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Journal of Power Sources 274 (2015) 1063e1069

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Journal of Power Sources

journal homepage: www.elsevier .com/locate/ jpowsour

Coating of a-MoO3 on nitrogen-doped carbon nanotubes byelectrodeposition as a high-performance cathode material forlithium-ion batteries

Hui Zhang a, b, c, Xiaojiang Liu b, Ruilin Wang a, Rui Mi c, Shaomin Li c, Yanhua Cui b, *,Yafeng Deng b, Jun Mei c, Hao Liu c, *

a College of Materials Science and Engineering, Sichuan University, Chengdu 610065, PR Chinab Institute of Electronic Engineering, China Academy of Engineering Physics, Mianyang 621000, PR Chinac Chengdu Green Energy and Green Manufacturing Technology R&D Center, Chengdu Development Center of Science and Technology of CAEP,Chengdu 610207, PR China

h i g h l i g h t s

� a-MoO3/N-CNTs nanocomposite has been prepared by a facile electrodeposition method.� Unique nanocomposite structure consists of N-CNTs with a-MoO3 nanoparticles homogeneously distributed on the surface.� The reversible capacity of this composite is 250 mA h g�1 at 30 mA g�1.� Outstanding cycling performance and high rate capability.

a r t i c l e i n f o

Article history:Received 26 July 2014Received in revised form17 October 2014Accepted 25 October 2014Available online 28 October 2014

Keywords:a-MoO3

Nitrogen-doped carbon nanotubeElectrodepositionCathode materialLithium-ions battery

* Corresponding authors.E-mail addresses: cuiyanhua@netease.com (Y. Cui),

http://dx.doi.org/10.1016/j.jpowsour.2014.10.1360378-7753/© 2014 Elsevier B.V. All rights reserved.

a b s t r a c t

In this work, a-MoO3 nanoparticles are grown on the surface of nitrogen doped carbon nanotubes (N-CNTs) via a facile electrodeposition method. The morphology of as-prepared nanocomposite shows thatthe interconnected a-MoO3 nanoparticles are homogeneously distributed on the surface of N-CNTs.When investigated as a cathode material for lithium ion batteries, the nanocomposite reveals a highreversible discharge capacity of 250 mA h g�1 at a current density of 30 mA g�1, simultaneously withgood cycling performance and excellent rate capability compared with pristine MoO3 powder. Kinetics ofthis process is investigated by electrochemical impedance spectroscopy to understand the difference inelectrochemical performance and the interface properties between the a-MoO3/N-CNTs nanocompositeand pristine MoO3 powder electrode. The correlation between the unique structural features of a-MoO3/N-CNTs nanocomposite and its excellent electrochemical performance is also discussed. The excellentperformance makes the as-prepared a-MoO3/N-CNTs nanocomposite a promising cathode material forhigh-performance lithium ion batteries.

© 2014 Elsevier B.V. All rights reserved.

1. Introduction

Lithium-ion batteries have gained increasing attention owing totheir potential applications in portable electronics devices becauseof their long lifespan, superior capacity, and environmentalbenignity [1]. The most widely used LiCoO2 cathode has a specificcapacity of 140 mAh g�1 [2,3], much smaller than that of the widelyused graphite anode (350e365 mAh g�1) [4e7]. Although consid-erable other cathode materials have been explored such as LiFePO4

mliuhao@gmail.com (H. Liu).

and LiMn2O4, the low specific capacity of these cathode materialsplaces limits on their application in high energy density devices. Onthe other hand, while metal oxides such as Co3O4, MoO3, Fe2O3 andMnO2 are possible choices due to their higher capacity than tradi-tional electrode materials, most of them are used as anode mate-rials instead because of their low discharge plateau [8e11]. Amongthem, MoO3 has been investigated widely as a promising cathodematerial due to its high theoretical capacity and prominent elec-trochemical properties [12]. MoO3 commonly exists in threedifferent structures, namely, orthorhombic a-MoO3, monoclinic b-MoO3, and hexagonal MoO3, among which a-MoO3 is the mostthermodynamically stable phase [13,14]. Meanwhile, orthorhombic

H. Zhang et al. / Journal of Power Sources 274 (2015) 1063e10691064

a-MoO3 presents an unique anisotropic layered crystal structureconsisting of bilayers MoO6 octahedron basic units and allowingreversible insertion/deinsertion of the lithium-ions, features whichmake a-MoO3 a promising lithium-ion battery electrode material[15,16].

