increasing reversible capacity. The study explains the progress of increasing capacity of C@SnS/SnO2@CNFs and likewise provides a perspective on optimization of the electrochemical performance of electrodes. KEYWORDS: C@SnS/SnO2@CNFs, increasing capacity, lithium-induced structural optimization, lithium-ion battery, carbon coated, heterostructures

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INTRODUCTION

heterostructures can form p−n heterojunctions which are able to encourage charge transfer.34 Many researchers have found the abnormal phenomenon of increasing capacity on cycling in the conversion materials. Yuan et al. had found ever-increasing pseudocapacitance in novel RGO-MnO-RGO (RGO = reduced graphene oxide) sandwich nanostructures via microstructure evolution upon cycling.35 FeMoO4 nanorods were shown to convert from nanorods to amorphous sheets during cycling, which induced increased capacities on cycling.36 Mesoporous Co3O4 hollow spheres converted to amorphous mesoporous spheres composed of enlarged pores and flower-like nanosheets during activation, which resulted in increased capacities on cycling.37 These results suggest that a large volume effect would make some positive sense in structural optimization to improve electrochemical performance when the conversion materials have certain structures and chemical composition. Herein, we synthesized a unique 3D hierarchical nanostructure C@SnS/SnO2@CNFs (CNFs = carbon nanofibers). The 3D hierarchical structure can provide a large surface, which is able to adequately contact with the electrolyte to facilitate the

Research studies of rechargeable Li-ion batteries (LIBs) have become popular in recent years, in order to effectively exploit the electrical energy generated from sun and wind but also meet the higher expectation of the LIBs for EVs (electric vehicles) and digital products. For anode materials, alloy/ dealloy materials and conversion materials usually provide high capacity and high energy density, but with poor cycling which results from electrode pulverization and separation from the current collector by the large volume effect during discharge and charge.1 SnS and SnO2 are two kinds of important conversion electrode materials, with the advantages and defects demonstrated above. However, poor cycling can be improved to some extent through constructing nanostructures. For SnO2, there are nanobox,2 nanotube,3 nanosheet,4,5 and hollow nanostructure,6,7 etc. And toward SnS, there are also a variety of nanostructures, such as nanoflower,8 nanobelt,9 nanorod,10,11 and nanosheet, 12 etc. To improve the poor intrinsic conductivity of SnO2 and SnS, many researchers have studied the carbon based nanocomposites,12−19 carbon coated nanocomposites,10,20−28 and conductive polymer coated nanocomposites,29−33 etc. Interestingly, the band gaps of SnO2 and SnS are 3.8 and 1.3 eV, respectively, so that SnS/SnO2 © XXXX American Chemical Society

Received: August 18, 2016 Accepted: October 21, 2016 Published: October 21, 2016 A

DOI: 10.1021/acsami.6b10391 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces Scheme 1. Schematic illustration of the formation of the C@SnS/SnO2@CNFs

diffusion of Li+ between the electrolyte and electrode materials. The carbon coating the SnS/SnO2 heterostructures can improve the electrical conductivity. Meanwhile, SnS/SnO2 heterostructures have the ability to encourage charge transfer. The 3D hierarchical structure, the coated carbon, and the SnS/ SnO2 heterostructures can alleviate the large volume effect without detachment from the current collector and prevent agglomeration. The C@SnS/SnO2@CNFs electrode exhibits excellent electrochemical performance when evaluated as the anode of Li-ion batteries. Curiously, the reversible capacity increased obviously upon cycling. In order to clarify the mechanism, we had analyzed the evolution of structure and electrochemical property. We found out that the lithiationinduced structural optimization can encourage charge transfer and enhance the reversibility of the conversion reactions of SnS and SnO2.

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ymethylcellulose sodium), were mixed in a weight ratio of 8:1:1 in the mixture of DI water and ethanol. Coin cells for LIBs were assembled with lithium foil as the counter electrode, 1 M LiPF6 in EC (ethylene carbonate)−PC (propylene carbonate)−EMC (ethyl methyl carbonate) (1:1:1, vol %) as the electrolyte and Celgard 2400 as the separator. The galvanostatic charge and discharge behaviors of cells were performed on a battery testing system (LANHE CT2001A, Wuhan LAND Electronics Co., People’s Republic of China) between 0.01 and 3.0 V. The electrochemical impedance spectroscopy (EIS) measurements were performed on a Princeton PARSTAT 4000 (AMETEK Co. Ltd.) from 0.1 Hz to 100 kHz. Cyclic voltammetry (CV) was tested on an Arbin battery testing system.

