journal homepage: www.elsevier.com/locate/nanoenergy Available online at www.sciencedirect.com RAPID COMMUNICATION High-rate and elevated temperature performance of electrospun V 2 O 5 nanofibers carbon-coated by plasma enhanced chemical vapour deposition Yan L. Cheah a , Robin von Hagen b , Vanchiappan Aravindan c , Raquel Fiz b , Sanjay Mathur b , Srinivasan Madhavi a,c,d,n a School of Materials Science and Engineering, NanyangTechnological University, Singapore 639798, Singapore b Institute of Inorganic Chemistry, University of Cologne, Greinstr. 6, 50939, Cologne, Germany c Energy Research Institute@NTU (ERI@N), Nanyang Technological University, Research Techno Plaza, 50 Nanyang Drive, Singapore 637553, Singapore d TUM-CREATE Center for Electromobility, Nanyang Technological University, 50 Nanyang Drive, Singapore 637553, Singapore Received 15 May 2012; received in revised form 12 July 2012; accepted 13 July 2012 Available online 24 July 2012 KEYWORDS Vanadium pentoxide (V 2 O 5 ); PECVD; Electrospinning; Carbon-coating; Li-ion battery; Cathode Abstract Vanadium pentoxide (V 2 O 5 ) nanofibers (VNF) are synthesized by electrospinning technique and homogeneously coated with carbon by plasma enhanced chemical vapour deposition. The morpho- logical features of the VNF are analyzed by field emission scanning and transmission electron microscopy showed the presence of carbon layer over the VNF crystallites. Powder X-ray diffraction (XRD) patterns of the calcined nanofibers reveal the formation of V 2 O 5 phase. Electrochemical Li- insertion behaviors of VNFs are explored as cathode in half-cell configuration by means of both potentiostatic and galvanostatic measurements. Carbon-coated VNF (C-VNF) showed the slightly less initial discharge capacity of 300 mA h g 1 with improved capacity retention of 465% after 50 cycles at 0.1 C rate, whereas native VNF showed only 40% capacity retention under the same testing conditions. Enhanced high rate and elevated temperature performance of C-VNF is noted with overall capacity and capacity retention (460%) characteristics than native fibers. Carbon-coating enables im- proved electronic conductivity profiles and prevents undesired side reactions with electrolyte counterpart without hindering the Li-ion mobility reflected in the superior battery performance of C-VNF. & 2012 Elsevier Ltd. All rights reserved. 2211-2855/$ - see front matter & 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.nanoen.2012.07.012 n Corresponding author at: Nanyang Technological University, School of Materials Science and Engineering, Singapore 639798, Singapore. Tel.: +65 6790 4606; fax: +65 6790 9081. E-mail address: [email protected] (S. Madhavi). Nano Energy (2013) 2, 57–64
AbstractVanadium pentoxide (V2O5) nanofibers (VNF) are synthesized by electrospinning technique andhomogeneously coated with carbon by plasma enhanced chemical vapour deposition. The morpho-logical features of the VNF are analyzed by field emission scanning and transmission electronmicroscopy showed the presence of carbon layer over the VNF crystallites. Powder X-ray diffraction(XRD) patterns of the calcined nanofibers reveal the formation of V2O5 phase. Electrochemical Li-insertion behaviors of VNFs are explored as cathode in half-cell configuration by means of bothpotentiostatic and galvanostatic measurements. Carbon-coated VNF (C-VNF) showed the slightly lessinitial discharge capacity of�300 mA h g�1 with improved capacity retention of465% after 50 cyclesat 0.1 C rate, whereas native VNF showed only �40% capacity retention under the same testingconditions. Enhanced high rate and elevated temperature performance of C-VNF is noted with overallcapacity and capacity retention (460%) characteristics than native fibers. Carbon-coating enables im-proved electronic conductivity profiles and prevents undesired side reactions with electrolytecounterpart without hindering the Li-ion mobility reflected in the superior battery performanceof C-VNF.& 2012 Elsevier Ltd. All rights reserved.
Elsevier Ltd. All rights reserved.012.07.012
g Technological University,ineering, Singapore 639798,65 6790 9081.
Figure 1 Schematic of PECVD setup showing formation ofplasma on VNF.