However, a major drawback of a-MoO3 is its poor electricalconductivity, which severely limits its lithium storage behavior.This is why bulk MoO3 powders have shown high initial capacitybut exhibited significant capacity fading with cycling [17]. Toaddress this problem, different nano-morphologies of a-MoO3,including nanowires [8], nanobelts [13,18,19], nanorods [16] andnanoparticles [20], have been reported recently, and these nano-structures show higher capacity and better cycling stability thanthe bulk a-MoO3 [17]. In addition, another effective strategy toovercome the low conductivity is to composite MoO3 with highelectro-conducting material such as carbon in the form of carboncoatings [12] or carbon supportings [21]. Among various carbonmaterials, carbon nanotubes (CNTs) have been recognized as verypromising materials because of their excellent physicochemicalproperties and unique structure [22]. Wang et al. [18] synthesizeda-MoO3/CNTs nanocomposite via a hydrothermal approach. How-ever, as observed from SEM, the a-MoO3 and CNTs synthesized bythis method are just simply mixed and not composited. Althoughconsiderable efforts have been made to achieve controllable a-MoO3/CNTs nancomposite, it is still challenging to synthesizehighly-homogeneous nanocomposite in a facile way for furtherapplication in lithium-ion batteries due to the inert nature ofgraphitized structure of pure CNTs. To meet this challenge, nitrogendoping has been taken as a feasible way to promote the conduc-tivity of CNTs, as well as to chemically activate the passive surface ofCNTs and to contribute extra defects. Therefore, compared to that ofCNTs, the surface of nitrogen doped carbon nanotubes (N-CNTs) aremore favorable for the composition of the nanotubes with metaloxides [22,23].

Herein, a-MoO3/N-CNTs nanocomposite composed of N-CNTswith homogeneously distributed a-MoO3 on the surface was syn-thesized by a simple electrodeposition method. By manipulatingdeposition parameters, electrodeposition has the prominentadvantage of the preservation of the morphology of supportingmaterials from being destroyed, thus allowing the MoO3 nano-particles to grow on the surface of N-CNTs. The contact resistancebetween the N-CNTs and a-MoO3 of the resulting composite cantherefore be efficiently reduced. To our knowledge, this is the firstreport about the synthesis and electrochemical properties ofcoating pure a-MoO3 on N-CNTs by electrodeposition. The elec-trochemical lithium insertion/deinsertion characteristic of the as-synthesized nanocomposite shows that this homogeneous nano-composite exhibits high reversible specific capacity, high ratecapability and excellent cycling stability. The excellent performancemakes the nanocomposite a promising cathode material forrechargeable lithium-ion batteries in the application of portableelectronics devices. Considering the three-dimensional architec-ture and binder and additive free composition, this electrode couldalso be further explored as an electrode for three-dimensionallithium-ion batteries.

2. Experimental

2.1. Preparation of N-CNTs

The N-CNTs were grown directly on nickel foam substrate by achemical vapor deposition (CVD) method with a tubular furnace, asreported in our earlier work [24]. In brief, before the synthesis, thecircular nickel foam with diameter of 16 mm was degreased withacetone, activated for 1 min in 10 wt.% H2SO4 solution, and then

washed with deionized water. The nickel foam was then placed inthemiddle of the oven inside the quartz tube. Ferrocene (Fe(C5H5)2)and melamine (C3H6N6), employed as catalyst precursor and ni-trogen additive, were located at the entrance of the furnace in aquartz tube. During the CVD process, argon gas was introduced intothe quartz tube at a flow rate of 900 sccm for 10min to expel the airin the quartz tube before the furnace was heated. Then the systemwas heated at a rate of 25 �C/min until 950 �C. Ethylene wasintroduced into the system at a flow rate of 18 sccm as carbonsource as soon as the target temperature was reached. The growthof N-CNTs lasted for 15 min, and then the ethylene gas was turnedoff, and the furnace was cooled down to room temperature inflowing argon gas. Details of the experimental procedures can befound elsewhere [22,24].

