Tunneled Mesoporous Carbon Nanofibers with Embedded ZnO

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Tunneled mesoporous carbon nanofibers with embedded ZnO nanoparticles for ultrafast lithium storage Geon-Hyoung An, Do-Young Lee, and Hyo-Jin Ahn ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b01286 • Publication Date (Web): 21 Mar 2017 Downloaded from http://pubs.acs.org on March 27, 2017

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ACS Applied Materials & Interfaces

Tunneled mesoporous carbon nanofibers with embedded ZnO nanoparticles for ultrafast lithium storage Geon-Hyoung An,† Do-Young Lee,‡ and Hyo-Jin Ahn∗,†, ‡ †

Program of Materials Science & Engineering, Convergence Institute of Biomedical Engineering and

Biomaterials and ‡Department of Materials Science and Engineering, Seoul National University of Science and Technology, Seoul 139-743, Korea

KEYWORDS: Li-ion battery, Anode, Tunneled mesoporous structure, Carbon nanofiber, Ultrafast

ABSTRACT: Carbon and metal oxide composites have received considerable attention as anode materials for Li-ion batteries (LIBs) owing to their excellent cycling stability and high specific capacity based on the chemical and physical stability of carbon and the high theoretical specific capacity of metal oxides. However, efforts to obtain ultrafast cycling stability in carbon and metal oxide composites at high current density for practical applications still face important challenges because of the longer Liion diffusion pathway, which leads to poor ultrafast performance during cycling. Here, tunneled mesoporous carbon nanofibers with embedded ZnO nanoparticles (TMCNF/ZnO) are synthesized by electrospinning, carbonization, and post-calcination. The optimized TMCNF/ZnO shows improved electrochemical performance, delivering outstanding ultrafast cycling stability, indicating a higher

∗

Corresponding author. E-mail address: [email protected] (H.-J. Ahn) ACS Paragon Plus Environment

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specific capacity than previously reported ZnO-based anode materials in LIBs. Therefore, the unique architecture of TMCNF/ZnO has potential for use as an anode material in ultrafast LIBs.

INTRODUCTION Lithium-ion batteries (LIBs) potentially offer higher energy densities with excellent cycling stability, no memory effect, and greater environmental friendliness than commercial secondary batteries and have been applied mainly in portable electronic devices such as tablet PCs, laptops, and smartphones.1–7 Despite the increasing commercialization and utilization of LIBs, the development and adoption of LIBs for use in fully electric vehicles (EVs) is still limited owing to the short driving range of 150–200 miles.8,9 To overcome this problem, the specific capacity and cycling stability of LIBs are continually developed using advanced materials for the electrodes, electrolytes, and membranes.10–12 However, despite various efforts, progress in improving LIBs for use in EVs is still quite slow owing to the limited theoretical capacity of the electrode materials. The LIB anode is a significant focus of efforts to improve the ultrafast capability, specific capacity, and cycling stability of LIBs, and also makes the largest contribution to a battery's weight, volume, and cost.13–15 To date, many studies have attempted to enhance the specific capacity and cycling stability of LIB anodes using materials other than pure graphite, which has a low theoretical specific capacity of 372 mA h g−1.16–28 Accordingly, a composite architecture consisting of carbon and a metal oxide (i.e., a carbon coating on a metal oxide or a metal oxide loaded on carbon) are generally prepared as LIB anodes because of the synergistic effects obtained by combining the excellent cycling stability of carbon and the high theoretical specific capacity of metal oxides.29–33 However, this approach still faces significant challenges, including poor ultrafast performance of the resulting materials due to the longer Li-ion diffusion pathway to core areas of the electrode material, which leads to a slow electrochemical reaction. Thus, further development of mesoporous structure with a shorter Li-ion diffusion pathway in anode materials is necessary to improve the ultrafast performance of next-generation LIBs.34–36

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Herein, we introduce tunneled mesoporous carbon nanofibers with embedded ZnO nanoparticles (TMCNF/ZnO) as an anode material for LIBs; the material is fabricated by electrospinning, carbonization, and post-calcination. This unique architecture with the mesoporous structure using the molten ZnO provides a shorter Li-ion diffusion pathway and physical buffers for the volume change of ZnO nanoparticles containing a typical high-capacity metal oxide (ZnO: 978 mA h g−1) during cycling.37,38 As expected, the TMCNF/ZnO showed ultrafast performance as well as high capacity and excellent cycling stability when it was evaluated as an anode material for high-performance LIBs.

