Ž .Electrochemistry Communications 2 2000 743–748www.elsevier.nlrlocaterelecom

Spectroelectrochemical studies on highly polarized LiCoO electrode in2

organic solutions

Takashi Itoh, Naomi Anzue, Mohamed Mohamedi, Yasunari Hisamitsu, Minoru Umeda,Isamu Uchida)

Department of Applied Chemistry, Graduate School of Engineering, Tohoku UniÕersity, Aramaki-Aoba, Aoba-ku, Sendai 980-8579, Japan

Received 3 August 2000; received in revised form 16 August 2000; accepted 16 August 2000

Abstract

In situ Raman spectroscopy was conducted on thin film electrodes of pure LiCoO in order to observe the nature of the changes in2Ž .interfacial structure between LiCoO and organic solutions propylene carbonate and ethylene carbonate containing 1 M LiClO when2 4

Ž q .LiCoO is scanned up to highly anodic potentials ;5.0 V Li rLi . Raman spectra and cyclic voltammograms were recorded2

simultaneously during the potential scan. We observed a sudden increase in the background signals of the Raman spectra at potentialsmore positive than 4.7 V. The increased background did not change after potential cycling. The change was irreversible, indicating thatsurface film formation occurred at positive potentials. As organic compounds fluoresce by visible light, the increased background isascribed to the formation of a film on the LiCoO electrode surface in organic solutions. q 2000 Published by Elsevier Science S.A.2

Keywords: Raman spectroscopy; Lithium ion battery; Lithium cobalt oxide; Surface film formation; Cathode material; Organic solution

1. Introduction

The electrochemical reactions of battery materials canbe classified as either disproportionation or topochemical

w xreactions 1 . A typical cathode material, lithium cobaltoxide, is classified as a topochemical reaction. This mate-rial has been studied in organic electrolyte solutions by

w xconventional electrochemical methods 2 , as well as byŽ .special techniques such as X-ray diffraction XRD analy-

w x w xsis 3–5 and spectroscopic measurements 6–8 .We reported previously that the variation of Raman

intensities for Li CoO electrodes, as measured by in situx 2

Raman spectroscopy in a potential range of 3.0 V to 4.2 V,was due to a change in optical skin depth associated with

w xelectric conductivity 7 . As a continuation of this research,the observation of Raman spectra at anodic potentialshigher than 4.2 V, possibly up to 5.0 V, is interesting in theunderstanding of film formation on highly polarizedLiCoO surfaces because oxidation of the organic solution2

) Corresponding author. Tel.: q81-22-217-7220; fax: q81-22-214-8646.

Ž .E-mail address: uchida@est.che.tohoku.ac.ip I. Uchida .

at the electrode surface and oxidative damage of theLi CoO electrode is expected. However, the occurrencex 2

w xof such reactions is thus far speculative 9 . In order tosolve this problem, as a first stage, we analyzed thischemical reaction by use of in situ Raman technique.

In this paper, we report on the in situ measurement ofthe Raman spectra of a thin film electrode of pure LiCoO2

Ž q.over the potential range of 3.0 V to 5.0 V LirLi inpropylene carbonate and ethylene carbonate mixed solvent

Ž .systems. Fourier transform Raman FT–Raman andŽ .Fourier transform infrared FT–IR measurements were

also made for samples treated at 5.0 V. We clarify theformation of a surface film on pure LiCoO electrodes in2

this potential region.

2. Experimental

An air-tight electrochemical cell was used in this study,equipped with a sapphire plate as an optical windowŽ .30 mm diameter, 300 mm thick . The LiCoO electrode2

Ž .was a thin film 5 mm diameter, ca. 0.1 mm thick preparedw xon a gold substrate by the molten carbonate method 2 .

