Optical and electrochemical properties of vanadium pentoxide gel thin filmsNguyen Thi Be Bay, Pham Minh Tien, Simona Badilescu, Yahia Djaoued, Georges Bader, Fernand E. Girouard,

Vovan Truong, and Le quang Nguyen Citation: Journal of Applied Physics 80, 7041 (1996); doi: 10.1063/1.363777 View online: http://dx.doi.org/10.1063/1.363777 View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/80/12?ver=pdfcov Published by the AIP Publishing

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Optical and electrochemical properties of vanadium pentoxidegel thin films

Nguyen Thi Be Bay, Pham Minh Tien, Simona Badilescu, Yahia Djaoued,Georges Bader, Fernand E. Girouard, Vo-van Truong, and Le quang NguyenDepartment of Physics, Universite´ de Moncton, Moncton, NB, Canada E1A 3E9

~Received 26 February 1996; accepted for publication 9 July 1996!

Optical and electrochemical properties of vanadium pentoxide~V2O5! films prepared by sol-gelmethod from organic and inorganic precursors were studied. The organic gel was obtained byhydrolysis and polycondensation of vanadium isopropoxide dissolved in isopropanol. The inorganicgel was prepared from aqueous solution of sodium vanadate. Lithium insertion in LiyV2O5 films~0,y<2.0! was investigated by cyclic voltammetry and electrochemical potential spectroscopy,showing good reversibility up toy;2.0 and structure transitions froma phase toe phase anddphase in the range 0,y<1.0. Highest values for chemical diffusion coefficient were about 3310210

cm2/s at y;0.8, as determined from complex impedance measurements. ©1996 AmericanInstitute of Physics.@S0021-8979~96!04120-5#

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I. INTRODUCTION

The electrochemical insertion of Li2 ions into vanadiumpentoxide~V2O5! has been widely studied in order to makreversible thin film cathodes in rechargeable batteries1,2 andelectrochromic devices.3,4 Thin films of vanadium pentoxidehave been prepared by various methods such as vacuevaporation,5 sputtering,6 chemical vapor deposition,7 andrecently, by sol-gel methods which provide a much easprocessing of large-scale films. Vanadium pentoxide gels cbe synthesized by hydrolysis and polycondensation of vadium alkoxides8 or by acidification of vanadate salts ansubsequent polymerization of the decavanadic acids.9

The studies of V2O5 for use in rechargeable batteriehave mostly considered bulk material or very thick films othickness greater than 10mm. With recent interest in produc-ing ultrathin solid-state batteries10,11 there is an increasingneed to investigate lithium insertion in thin films of thicknesless than 500 nm. Such thin films have been studied foras ion-storage layers in electrochromic smart windows3,4

However, works have principally focused on their opticmodulation and cycling capability, and little effort has beemade to thoroughly discuss structure-related and diffuskinetics aspects of lithium insertion into V2O5 thin films.These factors are in fact of great importance in both micrbattery and electrochromic device applications.

Our present work therefore is to study optical and eletrochemical properties of V2O5 thin films prepared by sol-gelmethods, from both the organic~vanadium alkoxide! and in-organic~vanadate salt! precursors, in order to compare anassess their performances with respect to potential appltions in thin film rechargeable batteries or electrochromdevices.

II. EXPERIMENTAL PROCEDURES

The organic precursor used in this work was vanadiuisopropoxide VO~OC3H7!3 dissolved in isopropanol. Thealkoxide/solvent ratio was 2.5 wt%, which allowed us t

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obtain a clear, stable solution suitable for making films. Filhydrolysis was carried out in air, leading to formation of thV2O5 layer.

Vanadium oxide gel was also prepared by acidificatioof sodium vanadate NaVO3 and subsequent polymerizationof the resulted decavanadic acid. The acidification was reized by ion exchange of sodium vanadate with a sulfonacid resin. The collected solution was brick red, stable, aready for film deposition.

