Synergistically Enhanced Electrochemical Performance of

Device & System Research Center, Samsung Advanced Institute of Technology, ... (6, 7) Among emerging anode materials, MoS2 exhibits high Li storage ca...
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Synergistically Enhanced Electrochemical Performance of Hierarchical MoS2/TiNb2O7 Hetero-nanostructures as Anode Materials for Li-Ion Batteries De Pham-Cong,†,∥ Jun Hee Choi,‡,∥ Jeongsik Yun,§ Aliaksandr S. Bandarenka,§ Jinwoo Kim,# Paul V. Braun,# Se Young Jeong,† and Chae Ryong Cho*,† †

Department of Nanoenergy Engineering and College of Nanoscience and Nanotechnology, Pusan National University, Busan 46241, Republic of Korea ‡ Device & System Research Center, Samsung Advanced Institute of Technology, Samsung Electronics, Suwon 16676, Republic of Korea § Physik-Department ECS, Technische Universität München, James-Franck-Straße 1, 85748 Garching, Germany Nanosystems Initiative Munich (NIM), Schellingstraße 4, 80799 Munich, Germany # Department of Materials Science and Engineering, Frederick Seitz Materials Research Laboratory, University of Illinois at Urbana−Champaign, Urbana, Illinois 61801, United States S Supporting Information *

ABSTRACT: As potential high-performance anodes for Li-ion batteries (LIBs), hierarchical heteronanostructures consisting of TiNb2O7 nanofibers and ultrathin MoS2 nanosheets (TNO@MS HRs) were synthesized by simple electrospinning/hydrothermal processes. With their growth mechanism revealed, the TNO@MS HRs exhibited an entangled structure both for their ionic and electronic conducting pathways, which enabled the synergetic combination of one- and two-dimensional structures to be realized. In the potential range of 0.001−3 V vs Li/Li+, the TNO@MS HR-based LIBs exhibited high capacities of 872 and 740 mAh g−1 after 42 and 200 cycles at a current density of 1 A g−1, respectively, and excellent rate performance of 611 mAh g−1 at 4 A g−1. We believe that the fabrication route of TNO@MS HRs will find visibility for the use of anode electrodes for high capacity LIBs at low cost. KEYWORDS: TiNb2O7 nanofibers, MoS2 nanosheets, hierarchical nanostructure, anode, synergetic behavior

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interaction between its (002) planes, enabling the easy intercalation of Li+ ions without a significant increase in volume.8−11 Moreover, hierarchical nanostructures such as three-dimensional branched, hyperbranched, and multibranched structures are considered to be more attractive in many nanoscale device applications.12−15 However, they still suffer from poor cycling performance. Recently, several research groups have assembled MoS2 on various inactive one-dimensional conducting matrices such as carbon nanotubes, mesoporous CMK-3, carbon nanofibers, and carbon fiber cloth as anode materials for the reversible storage of Li+. These

he Ti−Nb-based compound oxides such as Ti2Nb2O9, TiNb6O17, LiTiNbO5, and TiNb2O7 are known to have multiple redox couples of Ti4+/Ti3+, Nb5+/Nb4+, and 4+ Nb /Nb3+ and low volume expansion during the charge− discharge process.1−5 As a result, these oxides exhibit highly stable and reversible characteristics as anode materials for Liion batteries (LIBs). However, the theoretical capacity of TiNb2O7 is relatively low: 387.6 mAh g−1 for a five-electron transfer (Ti4+/Ti3+ and Nb5+/Nb3+) or 310 mAh g−1 for a fourelectron transfer (Nb5+/Nb3+).4 It has generally been acknowledged that nanostructured electrode materials provide excellent capacity because of the reduced diffusion length of Li ions and increased contact area between active materials and electrolytes.6,7 Among emerging anode materials, MoS2 exhibits high Li storage capacity thanks to the weak van der Waals © 2016 American Chemical Society

Received: November 14, 2016 Accepted: December 31, 2016 Published: December 31, 2016 1026

