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Cyclic Stability of Electrochemically Embedded NanobeamV2O5 in Polypyrrole Films for Li Battery CathodesYouna Kim,a Quang-Thao Ta,a Hung-Cuong Dinh,a,* Paul K. Aum,a

In-Hyeong Yeo,b,** Won Il Cho,c,** and Sun-il Mhoa,**,z

aDivision of Energy Systems Research, Ajou University, Suwon 443-749, KoreabDepartment of Chemistry, Dongguk University, Seoul 100-715, KoreacAdvanced Battery Center, Korea Institute of Science and Technology, Seoul 130-650, Korea

By utilizing a hydrothermal method, V2O5 crystalline powder of nanosized beam shape—termed nanobeam V2O5—was synthe-sized from a V2O5 xerogel. Then, using an electrochemical polymerization method, a composite film of nanobeam V2O5 embed-ded in a conductive polypyrrole—termed electrochemically embedded V2O5/polypyrrole �ee-V2O5/PPy�—was grown on anodesfrom an electrolyte solution containing pyrrole and dispersed nanobeam V2O5. Due to the oxidative catalytic action of V2O5 forthe polymerization of pyrrole, a thorough polypyrrole coating simultaneously grew on all the nanobeam V2O5 particles. Bothelectrochemically and catalytically polymerized polypyrrole can embed and connect the isolated nanobeam V2O5 particles, pro-viding mechanical flexibility during the expanding/shrinking cycles of discharge/charge. The cathode film with 2.6 mg loading ofee-V2O5/PPy had a specific capacity of 294 mAh g−1 at 0.1 C, which is 99.6% of the theoretical capacity for 2 equiv. of Liintercalation, and had a specific capacity of 195 mAh g−1 at 5 C. With 6.4 mg of loading, the system revealed superior cyclicstability, with a degradation rate of 2.1% and a specific capacity of 176 mAh g−1, after 40 battery cycles at the rate of 1 C. Theee-V2O5/PPy cathode performed excellently in terms of both cyclic stability and specific capacity.© 2010 The Electrochemical Society. �DOI: 10.1149/1.3523315� All rights reserved.

Manuscript submitted August 23, 2010; revised manuscript received November 10, 2010. Published December 16, 2010. This wasPaper 318 presented at the Vancouver, Canada, Meeting of the Society, April 25–30, 2010.

0013-4651/2010/158�2�/A133/6/$28.00 © The Electrochemical Society

Layer-structured V2O5 has been intensively studied for its poten-tial application as a cathode material in lithium batteries for smallelectrical devices due to its intrinsic material properties, including ahigh theoretical capacity with lithium intercalation/deintercalationand structural flexibility/reversibility.1-10 In addition to these intrin-sic material properties, the V2O5 morphology �amorphous or nano-crystalline� plays a key role in the electrochemical activities of thecathode. Amorphous V2O5 materials, such as aerogels or xerogels,have been introduced as intercalation cathodes for rechargeablelithium batteries because they adopt nanoscale solid-phase layeredstructures.3,4 However, either the low crystallinity of the V2O5 gelsor the presence of water in the gels often hinders consistent electro-chemical behavior, which limits the applications of these gels asbattery cathode materials. In contrast to the amorphous structures,micronscale and nanoscale structures with high crystallinity havebeen intensively studied due to their morphology, catalytic activities,and electrochemical performance as battery materials.5-10 Makingthe material structures on the micro- or nanoscale is expected toincrease their surface-to-volume ratios, which improves the accessi-bility of Li+ to the crystalline lattice and also shortens diffusionpaths. Overall, the kinetic barriers of the metal oxide electrodes,such as those made of V2O5, can be reduced, and the specific ca-pacity of the battery can be increased.3-10 In this work, to produce amaterial with a high surface-to-volume ratio and high crystallinity,nanosized beam-shape V2O5 crystalline particles—termed nano-beam V2O5—were prepared from a V2O5 xerogel using the hydro-thermal method.11,12

The limitation of the low electrical conductivity of V2O5 can beresolved by embedding dispersed nanobeam V2O5 within conduc-tive materials, such as conductive polymers. Conductive polymersare attractive materials in terms of cyclic stability as they are notonly electrically conductive but also mechanically flexible. Compos-ite films consisting of layer-structured metal oxide crystalline par-ticles embedded within a conductive polymer offer the prospect ofincreasing performance as cathode materials for rechargeable Li bat-teries due to their large surface accessibility, improved interparticle

* Electrochemical Society Student Member.** Electrochemical Society Active Member.

z E-mail: mho@ajou.ac.kr

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drift/diffusion pathways, shorter Li+ diffusion length within eachparticle, and mechanical flexibility.13-23 With these factors in mind,polypyrrole �PPy� was investigated as the matrix of a composite filmcontaining nanobeam V2O5 in this work.