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RESULTS AND DISCUSSION The preparation progress for C@SnS/SnO 2 @CNFs is illustrated in Scheme 1. Figure 1 shows the XRD patterns of the SnS2@CNFs, Cp@ SnS2/SnO2@CNFs and C@SnS/SnO2@CNFs. In the XRD

EXPERIMENTAL SECTION

All the chemical reagents used were of analytical grade. Preparation of Carbon Nanofibers. A DMF (N,N-dimethylformamide) solution of 8 wt % PAN (polyacrylonitrile, Mw = 150000) mechanically stirred for 24 h was electrospun into nanofibers under a flow rate of 0.2 mL h−1 at 10 kV with a tip-to-collector distance of 19 cm. Electrospun PAN nanofibers were preoxidized at 230 °C for 2 h. After that, the preoxidized nanofibers were treated at 600 °C in Ar for 1 h in order to carbonize the PAN. Preparation of SnS2@CNFs and SnS/CNFs. First, 2 mmol of SnCl4·5H2O and 8 mmol of TAA (thioacetamide) were dissolved in 30 mL of ethanol. Subsequently, the solution was mechanically stirred for 35 min. Then the solution was transferred into a 50 mL Teflonlined stainless steel autoclave. A 10 mg sample of CNFs was added into the above solution.The solution was heated at 180 °C in an oven for 12 h and cooled to room temperature. The as-prepared sample was rinsed with ethanol and deionized (DI) water, then dried at 60 °C in the oven, and named as SnS2@CNFs. The sample of SnS2@CNFs was calcined in a tube furnace at 500 °C for 4 h in the Ar/H2 (5%) mixture and transformed to be SnS/CNFs. Preparation of C@SnS/SnO2@CNFs. Then the sample of SnS2@ CNFs was added into a 30 mL solution dissolving 1 g of C6H12O6· H2O. The solution was transferred into a 50 mL Teflon-lined stainless steel autoclave and heated at 180 °C in an oven for 4 h. After cooling to room temperature, the obtained product was rinsed with DI water and ethanol and named Cp@SnS2/SnO2@CNFs (Cp = carbon precursor). Finally, the sample of Cp@SnS2/SnO2@CNFs was calcined in a tube furnace at 500 °C for 4 h in the Ar/H2 (5%) mixture and named C@SnS/SnO2@CNFs. As for C@SnS/SnO2@ CNFs-1, the preparation progress was the same as that for C@SnS/ SnO2@CNFs except that the C6H12O6·H2O was 3 g. Materials Characterization. The crystal structures of as-prepared materials were analyzed by X-ray diffraction (XRD; Rigaku D/Max2500, Cu Kα radiation, λ = 1.5406 Å). The morphologies and structures of samples were observed using a Nova Nano SEM230 field emission scanning electron microscope (FESEM) and a transmission electron microscope (TEM; JEOL JEM-2100F). The thermal gravimetric analysis was measured by a thermogravimetric analyzer (TGA; SHIMADZU DTG-60AH) in air at a heating rate of 10 °C/ min to 800 °C. Electrochemical Characterization. For fabrication of working electrodes, the active materials, acetylene black and CMC (carbox-

Figure 1. XRD patterns of SnS2@CNFs, Cp@SnS2/SnO2@CNFs, and C@SnS/SnO2@CNFs.

pattern of the SnS2@CNFs, all of the diffraction peaks are indexed as the pure hexagonal SnS2 structure. Compared with the SnS2@CNFs, some extra diffraction peaks can be assigned to the SnO2 structure and the other diffraction peaks are indexed as the hexagonal SnS2 structure in the XRD pattern of Cp@SnS2/SnO2@CNFs. After heat treatment, the diffraction peaks can be assigned to the SnO2 structure and the orthorhombic SnS structure in the XRD pattern of C@SnS/ SnO2@CNFs. Furthermore, no carbon-related peaks appear, indicating the carbon nanofibers and the carbon coated on the surface of SnO2 and SnS are amorphous. From the results of the XRD, we can easily find that part of SnS2 transformed to SnO2 when the SnS2@CNFs hydrothermal reacted with D-glucose. The following chemical reaction (eq 1) was suggested to be involved when part of SnS2 converted to SnO2.