Hereafter, layers prepared from the organic precursand from the vanadate salt will be referred to as organic ainorganic V2O5 films, respectively. In both cases, films werformed by a dip-coating process on substrates that were loered into the solution, and then withdrawn at a uniform raof 4 mm/s using a computer-controlled motor drive. Thmaximum film thickness deposited in a single dipping waabout 80 nm, as determined from ellipsometry. This migalso vary slightly depending on the substrate. In orderobtain thicker films the dipping process could be repeatmany times, with an intermediate heating step under alamp at about 180 °C for 3–10 min to prevent dissolutionprior layers by the solution. The films were then heated250 °C for 4 h in anelectric furnace to remove residual solvents and/or organic components. Films of thickness arou160 nm were used in the experiments.

Spectral normal transmittance and near normal refletance of V2O5 films, deposited on glass or ITO coated glassubstrates, were recorded in the wavelength range 300–2nm using a dual beam spectrophotometer~Cary 2400! withspecial reflectance attachment. A calibrated Al mirror servas reflectance standard.

Electrochemical measurements were performed usingEG&G ~Princeton Applied Research! system, consisting of alock-in amplifier ~M 273! and a potentiostat/galvanostat~M5210!. The ordinary three-electrode cell was used in all eperiments, with V2O5 film deposited on ITO coated substratas the working electrode, saturated calomel~SCE! as thereference, and graphite rod as the counter-electrode.electrolyte was 1 M lithium perchlorate~LiClO4! in propy-lene carbonate.

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FIG. 1. Spectral transmittance of 160-nm-thick LiyV2O5 films: ~a! y50.0,~b! y;1.5 corresponds to a charge density of;17 mC/cm2. Parts A and Brefer to films prepared from organic and inorganic precursors.

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III. OPTICAL PROPERTIES

Figure 1~A! shows transmittance spectra for a 160-nmthick V2O5 film in the as-deposited and lithiated states. Thlithiated state corresponds to a charge density of aboutmC/cm2 and a lithium compositiony;1.5. Similar data areplotted in Fig. 1~B! for an inorganic film of comparablethickness. Both types of film exhibit several common fetures.

First, the optical absorption edge atl,500 nm moves toshorter wavelengths as the lithium is inserted. Initially, thas-deposited film appears yellow due to fundamental absotion in the 400–500 nm range. Upon lithium insertion thabsorption edge moves to higher energies, resulting in higtransmittance in the blue. Consequently, a yellow to blueto colorless optical modulation is observed.

Second, in the region 1500 nm,l,2500 nm the trans-mittance is rather independent of lithium concentration.

These findings are in overall agreement with publishresults on sputter-deposited, evaporated, and sol-gel pcessed V2O5 films.

3,4 However, while it is commonly ob-served that lithiation leads to an increasing absorptiaround 1100 nm,3 we find here a decrease of absorptionthat region.

All the above features indicate that these LiyV2O5 filmsremain transparent asy increases, and therefore could probably be used as optically passive ion-storage layers in el

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trochromic smart windows. Quantitatively, this point can bassessed by considering the integrated luminous~lum! andsolar~sol! transmittance.12 These quantities are defined from

Tlum~sol!5*dlF lum~sol!~l!T~l!

*dlF lum~sol!~l!, ~3.1!

whereFlum is the luminous efficiency for photopic visionandFsol is the solar irradiance spectrum. For LiyV2O5 filmsto be efficiently used as ion-storage layers in electrochromsmart windows,Tlum~sol! should remain above 50%, irrespective of the amount of inserted lithium. In our case, they aindeed higher than 65% at compositions 0,y,2.0.

