Volume 55, Number 9, 2001 APPLIED SPECTROSCOPY 11310003-7028 / 01 / 5509-1131$2.00 / 0q 2001 Society for Applied Spectroscopy

Raman Microscopy Applied to Rechargeable Lithium-IonCells—Steps Towards in Situ Raman Imaging withIncreased Optical Ef� ciency

JAN-CHRISTOPH PANITZ,* PETR NOVAK,† and OTTO HAASPaul Scherrer Institute, Laboratory for Electrochemistry, CH-5232 Villigen PSI, Switzerland

Important improvements have been made in an in situ Raman celldeveloped to monitor the processes of lithium intercalation into thecarbon and metal oxide materials used in lithium-ion cells. By re-ducing the electrolyte gap in the cell and changing the optical ar-rangement, the signal-to-noise ratio could be improved by a factorof about 20. The optimized cell gives optical access to either elec-trode (anode or cathode) employed in commercial lithium-ion bat-teries. The optical ef� ciency is such that Raman maps of electrodesurface segments can be recorded under in situ conditions with anacquisition time of about 30 s per spectrum.

Index Headings: Raman microscopy; Lithium-ion batteries; Carbon;Lithium insertion; Electrolyte.

INTRODUCTION

Rechargeable lithium-ion batteries have rapidly in-creased their market share in recent years. Their majorapplications are in the � eld of portable electronic devices,notably mobile phones and laptop computers. The advan-tage of these batteries is an attractive speci� c energycombined with a high number of charge/discharge cy-cles.1,2 The commercial success of this type of battery hasaroused the interest of a large number of investigators,and a gamut of electroanalytical techniques and other insitu methods has been mobilized in order to gain a betterunderstanding of the processes occurring in rechargeablelithium-ion batteries.

Raman spectroscopy is an excellent technique for thecharacterization of carbonaceous materials.3–8 This meth-od has also been employed to study lithium intercalationinto carbons.8,9 More speci� cally, Raman microscopy hasbeen used to investigate the electrochemical processesand structural changes taking place in a lithium/polymerelectrolyte/vanadium oxide battery.10 Recently we report-ed investigations of lithium insertion into graphite usingRaman microscopy.11,12 With an in situ cell developed inour laboratory, it was possible to monitor the process ofintercalation (and de-intercalation) of lithium into a singleparticle on a model negative electrode (anode) made ofsynthetic graphite.11 With this cell we also demonstratedthe feasibility of Raman microscopy as a tool to inves-tigate the lithium insertion process into single particlesof the oxide material in positive electrodes (cathodes).13

The original in situ cell exhibited satisfactory electro-chemical performance, but its optical ef� ciency had to befurther improved in order to establish Raman microscopy

Received 28 November 2000; accepted 1 May 2001.* Present address: Chemetall GmbH, Trakehner Strasse 3, DE-60487

Frankfurt am Main, Germany.† Author to whom correspondence should be sent.

as a standard tool for in situ investigations. In addition,the excess electrolyte volume used in the Raman cell hadto be adapted (reduced) to that employed in today’s lith-ium-ion batteries. In this contribution, we report on theprogress made during optimization of the in situ cell. Anew setup is described, and � rst results obtained whenstudying a commercial anode sheet are discussed.

EXPERIMENTAL

The schematic of the new in situ cell is shown in Fig.1. In this new design, the thickness of the electrolytelayer is reduced from ;0.5 mm in the previous design11

to ;0.2 mm, and the thickness of the optical window isreduced from 1 to 0.1 mm. The cell design provides op-tical access to either electrode of a lithium-ion cell. Inthe following, the arrangement for optical access to thenegative electrode (anode) is described in detail. For theoptical access to the positive electrode (cathode), the ar-rangement of the electrodes must be swapped and thematerial for the base must be aluminum instead of copper.

Commercial electrode sheets with a charge capacity of;2.8 mAh/cm 2 were used for the measurements. The an-ode mass composite of this particular electrode type con-sisted of carbon particles [pyrolytic carbon, graphitized,and mesoporous carbon microbeads (MCMB)] and abinder, coated on a copper current collector. The anodesheet was � xed on a copper base (Fig. 1) using a con-ductive adhesive [� ne graphite particles dispersed in po-lyisobutene (Oppanol B200, BASF, Germany)], and cov-ered with a commercial separator sheet (Celgard). In sim-ilar fashion, the cathode sheet (containing LiCoO2 as theelectroactive component of the composite coated on analuminum current collector) was covered with anotherseparator. The cathode and the two separator sheets fea-ture small apertures (about 1 mm diameter) at suitablepositions on top of each other (Fig. 1). Such small ap-ertures do not in� uence the electrochemistry in the regionsigni� cantly because only the current density distributionon the electrode surface is slightly distorted. The cathode/separator assembly was � xed with an electrochemicallyinert adhesive (Oppanol B200 dissolved in petroleumether) onto a standard microscope cover glass (thickness,0.1 mm). After drying both anode and cathode pre-assembled components for 24 h at 80 8C and 0.2 mbar,the components were brought together in a glove boxunder argon into an arrangement as shown in Fig. 1, and20 mL of an electrolyte salt solution in a 1:1 w/w mixtureof ethylene carbonate (EC, Merck Selectipur) and di-methyl carbonate (DMC, Merck Selectipur) were addedper square centimeter of each electrode surface. From