Lithium foils on nickel mesh were used as counter and

1388-2481r00r$ - see front matter q 2000 Published by Elsevier Science S.A.Ž .PII: S1388-2481 00 00112-0

( )T. Itoh et al.rElectrochemistry Communications 2 2000 743–748744

reference electrodes; potential in this paper is given withrespect to Lir1 M Liq. The organic electrolyte solutionused in this study was a combination of propylene carbon-

Ž . Ž .ate PC and ethylene carbonate EC containingŽ .1 M LiClO Li-ion battery grade, Mitsubishi Kagaku Co. .4

The water content in the organic solution was less thanŽ .20 ppm. An argon ion laser 514.5 nm, 40 mW was fo-

cused on the working electrode surface. The beam spotsize was 1.0=0.1 mm at the electrode surface. Scatteredlight was dispersed by a triple grating spectrograph andcollected by an intensified diode-array detector. This sys-tem enabled us to obtain a spectrum of 800 cmy1 band-

Ž y1 .width typically 100 to 900 cm in one pass, and eachpass took 30 s. The Raman spectra and cyclic voltammo-

( .gram CV were measured simultaneously during continu-Ž .ous scanning 0.2 mVrs . Details of the in situ spectro-

w xscopic procedure have been reported in reference 7 .FT–Raman spectra using a YAG laser, and FT–IR spectra,were obtained using a JASCO RFT-600 spectrometer.

3. Results

Ž .A scanning electron microscope SEM image of theLiCoO electrode before CV measurement is presented in2

Fig. 1. The octahedral crystalline particles are clearlyvisible on the surface. This electrode is used throughoutthis work.

The variation of CV with different reversal potentialsŽ . Ž . Ž .are shown in Fig. 2: a 4.2 V, b 4.6 V and c 5.0 V, all

at 0.2 mVrs. The open circuit potential was 3.2 V. Anodicand cathodic current peaks were observed in a potentialrange of 3.8 V and 4.2 V. The anodic and cathodic peaksare attributed to the intercalation and deintercalation of Li

w xions to and from the Li CoO electrodes, respectively 2 .x 2

These CVs are somewhat broad compared with thosew xpreviously reported 2 and involve ohmic polarization

because the LiCoO electrode was pressed firmly onto the2

optical window in order to attenuate solvent signals. This

Fig. 1. SEM image of LiCoO thin film electrode on Au.2

Ž . Ž . Ž . Ž .Fig. 2. CV for different reversal potentials a 4.2 V — , b 4.6 V . . .Ž . Ž .and c 5.0 V --- , scan rate 0.2 mVr s.

electrode configuration, with a thin layer of electrolytebetween the optical window and the LiCoO electrode2

surface, is necessary to obtain good Raman signals withoutinterference by electrolyte molecules. Therefore, the dis-crepancy in CV was caused by the increased solutionresistance due to the narrow gap between the window andelectrolyte.

Ž .The CV shape in Fig. 2 c is different from these in Fig.Ž . Ž .2 a and b , specially decreasing potential side. We ob-

served two current peaks of lithium insertion to LiCoO2

electrode at 4.08 V and 3.92 V. This result suggested thattwo lithium insertion process were appear after highlypolarized LiCoO electrode.2

We carried out in situ Raman measurements duringpotential cycling. The evolution of the Raman spectra fromthe thin film LiCoO electrode for wavenumbers between2

y1 y1 Ž .100 cm and 900 cm is shown in Fig. 3 for a increas-Ž .ing and b decreasing potentials. Several characteristic

Ž . Ž .features are observed in Fig. 3 a and b . In the potentialrange between 3.0 V and 4.0 V, LiCoO Raman peaks2

y1 y1 Žappeared at 485 cm and 597 cm as indicated by.vertical arrows in the figure . These Raman peaks are

attributed to the E and A modes of LiCoO , respec-g 1g 2w x y1tively 8 . The other peaks, except for those at 485 cm

and 597 cmy1, can be ascribed to the organic solution andw xthe optical sapphire window 7 .

w xAs noted previously 7 , the peaks due to Li CoOx 2

disappear at potentials between 4.2 V and 4.6 V. This resulthas been explained in terms of a decrease in the opticalskin depth caused by an increase in the electric conductiv-

w xity of Li CoO over this potential range 7,10 .x 2

When the potential exceeded 4.6 V and approached5.0 V, the Raman spectra changed rapidly and signifi-