IV. LITHIUM INSERTION AND PHASE TRANSITION

There have been several studies showing the presencdifferent ternary phases LiyV2O5 ~0,y<2! when lithium isinserted into V2O5 using chemical and electrochemical intercalation techniques.1,13–15At low lithium content ~y,0.15!phase a—with lattice constants very close to those oV2O5—is formed by insertion of lithium between the V2O5layers. There are two other distinct phases, phasee occurringin the range 0.15,y,0.9 and phased appearing at compo-sitions 0.9,y,1.0. These also have lattice parameteclosely related to those of the parent oxide, indicating tpochemical insertion and consequently, very good reversibity of the intercalation process. Beyond Li1.0V2O5, a mixtureof various metastable phases develops from which lithiucannot be entirely deintercalated.

Phase changes occurring during lithium insertion infilm can be observed from the open circuit potential~emf!and incremental capacity~2dy/dV! data plotted againstcompositiony. These data were collected using the electrchemical potential spectroscopy~EPS! method16 and areshown in Fig. 2 for heat treated organic and inorganic V2O5thin films deposited on ITO coated glass substrates. Regiowhere two phases coexist correspond to plateaus in the ecurve and peaks in the incremental curve. Moreover,minima of the incremental capacity relate to the completioof certain structures with long range homogeneity or crystaline phases. Thus, as lithium is inserted, there is first a trasition from a phase toe phase in the range 0,y,0.33 fororganic V2O5 film and 0,y,0.4 for inorganic one. Theephase is achieved at a lithium content ofy;0.35 ~organic!and y;0.4 ~inorganic!, lower than the valuey;0.45 re-ported for bulk V2O5 electrode.

1 Upon further lithium inser-tion, the emf curves show a second plateau at 0.33~0.4!,y,1.0, indicating phase transition frome phase tod phase,which is completed aty;1.0, in good agreement with pub-lished results.1,17 Finally, with lithium content between 1.0,y,2.0 some structural reorganizations are observed, cresponding to partially irreversible phases. However,shown from cyclic voltammetry~Sec. V!, there still is animportant charge density, up to about 15 mC/cm2, that can beinserted and extracted reversibly.

These findings evidence the existence of long ranatomic ordering or crystalline phases in the heat treat

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FIG. 2. Equilibrium potential and incremental capacity vs compositiony ofgraphite/LiyV2O5 cell at room temperature. Film thickness was;160 nm.Parts A and B refer to heat treated organic and inorganic films.

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films. For inorganic films, this ordering is less pronouncethan in organic ones, as the corresponding incremental cushows weak and rounded off peaks.

V. CYCLIC VOLTAMMETRY

Cyclic voltammograms of organic and inorganic V2O5films for a sweep rate of 10 m V/s are shown in Fig. 3. Foboth types of film, measurements were taken for two sampprepared under identical conditions, but one was subjectedpost-deposition thermal treatment at 250 °C for 4 hwhereas the other one was studied in the as-deposited sBased on these data, many remarks can be made.

First, all samples show excellent reversibility. The quatity of lithium inserted and extracted on the cathodic ananodic sweeps are equal.

Second, the charge density increases considerably wthermal treatment, raising from about 10 to 15 mC/cm2. Thismay be attributed to the removal of residual solvents andorganic components from the films, leaving more room fthe intercalated lithium.

Voltammograms for heat treated samples show very wdefined peaks. As the current in voltammetric experimentsdirectly proportional to the incremental capacity2dy/dV,these peaks also indicate phase transitions that occur infilms during insertion or extraction of lithium. In agreemen

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FIG. 3. Cyclic voltammograms of 160-nm-thick V2O5 films: ~a! as-deposited,~b! heat treated at 250 °C for 4 h. The sweep rate was 10 mVParts A and B correspond to organic and inorganic films.

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to EPS data~Sec. IV!, transitions froma phase toe phaseand then, frome phase tod phase are observed. Cogan3 hasobtained similar results for sputtered polycrystalline V2O5films. The third peak in the voltammogram for organic film@Fig. 3~A!#, appearing at about21 V ~versus SCE! duringthe lithiated sweep, is ascribed to the transition to sommetastable phase with lithium compositiony greater than1.0.

