Bare Ta-doped V2O5 Energy & Environmetal Science 2011

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Cite this: Energy Environ. Sci., 2011, 4, 1712

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Energy storage studies of bare and doped vanadium pentoxide,(V1.95M0.05)O5, M ¼ Nb, Ta, for lithium ion batteries

A. Sakunthala,ab M. V. Reddy,*a S. Selvasekarapandian,bc B. V. R. Chowdari*a and P. Christopher Selvind

Received 1st October 2010, Accepted 4th February 2011

DOI: 10.1039/c0ee00513d

The bare V2O5 and doped (V1.95M0.05)O5 (M ¼ Nb, Ta) nano/submicron sized compounds were

prepared by the simple polymer precursor method. The compounds were characterized by different

physical and electroanalytical techniques. The effects of doping and different synthesis conditions on

the energy storage performance of the V2O5 compounds were discussed. All compounds delivered

a discharge capacity (at the end of the 2nd cycle) in the range 245 to 261 (�3) mA h g�1, except for the

tantalum doped compound (V1.95Ta0.05)O5 which exhibited a discharge capacity of 210 (�3) mA h g�1,

cycled in the range 2.0–4.0 V at a current rate of 120 mA g�1. An excellent cycling stability of 96% till

twenty cycles was achieved for the compound V2O5 prepared by the polymer precursor method.

Electrochemical impedance spectroscopy studies at different voltages during discharge and charge

cycles were discussed in detail.

Introduction

Lithium ion batteries are the preferable choices of power sources

for electronic devices due to their higher energy density.1–4 Owing

to the drawbacks associated with the commercially used layered

lithium cobalt oxide (LiCoO2) such as low energy density, safety

issues, relatively more toxic and expensive nature, interest has

developed for alternative cathode materials.1 Vanadium based

cathode materials such as lithium trivanadate (LiV3O8) and

vanadium pentoxide (V2O5) are considered as the attractive

alternatives due to their unique advantages such as high energy

aDepartment of Physics, National University of Singapore, Singapore117542. E-mail: [email protected]; [email protected]; Fax: +65-67776126; Tel: +65-651662605bDRDO-BU, Centre for Life Sciences, Bharathiar University, Coimbatore,641046, IndiacKalasalingam University, Krishnankoil, Virudhunagar, 626190, TamilNadu, IndiadNGM College, Pollachi, Tamilnadu, India

Broader context

In recent years, lithium ion batteries are one of the major power so

due to its advantage in terms of higher energy density along with lon

cathode materials such as LiCoO2, LiFePO4, LiMn2O4, etc., vanadi

lithium batteries more than twice (vs. Li) as high as the other catho

cathode is very poor and could be enhanced by different approach

metal ions or conductive coating. In this paper, we report submic

electrochemical studies were found to give better cycling stability. W

on V2O5 and reported its energy storage properties.

1712 | Energy Environ. Sci., 2011, 4, 1712–1725

density, low cost and relatively less toxic in nature.5–7 They have

a moderate working voltage (�3 V), particularly suitable for the

lithium polymer batteries, which will not degrade the polymer

electrolyte even at high temperatures.8 The compounds possess

good chemical stability as well as excellent safety characteristics

during overcharging due to the multiple oxidation states of

vanadium atom.5,9,10 LiV3O8 is made up of V3O8 layers stacked

one above the other forming a monoclinic structure.11 A

reversible phase transition takes place during the discharge/

charge cycling, where the presence of lithium ions in the octa-

hedral site between the layers acts like a pillar and gives an

excellent stability to the structure.12 Vanadium pentoxide (V2O5)

is made up of V2O5 layers stacked along the c-axis of the

orthorhombic structure. Each layer is in turn made up of VO5

square pyramids sharing edges and corners (Fig. 1).13 Unlike

LiV3O8, which has a pillar like Li-ion in the octahedral site, the

V2O5 layers are held together only by the presence of weak van

der Waals forces. So the compound easily undergoes different

structural phase transitions such as a, 3, d, g and u phases

urces for any kind of portable electronic devices. This is mainly

g cycle life over any other type of batteries. Among the popular

um based cathode V2O5 could enhance the energy density of the

de materials. But the cycle life of the battery with V2O5 as the

es such as proper preparation methods and conditions, doping

ron sized particles of V2O5 synthesized using simple methods,

e also studied the effect of doping of tantalum or niobium ions

This journal is ª The Royal Society of Chemistry 2011

http://dx.doi.org/10.1039/c0ee00513d






Fig. 1 Structural relationship between d and g phases, showing the possible shift of oxygen atoms and the consecutive rearrangement of the VO5

pyramids (taken copyright permission from Elsevier).13,15

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depending upon the degree of Li intercalation (x).13–18 The first