RESULTS AND DISCUSSION For comparison, conventional carbon nanofibers (CNFs) were prepared without ZnO particles; they exhibited interconnected network structures with smooth surfaces, as shown in Figure S1. Figure 1 shows low-magnification (Figure 1a–c) and high-magnification (Figure 1d–f) field–emission scanning electron microscopy (SEM) images of TMCNF/ZnO-10, TMCNF/ZnO-15, and TMCNF/ZnO-20, respectively. All the samples exhibit interconnected network structures that can be expected to support efficient electron transfer during cycling. As shown in Figure S2, the PAN nanofibers with ZnO particles presented rough surfaces, indicating that the ZnO particles were embedded within the PAN nanofibers. After carbonization, remarkably, we discovered a striking morphological change in the inner and outer structure of TMCNF/ZnO. In addition, TMCNF/ZnO was oxidized without any morphological changes, as shown in Figure S3. TMCNF/ZnO-10 samples (Figure 1a) with diameters of 195–213 nm show smooth surfaces. However, the inner structure of TMCNF/ZnO-10 contains mesopores, as shown in Figure 1d. Further, TMCNF/ZnO-15 (Figure 1b and e), with diameters of 213–231 nm, exhibited a unique architecture of tunneled CNFs with craters. The crater formation at the outer surface of TMCNF/ZnO-15 (Figure 1b) is ascribed to partial decomposition of carbon related to the partial oxidation–reduction reaction between the oxygen in ZnO and the CNFs during carbonization. In addition, in the inner structure, the tunneled CNFs in TMCNF/ZnO-15 contained mesopores, as


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demonstrated in the cross-sectional SEM image in Figure 1e. These results indicate that the ZnO particles in the PAN nanofibers play a significant role in forming the tunneled CNFs with craters. The unique architecture including tunneled CNFs with craters could effectively provide a shorter diffusion Li-ion pathway during cycling, leading to excellent ultrafast performance. However, for TMCNF/ZnO20 (Figure 1c and f), nanofiber structures are formed using only nanoparticles. The formation of nanofiber structure consisting of nanoparticles is due to the large quantity of ZnO particles in the PAN nanofibers before carbonization, which causes an excessive oxidation–reduction reaction between the oxygen in ZnO and the CNFs, leading to decomposition of the CNFs. Thus, the optimized tunneled CNFs with craters were obtained by using 15 wt% ZnO particles in the electrospinning solution. To further investigate the nanostructural properties, transmission electron microscopy (TEM) measurements were made. After electrospinning, the ZnO particles were closely embedded in the PAN nanofibers (Figure S4). As a result, a surface PAN layer with a thickness of ~9-21 nm was observed using high-magnification TEM (Figure S4b). TMCNF/ZnO-10 (Figure 2a and d) showed mesopores in the CNFs. In addition, the dark spots in TMCNF/ZnO-10 (Figure 2d) are welldispersed ZnO nanoparticles 9–12 nm in size inside the CNFs. Remarkably, TMCNF/ZnO-15 (Figure 1b and e) exhibited a distinct tunneled structure consisting of mesopores in the CNFs. These results indicate that tunneled structure with craters was generated by the embedded ZnO particles in the PAN nanofiber. During carbonization, the ZnO particles were reduced to Zn and reached thermodynamic stability. Thereafter, the Zn particles were dissolved during carbonization due to the low melting point (419.5 °C) of Zn and were partially evaporated, leading to the formation of the mesoporous tunneled structure. In addition, ZnO nanoparticles with diameters of 9–12 nm was embedded in the tunneled CNFs, indicating that Zn that dissolved during carbonization was dispersed along the CNFs and formed the nanoparticle structure after cooling. However, TMCNF/ZnO-20 showed a general nanofiber structure consisting of nanoparticles owing to the excessive oxidation–reduction reaction between the oxygen in ZnO and the CNFs due to the large quantity of ZnO particles, indicating that the Zn that dissolved during ACS Paragon Plus Environment

