Stannous ions reducing graphene oxide at room temperature to produce SnOx-porous, carbon-nanofiber flexible mats as binder-free anodes for lithium-ion batteries

Key Laboratory for Micro-Nano Optoelect School of Physics and Microelectronics, Sta and Chemometrics, Hunan University, Ch zhangming@hnu.edu.cn State Key Laboratory for Power Metallurg 10083, P. R. China Department of Materials Science & Engine Washington, 98195, USA. E-mail: gzcao@uw Beijing Institute of Nanoenergy and Nan Beijing 100083, China † Electronic supplementary information samples, FT-IR spectra for rGO and GO DOI: 10.1039/c5ta02107c Cite this: J. Mater. Chem. A, 2015, 3, 12672


Introduction
Great attention has been devoted to lithium-ion batteries (LIBs) with high energy density and long cycle stability to meet increasing demands in many elds, 1 such as portable electronics 2 and electric vehicles, [2][3][4][5][6] as well as large-scale stationary energy storage systems. 4,7Nevertheless, the energy density, power density, and cyclic stability of commercial LIBs whose theoretical capacities are just 137 and 372 mA h g À1 for LiCoO 2 (50% delithiation) and graphite need to be further improved. 1in oxides/tin as anodes for LIBs have attracted considerable attention owing to their high theoretical specic capacities, 8 such as SnO 2 (781 mA h g À1 ), [9][10][11] SnO (875 mA h g À1 ) 12 and Sn (992 mA h g À1 ), 1,5,[13][14][15] whose theoretical capacities are 2.5 times higher than that of commercial graphite (372 mA h g À1 ). 13,16wever, tin oxides suffer from signicant capacity fading due to enormous volume changes during the lithiation/delithiation process. 5,10,17Several approaches have been developed to address this issue.8][19] This strategy, however, results in a relatively high irreversible capacity in the initial cycle, arising from formation of enlarged solid electrolyte interface (SEI) lms because of the high specic area of nanomaterials. 20Another approach is to integrate tin oxides with carbonaceous materials to accommodate their huge volume changes, including amorphous carbon, mesoporous carbon, graphene, carbon nanotubes (CNTs) or carbon nanober mats. 4,5For example, SnO 2 -graphene composites have been synthesized using tin salts and graphene oxide (GO) as raw materials by many methods, 11 such as NaBH 4 reduction, 21 hydrothermal growth, 22 and in situ deposition. 5Additional reductants are commonly used in the above routes to reduce GO.Although GO has been demonstrated to be reduced by stannous ions at a relatively high temperature without any reductants, 23,24 the reduction of GO by stannous ions at room temperature with little energy consumption has not yet been reported.In addition, GO remains highly resistive even aer reduction, which is not optimal for energy storage applications.GO can also control morphology and size of metal oxides. 25o realize the high power density and good cyclic stability of tin oxides based on composites as anodes for LIBs, they should possess two important features: high electrical conductance a Key Laboratory for Micro-Nano Optoelectronic Devices of Ministry of Education, School of Physics and Microelectronics, State Key Laboratory for Chemo/Biosensing and Chemometrics, Hunan University, Changsha, 410082, P. R. China.E-mail: zhangming@hnu.edu.cn and fast diffusion channels for Li + . 5,26,27Tin oxides/graphene composites usually have relatively high conductance due to the presence of graphene. 26,28However, diffusion of Li + may be blocked by the binder (e.g., PVDF, polyvinylidene diuoride) or stacking of graphene sheets in tin oxides/graphene composites. 10,29This issue could be partially solved by introducing some carbon nanoparticles between graphene sheets to enlarge the interspace. 30Li + diffusion blocked by binders cannot be easily improved without binder-free electrodes, 29,31 such as growth nanomaterials on nickel foams.However, the micrometer-sized nickel foam with relatively low specic surface area cannot load nanomaterials at sufficiently high content of active materials; thus, the great interest in exploring tin oxides for fabrication of binder-free anodes for LIBs. 29,31lthough modifying LIBs with either carbon bers or graphene is a useful strategy to maintain structural stability and improve electrochemical properties, the double-protection strategy of tin oxides by both carbon bers and graphene have not been reported.In this study, exible hierarchical mats of SnO x -graphene-carbon nanobers (SGCFs) were synthesized by reducing GO with stannous ions at room temperature, and followed by electrospinning and thermal treatment.The ultrasmall tin oxides with diameters of approximately 4 nm were anchored on carbon bers by chemical bonds.Even though the content of graphene and SnO x in the nanobers was very low, the exible mats exhibited a high specic capacity at a large current density, and excellent cyclic stability as binder-free anodes for LIBs.Such superior electrochemical results were ascribed to the carbon and graphene double-protection strategy and the ultra-small size of tin oxides.