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revealed the thermal decomposition of polyvinyl pyrolidone (PVP) and other organics, and crystallization into TiNb2O7 occurs simultaneously up to a Tc of 700 °C (Figure S2a, Supporting Information). The crystallinity of the TiNb2O7 nanofibers was still low at a Tc of 700 °C but remarkably increased at a Tc of 850 °C and above; this temperature dependence is observed in the X-ray diffraction (XRD) patterns (Figure S2b, Supporting Information). This finding indicates that the faceted nanograins have high crystalline quality (see Figure S1). Therefore, the TiNb2O7 nanofibers annealed at a Tc of 850 °C were used for further experiments. The macroscopic color resemblance between the TNO@MS HRs and MoS2 indicates uniform MoS2 coating on the TiNb2O7 nanofibers during hydrothermal growth (Figure S3, Supporting Information). Energy-dispersive X-ray spectroscopy (EDS) analyses of the TNO@MS HRs revealed that thermal annealing in an Ar ambient after the hydrothermal growth is required to achieve stoichiometric ratios of both TiNb2O7 and MoS2 (Figure S4, Supporting Information). The TNO@MS HRs consisted of TiNb2O7 nanofibers fully covered by ultrathin MoS2 nanosheets (Figure 1c). To examine the growth mechanism of the TNO@MS HRs, the temporal evolution of the MoS2 morphologies was examined at various hydrothermal growth times (th), as shown in Figure 1a−c. Notably, at th of 1 h, a large number of MoS2 nuclei were observed to form on the surface of the

structures are known to exhibit additional advantages of outstanding electric conductivity, high charge mobility, and large specific surface area.16−21 However, all of the discussed approaches have not yet been successful in satisfying various requirements for next-generation LIB applications. Here, we demonstrate the fabrication of hierarchical heteronanostructures consisting of TiNb2O7 nanofibers and ultrathin MoS2 nanosheets (TNO@MS HRs) using simple and cost-competitive electrospinning/hydrothermal processes. We examine the synergetic advantages of TNO@MS HRs based on the following points: (i) the low volume change of the TiNb2O7 core during lithiation/delithiation should yield excellent cyclic stability at a relatively high rate without the formation of a solid-electrolyte interphase (SEI) layer, (ii) the ultrathin and independent MoS2 shell morphology should result in exceptionally high capacity even beyond the theoretical limit because of the large available sites for Li+ ion storage and high surface contact areas with electrolytes, and (iii) the entangled three-dimensional hierarchical TiNb2O7@MoS2 nanostructures should fully utilize both (i) and (ii) in a synergetic manner.

RESULTS AND DISCUSSION We propose a fabrication strategy for TNO@MS HRs as illustrated in Scheme 1. Uniform polymeric nanofibers with Scheme 1. Schematic Illustration of the Formation of the TNO@MS HRs: (a) As-Electrospun Polymeric TiNb2O7 Nanofibers, (b) TiNb2O7 Nanofibers Formed by Process I, and (c) TNO@MS HRs Composed of MoS2 Nanosheets Grown on TiNb2O7 Nanofibers Formed by Process II

smooth surfaces were first fabricated using electrospinning (Scheme 1a) and subsequent calcination at 850 °C for 4 h, which converted the polymer fibers into polycrystalline TiNb2O7 nanofibers through organic burning and successive crystallization (Scheme 1b). The TNO@MS HRs were subsequently fabricated with successive hydrothermal growth of MoS2 on the TiNb2O7 nanofibers followed by thermal annealing in an ambient Ar atmosphere (Scheme 1c). The TiNb2O7 nanofibers were composed of serially interconnected and faceted nanograins with an average diameter of ∼200 nm. The faceted nanograin morphology was clearly observed when calcination of the aselectrospun nanofibers was performed at 850 °C or above (Figure S1, Supporting Information). With further increase in the calcination temperature (Tc) to 950 °C, grain growth was clearly observed with a slight degradation of the diameter uniformity (Figure S1g,h, Supporting Information). Thermogravimetric analysis (TGA) of the as-electrospun nanofibers

Figure 1. (a−d) SEM images and (e, f) nucleation and growth models as a function of reaction time (1, 4, 8, and 16 h) of the TNO@MS HRs; (g) XRD patterns and (h) Raman spectra of the MoS2 nanosheets, TiNb2O7 nanofibers, and TNO@MS HRs. 1027

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bonding states of the elements of the TiNb2O7 nanofibers and TNO@MS HR samples were investigated using X-ray photoelectron spectroscopy (XPS) techniques (Figures S5 and S6, Supporting Information). Although Ti, Nb, and O elements in the high-resolution XPS spectra were clearly detected in both samples, Mo and S elements were only detected in the TNO@ MS HRs. The microstructures of the TiNb2O7 nanofibers and TNO@ MS HRs were investigated using transmission electron microscopy (TEM) (Figure 2 and Figure S7, Supporting