2.2. Preparation of a-MoO3/N-CNTs nanocomposite

For the preparation of a-MoO3/N-CNTs electrode, a simpleelectrodepostion method was used. The electrodeposition solutionwas firstly prepared by dissolving 1 g molybdenum powder(Aladdin, 99.5%) in 3.5 mL of 30% aqueous hydrogen peroxide so-lution (V/V, 30%). After the powders were completely dissolved andthe exothermic reaction had ended, the solution was diluted to50 mL with distilled water resulting in an approximate molybde-num content of 0.2 M. In the electrodeposition process, the assynthesized N-CNTs/Ni was directly used as the working electrode,and a Pt foil and Ag/AgCl electrode were employed as the counterand reference electrodes separately. The a-MoO3 was electro-deposited by a potentiostatic technique with a constantpotential �0.6 V for 2 min. The product was then heated at certaintemperatures of 300 �C, 350 �C and 400 �C in air for 2 h, respec-tively. The mass of the a-MoO3 loading on the N-CNTs/Ni substrateis about 0.8e1 mg cm�2. The weight ratio of a-MoO3 to N-CNTs wasestimated to be 1:1.1 (see Supporting Information).

2.3. Characterization

Morphological and structural information were obtained fromX-ray diffraction (XRD, D/max 2200/PC, Rigaku, 40 kV, 20 mA, CuKa radiation), scanning electron microscopy (SEM, Hitachi S-5200).Transmission electronmicroscope (TEM) and selected area electrondiffraction (SAED) measurements were carried out by 200 kV sideentry JEOL 2010 TEM. Raman spectroscopy was performed using amicro-Raman 2000 system (Renishaw, Britain) with a 10 mWheliumeneon laser excitation source of wavelength 633 nm.Thermogravimetric analysis (TGA) was performed by MettlerToledo TGA/DSC 1.

2.4. Electrochemical measurements

For the electrochemical measurements, the as-prepared a-MoO3/N-CNTs which was treated at 400 �C was used as the cathodeof the CR-2032-type coin cells directly. And lithium foil and Celgard2500 were employed as anode and separator respectively. Theelectrolyte was a 1:1 (in volume) mixture of ethylene carbonate(EC) and dimethyl carbonates (DMC) containing 1 M LiPF6. The CR-2032-type coin cells were assembled in an argon-filled glove box(moisture and oxygen concentration <0.1 ppm). To make a com-parison, pristine MoO3 cathode with similar loading density of thecomposite electrode (1.0e1.3 mg cm�2) was utilized as a blanksample. The purchase MoO3 powder (Aladdin, 99.95%) was mixedwith 10 wt% carbon black and 10 wt% polytetrafluoroethylenebinder, pasted on an aluminum foil, and then dried under vacuumat 100 �C for 12 h. Cyclic voltammetry measurements were carriedout on the Princeton V3 electrochemical workstation over the

H. Zhang et al. / Journal of Power Sources 274 (2015) 1063e1069 1065

potential range of 1.5e3.5 V at a scan rate of 0.1 mV s�1. Electro-chemical impedance spectroscopy (EIS) measurements were per-formed using Princeton P4000 electrochemical workstation. Theimpedance spectra were obtained by applying an AC voltage of10 mV over the frequency range from 0.1 to 100 kHz at roomtemperature. The cells were charged and discharged over a voltagerange of 1.5e3.5 V (vs Liþ/Li) at different rates using an Arbin BT-2000 Battery Test System. Considering the fact that N-CNTs isnon-active at the voltage range of 1.5 Ve3.5 V, the mass of the non-active N-CNTs was excluded in the composite electrode as a cath-ode material.

Fig. 2. The Raman spectra of the N-CNTs, pristine MoO3 powder and a-MoO3/N-CNTsnanocomposite electrode.

