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Hierarchical MoO/MoC/C Hybrid Nanowires as HighRate and Long-Life Anodes for Lithium-Ion Batteries Lichun Yang, Xiang Li, Yunpeng Ouyang, Qingsheng Gao, Liuzhang Ouyang, Renzong Hu, Jun Liu, and Min Zhu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b05049 • Publication Date (Web): 12 Jul 2016 Downloaded from http://pubs.acs.org on July 13, 2016

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ACS Applied Materials & Interfaces

Hierarchical MoO2/Mo2C/C Hybrid Nanowires as High-Rate and Long-Life Anodes for Lithium-Ion Batteries Lichun Yang,a Xiang Li,a Yunpeng Ouyang,a Qingsheng Gao,b Liuzhang Ouyang,a Renzong Hu,a Jun Liu,a Min Zhu*a a

School of Materials Science and Engineering, Guangdong Provincial Key

Laboratory of Advanced Energy Storage Materials, South China University of Technology, Guangzhou, 510641, P. R. China. b

Department of Chemistry, Jinan University, Guangzhou, 510632, P. R. China.

KEYWORDS: molybdenum dioxide, molybdenum carbide, hybrid materials, capacitive behavior, lithium-ion battery

ABSTRACT

Hierarchical MoO2/Mo2C/C hybrid nanowires (MoO2/Mo2C/C HNWs) have been fabricated through facile calcination of Mo3O10(C6H5NH3)2·2H2O nanowires which serve as both precursors and self-templates. In the MoO2/Mo2C/C HNWs, nanoparticles dispersed in the nanowires are beneficial for Li+ transportation due to the decreased diffusion paths. Moreover, hybridization with Mo2C and carbon 1 ACS Paragon Plus Environment


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facilitates the electron transfer and increase the structural stability without sacrifice of capacity. As anode materials for lithium-ion batteries, the MoO2/Mo2C/C HNWs exhibit a reversible capacity of 950 mAh g-1 after 320 cycles at a current density of 200 mA g-1. Even when cycled at 2000 mA g-1, they maintained a reversible capacity of 602 mAh g-1 after 500 cycles. By incorporation of Mo2C and C with MoO2, the MoO2/Mo2C/C HNWs show high-rate capability and long cycle-life, and can be a promising candidate for lithium-ion battery anodes.

Introduction Lithium-ion batteries (LIBs) have prevailed in portable electronic devices as power sources on account of their high energy density. However, the theoretical capacity of the conventional graphite anode for LIBs is only 372 mAh g-1, which hinders the application of LIBs as power sources for hybrid electric vehicles (HEVs) and electric vehicles (EVs).1 To meet the ever-growing demand for LIBs with high reversible capacity, stable cycle performance and capacitor-like rate capability, transition metal oxide-based anode materials have been widely studied because they can store much more Li ions than commercial graphite through conversion mechanism.2-4 Among various transition metal oxides, molybdenum dioxide (MoO2) with metallic conductivity and high theoretical capacity (838 mAh g-1) received great attention.5-7 Nevertheless, sluggish transportation of Li+ in the bulk causes poor kinetics for reversible Li+ storage/release, which hinders the capacity delivery. Therefore, MoO2 bulks suffer from poor cyclic stability and rate capability.8