TiNb2O7 nanofibers (Figure 1a). This result suggests typical heterogeneous nucleation, and the growth mechanism remains valid for the TNO@MS HRs. The nuclei could function not only as growth sites of ultrathin MoS2 nanosheets but also as strong binding sites between the TiNb2O7 nanofibers and MoS2 nanosheets for facilitated electron and ion transport. With a further increase of th to 4, 8, and 16 h, ultrathin MoS2 nanosheets were clearly visible and grown, fully covering the TiNb2O7 nanofibers (Figure 1b−d). From the nuclei, a large number of independently separated ultrathin MoS2 nanosheets consisting of several (002) atomic planes appeared to grow not in parallel but inclined or vertical to the surface of the TiNb2O7 nanofibers, as shown in the schematic (Scheme 1c and Figure 1b−f). In addition, the ultrathin MoS2 nanosheets appeared to grow in the (002) plane direction without any thickness growth. This result could be attributed to the sp2 atomic bonding of molybdenum and sulfur atoms in the (002) plane being energetically favorable compared with the van der Waals bonding between the planes. This MoS2 morphology of maintaining its initial thickness is the key factor in explaining high-performance LIBs, which will be discussed in detail. XRD measurements were performed to investigate the crystallinity of the MoS2 nanosheets, TiNb2O7 nanofibers, and TNO@MS HRs (Figure 1g). The XRD pattern of the bare MoS2 nanosheets displays distinct (100), (103), and (110) peaks of 2H-MoS2 (JCPDS 37−1492) at 33.2°, 39.4°, and 58.7°, respectively.22 The XRD pattern of the TiNb2O7 nanofibers indicates the presence of single-phase TiNb2O7 without any other phases such as TiO 2 , Nb 2 O 5 , or Ti2Nb10O29, which agrees well with the results of previous reports.23,24 The TiNb2O7 phase belongs to the monoclinic symmetry system with C2/m space group (JCPDS 70−2009). However, inspection of the XRD pattern of the TNO@MS HRs reveals that only the (100) peak of MoS2 is slightly distinguishable compared with those of the TiNb2O7 nanofibers. This result implies that the (100) planes of MoS2 are preferentially oriented parallel to the substrate surface, which agrees with our assumption that most (002) planes are not parallel to the surface of nanofibers and hence the surface of the substrate. To further explore the properties of the hierarchical hybrid structures, phonon spectra of the TNO@MS HRs, bare TiNb2O7 nanofibers, and MoS2 were obtained using Raman spectroscopy in the range of 50−1200 cm−1 (Figure 1h). The phonon vibrational modes of the pure TiNb2O7 nanofibers at 894 and 1011 cm−1 can be assigned to the corner-shared and edge-shared NbO6 octahedra, respectively. The strong band observed at 647 cm−1 is assigned to the vibration of the TiO6 octahedra. The peaks of the octahedral coordinations for both cations clearly verify the presence of TiNb2O7.25 The peak intensities for the TNO@MS HRs were relatively low compared with those for the TiNb2O7 nanofibers. However, two additional peaks at 368 and 389 cm−1 clearly verify the presence of MoS2 in the TNO@MS HRs, which are assigned to the representative modes of E2g and A1g of MoS2, respectively.26 It should be noted that the positions of these two peaks are shifted relative to those of bare MoS2 at 370 and 396 cm−1, indicating that the MoS2 nanosheets were surface-strained.27 We believe that the strained MoS2 is direct evidence of the presence of strong or robust interfaces between TiNb2O7 and MoS2. We can conclude that the TNO@MS HRs were synergistically combined, forming 1D−2D hybrid structures where each TiNb2O7 nanofiber was not only fully covered but also tightly bonded with the MoS2 nanosheets. The chemical

Figure 2. (a) TEM image of the TNO@MS HR (the inset presents an electron diffraction pattern of the sample). (b) HRTEM images of TNO@MS HR (the A and B regions represent TiNb2O7 and MoS2, respectively). (c) Cross-sectional, bright-field image of TNO@MS HR and (d) overlapped elemental mapping of oxygen (red) and sulfur (green). Separate mapping images are presented in Figure S9e,f. Fast-Fourier transform images of regions A and B obtained from the SAED patterns as shown in the insets, respectively.

Information). The selected-area electron diffraction (SAED) patterns (Figure S7d,f) confirmed the presence of the rutile phase, polycrystalline TiNb2O7, which agrees with the XRD results. Figure 2a demonstrates that the TNO@MS HR consists of MoS2 nanosheets as a shell and the TiNb2O7 nanofiber as a core. As discussed, the MoS2 nanosheets not only cover the TiNb2O7 nanofibers but also form a robust binding with them. The shell has a length of a few hundred nanometers and a thickness of a few nanometers, confirming the explained growth mechanism (Figure 2b). The core−shell 1028