PPy is an electrically conductive polymer and has received con-siderable attention due to its high electrical conductivity, stability inelectrochemical environments, and straightforward preparationmethods.18-30 Currently, research is focused on developing a com-posite film of thoroughly coated micro- or nanosize crystallite sur-faces with this polymer. A few recent attempts to develop mechani-cally flexible rechargeable batteries with completely organicpolymers have been reported, but the preparation of flexible elec-trodes with embedded inorganic oxide materials remains achallenge.30,31

In this work, we produced two kinds of composites, an electro-chemically embedded V2O5/ polypyrrole �ee-V2O5/PPy� film and aV2O5/PPy powder, both of which contained nanobeam V2O5 andPPy. The fabrication process of the ee-V2O5/PPy involves the hy-drothermal synthesis of nanobeam V2O5 from a V2O5 xerogel, fol-lowed by the anodic growth of the composite film in the electrolytesolution of pyrrole with dispersed nanobeam V2O5 utilizing an elec-trochemical polymerization method. While the polypyrrole film isgrowing electrochemically by applying an anodic potential, V2O5nanobeam particles dispersed in the electrolyte solution become em-bedded in the polymer film, forming a V2O5/PPy composite film.Within this composite film, smooth and thorough coatings of poly-pyrrole simultaneously formed on all nanobeam V2O5 particles dueto the oxidative catalytic action of V2O5 on the polymerization ofpyrrole. Both electrochemically and catalytically polymerized poly-pyrrole conductive polymers embed and connect the isolated nano-beam V2O5 particles into a valid electrically conductive network.Consequently, three processes—�i� electrochemical polymerization,�ii� catalytic polymerization, and �iii� particle embedding—occurredsimultaneously at the anode surface. Owing to these simultaneousprocesses, ee-V2O5/PPy was directly grown on stainless steel �SS�substrates. Henceforth, the requirement of mechanically pastingee-V2O5/PPy composite film on a battery cathode can be completelyavoided. In addition, the nanobeam V2O5/PPy conductive polymercomposite powder—V2O5/PPy powder—was produced only by thecatalytic action of V O on the polymerization of pyrrole in the

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same electrolyte solution. The performance of the cathode preparedfrom the V2O5/PPy powder was compared to that of the cathodebased on the ee-V2O5/PPy.

Experimental

Material preparation.— Nanobeam V2O5 was synthesized froma V2O5 xerogel using a cost-effective hydrothermal method.11 Abrown V2O5 xerogel was prepared according to the method de-scribed in Ref. 11 and 12. V2O5 powder �99.5 %� was heated to800°C and maintained at this temperature for 10 min to melt V2O5,which was then immediately poured into distilled water. A red V2O5xerogel was obtained upon aging for 1 week. To prepare nanobeamV2O5, the V2O5 xerogel �50 mL� was placed in a Teflon-lined au-toclave in a stainless steel shell along with a strong oxidant�KMnO4�. The oxidizing agent was added to prevent the formationof vanadium oxides containing vanadium ions with low oxidationnumbers. The autoclave was kept at 200°C for 48 h for the hydro-thermal reaction.

With the nanobeam V2O5 in hand, we synthesized ee-V2O5/PPyon an SS gauze electrode �SUS304-150 mesh, 90 �m thick, 17 mmdiameter disk� by the electrochemical polymerization of pyrrole�0.20 M� in 0.1 M LiClO4 containing dispersed V2O5 nanobeamparticles �14 mg/mL�. During this electrochemical growth, the SSgauze electrode was applied with an oxidative potential of +1.75 Vversus a Ag/AgCl �saturated KCl solution� reference electrode. Thethickness of the composite film grown on the SS substrate increasedfrom approximately 100 to 400 �m as the duration of anodizationwas increased from approximately 5 to 20 min.

