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Pipe-Wire TiO2-Sn@Carbon Nanofibers Paper Anodes for Lithium and Sodium Ion Batteries minglei mao, Feilong Yan, chunyu Cui, Jianmin Ma, Ming Zhang, Taihong Wang, and Chunsheng Wang Nano Lett., Just Accepted Manuscript • Publication Date (Web): 05 May 2017 Downloaded from http://pubs.acs.org on May 5, 2017

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Pipe-Wire TiO2-Sn@Carbon Nanofibers Paper Anodes for Lithium and Sodium Ion Batteries Minglei Mao, †‡ Feilong Yan, † Chunyu Cui, † Jianmin Ma, † Ming Zhang,*† Taihong Wang, † and Chunsheng Wang*‡ †State Key Laboratory of Chemo/Biosensing and Chemometrics, School of Physics and Electronics, Hunan University, Changsha 410082, China ‡Department of Chemical and Biomolecular Engineering, University of Maryland, College Park, MD 20742, USA

ABSTRACT

Metallic tin has been considered as one of the most promising anode materials both for lithium (LIBs) and sodium ion battery (NIBs) because of high theoretical capacity and an appropriate low discharge potential. However, Sn anodes suffers from a rapid capacity fading during cycling due to pulverization induced by severe volume changes. Here we innovatively synthesized pipewire TiO2-Sn@carbon nanofibers (TiO2-Sn@CNFs) via electrospinning and atomic layer deposition to suppress pulverization-induced capacity decay. In pipe-wire TiO2-Sn@CNFs paper, nano-Sn is uniformly dispersed in carbon nanofibers, which not only act as a buffer material to prevent pulverization, but also serve as a conductive matrix. In addition, TiO2 pipe as the protection shell outside of Sn@carbon nanofibers can restrain volume variation to prevent Sn

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from aggregation and pulverization during cycling, thus increasing the coulombic efficiency. The pipe-wire TiO2-Sn@CNFs show excellent electrochemical performance as anodes for both LIBs and NIBs. It exhibits a high and stable capacity of 643 mAh/g at 200 mA/g after 1100 cycles in LIBs and 413 mAh/g at 100 mA/g after 400 cycles in NIBs. These results would shed light on the practical application of Sn-based materials as high capacity electrode with good cycling stability for next generation LIBs and NIBs

KEYWORDS: pipe-wire structure, TiO2-Sn@carbon nanofibers, binder-free flexible anode, electrospinning, lithium and sodium ion batteries

Li-ion batteries have been widely used as a power source for portable electronic devices and are attractive to power electric vehicles.1-5 However, for energy storage in grid harvesting from renewable energy sources, NIBs are more competitive due to low cost, earth abundant, and environmental benignity of sodium resources.6, 7 Metallic tin has been considered as one of the most promising anode materials for LIBs because of the high theoretical capacity (992 mAh/g for Li4.4Sn) and a relatively low discharge potential.8-11 Meanwhile, Sn is also a promising anode candidate for NIBs (theoretical capacity: 847 mAh/g for Na15Sn4) that could effectively mitigate the strained lithium resources, particularly useful for large-scale energy storage.12 However, the practical application of Sn suffers from its pulverization issue induced by the severe volume changes both in LIBs (300%) and NIBs (520%), resulting in losing of electrical conductivity and a quick capacity fading during cycling.13-15 One of the most effective strategies to prevent the pulverization of Sn is to minimize Sn size into nanometer size, which could reduce the mechanical stress generated during alloying and


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dealloying reaction with Li and Na ions, thus inhibiting the tendency to fracture and crack.16 Moreover, nanostructure can provide excellent rate capability by shortening the path lengths for Li and Na ion transport.17, 18 However, the aggregation of Sn nanoparticles (NPs) still limits the cycle stability.19 Extensive effort has been conducted to migrate this challenge.8,