Ž .cantly, as shown in Fig. 4 a . Furthermore, the spectra didnot return to its initial form when the scan was thenreversed, exhibiting high background signal levels, as re-

Ž .vealed in Fig. 4 b . During the cathodic scan, only Ramanpeaks due to the optical window were observed and the

( )T. Itoh et al.rElectrochemistry Communications 2 2000 743–748 745

Ž . Ž .Fig. 3. In situ Raman spectra of LiCoO electrode on Au in PCqEC containing 1 M LiClO with a increasing and b decreasing potential between2 4

3.0 V and 4.6 V at scan rate 0.2 mVrs.

increased background remained and was present over theentire range between 5.0 V and 3.0 V.

4. Discussion

According to XRD studies into the LiCoO electrode2w xstructure conducted by Reimers and Dahn 3 , and Ozuku

w xet al. 4,5 , a phase transition occurs when the concentra-

tion of lithium in the Li CoO decreases to close tox 2

xs0.5. Changes in the unit cell constant and cell volumeas a function of lithium concentration in Li CoO hexago-x 2

nal phase have been noted, which leads to lithium orderingat xs0.5 coupled with lattice distortion from hexagonalto monoclinic symmetry. However, it is difficult to obtainthe same evidence for such a change by visible laserRaman spectroscopy because of the high conductivity of

w xLi CoO above 4.2 V 6,7,10 . Therefore, we could notx 2

( )T. Itoh et al.rElectrochemistry Communications 2 2000 743–748746

Ž . Ž .Fig. 4. In situ Raman spectra of LiCoO electrode on Au in PCqEC containing 1 M LiClO with a increasing and b decreasing potential between2 4

3.0 V and 5.0 V at scan rate 0.2 mVrs.

obtain any Raman peaks related to Li CoO in the poten-x 2

tial range 4.2 V to 5.0 V in this study, although we ob-served an increase in the background of the Raman spec-tra..

w xAccording to in situ FT–IR spectra 9 obtained previ-ously for the electrochemical oxidation of propylene car-bonate on LiCoO electrodes in a potential range of 4.1 V2

to 4.8 V, observed IR peaks correspond to carbonate ionsand organic compounds on the electrode surface, which

were formed by the ring-opening reaction of propylenew xcarbonate in this potential region 9,11,12 .

In order to confirm the presence of fluorescence fromthe surface film, we used FT–Raman spectroscopy. Ex situFT–Raman spectra are shown in Fig. 5 for LiCoO elec-2

Ž . Ž . Ž .trodes treated at a 4.2 V, b 4.6 V and c 5.0 V. Thesamples were washed with pure PCqEC solvent. The485 cmy1 and 597 cmy1 peaks in the Raman spectra can

Ž . Ž . Ž .be seen in Fig. 5 a and b . By contrast, in Fig. 5 c , no

( )T. Itoh et al.rElectrochemistry Communications 2 2000 743–748 747

Ž . Ž . Ž . Ž . Ž .Fig. 5. Ex situ FT–Raman spectra of LiCoO electrode on Au after CV with different reversal potentials: a 4.2 V — ; b 4.6 V ; and c 5.0 V2 — —Ž .--- .

Raman peaks were observed and the background in the FTspectra was low. A feature of FT–Raman spectroscopy isthat fluorescence in the Raman spectra is removed, there-fore the surface of the electrode fluoresced when thepotential was more positive than 4.7 V, as seen from thelow background. Furthermore, the LiCoO electrode was2

damaged, as indicated by the absence of peaks in theRaman spectra.

In addition, we also performed ex situ FT–IR spectraafter CV measurements for the same sample mentioned

Ž . Ž . Ž .above, as shown in Fig. 6 at a 4.2 V, b 4.6 V and c5.0 V. Two absorption peaks were visible at 2848 cmy1

and 2920 cmy1, which are attributable to –CH – symmet-2w xric and asymmetric stretching vibrations, respectively 13 .