Graphene oxide (GO) was prepared according to our techniques described in the literature. 29GO was dispersed in DMF at a concentration of about 0.4 mg mL À1 , and the suspension was processed with ultrasonic waves for 30 min.To prepare the precursor solution for electrospinning, 0.43 g PAN were dissolved in GO-DMF solution and vigorously stirred for 3 h at 30 C, forming the brown solution; 1.5 mmol SnCl 2 $2H 2 O were dissolved in DMF and vigorously stirred for 1 h at room temperature, forming the transparent solution in Fig. 1a.Next, the vigorously stirred solution was added in the brown solution with a dropper.To prepare comparative carbon bers, a precursor without GO was also prepared.A precursor for nanobers without graphene was prepared in a similar way, also without GO.All of the precursor solution was transferred into a 5 mL syringe with a stainless steel needle (with 0.71 mm inner diameter).A syringe pump controlled the ow rate of the precursor solution to about 0.2 mL h À1 .A piece of aluminum foil used as the collector was vertically positioned about 15 cm away from the needle.The needle was connected to a high-voltage DC power supply to obtain 11-14 kV.Under these conditions, PAN-SnCl 2 $2H 2 O and PAN-GO-SnCl 2 $2H 2 O nano-bers were generated and formed mats.Aer preoxidation at 230 C in air for 6 h, the resulting brown lms (PAN-GO-SnCl 2 $2H 2 O) were treated at 600 C and 700 C in argon for 2 h in order to carbonize the PAN and decompose SnCl 2 $2H 2 O, and the product is denoted as SGCF-600 and SGCF-700, respectively.The SnO x -C nanobers (SCFs) were treated in a similar way at 700 C in order to obtain the corresponding samples, and the product is denoted as SCF-700.

Material characterization
The morphologies of the samples were studied by a eld emission gun scanning electron microscope (Hitachi S-4800, 5 kV).Transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) investigations were carried out by a JEOL JEM-2100F microscope.The samples were also analyzed using X-ray photoelectron spectroscopy (XPS, Surface Science Instruments S-probe spectrometer).Elemental analysis of the samples was achieved using energy dispersive spectroscopy (EDS).The thermal gravimetric analysis was performed on a thermogravimetric analyzer (TGA, PerkinElmer, Diamond TG/DTA) in air with a heating rate of 3.5 C min À1 .The samples were heated from room temperature to 800 C.

Electrochemical measurements
The mats (including the SGCF-600, SGCF-700, and SCF-700) were directly used as binder-free anodes for electrochemical measurements towards the storage of Li + .A Celgard 2400 microporous polypropylene membrane was used as a separator to assemble coin cells (CR 2025).The electrolyte used was a solution of 1 M LiPF 6 in ethylene carbonate-dimethyl carbonate (1 : 1 by volume).Pieces of pure lithium foil were used as both counters and reference electrodes.All cells were assembled in an argon-lled glove box with the moisture and oxygen levels less than 1 ppm.Discharge and charge measurements were carried out using an Arbin BT2000 and a LAND battery testing system with cut-off potentials of 0.005 V for discharge and 3 V for charge.The cyclic voltammetry results were collected on an electrochemical workstation (CHI660B).