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viewed in Figure 3b. The mesoscopic and atomistic schematics clearly reveal Li+ conducting channels fully connected from MoS2 to TiNb2O7 and favorable electron transfer paths in MoS2 and TiNb2O7 (Figure 3a−c). As long as the (002) planes of the MoS2 nanosheets are not parallel to the surface of TiNb2O7 nanofiber, the absence of an epitaxial relationship does not appear to significantly reduce ionic or electronic transport across the heterointerface. Such structural features might enable the high performance of the anode by synergistically combining the high charge capacity of MoS2 and facile electron transport of TiNb2O7. The elemental mapping for the TiNb2O7 nanofiber and TNO@MS HR was performed using EDS. In the bare TiNb2O7 nanofiber, Ti, Nb, and O elements were uniformly distributed (Figure S8, Supporting Information). Inspection of the compositional mapping of the TNO@MS HR clearly reveals that TiNb2O 7 nanofibers and MoS 2 nanosheets play roles as a core and shell, respectively (Figure S9a−f, Supporting Information). The uniformity of the Mo and S elements was also maintained in the axial direction, confirming the uniform distribution of the MoS2 nanosheets across the entire TiNb2O7 nanofiber (Figure S9g−l, Supporting Information). To quantitatively examine the synergetic effect of the TNO@ MS HRs on the performance of an anode material for LIBs, their electrochemical performance was measured and compared with that of bare TiNb2O7 nanofibers, bare bulk MoS2, and other MoS2- and TiNb2O7-based hybrid nanostructures (Figure 4). The cycle life was measured for TNO@MS HR-, TiNb2O7 nanofiber-, and bulk MoS2-based anodes in the voltage range of 0.001−3.0 V at a high current density of 1 A g−1 (Figure 4a). Although a discharge potential of below 1.0 V vs Li+/Li to the electrode typically leads to the formation of a SEI layer because of the decomposition of an ethyl/methyl carbonate-based electrolyte, no distinguishable capacity fading behavior was observed. The TNO@MS HR-based anodes exhibited both high charge capacity and cycling stability, delivering discharge/ charge capacities (Cdis/Cch) of 1026/808 mAh g−1 after the first cycle, 870/872 mAh g−1 after the 42th cycle, and 733/740 mAh g−1 after the 200th cycle. However, the TiNb2O7 nanofiberbased anodes exhibited relatively low charge capacity: Cdis/Cch values of 399/306 mAh g−1 after the first cycle and 237/236 mAh g−1 after 200th cycle. The MoS2 bulk-based anodes exhibited a high initial Cdis/Cch of 1343/989 mAh g−1, which rapidly decreased to 159/158 mAh g−1 after the 200th cycle. This behavior is attributed to the layer-by-layer dissociation of the MoS2 nanosheets due to their structural transition from the 2H to the 1T phase by the intercalation of Li ions.21,28 Compared with both the TiNb2O7 nanofiber- and bulk MoS2based anodes, the TNO@MS HR-based anodes exhibited highly stable and reversible Cdis, Cch, and average capacities (Cave = (Cdis + Cch)/2) for various current densities (Figure 4b,c). Cave of the TNO@MS HR-based anodes were 820 and 580 mAh g−1 at 0.2 and 4 A g−1, respectively. The experimental Cave values were even larger than the theoretical values for MoS2 (Ctheo,MoS2 = 670 mAh g−1)29,30 and TiNb2O7 (Ctheo,TNO = 387 mAh g−1). It should be noted that our TNO@MS HRbased anodes exhibit superior electrochemical performances compared to recently reported TiNb2O7- or MoS2-based anodes and even compared to other MoS2-based hybrid nanostructures (see Figure 4d and Table S1, Supporting Information).8,16,18,20

feature was verified using cross-sectional analyses (Figure 2c,d). Note that most of the MoS2 nanosheets were not parallel but inclined to the surface of the TiNb2O7 nanofiber, which is consistent with the XRD analyses. To study the TNO@MS HRs on the atomic scale, we examined TiNb2O7/MoS2 heterointerfaces. Because of the absence of crystal symmetry between TiNb2O7 and MoS2, the heterointerface intrinsically lacks epitaxial relationships (TiNb2O7 is indexed to the monoclinic Bravais lattice (a = 11.90 Å, b = 3.80 Å, c = 20.37 Å, α = γ = 90°, β = 120°, space group: C2/m), and MoS2 is indexed to the hexagonal lattice (a = b = 3.16 Å, c = 12.30 Å, α = 90°, β = 120°, γ = 90°, space group: P63/mmc)). Therefore, several crystallographic orientations are available between them. Figure 3a,b presents two exemplary atomistic views of the heterointerface based on the TEM observations. The first