two, a-LixV2O5 (x < 0.01) and 3-LixV2O5 (0.35 < x < 0.7), phases

are similar to that of un-intercalated V2O5, but result in weak

puckering of V2O5 layers.15 The structure with more puckered

V2O5 layers called d-phase results with further lithium interca-

lation (x < 1). Here the c parameter is doubled due to the gliding

of the V2O5 layers (Fig. 1). Both the a to 3 and 3 to d phases are

reversible because the V–O bond is not broken in any case. When

the lithium intercalation is more, i.e. x > 1, an irreversible

structural modification to g-phase occurs. Fig. 1 shows

the difference in crystal structures of d and g-V2O5 phases. In

g-phase, the layer puckering becomes more pronounced than in

d-phase where the VO5 pyramids alternate up and down

individually instead of forming pairs as in the case of d-phase.15

A further deep discharge to x > 2 results in u-Li3V2O5

(when discharged below 2 V), where the Li-ion diffusion will be

very slow.13,14


Apart from lithium batteries, V2O5 finds many applications

such as electrochemical supercapacitors, sensors, optical and

thermally activated electrical switching devices, memory devices

and electrochromic devices.19–21 Avellaneda21 had reported

2.5 mol% of tantalum (Ta) doped V2O5 thin films, where they

showed the doped compound to deliver better electrochromic

properties than the undoped V2O5 compound. It was also

reported as a good candidate to be an electrode in lithium

microbatteries.21 Iida and Kanno22 had investigated the doping

of different elements such as Nb, Ce, Nd, Dy, Sm, Ag, and/or Na

ions on V2O5 thin films and they observed electrochromic

properties of V2O5 films doped with Nb and/or Na to be superior

to those of other elements.

In the present work, we studied the effect of doping of Ta and

Nb-ions on V2O5 matrix, the bare and doped V2O5 compounds

were prepared using the polymer polyvinylpyrrolidone (PVP) as

the dispersing agent. The effects of different preparation

Energy Environ. Sci., 2011, 4, 1712–1725 | 1713


Fig. 2 Rietveld refined XRD patterns of the compounds: (a) V2O5-B

and V2O5-WP, (b) (V1.95Nb0.05)O5 and (V1.95Ta0.05)O5, (c) V2O5-H and

V2O5-S.

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methods on the electrochemical performance, cycled in the range

2.0–4.0 V at a current rate of 120 mA g�1, were discussed.

Experimental section

Material preparation

Polymer precursor method. The undoped V2O5 and the

niobium or tantalum doped V2O5 were synthesized as follows:

ammonium metavanadate, NH4VO3 (Fischer, Purity, 99%),

tantalum ethoxide (Aldrich, Purity, 99%) and niobium ethoxide

(Aldrich, Purity, 99%) were used as the raw materials. The

polymer polyvinylpyrrolidone (PVP) (K40, Aldrich, molecular

weight 40 000) was used as the dispersing agent. For the prepa-

ration of the bare compound, 0.2 M (2 g) of NH4VO3 was mixed

with the 0.3 M (3 g) of the polymer PVP dissolved in 100 ml of

distilled water. The solution was refluxed at 100 �C for about 2 h

in ambient atmospheric pressure. The resultant solution after

reflux was dried on a hot plate and the dry residue was calcined

at 400 �C for 6 h in air. The sample was named as V2O5-B

(B—bare).

The tantalum doped compound (V1.95Ta0.05)O5 was prepared

by mixing 0.195 M NH4VO3 and 0.005 M tantalum ethoxide

with 0.3 M of polymer PVP dissolved in 100 ml of distilled water,

followed by drying and calcination as explained above. The Nb

doped compound (V1.95Nb0.05)O5 was prepared similar to

(V1.95Ta0.05)O5, by taking niobium ethoxide as the raw material.

The total metal ion to polymer ratio was 1 : 1.5 in all the cases.

To understand the role of polymer, the compound V2O5 was

also prepared without using polymer PVP in the preparation step

as follows. A 0.2 M of NH4VO3 was mixed with 100 ml of

distilled water and refluxed for 2 h. The resultant solution was

dried on a hot plate and calcined at 400 �C for 6 h in air. The

sample was named as V2O5-WP.

H2O2 method. 2 g of the commercial V2O5 powder and 0.2 g of

PVP were mixed together in 100 ml of H2O2, in the presence of 2

g of long chain alkyl amine (1-hexadecylamine, C16H33NH2)

dissolved in 15 ml of acetone. It was similar to the procedure

reported by Chandrappa et al.,23 except for the addition of 0.2 g

of polymer PVP. The continuous stirring resulted in a volumi-

nous yellow foam, which was collected and calcinated at 400 �C

for 6 h in air. The sample was named as V2O5-H.

Surfactant method. 5 g of the commercial V2O5 powder was

stirred with 15 ml of the surfactant Igepal (polyoxyethelene

nonylphenol, Aldrich, Purity, 99%) for about 30 min. The

resultant solution was as such calcinated at 400 �C for 6 h in air.

The resultant product was named as V2O5-S and subjected to

further studies. In all the cases, the controlled heating rate of the

precursors from the room temperature to the desired calcination

temperature was 3 �C min�1 in air and the same was maintained

during cooling of the samples.

Material characterization and electrode fabrication

The as prepared samples were characterized by X-ray diffraction

(XRD) using Philips X’PERT unit (PANalytical). The powder

XRD patterns were recorded in the angular range 10–80 (2q, in

deg) employing Bragg–Brentano parafocusing optics. Line focus


Ni-filtered CuKa-radiation from an X-ray tube (operated at

40 kV and 30 mA) was collimated through a soller slit of 0.04 rad,

a fixed divergence slit of 1� and an incident beam mask (5 mm)

before incident on the sample. Then the diffracted X-ray beam

from the sample was well collimated by passing it through

a programmable anti-scattering slit of 1� to reduce air scattering,

a programmable receiving slit of 0.3 mm and a soller slit of 0.04

rad before getting it reflected by the curved graphite crystal (002)

monochromator mounted at a goniometer circle of radius 240

mm for high resolution diffraction studies. A Xe-gas filled

proportional counter detector with electronic pulse height

discrimination was mounted behind the curved graphite crystal

(002) monochromator to receive diffracted X-ray signals.