B

SnS2 + H 2O → SnO2 + H 2S

(1)

SnS2 (s) + H 2(g) → SnS(s) + H 2S(g)

(2)

DOI: 10.1021/acsami.6b10391 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 2. FESEM images of carbon nanofibers (a), SnS2@CNFs (b, c), Cp@SnS2/SnO2@CNFs (d), and C@SnS/SnO2@CNFs (e, f).

After Cp@SnS2/SnO2@CNFs at 500 °C for 4 h in the Ar/H2 (5%) mixture, the remaining SnS2 converted to SnS (eq 2). The carbon precursor coating the SnS2 and SnO2 was carbonized through the calcination. Figure 2 shows the morphology and microstructure of the carbon nanofibers, the SnS2@CNFs, Cp@SnS2/SnO2@CNFs, and C@SnS/SnO2@CNFs were investigated via FESEM. Figure 2a shows a typical image of the carbon nanofibers obtained by the electrospining method. The carbon nanofibers are about 200 nm in diameter, with the smooth surface. Figure 2b shows that SnS2 nanowalls cover on the surface of carbon nanofibers uniformly. In Figure 2c, SnS2 nanowalls interconnect adjacently with each other and grow densely around the carbon naonofiber, with a smooth surface and edge. Compared with SnS2@CNFs, the morphology of Cp@SnS2/SnO2@CNFs is almost the same as that of SnS2@CNFs except that the surface and edge of Cp@SnS2/SnO2@CNFs are uneven, as shown in Figure 2d. Some nanoparticles on the surface and edge are

suggested to result from SnO2 converting from part of SnS2 and the formation of carbon precursor. The FESEM image of the C@SnS2/SnO2@CNFs with low and high magnification is shown in Figure 2e,f, respectively. There are not aggregated large particles after calcination, in Figure 2e. The nanowalls of carbon coated SnS/SnO 2 heterostructures grow densely around the carbon nanofiber, as shown in Figure 2f. Nanowalls have uneven surface and edge where some nanoparticles can be observed. In addition, there are a few carbon nanospheres among the nanowalls. The nanowalls of carbon coated SnS/SnO2 heterostructures have a large surface that can increase electrode−electrolyte contact area and accelerate Li+ transport between electrolyte and active materials. The carbon coated SnS/SnO2 can accelerate the diffusion of electrons and also alleviate the volume effect caused by the insertion and extraction of lithium ion. A few carbon nanoparticles among the nanowalls also can enhance the electricity. The gaps among the nanowalls can also accommodate the volume expansion and provide enough space to C

DOI: 10.1021/acsami.6b10391 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 3. FESEM images of SnS2 synthesized without CNFs (a), Cp coated SnS2 (b), SnS/CNFs (c), and the C@SnS/SnO2@CNFs-1 (d).

Figure 4. TEM (a, c), HRTEM (b), and element mapping (d−g) images of C@SnS/SnO2@CNFs before cycling. The inset is the SAED patterns of the corresponding area circled in panel a.

CNFs. After SnS2 hydrothermal reacted with 1 g of D-glucose, as-synthesized Cp coated SnS2 were aggregated compared with the flower-like SnS2, as shown in Figure 3b. Comparing the morphology between the Cp coated SnS2 and Cp@SnS2/ SnO2@CNFs, we can find that Cp@SnS2/SnO2@CNFs remains the general morphology as SnS2@CNFs without agglomeration, which indicates that the unique nanostructure is able to remain the morphology after coating carbon precursor. It is clear that SnS2 nanowalls exfoliated from the carbon nanofibers and grown into bulk monocrystallines SnS during heat treatment at 500 °C for 4 h in the Ar/H2 (5%) mixture, as

facilitate the diffusion of electrolyte into the inner region of the electrode. From the TGA (Supporting Information Figure S1), the amounts of CNFs and coating carbon in the C@SnS/SnO2@ CNFs are about 17.90 and 16.42 wt %, respectively. From the EDS spectrum (Figure S2), the ratio between SnS and SnO2 is about 1.7 in the SnS/SnO2 heterostures. Figure 3 shows the FESEM images of the SnS2 synthesized without CNFs, Cp coated SnS2, SnS/CNFs, and C@SnS/ SnO2@CNFs-1. Figure 3a shows a typical image of the flowerlike SnS2 obtained by the solvethermal method without adding D