Figure 3. Atomistic and mesoscopic views of the TNO@MS HRs. TNO and MoS2 heterointerfaces with two different orientation relationships: (a) [−110]TNO//[002]MoS 2 , (001)TNO// (100)MoS2 and (b) [−110]TNO//[100]MoS2, (001)TNO// (010)MoS2. Li+ ions reach the TNO structure by moving between MoS2 layers. (c) Mesoscopic views of the TNO@MS HRs. The pathways of Li+ ions and electrons are displayed.

heterointerface (marked with a yellow line) is in between regions A and B (Figure 2b) having the zone axes of [110] for TiNb2O7 (Figure 2e) and [002] for MoS2 (Figure 2f). Making the zone axes parallel to each other with the arbitrarily selected zone planes, we can present an atomistic view of the first heterointerface (Figure 3a). In the same manner, the second heterointerface (marked with a white line in Figure 2c) can be 1029

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Reaction 3 can yield a Ctheo,MoS2 of 1671 mAh g−1 if the weight of Mo is not considered.32 It is believed that reaction 3 can to some extent contribute to increasing Ctheo,MoS2 beyond the experimental Cave of the TNO@MS HR-based anodes. Next, we examine the high retention characteristics of the TNO@MS HR-based anodes. As demonstrated in Scheme 1c and Figures 1e and 3, the TNO@MS HRs consist of highsurface area MoS2 nanosheet shells and TiNb2O7 nanofiber backbone cores that provide abundant intercalation channels of surrounding Li ions (Li+) and fast electronic conduction channels, respectively. The Li+ and electron conduction pathways are well guided from the MoS2 shells to the TiNb2O7 core through the robust binding sites. More importantly, most of the MoS2 shells exist in the form of independent atomically thick layers, i.e., several (002) planes (Figure 1a−f), which can secure highly active and defective sites that can store Li+ beyond the theoretical limit. Such an ultrathin MoS2 morphology in the TNO@MS HR might be optimal to avoid aggregation and retain a large surface area and small dimensions with minimized volume expansion even under high Li+ loading. In addition, the ultrathin MoS2 nanosheets shorten the diffusion paths of Li+ ions, thus improving the dynamic performance of Li+ insertion/extraction. The diffusion coefficients of Li+ insertion/extraction were 2.44 × 10−12/1.60 × 10−12 and 1.60 × 10−13/0.90 × 10−13 cm2 s−1 in the TNO@ MS HR- and TiNb2O7-based anode, respectively. These values were calculated from the plots of peak current vs the square root of the scan rate using the Randles−Sevcik equation33 (see Figure S10 and Table S2, Supporting Information). It is worth noting that comparison of irreversible capacity loss during the first cycle also suggests that the rate performance between the TNO@MS HR- and the pure TiNb2O7-based anodes is comparable (see Figure S11, Supporting Information). It is concluded that the TNO@MS HRs exhibit high capacity, excellent cycling stability, and outstanding rate capability for LIB anodes under high current densities and a voltage window of 0.001−3.0 V. To further study the electrochemical process in detail, additional comparative measurements were performed for the TiNb2O7- and TNO@MS HR-based anodes. Figure 5a,b presents the cyclic voltammetry (CV) measurements of the TiNb2O7- and TNO@MS HR-based anodes. For the bare TiNb2O7-based anode (Figure 5a), the overall Li insertion (reduction, cathodic)/extraction (oxidation, anodic) reaction can be described by the following equation:

Figure 4. (a) Specific capacity as a function of the cycle number of TNO nanofibers, bare bulk MoS2, and TNO@MS HRs in the voltage range of 0.001−3.0 V at a current density of 1 A g−1. (b, c) Charge/discharge and average capacities for various current densities. (d) Comparison of the average capacity value at various current densities of the previously reported results related to MoS2based composites and our work. (e) Excess number of Li atoms calculated considering the number of MoS2 layers. A Li atom is additionally considered to bind on both sides of the surfaces of MoS2 layers.