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example, a nanostructured Sn/C composite provides a high capacity of 500 mAh/g for more than 100 cycles where the carbon matrix acts as a buffer to accommodate the volume change and prevent Sn particle aggregation.20. We demonstrated that nano-Sn dispersed evenly in a carbon matrix could retain a high capacity of 710 mAh g-1 in 130th cycle.8 However, the high surface of nano-Sn/C composites also increase the irreversible capacity, reducing the initial coulombic efficiency. A rigid shell over nano-Sn/C composite can significantly enhance both coulombic efficiency and cycling stability. It has been reported that TiO2 coating on active materials can significantly enhance the cycling stability and coulombic efficiency for sulfur and other high capacity electrode materials. 25, 26 Herein, pipe-wire TiO2-Sn@CNFs were successfully synthesized via electrospinning and atomic layer deposition (ALD), where the nano-Sn was uniformly dispersed in carbon nanofibers to buffer the stress/strain and maintain 3D conductive matrix. Depositing thin TiO2 film on Sn@carbon nanofibers (Sn@CNFs) surface using ALD techniques can reduce its surface to electrolyte, thus enhancing the coulombic efficiency. In addition, the TiO2 pipe can also reduce the Sn aggregation and restrain volume variation to prevent Sn from pulverization during cycling. Thus, pipe-wire TiO2-Sn@CNFs show excellent electrochemical performance as an anode material both in LIBs and NIBs. The synthesis process of pipe-wire TiO2-Sn@CNFs is schematically shown in Figure 1a. First, Sn@CNFs was synthesized by electrospinning. The nanofibers showed a narrow diameter


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distribution and lengths up to several millimeters (Figure.1b). The Sn@CNFs was then coated by a thin layer of TiO2 via ALD, producing core-shell TiO2-Sn@CNFs (Figure 1c). The following calcination of these core-shell nanofibers at 400 °C for 1h in air led to the partial oxidation and constriction of carbon nanofibers, forming pipe-wire TiO2-Sn@CNFs with an average diameter of around 160-220 nm (Figure.1d). In addition, the flexibility of pipe-wire TiO2-Sn@CNFs paper was tested by bending experiment (Figure 1e-g). The paper can still keep intact and flexible even with 180 degree bend, showing excellent flexibility. The 1D pipe-wire structure and uniform morphology can be confirmed by transmission electron microscopy images (Figure 2a), where TiO2 pipe about 15 nm thick is coated on the Sn@CNFs, leaving some space to relieve the volume expansion. From the high resolution TEM image (Figure 2b), (200) crystal facet of metallic Sn with space of 0.29 nm can be seen. Sn nanoparticle is dispersed evenly in the carbon matrix. The faint polycrystalline rings in the selected area electron diffraction (SAED) pattern (inset in Figure 2b) further confirm the presence of crystalline Sn. Energy dispersive-spectroscopy (EDS) elemental mapping images (Figure 2c-f) well match results shown in the Figure 2a, demonstrating that TiO2 are uniformly distributed along the axial direction of Sn@CNFs. Metallic tin are evenly embedded in the carbon, and a tunnel can be easily observed. Pipe-wire TiO2-Sn@CNFs with different TiO2 layer thickness (5, 15, and 25nm) are shown in Figure S1. Compared to intact and uniform morphology of pipe-wire TiO2-Sn@CNFs with 15nm TiO2 layer, TiO2 layer of 5 nm thickness split and crack after annealing, while TiO2 layer of 25 nm thickness fracture and form TiO2 particles, attributed to high stress and an excess of TiO2.27, 28 X-ray diffraction (XRD) pattern is measured and showed in Figure. 3a. All peaks can be indexed to metallic Sn (tetragonal phase, JCPDS Card No, 04-0673) and TiO2 (anatase phase,


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JCPDS Card No, 21-1272). Graphite peaks are completely absent, suggesting the carbon exists in amorphous due to the low calcination temperature. The components in pipe-wire TiO2Sn@CNFs was confirmed by the EDS analysis (Figure. S2). The Cu peaks are from the copper net, which is used in EDS characterization. To quantify the amount of components in the composites, thermogravimetric analysis (TGA) and derivative thermogravimetry (DTG) analysis of pipe-wire TiO2-Sn@CNFs were carried out in air (Figure. 3b). A mass loss of about 2.68% observed below 150 °C on the TGA curve may correspond to the loss of free water, physically adsorbed water. The weight addition from 150 to 450 °C could be attributed to the oxidation of metallic tin to tin dioxide.8 The composites show an obvious weight loss of 16.24% between 450 and 620 °C, which is ascribed to the oxidation process of carbon. The final product with a retention rate of 92.1% is mixture of SnO2 and TiO2. According to the transformation of metallic tin to SnO2 with a weight change of 127%, the weight ratio of metallic tin in composites can be estimated to be 40.8%. To determine the chemical composition of pipe-wire TiO2-Sn@CNFs and to identify the chemical status of the C element in the samples, X-ray photoelectron spectroscopy (XPS) measurements were carried out using an ESCALAB 250 in the region of 0-1351.2 eV (Figure 4). Figure 4a exhibits the XPS survey spectra of pipe-wire TiO2-Sn@CNFs and peaks of Sn 3d, Ti 2p, O 1s and C 1s can be clearly observed. Figure 4b shows a high-resolution C 1s XPS spectra of pipe-wire TiO2-Sn@CNFs. Three peaks which correspond to carbon atoms in different functional groups appear clearly. The main peak centered at 284.8 eV corresponds to extensively delocalized sp2-hybridized carbon atoms, and independent peaks with binding energies of 286.2, and 288.9 eV can be assigned to carbon atoms in C=O, and O-C=O, respectively. The highresolution Sn 3d XPS spectra of pipe-wire TiO2-Sn@CNFs exhibit two peaks at 486.6 and 495.1