3. Results and discussion

X-ray diffraction patterns, as shown in Fig. 1a, demonstrate theevolution from amorphous to crystalline a-MoO3 in a-MoO3/N-CNTs composite materials as the annealing temperature increasesfrom 300 �C to 400 �C. The product sintered for 2 h at 300 �C isamorphous with only noticeable nickel substrate peaks at 44.5�,51.8�, 76.4� and a broad carbon peak at 26.3� coming from the N-CNTs. As the annealing temperature increases to 350 �C, two peaksemerge, which are attributed to the (110) and (021) plane of alayered orthorhombic a-MoO3. Further increasing the annealingtemperature to 400 �C, XRD pattern shows more noticeable (110)and (021) peaks, as well as some other peaks, which could also beassigned to orthorhombic a-MoO3. These features indicate that thecrystallinity increases at elevated annealing temperature and thephase transforms from an amorphous structure to a more ther-modynamically stable layered orthorhombic structure at a tem-perature of 400 �C and above. As is shown in Fig. 1b, the uniquelayered orthorhombic crystal structure is in favor of the diffusion oflithium ions. The a-MoO3 belongs to space group Pbnmwith latticeconstants a ¼ 3.962 Å, b ¼ 13.885 Å, c ¼ 3.699 Å. The structure isbased on a series of bilayers which consist of two sublayers ofdistorted MoO6 octahedra and are stacked along the b lattice di-rection of the orthorhombic unit cell [25,26]. The crystal structureof a-MoO3 is edge-shared MoO6 octahedra which contains threekinds of oxygen ions for MoeO bonding. The interactions betweenbilayer and the adjacent bilayer in the b lattice direction are non-covalent, which are mainly governed by both weak electrostaticand van der Waals contributions [27e29].

Raman spectra are very useful in determining the molecularstructure of different derivatives of molybdenum oxide becauseeach of them occupies different atomic arrangements and showsobviously different Raman vibration bands. The Raman spectrum of

Fig. 1. a. X-ray diffraction of the electrodeposit on N-CNTs sintered for 2 h

the a-MoO3/N-CNTs nanocomposite is shown in Fig. 2, whichshows the typical Raman scattering bands of a-MoO3 and N-CNTs.The frequency at 995 cm�1 is attributed to the symmetric stretch-ing vibrations of O]Mo]O unit, whereas the frequencies at818 cm�1 and 663 cm�1 is respectively assigned to the character-istic peaks of corner-sharing OeMoeO and edge-sharing OeMoeO[30,31]. The other two visible bands observed at around 1324 and1591 cm�1 are in good agreement with the D band and G band of N-CNTs, respectively [32,33]. For comparison, the Raman spectra ofthe pristine MoO3 and N-CNTs were also given. The Raman spectrastudy, as well as the XRD result, indicates that the obtained samplesare composed of a-MoO3 crystals and N-CNTs.

Themorphology of N-CNTs and a-MoO3/N-CNTs nanocompositewere characterized by SEM. Fig. 3 illustrates representative SEMimages of the morphology of N-CNTs and a-MoO3/N-CNTs nano-composite. As shown in Fig. 3a and b, comparing with bare N-CNTs,a-MoO3/N-CNTs preserves the one dimensional structure guaran-teeing the high surface area of electrodes. The a-MoO3 particles areuniformly deposited on the surface of N-CNTs, instead of homo-geneously mixed with carbon materials as report before [18,31].Fig. 3c and d shows that the diameter increased from 140 nm to

in atmosphere at 300, 350, 400 �C; b. The crystal structure of a-MoO3.

Fig. 3. SEM images of the (a, c) N-CNTs and (b, d) a-MoO3/N-CNTs nanocomposite structure at different magnifications. (e) TEM image and SAED pattern (inset) and (f) HRTEM of a-MoO3/N-CNTs nanocomposite.

H. Zhang et al. / Journal of Power Sources 274 (2015) 1063e10691066

250 nm after a-MoO3 was coated, which means that the thicknessof the deposited a-MoO3 is about 55 nm. This unique structurecould ensure a better electrical connection between a-MoO3nanoparticles and N-CNTs than a simple mixture. Fig. 3e presentsthe TEM image and SAED pattern of a-MoO3/N-CNTs nano-composite, which further confirm the unique morphology anddetailed structure of the nanocomposite. From the TEM image, itcan be clearly observed that the morphology of the nanocompositeis in good agreement with the SEM results even if the electrons donot transmit the sample due to the increased thickness of thenanocomposite. As expected, the SAED pattern (Fig. 3e inset) showspolycrystalline nature of the nanocomposite. The diffraction ringsand spots from inside to outside can be indexed to the (040), (111),(200), and (202) planes of a-MoO3, respectively. The high-resolution TEM (HRTEM) image in Fig. 3f shows crystal latticefringes with spacings of 0.35 nm, corresponding to the interplanardistance of (040) planes of a-MoO3.