Y.L. Cheah et al.58
Introduction
Lithium-ion batteries (LIB) are widely used in portableapplications like personal digital assistants (PDAs), laptopsand hand phones etc., [1–6]. Current research activities onLIBs are focused the development of high energy densitypower packs and their application towards hybrid electricvehicle (HEV) and electric vehicles (EV) [7–9]. Since thecommercialization of LIBs by Sony Inc LiCoO2 has dominatedas cathode material in such power packs [10]. However, thelimitations of Li-ion (0.5 mol) extraction from LiCoO2 due tothe weak Co–O bonding results in the loss of energy density.Apart from this, the cost and toxicological properties of Co arealso other main concerns. Hence, the search for alternatecathodes with low cost and high performance is warranted. Inthis line, LiMn2O4, LiFePO4, LiMnPO4, LiNi0.5Mn1.5O4 and LiMn1/3Ni1/3Co1/3O2 are found to be promising materials for thereplacement of LiCoO2 [7,9,11–15]. Unfortunately, said mate-rials are exhibiting the practical specific capacity o200 mA hg�1 which is insufficient for the development of high energydensity Li-ion power packs. Beyond one electron reactioncathodes such as Li2MSiO4 and Li2MPO4F (M=Co and Mn) arealso proposed, but achieving the capacity more than 200 mAh g�1 remains questionable due their own setbacks [16–19].Vanadium pentoxide (V2O5) is a promising material due to itslayered structure, for accommodation of more than two Li-ions with theoretical capacity �400 mA h g�1 and possessescertain advantage over the abovementioned cathodes [20].Cost effectiveness, ease of synthesis and natural abundance onearth’s crust are also other important advantages. The highcapacity of V2O5 cathode is mainly due to the variableoxidation states of vanadium. During electrochemical lithiumintercalation, V2O5 undergoes phase transformations accordingto following equilibrium V2O5+xLi
+ +xe�2LixV2O5 [20], viathe reduction of V5+ to V4+ and subsequently V3+ to accom-modate Li-ions into its layered structure [21].
On the other hand, V2O5 possesses certain issues such assevere capacity fading and poor rate capability which preventits use in practical LIBs. Observed problems are generally dueto poor Li-diffusion kinetics, structural instability during Li-intercalation/de-intercalation and inherent poor electronicconductivity (�10�2–�10�3 S cm�1) [20,22]. To address thesaid issues several approaches have been adopted such asreducing Li-ion diffusion pathways by fine-tuning the morphol-ogy of V2O5 by creating 1D and 3D nanostructured networks[23–25]. Electronic conductivity has been improved via surfacemodifications by conductive coatings or making compositeswith either carbon [26–29] or conductive additives for examplepolyaniline (PANI) [30,31], polypyrrole (Ppy) [32], poly(vinyl-pyrrolidinone) (PVP) [33] and poly(ethylene oxide) (PEO)[4,34]. The underlying concept of making composites is notonly to improve the conductivity but also to provide necessarystability during electrochemical cycling [33]. In addition, themorphology of V2O5 nanostructures and herein preferably one-dimensional nanostructures has shown to play a vital role in toachieving high capacity [21]. Hence, in the present study, asimple and inexpensive electrospinning technique has beenused to synthesize one dimensional V2O5 nanofibers (VNF).Further, electrospun VNF provides high aspect ratio and a largesurface area-to-volume ratio, which enables facile diffusion ofLi-ions via the improved electrode/electrolyte contact area[23,35,36].
Carbon coating was employed to improve the electronicconductivity of VNF using plasma enhanced chemical vapourdeposition method (PECVD). A main advantage of using PECVDtechnique was that morphological features of the mm long onedimensional fibers build up of sintered V2O5 nanofibersretained during the gas phase process. Additionally, it enableda homogeneous coating with desired thickness of carbon layerand no work up step was needed unlike in time consumingconventional wet-chemical coating procedures [37]. Electro-chemical Li-insertion properties of carbon-coated VNFs withdifferent durations (15 and 30 min) are carried out in half-cellconfiguration (Li/VNFs) and compared with bare VNFs. Tostudy protective properties of carbon layer towards electro-lyte solutions elevated temperatures studies are conductedand described in detail.