In the electrolyte solution, due to the oxidative catalytic action ofV2O5, pyrrole is polymerized into PPy at the surface of the nano-beam V2O5 particles, forming V2O5/PPy composite particles. Whenthese composite particles are not deposited onto the anode surface, acomposite powder—V2O5/PPy powder—of nanobeam V2O5 par-ticles embedded within PPy will be produced in the electrolyte so-lution from which it will precipitate. This composite powder isbrown-black, thus confirming the growth of PPy on the surface ofthe dispersed nanobeam V2O5, which has a yellow color. V2O5 ex-hibits oxidizing power, and the oxidation of pyrrole should takeplace on the surface of the V2O5 particles, resulting in PPy-coatedV2O5.

Characterization.— Crystallographic information was obtainedfrom the X-ray diffraction �XRD� data collected over the 2� rangeof 3–70 using a Rigaku D/Max-III �Japan� powder diffractometerequipped with Cu–K� radiation �� = 1.54056 Å�. The compositionand morphologies of V2O5/PPy composites were characterized us-ing an Fourier transform-infrared �FT-IR� spectrophotometer �Nico-let iS10, USA�, a thermal gravity analyzer �Mac Science, MTC1000, Japan�, and a scanning electron microscope �SEM, JEOLJSM-6700F, Japan�. The battery cells were assembled in a dry room;they consisted of �i� lithium metal as an anode, �ii� a polypropyleneseparator soaked in a 1.0 M LiPF6 electrolyte solution in ethylenecarbonate/dimethyl carbonate, and �iii� a cathode prepared fromee-V2O5/PPy. The battery cathode was prepared by rolling pristineee-V2O5/PPy on SS gauze to a total thickness of approximately180 �m �including the thickness of the SS gauze, which was90 �m�, regardless of the loading amount of ee-V2O5/PPy compos-ite film. Therefore, with increased loading of the composite film, theporosity of the resulting film is expected to be lower. In this work,the total loading amount varied from 2.6 to 16.8 mg on the batterycathode. For comparison, V2O5/PPy composite powder was pastedonto an aluminum foil substrate. The loading amount of V2O5/PPypowder was 6.45 or 6.64 mg. The resulting material went throughthe same rolling step in the preparation of the battery packages ofthe same thickness.

The electrochemical performances of the ee-V2O5/PPy andV2O5/PPy powder composite electrodes were tested after assem-bling coin-type �CR2032� battery cells with a battery test system

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�Maccor Series 4000�. These cells were cycled galvanostatically inthe multichannel battery test mode. Unless otherwise indicated, allcyclic stability tests were conducted using freshly fabricated batter-ies. These tests involved five cycles at 0.2 C, two cycles at 0.5 C,and two cycles at 1 C for initial conditioning. After the 9th cycle, aseries of cyclic tests was performed at 1 C until the 50th cycle ormore. The degradation rate over 40 cycles was calculated based onthe specific capacity differences between the 9th and 50th cycles.

Results and Discussion

Structural, compositional, and morphological characteriza-tions.— The structural, compositional, and morphological character-izations of the composites of nanobeam V2O5, ee-V2O5/PPy, andV2O5/PPy powder afforded insight into their electrochemical behav-ior as cathode materials. The XRD patterns of the V2O5 nanostruc-ture, prepared using the hydrothermal process, are shown in Fig. 1.All of the diffraction peaks of the nanobeam V2O5 �Fig. 1a� wereindexed to the orthorhombic system with the lattice constants a= 11.48 Å, b = 4.36 Å, and c = 3.55 Å �JCPDS 41-1426�. TheXRD pattern �Fig. 1b� of the ee-V2O5/PPy composite films indicatesthat the V2O5 diffraction peaks appear on top of the broad bandoriginating from the amorphous polypyrrole. A similar XRD patternwas obtained for the V2O5/PPy powders �Fig. 1c�, which were syn-thesized from the electrolyte solution by making use of the dual roleof V2O5 as a catalyst for the polymerization of pyrrole and as anembedded component of the composite powders.

The XRD pattern of vanadium-oxide xerogel is included in Fig.1d for comparison. The XRD pattern of xerogel shows the pro-nounced �001� peak at the angle of 2� = 7.85°. This pronouncedpeak indicates that the xerogel is not completely amorphous andinvolves lamellar ordering with the bilayer structure of possible in-tercalation. From this V2O5 xerogel, V2O5 nanobeams were pre-pared by the hydrothermal method. No peak was observed at anglessmaller than 10°, and all diffraction peaks of the V2O5 nanobeamscan be indexed to the orthorhombic V2O5 lattice as shown in theXRD pattern �Fig. 1a�. For the composite films or powders of V2O5nanobeams and PPy prepared by either the electrochemical methodor the catalytic method, no peak at the angles smaller than 10°appeared, as shown in Figs. 1b and 1c. Hence, we have concludedthat the pyrrloe or PPy intercalation into crystal lattices did notoccur significantly to be shown in XRD patterns.