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To improve the electrochemical performance of MoO2, a lot of efforts have been made, which can be summarized into downsizing and hybridization. Construction of nanostructures with greatly reduced paths for Li+ diffusion can effectively enhance the kinetics for Li+ storage/release.5, 9-12 Furthermore, hybridization with other materials, e.g., carbon,13 sulfur,14 Mo2N,15 etc. facilitates the electronic transport as well as buffers the volume variation during the charge/discharge process. There have been many reports on nanocomposites of MoO2 with various carbon forms, including carbon nanotubes,16 graphene sheets,17-24 amorphous carbon,25-30 etc. However, high content of carbon materials with relatively low capacity usually brings down the overall capacity of the composite. Therefore, hybridizing with alternative materials processing high conductivity as well as high capacity is desirable to enhance the reversible Li+ storage performance of MoO2 to a higher degree. Transition metal carbides (TMCs) have been interesting active materials for electrodes owing to their advantageous properties including extreme hardness, high electrical conductivity and chemical stability. Especially, as an emerging anode material, Mo2C with a high specific conductance of 1.02×102 S cm-1 has recently drawn much attention because they exhibit high capacity (600-1000 mAh g-1) which far exceeds that of amorphous carbon.31-34 Therefore, when hybridized with MoO2, Mo2C can increase the conductivity of the composite and contribute high capacity at the same time.35-37 For example, MoO2/Mo2C heteronanotubes exhibit reversible capacities of 790 and 510 mAh g-1 at current densities of 200 and 1000 mA g-1, respectively, after 140 cycles.36 And MoO2/Mo2C/C spheres maintain reversible 3 ACS Paragon Plus Environment


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capacities of ~900 and ~700 mAh g-1 after 200 cycles at current densities of 100 and 500 mA g-1, respectively.37 Herein, we develop a facile synthesis of hierarchical MoO2/Mo2C/C hybrid nanowires (denoted as MoO2/Mo2C/C HNWs) using Mo3O10(C6H5NH3)2·2H2O nanowires as an precursor and self-template (Scheme 1). The precursor possesses a uniform organic/inorganic hybrid structure at molecular level. In the high temperature annealing process, the aniline molecules act as carbon source and reductant in the Ar flow, resulting in partial carbonization of molybdate ions. In the composite, Mo2C reduces the charge transfer resistance and contributes capacity. Meanwhile, residual free carbon provides pathways for fast electron transfer as well as buffers the volume change during the discharge/charge cycles. MoO2/Mo2C/C HNWs take advantages of Mo2C and carbon, therefore exhibit high rate performance and long-term cycle stability without sacrifice of capacity. At a current density of 200 mA g-1, they delivered a reversible capacity of 950 mAh g-1 after 320 cycles, and even at a high current density of 2000 mA g-1, they maintained a reversible capacity of 602 mAh g-1 after 500 cycles. Moreover, through electrochemical impedance spectra (EIS) and cyclic voltammetry (CV), we evidenced enhanced capacitive effect during the lithiation/delithiation process of the MoO2/Mo2C/C HNWs, which originated from the hybridization with Mo2C and carbon in nanoscale and attributed to the fast kinetics of Li+ storage/release.


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Scheme 1. Schematic illustration of the synthesis and structure of the MoO2/Mo2C/C HNWs.

Experimental Section Synthesis of MoO2/Mo2C/C HNWs The precursor of Mo3O10(C6H5NH3)2·2H2O nanowires was synthesized based on the previous work.38 First, 1.24 g (NH4)6Mo7O24·4H2O and 1.67 g aniline was dissolved in 40 mL de-ionized water. Then HCl (1 M) was added dropwise into the mixture under magnetic stirring until a white precipitate appeared (at pH 4-5). The mixture was transferred to an oil bath at 50 °C, stirred for 2 hours, and then filtered to harvest Mo3O10(C6H5NH3)2·2H2O nanowires. The obtained Mo3O10(C6H5NH3)2·2H2O nanowires were washed with ethanol, dried at 60 °C and then annealed at 650 oC for 5 h in an Ar flow (200 mL min-1) to synthesize MoO2/Mo2C/C HNW. After the furnace cooling down, the MoO2/Mo2C/C HNWs were finally harvested. Physical characterization The structure characterization of the MoO2/Mo2C/C HNWs were conducted by X-ray diffraction (XRD) (Bruker D8 diffractometer, Cu Kα).The contents of MoO2, Mo2C 5 ACS Paragon Plus Environment