Experimental
V2O5 synthesis and characterization
A simple and cost-effective electrospinning procedure wasadopted to synthesize the VNF and described elsewhere indetail [38]. The sintered VNF were coated with amorphouscarbon by plasma-enhanced chemical vapor deposition(PECVD) using acetylene (C2H2) as the carbon precursor forat 500 1C as shown schematically in Fig. 1. PECVD processwas performed on a commercial vacuum system (PlasmaElectronics, CVD-PECVD DOMINO) working with radio-fre-quency excitation (13.56 MHz). 80 sccm of C2H2 were intro-duced into the reaction chamber through a mass flowcontroller (MKS MFC 1179) controlled by a software inter-face during the reaction and the reactor pressure wasmaintained at 14 Pa. C2H2 was decomposed directly on theVNFs deposits, which were loaded in a ceramic boat placedon the heatable RF electrode. The temperature was set to500 1C and the plasma power to 50 W (resulting in a bias of214–218 V). The VNF were coated with carbon using thissetup for two different durations namely, 15 and 30 min andhereafter denoted as C-VNF-15 and C-VNF-30, respectively.
Morphological features of carbon-coated and bare VNFwere studied using field emission scanning electron micro-scope (FE-SEM, JEOL JSM-7600F) with an acceleratingvoltage of 5 kV. Presence of carbon layer was investigated
Figure 2 Field emission secondary electron microscopic (FE-SEM) images of (a) bare VNF, (b) C-VNF-15 and (c) C-VNF-30with corresponding transmission electron microscopic (TEM)images of C-VNFs (insets).
High-rate and elevated temperature performance of electrospun V2O5 nanofibers 59
using a transmission electron microscope (TEM, JEOL 2100F)in high resolution mode operating at 200 kV. Structuralproperties of carbon-coated and bare VNF were examinedby Bruker X-ray diffractometer using Cu Ka radiationbetween 101 and 801. Rietveld refinement was conductedfor the obtained reflections using Topas V3 software [39,40]by fundamental parameters approach [41]. Thermogravi-metric analyses (TGA) of the C-VNF were recorded usingPerkin Elmer Q500 with the scan rate of 5 1C min�1 fromroom temperature to 700 1C under 60% air+40% nitrogenatmosphere.
Electrode fabrication process
Composite cathodes were formulated by mixing active mate-rial (VNFs), binder (Kynar 2801), and conductive additive(Super P Li carbon, Timcal) in the weight ratio of 60:20:20,respectively. 1-methyl-2-pyrrolidinone (NMP, Sigma-Aldrich)was used as solvent for the binder to forms a slurry. Theresulting viscous slurry was stirred continuously overnight andsubsequently coated on aluminium foil using a doctor blade.Then the Al foils were dried in a vacuum oven for several hoursto remove the solvent molecules. Later, slurry containing Alfoil was pressed in between the twin rollers to enablenecessary adherence towards current collector. The 16-mmdiameter blanks were punched out from the dried coatings onAl foil with same area of counter electrode (lithium foil,�0.59 mm thick, Hohsen Corporation, Japan) was used tofabricate the test cells. The lithium insertion properties wereevaluated in half-cell configurations (vs. Li) using standard two-electrode coin cell (CR 2016) assembly. Test electrodes wereseparated by Celgard 2400 separator and 1 M LiPF6 in ethylenecarbonate (EC): diethyl carbonate (DEC) (1:1 wt%, Danvec) wasused as electrolyte solution. The coin-cell assembly processwas conducted in an Argon filled glove box (MBraun).
Galvanostatic discharge–charge profiles of carbon-coatedand bare VNF based test cells were conducted between 1.75and 4 V vs. Li in ambient and elevated temperature (551 C)conditions using battery testing systems (Neware). Cyclicvoltammetry (CV) and electrochemical impedance spectrabetween 5 and 1 MHz were recorded using Solartron 1470Epotentiostat coupled with SI 1255B Impedance/gain-phaseanalyzer using standard two-electrode coin cell configura-tion. For CV analysis, metallic lithium serves as both counterand reference electrode at slow scan rate of 0.1 mV s�1. TheNyquist plots Z0 0 vs. Z0 were derived using Zview software(Version 2.2, Scribner Associates Inc., USA).