Synthesis mechanism of SnO x -graphene
A new strategy for preparing graphene by reducing GO with stannous ions was explored for the synthesis of SnO x -G nanocomposite.As shown in Fig. 1a, a light yellow DMF solution containing GO and polyacrylonitrile (PAN) were prepared (le) and SnCl 2 $H 2 O was dissolution in DMF to form a colorless solution (right).A brownish mixture, the typical color of GO, was achieved by mixing above two solutions, 23,24 as shown in Fig. 1b.The light brown color can be attributed to the dilution of colorless SnCl 2 $H 2 O solution.Then the mixed solution was stirred at room temperature.Aer stirring for 1 h, the solution (Fig. 1c) became darker as compared to the freshly mixed solution.When stirring for 10 h, the solution turned from brown to black, a typical color of graphene (Fig. 1e).In this solution, stannous ions have stronger reduction power than components such as DMF, PAN, H 2 O, and Cl À . 23,32The solution color transferring from light brown to black is an indication that GO was reduced by stannous ions at room temperature. 23The scheme in Fig. 1f presents the chemical reaction process of stannous ion reducing GO at room temperature and the formation of SnO 2 -graphene (SnO 2 -G) composites.A possible reaction mechanism for the reduction process is shown in eqn (1) and (2).First, the standard electrode potential in Sn 4+ /Sn 2+ acid (4 showing relatively strong reducibility.In addition, the standard electrode potential of Sn 4+ /Sn 2+ shows a lower value compared with the same value in acid in the base (4 Q B ¼ À0.93 V), indicating strong reducibility.Previous reports also demonstrated that stannous ions can reduce Ag + to silver metal. 33,34Second, owing to the strong reducibility of Sn 2+ , GO could be reduced to graphene and SnO 2 nanoparticles were xed onto the graphene sheets.The organic solvent may be a special factor in improving the reducibility of Sn 2+ at room temperature as compared with previous studies about SnCl 2 $2H 2 O reducibility in a water solution. 23,32Furthermore, the intensity of the C]O stretching peak at 1730 cm À1 in the FT-IR spectrum of rGO was diminished aer reduction (Fig. S2 †).A weak signal for the C-OH stretching vibration at 3420 cm À1 could be ascribed to the vibrations of the adsorbed water molecules.So it could be concluded that GO was reduced by the stannous ions. 23In addition, the diameters of resulting SnO 2 nanoparticles may be decreased by the oxygen-containing groups and the nonaqueous reaction system. 35

Synthesis and characterization of SGCF exible mats
Aer electrospinning of the precursor solution containing PAN and SnO 2 -G, the as-prepared bers (PAN-SnO 2 -G) were preoxidized in air followed by treatment at a high temperature in Ar.The electrospun bers with large length to diameter ratios are easily woven into mats without any binders. 29,31The resulting exible mats were hammered into circular lms with diameters of 13 mm, as shown in Fig. 2a and d.As-prepared PAN-SnO 2 -G bers without any obvious nanoparticles are shown in Fig. S1a, † indicating the ultra-small diameters of SnO 2 .Fig. 2b and e display the digital photos of the preoxidized samples treated at 230 C in air for 6 h.In spite of diameters reduced to 12 mm and color change from white to brown, the circular lms were without any obvious variation of shape and thickness.The corresponding SEM image shown in Fig. S1b † conrms the stability of bers.Aer thermal treatment at 700 C in Ar for 2 h, the nal black SGCF mats (Fig. 2c and d) without cracks are about 8 mm in diameter, showing good structural stability and exibility of the mats built by SGCFs.Fig. 2g is a typical low-magnication SEM image of the continuous SGCF-700 nanobers, which shows regular diameter in the range of 120-160 nm and length of several micrometers.In addition, the nanobers weave into a multilayer and hierarchical network without binders.An amplied SEM image of SGCF-700 is displayed in Fig. 2h, revealing that the surface of the SGCF-700 nanobers is smooth and uniform.Graphene is not directly observed in the SEM image, because graphene with an approximate concentration of 1.2 wt% (based on a carbonization yield of 45 wt% for PAN) is embedded in the carbon matrix pyrolysized from PAN.As a comparison experiment, no GO was added and the nal samples treated at 700 C were SnO x -C nanobers, which were marked as SCF-700 and shown in Fig. 2i (Fig. S1c †).The diameter of SCF-700 is a bit smaller than that of SGCF-700, and some cracked bers are found, although the bers also build a binder-free network.The exibility of SGCF-700 mats was tested by bending experiments, as shown in Fig. 2j-l.The SGCF-700 mat with a 180 degree bend was not broken (Fig. 2j).The degree of bending can decrease with the improvement of tweezer interspace (Fig. 2k).Aer the bending experiments, the mat without cracks is an integral structure, conrming the excellent exibility of SGCF-700 mats.