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and C are determined on the basis of the thermogravimetric analysis coupled with differential scanning calorimetry (TGA/DSC, NETZSCH STA449F3) under air flow and CHN elemental analysis (Vario EL Elementar). N2 sorption isotherms were recorded on a Quanta chrome Autosorb-iQ-MP adsorption analyzer at -196 °C (77K), according to which specific surface area and pore distribution are determined by Brunauer-Emmett-Teller (BET) and Barrett-Joyner-Halenda (BJH) methods, respectively. The morphologies and microstructure of the materials were investigated by scanning electron microscopy (SEM, ZEISS ULTRA55) and transmission electron microscopy (TEM, JEOL JEM 2100F). To investigate the morphology of free carbon, MoO2 and Mo2C were removed by immersing the MoO2/Mo2C/C HNWs into the solution of H2O2 (30 wt%) for two hours, and the remaining product was then subjected to TEM observation. Electrochemical measurements The MoO2/Mo2C/C HNWs was assembled into CR2016 coin-type cells for the test of electrochemical performance. The working electrodes were composed of MoO2/Mo2C/C HNWs as active material (80 wt%), Super-P as conductivity agent (10 wt%), and carboxymethyl cellulose sodium salt (Na-CMC) as binder (10 wt%). The mass loadings on the Cu foil current collectors are 0.8~1.0 mg cm-2. Coin cells were assembled in an argon-filled glove box, using Li metal foil as counter and reference electrodes, polyethylene membranes (Teklon@Gold LP) as separators, and 1M LiPF6 in a mixture of diethyl carbonate and ethylene carbonate and (DEC:EC=2: 1 w/w) with fluoroethylene carbonate (FEC, 10 wt%) (Tinci Co., Ltd, China) as electrolyte. 6 ACS Paragon Plus Environment

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The galvanostatic discharge/charge measurements were performed with a cut-off voltage ranging from 0.05 to 3 V vs. Li/Li+ on a battery tester (LAND, CT2001A). The capacities were calculated based on the total weight of the MoO2/Mo2C/C composite. EIS and CV measurements were performed on an electrochemical work station (Gamry Interface 1000). EIS of the half cells apply a sine wave with amplitude of 5 mV ranging from 0.01 Hz to 100 kHz at selected discharge/charge voltages in different cycles. The CV measurements were performed at a sequence of scanning rates with a cut-off voltage of 0-3.0 V vs. Li/Li+. The commercial MoO2 nanoparticles with BET surface area of 3.7 m2 g-1 (denoted as MoO2 NPs, Figure S1) were also assembled into cells and tested for comparison. Results and discussion

Figure 1. (a) XRD pattern, (b) TGA-DSC, (c) N2 sorption isomers, (d) SEM, (e) TEM and (f) HR-TEM images of the MoO2/Mo2C/C HNWs. The inset of Figure 1e is the SAED pattern obtained on the nanowire. 7 ACS Paragon Plus Environment


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Figure 1a shows the XRD pattern of the as-prepared nanowires. The diffraction peaks at 2θ = 26.1o, 37.1o, 53.7o, 60.4o, 66.8o and 79.0o can be indexed to the (011), (020), (022), (031), (131) and (040) planes of monoclinic MoO2 (JCPDS No. 65-1273), respectively. And the peaks at 2θ = 34.3o, 38.0o, 39.3o, 52.1o, 61.4o, 69.5o, 74.5o and 75.5o are ascribed to the (021), (200), (121), (221), (040), (321), (240) and (142) of orthorhombic Mo2C (JCPDS No. 31-0871). The XRD pattern demonstrates the hybrid composition of the final product. In TGA-DSC measurement (Figure 1b), the weight loss below 200 oC is related to the vaporization of the absorbed water. Above 200 oC, the weight rises to a maximum before falls to a stable value, which is attributed to the oxidation of MoO2 and Mo2C and free carbon. The CHN elemental analysis determined the content of C in the composite is 5.9 wt%. Combining the TGA and CHN elemental analysis, the composition of the composite can be determined as 72.2 wt% MoO2, 23.3 wt% Mo2C and 4.6 wt% free carbon. Based on the N2 sorption isotherms (Figure 1c), the BET surface area of the MoO2/Mo2C/C HNWs is determined to be 91.8 m2 g-1, indicating well-exposed surface which facilitates electrochemical reactions. The MoO2/Mo2C/C composite was further investigated by SEM and TEM (Figures 1d ~1f). The SEM image (Figure 1d) shows the composite is nanowires, the morphology of which inherits from the 1D precursor of Mo3O10(C6H5NH3)·2H2O (Figure S2). The nanowires with rough surface are 100-200 nm in diameter and several micrometers in length. The TEM image reveals a single nanowire is composed of nanoparticles (Figure 1e), forming a hierarchical structure. The as-obtained 8 ACS Paragon Plus Environment