DOI: 10.1021/acsami.6b10391 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 5. (a) CV curves of C@SnS/SnO2@CNFs between 0.01 and 3.0 V at a scanning rate of 0.1 mV/s. (b) Voltage profiles of C@SnS/SnO2@ CNFs at a current density of 200 mA g−1. (c) Cycling performance of C@SnS/SnO2@CNFs, and SnS2@CNFs at a current density of 200 mA g−1 between 0.01 and 3.0 V. (d) EIS of C@SnS/SnO2@CNFs before cycling and after 200 cycles at 0.2 A g−1. (e) High rate cycling performance of C@ SnS/SnO2@CNFs (0.2 A g−1 for the first five cycles, 1.0 A g−1 for the next 500 cycles). (f) Rate capability of C@SnS/SnO2@CNFs, SnS/CNFs, and SnS2@CNFs.

and O distribute in the whole structure uniformly, which confirms that SnS and SnO2 distribute in the whole structure uniformly. Figure 5a shows the CV curves of C@SnS/SnO2@CNFs electrode for the first three cycles and after 200 cycles at 200 mA g−1. For the first discharge curve, a weak reduction peak appears at 1.56 V, corresponding to the formation of LixSnS which lithium ions intercalate to SnS layers.32 The obvious cathodic peaks at 1.16 and 0.84 V are ascribed to the conversion reaction of LixSnS and the formation of SEI film, respectively. The slope plateau ranging from 0.67 to 0.38 V can be attributed to the reduction of SnO2, and the partial slope plateau is due to the alloying between Sn and Li. The SEI film also forms between 0.67 and 0.01 V in the first cathodic curve. During the following charge curve, the oxidation peaks at 0.58, 0.65, and 0.81 V can be due to the dealloying of LixSn.19 The weak oxidation peak at about 1.33 V in the first several cycles can be ascribed to the partial delithiation of Li2S (eq 3) and Li2O (eq 4).6,15,38,39 The anodic peak at 1.91 V can be attributed to Li+ extracting from LixSnS. After the first CV curve, the reduction peak at 0.52 V in the discharge curve can be ascribed to conversion of SnO2 into Sn. After 200 cycles, an oxidation peak at 1.33 V is obviously strengthened, which

shown in Figure 3c depicting SnS/CNFs. Different from SnS/ CNFs, C@SnS/SnO2@CNFs shown in Figure 2e,f still generally remained the morphology before calcination, which was ascribed to the carbon coating tin compound inhibiting the SnS from agglomeration and achieving the decomposition from SnS2 into SnS in situ. In Figure 3d, many carbon nanospheres and the obviously thicker nanowalls compared with C@SnS/ SnO2@CNFs were observed, which was caused by too much Dglucose added. The detailed microstructure features of C@SnS/SnO2@ CNFs were further elucidated using transmission electron microscopy (TEM). As shown in Figure 4a, nanowalls composed of many nanocrystals grow densely and radially on the carbon nanofiber to form the unique 3D structure. The pattern from selected area electron diffraction (SAED) further confirms that the nanowalls are polycrystalline structures (Figure 4a, inset). The high resolution TEM (HRTEM) image presented in Figure 4b reveals the (110) plane of SnO2 and (120) plane of SnS, with corresponding interplane spacings of 0.335 and 0.342 nm, respectively. Figure 4d reveals that element C distributes in the whole structure but is more in the center of the structure, which could be ascribed to the existence of the carbon nanofiber. Panels e−g of Figure 4 show that Sn, S, E

DOI: 10.1021/acsami.6b10391 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 6. TEM (a) and HRTEM (b−d) images of C@SnS/SnO2@CNFs after 200 cycles at a current density of 200 mA h g−1. The inset is the SAED patterns of the corresponding area circled in panel a.