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carbonization should form nanoparticles after cooling. Thus, a tunneled structure with craters was successfully formed by carbonization using the optimum amount (15 wt%) of ZnO particles. To verify the distributions of carbon, zinc, and oxygen in TMCNF/ZnO-15, energy-dispersive X-ray spectrometry (EDS) mapping was performed, as shown in Figure S5. All of the atoms are uniformly dispersed, indicating that the ZnO nanoparticles are uniformly embedded in the tunneled CNFs. Figure 3a shows the X-ray diffractometry (XRD) patterns of the samples. All the samples exhibit broad peaks around 25° corresponding to the (002) plane of graphite.12,33 For TMCNF/ZnO-10 and TMCNF/ZnO-15, the diffraction peaks of ZnO are not visible, implying that the nanosized ZnO is inside the tunneled CNFs. TMCNF/ZnO-20 exhibits diffraction peaks at 31.7°, 34.4°, and 56.5°, which correspond to the (100), (002), and (110) planes, respectively, of ZnO phases with a hexagonal structure, implying the existence of a large quantity of ZnO particles. In addition, X-ray photoelectron spectroscopy (XPS) measurements were made to investigate the chemical bonding states of TMCNF/ZnO-15. The results show two Zn 2p signals at ~1022.0 and ~1045.1 eV, with a spin energy separation of 23.1 eV, which correspond to Zn 2p3/2 and Zn 2p1/2 in the ZnO phase, respectively, as shown in Figure S6. These XPS results show the ZnO phase in TMCNF/ZnO-15. Moreover, the amount of ZnO in TMCNF/ZnO was identified using thermogravimetric analysis (TGA) measurements, as shown in Figure 3b. TMCNF/ZnO-10, TMCNF/ZnO-15, and TMCNF/ZnO-20 exhibit weight losses of 18.8, 26.3, and 34.8%, implying the existence of ZnO nanoparticles in the tunneled CNFs. To determine the types of pores in the samples, N2 adsorption/desorption isotherms were observed using Brunauer– Emmett–Teller (BET) measurements, as shown in Figure 3c. The isotherms for the CNFs show type I characteristics, implying the existence of micropores (pore width, 0.4) and indicating that ZnO particles in the PAN nanofibers could


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produce the mesopores in the CNFs during carbonization.39–41 However, TMCNF/ZnO-20 exhibits type I characteristics owing to decomposition of the CNFs during carbonization. The detailed BET results (the specific surface area, total pore volume, average pore diameter, and pore volume fraction) are presented in Table 1. The specific surface area of TMCNF/ZnO-15 (658 m2 g−1) is higher than those of TMCNF/ZnO-10 and TMCNF/ZnO-20. In addition, the mesopore distribution increases from TMCNF/ZnO-10 to TMCNF/ZnO-15 with increasing quantity of ZnO particles. The mesopore distribution of TMCNF/ZnO-15 (81.1%) exceeds that of the other samples as well as previously reported values for CNFs.39–45 These results indicate that the optimized condition for obtaining tunneled mesoporous CNFs is 15 wt% of ZnO particles in the electrospinning solution. Figure 3d shows the pore volumes and pore size distributions observed using Barrett–Joyner–Halenda (BJH) measurements. TMCNF/ZnO-15 showed mesopore sizes ranging from 20 to 40 nm, in good agreement with the TEM results. The high specific surface area and high mesopore distribution of TMCNF/ZnO-15 are factors that are known to be important for obtaining a shorter Li-ion diffusion pathway during ultrafast cycling. Based on the SEM, TEM, XRD, XPS, BET, and BJH results, a possible formation mechanism of the unique architecture of TMCNF/ZnO-15 is illustrated in Figure 4. As shown in Figure 4a, the ZnO particles were closely embedded in PAN nanofibers by electrospinning. The subsequent development of mesoporous tunneled CNFs is attributed to the phase transition from ZnO to Zn as well as dissolution of Zn during carbonization, as shown in Figure 4b. That is, during carbonization in a nitrogen atmosphere, ZnO particles should be reduced to Zn and reach thermodynamic stability. The craters developed on an outer surface because of a partial oxidation–reduction reaction between the oxygen in ZnO and the CNFs, and Kirkendall effect using the outward diffusion of species from the ZnO and CNF.24 Therefore, the mesoporous structure was mainly devolved using the molten of ZnO nanoparticles. Thereafter, the reduced Zn particles were dissolved at 800 °C because of the low melting point (419.5 °C) of Zn and were partially evaporated, leading to the development of the tunneled mesoporous CNFs. Next, dissolved Zn was uniformly dispersed along the micropores in a tunneled CNF, leading to the formation ACS Paragon Plus Environment