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selected area electron diffraction (SAED) pattern on the nanowire is well indexed by MoO2 and Mo2C (inset of Figure 1e), which is in accordance with the XRD result. As further analyzed by high-resolution TEM (HR-TEM, Figure 1f), the lattice fringes of two closely packed nanoparticles can be clearly observed. The lattice fringes of approximate 0.34 nm is ascribed to the (011) plane of MoO2, and those of 0.23 and 0.18 nm are associated with the (121) and (221) planes of Mo2C, respectively. After the removal of MoO2 and Mo2C nanoparticles by H2O2, one dimensional amorphous carbon residual was obtained (Figure S3), demonstrating the continuous light-colored wire-like matrix in MoO2/Mo2C/C (Figure 1e) is free carbon. The TEM observation demonstrates the hierarchical structure of the nanowire in which irregular MoO2 and Mo2C nanoparticles directly contact or interconnected by continuous carbon matrix. The small dimensions of the built-up unit of MoO2 and Mo2C nanoparticles are beneficial for Li+ diffusion, while the continuous nanocarbon facilitates charge transfer and avoids agglomeration of nanoparticles.



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Figure 2. Electrochemical performance of the MoO2/Mo2C/C HNWs as anode materials for LIBs: (a) CV curves tested with a cut-off voltage of 0-3V vs. Li/Li+ at 0.5 mV s-1, (b) discharge/charge profiles tested at a current density of 200 mA g-1 , and comparison of the electrochemical performance of the MoO2/Mo2C/C HNWs and the MoO2 NPs: cyclic performance tested at (c) 200 mA g-1, (d) 2000 mA g-1, and (e) rate capability. To study the reversible Li+ storage performance of the MoO2/Mo2C/C HNWs, CV measurements were carried out (Figure 2a). In the first cycle, two redox couples located at 1.54/1.70 V and 1.25/1.43 V can be observed on the CV profiles of the MoO2/Mo2C/C HNWs. These redox peaks are attributed to the partial insertion of Li+ into MoO2 to from LixMoO2 (Eq. 1), which results in reversible phase transition (monoclinic-orthorhombic-monoclinic).8, 36 The cathodic current increases when the 10 ACS Paragon Plus Environment

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potential is below 1 V, corresponding to the formation of the solid-electrolyte interphase (SEI) and the conversion reaction of LixMoO2 with Li+ (Eq. 2).5, 11 As reported, Mo2C shows activity towards reversible storage of Li+ through the conversation reaction (Mo2C+xLi++xe-→2Mo+LixC).34,32 Accordingly, the cathodic peak corresponding to the conversion reaction of Mo2C with Li+ overlaps with that of 1.25 V, while the anodic peak at 1.47 V is related to the Li+ release from LixC.32-33 As the CV cycle proceeds to the 50th, the cathodic peaks located at 1.25 and 1.54 V merge to one peak at 1.34 V, and the anodic peak located at 1.70 V becomes vague. At the same time, the current intensity below 0.6 V increases and an evident cathodic peak emerges at around 0.2 V. This phenomenon indicates that LixMoO2 converts to metallic Mo and Li2O more completely upon cycling, and the conversion reaction become the dominant mechanism for the reversible storage of Li+ in the activation process.11, 36 Note that after the activation process, the anodic current rises in the region of 2-3 V, indicating the increase of reversible capacity, which may be associated with partial decomposition of the polymeric species in the SEI film.