Experimental control and data acquisition were fully automated

through computer. We have not used any internal standard for

quantitative phase analysis. The unit-cell lattice parameters were

obtained by the Rietveld refinement of the powder XRD data

using the commercially available software TOPAS Version 2.1

(2000 Bruker AXS, Germany).3f The morphology of the powders

was examined by means of Scanning Electron Microscope

(SEM) using JEOL JSM-6700F instrument. Transmission elec-

tron microscope (TEM) measurements were made by JEOL



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JEM-3010 instrument. Tristar 3000 (Micromeritics, USA) and

AccuPyc 1330 pycnometer (Micromeritics, USA) were used to

study Brunauer, Emmett and Teller (BET) surface area and

density, respectively of the compounds.

X-Ray absorption spectroscopy (XAS) studies were carried

out at the X-ray demonstration and development (XDD) beam

line of the Singapore Synchrotron Light Source (SSLS), National

University of Singapore (NUS), Singapore.24,25 The beam line

provides a focussed 2.5 to 10 keV source selected using a Si (111)

monochromator. The samples were ground into fine powders,

coated on a Scotch tape and folded to form multiple layers, and

subjected to the vanadium K-edge measurement. The data from

5376 eV to 6215 eV were collected at room temperature in the

transmission mode, with intensity normalized against two ioni-

zation chambers, filled with pure nitrogen recording the incident

Fig. 3 Lower magnification SEM images of (a) V2O5-B and (b) V2O5-WP. S

V2O5-WP, (e) (V1.95Nb0.05)O5, and (f) V1.95Ta0.05O5. Scale bar: 100 nm.


and transmitted X-ray intensities simultaneously. The electron

energy in the storage ring was about 700 MeV with a current of

about 200 mA. Vanadium K-edge data for the vanadium foil

(6 mm) were collected separately just before the data were

collected for all the other samples under study, and used for

energy calibration. The position of the vanadium foil edge was

taken as 5465 eV. The spectra collected were analyzed using the

WinXAS 2.3 code24 (http://www.winxas.de, by Prof. Thorsten

Ressler, Technische Universit€at Berlin, Germany).

Electrochemical studies were carried out with 2016 type of coin

cells. The electrodes were fabricated with the active material

V2O5 (bare or doped), Super P conductive carbon black and

binder (Kynar 2801) in the weight ratio of 70 : 15 : 15 using

N-methylpyrrolidone (NMP) as a solvent to the binder. Etched

Al-foil (20 mm) was used as the current collector. Lithium metal

cale bar: 1 mm. Higher magnification SEM images of (c) V2O5-B and (d)



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foil was used as the negative electrode (anode) and 1 M LiPF6 in

ethylene carbonate (EC) + dimethyl carbonate (DMC) (1 : 1

volume ratio) (Merck, Selectipur LP40) was used as the elec-

trolyte. Cyclic voltammetry studies carried out at a scan rate of

0.058 mV s�1 using Macpile (Biologic, France). Charge–

discharge cycling was carried out at ambient temperature by

using Bitrode battery tester (model, MCV16-0.5, USA). Imped-

ance spectroscopy was carried out with Solartron Impedance/

gain-phase analyzer (model SI 1255) coupled with a Potentiostat

(SI 1268) at room temperature (25 �C). The frequency was varied

from 0.18 MHz to 3 mHz with an alternating current signal

amplitude of 10 mV. The Nyquist plots (Z0 vs. �Z00) were

collected and analyzed using Z-plot and Z-view software

(Version 2.2, Scribner associates Inc., USA). Galvanostatic

cycling and impedance studies were carried out at a current

density of 120 mA g�1 in a voltage range of 2.0–4.0 V vs. Li,

cycled at room temperature (25 �C). Impedance studies were

recorded after stabilizing at each voltage for 2 h.

Results and discussion

Structural analysis

Fig. 2(a) shows the Rietveld refined XRD patterns of the

compounds V2O5-B and V2O5-WP prepared with and without

using polymer in the preparation step. A pure phase of the

compound was obtained in both the cases. The lattice parameter

values were found to be a ¼ 11.480 �A, b ¼ 3.560 �A, c ¼ 4.372 �A

Fig. 4 (a) Low magnification and (b) high magnification SEM images of V2

V2O5-S. Scale bar: low magnification at 1 mm; high magnification at 100 nm.


for the compound V2O5-B and a ¼ 11.474 �A, b ¼ 3.560 �A,

c ¼ 4.371 �A for the compound V2O5-WP. The values were found

to match with the JCPDS card no. 41-1426.