To understand the lithium storage properties of the a-MoO3/-NCNTs nanocomposite, cyclic voltammograms and galvanostaticchargeedischarge measurements were performed. Fig. 4a showsthe initial three consecutive cyclic voltammograms of the electrodemade from pristine MoO3. In the cathodic polarization process ofthe first cycle, two peaks are observed at 2.65 and 2.25 V vs Li/Liþ.However, in the following anodic polarization, there is only onepeak located at around 2.5 V. The first cathodic peak in the firstcathodic polarization process is related to lithium ion irreversible

insertion into the octahedral sites of MoO3, which tends to triggerunrecoverable structural transformation of MoO3 [34], and thesecond corresponds to the lithium ion reversible intercalation inthe interlayer spacing between the MoO6 octahedron layers(van-der-waals gap) [15]. From the second cycle, the pristine MoO3displays only one pair of redox peaks at about (2.5 V, 2.0 V), whichhave been reported similarly by others' work [17]. The separationbetween the redox couple is about 0.25 V in the first cycle andenlarges in the subsequent cycles, suggesting a bad reversibility ofcycle performance.

Fig. 4b shows the cyclic voltammograms of the electrode madefrom a-MoO3/N-CNTs composites in the initial three consecutivecycles. In the cathodic polarization process of the first cycle, twostrong peaks are observed at 2.7 and 2.25 V vs Li/Liþ, correspondingto the multistep lithium intercalation processes. In the followinganodic polarization, there are also two peaks located at around 2.8and 2.5 V, corresponding to the multistep lithium extraction pro-cesses. These two sets (anodic, cathodic) of current peaks,appearing at around the potentials of (2.5 V, 2.25 V) and (2.8 V,2.75 V), represent two stages of reversible electrochemical lithiuminsertion/deinsertion, which occurred in different structural sites ofa-MoO3. Besides, these two sets of current peaks can still beobserved in the second and subsequent cycles. The peak appearingat around 2.75 V during the cathodic polarization of the first cycle isstronger than that of the second and subsequent cycles, whichmight be attributed to the irreversible lithium insertion into the

Fig. 4. Cyclic voltammograms of the electrodes made from pristine MoO3 powder (a) and a-MoO3/N-CNTs nanocomposite (b) at a scan rate of 0.1 mV s�1 in the voltage range of1.5e3.5 V vs Li/Liþ.

H. Zhang et al. / Journal of Power Sources 274 (2015) 1063e1069 1067

octahedral sites of MoO3 [11]. The first set of peaks (2.5 V, 2.25 V),which is much more obvious than the second set, corresponds tothe reversible insertion/deinsertion of lithium ions in the interlayerspacing (van der Waals spacing) between the MoO6 octahedronlayers. The separation between the redox couple is relatively small(0.34 V) in the first cycle and further reduces in the second andsubsequent cycles. It is also noticeable that the anodic and cathodicpeaks of second and following cycles are generally overlapping,suggesting a good reversibility of lithium ion insertion/deinsertionin the interlayer spacing between the MoO6 octahedron layers of a-MoO3.

Fig. 5 shows the first and second discharge/charge curves of theelectrodes made from the as-prepared a-MoO3/N-CNTs nano-composite and pristine MoO3 powder, respectively. Both of themexhibit evident charge and discharge voltage plateaus at about 2.5and 2.25 V respectively in the initial two cycles, corresponding tolithium ion deinsertion and insertion processes in the a-MoO3

lattice. The a-MoO3/N-CNTs nanocomposite shows a shortdischarge voltage plateau at about 2.75 V. The pristine MoO3 shows

Fig. 5. Discharge/charge voltage profiles for the electrodes made from the as-prepareda-MoO3/N-CNTs nanocomposite and pristine MoO3 powder at a current density of30 mA g�1 in the voltage range of 1.5e3.5 V.

an unconspicuous discharge voltage plateau at about 2.75 V in thefirst cycle but disappear in the second one, corresponding tolithium ion irreversible insertion into the octahedral sites of MoO3[16]. These results are also in accordance with the anodic andcathodic peaks in the CV.