Results and discussions
Morphological and compositional studies
Fig. 2 represents the FE-SEM pictures of VNF and C-VNFsynthesized by electrospinning technique. As observed fromFig. 2a, large aspect ratio fibers with diameters ranging from500 to 800 nm are evidenced. Observed fibrous morphologycomprises nanosized V2O5 crystals which forms a porous butcontinuous chain like morphology of resultant fibers aftercalcination. The observed morphology of VNF is expected andconsistent with our previous results and the porosity willassure a good wetting behavior with the electrolyte. After the
calcination process, bare VNF are subjected to carbon coatingby PECVD. PECVD is a non-destructive technique to produce athin layer of carbon over the VNF, more specifically on V2O5
nanocrystals. In addition, this PECVD technique enables theretention of fibrous morphology as evidenced from the FE-SEMpictures (Fig. 2b and c). To ensure the formation of carbonlayer, a TEM study was conducted and presented as inset inFig. 2b and c. It can be seen from the TEM images that C-VNFsshow variation in thicknesses of the carbon-layers due to thedifference in duration inside the plasma. The C-VNF-15 (insetFig. 2b) showed a heterogeneous carbon coating over the V2O5
nanocrystals which indicates the duration is not sufficient toenable homogeneous coating. Hence, coating duration isincreased to 30 min (C-VNF-30) to achieve homogenous carbon
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Figure 3 (a) X-ray diffraction (XRD) patterns of bare VNF, C-VNF-15 and C-VNF-30 showing formation of V2O5 structure (�asterisksindicate VO2 peak) and (b) thermo-gravimetric analyses (TGA)showing weight percentages of C-VNF-15 and C-VNF-30.
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Figure 4 Cyclic voltammograms showing first cycle of (a) bareVNF, (c) C-VNF-15, (e) C-VNF-30 and subsequent cycles of(b) bare VNF, (d) C-VNF-15 and (f) C-VNF-30 respectively, inwhich metallic lithium serves as both counter and referenceelectrodes in two electrode coin cell configuration at scan rateof 0.1 mV s�1 between 1.75 and 4.0 V vs. Li.
Y.L. Cheah et al.60
layer with thickness of �5 nm over V2O5 nanocrystals asevidenced from Fig. 2c inset.
Structural properties of bare and carbon-coated VNFswere analyzed by XRD and presented in Fig. 3a.Rietveld refinements were carried out for all the threesamples (VNF, C-VNF-15 and C-VNF-30) using TOPAS software.The observed reflections for VNF can be indexed accordingto layered Shcherbinaite structure (a=11.5181(6) A,b=4.3804(3) A, c=3.5671(2) A and crystallite size=98.2(2) nm)with Pmn21 space group. The observed patterns clearlyindicate the formation of single phase V2O5. However, inC-VNF-15 and C-VNF-30, there are reflections showing traceamount of impurity peaks at �281, which corresponds to theformation of VO2 phase. Presence of VO2 impurity phase is dueto the carbothermal reduction of V2O5 (V5+ to V4+) duringcarbon coating procedure by PECVD. During the carbon coatingprocedure, the precursor C2H2 is decomposed (C2H2+V2O5-2C+(V2O5, VO2 (on the surface))+H2O) and formed as carbonon the surface under inert atmosphere, leading to the forma-tion of VO2 impurity. Evidently, increasing the plasma durationfrom 15 to 30 min. leads to an increase in intensity of the peak(2y=281) related to VO2 phase. Rietveld refinement has beenused to estimate the amount of VO2 formed during carboncoating and found to be �4.5 and �7.5 wt% for C-VNF-15 andC-VNF-30, respectively. In addition, no extra peaks related tocarbon are detected in the XRD observations, which corre-sponds the formation of amorphous carbon layer over the V2O5
nanocrystals. In order to estimate amount of carbon present in
C-VNF-15 and C-VNF-30, TGA was carried out in 60% air+40%nitrogen atmosphere and presented in Fig. 3b. Observedweight loss (%) during TGA studies are directly related to theamount of carbon present in the compound. C-VNF-15 andC-VNF-30 showed the weight loss of �1.4 and �4.2 wt%,respectively. The TGA results suggested that increasing coatingduration provided higher amount of carbon content, which isconsistent with the increase in the intensity of VO2 phase notedin XRD observations. Further, Raman spectroscopy also con-firmed the presence of amorphous carbon layer over the V2O5
fibers (supplementary information, Fig. S1).