The detailed morphological and structural features of the SGCFs were also characterized by TEM.Fig. 3a shows a typical TEM image of SGCF-700.No aggregated SnO x nanoparticles are found on the smooth surface of the bers, indicating that the particles were embedded in SGCFs.This phenomenon can be conrmed by more TEM images (Fig. S3 †) at different magni-cations.The fracture of SGCFs could be ascribed to ultrasonication during sample preparation for the TEM test.The high-magnication TEM image is shown in Fig. 3b.The lighter color in the bers represents the carbon matrix and the darker color represents SnO x .This image reveals that the ultra-small nanoparticles with diameters of approximate 4 nm (black dots) were well dispersed in SGCFs, suggesting good compatibility between carbon and nanoparticles.The size of SnO x is very close to the perfect diameter of SnO 2 nanoparticles as anodes for LIBs reported in a previous paper. 36To further explore composite structure, high-resolution TEM (HRTEM) studies were carried out, as presented in Fig. 3c and d.An ultra-small SnO x was embedded in the SGCFs.Two planes with d-spacing of 0.275 nm in Fig. 3c and 0.26 nm in Fig. 3d were observed on the surface of SGCFs.Those values are highly consistent with the (011) planes of SnO and the (002) planes of SnO 2 , 5,37 indicating the presence of ultra-small SnO x nanoparticles in SGCFs and transformation of SnO 2 to SnO during the treatment at 700 C. The ultra-small size of SnO x nanoparticles could be attributed to the inhibition of carbon matrix and graphene on nanoparticle growth. 35The presence and distribution of Sn elements were further veried by TEM element mapping (Fig. S4a-d †).Fig. S4c † conrmed the homogenous distribution of Sn on CNFs.Other elements, such as C, N, and O, also were uniformly distributed on CNFs.Taking into account the TEM results, it can be concluded that ultrane SnO x nanoparticles are homogenously distributed on CNFs.
The chemical composition at the surface of SGCF-700 was also investigated by X-ray photoelectron spectroscopy (XPS).As shown in Fig. 4a, the Sn 3d 5/2 peak can be deconvoluted to two peaks.The peaks at 486.7 and 487.4 eV are ascribed to Sn 2+ and Sn 4+ , corresponding to atom ratios of 61.3% and 38.7%, respectively. 1,5This result is consistent with the TEM conclusion that there are two tin-based compounds in the SGCF-700.The O 1s spectrum is shown in Fig. 4b, which could be deconvoluted to four peaks.The peaks at 531.3, and 533.2 eV are assigned to oxygen in O 2 and H 2 O, respectively, which should be attributed to the adsorbed H 2 O and O 2 on the carbon matrix-graphite. 1 Another peak at 533.6 eV originated from C-O bonding indicates that the oxygen also bonds with carbon.Other peaks at 531.3 and 533.2 eV can be indexed to oxygen and water. 1 The remaining peak may be evidence of Sn-O bonding (532.4 eV), which was directly explained by the anchoring of ultra-small tin oxides with 4 nm diameters on carbon bers by chemical bonds. 1 The full XPS spectrum of SGCF-700 in Fig. 4c demonstrated the presence of nitrogen and carbon in SGCFs, but not tin and oxygen.The ne XPS spectrum of N 1s shows two peaks at about 398.4 and 400.8 eV that could be indexed to pyridinetype and conjugated nitrogen.Both of the above nitrogen types have positive effects on the storage of lithium ions, especially pyridine-type nitrogen. 29In addition, nitrogen doping could improve carbon conductivity, and result in enhanced properties of lithium ion storage. 1,4The EDS spectrum in Fig. 4d shows the presence of carbon, oxygen, tin, and nitrogen in SGCFs, which is  consistent with XPS results in Fig. 4c.The copper can be assigned to the sample holder for TEM characterization.Thus, SnO x nanoparticles (including SnO 2 and SnO) were chemically embedded in nitrogen-doped carbon bers.
