MnOx/SWCNT macro-films as flexible binder-free anodes for high-performance Li-ion batteries

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MnOx/SWCNT macro-films as flexible binder-freeanodes for high-performance Li-ion batteries

Jinwen Qina, Qing Zhangb, Zeyuan Caob, Xin Lib,Changwen Hua, Bingqing Weib,n

aKey Laboratory of Cluster Science, Ministry of Education of China, Department of Chemistry,Beijing Institute of Technology, Beijing 100081, PR ChinabDepartment of Mechanical Engineering, University of Delaware, Newark, DE, 19716, USA

Received 15 November 2012; accepted 25 December 2012

KEYWORDSManganese oxides;Single-walled carbonnanotubes;Anode materials;MnOx/SWCNT hybridmacro-films;Binder-freeelectrodes;Li-ion batteries

nt matter & 20130.1016/j.nanoen.2

thor. Tel.: +1 302.

[email protected] (B.

icle as: J. Qin, et), http://dx.doi.o

AbstractWe present a low-temperature synthetic route toward the binder-free MnOx (the mixture ofMn3O4 and MnO2)/single-walled carbon nanotube (SWCNT) macro-films, which could beemployed as the single component of anodes in Li-ion batteries. Morphological characteriza-tions reveal that the interconnected network of SWCNT macro-films in the free-standing MnOx/SWCNTs intrinsically possesses high electrical conductivity and exhibits good mechanicaladhesion on Cu foils (current collectors). The MnOx nanoparticles directly grown onto theexternal surface of SWNT macro-films have very strong bonds with the nanotubes, avoidingagglomeration during their electrochemical operations. These MnOx/SWNT macro-films areevidenced to possess an excellent capacity of �1000 mAh g�1, showing promise as flexibleanode materials for light weight and high performance Li-ion batteries.& 2013 Elsevier Ltd. All rights reserved.

& 2013 Elsevier Ltd. All rights reserved.

Introduction

The design and fabrication of low weight and high specificcapacity Li-ion batteries have been considered as the trendof development in energy storage devices. In conventionalprocessing of Li-ion battery anodes, the active materials are

Elsevier Ltd. All rights reserved.012.12.009

831 6438;

Wei).

al., MnOx/SWCNT macro-films asrg/10.1016/j.nanoen.2012.12.009

usually attached to the current collectors using polymerbinder poly(vinylidene fluoride) (PVDF), solvent N-methyl-2-pyrrolidone (NMP) and conductive additives (carbonblack) for improving mechanical adhesion and electricalcontact. However, the addition of polymer binder inhibitsthe ion transport in the electrolyte and degrades theelectrical conductivity, leading to an obvious capacity loss.As a consequence, much effort has been devoted to preparebinder-free electrodes using spray deposition [1], vacuumfiltration [2], cross stack [3], layer-by-layer assembly [4],and hierarchical bottom-up approach [5]. Conventional

flexible binder-free anodes for high-performance Li-ion batteries,

dx.doi.org/10.1016/j.nanoen.2012.12.009



mailto:[email protected]





J. Qin et al.2

binder-free electrodes prepared by above methods com-monly consist of active materials embedded in the carbonmatrix, which may be a superior solution to overcome thelimitation of adding binders. Among the carbon matrixmaterials, carbon nanotubes (CNTs) as one of the promisingcarbon materials have drawn tremendous attention todesign electrodes for energy storage devices due to theirunique physical and chemical characteristics, involving highspecific surface areas, high electron conductivity, chemicaland mechanical stability, etc. Various metal oxidesanchored on CNTs have been investigated as the electrodesin Li-ion battery, such as SnO2 [6], V2O5 [7], TiO2 [8,9], Cu2O[10], CoO [11], and Fe2O3 [12].