Electrochemical studies
Li-insertion properties of electrospun bare and carbon-coated VNFs were evaluated by means of both galvanostaticand potentiostatic modes in half-cell (Li/VNF or C-VNF)assembly. Generally, Li-insertion into V2O5 lattice resultedin phase transition from V2O5 to LixV2O5 according to thefollowing equilibrium,
V2O5+xLi+ +xe�2LixV2O5 (1)
During Li-insertion, V2O5 undergoes structural distortion toaccommodate the Li-ions into spaces between the layersformed by VO5 octahedral units. To investigate the phasetransformations observed during electrochemical Li-interca-lation/de-intercalation, cyclic voltammograms (CV) wererecorded at slow scan rate of 0.1 mV s�1 between 1.75–
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Figure 5 Typical galvanostatic traces of first two charge–discharge curves of (a) bare VNF, (b) C-VNF-15 and (c) C-VNF-30 cells between 1.75 and 4.0 V vs. Li at 0.1 C rate in roomtemperature.
High-rate and elevated temperature performance of electrospun V2O5 nanofibers 61
4.0 V vs. Li and shown in Fig. 4. All the test cells fabricatedexhibited the open circuit voltage (OCV) �3 V vs. Li andsubsequently discharged to insert Li. Fig. 4 shows the typicalCV traces of VNF recorded in the slow scan rate. The half-cells exhibited several oxidation/reduction peaks duringcathodic and anodic scan, which clearly evident from Fig. 4a,c, and e which corresponds to the intercalation/de-intercala-tion processes of Li-ions in V2O5 lattice. In bare VNF, mainreduction (cathodic) peaks are noted at �3.15, �2.20, �1.99and �1.90 V vs. Li, whereas only one broad oxidation (anodic)peak is observed �2.66 V vs. Li. Similarly, in the case of C-VNF-15 and C-VNF-30, reduction peaks are observed at �2.24,�1.99 and �1.90 V vs. Li, and a broad oxidation peak at�2.66 V vs. Li. Cathodic peak observed at �3.15 V vs. Li inbare VNF is not present for both C-VNFs. The appearance ofsuch cathodic peak is indicative of the multistep reductionprocess of V5+ in V2O5 and this leads to the formation of e-LixV2O5 phase [23]. The CV traces of the C-VNFs do not havethis peak, which is in agreement with the reduction of V5+ toV4+ as observed in the XRD. The absence of V5+ thus eliminatesthe related phase transformation related to the electrochemi-cal reduction of V5+ to V4+. Other cathodic peaks �2.20 and�1.90 V vs. Li, as observed in all VNFs belong to partialreduction of V4+ to V3+ and this reduction process is associatedwith the formation of d- and g-LixV2O5 mixed phases [23].Appearances of additional reduction peak at �1.99 V vs. Li inC-VNFs could be attributed to the increased instances ofreduction of V4+. Cycling below 2.0 V vs. Li leads to theformation of irreversible g-LixV2O5 phase which is confirmed bythe appearance of reduction peak at �1.90 V vs. Li and notedfor all three VNFs tested [23].
CV studies are continued up to 10 cycles and selectedcycles (2, 5 and 10) are presented in Fig. 4b, d and f. Theobserved traces in the successive cycles only showed onepair of broad reduction and oxidation peaks, which indicatesthat the phase transformations occur reversibly in the g-LixV2O5 phase formed at the end of the first discharge. Inthe bare VNF, the main reduction peak occurred at �2.30 Vvs. Li for consecutive cycles, whereas the oxidation peakremains at �2.70 V vs. Li, which is similar to the firstcycle. Likewise, in C-VNF-15 and C-VNF-30, the reductionand oxidation peaks appear at �2.30 and �2.70 V vs. Li,respectively. Further, these redox peaks also appears to besharper with less fade in current density compared to bareVNF and indicates good reversibility during successivecycling. The improved performance is mainly attributed tothe enhanced electronic conductivity enabled by carboncoating. A decrease in current density is observed forcarbon-coated VNFs, for example current density of �110,�90 and �70 mA g�1 peak potential at �2.70 V vs. Li forbare VNF, C-VNF-15 and C-VNF-30, respectively. Reddy et al.[4] also noticed similar kind of reduction in current densityduring CV analysis for V2O5-PEG nanobelt composites whencompared to bare V2O5 nanobelts.