Thermogravimetric analysis (TGA) was carried out to gain insight into the chemical composition of SGCF-700 in air; results are shown in Fig. 5. From the TGA curve, SGCF-700 has 68.22% weight loss from 300 to 633 C that can be assigned to combustion of graphene and carbon in air forming CO 2 . 4This carbon weight loss agrees well with XPS analysis results.The nal product of TGA is SnO 2 with weight of 33.8%.Considering the transformation of SnO to SnO 2 with weight increase of 11.9% and Sn 2+ with an atomic ratio of 61.3%, the weight ratio can be estimated at 18.5 wt% for SnO, 13.1 wt% for SnO 2 , 67.3 wt% for carbon bers, and 0.1 wt% for graphene.Calculation details are shown in the ESI.† Nitrogen adsorption studies were carried out in order to investigate the surface area and pore volume of as-prepared porous SGCF-700 mats (Fig. S5 †).BET results show that porous SGCF-700 nanobers have a surface area of about 14.9 m 2 g À1 .In addition, porous SGCF-700 nanobers also have a pore volume of about 0.208 cm 3 g À1 .Fig. S5b † demonstrates that most of the pores are smaller than 50 nm in diameter, and that SGCF-700 mats are mainly composed of mesopores (1-50 nm).The BET results demonstrate the formation of porous structure in the SGCF-700 mat.
CV tests were performed to investigate the mechanism of electrochemical reactions, as shown in Fig. 6.The cathodic peak of SGCF-700 at about 0.57 V in the rst cycle (Fig. 6a) is ascribed to formation of an SEI lm and the lithiation of SnO 2 (eqn (3)), which disappears in the following cycles.][40] During the anodic scan, three peaks were found.The peak at about 0.1 V could be attributed to the delithiation of graphite carbon.Another anodic peak at 0.65 V can be ascribed to delithiation of Li-Sn alloys (eqn ( 4)).The anodic peak at about 1.35 V may be deconvoluted to two aspects.The rst is the reversible transfer of Sn 0 to SnO 2 , which has been observed in some SnO 2 -carbon-graphene composites for LIB. 23,41,42Another possible reaction representing this peak is the reversible lithium ions extracting from defective sites or micropores of carbon matrix. 43Further investigation needs to be carried out to clarify details of the mechanism for this peak.Although the CV curves of SCF-700 are similar to those of SGCF-700, there are some differences between them.First, changes of anodic peaks for SGCF-700 at 0.65 and 1.35 V are smaller than those of SCF-700 in the initial four cycles, showing an improved cyclic stability of SGCF-700 by introducing graphene, although both show good cyclic stability from the second to the fourth cycle.Second, the peak current ratios of peak 3 to peak 2 of SGCF-700 are higher than those of SCF-700, indicating a possible improved reversibility of the transfer of Sn 0 to SnO 2 and lithium ions extracting from the defective sites or carbon matrix micropores. 44O x + 2xLi + + 2xe À / Sn + xLi 2 O (3) The charge-discharge curves of SGCF-700 mats are shown in Fig. 7a.It is worth noting that the poorly dened plateau at about 0.8 V associated with the reduction of SnO 2 to Sn becomes indiscernible in the following cycles, which suggests  irreversibility of this reaction within the narrow voltage window. 45The potential proles of SGCF-700 electrodes at a current density of 200 mA g À1 presented excellent discharge and charge capacities of 1272.6 and 828.7 mA h g À1 in the rst cycle corresponding to a Coulombic efficiency (CE) of 65.1%.While increasing the number of cycles, the discharge capacity gradually decayed to 891.6, 639.8, 601.6, and 598 mA h g À1 in the 2nd, 100th, 200th, and 400th cycles, respectively.The large capacity loss (34.9%) in the rst cycle is generally attributed to irreversible formation of the SEI lm during the rst discharge process.