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of embedded Zn nanoparticles (Figure 4c). Finally, after post-calcination at 250 °C in air, Zn nanoparticles were oxidized to ZnO nanoparticles without any morphological changes, as shown in Figure 4d. That is, we synthesized a unique architecture in TMCNF/ZnO-15. To investigate the electrochemical performance of LIBs, charge–discharge tests were performed using coin-type cells. The cycling stability was investigated at a current density of 100 mA g−1 in a voltage range of 0.05–3.00 V (vs. Li/Li+), as shown in Figure 5a. The specific charge and discharge capacities of the electrodes were 1162 and 1670 mA h g−1 for TMCNF/ZnO-10, 1319 and 1750 mA h g−1 for TMCNF/ZnO-15, and 1180 and 1730 mA h g−1 for TMCNF/ZnO-20 in the first cycle, respectively. These values are higher than those of the CNF electrode (389 and 612 mA h g−1) and a commercial ZnO electrode (730 and 1590 mA h g−1). Moreover, the charge–discharge voltage curves and cyclic voltammetry (CV) curves of the electrodes indicated typical electrochemical behavior, as show in Figure S7 and S8, respectively. The first discharge capacities of the commercial ZnO, TMCNF/ZnO-10, TMCNF/ZnO-15, and TMCNF/ZnO-20 electrodes were higher than the theoretical capacity because of the immediate formation of solid–electrolyte interface (SEI) layers, which can store charge via interfacial charging at the Zn/Li2O interface.14,31 SEI layers are typically formed on the electrode surface within the first few cycles because of reductive disassembly of the electrolyte components, leading to a high initial irreversible reaction that results in capacity losses and low Coulombic efficiency. Nevertheless, the TMCNF/ZnO-15 electrode presented a higher Coulombic efficiency (75.3%) than the CNF electrode (63.6%), commercial ZnO electrode (45.9%), TMCNF/ZnO10 electrode (69.6%), and TMCNF/ZnO-20 electrode (68.2%), as shown in Figure S7. This indicates that the tunneled structure with embedded ZnO nanoparticles can play a crucial role in determining the electrochemical performance of LIBs during the first cycle. Furthermore, the Coulombic efficiency of all the electrodes reached nearly 100% after 7 cycles, implying high reversible electrochemical performance. However, the specific capacity of the commercial ZnO electrode dropped rapidly to 145 mA h g−1 after 100 cycles, implying structural disintegration of ZnO due to the large volume change ACS Paragon Plus Environment

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owing to Li-ion insertion/extraction during cycling. The CNF electrode retained its specific capacity well for up to 100 cycles, implying excellent cycling stability. However, it exhibited a lower specific capacity (335 mA h g−1) than the TMCNF/ZnO-10 and TMCNF/ZnO-15 electrodes because of the low theoretical capacity. Therefore, a composite architecture consisting of carbon, which has excellent cycling stability, and ZnO, which has a high theoretical capacity, is essential to obtaining highperformance LIBs. To overcome the low theoretical capacity of carbon, we prepared a unique architecture of tunneled CNFs with embedded ZnO nanoparticles. Notably, the TMCNF/ZnO-15 electrode exhibited impressive cycling stability with a high specific capacity of 832 mA h g−1 after 100 cycles, outperforming the other samples. This electrochemical performance surpasses that previously reported for ZnO-based anode materials, as summarized in Table S1.46–54 The impressive cycling stability and high specific capacity of the TMCNF/ZnO-15 electrode is attributed mainly to two effects. First, the CNF matrix could efficiently accommodate the large volume change of the ZnO nanoparticles during cycling, leading to the excellent cycling stability. Second, the well-dispersed ZnO nanoparticles in the CNF matrix could support the development of numerous electroactive sites because of the large contact areas between ZnO nanoparticles and Li ions, leading to the high specific capacity. However, the TMCNF/ZnO-10 electrode displayed a relatively low specific discharge capacity of 691 mA h g−1 after 100 cycles, which is lower than that of the TMCNF/ZnO-15 electrode owing to the small quantity of ZnO nanoparticles in the CNF matrix. Furthermore, because of its nanofiber structure consisting of ZnO nanoparticles without CNFs, TMCNF/ZnO-20 presented poor cycling stability with a low specific discharge capacity of 359 mA h g−1 after 100 cycles, which suggests a large volume change in the ZnO nanoparticles. Figure 5b shows the high-rate performance obtained at current densities of 100, 300, 700, 1000, 1500, and 2000 mA g−1. Strikingly, the TMCNF/ZnO-15 electrode exhibited a remarkable highrate performance of 591 mA h g−1 at 2000 mA g−1, subsequently returning to 831 mA h g−1 (93% of the original specific capacity) when the current density recovered to 100 mA g−1. Moreover, the TCNF/ZnO-15 showed the high specific capacity of 387 mAh g-1 at high current density of 5000 mA g-1