Galvanostatic discharge–charge profiles of half-cells (Li/VNFs) are cycled between 1.75 and 4.0 V vs. Li at 0.1 C rate(1 C is assumed to be 350 mA h g�1) in room temperatureare shown in Fig. 5. As expected, all the three samplesshowed multiple plateaus during first discharge, which isconsistent with the CV analysis and representative phasetransitions occurring during Li-intercalation/de-intercala-tion [4,5,25,42]. The first discharge plateau observed for
bare VNF at �3.20 V vs. Li is attributed to the reduction ofV5+ to V4+ which enables the intercalation of Li-ions intolayered V2O5 [24,33,43]. Plateau at �3.20 V vs. Li is notpresent in both C-VNFs, which is due to the absence of V5+
and consistent with CV analysis. Upon further discharge, twoprominent plateaus at �2.28 and �2.0 V vs. Li are observedfor all the three VNFs. The plateau at �2.28 V is indicatesformation of d-LixV2O5 phase and V4+ is partially reduced toV3+ forming d- and g-LixV2O5 mixed phases. The plateau�2.0 V vs. Li is believed to be the transformation of d-LixV2O5 in to g-LixV2O5 phase. Further discharge to 1.75 V vs.Li causes the irreversible formation of g-LixV2O5 phase, andthis is in good agreement with CV traces obtained above.Bare VNF gives initial discharge capacity of 316 mA h g�1,whereas C-VNF-15 and C-VNF-3 exhibited a capacity of �280and �300 mA h g�1, respectively. Apparently, increasingcarbon content leads to the higher initial discharge capacity(C-VNF-15 to C-VNF-30). On the other hand, lower initialcapacity of the carbon-coated samples can be attributed tothe reduced occurrence of the first phase transformation at�3.20 V vs. Li. Chen et al. [30] also noted similar kind ofabsence of plateau region at �3.20 V vs. Li in PANI-V2O5
composites and it is mainly due to the formation of VO2
phase by reduction of V5+ from V2O5.Fig. 6 illustrates the plot of discharge capacity vs. cycle
number obtained from the galvanostatic discharge–chargecurves of VNF cells cycled between 1.75 and 4.0 V vs. Li at0.1 C rate (35 mA g�1) at room temperature. The bare VNFdelivered an initial discharge capacity of 316 mA h g�1
(�2.2 mol of lithium) and retained 43% of its capacity after50 cycles (Fig. 6a). The carbon coated VNFs (C-VNF-15 andC-VNF-30) cycled under the same conditions retained �67and �65% of the initial capacity after 50 cycles, in spite ofthe different amounts/thicknesses of carbon coating. Theobtained discharge capacities are normalized and presentedin Fig. 6b. It is evident that, carbon coated VNFs (C-VNF-15and C-VNF-30) exhibited good capacity retention character-istics compared to bare VNFs irrespective of the initialcapacity values during extended cycling. There is only slight
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Figure 7 Plots of specific discharge capacity vs. cycle numberof VNF and C-VNF-15 cycled at (a) 0.1 C rate at 551 C and (b) 1 Crate at room temperature.
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Figure 8 Electrochemical impedance spectra (EIS) bare VNF,C-VNF-15 and C-VNFA30 with enlarged high frequency region(inset).
Y.L. Cheah et al.62
variation in the capacity retention properties noted forC-VNFs, hence C-VNF-15 was chosen for high temperatureand rate performance studies.