Fig. 7b displays the cyclic properties of SGCF-700 mats in the potential range of 0.005-3.0V at a current density of 200 mA g À1 .The CE of the second cycle exceeds 92.3%.The high CE indicates that embedding SnO x nanoparticles in the carbon matrix can largely prevent detrimental reactions between SnO x and the electrolyte, which is in good agreement with the CV results.High capacity should benet from the ultra-small SnO x nanoparticles and porous carbon network structure, which enable full utilization of SnO x nanoparticles.More importantly, aer several cycles the reversible capacity is stabilized at 100%.
The slow shrinkage of charge-discharge curves aer the rst cycle implies the high cycling stability of the as-prepared composite.The reasons for this enhancement are caused by the synergistic effect of electrospinning and optimal heat treatment.For comparison, the SGCF-600 composite anode delivered an initial discharge capacity of 1277 mA h g À1 with a CE of 70.9%.The properties of SGCF-700 are just a little better than those of SGCF-600.But aer 135 cycles, SGCF-600 anodes only delivered a capacity of 496 mA h g À1 , which is much lower than SGCF-700 anodes.The inferior properties of SGCF-600 could be attributed to the low degree of carbonization and poor nanober conductance.This result agrees with a recent study on PAN-based carbon for LIBs. 29The porous carbon nanober mats prepared by electrospinning act as a buffering matrix that  can prevent the agglomeration of SnO 2 particles, volume expansion, and mechanical stress during Li alloying and dealloying. 31The porous carbon nanober is used as an electrical conductor to compensate for the drawbacks of semiconductors such as SnO 2 and Li 2 O.And it helps promote decomposition of SnO x as a semiconductor into Sn and Li 2 O.Moreover, the porous carbon nanobers allow the charge transfer and mass transfer to facilitate lithiation/delithiation due to high electronic conductivity. 5,29ig. 7c compares the cyclic stability and specic capacities of SGCF-700 and SCF-700 mats.It is clear that SGCF-700 mats exhibit much better cycling stability than SCF-700 electrodes.SCF-700 mats delivered an initial discharge capacity of 1637 mA h g À1 with a CE of 43.0%.Although the initial discharge of SGCF-700, 1273 mA h g À1 , is lower than that of SCF-700.SGCF-700, a discharge capacity of 640 mA h g À1 in the 100 th cycle, 1.25 times that of SCF-700 in the same cycle.Even aer 480 cycles, SGCF-700 mats still delivered a discharge capacity of 600 mA h g À1 , which is almost 50% higher than that of traditional graphitic anode (372 mA h g À1 ).In addition, the CE for SGCF-700 was also higher than that of SCF-700 in the fourth cycle in spite of CE being almost 100% in subsequent cycles.Besides, the high CE in SGCF-700 anodes aer four cycles suggests high reversibility of SGCF-700 mats for the storage of lithium ions.Considering the difference between SGCF-700 and SCF-700, the improved properties of SGCF-700 could be attributed to the presence of graphene, which can affect the growth of nanoparticles resulting in ultra-small nanoparticles and suppress the agglomeration of tin alloys. 10,28Another reason for the high properties of SGCF-700 may be the ultra-small diameter (4 nm) of SnO x nanoparticles, which is close to the optimizing size (about 3-4 nm) of SnO 2 for Li + storage. 36ig. 7d shows the rate capacities of SGCF-700 and SCF-700.SGCF-700 maintains reversible capacities of 850, 737, 613, 504, and 368 mA h g À1 at current densities of 100, 200, 500, 1000, and 2000 mA g À1 , respectively.These values are higher than those of SCF-700 at corresponding current densities.This improvement could be attributed to the enhanced conductance arising from graphene.The synergistic effects arising from the ultra-small SnO x particles within the porous carbon nanobers and graphene are further discussed for comparison.It is proven that the graphene can be used to improve the mechanical properties of porous carbon nanober mats made by electrospinning.