The IR spectra of the nanobeam V2O5/PPy composites areshown in Fig. 2. The composite films of ee-V2O5/PPy andV O /PPy powder exhibited the absorption bands of both V O and

Figure 1. XRD patterns of the �a� V2O5 nanobeam, �b� ee-V2O5/PPy filmprepared by the electrochemical method, and �c� V2O5/PPy powder precipi-tated from the LiClO4 electrolyte solution.

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PPy in their IR spectra. As indicated in the IR spectra ofee-V2O5/PPy, the three main vibration bands of V2O5 are clearlypresent at 1020, 850, and 600–480 cm−1; these signals correspond tothe terminal oxygen symmetric stretching mode �vs� of V v O andthe bridge oxygen asymmetric and symmetric stretching modes �vasand vs� of V–O–V, respectively.21-23 Additionally, the characteristicbands of PPy in the 500–1600 cm−1 range confirm the presence ofPPy in the composites.16,21-23 The IR spectrum of the PPy film pre-pared by the electrochemical oxidation method is different from thatof the PPy powder prepared from the electrolyte solution. The spec-trum of the composite film prepared by the electrochemical oxida-tion method shows extra peaks compared to that of the compositepowder prepared from the electrolyte solution. The extra peaks at1145, 1112, 1089, and 626 cm−1 indicate the presence of perchlorateions�Q1� �ClO4

−� that were incorporated into the polypyrrole filmdue to the charge compensation during the oxidative growth of thepolymer film at the oxidation potential �+1.75 V�. The V2O5/PPypowder samples, which were prepared only by the catalytic action ofV2O5 in the LiClO4 electrolyte solution, did not show any absorp-tion bands associated with perchlorate ions, which are similar to thePPy powder samples prepared using other oxidative catalysts.20,21

The nature of the electrolyte anion is known to have an impact onthe quality of the polymer films.16,25-31 The relative intensities of theIR peaks corresponding to V2O5 and PPy are indicative of theirrelative contents in the composites. The intensities of the V2O5peaks relative to those of the PPy peaks were larger in the compositepowders than in the electrochemical composite films. The weightratios of the V2O5/PPy composites were determined by the thermo-gravimetric analysis. Because the combustion of the polymer is nor-mally completed below 500°C, the V2O5 nanobeam content in thecomposites can be estimated. The V2O5 nanobeam contents in theelectrochemical composite films prepared in this work were typi-cally lower than those in the composite powders. The compositionof the composite powders corresponded to the proportion of eachcomponent initially added to the solution �80 � � 0.5� wt % ofV2O5 nanobeams�. In the ee-V2O5/PPy composite film, a V2O5nanobeam content of 56 � � 3� wt % was obtained.

The SEM images of the V2O5 nanobeam and V2O5/PPy com-posites are shown in Fig. 3. The morphology of V2O5 prepared bythe hydrothermal method consisted of V2O5 beams with relativelyuniform widths �30–80 nm�, lengths �approximately 5 �m�, andrectangular tips �inset of Fig. 3a�. The morphologies of bothee-V O /PPy and the V O /PPy powder composite were similar to

Figure 2. FT-IR spectra of �a� ee-V2O5/PPy film prepared by the electro-chemical method and �b� V2O5/PPy powder prepared from the LiClO4 elec-trolyte solution. The absorbance bands corresponding to V2O5 and PPy areindicated in the spectra.

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those of nanobeam V2O5; i.e., the prepared V2O5/PPy compositeexhibited the beam shape of V2O5 �Figs. 1b and 1c�. This result is incontrast with the electrochemically grown pure PPy films, whichshow a globular shape; these films result in cauliflowerlike shapeswhen they were grown into thicker films �Fig. 3d�. In the V2O5/PPycomposites, the V2O5 nanobeam particle structures were fullycoated with the PPy films due to the oxidative catalytic action ofV2O5 on the polymerization of pyrrole. No globular orcauliflowerlike-shaped isolated PPy was found in the composites.Herein, this work has demonstrated the electrochemical synthesis ofmechanically flexible composite films of dispersed nanobeam V2O5embedded in conductive PPy. This method is inexpensive and didnot require the use of patterned templates or catalysts other thanV2O5 itself.