MoO2+xLi++xe- ↔LixMoO2

(1)

LixMoO2+(4-x)Li++(4-x)e-↔2Li2O+Mo

(2)

The discharge/charge curves of the MoO2/Mo2C/C HNWs recorded at 200 mA g-1 over a potential of 0.05-3V are shown in Figure 2b. In the initial cycle, there are two discharge plateaus at 1.56 and 1.27 V and three charge plateaus located at 1.38, 1.52 and 1.69 V, which are in accordance of the redox peaks in the CV profile (Figure 2a). The MoO2/Mo2C/C HNWs exhibit the initial discharge and charge capacities of 924 11 ACS Paragon Plus Environment


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mAh g-1 and 639 mAh g-1, respectively. The capacity loss in the first cycle is owing to the irreversible processes, including the trapping of Li+ in the MoO2 lattice and the generation of SEI. From the 2nd cycle to 100th cycle, the capacity gradually increases to 944 mAh g-1, which even exceeds theoretical capacity of MoO2 based on the conversion reaction. This phenomenon is commonly observed for the MoO2 based materials.5, 11, 22, 30 The reasons for the capacity increase can be as follows: (1) The phase-transition plateaus in the discharge/charge profiles (Figure 2b) and the corresponding peaks in the dQ/dV curves (Figure S4) become indistinct upon cycling, indicating further downsizing or amorphization of the MoO2 and Mo2C nanoparticles due to the electrochemical milling effect during the repeated discharge/charge processes, which provides more active sites for Li+ storage/release and facilitates the Li+ transfer. (2) The discharge capacity increases mainly in the potential below 1.0 V vs. Li/Li+, and the charge capacity correspondingly rises in the potential of 0-1.0 V vs. Li/Li+ (Figure S4 and S5), indicating more complete reversible transformation of LixMoO2 to metallic Mo. Due to the increase of the amount metallic Mo, the charge transfer resistance is reduced accordingly during the activation process as will be verified by EIS analysis later, which further enhances the Li+ storage/release. (3) In the potential region of 2.0-3.0 V vs. Li/Li+, the capacity differential value rises in the dQ/dV curves (Figure S4) and reversible capacity increases markedly (Figure S5). This observation is possibly related with the reversible decomposition/formation of polymeric gel-like film, which accounts for the extra capacity 39-40


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The cyclic performance of the MoO2/Mo2C/C HNWs tested at the current density of 200 mA g-1 is presented in Figure 2c. The capacity of the MoO2/Mo2C/C HNWs greatly increases in the initial 50 cycles, and become stable after 100 cycles. It retains as high as 950 mAh g-1 even after 320 cycles. Like that of the MoO2/Mo2C/C HNWs, the capacity of the MoO2 NPs also rises upon the cycling. However, after reaching the maximum value, it quickly fades subsequently, and only retains 440 mAh g-1 in the 280th cycle. When the current density increases to 2000 mA g-1, the MoO2/Mo2C/C HNWs also exhibit high capacity and stable cycling performance (Figure 2d). After an activation process, they reach the highest reversible capacity of 640 mAh g-1 in the 200th cycle, and still maintain 602 mAh g-1 in the 500th cycle. The capacity fading is only 0.02% per cycle during the 200th -500th cycle. In contrast, the MoO2 NPs exhibit a reversible capacity of 341 mAh g-1 in the first cycle, and only maintain 128 mAh g-1 after 500 cycles. Apart from the superior cyclic performance, the MoO2/Mo2C/C HNWs also exhibit excellent rate performance at various current densities, as shown in Figure 2e. In the initial 30 cycles, the MoO2/Mo2C/C HNWs show reversible capacities from 559 to 635 mAh g-1 at 1 A g-1, due to the activation process. They still maintain reversible capacities ~595 mAh g-1 even at a high rate of 5 A g-1 during the 120-150th cycles. When the current density drops back to 1 A g-1, the capacity stables at ~810 mAh g-1 during the 180-210th cycles. On the contrary, the specific capacity of MoO2 NPs merely retains ~50 mAh g-1 at 5 A g-1, and regains ~350 mAh g-1 when the rate returns to 1 A g-1 after 210 cycles. Obviously, the overall Li+ storage/release 13 ACS Paragon Plus Environment