Rietveld refined XRD patterns of the doped compounds

(V1.95M0.05)O5 (M¼Nb, Ta) are shown in Fig. 2(b). The 2q peak

values of the doped compounds matched with the bare

compound. The lattice parameter values were found to be

a ¼ 11.514 �A, b ¼ 3.565 �A, c ¼ 4.378 �A and a ¼ 11.498 �A,

b ¼ 3.560 �A, c ¼ 4.370 �A, for the niobium and tantalum doped

compounds, respectively. The lattice parameter values were

slightly higher compared to the bare compound V2O5-B, which is

due to the doping effect. No new peaks were observed in the case

of niobium doped compound. This indicated that the Nb5+ ions

are occupied at the vanadium site. In the case of Ta5+ doped

compound, a low intensity peak with 2q values at 15 and 20�

appeared which corresponds to the impure phase tantalum oxide,

Ta1.52O3.12 (JCPDS 84-0435). The impure phase was found to be

less than 0.2%.

Fig. 2(c) shows the Rietveld refined XRD patterns for the

compounds V2O5-H and V2O5-S. No impurity was observed and

the 2q peak values matched with the JCPDS card no. 41-1426.

The lattice parameter values were found to be a ¼ 11.506 �A,

b ¼ 3.563 �A, c ¼ 4.373 �A and a ¼ 11.508 �A, b ¼ 3.563 �A,

c ¼ 4.378 �A, for the compounds V2O5-H and V2O5-S, respec-

tively. The estimated error values for the lattice parameter values

are in the range �0.005 to �0.002 for all the samples. The

differences in the lattice parameter values of V2O5-B, V2O5-WP,

V2O5-H and V2O5-S were due to the different preparation

O5-H. (c) Low magnification and (d) high magnification SEM images of



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methods which lead to slight differences in colour and chemical

valence states of V2O5 powders.

Morphology analysis

The low and high magnification SEM images of the compounds

V2O5-B and V2O5-WP are shown in Fig. 3(a)–(d). The differences

in the particle nature of the above two compounds are clearly

seen in Fig. 3(a) and (b). Homogeneous particle morphology was

observed for the compound V2O5-B (Fig. 3(a)), whereas irregular

or non-homogeneous agglomerates were observed in the case of

the compound V2O5-WP (Fig. 3(b)). The particle size varies from

100–300 nm (Fig. 3(c) and (d)) for both the compounds, with less

agglomeration combined with smaller particle size in the case

of V2O5-B when compared to the compound V2O5-WP.

The differences in morphology of the compounds V2O5-B and

V2O5-WP show that the PVP in the preparation step acted as

a good dispersing or capping agent. The polymer is easily soluble

in water and the metal ion gets nicely dispersed in the presence of

the PVP. The SEM images of the compounds (V1.95Nb0.05)O5

and (V1.95Ta0.05)O5 are shown in Fig. 3(e) and (f). With doping

(Nb, Ta), the particles were found to be in submicron size

(<1 mm), reflecting the increase in crystallinity by doping. SEM

images of the compound V2O5-H prepared by the H2O2 method

using the commercial V2O5 were found to be closely packed with

small bar like particles of submicron size (Fig. 4(a) and (b)). The

Fig. 5 (a–c) Transmission electron microscopy images, scale bar: 100 nm; (

resolution TEM images, scale bar: 5 nm, of the compounds V2O5-B, (V1.95N


compound V2O5-S (Fig. 4(c) and (d)) was found to have homo-

geneous morphology with an average particle size of less than

300 nm.

While morphology of the compounds such as shape and

agglomerations of the bulk sample could be seen by SEM images,

TEM images will give more clear images of particle size and

morphology. A drop of the solution (micrograms of powder

samples dispersed in an ethanol solvent) was taken over the

Cu-grid, dried and subjected to TEM analysis. In the present

work, TEM studies were taken for all the compounds under

study. The TEM images of the compounds V2O5-B,

(V1.95Nb0.05)O5, and (V1.95Ta0.05)O5 are shown in Fig. 5(a)–(c).

The particle was found to have a diameter around 200 nm and

length around 550 nm in the case of compound V2O5-B

(Fig. 5(a)). The selected area electron diffraction pattern shows

the single crystalline nature (Fig. 5(a0)). The high resolution

image (Fig. 5(a00)) shows the d-spacing corresponding to the (200)

plane. The TEM image of the compound (V1.95Nb0.05)O5

(Fig. 5(b)) shows particles around 300 nm and the compound

(V1.95Ta0.05)O5 (Fig. 5(c)) shows that the particles were highly

agglomerated. The selected area electron diffraction patterns of

Nb and Ta-doped V2O5 show the polycrystalline nature of the

particles (Fig. 5(b0) and (c0)). The high resolution images

(Fig. 5(b00) and (c00)) show the d-spacing corresponding to the

(200) plane. The TEM image of the compound V2O5-H

(Fig. 6(a)) shows that the particles were highly agglomerated and

a0–c0) selected area electron diffraction (SAED) images and (a00–c00) high

b0.05)O5 and V1.95Ta0.05O5, respectively.



Fig. 6 (a–c) Transmission electron microscopy images, scale bar: 100 nm; (a0–c0) SAED images and (a00–c00) HRTEM images, scale bar: 5 nm, of the

compounds V2O5-H, V2O5-S and V2O5-WP, respectively.

Fig. 7 Vanadium K-edge XANES spectra of the compounds V2O5-B,

(V1.95Ta0.05)O5 prepared using the polymer precursor method and the

standard vanadium foil and V2O3.