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eV, corresponding to Sn 3d5/2 and Sn 3d3/2 spin orbit peaks of metallic tin (Figure 4c). As for the high-resolution spectra of Ti 2p (Figure 4d), two peaks at 464.5 and 458.8 eV are attributed to Ti 2p1/2 and Ti 2p3/2, respectively. The XPS results further confirm coexistence of metallic tin and TiO2 in pipe-wire TiO2-Sn@CNFs, which agrees well with the XRD results. Electrochemical performance was evaluated in coin-type cells by directly using pipe-wire TiO2-Sn@CNFs paper as working electrode and Li or Na metal as the counter-electrode. The binders, conductive additives, and Cu current collector are all free, which can reduce the electrode weight, thus increasing the energy/power densities of the cell. Figure 5a shows cyclic voltagrams (CVs) of pipe-wire TiO2-Sn@CNFs electrode in the 1st, 2nd and 10th scans at a scan rate of 0.2 mV/s between 0.01 and 3V versus Li/Li+. The first cathodic scan shows a reversible peak at 1.52V and wide irreversible current peak in the range of 0.9 to 0.6 V. The peak at 1.52V is due to the Li intercalation, which is in accordance with that reported for anatase TiO2.29 The irreversible peak in 0.9-0.6 V corresponds to the decomposition of electrolyte to form solidelectrolyte interface (SEI) films, resulting in irreversible capacity loss.30 The further cathodic scan of Sn anodes presents three reversible peaks at 0.27, 0.48, and 0.62 V, which are related to the lithiation reactions between Sn and Li (Figure 5a). In the anodic scan, anodic peaks at 0.41, 0.72, 0.81, and 0.86 V are attributed to the delithiation of LixSn (Figure 5a). These delithiation potentials are in good agreement with the calculated potential plateaus by Ceder’s group if overpotential is considered due to slow Li diffusion in Sn during the CV scan.31 The sharp peak at 2.17 V is attributed to Li-ion extraction from anatase TiO2. The two-phase reaction occurs during electrochemical Li+ insertion/extraction according to the following reaction: TiO 2 + xLi + + xe-

Li x TiO2


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Small deviations in the peak positions are noticed in the subsequent cycles, which can be contributed to the structural rearrangement in the TiO2 crystal lattice. Similar to cyclic voltammograms of pipe-wire TiO2-Sn@CNFs in LIBs, wide peaks in the range of 0.7 to 1.0 V are also observed in the first anode scan of pipe-wire TiO2-Sn@CNFs in NIBs, which are also attributed to the desodiation of partially amorphous NaxSn (Figure 5b)32. In the first cathodic scan, a wide irreversible current peak from 0.1 to 0.8 V versus Na/Na+ corresponds to the formation of SEI film and NaxSn. The peaks located at 0.2 and 0.55 V in the subsequent cathodic scans are assigned to the sodiation process.33 Different from TiO2-Sn@CNFs in LIBs, no sodiation/desodiation peaks of TiO2 at high potential were observed, demonstrating that TiO2 is inactive for Na-ions.34 The galvanostatic charge/discharge behavior of pipe-wire TiO2-Sn@CNFs anodes in 0.01-3 V for LIBs and 0.01-2 V for NIBs at 100 mA/g are shown in Figure 5c and d, respectively. The first discharge and charge capacities of pipe-wire TiO2-Sn@CNFs for LIBs are 1644 and 1099 mAh/g, corresponding to a Coulombic efficiency (CE) of 66.8%. The high CE can be attributed to embedment of Sn NPs in CNFs, which greatly avoids detrimental reactions between Sn nanoparticles and electrolyte. The high capacity can be derived from the synergistic effect of nanoscale Sn grain, pipe-wire structure, and close interaction of pipe-wire TiO2-Sn@CNFs. In addition, the reversible capacity can be still stabled at 950 mAh/g after several cycles. The overlap of discharge curves (lithiation or sodiation) after the first cycle implies the high cycling stability of TiO2-Sn@CNFs. While Coulombic efficiency of as-prepared anode for NIBs (58.3%) is relatively lower than that for LIBs, which can be mainly attributed to stable SEI films. The ion extraction voltages for Na are lower than that for Li, implying that Sn is more suitable as anodes for NIBs. The charge capacity of pipe-wire TiO2-Sn@CNFs anodes in NIBs is 610 mAh/g,