Fig. 6 illustrates the cycling behaviors and the coulombic effi-ciency of the as-prepared a-MoO3/N-CNTs nanocomposite andpristine MoO3 powder at 30 mA g�1. Both of them exhibit a seriouscapacity fading in the second cycle, possibly due to irreversiblelithium ion insertion in MoO3 crystal lattice. The capacity deliveredby a-MoO3/N-CNTs is about 350 mAh g�1 at the first cycle and thenlevels off at about 250 mAh g�1 after 50 cycles. In contrast, thepristine MoO3 powder gives a lower initial capacity of 295mAh g�1,then drops to 112 mA g�1 quickly during subsequent cycles. Thecoulombic efficiency increases and stabilizes at more than 95% afterthe initial several cycles. The cycle performance conducted at150 mA g�1 is also given in Fig. S1.

Fig. 7 shows the rate capabilities of the as-prepared a-MoO3/N-CNTs nanocomposite and pristine MoO3 powder at different char-geedischarge current densities of 30, 60,150, and 300mA g�1. With

Fig. 6. Cycle performances of the electrodes made from the as-prepared a-MoO3/N-CNTs nanocomposite and pristine MoO3 powder at a current density of 30 mA g�1 inthe voltage range of 1.5e3.5 V.

Fig. 7. Rate performances of the electrodes made from the as-prepared a-MoO3/N-CNTs nanocomposite and pristine MoO3 powder at a stepwise increasing currentdensities of 30, 60, 150, 300 and then back to 30 mA g�1.

Fig. 8. Nyquist plots of electrochemical impedance spectroscopy (EIS) of a-MoO3/N-CNTs nanocomposite, pristine MoO3 powder electrodes before cycles and after cycles(the inset), and Randles equivalent circuit used for analysis of impedance spectra.

Table 1Kinetic parameters of electrodes.

Sample Rs(U)a Rct(U)a Rs(U)b Rct(U)b

MoO3 powder 2.274 125.3 12.42 545.6a-MoO3/NCNTs 2.42 42.6 7.741 48.83

a The fitting value of the kinetic parameters of electrodes before cycles.b The fitting value of the kinetic parameters of electrodes after cycles.

H. Zhang et al. / Journal of Power Sources 274 (2015) 1063e10691068

the increase of the current density, capacities generally decrease,which is a result of the increase of internal resistance at high cur-rent density where thermodynamic equilibrium cannot be reached[35]. Except for the initial capacity fading, the a-MoO3/N-CNTscomposites exhibit quite stable capacity retention. The a-MoO3/N-CNTs nanocomposite can deliver a higher average capacity of280 mAh g�1 compared with 200 mAh g�1 for pristine MoO3powder at 30 mA g�1 in the initial 10 cycles. With the increasingcurrent density, the capacity difference between a-MoO3/N-CNTsnanocomposite and pristine MoO3 powder become bigger andbigger, which shows that the former has more evident advantage athigh current densities. The average capacity of pristine MoO3powder is just about 52 mAh g�1 at 300 mA g�1 in the range of31e40 cycles, much less than that of the a-MoO3/N-CNTs com-posites (190 mAh g�1). When the current density returns to theinitial 30mA g�1 after 40 cycles, a stable average specific capacity of230 mAh g�1, about 82% of the average value in the first 10 cycles,can be recovered. Obviously, the as-prepared a-MoO3/N-CNTsnanocomposite reveals a better rate performance than pristineMoO3 powder. The as-prepared electrode exhibits better lithiumstorage performance than MoO3 powder electrode, and also betterthan results reported by others in the same voltage range andcurrent density [15,17].

To understand the difference in electrochemical performanceand the interface properties between the a-MoO3/N-CNTs nano-composite and pristine MoO3 powder electrodes, the EIS wasmeasured and fitted to the equivalent circuit. Fig. 8 shows theNyquist plots of the two electrodes before cycles, respectively. Onesemicircle in the high frequency region and one straight line in thelow frequency region can be observed in Fig. 8 from all spectra.Generally, the intercept at the Zre axis at the high-frequency regionreflects the resistance of electrolyte (Rs). Obvious semicircles areobserved in the high-frequency region, which can be attributed tothe charge transfer impedance (Rct) at the interface between theelectrolyte and the working electrode. The diameter of the semi-circle in high-frequency region represents the Rct. The straight linein the low frequency region is associated with Warburg diffusion[36]. A simple equivalent circuit is established to interpret theimpedance spectra. As shown in Table 1, the fitting results showthat the a-MoO3/N-CNTs nanocomposite exhibits a small Rs valueof 2.42 U, similar to pristine MoO3 powder (2.274 U). In addition,