First, we discuss the reason for the high capacity beyond the Ctheo,MoS2 of 670 mAh g−1, corresponding to each MoS2 unit taking up 4 Li ions (Li+) into the intercalated forms between MoS2 layers using the following successive reactions (eqs 1 and 2): MoS2 + x Li+ + x e− ↔ LixMoS2

( ∼1.1 V vs Li/Li+) (1)

LixMoS2 + (4 − x)Li+ + (4 − x)e− ↔ Mo + 2Li 2S ( ∼0.6 V vs Li/Li+)

(2) +

We propose that excess Li ions are stored outside the topmost and bottommost MoS2 layers through various defective sites.31 For example, assuming that the same amount of Li+ can be stored in the topmost or bottommost layers as in between the layers, the excess Li+ in the nanoshell can be calculated. Compared with the capacity in the bulk, 200%, 100%, 66.7%, and 50% excess Li+ can be stored for the shells consisting of two, three, four, and five (002) planes, respectively (Figure 4e). It is evident that a thinner MoS2 shell is preferable if this storage mechanism is valid. Considering these values, the maximum calculated Cave according to this mechanism (∼795 mAh g−1 for five-(002) plane-thick MoS2) remains lower than the experimental value, indicating that this mechanism cannot be the sole reason for the high capacity. As an auxiliary mechanism, an additional lithiation should be considered through reaction eq 3 following eqs 1 and 2: S + 2Li+ + 2e− ↔ Li 2S

( ∼2.2 V vs Li/Li+)

TiNb2 O7 + x Li+ + x e− ↔ LixTiNb2 O7

(0 ≤ x ≤ 5) (4)

The peaks at 1.60 and 1.69 V vs Li/Li during the first cathodic and anodic sweep correspond to the Ti4+/Ti3+ and Nb5+/Nb3+ redox couples, respectively. The oxidation peak of 1.02 V is assigned to Nb4+/Nb3+ redox couples. For the TNO@ MS HR-based anode (Figure 5b), additional MoS2-related peaks appeared, and their overall reaction can be described by eqs 1 and 2. The transient peaks at 0.49 and 0.83 V during the first cathodic scan and at ∼1.24 V during the second cathodic scan might correspond to the decomposition of MoS2 into Mo nanoparticles embedded in a Li2S matrix by a series of reactions (eqs 1 and 2).34 The oxidation/reduction peaks at 2.22/1.85 V vs Li/Li+ should correspond to Li+ extraction/intercalation by the oxidation/reduction of Li2S (reverse/forward reaction in eq 3).32,35 The oxidation peak corresponding to Li+ extraction from the LixMoS2 lattice (reverse reaction in eq 1) may overlap +

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When the current density returned to 0.2 A g−1, the capacity of the TNO@MS HRs increased up to 889 mAh g−1, which is even higher than its initial value. It should be pointed out in Figure 5f that the average lithiation voltage of TNO@MS HRbased anodes is quite high (∼1.6 V). This means that the overall-energy-density value of TNO@MS HR-based LIBs might not be as remarkable as that of charge capacity (see Figure S12, Supporting Information). In order to fully utilize the merit of high capacity, we are currently investigating the reduction of the lithiation voltage by incorporating other materials such as carbon into TiNb2O7 nanofibers during electrospinning process. We further examined the electrochemical performance of the TNO@MS HR-based anodes in other electrolytes such as LiClO4/propylene carbonate to evaluate their robustness in different environments (Figure 6). With slight differences in the

Figure 5. (a, b) CV measurements at a scan rate of 0.1 mV s−1 in the region of 3.0−0.001 V vs Li/Li+ for TNO nanofibers and TNO@MS HRs. (c,d) Nyquist plots of the fresh TiNb2 O 7 nanofibers and those after cycling and TNO@MS HR anodes. (e, f) Charge/discharge curves of the TiNb2O7 nanofibers and TNO@ MS HR anodes at various current densities in the voltage range of 0.001−3.0 V.

with that of TiNb2O7 at 1.69 V. In particular, the clear and stable existence of the 2.22/1.85 V peaks indicates that reaction 3 is stable and reversible, supporting the previously discussed reason for the high Cave of the TNO@MS HR-based anode. These peaks should be reversible as there is no significant change in the second cycle. Next, the Nyquist plots of the TiNb2O7- and TNO@MS HR-based anodes obtained using electrochemical impedance spectroscopy (EIS) are displayed in Figure 5c,d. The diameter of the semicircle at high frequencies is remarkably reduced in the plot of the TNO@MS HRs compared with that of the bare TiNb2O7 nanofiber, indicating the greatly decreased charge-transfer resistance at the electrode/electrolyte interface due to robust binding to fully utilize the high surface area and high electrical conductivity of MoS2. Irrespective of the large surface area, bare MoS2 nanoparticles or powders are also reported to exhibit low charge-transfer resistance at the electrode/electrolyte interface because of their high electrical conductivity.36 This enhanced charge-transfer in the TNO@MS HR could maximize the LIB performance. Finally, parts e and f of Figure 5 display the charge/discharge curves of the TiNb2O7 nanofiber- and TNO@MS HR-based anodes at various current densities in the voltage range of 0.001−3.0 V for a detailed investigation of Figure 3b,c. It can be clearly observed that the rate performance of the TNO@MS HR-based anodes is excellent even compared with other anode materials including the TiNb2O7 nanofibers. The Cdis values of the TNO@MS HRs were 844−611 mAh g−1 in the current density range of 0.2−4 A g−1, respectively. However, the Cdis values of the TiNb2O7 nanofibers were relatively poor: 407−135 mAh g−1 under the same conditions.