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bar like reflecting SEM image (Fig. 4(b)). The TEM image of the

compound V2O5-S (Fig. 6(b)) shows that the particles were

around 300 nm. The selected area electron diffraction patterns

show the polycrystalline nature of the particles (Fig. 6(a0) and

(b0)). The high resolution images (Fig. 6(a00) and (b00)) show the

d-spacing corresponding to the (200) plane. The TEM image of

the compound V2O5-WP (Fig. 6(c)) shows that the particles were

irregular with no shape and particle size was greater than 300 nm.

The particles were polycrystalline in nature and d-spacing

corresponds to (200) plane (Fig. 6(c0) and (c00)). The surface areas

of the compounds V2O5-B, V2O5-WP, (V1.95Nb0.05)O5,

(V1.95Ta0.05)O5, V2O5-H and V2O5-S were 7.54, 5.93, 10.32,

10.46, 8.29 and 7.14 m2 g�1, respectively. The experimental

powder densities of all compounds are in the range 3.545 to 4.015

g cm�3.

X-Ray absorption near edge structure studies

The X-ray absorption near edge structure (XANES) analysis is

a well established technique to know the oxidation state of the

probe element and is strongly affected by the co-ordination

environment of the absorbing atom.26,27 Vanadium K-edge

XANES spectra of the standard vanadium foil, V2O3 and the


compounds V2O5-B and (V1.95Ta0.05)O5 are shown in Fig. 7. All

the spectra were energy calibrated using the spectrum of vana-

dium metal foil (5465 eV). The spectra were then background

corrected and normalised using the two polynomial fit to obtain

the XANES spectra, using WinXAS software. The pre-edge



Fig. 8 Comparative 2nd cycle cyclic voltammograms of the compounds:

(a) V2O5-B, V2O5-WP and (V1.95M0.05)O5 (M ¼ Ta or Nb) and (b) V2O5-

B, V2O5-H and V2O5-S. The peak currents were normalized to their active

mass. Voltage range: 2–4 V. Scan rate: 0.058 mV s�1.

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observed is a product of transition from V 1s states to V 3d states.

This transition, however, is forbidden by the dipole selection

rules in centrosymmetric systems. But it is allowed in the case of

non-centrosymmetric systems due to hybridization between V 3d

and O 2p states.28–33 The K-edge energies were found to be 5484,

5490, 5490 eV for V2O3, V2O5-B and (V1.95Ta0.05)O5, respec-

tively. The compounds V2O5-B and (V1.95Ta0.05)O5 were found

to have a higher energy of absorption when compared to the

standard V2O3. The K-edge energy of (V1.95Ta0.05)O5 was the

same as V2O5-B, as seen from the overlapping of XANES spectra

for both the compounds (green and black lines). This indicated

that the vanadium has a 5+ oxidation state in V2O5-B and

(V1.95Ta0.05)O5. This shows that doping does not affect the

Table 1 Second cycle anodic/cathodic peak voltages and integrated total ar

Compound 2.0–2.8 V anodic/cathodic voltage/V 2.8–4.

V2O5-B 2.58/2.24 3.37,V2O5-WP 2.51/2.23, 2.33, 2.41 3.33,(V1.95Nb0.05)O5 2.52/2.25 3.34,(V1.95Ta0.05)O5 2.50/2.28, 2.37, 2.43 3.33,V2O5-H 2.57/2.16, 2.25 3.2, 3V2O5-S 2.63/2.15, 2.53 3.42,


oxidation state of vanadium. Due to technical difficulties at SSLS

light source we were not able to do XANES studies of

(V1.95Nb0.05)O5. Further studies are needed on optimizing the

doping concentration of tantalum and niobium ions and the in

situ studies during Li-intercalation/de-intercalation to under-

stand the role of a Ta or Nb-ion in stabilizing the V2O5 structure.

Cyclic voltammetry studies

The comparative second cycle cyclic voltammograms of the

compounds V2O5-B, V2O5-WP and (V1.95M0.05)O5 (M¼ Ta, Nb)

normalized with respect to their active masses are shown in

Fig. 8(a). The comparative second cycle cyclic voltammograms

of the compounds V2O5-B, V2O5-H and V2O5-S normalized with

respect to their active masses are shown in Fig. 8(b). All the

compounds show very good reversible peaks. It was observed

that the cyclic voltammograms of the compounds V2O5-B and

V2O5-S were found to be broader than the other compounds.

This may be due to the difference in the crystallinity of the

samples. The cathodic (reduction)/anodic (oxidation) peak

voltage values are given in Table 1.

One pair of anodic/cathodic peak in the voltage region of

2.0–2.8 V, and three pairs of anodic/cathodic peaks in the voltage

region between 2.8 and 4.0 V were found to appear in the case of

the bare V2O5-B and niobium doped (V1.95Nb0.05)O5

compounds. All the other compounds have minor peaks (as

italicized in Table 1) apart from main redox peaks. The peak

around �2.2 V in the low voltage region corresponds to the

g-phase and the peaks observed between 2.8 and 4.0 V corre-

spond to the mixture of a, 3 and d-phases.10,13–15 The total areas

under the CV curves are noted in Table 1, indicating a slightly

lower value for the compound (V1.95Ta0.05)O5 when compared to

all the other compounds.