electrolyte, could be the reasons for the disappointing electrochemical performance. Figure 5f shows the rate capability of C@SnS/SnO2@CNFs, SnS/CNFs, and SnS2@CNFs. With increasing the current density to 0.2, 0.5, 1.0, 1.5, and 2.0 A g−1, the reversible capacities of C@SnS/SnO2@CNFs are about 758.3, 631.1, 538.4, 451.0, and 398.1 mA h g−1. When the current density returns to 0.2 A g−1, the reversible capacity remains 730.1 mA h g−1. Compared with other two electrode materials, C@SnS/ SnO2@CNFs is much superior to SnS2@CNFs and has a better capacity retention than SnS/CNFs. Its excellent rate capability can be ascribed to its unique structure. The 3D hierarchical structure is able to adequately contact with the electrolyte to facilitate the diffusion of Li+. The coated carbon can improve the electrical conductivity, and SnS/SnO2 heterostructures can encourage charge transfer.34 The 3D hierarchical structure, the coated carbon, and the heterostructure of SnS/SnO2 can alleviate the volume effect and prevent agglomeration, so that C@SnS/SnO2@CNFs has good capacity retention. To investigate the phenomenon of increasing capacity, the structures of the electrode materials after cycling were observed through TEM. Figure 6a shows the TEM image of the morphology of C@SnS/SnO2@CNFs after cycling 200 cycles at a current density of 200 mA g−1. The obvious nanocrystals in Figure 4a cannot be observed in Figure 6a. After cycling 200 cycles, the nanowalls converted to the thinner and larger nanosheets with high porosity, which wrapped around the carbon nanofibers. The pattern from SAED confirms that the nanosheets are amorphous structures (Figure 6a, inset). The conversion of nanowalls into nanaosheets can be ascribed to the huge volume change caused by the insertion and extraction of Li+, and also benefit from carbon coated SnS/SnO2 preventing the crystals from agglomeration. The HRTEM image of the active materials cycling 200 cycles is shown in Figure 6b−d. Figure 6b shows many mesopores and micropores. In Figure 6c, the SnS nanocrystal not completely amorphized is observed. The amorphous nanosheets wrapping on the CNFs can be found in Figure 6d. From the TEM analysis of the electrode materials before cycling and after 200 cycles, the increasing capacity can be ascribed to the progress of the conversion of electrode materials

indicates that the reversibility of the conversion of SnS (eq 3) and SnO2 (eq 4) are significantly enhanced after cycling 200 cycles. In Figure 5b, compared with several charge curves of first, second, 50th, 100th, and 200th cycles, the plateau at about 1.33 V becomes obvious and the curve above 2.0 V flattens along with cycling, which indicate the reversibility of the conversion of SnS (eq 3) and SnO2 (eq 4) are significantly enhanced along with cycling. Li 2S + Sn ↔ SnS + 2Li+

(3)

2Li 2O + Sn ↔ SnO2 + 4Li+

(4)

Figure 5c presents the cycling performances of C@SnS/ SnO2@CNFs and SnS2@CNFs at a current density of 200 mA g−1 between 0.01 and 3.0 V. The initial discharge and charge capacity of C@SnS/SnO2@CNFs are 1265.3 and 917.9 mA h g−1. The initial Coulombic efficiency for C@SnS/SnO2@CNFs is as high as 72.54%. After the initial several cycles, the Coulombic efficiency is higher than 98.5% in the sequent cycles. The discharge capacity gradually increases on cycling up to 1398.1 mA h g−1 in the 200th cycle. As shown in Figure 5d, the impedance resisitance of C@SnS/SnO2@CNFs electrode decreases obviously after 200 cycles. The Nyquist plots show only a semicircle in the high-frequency range, which is ascribed to the charge-transfer resisitance at the interface of electrolyte/ electrode. In this equivalent circuit, the charge-transfer resisitance (Rct) of C@SnS/SnO2@CNFs before cycling and after 200 cycles are 590 and 112.8 Ω, respectively, which indicate that structural optimization improves the electron transfer and the diffusion of Li ion.40,41 Figure 5e exhibits the high rate cycling performance of C@SnS/SnO2@CNFs at the current density of 1.0 A g−1. The reversible capacity faded from 508.2 (seventh cycle) to 352.2 mA h g−1 (150th cycle), which was ascribed to the formation of unstable SEI films.37 It is interesting that the capacity gradually increased to 826.8 mA h g−1 in the 500th cycle. In contrast, SnS2@CNFs exhibits low capacities and poor cycling performance as shown in Figure 5c. The poor electricity confirmed by the bad rate performance and no coated carbon which cannot protect SnS2 nanowalls from the erosion of F