Among transition metal oxides, manganese oxide is animportant class of electrode materials for Li-ion batterybecause of its high specific capacity (the theoretical capa-city of MnO2 and Mn3O4 is 1230 and 937 mAh g�1, respec-tively), low cost and environmental friendliness [13–15].However, most researches on manganese oxides have to usecarbon-based material additions and polymer binders[14,16,17]. Comparatively little investigation has beenmade on manganese oxides as a single component in Li-ionbatteries due to their extremely low conductivity and poorcycling stability. Recently, research efforts have focused onCNTs as a prominent candidate to fabricate binder-freeelectrodes with excellent electrochemical performance[4,18–20]. Our group has recently succeeded in fabricationSWNT macro-films, which possess intrinsic electronic junc-tions in the network of the macro-films, suggesting a lowersheet resistance [21]. It is therefore facile to fabricatebinder-free electrodes through surface modification ofthese SWNT macro-films.

In this paper, we report a low-temperature synthesis ofmanganese oxides which strongly bond on the flexible SWNTmacro-films substrate to fabricate binder-free electrodesfor Li-ion battery applications. The SWNT macro-filmsdeposited with manganese oxides provide excellent elec-tronic conductivity by avoiding agglomerative binder andenhance the energy storage as the additional lithiumstorage material (both SWCNTs and MnOx are active materi-als for lithiation and delithiation). Meanwhile, the SWNTmacro-films combined with manganese oxides offer wonderfulcontacts with the current collectors, the Cu foils, exhibiting aconsiderable adhesion. The as-prepared MnOx/SWNT macro-films exhibits a high capacity of �1000 mAh g�1 and a goodcapacity retention.

Experimental section

Synthesis of SWNT macro-films

SWNT macro-films were grown via the chemical vapordeposition of ferrocene and sulfur in the atomic ratio of10:1 under argon/ hydrogen atmosphere. Here, ferroceneacted as the carbon source as well as the catalyst and sulfuras an additive to promote SWCNT growth. More specific,the CVD furnace was heated to 1100–1150 1C under amixture gas flow of argon (1500 mL/min) and hydrogen(150 mL/min). After 30 min reaction time, furnace wascooled to room temperature under argon flow. The prepared

Please cite this article as: J. Qin, et al., MnOx/SWCNT macro-films asNano Energy (2013), http://dx.doi.org/10.1016/j.nanoen.2012.12.009

SWNT macro-films were heated in air at 425 1C for 30 min toremove amorphous impurities.

Synthesis of oxidized SWCNT (O-SWCNT)macro-films

The prepared SWNT macro-film was firstly suspended in20 mL of concentrated H2SO4 for 1 h, and then was furtheroxidized at room temperature for 30 min after 0.1 g KMnO4

was added. The color of the SWNT macro-film changed fromblack to brown. The O-SWCNT macro-film was washed withdistilled water.

Synthesis of MnOx/SWNT macro-films

0.183 g Mn(CH3COO)2.4H2O was dissolved in distilled waterto which the obtained O-SWNT macro-film was added in,and then the system was rested at room temperature for1 h. 10 mL of 0.02 M KMnO4 was added dropwisely to theabove system, followed by 30 min rest. Finally, brownsuspension Mn3O4 and the O-SWNT macro-film coated withMn3O4 (Mn3O4/O-SWCNTs) were obtained. 20 mL of 2.5 Mfreshly prepared NH2NH2 solution (NaOH was added untilpH=11.5) was added to reduce the mixture. The brownmixture immediately turned into black with bubbling andwas then kept 60 1C for 4 h. The resulting precipitates andhybrid macro-film were washed three times with distilledwater to remove residual ions. The precipitate MnOx driedat 80 1C for 3 h in air, while the obtained MnOx/SWNTmacro-films were transferred onto copper foils and driedunder the same conditions. A reduced O-SWCNT macro-filmwithout adding manganese sources was also prepared forcomparison.