Electrochemical performance

Galvanostatic charge/discharge plateaus. The charge/discharge

plateaus of all the compounds at cycle numbers 1, 2, 20 and 60

are shown in Fig. 9(a)–(f). Good reversible plateau regions were

observed for all the compounds. Interestingly the compounds

V2O5-B and V2O5-WP maintained a well defined good reversible

plateau region even at the 60th cycle, when compared to the other

compounds under study. Fig. 10 shows the differential capacity

plots corresponding to the charge/discharge profile of the

compound V2O5-B at cycle numbers 2, 20 and 60. The 2nd cycle

differential plot shows the peaks around 2.5, 3.37, 3.48, 3.66 V

during charge, and peaks around 2.28, 3.14, 3.33 and 3.56 V

during discharge. We note discontinuity of voltage values in

differential capacity vs. voltage plots which is due to the less

ea of bare and doped V2O5

0 V anodic/cathodic voltage/V Total area under the curve

3.49, 3.67/3.12, 3.33, 3.59 3163.46, 3.54, 3.67/3.15, 3.36, 3.44, 3.56 3633.46, 3.64/3.15, 3.35, 3.60 3243.45, 3.65/3.17, 3.37, 3.46, 3.59 304.38, 3.50, 3.67/3.12, 3.33, 3.60 3553.55, 3.68/3.08, 3.28, 3.58 534



Fig. 9 Discharge/charge profiles of the compounds: (a) V2O5-B, (b) V2O5-WP, (c) (V1.95Nb0.05)O5 (d) (V1.95Ta0.05)O5, (e) V2O5-S and (f) V2O5-H at

different cycle numbers. Current density: 120 mAg-1; voltage range: 2.0-4.0 V

Fig. 10 Differential capacity plot of the compound V2O5-B at different

cycle numbers.


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number of points collected during the data collection. Whereas the

continuous curves are clearly seen in CV(Fig. 8), and the anodic/

cathodic peaks were found analogous to the cyclic voltammetry

results as discussed above. Good reversible peaks were observed

even after sixty cycles, but with a decrease in peak intensity. The

peaks were shifted towards the higher voltages, indicating struc-

tural modifications with increase in cycle number. To understand

the lithium ion kinetics during intercalation/de-intercalation, the

electrochemical impedance spectroscopy studies at different cycle

numbers were carried out only for the compound V2O5-B due to

its best performance when compared to all the other samples. The

results are discussed in the later section.

Cycling stability

A comparison of the capacity vs. cycle number plots of the

compounds V2O5-B, V2O5-WP and (V1.95M0.05)O5 (M¼ Ta, Nb)



Fig. 11 The capacity vs. cycle number plots of the compounds:

(a) V2O5-B (symbol: P), V2O5-WP (symbol: O) and (V1.95M0.05)O5

(M ¼ Nb or Ta) (symbols: ,, B) (b) V2O5-H (symbols: ,) and V2O5-S

(symbol: B). Current density: 120 mA g�1; voltage range: 2.0-4.0 V.

Closed symbols: charge capacity. Open symbols: discharge capacity.

Table 2 Galvanostatic cycling data of the bare and doped compoundV2O5 prepared under different preparation conditions; current density:120 mA g�1; voltage range: 2.0 to 4.0 V

CompositionCapacity/(�3) mA h g�1

(cycle number)

Capacity retention (%)(2nd to 20th cycle);(2nd to 60th cycle)

V2O5-B 250(2)–239(20)–182(60) 96; 73V2O5-WP 261(2)–232(20)–176(60) 89; 67(V1.95Nb0.05)O5 245(2)–202(20)–124(60) 82; 51(V1.95Ta0.05)O5 210(2)–175(20)–118(60) 83; 56V2O5-S 255(2)–217(20)–131(60) 85; 51V2O5-H 260(2)–212(20)–140(60) 82; 54

Fig. 12 Electrochemical impedance spectroscopy (EIS) studies on the

compound V2O5-B. Nyquist plots (Z0 vs. �Z00) during (a) 1st discharge

and (b) 1st charge cycles. (c) Equivalent electrical circuit used to fit the

experimental impedance spectra. Symbols represent experimental

spectra, line is fitted spectra.

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at a current density of 120 mA g�1 is shown in Fig. 11(a). The

cycling data of the compounds V2O5-H and V2O5-S are shown in

Fig. 11(b). The capacity values are given in Table 2. On

comparison, the bare compound V2O5-B prepared using the

polymer PVP shows a good electrochemical performance. The

compound delivered a discharge capacity value of 250 mA h g�1

(1.7 mol of Li, assuming a theoretical capacity of 147 mA h g�1

for 1 Li) at the end of the 2nd cycle with an excellent stability of

96% and 73% at the end of the 20th and 60th cycles, respectively.