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performance of the MoO2/Mo2C/C HNWs is significantly higher than that of the MoO2 NPs. Up to now, it is still a challenge for MoO2-based anode materials to have both high capacity and long cycle life at high current density. Table S1 compares the recently-reported MoO2-based materials with discharge/charge cycles exceeding 100 cycles and/or at current densities higher than 500 mA g-1. Obviously, our MoO2/Mo2C/C HNWs are featured by the high capacity retention and long-cycle life even at high current density, which verifies the effectiveness of Mo2C and C hybridization with MoO2 in the nanowires. In the nanowires, MoO2 nanoparticles with small dimensions facilitate the Li+ diffusion, and Mo2C nanoparticles increase the conductivity of the composite and contribute capacity at the same time. Moreover, the continuous carbon matrix connecting the active nanoparticles not only offers fast ways for electron transfer but also plays a buffer role to accommodate the volume expansion/retraction during repeated cycles. The stability of the structure is verified in Figure S6, which shows the well maintained hierarchical morphology of MoO2/Mo2C/C HNWs after 100 cycles of discharge/discharge at a current density of 2000 mA g-1. By combining the advantages of Mo2C and carbon, MoO2/Mo2C/C HNWs exhibit high rate capability and long-term cycle stability without sacrifice of capacity. To understand the superior electrochemical performance of the MoO2/Mo2C/C HNWs, we measured the EIS of the MoO2/Mo2C/C HNWs and the MoO2 NPs at the same potential of 2 V in the first discharge cycle. As displayed in Figure 3a, each Nyquist plot consists of a depressed semicircle in the high and medium frequency 14 ACS Paragon Plus Environment

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corresponding to the resistance of charge transfer, and a declined line in the low frequency corresponding to the Warburg impedance which is related to the Li+ diffusion in the electrode materials. Obviously, the diameter of the semicircle of the MoO2/Mo2C/C HNWs is much smaller than that of the MoO2 NPs, implying the improved conductivity. The high conductivity of the MoO2/Mo2C/C HNWs is due to the hybridization with Mo2C and C, which greatly decreases the resistance of charge transfer, leading to significantly improved electrochemical performance. Figure 3b presents the Nyquist plots for the MoO2/Mo2C/C HNWs in the fully lithiated stated (discharged to the potential of 0.05 V vs. Li/Li+) in various cycles. After the first discharge process, two depressed semicircles appear on the Nyquist plots, indicating formation of a new interface film, namely SEI. In the subsequent cycles, the diameters of the two semicircles decrease, and become stable from the 60th cycle, indicating the decreased resistance of charge transfer and SEI after the activation process. The reduction of the charge transfer resistance may be attributed to the more complete conversion reaction of MoO2 with Li+ which increases the amount of metallic Mo during the activation process, and the decrease of the SEI resistance implies the stabilization of the SEI film after the activation process. The reduction of both charge-transfer and SEI resistance is beneficial for the Li+ diffusion, which accounts for the capacity increase upon cycling and superior rate capability for the MoO2/Mo2C/C HNWs.



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Figure 3. Nyquist plots of half cells with working electrodes of (a) MoO2/Mo2C/C HNWs and MoO2 NPs at the potential of 2 V vs. Li/Li+ in the first discharge process, and (b) MoO2/Mo2C/C HNWs at the potential of 0.05 V vs. Li/Li+ in various cycles. To further illustrate the exceptional electrochemical performance of the MoO2/Mo2C/C HNWs, we investigated the capacitive effect by CV. The total stored charge of nanostructured materials can be categorized into tree components according to the mechanism: 41 non-faradaic contribution from the double-layer capacitance, and faradaic contributions from diffusion-controlled Li+ insertion and from charge transfer processes on the surface 41 Faradic process with a capacitive behavior is referred to as pseudocapacitance. The capacitive effect of the battery system can be calculated according to:41-42 i=avb log i=blog v+log a