Characterization

Transmission electron microscopy (TEM) observation wasperformed on the differently obtained samples using afield-emission transmission electron microscope (JEOLJEM-2010F). Scanning electron microscopy (SEM) imageswere obtained by a JEOL JSM-7400F. X-ray diffraction(XRD) was performed on a Philips X’pert diffractometerusing Cu Ka radiation. Thermogravimetric analysis (TGA)was carried out in air on a Mettler Toledo TGA/DSC 1 witha heating rate of 2 1C/min. Raman spectroscopy wasperformed with a Renishaw Invia with a 785 nm wavelengthlaser and X-ray Photoelectron Spectroscopy (XPS) wasrecorded using PHI 5600.

Electrochemical test

The MnOx/SWNT macro-film with a typical mass of 0.15 mgwas transferred to a copper foil without using carbon black,PVDF binder, and solvent NMP. Half Li-ion battery cells wereassembled in argon-filled glovebox using the MnOx/SWNTmacro-films as working electrodes, lithium metal foils as thecounter/reference electrodes and 1 M LiPF6 in 1:1 (v/v)mixture of ethylene carbonate (EC) and dimethyl carbonate(DMC) as the electrolyte. Commercial Celgard separator wasused as a separator. The assembled cells were aged for 24 h





Figure 1 Schematic diagram showing the low-temperaturesynthesis of MnOx/SWCNTs macro-film.

3MnOx/SWCNTs macro-film as flexible binder-free anodes for high-performance Li-ion batteries

at room temperature before electrochemical tests. Thecycling performance was evaluated by charging and dis-charging between 3.0 and 0.01 V vs. Li/Li+ using an ArbinBT4 battery test station. Cyclic Voltammetry (CV) measure-ments and electrochemical impedance spectroscopy (EIS)were carried out on PARSTAT 2273 Potentiostat/Galvanostat.

Results and discussion

The approach to achieve the low-temperature synthesis ofmanganese oxides anchored on the SWNT macro-film is illu-strated in Figure 1. To fabricate the O-SWNT macro-film, aSWNT macro-film prepared using the CVD method is oxidizedwith concentrated H2SO4 and KMnO4 treatment to increasefunctional groups [21,22]. The oxygen functional groups on theO-SWNT macro-film act as anchoring sites and nucleation sitesfor the growth of Mn3O4. The Mn3O4/O-SWNT macro-film isprepared through a redox reaction between KMnO4 andMn(CH3COO)2 in the presence of O-SWCNTs with OH� functionalgroups [23], as suggested by the following reactions:

2MnO�4 þMn2þþ2Cþ2OH�- Mn3O4þ2HCO

�3 ð1Þ

The obtained Mn3O4/O-SWNT macro-film is reduced intoMn(OH)2/SWCNTs in alkaline solution using NH2NH2 as thereductant at 60 1C for 4 h. Finally, after the Mn(OH)2/SWNTmacro-film is heated at 80 1C for 3 h in air, the mixture ofMn3O4 and MnO2 (referred as MnOx) deposited on the SWCNTmacro-film is obtained [24]. The possible reactions aredescribed as follows:

6MnðOHÞ2þO2-2Mn3O4þ6H2O ð2Þ

2MnðOHÞ2þO2-2MnO2þ2H2O ð3Þ

The formation of amorphous Mn3O4 and Mn(OH)2 in thepresence of SWNT macro-film is characterized by XRDmeasurements (Supporting Information, Figure S1).Figure 2 shows the TEM images of the SWCNTs, O-SWCNTs,Mn3O4/O-SWCNTs and MnOx/SWCNTs. The initial SWCNTswith crystalline structures generally entangle together withsmooth walls and contain impurities, as shown inFigure 2(a). When SWCNTs are oxidized by concentratedH2SO4 and KMnO4, impurities consisting of catalysts andamorphous carbon can be stripped off and oxygen attacksthe outside wall of SWCNTs, resulting in the breaking ofSWCNTwalls. Some outer graphitic layers have been etchedoff, leading to rough convex–concave surfaces as markedwith the red arrows (see Figure 2b). This is in consistencywith previous reports that the crystalline structures ofSWCNT are partially destroyed and external surfaces arepreferentially oxidized. The TEM image in Figure 2(c) showsthe amorphous Mn3O4 nanoflakes grown directly onto theexternal surface of O-SWCNTs. Moreover, the final hybridMnOx/SWCNTs structure composed of SWCNT bundles andMnOx nanoparticles with diameter of 20–50 nm is shown inFigure 2(d). The as-prepared MnOx/SWCNTs were furthercharacterized using HRTEM (Figure 2e). The crystal latticefringes with an interlayer spacing of 0.322 and 0.230 nm,corresponding to (112) crystal planes of Mn3O4 and (311)crystal planes of MnO2, respectively, are clearly observed.The XRD spectra of the as-prepared MnOx, reference toMnO2 (JPDF no. 44-992) and Mn3O4 (JPDF no. 3-1041) phases