To demonstrate the superior electrochemical performance of SnO x -carbon-nanober (CNF) freestanding electrodes developed in this study, Table 1 compares the capacities and cycles of the electrodes prepared using similar SnO x -based materials and approaches taken from recent reports.Indeed, the SnO x -graphene-carbon ber composites obtained in this study present relatively high capacities and superior cyclic stability.
In order to investigate the lithium-driven structural and morphological changes during the charge and discharge processes, we performed ex situ SEM analyses on the SGCF-700 electrodes before cycling and aer the 1000th cycle and SCF-700 electrodes aer the 100th cycle at a current density of 200 mA g À1 ; the SEM images are shown in Fig. 8.It can be seen in Fig. 8 that the structure and morphology of SGCF-700 electrodes maintain integrity (Fig. 8a and S5a †).However, in the case of SCF-700 electrodes, much of the aggregate was on the porous carbon nanober surfaces (Fig. 8b), and protuberant particles with diameters of 1-4 mm were dispersed in SGCFs (Fig. S5b †).The stark contrast between the two samples demonstrates that graphene not only can obviously improve specic capacity, but also can maintain cycling stability for LIBs.

Conclusions
Flexible hierarchical porous mats of SGCFs were synthesized by reducing GO with stannous ions at room temperature and following electrospinning route.The SnO x nanoparticles (including SnO and SnO 2 ) with ultra-small sizes were chemically embedded in the graphene-carbon nanober mats.As binderfree anodes for LIBs, the exible SGCF mats obtained at 700 C can deliver a discharge capacity of 545 mA h g À1 aer 1000 cycles at a current density of 200 mA g À1 while it is only 509 mA h g À1 for the mats without graphene in the 100th cycle.The rate capacities of SGCF mats are also higher than those of SCFs at the same current densities.The excellent properties of SGCF mats can be ascribed to hierarchical networks serving as the highway for electrons and diffusion channels for ions, and to graphene for maintaining structural stability of SnO x nanoparticles and improving the conductivity of CNFs.It is believed that this strategy may be used for composite synthesis based on other compounds of valence-variable elements, graphene, and carbon bers for the storage and transformation of energy.

Fig. 2
Fig. 2 Digital photographs of as-prepared mats (a and d); mat treated at 230 C (b and e), and SGCF-700 mats (c and f).Images show size shrinkage of mats.Low-(g) and high-magnification (h) SEM images of the SGCF-700 mat.(i) SEM image of SCF-700 mats.(j-l) are digital photographs of SGCF-700 that show flexibility of SGCF-700 mats.

Fig. 3
Fig. 3 (a) and (b) Display low-and high-magnification TEM images of SGCF-700.Ultra-small particles in (b) are marked as circles.(c) and (d) Show HRTEM images of the nanoparticles indicated by circles in (b) conforming to presence of both SnO and SnO 2 .

Fig. 5
Fig. 5 Thermogravimetric analysis (TGA) of SGCF-700 at heating rate of 3.5 C min À1 in air with flow rate of 20 mL min À1 .

Fig. 7
Fig. 7 (a) Charge-discharge voltage profiles of SGCF-700 mats at the 1st, 2nd, 100th, 200th, 400th cycles at a current density of 200 mA g À1 .(b) and (c) Show cyclic properties of SGCF-700 mats in comparison with SGCF-600 and SCF-700 at current densities of 200 mA g À1 .(d) Rate capability of SGCF-700 as compared with SCF-700 at current densities of 100, 200, 500, 1000, 2000, and 100 mA g À1 , respectively.All specific capacities in this study were calculated based on weight of whole mats.

Table 1
Comparison of capacities for various SnO x -carbon electrodes