To explore the protective nature of carbon-coatingtowards electrolyte counterpart, C-VNF-15 and VNF cellswere subjected to high temperature (551 C) studies. Thetest cells were galvanostatically cycled at 0.1 C ratebetween 1.75 and 4.0 V vs. Li at elevated temperatureand presented in Fig. 7a. As expected, in the elevatedtemperature conditions improved discharge capacity isnoted for C-VNF-15 in the first cycle when compared toroom temperature conditions (�280 to �298 mA h g�1). Onthe other hand, bare VNF showed the discharge capacity of�284 mA h g�1 at second cycle, which is �32 mA h g�1 lessthan its room temperature performance. The reduction ofcapacity in elevated temperature condition is mainlyattributed to the severe reactivity of VNFs with electrolytecounterpart. In terms of cycling efficiency at elevated
temperatures, �65 and 45% capacity retention is observedafter 50 cycles for VNF and C-VNF-15, respectively. Inaddition, the cells were also subjected to high current rateto study the effect of carbon coating towards electronicconducting properties of VNFs. Fig. 7b represents the highrate cycling performance of VNF and C-VNF-15 between1.75 and 4.0 V vs. Li at 1 C (350 mA g�1) at room tempera-ture. Drastic differences between the cycling properties ofC-VNF and VNFs are noted under high current cycling. TheC-VNF-15 delivered higher initial capacity of �250 mA h g�1
and �64% capacity is retained after 50 cycles, whereas bareVNF showed only �100 mA h g�1 of discharge capacity underthe same test conditions.
To further elucidate the effect of carbon coating on theelectrochemical performance of VNF, electrochemical impe-dance spectroscopy (EIS) was carried out on bare VNF,C-VNF-15 and C-VNF-30, respectively, and presented inFig. 8. EIS is a simple and versatile technique to understandthe electrochemical reactions occurring within the cells.The high-to-middle frequency semicircle were observedwhich are related to the formation of solid electrolyteinterphase (SEI) layer and surface film capacitance, whilethe middle-to-low frequency region refers to the chargetransfer (CT) and interfacial capacitance across the elec-trode/electrolyte interface. The inclined vertical line at 451in low frequency regions refer to the lithium-diffusion-related kinetics [16,23,44]. It is clearly observed that thediameter of the high-frequency semicircles showed adecreasing trend with increasing carbon coating, suggestingthat there is CT occurring in the C-VNF electrodes. Thisconfirms that the electronic conductivity of VNF increasedwith larger amount of carbon coating and it was well-reflected in the electrochemical cycling studies.
From the galvanostatic studies, it is evident that amor-phous carbon-coating plays a dual role to improve theelectrochemical properties of VNFs. First, the electronicconductivity has been improved by the presence of thecarbon layer, which has been clearly seen from electro-chemical properties of C-VNFs at high current densities.Unfortunately, low current rates improvement of electro-chemical properties is not obvious which is mainly due tothe presence of VO2 phase. Improvement in conductivityenables the facile Li-intercalation/de-intercalation duringcharge–discharge processes. Second, the homogeneous
High-rate and elevated temperature performance of electrospun V2O5 nanofibers 63
coating effectively prevents the nucleophilic attack on theelectrode material by F� atoms from HF, especially atelevated temperature operations [45] and it is clearly seenfrom Fig. 7a. Apart from the carbon coating, presences ofvoids in the fibers enable more contact area towardselectrolyte solution leads to the faster diffusion of Li-ionscannot be ruled out. At the same time, capacity fading ofVNFs are inevitable, which can be related to vanadiumdissolution [46], poor compatibility of vanadium compoundstowards linear carbonates (DEC) [47,48] and also intrinsicnature of the native compound [21]. Further studies are inprogress to suppress the capacity fading by minimizing theformation of VO2 phase thus improving the electrochemicalperformance of the VNFs.
Conclusion
A tandem electrospinning—PECVD technique was employedto fabricate carbon-coated (PECVD) polycrystalline V2O5
nanofibers (VNF). FE-SEM analysis clearly indicated theretention of morphological features of VNF upon carboncoating and a conformal coverage over the crystallites ofVNF due to the penetration of plasma cloud through thefiber meshes. HR-TEM and Raman studies confirmed thepresence of amorphous carbon layer on the surface of theVNF nanocrystallites. At the same time, XRD revealed areduction of vanadium from 5+ to 4+ and formation of traceamount of VO2 impurities along with native compound,which is possibly due to carbothermal reduction facilitatedby high temperatures and reactivity of nanostructured V2O5.Electrochemical studies showed the carbon-coated VNFs toexhibit improved electrochemical profiles in ambient andelevated temperature conditions than bare VNFs. Suchenhancement is mainly attributed to the homogeneouscarbon coating by PECVD, which improves the electronicconducting profiles of VNF and prevented undesired sidereactions with electrolyte counterpart. Improvement in theconductivity profiles of C-VNFs were validated though EISmeasurements.