Performance of the ee-V2O5/PPy composite films as cath-odes.— After assembling the ee-V2O5/PPy composite-film cathodesinto the coin-type Li cells, the performance of these cells as Libatteries was tested using a battery test system, as shown in Fig. 4a.All of the 40 consecutive discharge/charge curves of the lithiumbattery after the initial conditioning, assembled with theee-V2O5/PPy composite-film cathode, showed reversible lithiuminsertion/extraction during the discharge/charge processes. Distinctplateaus only appeared at voltages above 3 V; these patterns areknown to be due to the reversible structural modification of V2O5 toLiV2O5 �x = 1 for LixV2O5�, which is induced by the insertion of 1equiv. of lithium �specific capacity of 147 mAh g−1� during the dis-charge process. The decreasing trend of the cell voltages with sev-eral voltage plateaus in the first discharge curves for the V2O5 cath-odes is well known.1-8 Normally, more plateaus appear at lower cellvoltages, corresponding to the insertion amounts greater than 1equiv. For example, the potential plateau appearing in the vicinity of2.3 V is considered to correspond to the phase modification ofLixV2O5 for 2 equiv. of Li insertion �x = 2� into V2O5. The plateauat approximately 2.3 V typically extends to greater values of thecapacity �up to approximately 295 mAh g−1� in the first dischargecurve, and this plateau usually disappears in the subsequent dis-

Figure 3. SEM images of �a� nanobeam V2O5, �b� ee-V2O5/PPy film pre-pared by the electrochemical method, �c� V2O5/PPy powder precipitatedfrom the LiClO4 electrolyte solution, and �d� PPy film grown electrochemi-cally. The scale bar in the figures indicates 1 �m and that in the inset indi-cates 50 nm.

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charge curves.1,2 The severe loss of capacity during the repeateddischarge/charge processes is considered to correlate with severalsteps of irreversible structural modifications induced by extensive Liinsertion into and extraction from the lattices.16,19-23 However, nomore potential plateaus below 2.5 V appeared for the lithium batteryassembled with the ee-V2O5/PPy composite-film cathode in thiswork. As a result of the reversible structural modifications ofLixV2O5 in the composite film with lithium insertion/extraction, ex-cellent cycle stability of the rechargeable battery was observed dur-ing the repeated discharge/charge processes. The excellent cyclingstability of the composite-film cathodes was also confirmed by thesimilarity of the differential capacity �dC/dt; current� curves over 40discharge/charge cycles, as shown in Fig. 4b. The cycling stabilityof the composite-film cathodes is thought to result from the syner-gistic effect of the inorganic, electrochemically active material andthe conductive organic polymer. The polymer improves the accessi-bility of the Li+ ions, the conductivity, and the flexibility. Thequasitwo-dimensional morphology of nanobeam V2O5 provides alarge surface area and a short diffusion length for lithium diffusion.In addition, the surface of V2O5 particles was coated with PPy,which �i� provides the flexibility required to absorb the minute vol-ume changes during lithium insertion/extraction and �ii� improves

(a)

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Figure 4. �a� Discharge–charge curves of 50 cycle battery testing, except theinitial conditioning cycles, at a rate of 1 C �1 C = 295 mA/g� obtained fromthe lithium battery assembled with the cathode consisting ee-V2O5/PPy film�loading of 8.5 mg�. �b� Differential capacities �dC/dt� of ee-V2O5/PPy filmelectrodes, where �———� line is for the 2nd cycle, �� is for the 25th cycle,and �-.-.-.-.-.-� is for the 50th cycle �loading of 8.5 mg�.

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the electronic conductivity by connecting the isolated V2O5 particlesin the electrode. The minute volume changes caused by the lithiuminsertion/extraction between the layers of the quasitwo-dimensionalmorphology of the nanobeam V2O5 particle can be accommodatedby the flexible conductive polymer coated on the surfaces of theparticles in the composite film. This property is a crucial factor forthe reversible discharge/charge behavior of the batteries. Of note,the discharge capacities showed trivial fading �a small capacity lossof �10%� during repeated charge/discharge cycles for the batteriescontaining ee-V2O5/PPy composite-film cathodes. The several irre-versible steps corresponding to the structural changes of LixV2O5induced by the lithium insertion/extraction into/from the V2O5 lat-tices are reported to be correlated with the severe decrease in thespecific capacity during the discharge/charge processes.16,19-23 Thecapacity of the lithium batteries containing the ee-V2O5/PPy com-posite film remained fairly constant, as compared to the reportedcapacity losses of V2O5 �more than 25%� that result from the inter-facial properties and irreversible structural changes induced by re-dox cycling.