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which is smaller than that (1099 mAh/g) in LIBs. The lower capacity of pipe-wire TiO2Sn@CNFs

anodes

in

NIBs

than

LIBs

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mainly

due

to

poor

electrochemical

sodiation/desodiation kinetics.30 In addition, Sn has different theoretical capacity in NIBs and LIBs because of its different alloy composition with Na and Li. The cycling performance of pipe-wire TiO2-Sn@CNFs electrodes in LIBs is measured at the current density of 100 mA/g. As illustrated in Figure 6a, pipe-wire TiO2-Sn@CNFs electrodes show a stable cycling performance beyond initial several cycles. The capacity fading in the first several cycles may be derived from the formation of SEI film and partial irreversible insertion of Li+ into carbon nanofibers. After 200 cycles, the electrode can still deliver a discharge capacity of 831 mAh/g. Prolonged cycle life of pure CNFs, Sn@CNFs, core-shell TiO2-Sn@CNFs, and pipe-wire TiO2-Sn@CNFs at 200 mA/g in Li ion battery are depicted in Figure 6b. After 1100 cycles, pipe-wire TiO2-Sn@CNFs electrodes can still retain 643 mAh/g in capacity, superior to core-shell TiO2-Sn@CNFs (544 mAh/g after 44 cycles), Sn@CNFs (484 mAh/g after 370 cycles), and pure CNFs (142 mAh/g after 500 cycles), highlighting pipe-wire structure of TiO2Sn@CNFs contributing to long cycle life and high specific capacity. The slightly increasing in capacity of pipe-wire TiO2-Sn@CNFs from 200 to 350 cycles could be attributed to the elevating temperature and an activation process. The pipe-wire TiO2-Sn@CNFs show the best electrochemical performance in Sn/C composites reported previously.8, 16, 35-40 Also, it should be pointed out that a high Coulombic efficiency of >99% is achieved after several cycles, indicating high reversibility of pipe-wire TiO2-Sn@CNFs. The cycle stability of pipe-wire TiO2-Sn@CNFs in NIBs was also evaluated by galvanostatically discharging and charging at 100 mA/g for 400 cycles (Figure 6c). The reversible capacities are 490 and 413 mAh/g at the end of 10 and 400 cycles, respectively, with


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about 15.7% capacity decrease over 400 cycles. While capacities of core-shell TiO2-Sn@CNFs, Sn@CNFs and pure CNFs are 285 (after 100 cycles), 255 (after 50 cycles), and 142 mAh/g (after 200 cycles), much lower than that of pipe-wire TiO2-Sn@CNFs. Remarkably, Coulombic efficiency approaches 100% (>99.5%), implying that superior cycle stability of pipe-wire TiO2Sn@CNFs electrode for reversible Na+ storage was highly repeatable. Compared to core-shell TiO2-Sn@CNFs and Sn@CNFs, pipe-wire TiO2-Sn@CNFs present much higher capacity and more stable cycle performance both in LIBs and NIBs, manifesting essential function of TiO2 pipe providing sufficient protection and room for volume fluctuation. Clearly, it can be seen from Figure 6a and c that pipe-wire TiO2-Sn@CNFs electrode show more stable cycle life in NIBs than LIBs, ascribed to the formation of amorphous NaxSn phase during charge/discharge. Therefore, Sn nanoparticles experience well-proportioned volume expansion in NIBs, while LixSn nanocrystals undergo unidirectional volume variation in LIBs, causing faster capacity decay.32, 41 In order to further confirm importance of TiO2 pipe, pipe-wire TiO2-Sn@CNFs with 5, 15, and 25 nm TiO2 pipe were galvanostatically tested at 200 mA/g in LIBs. As shown in Figure 6d, three electrodes display the similar trend. Meanwhile, pipe-wire TiO2-Sn@CNFs electrodes with 15 nm TiO2 pipe obtain higher capacity than those with 5 and 25 nm TiO2 pipe, which could be attributed to uniform and stable TiO2 pipe with 15 nm thickness in accordance with Figure S1. Figure 6e depicts the rate capability and cycling performance of the as-synthesized pipe-wire TiO2-Sn@CNFs in LIBs. At a low current density of 100 mA/g, pipe-wire TiO2-Sn@CNFs delivers a reversible capacity as high as 1020 mAh/g. As the current rate increased gradually to 200, 400, 600, 800, and 1000 mA/g, the corresponding reversible capacities reach 780, 570, 430,