(3) (4)

where i is the current density, v is the scan rate, and a and b are adjustable parameters. With regard to the typical diffusion controlled insertion of Li+, b-value is 0.5, whereas for the ideal capacitive behavior which is related to fast kinetics, b-value is 1.12, 41-42 Figure 4a shows the CV curves of the MoO2/Mo2C/C HNWs at various 16 ACS Paragon Plus Environment

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sweep rates (0.3-1 mV s-1) after ten successive CV cycles at 1 mV s-1. The peaks corresponding to the insertion (located at 1.56, 1.27 V) and attraction (located at 1.43, 1.70 V) of Li+ are denoted as Peak 1, Peak 2, Peak 3 and Peak 4, respectively. From the slops in the plots of log i vs. log v (Figure 4b), the b-values for Peaks 1-4 can be determined as 0.97, 0.98, 0.94 and 0.93, respectively. These values are close to 1.00, indicating the Li+ storage/release is a pseudocapacitive behavior rather than diffusion controlled process. By contrast, the b-values for Peak 1-4 of the MoO2 NPs are calculated (Figure S7) to be 0.55, 0.85, 0.61 and 0.65, which are larger than 0.5, however, much lower than those of the corresponding value for the MoO2/Mo2C/C HNWs. The higher b-values of the MoO2/Mo2C/C HNWs suggest that the hybridization with Mo2C and C enhances pseudocapacitive effect and kinetics for Li+ storage/release, which is attributed to the superior rate capability.

Figure 4. (a) CV curves of the MoO2/Mo2C/C HNWs recorded after ten CV scans at 1 mV s-1 and (b) log i vs. log v plots derived from peak currents at various scan rates in the CV profiles in Figure 4a.

Conclusions 17 ACS Paragon Plus Environment


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In summary, we have successfully fabricated MoO2/Mo2C/C HNWs through facile carbothermal reduction method. The MoO2/Mo2C/C HNWs adopt the morphology of the organic/inorganic hybrid precursor, and are built up by MoO2, Mo2C nanoparticles and residual free carbon. The hierarchical structure avoids agglomeration of active nanoparticles, and the nanoparticles dispersed in the nanowires are beneficial for Li+ transportation due to the decreased diffusion paths. Moreover, hybridization with Mo2C and carbon facilitates the electron transfer and increases the structural stability without sacrifice of capacity. Therefore, the MoO2/Mo2C/C HNWs show a capacitive behavior which relates to fast kinetics for lithiation/delithiation. When cycled at 200 mA g-1 for 320 cycles, the MoO2/Mo2C/C HNWs reaches a reversible capacity of 950 mAh g-1. Even when the current density increases to 2000 mA g-1, the reversible capacity maintains 602 mAh g-1 after 500 cycles. The MoO2/Mo2C/C HNWs combine advantages of MoO2, Mo2C and C, exhibiting high-rate capability and long cycle-life, and can be a promising candidate for LIB anodes.

Supporting Information. SEM image and N2 sorption isomers of MoO2 NPs; SEM image of Mo3O10(C6H5NH3)2·2H2O nanowires; capacity differential vs. potential plots for different cycles and capacity contribution of the MoO2/Mo2C/C HNWs in different potential region; TEM image of the MoO2/Mo2C/C HNWs after discharge/charge cycles; determination of b-values for the MoO2 NPs through CV measurement. This material is available free of charge via the Internet at http://pubs.acs.org. 18 ACS Paragon Plus Environment

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Corresponding Author *M. Zhu.

E-mail: [email protected]. Tel.:+86-20-87113924; fax:

+86-20-87111317

ACKNOWLEDGMENT We acknowledge financial support from the National Natural Science Foundation of China (Grant No. 51402110, 21373102 and 51231003), and RSA-China collaborative research grant (CS08-L11).

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