are also shown in Figure S2. It is therefore evident that theMnOx nanoparticles embedded in the SWCNT network arethe mixture of Mn3O4 and MnO2 phases. In addition, theradial breathing mode (RBM) band located between 75 and300 cm�1 in Figure 2(f) is unique for SWCNTs indicating thecharacteristics of CNTs in our MnOx/SWCNTs. Based on RBManalysis, the calculated diameters are widely distributed,ranging from 0.9 to 2.0 nm [21,25].

The morphologies of the as-prepared Mn3O4/O-SWCNTsand MnOx/SWNT macro-films are displayed in Figure 3.Figure 3(a) shows an interconnected network structure ofO-SWCNTs anchored with Mn3O4 particles, and the poresbetween O-SWCNT bundles are filled with generated Mn3O4.After NH2NH2 reduction and the heating treatment, theMnOx/SWCNT macro-film maintains the interconnected net-work with porous structures and the MnOx nanoparticles alsouniformly distribute within the SWCNT film without aggre-gation in Figure 3(b). As shown in Figure 4, the opticalphotographs of the as-synthesized free-standing Mn3O4/O-SWCNTs and MnOx/SWNT macro-films are flexible thin filmsthat can float on the water surface. It can be observed thatthe color of Mn3O4/O-SWCNTs have changed from brown toblack after NH2NH2 reduction. Figure 4(c) also presents theeasy transferring of the MnOx/SWCNT macro-film onto a Cufoil with the considerable mechanical adhesion.

In order to determine the stability and actual content ofSWCNTs in the MnOx/SWNT macro-films, TGA analysis wasperformed from 25–1000 1C at a heating rate of 2 1C/minunder air flow. We assume that the SWCNTs finally areburned up and MnOx is turned into Mn2O3 [26,27]. As shownin Figure S3, the weight loss of 19.3% below 200 1C isascribed to the loss of adsorbed moisture, and the 51.7%weight loss in the temperature range of 200–650 1C is mainlyattributed to the oxidation and damage of SWCNT structuresin air [28,29]. And the weight gain of about 7.6% is relatedto the oxidation of manganese oxides into Mn2O3 and





Figure 2 TEM images of the (a) SWCNTs, (b) O-SWCNTs, (c) Mn3O4/O-SWCNTs and (d) MnOx/SWCNTs, (e) HRTEM image ofas-prepared MnOx/SWCNTs and (f) RBM band of MnOx/SWCNTs in Raman spectrum.

J. Qin et al.4

residual metal catalysts into metal oxides at temperaturehigher than 650 1C. Based on the TGA analysis, the SWCNTcontent in MnOx/SWCNT is measured to be about 51.7%.