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360, and 280 mAh/g, respectively. The capacity recovers to 1010 mAh/g after the current density is reduced to 100 mA/g, and maintains 950 mAh/g after 100 cycles. The detailed reaction kinetics of pipe-wire TiO2-Sn@CNFs in LIBs and NIBs were investigated before and after 200 cycles using electrochemical impedance spectroscopy (EIS) in the frequency range of 100 kHz to 0.01 Hz at 10 mV amplitude (Figure 6f). The equivalent circuit model of the studied system is inset in Figure 6f. The impedance spectra are composed of one depressed semicircle at high frequencies corresponding to the interfacial resistance including charge transfer and SEI resistance, and a straight slopping line at an approximate 45° angle to the real axis related to the Li+-diffusion process within the bulk of the electrode material. As shown in Figure 6f, interfacial resistance Rf+Rct only slightly increase after 200 cycles both in LIBs and NIBs, indicating superior cycling stability. Rf represents the resistance of solid-electrolyte interphase, while Rct represents the resistance of charge transfer. The fast ion-transfer kinetics of pipe-wire TiO2-Sn@CNFs electrodes benefit from Sn nanograins homogeneously distributed in conductive carbon nanofibers, which could facilitate Li+ and Na+ diffusion. In addition, pipewire TiO2-Sn@CNFs anodes experience larger interface impedance in LIBs than NIBs, which could be attributed to either higher SEI film resistance or larger charge-transfer resistance, and is also an evidence that Sn is more suitable for NIBs. The morphology changes of pipe-wire TiO2-Sn@CNFs after 200 discharge/charge cycles at 100 mA/g in LIBs were investigated (Figure 7a-c). In comparison to the morphology of the fresh pipe-wire TiO2-Sn@CNFs anode in Figure 1d, no obvious morphology change was observed after 200 cycles, indicating that pipe-wire structure of TiO2-Sn@CNFs can effectively cope with the mechanical strain that is induced by iteration of the Li+ intercalation/extraction, resulting in stable cycling performance. It can be clearly seen from Figure 7b that Sn@CNFs after 200 cycles


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become thicker compared to the fresh one (Figure 1b), while the overwhelming majority of TiO2 pipe keep intact. Particles on the surface of TiO2 pipe may originate from salt precipitation from residual electrolyte (Figure 7a, b). Meanwhile, Sn nanoparticles still keep evenly dispersed in CNFs without evident aggregation, revealing that CNFs can act as a good buffer material to prevent Sn aggregation (Figure 7c). Although volume of Sn varies owing to mechanical stress with cycling, pipe-wire structure provides sufficient room and TiO2 pipe restrict the volume expansion, both of which contribute to excellent cycle life. In summary, we have innovatively designed and fabricated a pipe-wire structure for highcapacity and electrochemically stable TiO2-Sn@CNFs electrodes. The structure takes advantage of high capacity of Sn, excellent electrochemical stability of TiO2, and high electrical conductivity of carbon nanofibers together. The pipe-wire arrangement consists of Sn nanoparticles embedded in CNFs matrix completely protected by a thin, conformal, and selfsupporting TiO2 pipe. The rationally designed vacant space between TiO2 pipe and Sn@CNFs allows for expansion of Sn without deforming TiO2 pipe or disrupting SEI on the surface. The pipe-wire TiO2-Sn@CNFs anodes exhibit a high reversible capacity and stable cycling performance of 643 mAh/g at 200 mA/g after 1100 cycles in LIBs and 413 mAh/g at 100 mA/g after 400 cycles in NIBs. These results would shed light on the practical application of Sn-based materials as high capacity electrode with good cycling stabililty for next generation LIBs and NIBs.