the nanocomposite exhibits a diminished Rct of 42.6 U comparedwith 125.3 U for the pristine one, indicating that kinetics over Lidiffusion across electrolyte and the followed charge transferbecome easier. The inset left shows the Nyquist plots of the twoelectrodes after cycles. The Rct of pristine MoO3 powder is about545.6U, much bigger than that before cycles. However, the Rct of a-MoO3/N-CNTs is about 48.83 U, similar to that before cycles. Itshows that charge transfer of pristineMoO3 powder become harderwith the increase of cycle number, while as-prepared a-MoO3/N-CNTs have little change of charge transfer impedance before andafter cycles. It is believed that the reduced original Rct whichmaintained a low value with the increase of cycles could beimportant reasons for the improved electrochemical performances.

Compared with pristine MoO3 powder, our as-prepared a-MoO3/N-CNTs nanocomposite exhibits excellent storage perfor-mance which could be attributed to several unique features thatsuitably mitigate the drawback of pristine MoO3 powder. First, thea-MoO3 nanoparticles grown on the surface of N-CNTs reduce thelithium ion diffusion distance within the MoO3 crystals signifi-cantly, allowing more efficient insertion/deinsertion of lithium ions[16]. In addition, the interconnected a-MoO3 and N-CNTs frame-work with good conductivity not only provides the electron con-duction pathway but also reduces the internal resistance of thecells. Furthermore, the intrinsically flexible N-CNTs that tightlyencompassed with a-MoO3 nanoparticles provide the structuralintegrity by not only buffering the stress and strain caused by thevolume change during the lithium ion insertion/extraction processin the MoO3 crystal lattice but also preventing the aggregation ofthe MoO3 nanoparticles upon cycling [12]. Finally, the three-dimensional and porous architecture makes a better contact ofthe active materials and electrolyte. This is another important

H. Zhang et al. / Journal of Power Sources 274 (2015) 1063e1069 1069

factor making our as-prepared electrode an excellent lithiumstorage performance. Since a-MoO3 is directly deposited on three-dimensional N-CNTs requiring no additional polymeric binders orcarbon black, the unique structuremakes the as-prepared electrodea promising three-dimensional cathode for lithium-ion batteries.

4. Conclusions

In summary, a-MoO3/N-CNTs nanocomposite has been preparedby a facile electrodeposition method. The unique nanocompositestructure consists of N-CNTs with interconnected a-MoO3 nano-particles homogeneously distributed on the surface. The a-MoO3nanoparticles grown on the surface of N-CNTs reduce the lithiumion diffusion distance within the MoO3 crystals significantly andthe N-CNTs frameworkwith good conductivity provides an electronconduction pathway. Electrochemical test results reveal that thenanocomposite exhibits observably improved cycling performanceand rate capability compared with pristine MoO3 powder. Thenanocomposite affords a reversible discharge capacity of250 mAh g�1 after 50 cycles at a current density of 30 mA g�1.Moreover, it exhibits a fairly good rate performance with dischargecapacities of 240mAh g�1 at 60mA g�1, 212mAh g�1 at 150mA g�1,and 190mAh g�1 at 300mA g�1. The excellent performance of high-capacity retentionwith low-capacity fading makes the as-prepareda-MoO3/N-CNTs nanocomposite a promising cathode material forhigh-performance lithium ion batteries.

Acknowledgments

The authors appreciate the financial support of Laboratory ofPrecision Manufacturing Technology, CAEP (ZZ13007), Project2013A030214 supported by CAEP, and Science & TechnologyDepartment of Sichuan Province (2013GZX0145-3). We areindebted to Margaret Yau, Xichuan Liu, and Jiabin Liu for their kindhelp and fruitful discussions.

Appendix A. Supplementary data

Supplementary data related to this article can be found at http://dx.doi.org/10.1016/j.jpowsour.2014.10.136.

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