The as-deposited organic film appears to have lessdered structure than the inorganic one, its corresponding vtammogram being practically structureless. However, tcurve shape changes dramatically after heat treatment ofsample, reflecting a strongly enhanced long range atomicdering. We have previously been able to reach the same cclusion basing on infrared data.18

VI. LITHIUM DIFFUSION

Lithium diffusion coefficient at room temperature wadetermined using complex impedance measurements infrequency range from 0.1 to 105 Hz, with LiyV2O5 film de-posited on ITO coated substrate as the working electrode ithree-electrode cell. Initially, various dc voltages were aplied between the working and counterelectrodes until eqlibrium was established, enabling ac impedance measuments at different lithium compositionsy. Figure 4 shows a

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FIG. 4. A typical complex impedance Nyquist plot of LiyV2O5 film and thesimulated spectrum. The film was held at a dc voltage of20.4 V ~vs SCE!which corresponds toy;0.94.

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typical impedance plot for Li1.0V2O5, together with the simu-lated spectrum based on a generalized Randles circuit whtakes into account interfacial adsorption.19

A complex nonlinear least square~CNLLS! procedurewas used to fit experimental data to the equivalent circwhich contains the finite diffusion impedanceZD ,

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ZD5coth~BAjv!

Y0Ajv, ~6.1!

wherej 5 A21,v is the ac voltage frequency,B andY0 aretwo diffusion dependent quantities:

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TABLE I. Thermodynamic and kinetic data for organic LiyV2O5 film. Cal-culation was done assuming a value of 3.36 g/cm3 for film density.

V vs SCE~V! y

D̃~cm2/s!310210 W

DK

~cm2/s!310212

sLi

~V21 cm21!31028

20.1 0.749 2.63 51.666 5.08 5.7720.2 0.832 3.05 42.647 7.15 2.3520.3 0.900 2.15 87.683 2.45 2.2520.4 0.942 2.78 70.234 3.96 5.7220.5 1.010 1.33 50.435 2.63 3.7720.6 1.080 1.67 63.989 2.61 6.0920.7 1.220 0.45 15.271 2.96 4.65

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TABLE II. Thermodynamic and kinetic data for inorganic LiyV2O5 film.Calculation was done assuming a value of 3.36 g/cm3 for film density.

V vs SCE~V! y

D̃~cm2/s!310210 W

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20.1 0.61 1.5 22.39 6.7 0.1820.2 0.71 2.1 2.66 79 0.2920.3 0.79 1.9 3.80 50 1.520.4 0.89 1.2 3.52 34 3.020.5 1.01 0.14 28.03 0.5 0.4920.6 1.16 0.20 25.01 0.8 0.6320.7 1.40 0.17 11.34 1.5 0.72

ich

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where D̃ is the lithium chemical diffusion coefficient,d isthe film thickness,A is the active surface area of the sampleF is the Faraday number, andVM is the material molar vol-ume.

The chemical diffusion coefficientD̃ can be either ob-tained from Eqs.~6.2! or ~6.3!, using the values ofB andY0from CNLLS fitting. The component diffusion coefficientDK is then deduced from:

D̃5WDK . ~6.4!

with W being the enhancement factor determined from EPdata,

W52F

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and the partial conductivity of lithium can be obtained usinthe following expression:

sLi52Fr

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. ~6.6!

The density of bulk V2O5, r53.36 g/cm3, was used for filmdensity in the calculation. Numerical values forD̃, DK , W,and sLi of organic and inorganic LiyV2O5 films at variouscompositionsy are given in Tables I and II. These values fothe chemical diffusion coefficientD̃, ranging from1.33310210 to 3.05310210 cm2/s for organic films and from1.4310211 to 2.1310210 cm2/s for inorganic ones, are of twoorders lower than those reported for bulk V2O5.

21 The mostrapid diffusion occurs in the range 0.7,y,0.8 for both typesof film. At all lithium contents considered, diffusion in or-ganic films is faster than in inorganic ones. This may battributed to the higher level of long range atomic orderingheat treated organic films, as pointed out by EPS and cycvoltammetry.