ASSOCIATED CONTENT Supporting Information is available free of charge on the ACS Publications website at http://pubs.acs.org.


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Details of experimental methods, characterizations, electrochemistry, and additional figures (Figures S1-S2) (PDF) AUTHOR INFORMATION

Corresponding Author *Email: [email protected], [email protected]

Author Contributions M. Mao and F. Yan contributed equally.

Notes The authors declare no competing financial interest. ACKNOWLEDGMENT The authors thank Prof. Bingan Lu and Prof. Huigao Duan from Hunan University for the help on ALD and Mr. Kai Fan on schematic illustration of the preparation progress. This work was supported as part of the Nanostructures for Electrical Energy Storage (NEES), an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences under Award number DESC0001160. The present work has been supported by the National Natural Science Foundation of China (Grants 51404103, 51574117 and 61376073), Hunan University Fund for Multidisciplinary Developing (2015JCA04), Hunan Provincial Innovation Foundation for Postgraduate (No. CX2016B120) and Project funded by China Postdoctoral Science Foundation. M. Mao’s fellowship was supported by China Scholarship Council (grant No. 201606130050). The authors also thank Dr. Xiulin Fan for the help on the TEM after cycling.


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Figure 1. The preparation process of pipe-wire TiO2-Sn@CNFs. (a) Schematic illustration of the preparation process of pipe-wire TiO2-Sn@CNFs. (b-d) SEM images of Sn@CNFs, core-shell


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TiO2-Sn@CNFs, and pipe-wire TiO2-Sn@CNFs, respectively. (e-g) Digital photographs of pipewire TiO2-Sn@CNFs paper.

Figure 2. (a) SEM, (b, c) HRTEM images, and (d-f) corresponding elemental mapping images of pipe-wire TiO2-Sn@CNFs. Inset: SAED image.


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Figure 3. (a) XRD pattern of pipe-wire TiO2-Sn@CNFs, (b) TGA (black line) and DTG (blue line) curves of pipe-wire TiO2-Sn@CNFs in air at a rate of 10 °C/min.


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Figure 4. (a) Survey XPS spectra of pipe-wire TiO2-Sn@CNFs. (b-d) High-resolution XPS spectra of C, Sn 3d, and Ti 2p, respectively.


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Figure 5. Cyclic voltammetry curves of pipe-wire TiO2-Sn@CNFs in (a) LIBs and (b) NIBs at a scan rate of 0.2 mV/s. Galvanostatic charge/discharge profiles of pipe-wire TiO2-Sn@CNFs in (c) LIBs and (d) NIBs at a current density of 100 mA/g.


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Figure 6. (a) Cycling performance of pipe-wire TiO2-Sn@CNFs at 100 mA/g in LIBs. (b,c) Prolonged cycle life of pipe-wire TiO2-Sn@CNFs, core-shell TiO2-Sn@CNFs, Sn@CNFs, and CNFs in LIBs and NIBs, respectively. (d) Prolonged cycle life of pipe-wire TiO2-Sn@CNFs with 5, 15, and 25 nm TiO2 shell at 200 mA/g in LIBs. (e) Rate capability of pipe-wire TiO2Sn@CNFs in Li-ion battery. (f) Nyquist plot of pipe-wire TiO2-Sn@CNFs anodes before and after 200 cycles in LIBs and NIBs, respectively. Inset is the equivalent circuit model of the studied system. The capacity is based on the mass of entire electrodes.


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Figure 7. (a, b) SEM, and (c) HRTEM images of pipe-wire TiO2-Sn@CNFs after 200 cycles in LIBs. The red ovals in (c) highlight Sn nanograins.

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For Table of Contents Only

Pipe-Wire TiO2-Sn@Carbon Nanofibers Paper Anodes for Lithium and Sodium Ion Batteries Minglei Mao, Feilong Yan, Chunyu Cui, Jianmin Ma, Ming Zhang*, Taihong Wang, and Chunsheng Wang*

TOC figure

We innovatively synthesized pipe-wire TiO2-Sn@CNFs, where nano-Sn is uniformly dispersed in carbon nanofibers, which act as a buffer material and conductive matrix, and TiO2 pipe as the protection shell outside of Sn@carbon nanofibers. The pipe-wire TiO2-Sn@CNFs exhibits a high and stable capacity of 643 mAh/g at 200 mA/g for 1100 cycles in LIBs and 413 mAh/g at 100 mA/g for 400 cycles in NIBs.


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