Acknowledgments
This work was supported by National Research Foundation,Singapore through Clean Energy Research Project(NRF2009EWT-CERP001-036) and German Federal Ministryof Education and Research (BMBF; KoLiWIn 03SF0343F).Authors also acknowledge Timcal for gratis providing SuperPTM Li Carbon black.
Appendix A. Supporting information
Supplementary data associated with this article can be foundin the online version at http://dx.doi.org/10.1016/j.nanoen.2012.07.012.
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Yan Ling Cheah received her Bachelor ofEngineering (Hons) in Materials Engineeringfrom Nanyang Technological University,Singapore, in 2007 and Double Masters inMaterials Science, under the prestigiousErasmus Mundus Scholarship, from Uni-versit�e de Rennes 1 (University of Rennes1), France and Universit�a degli Studi diTorino (University of Turin), Italy, in 2008.In the same year, she returned to the School
of Materials Science and Engineering, Nanyang Technological Uni-versity, to pursue a Ph.D. Her current research focuses on cathodematerials on lithium ion batteries and has published several paperson vanadium pentoxide cathode materials.
Robin von Hagen studied chemistry at theUniversity of Cologne (2004–2009) andreceived his diploma in 2009. He currentlyis pursuing his Ph.D. in inorganic and mate-rials chemistry under the supervision ofProf. Sanjay Mathur and his research isfocusing on electrospun ceramic and com-posite nanofibers for energy and environ-mental applications.
Vanchiappan Aravindan is currently work-ing as Research Fellow in Energy ResearchInstitute@NTU (ERI@N), Nanyang Technologi-cal University, Singapore. He received hisPh.D. in 2009 and completed the Post-Doc-toral program at The Research Institute forCatalysis, Chonnam National University,Gwang-ju, South Korea with Prof. Yun-SungLee. His research interests are mainly on thedevelopment of high performance electrode
and electrolyte materials for aqueous and non-aqueous Li-ion batteriesand supercapacitors. He has authored and co-authored several peerreviewed publication on energy storage applications. He may bereached at [email protected].
Raquel Fiz received her Bachelor’s (2006)and Master’s (2008) degree in Mechanicaland Industrial Engineering, from the Univer-sidad de Leon, Spain. Currently she is pursu-ing her Ph.D. in Inorganic and MaterialsChemistry under the supervision of Prof.Sanjay Mathur at the University of Cologne,Germany. Her research interests are focusedon the fabrication, functionalization andcharacterization of quasi-one-dimensional
semiconductor nanostructures and their integration into functionalnanosystems.
Sanjay Mathur was born in 1968 and is thechair Professor of Inorganic and MaterialsChemistry at the University of Cologne since04/2008. He did his habilitation (Prof. Dr. MVeith) at the University of Saarbrucken andwas a Professor of Chemistry at theWurzburg University. He is a titular memberof the Inorganic Chemistry Devision of theIUPAC, and is the chair of the EngineeringCeramics Division of the Amercian Ceramic
Society. He is an academician of the World Academy of Ceramicsand holds visiting Professorships of the Central South University,China and National Institute of Science Education and Research(NISER), India. His research is focusing on nanochemistry usingpreorganized molecular architectures for the synthesis of functionalnanomaterials.
Madhavi Srinivasan is an Assistant Professorat the School of Materials Science and Engi-neering, Nanyang Technological University,Singapore. She has over eight years ofresearch and engineering experience in nano-material synthesis/characterization and theirapplications in the energy storage area. Cur-rently, her research focuses on enhancingenergy storage capabilities of devices suchas lithium ion batteries, supercapacitors and
advanced batteries by employing nanoscale materials. She graduatedfrom Indian Institute of Technology (IIT, India) and did her Ph.D.dissertation from National University of Singapore. She has co-authored several papers in the field of nanomaterials and chargestorage. She is also one of the three women scientists awarded theL’Oreal-UNESCO for women in science national fellowships awards inSingapore (2010). She may be contacted at [email protected].