To confirm the manganese oxides decoration of SWCNTs andunderstand the different chemical states of manganese oxidesand SWCNTs before and after reduction, XPS profiles of Mn3O4/O-SWCNTs and MnOx/SWCNTs shown in Figure 5 reveal the C1sand Mn2p peaks. Figure 5(a) and (c) shows the XPS spectra C1s ofO-SWCNTs and SWCNTs after NH2NH2 reduction. The XPS C1speaks of Mn3O4/O-SWCNTs show a main peak at 284.1 eV which


attributes to the sp2 carbon, a peak at 283.1 eV attributes tocarbon atom contacted with Mn forming metal carbides [30],whereas the peaks located at 285.5 (defects), 286.3 (C–O) and287.8 (C¼O) eV corresponding to carbon atoms attached bydifferent functional groups [31]. The peak at 285.5 eV isassigned to defects on the O-SWCNT structure in the presenceof carboxylic and hydroxyl functions [32]. Furthermore, thereare five typical signals of the C1s peaks located at 288.0, 286.5,285.6, 284.8 and 283.0 eV after NH2NH2 reduction. InFigure 5(c) a decrease of C=O and metal carbides are observed,





Figure 3 SEM images of the (a) Mn3O4/O-SWCNTs and (b) MnOx/SWCNTs.

Figure 4 Optical photographs of the (a) Mn3O4/O-SWCNTs, (b) MnOx/SWCNT macro-film and (c) MnOx/SWCNT macro-film pasting ona Cu foil.


while C–O and defects from disordered carbon structure are stillobserved. The above peaks increased in the MnOx/SWCNTsample is attributed to the oxygen groups and defectswhich offer the combining sites for MnOx growth on the


SWCNT surface. Since the mixed oxidation states of manganesecan be determined by XPS, the chemical states of Mn in theMn3O4/O-SWCNTs and MnOx/SWCNTs are analyzed, as shown inFigure 5(b) and (d). It can be observed that the Mn2p 1/2 peak is





300 298 296 294 292 290 288 286 284 282 280Binding energy (eV)

Experimental Data

287.8286.3 285.5

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Mn4+2P3/2

Mn3+2P3/2

Mn2+2P3/2

Figure 5 XPS spectra: (a) C1s peaks, (b) Mn2p peaks of Mn3O4/O-SWCNTs and (c) C1s peaks and (d) Mn2p peaks of MnOx/SWCNTs.

J. Qin et al.6

at 653.3 eV and Mn2p 3/2 appears two peaks, located at 641.7and 642.5 eV, which confirm that manganese oxides with a spin-energy separation of 11.9 eV on O-SWCNT sample are Mn3O4. Inaddition, the area ratio of Mn4+ vs. Mn2+ is about 2:3, which iscoincident with the theoretical value [33]. Upon comparing thechemical states of Mn in the Mn3O4/O-SWCNT sample, the Mn2p3/2 of MnOx/SWCNTs appears to have three peaks located at641.4, 642.6 and 643.8 eV, which could be attribute to Mn2+,Mn3+ and Mn4+, respectively [34]. As mentioned above, theoxygen in air acts as an oxidation agent to partially change thevalence of Mn to 3+ and 4+ during heating process. Therelative surface concentrations of each Mn state, calculatedfrom the peak intensity areas are Mn2+ (45.67%), Mn3+ (15.75%)and Mn4+ (38.58%), respectively.

To verify whether the O-SWCNTs are reduced to SWCNTsduring the NH2NH2 reduction, Raman spectra are also con-ducted in order to compare the D/G intensity ratio, which isusually used to estimate the defects and disorders of thegraphitized structures and fraction of sp3/sp2-bonded carbon.Figure S4 shows the Raman spectrum of Mn3O4/O-SWCNTs, thetwo peaks at 1346 cm�1 and 1596 cm�1 corresponding to theD and G bands, since the oxidation of SWCNTs is similar to thatof the formation of graphite oxide [35]. Upon the hydrazinereduction, the G band shifts to a lower value of 1592 cm�1, andthe D/G intensity ratio decreases from 0.408 to 0.089 indicatinga decrease of carbonaceous defects and sp3-hybridized carbonsso as to a reduction of O-SWCNTs to SWCNTs, in consistent withthe C1s spectrum of the XPS results. Thus, it is inferred from theabove investigations that the as-synthesized MnOx/SWCNTs arereduced from O-SWCNTs to increase conductivity. Furthermore,the intensive contact with Cu foil and the dispersed MnOx onSWCNTs without aggregation make the hybrid material apromising anode candidate for Li-ion battery.