The compound V2O5-WP delivered a slightly higher discharge

capacity of 261 mA h g�1 (1.78 mol of Li) at the end of the 2nd

discharge cycle. But at the end of the 20th cycle, the capacity loss

was 11% for V2O5-WP, but only 4% for V2O5-B. This difference


in stability is attributed to the difference in morphology of the

compounds as discussed above in SEM/TEM analysis. From

Table 2 it is seen that, at the end of the 2nd cycle, discharge

capacity value around 260 (�15) mA h g�1 was delivered by all

the compounds under study except for the compound

(V1.95Ta0.05)O5. It was also reflected by its slightly low value for

the total area under the peaks as discussed in CV studies. The

observed decrease in discharge capacity of tantalum doped

sample is due to the electrochemically inactive nature of Ta5+ ion.

No irreversible capacity loss (ICL) was found in the case of

niobium doped sample when compared to all the other

compounds under study. This was clearly observed from the 1st

cycle discharge–charge curves and the cycling data (Fig. 9(c)

and 11(a)). The discharge capacity of niobium doped sample

was close to the bare compound. Irrespective of the higher

discharge capacity and 100% reversibility during the 1st cycle for

the niobium doped compound, the stability achieved was more

or less the same as in the case of tantalum doped sample

(Table 2). Next to V2O5-B, a better stability till twenty cycles was

observed for the compound V2O5-WP, followed by V2O5-S,

(V1.95Ta0.05)O5, V2O5-H and (V1.95Nb0.05)O5, respectively. The

insets of Fig. 11(a) and (b) show a faster rate of capacity fading



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after 20 cycles, with better stability in the case of the compound

V2O5-B. Therefore, the present study suggests that the structural

stability of V2O5 was not improved with Ta and Nb-doping. Our

observed capacity values at the end of 45–50 cycles are higher

then the recently reported ultralong hierarchical V2O5 nano-

wires.34 The bare compounds prepared from ammonium meta-

vanadate as the source material maintained a better cycling

stability compared to the compounds prepared using the

commercial V2O5.

Comparison of electrochemical performance with literature

The electrochemical performance of the compound V2O5 in the

present study prepared under different preparation conditions

was compared with the literature reports and was found to be

better than the nanowires,34 hollow microspheres35 and nano-

strips.36 Mai et al.34 obtained ultralong hierarchical V2O5 nano-

wires using electrospunning followed by annealing at 480 �C in

air for 3 h, and they showed a reversible capacity of 187 mA h g�1

at the end of the 50th cycle cycled in the range 2.0–4.0 V, at

a current rate of 30 mA g�1. V2O5 hollow microspheres prepared

by the self-assembly method were found to deliver a discharge

capacity of 231 mA h g�1 at the end of the 15th cycle and 160 mA

h g�1 at the end of the 45th cycle, for a current density of 80 mA

g�1. The V2O5 nanostrips show the poor stability of 80% at the

end of the 20th cycle, cycled in the voltage range 2.75–4.0 V. The

compound V2O5-B in the present study delivered a better

discharge capacity value of 240 mA h g�1 at the end of 15 cycles

and 200 mA h g�1 at the end of 45 cycles with an excellent

capacity retention of 96% and 80% respectively for a current

density of 120 mA g�1. Also, the compounds V2O5-WP and

V2O5-S in the present study delivered discharge capacity values

of 240 mA h g�1 and 232 mA h g�1 at the end of 15 cycles with

better capacity retention of 91% and 92%, respectively. The

Table 3 Impedance data for the compound V2O5-B. Resistance in ohms and cCPEb ¼ �3 mF)

Voltage vs. Li/V Re/U Rsf+ct/U CPEsf+

1st discharge cycleOCV (3.4) 4 123 443.2 7 161 463.0 2 160 742.8 2 160 632.4 3 181 492.2 3 193 612.0 4 170 8520th discharge cycle2.8 4 249 542.2 5 460 6460th discharge cycle2.8 9 Rsf: 130; Rct: 479 CPEsf:2.2 11 Rsf: 180; Rct: 223 CPEsf:1st charge cycle2.2 4 183 782.4 4 149 1422.6 3.3 90 1522.8 3.2 78 1253.0 3.4 76 1153.2 4.4 74 1133.4 4.3 69 1103.6 4 63 1104.0 4.5 51 71


submicron sized V2O5 compounds prepared by simple chemical

route in the present study showed a better capacity and cycling

stability compared with the V2O5 nanowires, microspheres,

nanostrips34–36 and other literature report.37 Irrespective of

preparation method or doping or different morphologies in all

the cases capacity fading was noted during long term cycles, this

is due to structural transformations during cycling or maybe due

to the intrinsic nature of V2O5 material.

Electrochemical impedance spectroscopy

Electrochemical impedance spectroscopy (EIS) is one of the most

informative among the different electroanalytical techniques to

understand the kinetics of the Li+ ion insertion/deinsertion

process in intercalation compounds.38–46 In the present study EIS

was carried out only on the compound V2O5-B, because of its

stable Li-cycling behaviour. Fig. 12(a) and (b) show the first cycle

Nyquist plot for the V2O5 compound during various discharge

and charge voltages starting from the open circuit voltage (OCV,

3.4 V) of the cell. Only selected voltages are shown for clarity.

Fig. 12(c) shows the equivalent circuit used to fit the impedance

spectra.