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(Figure S9). Notably, the remarkable high-rate performance of TMCNF/ZnO-15 is higher than previously reported values for ZnO-based anode materials in LIBs (Figure 5c).46–49,51–55 The enhanced high-rate performance of the TMCNF/ZnO-15 electrode is ascribed mainly to a shorter Li-ion diffusion pathway through the tunneled CNF with a high mesopore distribution. In other words, a tunneled CNF could effectively provide rapid electrochemical reactions between the ZnO nanoparticles and Li ions, leading to the remarkable high-rate performance. To confirm the electrochemical kinetics related to Liion diffusion, electrochemical impedance spectroscopy (EIS) measurements were performed using fresh cells. Figure 5d presents Nyquist plots of the electrodes in the frequency range of 105 to 10−2 Hz at the open-circuit potential. The semicircle in the high-frequency region is attributed to the charge transfer resistance (Rct) at the electrode–electrolyte interface, and the inclined line in the low-frequency range (the Warburg impedance) corresponds to Li-ion diffusion in the electrode.14,15 The TMCNF/ZnO-15 electrode showed the lowest Rct and lowest Warburg impedance among the electrodes. The low Rct is ascribed to the high number of electroactive sites based on good dispersion of ZnO nanoparticles in the tunneled CNFs. The tunneled CNFs with high mesopore distributions could clearly reduce the Warburg impedance compared to that of the other electrodes, indicating a shorter Li-ion diffusion pathway. Therefore, the TMCNF/ZnO-15 electrode displayed outstanding ultrafast cycling stability with a specific discharge capacity of 452 mA h g−1 at a high current density of 2000 mA g−1 after 500 cycles (Figure 5e) due to the shorter Li-ion diffusion pathway as well as efficient accommodation of the volume change of the ZnO nanoparticles. Thus, the optimized TMCNF/ZnO-15 electrode can be used in an ultrafast discharge–charge condition in LIBs. The function of the tunneled CNFs in preserving the structural stability of the TMCNF/ZnO-15 electrode during cycling is demonstrated using a disassembled coin-type cell using TEM measurements. At a current density of 100 mA g−1 after 100 cycles, TMCNF/ZnO-15 clearly showed ZnO nanoparticles with a diameter of 11–15 nm embedded in a tunneled CNF and retained without any structural changes from the initial structure, as shown in low- (Figure 6a) and high-magnification (Figure 6b) TEM images.


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Similarly, at 2000 mA g−1 after 100 cycles, TMCNF/ZnO-15 maintained a tunneled CNFs with embedded ZnO nanoparticles despite the ultrafast cycling condition (Figure 6c). However, TMCNF/ZnO-15 displayed slightly larger ZnO nanoparticles (19–26 nm) in the tunneled CNFs, indicating that the ultrafast cycling condition caused aggregation of the ZnO nanoparticles, as shown in Figure 6d. These results demonstrating the excellent structural stability of TMCNF/ZnO-15 during cycling indicated that the reversible electrochemical reaction between the electrode and the electrolyte is facilitated from low to high current densities, resulting in improved ultrafast cycling performance with a high specific capacity. In the schematic illustration of the electrochemical reaction in Figure 6e, the tunneled mesoporous structure of TMCNF/ZnO-15 offers a shorter Li-ion diffusion pathway and adequately accommodates the volume change of the embedded ZnO nanoparticles without any structural changes, leading to improved ultrafast cycling performance.

CONCLUSIONS TMCNF/ZnO was successfully synthesized by a sequential process of electrospinning, carbonization, and post-calcination. By optimizing the amount of ZnO, we obtained a unique architecture of tunneled CNFs with embedded ZnO nanoparticles (2.1 to 2.7 nm) in the TMCNF/ZnO-15 sample; this structure featured craters on the outer surface of the CNFs and a high mesopore distribution of 81.1%. The optimized TMCNF/ZnO-15 electrode exhibited improved lithium storage performance, including excellent cycling stability with a high specific discharge capacity (832 mA h g−1 at 100 mA g−1 after 100 cycles), and a remarkable high-rate performance compared with CNF, commercial ZnO, TMCNF/ZnO10, and TMCNF/ZnO-20 electrodes. Moreover, the TMCNF/ZnO-15 electrode showed outstanding ultrafast cycling stability (452 mA h g−1 at 2000 mA g−1 after 500 cycles). This surprising electrochemical performance can be attributed primarily to the following factors: (i) the excellent cycling stability is attributed to the CNF matrix, which efficiently accommodates the large volume


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change of the ZnO nanoparticles during cycling; (ii) the high specific capacity is related to the welldispersed ZnO nanoparticles in the CNF matrix, which provide numerous electroactive sites; and (iii) the impressive high-rate performance and outstanding ultrafast cycling stability are ascribed to the tunneled CNFs with high mesopore distributions, which support rapid electrochemical reactions between the ZnO nanoparticles and Li ions because of the shorter Li-ion diffusion pathway. Therefore, we believe that this novel strategy using tunneled CNFs with embedded ZnO nanoparticles has great potential for producing anode materials for use in ultrafast LIBs.