Loading effect of the ee-V2O5/PPy composite-film cath-odes.— To investigate the effect of loading on the electrochemicalperformance of the ee-V2O5/PPy composite-film electrodes, variousloading amounts of the ee-V2O5/PPy composite on the cathode wereinvestigated in Li batteries. The loading of the cathode materialdepends upon the thickness of the electrochemical composite film,which can be adjusted by controlling the duration of the anodicpolymerization of pyrrole. As shown in Fig. 5, after the Li batterieswere assembled with the cathodes made of ee-V2O5/PPy compositefilms with various loading amounts, the discharge capacities andspecific capacities were tested for 50 or more consecutive discharge/charge cycles: five discharge/charge cycles at a rate of 0.2 C, twocycles at a rate of 0.5 C, two cycles at a rate of 1.0 C �during initialconditioning for the purpose of structural stabilization�, and all re-maining cycles at a rate of 1.0 C.

The capacities of the batteries increased as the loading amountsof ee-V2O5/PPy composite films in the cathodes increased from2.6 to 16.3 mg. However, the specific capacities decreased with in-creasing loading. These data reveal that the utilization efficiency ofthe ee-V2O5/PPy composite in the cathodes is reduced when theloading increases. When the loading of the ee-V2O5/PPy compositefilm rose above 9.5 mg, the increases in the capacity diminished,causing an abrupt decrease in the specific capacity, as can be seenfrom curves ¬ and − in Figs. 5a and 5b.

Cyclic performance of the ee-V2O5/PPy composite-film cath-odes.— To achieve more stable cycle lives and high efficiency forthe lithium batteries containing the ee-V2O5/PPy composite-filmcathodes, optimization of the loading amount was required. Asshown in Fig. 5c, 6.4 mg loading had the best cyclic stability, with adegradation rate of approximately 2.1% and a specific capacity of176 mAh g−1 after 40 battery cycles at a rate of 1 C.

As shown in Fig. 6, the cyclic performances under variousdischarge/charge rates, ranging from 0.1 to 5 C with five cycles ateach rate, were also tested for the ee-V2O5/PPy composite-filmcathodes with three different loadings—2.6, 6.5, and 11.3 mg. Boththe discharge capacities and the specific capacities decreased withincreasing rate for all of the film cathodes. Films with less loadingexhibited larger specific capacities than the thicker films, especiallyat high discharge rates. For example, the discharge capacity was271 mAh g−1 �92% of the theoretical capacity� at a rate of 0.1 C forthe film cathode with 11.3 mg loading, and it decreased steeply to121 mAh g−1 �41% of the theoretical capacity� at a high current rateof 5 C. The discharge capacity for a cathode with less loading�2.6 mg� was 294 mAh g−1 �99.6% of the theoretical capacity� at0.1 C and 195 mAh g−1 �66% of the theoretical capacity� at a rate of5 C. Cathodes that were loaded less exhibited better rate capabilitiesamong the electrochemical composite-film cathodes.

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of 0.1 C as the last step.

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(a)

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Figure 5. �a� The plots of the battery capacities at the discharge rate of 1 Cfor ee-V2O5/PPy film cathodes with various loadings; ¬ 16.8 mg, −

13.1 mg, ® 9.5 mg, ¯ 6.4 mg, ° 3.9 mg, and ± 2.6 mg. �b� The plots of thebattery specific capacities at the discharge rate of 1 C for ee-V2O5/PPycomposite-film cathodes with various loadings; ¬ 16.8 mg, − 13.1 mg, ®

9.5 mg, ¯ 6.4 mg, ° 3.9 mg, and ± 2.6 mg. �c� The three-dimensional plotof the decrease in specific capacities at the discharge rate of 1 C for theee-V2O5/PPy composite-film cathodes with various loadings; ¬ 16.8 mg, −

13.1 mg, ® 9.5 mg, ¯ 6.4 mg, ° 3.9 mg, and ± 2.6 mg.