The semicircle observed at the high frequency region was due

to the combined process of surface film (Rsf) and the charge

transfer resistance (Rct). Rsf arises due to the Li+ migration

through the solid electrolyte interphase (SEI) film, and the Rct is

due to the electrode/electrolyte interface. A constant phase

element (CPE) was used in the equivalent circuit instead of a pure

capacitance due to the observation of depressed semicircle

indicative of inhomogeneous surface. CPEsf and CPEdl are

constant phase elements corresponding to the surface film and

double layer capacitance and further details on equivalent elec-

trical circuit notations are discussed by Reddy et al.40,42,43 Ws is

the Warburg resistance associated with the solid state diffusion

apacitance in F (error values: Rsf+ct¼�5 U, CPEsf+dl:�5 mF; Rb¼�3 U,

dl/mF Rb/U CPEb/mF Ws (�10)/U

18 25 56558 13 295— — 672— — 84454 10 66871 8 55288 8 361

84 10 1120472 5 1111

21;CPEdl: 35 502 2 55616;CPEdl: 29 479 5 2288

84 8 46667 16 240— — 41525 21 31835 19 38229 29 434— — 267— — 283— — 511



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of Li+ ion through the V2O5 lattice and the CPEint is the inter-

calation capacitance. The Rb and CPEb components were

included only for the impedance spectra with a low frequency

semicircle corresponding to the bulk phenomena, it arises due to

Fig. 13 Nyquist plots of V2O5-B during the discharge cycle voltage

region of 4.0–2.8 V at (a) 20th cycle and (b) 60th cycle. Bode plots at (c) 20th

cycle and (d) 60th cycle.


electronic conductivity of active material and ionic conductivity

of the electrolyte filled in the pores of composite electrodes40,42

and also arises due to structural modifications during cycling.

The impedance data values are summarized in Table 3. The

impedance values corresponding to combined surface film and

charge transfer resistances (Rsf+ct), and the bulk resistance (Rb)

were found to increase with the discharge voltage and decrease

with the charge voltage during the first cycle. At the OCV, the

Rsf+ct value was 123 U. But at the end of the discharge, it

increased to 170 U. Similarly, the Rb value increased from 18 to

88 U at the end of discharge (2 V). During charge cycle, the Rsf+ct

value decreased to 51 U at 4 V, and the Rb value decreased to 29

U at 3.2 V. This indicated that the Li intercalation (discharge)

was not as smooth as Li de-intercalation (charge). The interca-

lation capacitance reflecting the occupation of lithium ion into

the inserted site was found to be between 0.1 and 1 F.

Fig. 13(a) and (b) show the Nyquist plots of the 20th and 60th

cycle discharge voltages between 2.8 and 4.0 V, respectively.

Fig. 13 (c) and (d) show the corresponding Bode plots. The width

of the semicircles was found to increase steadily with the decrease

in the discharge voltage. It clearly shows the higher impedance at

60th cycle when compared to 20th cycle. The surface film (Rsf) and

charge transfer resistances (Rct) were seen well separated for the

60th discharge cycle corresponding to 2.8 V as seen from Nyquist

and Bode spectra (Fig. 13(b) and (d)). The impedance values at

2.8 V for the 20th and 60th cycles are given in Table 3, which were

found to be higher compared to the first cycle values.

The Nyquist and Bode plots corresponding to 2.2 V for the

cycle numbers 1, 20 and 60 are compared in Fig. 14(a) and (b). At

2.2 V, the width of the semicircle corresponding to the 20th and

Fig. 14 Impedance studies of V2O5-B at the discharge voltage of 2.2 V at

different cycles. (a) Nyquist plot (Z0 vs. �Z00), and (b) Bode plot.



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60th cycles was more or less the same and much higher than the

2nd cycle. The impedance values are noted in Table 3, which

indicated an abrupt increase in impedance with increase in cycle

number. At 2.2 V (discharge), the Rsf+ct value observed was less

than 200 U at 1st cycle. But, the value was found to be more than

double its value in the case of 20th cycle. Only a single high

frequency semicircle corresponding to surface film + charge

transfer resistance was observed in the case of 20th cycle, but it

was seen well separated for the 60th cycle (Fig. 14(b)). The

impedance analysis indicated that after 20 cycles there was an

abrupt increase in impedance in the voltage region below 2.8 V

than in the voltage region above 2.8 V. Long term cycling

resulted in the clear appearance of surface film resistance and

similar behaviour was noted in literature.47

Conclusions

We studied the effect of different preparation methods and

doping of niobium and tantalum ions on V2O5 compound. The

compounds were characterized using X-ray diffraction, XANES,

SEM, TEM, density and BET surface area, cyclic voltammetry,

galvanostatic cycling and impedance techniques. The submicron

sized V2O5 compound (V2O5-B) synthesized by the polymer

precursor method was found to give a good stability of 96% till

twenty cycles. After 15 cycles, the capacity fading was noted

irrespective of the preparation method or doping. This may be

due to the intrinsic nature of the material or non-suppression of

structural transformations.

Acknowledgements

Authors thank Prof. G. V. Subba Rao, Dept. of Physics, NUS

for his helpful discussions. Authors thank Dr Aga Banas, SSLS,

for her help with XAS data collection. A. Sakunthala thanks the

Defence Research and Development Organisation (DRDO),

India, for grant of Senior Research Fellowship. Dr M. V. Reddy

and Prof. B. V. R. Chowdari thank to Ministry of education

(MOE) (Grant no. R-284-000-076-112) and National Research

Foundation (NRF), Singapore.

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