Figure 6(a) shows the CV curves of the MnOx/SWCNTs as abinder-free electrode at a scan rate of 0.1 mV/s and withina voltage range of 0.01–3 V. In the first cycle, it showsdelithiation peaks at 1.27 V and 2.1 V and a narrow lithiationpeak at 0.12 V. In the second cycle, the intensity of thenarrow lithiation peak is reduced due to the partiallyirreversible reduction of Mn from higher oxidation state tolow oxidation state and the formation of a stable solidelectrolyte interface (SEI) [13]. As shown in the CV curve,there are two redox couples located at 0.68/2.1 V and 0.12/1.27 V. The Mn3+ and Mn4+ in MnOx/SWCNTs is reduced toMn2+ at 0.68 V and further reduced to Mn at 0.12 V in the firstlithiation process. Mn is re-oxidized back to Mn2+ at approxi-mately 1.27 V and higher oxidation state of Mn at 2.1 V in thefirst delithiation process. During the second cycle, it appearsthat the two pairs of peaks have changed. The pair of peaksat low potential becomes broader, and the pair at highpotential in the second cycle goes smaller. It is probablyattributed to the irreversible formation of oxygen-rich MnOx

in the subsequent cycles compared to the initial MnOx [17].The electrochemical performance of the as-synthesized

MnOx/SWCNT macro-film is also characterized using galva-nostatic cycling in a coin cell with Li metal as the anode.Figure 6(b) exhibits the galvanostatic charge–dischargevoltage profiles of the MnOx/SWCNT electrode between0.01 and 3.0 V at a current density of 100 mA g�1. Theobserved potential plateaus in the charge–discharge curvesare consistent with redox peaks in the CV curves. The firstdischarge curve shows two well-defined voltage plateau ataround 0.85 and 0.3 V, which corresponds to the reductionprocess of MnOx to MnO and MnO to Mn, respectively.However, the second discharge is different and two voltageplateaus are observed at around 0.85 and 0.4 V, which is





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Figure 6 (a) CV curves of MnOx/SWCNTs as a binder-free electrode at a scan rate of 0.1 mV/s. (b) Charge–discharge voltage profilesfor MnOx/SWCNTs. (c) Capacities vs. Cycle number curves of MnOx/SWCNTs, SWCNTs and MnOx electrodes and (d) Cyclingperformance of MnOx/SWCNTs electrode at different current densities.


higher and more sloped than the first one, indicating thelithiation reaction of second cycle is easier. The dischargecapacity of the first cycle is 1433 mAh g�1, which includesthe irreversible reaction, such as the electrolyte decom-position. The subsequent charge–discharge cycles tend to bestabilized at about 1000 mAh g�1.

Figure 6(c) shows the galvanostatic charge–discharge capa-cities vs. cycle number at a rate of 100 mA g�1 for the MnOx/SWCNTs, SWCNTs and MnOx electrodes. The maximum dischargeof the MnOx/SWCNT electrode, except for the first discharge,reaches up to 997 mAh g�1. Interestingly, the sample exhibitsimproved reversible discharge capacities over 30 cycles [36]. Itis well known that the formation of Li2O and Mn leads to atheoretical capacity of 1230 mAh g�1 for MnO2 and 937 mAh g�1

for Mn3O4. If this is the case for the MnOx/SWCNTelectrode, thereaction mechanism of manganese oxides could be proposed asfollows [14,15]:

MnO2þ4Liþþ4e�22Li2OþMn ð4Þ

Mn3O4þ8Liþþ8e�24Li2Oþ3Mn ð5Þ

In comparison, pure MnOx nanoparticles and the SWNTmacro-films have been utilized as control samples to investigatethe electrochemical properties of the hybrid structure. TheMnOx electrode with a total mass of 4 mg is prepared byconventional coating process using 15% carbon black and 5%PVDF binder. Typically, the carbon black and the polymer binderare added, even to some hybrid materials in the preparation ofelectrodes in order to increase the electronic conductivity andto ensure adhesion to the current collectors, respectively.