These two modes of vibrations are considered to be due tosurface film formation in the ring-opening reaction of

w xpropylene carbonate on the LiCoO electrode 9,11,12 .2

The surface film may consist of functional –CH – groups,2

as well as –CH , )CsO, O–CsO, and C–O–C3w x9,11,12 . Unfortunately, SEM observation cannot distin-guish between surface film formation on the LiCoO elec-2

trodes and decomposition of the electrolyte.We have shown that an increase in background signal

level due to fluorescence in in situ Raman spectra occurs

Ž . Ž . Ž .Fig. 6. Ex situ FT–IR spectra of LiCoO electrode on Au after CV with different reversal potentials: a 4.2 V; b 4.6 V; and c 5.0 V.2

( )T. Itoh et al.rElectrochemistry Communications 2 2000 743–748748

in a potential range of 4.7 V and 5.0 V. The backgroundwas not seen to be dependent on potential, which indicatesthat the film on the LiCoO is stable during the cathodic2

potential scan. It is also feasible that gas evolution occursas a result of decomposition of the organic solution in thispotential region. We believe that gas production did notoccur in our experiments. In situ Raman spectroscopy isthe best method for obtaining information on the interfacebetween electrodes and electrolytes.

5. Conclusions

In this study, we report the in situ Raman spectra forLiCoO electrodes in 1 M PCqEC organic solution, in a2

potential range between 3.2 V and 5.0 V. At high positivepotentials, between 4.7 V and 5.0 V, the background of thespectra increases suddenly and then remains stable duringthe cathodic scan down to 3.0 V. These results are associ-ated with surface film formation due to the ring-openingreactions of PC on the LiCoO electrodes over this poten-2

tial range.

Acknowledgements

The present work was partly supported by Grants-in-AidŽfor Priority Area A No. 11118213 for ‘Electrochemistry

of Ordered Interfaces’ and No. 12022208 for ‘New Pro-.tium Function’ and Encouragement of Young Scientists

Ž .No. 12750724 , from the Ministry of Education, Science,Sports and Culture, Japan. We wish to thank Dr. MashioShibuya for operating the FT–Raman and FT–IR spec-trometer.

References

w x Ž .1 M.S. Whittingham, J. Electrochem. Soc. 123 1976 315.w x Ž .2 I. Uchida, H. Sato, J. Electrochem. Soc. 142 1995 139.w x Ž .3 J.N. Reimers, J.R. Dahn, J. Electrochem. Soc. 139 1993 2091.w x4 T. Ozuku, A. Ueda, M. Nagayama, Y. Iwakoshi, H. Komori,

Ž .Electrochem. Acta 38 1993 1159.w x Ž .5 T. Ozuku, A. Ueda, J. Electrochem. Soc. 141 1994 2972.w x6 M. Inaba, Y. Todzuka, H. Yoshida, Y. Grincourt, A. Tasaka, Y.

Ž .Tomida, Z. Ogumi, Chem. Lett. 1995 889.w x7 T. Itoh, H. Sato, T. Nishina, T. Matsue, I. Uchida, J. Power Sources

Ž .68 1997 333.w x8 M. Inaba, Y. Iriyama, Z. Ogumi, Y. Todozuka, A. Tasaka, J. Raman

Ž .Spectrosc. 28 1997 613.w x9 K. Kanamura, S. Toriyama, S. Shiraishi, M. Ohashi, Z. Takehara, J.

Ž .Electroanal. Chem. 419 1996 77.w x10 M. Shibuya, T. Nishina, T. Matsue, I. Uchida, J. Electrochem. Soc.

Ž .143 1996 3157.w x Ž .11 P. Novak, W. Vielstich, J. Electrochem. Soc. 137 1990 1681.w x Ž .12 L. Kavan, P. Krtil, M. Gratzel, J. Electroanal. Chem. 373 1994

123.w x13 N.B. Colthup, L.H. Daly, S.E. Wiberley, Introduction to Infrared

and Raman Spectroscopy, 3rd ed., Academic Press, San Diego 1990.