Supporting Information Material and methods; SEM images of CNFs, PAN nanofibers with embedded ZnO particles after electrospinning, TMCNF/ZnO-10, TMCNF/ZnO-15, and TMCNF/ZnO-20 before the post-calcination; TEM images of PAN nanofibers with embedded ZnO particles after electrospinning; TEM-EDS mapping data of TMCNF/ZnO-15; Zn 2p and O1s XPS spectra of TMCNF/ZnO-15; Charge and discharge voltage curves of CNFs, commercial ZnO, TMCNF/ZnO-10, TMCNF/ZnO-15, and TMCNF/ZnO-20; CV curves of TMCNF/ZnO-15; Cycling stability competition with reported ZnObased anode materials; extra references

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Figure caption Figure 1 (a–c) Low- and (d–f) high-magnification SEM images of TMCNF/ZnO-10, TMCNF/ZnO-15, and TMCNF/ZnO-20. Figure 2 (a–c) Low- and (d–f) high-magnification TEM images of TMCNF/ZnO-10, TMCNF/ZnO-15, and TMCNF/ZnO-20. Figure 3 (a) XRD data of CNFs, TMCNF/ZnO-10, TMCNF/ZnO-15, and TMCNF/ZnO-20. (b) TGA ACS Paragon Plus Environment

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data of CNFs, TMCNF/ZnO-10, TMCNF/ZnO-15, and TMCNF/ZnO-20. (c) N2 adsorption/desorption isotherms. (d) BJH pore size distributions between 0.2 and 50 nm. Figure 4 Schematic illustration of the fabrication of TMCNF/ZnO-15. (a) PAN nanofibers with embedded ZnO particles, (b) Carbon nanofiber with dissolved Zn, (c) Tunneled carbon nanofibers with embedded Zn nanoparticles, and (d) TMCNF/ZnO-15. Figure 5 Electrochemical properties of prepared electrodes. (a) Cycling stability of CNFs, commercial ZnO, TMCNF/ZnO-10, TMCNF/ZnO-15, and TMCNF/ZnO-20 at a current density of 100 mA g−1 up to 100 cycles. (b) High-rate performance at current densities of 100, 300, 700, 1000, 1500, and 2000 mA g−1. (c) Comparison of high-rate performance with previously reported results for ZnO-based anode materials in LIBs. (d) Nyquist plots of the electrodes in the frequency range of 105 to 10−2 Hz at the open-circuit potential. (e) Ultrafast cycling stability of TMCNF/ZnO-15 at a high current density of 2000 mA g−1 up to 500 cycles. Figure 6 (a) Low- and (b) high-magnification TEM images of TMCNF/ZnO-15 at a current density of 100 mA g−1 after 100 cycles. (c) Low- and (d) high-magnification TEM images of TMCNF/ZnO-15 at a high current density of 2000 mA g−1 after 500 cycles. (e) Proposed model of TMCNF/ZnO-15 during Li-ion insertion.


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Figure 1

Figure 2


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Figure 3

Figure 4


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Figure 5


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Figure 6


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Table 1. The specific surface area, total pore volume, average pore diameter, and pore volume fraction of CNFs, TMCNF/ZnO-10, TMCNF/ZnO-15, and TMCNF/ZnO-20.

Total pore volume Samples

2 -1

SBET [m g ]

3 -1

Average pore

Pore volume fraction

(p/p0=0.990) [cm g ]

diameter[nm]

Vmicro (%)

Vmeso (%)

CNF

361

0.18

2.0

84.5

15.5

TMCNF/ZnO 10

548

0.55

4.0

35.4

64.6

TMCNF/ZnO 15

658

1.06

6.4

18.9

81.1

TMCNF/ZnO 20

356

0.42

4.7

45.9

54.1


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Table of Contents Graphic


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Tunneled Mesoporous Carbon Nanofibers with Embedded ZnO

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