However, the addition of carbon black and polymer binder inthe electrode lowers the specific capacity. It is observed thatthe discharge capacity of MnOx nanoparticles is about 1068 mAhg�1 for the first cycle and fades rapidly below 100 mAh g�1

within 10 cycles. In order to clarify how much the SWCNTscontribute to the total reversible capacity of MnOx/SWCNTs, wehave measured the capacity of the as-synthesized SWCNTs usedin our experiments. As shown in Figure 6(c), the dischargecapacity of the as-synthesized SWCNTs remains at about 200–300 mAh g�1 within 10 cycles, which is much lower than that ofthe MnOx/SWCNT sample. However the contribution of SWCNTsto the total capacity cannot be negligible. Furthermore, it isfound that the MnOx/SWCNT electrode exhibits an excellentrate performance, as shown in Figure 6(d). After the electrodeis cycled at a rate of 100 mA g�1, the applied current density isincreased to 200, 400 and 800 mA g�1. It is clear that the MnOx/SWCNT sample has reversible capacities around 866, 688 and437 mAh g�1, respectively. After 36 cycles, the capacity ofMnOx/SWCNTs maintains 850 mAh g�1 at the current density of100 mA g�1.

When the SWCNT macro-film is introduced in the hybridmaterials (MnOx/SWCNTs) as a conductive matrix and as aninterconnected ‘‘binder’’, EIS measurements are performed toinvestigate the electrochemical characteristics of electrode/electrolyte interface. As shown in Figure 7, the shapes of theNyquist Plots of the MnOx/SWCNTs and SWCNT samples aresimilar, showing a semicircle in the high frequency regionfollowed by a straight sloped line in the low frequency region.Moreover, the semicircle of the MnOx/SWCNT sample is almostthe same as that of the pure SWCNTs, indicating the excellent





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Figure 7 Nyquist plots of MnOx nanoparticles, SWCNTs andMnOx/SWCNTs.

J. Qin et al.8

electron transfer even if there is the massive loading of MnOx

nanoparticles. However, the semicircle of the pure MnOx

nanoparticles is found to be five times larger than the others,suggesting poor electron transfer in the absence of SWCNTs. Atlow frequency region, the MnOx/SWCNTs showing the Warburgline with a 451 slope, is a result of ion diffusion dependence atelectrolyte/electrode interface. The results indicate that thepresence of the SWCNT macro-film in MnOx/SWCNTs indeeddecreases the resistance of the electrode system. The shorterideal straight line represents at lower frequencies is idealWarburg impedance, indicating a lower Li-ion diffusion resis-tance of MnOx/SWCNTs.

To sum up, the improved electrochemical performance isattributed to the unique structure of the hybrid MnOx/SWCNTmacro-films, which have several advantages: (1) binder-freeand interconnected network of SWCNT macro-film is favorablefor fast electron transportation from highly electrical conduc-tive SWCNTs to MnOx. The porous structures and enormousspace among SWCNT bundles facilitates electrolyte penetrationand ion diffusion. (2) MnOx nanoparticles stick tightly onto theSWCNT bundles due to abundant oxygen functional groups suchthat aggregation is prevented during the repeated lithiation anddelithiation processes. Moreover, The SWCNTs offer buffer spaceto relieve the tension and expansion of MnOx nanoparticlescaused by repeated processes of lithium insertion/extrusion.(3) SWCNTs in the MnOx/SWCNT macro-film provide additionallithium storage sites, leading to an improved lithium storagecapacity. SWCNT bundles have multiple active sites for Li-ioninsertion including the external surface of sidewalls, interstitialsites between nanotubes in a bundle and interior of nanotubesthrough lattice defects and the open ends. It is reported thatthe reversible theoretical capacity could exceed 1000 mAh g�1,which presents a dramatic improvement over graphite(372 mAh g�1) [37,38].