Figure 6. (a) CV measurements at a scan rate of 1 mV s−1 in the region of 3.0−0.001 V vs Li/Li+ for TNO@MS HRs. (b) EIS spectra of TNO@MS HRs at 1.0, 1.5, and 2.0 V vs Li/Li+; the open circles are experimental data points (with 95% accuracy in K−K check), and the lines represent the corresponding fitting. The inset shows the equivalent electric circuit elucidated to fit the EIS spectra. Ru, uncompensated resistance; Zdl, impedance of the double layer; Rct, charge transfer resistance; the other R and C elements are the adsorption (pseudo)resistances and (pseudo)capacitances, respectively, which are given by complex combinations of physicochemical parameters of the interfacial charge and mass transfer.37,38

oxidation/reduction peaks, the CV curves in Figure 6a primarily exhibit the similar behavior as those measured in our typical LiFP6-based electrolyte solution (see Figure 5b and Figure S10, Supporting Information). The shape of the EIS spectra and the trend of the impedance reduction (Figure 6b) are also similar to those in Figure 5c,d. As discussed, these findings indicate that the lithiation/delithiation occurs in multiple stages, which is attributed to differences in the diffusion rate between the electrode and electrolyte. It should be noted that the experimental EIS data were well fitted with the equivalent electric circuit model depicted in the inset of Figure 6b.37,38 1031

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using a doctor blade on clean Cu foil and then dried in a vacuum oven at 100 °C for 12 h. The mass of the active materials in each working electrode disk with a diameter of ∼1.4 cm was calculated to be approximately 1.0−1.2 mg cm−2. The dried electrode was punched under a pressure of 4 MPa in a glovebox filled with Ar gas to prepare the coin cells. Then a 1 M LiPF6 solution in ethylene carbonate/ dimethyl carbonate (1:1 vol %) with 10% fluoroethylene carbonate was used as an electrolyte. Pure Li foil (thickness of 1.3 mm and diameter of ∼1.6 cm) was used as the counter electrode, and the separator was Celgard 2300. The electrochemical properties of the samples were investigated using a multichannel potentiostat/ galvanostat (Wonatech, WMPG 1000) in the voltage range of 0.001−3.0 V vs Li/Li+ at room temperature at current densities ranging from 0.2 to 4 A g−1. CV measurements were performed in the voltage range of 0.001−3.0 V at various scan rates from 0.1 to 2.5 mV s−1. The ac impedance spectra were collected by applying a sine wave with amplitude of 5.0 mV in the frequency range of 100 kHz to 0.01 Hz. CV and EIS measurements were also performed to investigate the interfacial mass and charge transfer mechanism during intercalation/ deintercalation for the TNO@MS HRs under the same conditions (except using a 1 M LiClO4 solution in propylene carbonate (PC)). A “Compactstat electrochemical interface” (IVIUM Technologies) potentiostat enhanced with a “Compactstat Plus II compliance booster” (IVIUM Technologies) was used to obtain greater scan ranges. Data were collected with “IVIUM soft” v2.509″. The 1 M LiClO4/PC electrolyte was prepared under an Ar (5.0, Air Liquid, Germany) atmosphere. For the analysis and fitting of the impedance data, a homemade software “EIS Data Analysis 1.2” was used as described in detail elsewhere.39,40