VII. CONCLUSION

Vanadium pentoxide thin films were successfully produced by the sol-gel method from organic and inorganic prcursors. They exhibit optical and electrochemical propertisuitable for potential use in electrochromic smart windowUp to a composition ofy;2.0, films of thickness around 160nm are still capable of reversible cycling at a substant

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charge density of 15 mC/cm2, and thus could be consideredas candidates for reversible cathodes in thin film rechargable batteries.

EPS and cyclic voltammetry evidence the structure dference between organic and inorganic films. The adeposited inorganic films seem to have a more ordered strture than the organic ones. However, after undergoithermal treatment at 250 °C, the organic films show strongevidence of long range atomic ordering and transitions froa phase toe phase and then tod phase, together with a fastelithium diffusion.

1P. G. Dickens and M. F. Pye, inIntercalation Chemistry, edited by M. S.Whittingham and A. J. Jacobson~Academic, New York, 1982!, p. 539.

2C. Delmas, S. Brethes, and M. Menetrier, J. Power Source34, 113~1991!.3S. F. Cogan, inLarge-Area Chromogenics: Materials and Devices foTransmittance Control, edited by C. M. Lampert and C. G. Granqvis~Optical Engineering, SPIE, Bellingham, WA, 1988!, p. 170.

4A. Talledo, A. M. Andersson, and C. G. Granqvist, J. Appl. Phys.69,3261 ~1991!.

5L. Michailovits, J. Hevesi, L. Phan, and Zs. Varga, Thin Solid Films102,71 ~1983!.

6S. D. Hansen and C. R. Aita, J. Vac. Sci. Technol. A3, 660 ~1985!.

J. Appl. Phys., Vol. 80, No. 12, 15 December 1996 article is copyrighted as indicated in the article. Reuse of AIP content is su

98.215.252.144 On: Sun

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if-s-uc-ngermr

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7R. L. Remke, R. M. Walser, and R. W. Bene, Thin Solid Films97, 129~1982!.

8C. Sanchez, M. Nabavi, and F. Taulelle, Mater. Res. Soc. Symp. Pr121, 93 ~1988!.

9J. Livage, Mater. Res. Soc. Symp. Proc.32, 125 ~1984!.10G. Meunier and R. Dormoy, inMicroionics—Solid State Integrable Bat-teries, edited by M. Balkanski~North Holland, Amsterdam, 1991!, p. 73.

11J. B. Bates, G. R. Gruzalski, N. J. Dudney, C. F. Luck, X.-H. Yu, and SD. Jones, Solid State Technol.July, 59 ~1993!.

12C. M. Lampert, Vo-van Truong, J. Nagai, and M. Hutchins, Characteriztion Parameters and Test Methods for Electrochromic Devices in GlazApplications, International Energy Agency Task X-C, Glazing MaterialInterim Document~1991!.

13D. W. Murphy, P. A. Christian, F. J. DiSalvo, and J. V. Waszczak, InorChem.18, 2800~1979!.

14S. Hub, A. Tranchant, and R. Messina, Electrochim. Acta33, 997~1988!.15P. Hagenmuller, Prog. Solid State Chem.5, 71 ~1971!.16A. H. Thompson, J. Electrochem. Soc.126, 608 ~1979!.17F. Theobald, R. Cabala, and J. Bernard, J. Solid State Chem.17, 431

~1976!.18T. B. B. Nguyen, M. T. Pham, S. Badilescu, Y. Djaoued, G. Bader, F.Girouard, and Vo-van Truong~unpublished!.

19D. R. Franceschetti, J. Electrochem. Soc.129, 1755~1982!.20C. Ho, I. D. Raistrick, and R. A. Huggins, J. Electrochem. Soc.127, 343

~1980!.21J. Livage, P. Barboux, J. C. Badot, and N. Baffier, Mater. Res. Soc. SymProc.121, 167 ~1980!.

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