Conclusions

We have reported in this paper that the binder-free andfree-standing MnOx/SWNT macro-films can be synthesizedat a low temperature. The MnOx nanoparticles growndirectly on the flexible SWNT macro-films show a promisingelectrochemical performance in Li-ion batteries.


Furthermore, the MnOx/SWCNT macro-film exhibits excel-lent conductivity and a considerable mechanical adhesionon Cu foils. This hybrid thin film employed as the singlecomponent of anodes significantly decreases the weight by~95% compared to the conventional multi-component elec-trodes. Its high specific capacity of �1000 mAh g�1 and goodcycling performance are obtained due to the unique nano-architectures with the intrinsically interconnected frame-work of SWCNTs. It is expected that the hierarchicallystructured MnOx/SWNT macro-films would be a promisingcandidate for cost-effective and large-scale electrodes inenergy storage devices.

Acknowledgments

BQW would like to acknowledge the financial support fromNational Science Foundation (US NSF CMMI-1067960). JWQacknowledges the financial support from the State Scholar-ship Fund of China Scholarship Council (CSC, File no.2010603031).

Appendix A. Supplementary Information

Supplementary data associated with this article can befound in the online version at http://dx.doi.org/10.1016/j.nanoen.2012.12.009.

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Jinwen Qin is currently a Ph.D. student inthe Department of Chemistry at BeijingInstitute of Technology. She received herB.S. degree in chemistry from Beijing Insti-tute of Technology in 2008. Her research isfocused on carbon-based nanocompositesfor energy storage and catalysis.


Qing Zhang received her Bachelor’s degree
in Materials Physics at University of Scienceand Technology, Beijing in 2009. She joinedProf. Bingqing Wei’s group as a researchassistant at University of Delaware in 2009and she is currently working on energyretention mechanisms of electrochemicalcapacitors built with carbon-basednanomaterials.
Zeyuan Cao graduated from NorthwesternPolytechnical University with a Bachelor ofEngineering degree in 2010. After joiningDr. Bingqing Wei’s group in the same year,he started his research on the developmentof nanostructured electrode materials forrechargeable lithium batteries.

Xin Li graduated from Beijing Institute ofTechnology in 2005. He received his Masterof Science degree from Chalmers Univer-sity of Technology in 2007. After joiningDr. Bingqing Wei’s group, he startedresearch in energy storage devices usingnanomaterials. He graduated with his Mas-ter of Science degree from University ofDelaware in 2012. He is currently working asa research assistant at Stevens Institute of

Technology, under the supervision of Dr. Eui-Hyeok Yang. Hisresearch interest is energy storage devices using nanomaterials.

Changwen Hu is currently a full Professor inthe Department of Chemistry at BeijingInstitute of Technology. He received hisM.S. degree at Northeast Normal Universityand Ph.D. degree at University of Tokyo. Hiscurrent research interests include polyoxo-metalates, nitrogenous compounds andgreen catalysis. He also has been the leaderof Key Laboratory of Cluster Science, Minis-try of Education of China.

Bingqing Wei is currently a professor inthe Department of Mechanical Engineeringat the University of Delaware (UD). Hereceived his Ph.D. degree of mechanicalengineering from Tsinghua University,China. He worked at the Max-Planck Institutfur Metallforschung in Stuttgart, Germany;Rensselaer Polytechnic Institute in Troy,New York; and Louisiana State Universitybefore he moved to UD in 2007. Dr. Wei’s

research interest lies in nanomaterials and nanotechnology. Hisrecent research focuses on controllable synthesis of macroscalecarbon nanotube architectures with 1-, 2-, and 3-dimensions andtheir electrochemical device applications, such as lithium batteriesand supercapacitors.





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