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(b)

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Figure 6. �a� The discharge and charge curves recorded at various rates of0.1, 0.5, 1.0, 2.0, and 5.0 C for the ee-V2O5/PPy composite-film cathodewith 2.6 mg loading. Five cycles at each rate were recorded.�b� The cycla-bility test by sequentially increasing charge–discharge rates of 0.1 C, 0.5 C,1.0 C, 2.0 C, and 5.0 C for the ee-V2O5/PPy composite-film cathodes withvarious loadings; ¬ 2.6 mg, − 6.5 mg, and ® 11.3 mg. Five cycles at eachrate were recorded and the rate was set back to the initial rate of 0.1 C as thelast step. �c� The cyclability test by sequentially increasing discharge–chargerates of 0.1, 0.5, 1.0, 2.0, and 5.0 C for the ee-V2O5/PPy composite-filmcathodes with various loadings; ¬ 2.6 mg, − 6.5 mg, and ® 11.3 mg. Fivecycles at each rate were recorded and the rate was set back to the initial rate

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From the 30th cycle of the 0.1 C test, shown in Fig. 6b, aftertesting rates from 0.1 to 5 C, one can estimate the battery retentioncapabilities. The specific capacities returned to a level close to theinitial states of 0.1 C, although the specific capacity decreased withincreased rates as indicated by the 25th cycle. As shown in Fig. 6b,between the 5th and 25th cycles, the battery with 2.6 mg loadinghad the least degradation for higher rates �0.5–5 C�. This kind ofloading effect is clearly shown in Fig. 6c.

Performances of the V2O5/PPy powder composite-film cath-odes.— The powder form of the nanobeam V2O5/PPy conductivepolymer composite—V2O5/PPy powder—was also produced fromthe same electrolyte solution. As shown in Fig. 7, the V2O5/PPypowder composites, loaded with either 6.45 or 6.63 mg, had inferiorperformances in terms of both cyclic stability and specific capacity.After testing 40 battery cycles that included the initial conditioningcycles mentioned before and after the duration tests at a rate of 1 C,the degradation rate was found to be approximately 26%, and thespecific capacity was found to be approximately 94.7 mAh g−1. Thecathode made from the V2O5/PPy powder displayed inferior perfor-mances in terms of both cyclic stability and specific capacity whencompared to those containing the ee-V2O5/PPy composite film.

Conclusions

A composite of crystals embedded in a conductive polymer wasformed to improve the electrochemical characteristics. The combi-nation of the conductive and mechanically flexible polypyrrole withthe nanostructured V2O5 in the composite films produced larger ca-pacities due to the large surface accessibility, improved diffusionpathways, and shortened Li+ diffusion lengths. Nanobeam V2O5 andpolypyrrole composite films �ee-V2O5/PPy� were synthesized byanodic polymerization of pyrrole, and their electrochemical charac-teristics were investigated. The composition and thickness of thecomposite-film cathodes can also be controlled during the one-stepanodic polymerization process. The conductive polymers in thecomposite films connect the isolated V2O5 nanobeams and give riseto a conductive network in the electrode. The ee-V2O5/PPy compos-ite films have excellent rate capabilities and cycling stabilities in

Figure 7. The plots of the battery specific capacities at the discharge rate of1 C for the cathodes of V2O5/PPy powder with loadings of 6.45 mg �symbol�� and 6.63 mg �symbol ��.

address. Redistribution subject to ECS term144.213.253.16aded on 2015-06-23 to IP

rechargeable batteries. This fact enabled the battery cathode filmwith 2.6 mg loading of ee-V2O5/PPy to have excellent rate capabili-ties: the specific capacity was 294 mAh g−1 at 0.1 C �approximately99.6% of the theoretical capacity� and was 195 mAh g−1 at 5 C�approximately 66% of the theoretical capacity�. The electrode with6.4 mg loading displayed superior cyclic stability with a degradationrate of approximately 2.6% and a specific capacity of approximately176 mAh g−1 �after testing 50 full battery cycles at the rate of 1 C�.These improved results are thought to be due to the synergisticeffect of the enhanced diffusion path, the increased accessibility ofthe Li+ ions, the increased crystal surface-to-volume ratio, and themechanical flexibility of each component of the electrochemicallyproduced composite films, which consist of a nanocrystalline inor-ganic lattice and a conductive polymer.

Acknowledgments

This work was supported by the Korea Research Foundation�2009-0083734 and 2009-0094049�.

Ajou University assisted in meeting the publication costs of this article.

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