DOI: 10.1021/acsami.6b10391 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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from nanowalls containing polycrystals into thinner and larger amorphous nanosheets because of the large volume effect during charge and discharge. The 3D hierarchical structure, the coated carbon, and the heterostructures of SnS/SnO2 not only can enhance the electricity but also can alleviate the volume effect and prevent agglomeration,42 which benefits conversion into amorphous nanosheets by utilizing the volume effect. As shown from Figure 6a, nanosheets have amorphous structure, with high porosity and large surface, which not only can enhance the electricity and the diffusion of Li+ but also can excellently enhance the reversibility of the conversion of SnS (eq 3) and SnO2 (eq 4), confirmed by Figure 5a,b. The increasing capacity can be ascribed to the enhancing reversibility of the new mechanism (eqs 3 and 4) caused by the structural optimization on cycling. In Figure 6c, the SnS nanocrystal not completely amorphized is observed, which can also confirm the reversibility of the conversion of SnS. The defects of the nanosheets also can store Li ions,43 inducing the discharge and charge capacity higher than the initial discharge and charge capacity. Moreover, SnS/CNFs electrode also appears to have increasing capacity on cycling. In Figure S3a, SnS/CNFs electrode shows a high initial Coulombic efficiency of 83.47%, with the discharge capacity of 1032.2 mA h g−1 and the charge capacity of 861.6 mA h g−1. The reversible capacity fades to 448.6 mA h g−1 (79th cycle) and has a steady cycle to the 127th cycle. Subsequently, the reversible capacity increases to 763.2 mA h g−1 (290th cycle). With analysis of the TEM images (Figure S4) of SnS/CNFs before cycling and after 290 cycles, SnS transformed from bulk to nanoparticles in morphology and converted from monocrystalline to amorphous structure in crystallography on cycling, similar to the change of C@SnS/ SnO2@CNFs. As shown in Figure S3b, the plateau at about 1.33 V becomes evident and the curve above 2.0 V of the charge curves slows upon cycling, which indicate that reversibility of the conversion reaction of SnS is enhanced upon cycling. The morphology optimization provides a large active specific surface and the amorphization can boost the conversion reaction,44 which could contribute to the enhancing reversibility of the conversion reaction of SnS upon cycling.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b10391. TGA curves of the SnS2@CNFs and C@SnS/SnO2@ CNFs, EDS spectrum of C@SnS/SnO2@CNFs, cycling performance of SnS/CNFs at a current density of 200 mA g−1 between 0.01 and 3.0 V, discharge and charge curves of SnS/CNFs electrode operated from 0.01 to 3.0 V at a current density of 200 mA h g−1, and TEM images of the SnS/CNFs before cycling and after cycling 290 cycles at a current density of 0.2 A g−1 (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*(L.C.) E-mail: [email protected]. *(C.L.) E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research work has been financially supported by the National Natural Science Foundation of China (Grants 21373081 and 21303047), the Hunan Provincial Natural Science Foundation of China (Grant 14JJ3067), the Program for Shenghua Overseas Talents from Central South University, the Project of Innovation-Driven Plan in Central South University, and the Self-Established Project of the State Key Laboratory of Powder Metallurgy.

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REFERENCES

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CONCLUSION C@SnS/SnO2@CNFs composite is evaluated as an anode material for Li-ion batteries. The 3D hierarchical structure, the coated carbon, and the heterostructures of SnS/SnO2 not only can enhance the electricity but also can alleviate the volume effect and prevent agglomeration, which are beneficial to convert from nanowalls containing polycrystals into amorphous nanosheets with high porosity and larger surface by utilizing the large volume effect during discharge and charge. The above lithiation-induced structural optimization can boost charge transfer and enhance the reversibility of the conversion reactions of SnS (eq 3) and SnO2 (eq 4). The reversible capacity of C@SnS/SnO2@CNFs electrode increases from 744.3 (eighth cycle) to 1381.6 mA h g−1 (200th cycle) at the current density of 0.2 A g−1 because of the lithiation-induced structural optimization. We demonstrated the increasing capacity on cycling is due to the lithiation-induced structural optimization by utilizing the large volume effect during discharge and charge, which provides a perspective for optimization of the electrochemical performance of electrodes. G

DOI: 10.1021/acsami.6b10391 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces

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DOI: 10.1021/acsami.6b10391 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX