XPS and IR studies of transparent InVO4 films upon Li charge–discharge reactions

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Solid State Ionics 165 (2003) 89–96

XPS and IR studies of transparent InVO4 films upon

Li charge–discharge reactions

N. Ciminoa, F. Artusoa, F. Deckera,b,*, B. Orelc, A. Surca Vukc, R. Zanonia,b

aDipartimento di Chimica, Universita degli Studi di Roma ‘‘La Sapienza’’, Piazzale Aldo Moro, 5, Rome 00185, Italyb INFM Sezione RSF2, Italy

cNational Institute of Chemistry, Ljubljana, Slovenia

Abstract

InVO4 belongs to the family of orthovanadates, oxides with attractive properties as Li insertion electrodes, indicated for

electrochromic windows due to their transparency. The Li insertion reaction in indium vanadate films, deposited by the sol–gel

dipping method onto conducting glass substrates, was performed electrochemically and the changes in physical properties of the

material were studied by means of Infrared and X-ray Photoelectron spectroscopies. Results of photoelectron spectroscopy

showed the reduction of V5 + to V4 + and V3 + upon Li insertion, and the onset of new structures in the valence band, related to

the formation of Li carbonate on the electrode surface. The IR spectra for the Li-charged samples showed a drop in band

intensities, which was associated with a decrease of the film electrical conductivity, and the appearance of new bands for x>0.6.

The Li electrochemical de-insertion brought about the almost complete recovery of the oxide XPS initial spectrum, and the

partial recovery of its IR spectrum. Such results confirmed the excellent chemical stability of this oxide material upon Li

charge–discharge reactions (x < 1), even when the long-range crystalline order has been perturbed by the electrochemical bulk

insertion process.

D 2003 Published by Elsevier B.V.

PACS: 78.20.Jq; 81.20.Fw

Keywords: Indium orthovanadate; Electrochromism; IR; XPS; Li insertion electrode

1. Introduction similar to that of the coloring electrode. The class

The investigation on transparent counter-electro-

des for electrochromic windows has been extensive

during the last decades. The aim of such research has

been to combine in one electrode a large ion storage

capacity (above 35 mC cm� 2) a photopic transmit-

tance both in the charged and in the discharged state

of at least 85%, and a Li ion diffusion kinetics

0167-2738/$ - see front matter D 2003 Published by Elsevier B.V.

doi:10.1016/j.ssi.2003.08.020

* Corresponding author. Tel.: +39-6-49913169.

of vanadate films (CeVO4 [1], FeVO4 [2], Fe2V4O13

[3], InVO4 [4,5]) fulfills most of the above require-

ments and is worth a deeper investigation to eluci-

date whether these materials are chemically and

structurally stable and reversible, upon Li charge–

discharge insertion reactions. InVO4 belongs to the

family of orthovanadates, oxides with attractive

properties as insertion electrodes for Li batteries

(Denis et al. [6]), and indicated for electrochromic

windows due to its transparency. LixInVO4 has been

indeed checked as an anode for Li batteries up to

N. Cimino et al. / Solid State Ionics 165 (2003) 89–9690

xH1. The present spectroscopic study, intended to

disclose the structural and electronic changes of this

Li host electrode, was restricted to the 0 < x < 1

domain, where the oxide films are highly transparent

and (apparently) quite reversible. We focused on two

different physical properties of such oxide film, the

vibro-rotational lattice modes which appear in the IR

range, and the energy of the atomic and molecular

electronic orbitals of both host and guest species,

which appear in the X-ray domain and can be

detected by Photoelectron Spectroscopy (PES). IR

spectra can in fact show the perturbation in the long-

range crystalline order by the electrochemical inser-

tion process and, in some cases, the presence of

reduced vanadium and/or indium in the bulk elec-

trode material. PE spectra, on the other hand, are

very sensitive to the presence and nature of the

chemical bonds of each element present in the

surface top layer, including Li, and to the modifica-

tion induced in the existing valence band states such

as the formation of new bands or filling of the

existing ones. These two spectroscopic tools are

therefore complementary, with respect both to the

material properties investigated and to the oxide film

region analyzed. The objective of the present study

was to perform few charge–discharge electrochemical

cycles, and to follow both the IR and PE spectra of the

electrode material at different states of charge. For both

spectroscopies, therefore, we needed special care in the

sample preparation and in the sample transfer from the

electrochemical cell to the spectrometer, because XPS

is intrinsically an ex-situ, UHV spectroscopy, and

because ex-situ IR (often more sensitive than in-situ

IR) needs special substrates, transparent to infrared.

The aim of this investigation is to discuss some subtle

changes in the film electrode properties (such as:

charge capacity fading, small variation in photopic

transmission, irreversible Li incorporation) on the basis

of a deeper understanding of the material properties of

the host oxide film.

2. Experimental

The InVO4 films were prepared using a sol–gel

synthesis route. In(NO3)3�5H2O was first dissolved in

1-propanol, then V-oxoisopropoxide was added in a

In/V 1:1 molar ratio in precursors. The sol was

vigorously stirred for at least 1 h prior to the film

deposition with a dip-coating technique. The SnO2:F

glass substrates (for in-situ UV–visible measure-

ments) or silicon wafers (for IR measurements) were

dipped into the sol and the films were then deposited

with a pulling velocity of 10 cm min� 1. The films

were thermally treated at 500 jC for 1 h in air. The

film structure resulting from XRD spectrum is a

monoclinic InVO4-I crystalline phase with grain size

below 40 nm, with some admixture of the InVO4-III

orthorhombic phase. The morphology of the sol–gel

layers, as revealed by AFM (performed with a VT-

UHVAFM-STM by OMICRON), was very homoge-

neous and flat in the scanning range of 5� 5 Am2,

2� 2 Am2 and 1�1 Am2, very similar to the mor-

phology of the SnO2:F glass substrate.

All electrochemical measurements were performed

in three electrode cells hermetically sealed under argon

atmosphere in a glove box. The three electrode cells

had optical windows to allow the spectrophotometric

measurements of the film transmittance in the UV–

visible range. Lithium metal was used as counter and

reference electrodes while the working electrode was

the thin film oxide under investigation. The electrodes

were immersed in a 1 M solution of lithium perchlo-

rate in propylene carbonate. To promote Li insertion

and the charging/discharging of the electrode, cyclic

voltammetry (CV) and chronopotentiometry were ac-

complished between the potential limits of 1.6 and 4.8

V vs. Li/Li+, using an EG&G PAR 273 Potentiostat/

Galvanostat. The CV curves were recorded after few

cycles (typically 3 or 4) at a scan rate of 20 mV s� 1.

In-situ UV–visible spectroelectrochemical measure-

ments were obtained on an HP 8453 UV–visible

diode array spectrophotometer combined with an

EG&G PAR model 273 potentiostat–galvanostat.

Ex-situ IR absorbance spectra (TO modes) were mea-

sured using a Perkin Elmer System 2000. The IR

spectra were obtained after galvanostatic charging/

discharging with a current density of F 22.3 AAcm� 2 and with potential limits as in CV. For ex-situ

IR measurements were the InVO4 films deposited on

double-sided polished silicon wafers with an electrical

resistivity of 20 V cm and transparent to IR radiation

(up to 50%).

XPS measurements were conducted with the sam-

ples mounted on conductive stubs by means of a small

Cu spring pressed against the SnO2/F to minimize

N. Cimino et al. / Solid State Ionics 165 (2003) 89–96 91

charging under X-rays. The as-prepared sample was

stored in air after preparation, and measured as such.

To prevent air exposure of the Li-treated samples

during their transport from the dry-box, where the

lithiation/delithiation occurred, and the XPS instru-

ment, a vacuum-tight stainless steel vessel equipped

with a transfer mechanism and a UHV valve was

preventively introduced in the Ar-filled dry-box. The

vessel was then connected to its dedicated flange on

the XPS instrument, the dead volume was evacuated

before opening both the vessel and the XPS prepara-

tion chamber valve, and the samples were eventually

placed inside the holder in the XPS chamber. XPS

spectra were acquired with a VG ESCALAB Mk II

spectrometer (CNR, Area della Ricerca di Roma,

Montelibretti), equipped with a hemispherical analyz-

er operated in fixed analyzer transmission (FAT)

mode, with pass energy of 20 eV. Unmonochromat-

ized AlKa photons (hm = 1486.6 eV) were used to

excite photoemission. The binding energy (BE) scale

was calibrated by taking the Au 4f7/2 peak at 84.0 eV.

All BEs are referenced to a value for the residual

pump oil contamination-related C 1s peak taken at

285.0 eV. The spectra were collected by a A-PDP 11/

83 DEC computer and processed by means of VGS

5250-SI software (version of Feb. 1991). A series of

commercially available DOS and Windows routines

were used for data analysis. The accuracy of reported

BEs is F 0.2 eV, and the reproducibility of the results

is within these values.

3. Results and discussion

3.1. Electrochromic properties

The cyclovoltammetric curves of sol–gel InVO4

films (Fig. 1) revealed a single cathodic wave at 1.6 V,

evolving into a peak around 1.8 V, corresponding to

an inserted charge growing from � 25 mC cm� 2 (1st

cycle) to more than � 35 mC cm� 2 (20th cycle),

equivalent to x = 0.8 in the formula LixInVO4 and

assuming a 100% efficient Li insertion reaction. The

anodic potential sweeps of the same experiments (1st

and 2nd cycles) revealed an incomplete reoxidation

reaction, corresponding to an irreversible film lithia-

tion of the order of 20% of the cathodic charge.

Further cycling resulted, however, in a more balanced

cathodic–anodic reaction and indicated that the

monoclinic InVO4-I phase became progressively more

accessible to lithium ions. Such results are in agree-

ment with the experiments of Denis et al. [6], where

the monoclinic InVO4-I powdered electrodes was

tested for the Li battery. The irreversible lithiation is

particularly relevant here, because it will be discussed

in the light of the results from the different spectro-

scopic methods used in this work. The spectral

transmittance of the same films, after lithiation and

successive delithiation for 100 s at 1.6/4.8 V vs. Li in

1 M LiClO4/PC electrolyte, is shown in Fig. 2. Films

show a weak cathodic electrochromism below 1000

nm, which correspond to a drop in photopic transmit-

tance down to 72%, from the initial 89% of the as-

prepared film. The bleaching process in such electro-

des is almost complete upon delithiation (the photopic

transmittance returning to 87%). The above transmit-

tance changes may be ascribed to the partial filling of

a split-off band characteristic also for V2O5 with Li

insertion [7]. The high transmittance above 1000 nm,

on the other hand, is an indication that there is no

polaron hopping in this material in either the pristine

or the Li-treated state.

3.2. IR spectroscopy of InVO4

Comparison of the IR spectrum of an initial state of

InVO4 film (Fig. 3a) to those reported by Roncaglia et

al. [8] and Touboul and Popot [9] confirmed our XRD

measurements [4,10],[10] that the prevailing phase in

the sol–gel films is the monoclinic InVO4-I phase

(space group C2/m). Both spectra are characterized by

a large splitting of the r3 band of the VO4 groups [8],

which appeared in our spectrum at 998, 954, 889, 750,

738, and 629 cm� 1. The reason for the large splitting

is the existence of the two different VO4 groups with

different VUO bond lengths, connecting the In4O16

clusters together [11]. The former VO4 group has the

lengths of the VUO bonds 0.164 and 0.178 nm, the

latter 0.159 and 0.187 nm. The unusually long VUO

bond (0.187 nm) corroborates the assignment of the

band at 629 cm� 1 to the r3, although it is difficult to

judge without extensive normal coordinate calcula-

tions to what an extent the In–O stretching vibrations

are admixed. According to the length of the VUO

bonds the bands above 850 cm� 1 can be ascribed to

the terminal VUO bonds, the bands between 850 and

Fig. 1. Cyclovoltammetric response of an InVO4 film thermally treated at 500 jC for 1 h. Scan rate used was 20 mV/s and the electrolyte was 1

M LiClO4 in PC.

Fig. 2. In-situ UV–visible spectra of InVO4 film thermally treated at

500 jC for 1 h. The films were charged/discharged chronocoulo-

metrically for 100 s at 1.6/4.8 V vs. Li in 1 M LiClO4/PC electrolyte.


700 cm� 1 to the bridging VUO. . .In stretching with

the shorter VUO bonds, the bands between 700 and

500 to the bridging VUO. . .In modes with the longer

VUO bonds and the band at 478 cm� 1 to the V—

O—V deformations. The characteristic feature of the

monoclinic phase [8,9] is the band at 629 cm� 1 and it

cannot be found in the IR spectra of the orthorhombic

InVO4-III phase [12]. However, the shoulders at 946

and 750 cm� 1 in the spectrum in Fig. 3a indicate also

the presence of an admixed orthorhombic phase in our

sol–gel InVO4 samples.

After galvanostatic charging to x = 0.9, the IR spec-

trum of the sol–gel InVO4 film (Fig. 3b) drastically

changed and became similar to the IR spectra of other

highly charged orthovanadates and V2O5 films

[3,10,13]. Only two broad bands remained in the

spectrum, the weaker one at around 866 cm� 1 and a

more intense one at 434 cm� 1. The red frequency shifts

of the V–O stretching modes are a common feature of

the lithium insertion. In the intercalation domain x= 0.9

almost all V5 + species became reduced to lower

oxidation states and the V4 +–O or even V3 +–Omodes

could be found in IR spectra below 500 cm� 1. On the

other hand, without normal coordinate calculations it is

difficult to estimate only at the basis of IR spectrosco-

py, whether In-species are involved in the redox reac-

tions. In part, the band at 434 cm� 1 together with the

Fig. 3. Ex-situ IR absorbance spectra of the InVO4 film thermally treated at 500 jC for 1 h: (a) initial state, (b) galvanostatically charged to

x= 0.90 and (c) galvanostatically discharged from x= 0.9. Electrolyte was 1 M LiClO4 in PC.

Table 1

XPS binding energy values (eV) and atomic ratios for the reported

compounds

Samples In

3d5/2

V 2p3/2 Li

1s

V/

In

Oox/

V

Li/

Ccarb

Li/V

As-prepared 445.1 517.6 55.3 0.7 3.9

Lithiated 444.3 515.1 55.4 0.4 3.0 2.3 66.0

Delithiated 444.5 517.6 515.9 55.3 0.6 3.9 2.5 2.7

Oox and Ccarb are the oxide- and carbonate-related components in

the O 1s and C 1s complex peak, respectively.


increase in the background absorption belonged to the

Li+–O modes [14], which form in the structure of the

lithiated InVO4 films between the intercalated lithium

ions and the host’s oxygens. After delithiation, the

increased background and the low-intensity band at

485 cm� 1 indicated the incomplete removal of lithium

ions from the film’s structure (Fig. 3c). The spectrum of

the discharged sample resembles with its broad bands

very well the spectra of the amorphous films prepared

at lower temperatures (for example 300 jC) [10]

proving that the amorphisation of the structure of

InVO4-I took place. The amorphisation of the InVO4-

I powders was already found by Denis et al. [6] who

observed a steady decrease in the intensity of the

diffraction peaks in XRD spectra between 0.5 < x < 1

until their disappearance at x = 1.

3.3. XPS data

The XPS investigation was concentrated on the

characterization of the surface effects induced by the

Li lithiation/delithiation process. The electronic struc-

ture of the lithiated film, both in terms of binding

energies (BEs) and of surface composition, was ex-

plored and compared to that of the as-prepared sample,

and the same analysis was again repeated after Li

delithiation from the same film sample. Notice that,

in this sequence of measurements, any contact of the

sample with air was prevented, by making use of a

vacuum-tight vessel, which can be introduced in dry-

box for the treatments, extracted and fit to the XPS

machine.

The analysis of the V 2p and In 3d core lines of the

as-prepared samples clearly indicates the presence of

V+ 5 and In+ 3 oxidation states. The atomic ratios

shown in Table 1 show the presence of excess In,

while they are consistent with the expected vanadate

oxide VO4 composition. A reproducible relative en-

hancement of In has been detected in all the In

vanadates film measured so far, which could indicate

some In enrichment at the surface top. The lithiation

Fig. 4. (Left panel) V 2p ionization region, with the 3/2 spin–orbit component, and (right panel) In 3d ionization region with the 5/2, 3/2 spin–

orbit components for the In vanadate samples: as prepared (a); after lithium intercalation (b); after the deintercalation cycle (c). The two small

wiggles at the far left in (b) (Left panel) are an artifact of the software routine which removes the contribution from Al ka3,4 satellite.


process is responsible for the shift of the In 3d and V

2p core levels to lower BEs, associated with a large

broadening of the V 2p band, as well as for the

Fig. 5. (Left panel) C 1s and (right panel) O 1s ionization re

evolution of the O 1s and C 1s regions and for the

formation of new structures in the valence band

spectrum. All such aspects can be seen in Figs. 4–6

gion for the In vanadate samples. Labels as in Fig. 4.