CONCLUSION In summary, we rationally designed and synthesized TNO@MS HRs composed of two-dimensional MoS2 nanosheets grown around one-dimensional TiNb2O7 nanofiber cores with a robust MoS2/TiNb2O7 interface. The ultrathin/independent MoS2 growth morphology was well explained based on the typical nucleation and growth mechanism. The TNO@MS HR exhibited synergistic performance including high capacity, excellent rate capability, and outstanding cycling stability. We anticipate that this hierarchical nanostructure can be applied in high-power LIBs, supercapacitors, and various catalysts. EXPERIMENTAL SECTION Materials Preparation. First, the TiNb2O7 nanofibers were synthesized using a simple electrospinning technique. In a typical synthesis procedure, the sol−gel homogeneous transparent solution was prepared by mixing with 1.1 g of niobium(V) ethoxide (Nb(OC2H5)5), 0.6 g of titanium(IV) butoxide (Ti(OC4H9)4), and 0.7 g of PVP, Sigma-Aldrich, Mw = 1300000), which was added into a solution containing 9 mL of ethanol and 2 mL of acetic acid. After 3 h of stirring, the prepared solution was transferred into a 15 mL syringe with a 25-gauge blunt tip needle. A voltage of 15 kV was applied between the tip and aluminum foil at a distance of approximately 20 cm. The as-electrospun TiNb2O7 nanofibers were collected by controlling the syringe pump with the sol−gel solution at a flow rate of 0.5 mL h−1. The dried polymetric TiNb2O7 nanofibers were crystallized into one-dimensional TiNb2O7 nanofibers by annealing at 850 °C for 5 h with a 2 °C min−1 ramp rate in air via the thermal decomposition and combustion of the polymer. The TiNb2O7 nanofibers were used as a backbone for loading MoS2. In the next step, the MoS2 nanosheets were directly grown onto the TiNb2O7 nanofibers via a low-temperature hydrothermal method. In a typical preparation, 0.3 g of sodium molybdate dehydrate (Na2MoO4· 2H2O) was first added to 40 mL of a 0.03 M glucose solution and then stirred for 0.5 h. Subsequently, 0.6 g of thiourea (N2H4CS) was added to the prepared solution, which was further stirred for another 0.5 h. The solution mixed with 0.2 g of TiNb2O7 nanofibers was transferred into a 60 mL Teflon-lined autoclave and then maintained at 200 °C for 24 h. After a hydrothermal reaction, the autoclave was cooled to room temperature, and the TNO@MS HR product was obtained. The postannealing process was performed in a tube furnace at 800 °C for 2 h with a 2 °C min−1 ramp rate in an Ar/10%H2 mixture gas. For comparison with the TNO@MS HRs, MoS2 nanosheets were also prepared using the same hydrothermal process without the TiNb2O7 nanofiber backbone. Characterization. The morphology of the samples was characterized using scanning electron microscopy (SEM; Hitachi, S4700) at an operating voltage of 15 kV. The crystal structure of the active materials was determined using XRD (PANalytical, MPD-Pro) in θ− 2θ scan mode with 2θ ranging from 10° to 80° and a Cu Kα1 radiation source. To investigate the reaction process with increasing temperature of the as-spun polymeric nanofibers, TGA (TA Instruments, Q500) was performed from room temperature to 1000 °C with a heating rate of 5 °C min−1 in air. Crystalline structures of the samples were determined, and elemental mapping was performed using TEM (JEOL, JEM2011). Raman spectra were obtained using a Raman spectrometer (Dongwoo Optron, MonoRa500i) with a 532 nm excitation light source of an Ar laser. The chemical bonding states of the pristine TiNb2O7 nanofibers and TNO@MS HRs were analyzed using XPS (VG Scientific, ESCALAB250) with monochromic Al Kα radiation (1486.6 eV). The binding energy in the spectra was calibrated with carbon (C 1s = 284.6 eV). Electrochemical Measurements. The electrochemical properties of the MoS2 nanosheets, TiNb2O7 nanofibers, and TNO@MS HRs were evaluated using CR2032 coin cells with Li metal as the counter electrode. The working electrode was prepared by spreading a slurry composed of the active materials, carbon black, and carboxymethyl cellulose with a weight ratio of 70:15:15. The mixed slurry was coated

ASSOCIATED CONTENT S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.6b07666. Figures S1−S12 and Tables S1,S2 as referenced in the paper (PDF)

AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. ORCID

Aliaksandr S. Bandarenka: 0000-0002-5970-4315 Paul V. Braun: 0000-0003-4079-8160 Chae Ryong Cho: 0000-0002-6685-4012 Author Contributions ∥

D.P.-C. and J.H.C. contributed equally.

Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was supported by a research program (Grant No. NRF-2015R1D1A3A01018611) through the Ministry of Education of the Korean government. REFERENCES (1) Colin, J. F.; Pralong, V.; Hervieu, M.; Caignaert, V.; Raveau, B. Lithium Insertion in an Oriented Nanoporous Oxide with a Tunnel Structure: Ti2Nb2O9. Chem. Mater. 2008, 20, 1534−1540. (2) Lin, C.; Wang, G.; Lin, S.; Li, J.; Lu, L. TiNb6O17: A New Electrode Material for Lithium-Ion Batteries. Chem. Commun. 2015, 51, 8970−8973. (3) Catti, M.; Pinus, I.; Ruffo, R.; Salamone, M. M.; Mari, C. M. A Novel Layered Lithium Niobium Titanate as Battery Anode Material: 1032

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