Fig. 6. Extended region of the XPS valence band for In vanadate

samples. Labels as in Fig. 4.


and are discussed as follows. As far as the electronic

structures of V and In are concerned, the lithiation

process promotes a 0.8 eV lowering in the In 3d BE

(discussed below), and V reduction to a lower oxida-

tion state, an admixture of 4+ and 3+ that cannot be

defined in a quantitative way, due to the drop in signal

intensity. The BE shift of the indium peak, and the

absence of In 3d line broadening, is probably due only

to the electrostatic charge of its neighboring atoms,

because 4 of the 6 oxygens in the octahedral coordi-

nation of InO6 are shared with the VO4 tetrahedra and

may take up part of the negative charge of the

reduction (lithiation) reaction. These effects are nicely

reversible, because the shifts of the core levels due to

lithiation are reversed after delithiation and the peak

intensities are recovered after one full charge–dis-

charge cycle. A low intensity component of the

reduced V is still detectable in the delithiated sample,

which could be related to the irreversible Li charge

already measured electrochemically. As to the O 1s

and C 1s regions, the lithiation is responsible for the

appearance of new peaks at higher BEs, which are still

of sizeable intensity after delithiation. The appearance

of such peaks, corroborated by the analysis of the

valence band region (as previously shown for the

mixed Ni–V oxide film electrodes [15]), can be

explained by the presence of a carbonate surface layer,

mainly deposited during the cathodic reaction. The

new peaks in the C 1s, Li 1s and O 1s core levels and

the structures at 13 eV (valence band spectrum in Fig.

6) support an assignment of the surface carbonate to

Li2CO3, by comparison with the data from the rele-

vant literature [16].

Such layer is in large part removed by delithiation,

as deduced from XPS relative quantitative data, but is

expected to grow thicker on the electrode surface after

hundreds (thousands) charge–discharge cycles, as

already observed for other oxide film electrodes

[15]. Other features of the photoemission spectrum

from the valence band region (Fig. 6) are the core

lines Li 1s at 55.3 eV and In 4d at 18 eV, and the

valence band features deriving from the O 2s and O

2p levels, which appear as broad structures around 25

and 6 eV, respectively. Interestingly, the Li 1s core

level does not disappear completely after sample

delithiation, being barely detectable in the c spectrum.

4. Conclusions

Most of the results from IR and XP spectroscopies

can be explained by making the assumption that In

and VO4 can be considered as separate ions held

together mainly by electrostatic forces. The lithiation

process, in fact, strongly affected the VUO bonds,

with important changes in the V and O core levels,

consistent with a more ionic V–O interaction, and had

only a minor (electrostatic) effect on the In side. The

IR spectra were in good agreement with such findings,

because the lattice modes of the In–O groups were

found in the far IR, contrasting with the presence of

the strong modes in the range 1000–600 cm� 1 from

the VO4 units. Spectroscopic analysis of the heavily

charged films (x>0.6) brought about a new IR band at

low wavenumbers, similarly to what occurs with

heavily lithiated vanadium pentoxide and other lithi-

ated vanadates. The above findings exclude the pos-

sible reduction of In oxidization state, which might

appear in parallel to V reduction upon lithiation.


Because of the amorphization process taking place for

x>0.6, the IR spectra of delithiated films do not

correspond to that of the pristine sample, rather being

close to the spectrum of an amorphous InVO4 film. In

spite of this structural instability, the good chemical

stability of this oxide material upon Li charge–dis-

charge reactions (x < 1) was shown by XPS. The

delithiation process, in fact, was able to restore to a

large extent the XPS spectrum of the pristine InVO4

sample, with the noteworthy exception of the struc-

tures associated with the Li carbonate layer. The

presence of the surface layer is not only a direct

consequence of the cathodic electrochemical process,

but it may be associated to the final reaction product

of the highly reactive lithiated oxide film, with traces

of oxidizing species present in the inert atmosphere or

even in high vacuum. So far, it is not completely clear,

therefore, to what extent the formation of Li carbo-

nates is responsible for the irreversible lithiation (i.e.

the charge unbalance) measured in the first few cyclo-

voltammograms. The existence of small amounts of V

in the 4+ state even after electrochemical delithiation

(from XPS spectra) would suggest that some lithium

ions are indeed trapped inside the oxide material,

although they will eventually outdiffuse from the bulk

oxide to form a more stable compound on the elec-

trode surface.

Acknowledgements

This work was undertaken with the auspices of the

Bilateral Cooperation Agreement between Italy and

Slovenia and has been partially financed by MIUR

(Italy) and INFM (Roma). The authors would like to

thank Dr. M. Liberatore for experimental support and

Prof. C. Coluzza for valuable discussions. Prof. M.

Touboul (Amiens, France) is acknowledged for

supplying standards of indium vanadate.

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XPS and IR studies of transparent InVO4 films upon Li charge–discharge reactions

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