1132 Volume 55, Number 9, 2001

FIG. 1. Sketch (not in scale) of the new electrochemical cell for in situRaman microscopy.

earlier experiments we know that the most suitable lith-ium salt for Raman spectroscopic investigations is lithiumperchlorate (LiClO4), which was used (in Merck Selec-tipur grade) in a concentration of 1 M. A commercialpolymer adhesive based on methyl methacrylate (Angst& P� ster, Switzerland) was then applied in order to sealthe in situ cell. During the sealing of the cell a temporarybase of magnetic steel was used, which allowed the com-ponents to be kept in place with a permanent magnet.

The sealed cell was placed under the microscope ofthe Raman system in the same manner as described pre-viously.11 In the � rst cycle, the cell was charged to 3.8V at a rate of øC/30.‡ The rate was then increased toøC/10 and the cell charged to 4.2 V. Subsequently, thecell was cycled between voltage limits of 3.0 and 4.2 Vat the øC/10 rate.

A confocal Raman microscope (Labram, DILOR/In-struments S.A.) was used for acquisition of the Ramanspectra. The 530.9 nm line emitted by an external Kr1

ion laser was used for excitation of Raman spectra in thewavenumber range of 700 to 2040 cm21. Raman mapswere obtained with a lateral resolution of 8 mm whenusing a microscope objective with 503 magni� cation andextended working length, and a pinhole diameter of 400mm. Each point represents a Raman spectrum with aspectral resolution of 4 cm21. To avoid the damage in-� icted by irradiation with high power densities, the laserpower at the sample was limited to 350–500 mW. Excep-tionally, a power of 2.5 mW was employed to record aRaman map prior to the charge/discharge cycles. Gen-erally, the electrodes were found to be more sensitive toincident laser power while the electrochemical process(charge, discharge) occurred. The Raman band positionswere calibrated by recording the line spectrum of a neonlamp (PenRay, Oriel).

In the following, all spectra are shown as raw data.The signal-to-noise ratio [de� ned as maximum bandmagnitude divided by the standard deviation of the back-ground in an off-band spectral region (2000–2025 cm21)]achieved for the G-band of carbon (at ;1580 cm21) was

‡ In the technical literature the nominal charge capacity of a battery oran electrode is called C (in our case: C ø 3 mAh/cm 2). Thus, ourelectrode will theoretically deliver a current of 3 milliamps per cm 2

geometrical area for one hour. The ‘‘C/30’’ rate means ‘‘full nominalcharge capacity in 30 hours’’ or, in this example, 0.1 mA/cm 2 currentdensity.

compared with data published previously11 after process-ing of our spectra with the Dilor software package ac-cording to the following procedure. First, the backgroundwas subtracted using a fourth-order polynomial, and thena � ltration with a moving-average � lter (symmetric maskof three data points) was performed. Selected spectrawere analyzed in addition by non-linear least-squares � t-ting procedures using Voigt pro� les, as described in detailin Ref. 14. The integrated band areas obtained by thisanalysis were used to calculate intensity ratios.

RESULTS AND DISCUSSION

Both the electrochemical and optical performance ofthe new cell have been tested in a number of experiments.In the following, a few typical results are discussed indetail.

The charge/discharge curves recorded in the in situ cellare in good agreement with the charge/discharge curvesmeasured for identical electrodes in a practical lithium-ion battery. However, we noticed that the in situ cell hasa limited useful lifetime of about one week (after whichthe thin optical window cracks due to increased internalpressure caused by gas evolution and/or electrode expan-sion during cycling). Thus, a detailed investigation of theelectrodes with optical methods is possible during the� rst few cycles. Future efforts should be directed towardsimproving the long-term stability of the cell.

The Raman spectra discussed below were recordedwithin two days after cell assembly and refer to the � rsttwo electrochemical cycles of the cell. It should be point-ed out that the in situ cell presented here is useful formore than investigations by spectroscopical methods. Ex-periments using optical microscopy can also be carriedout to study the morphological changes taking place dur-ing charge/discharge processes.

Soon after cell assembly, an arbitrarily selected area ofthe anode was � rst examined in situ using Raman mi-croscopy. A Raman map was measured at open circuiton an area of 80 mm 3 80 mm that contained 12 3 12individual spectra. Figure 2 shows this Raman map, con-structed from the intensity ratio I(EC)/I(G) of the bandareas due to the Raman signals of EC (at ;900 cm21),divided by the band areas of the G-band area of carbonspecies (at ;1580 cm21). Lighter areas in the image cor-respond to a stronger signal from the electrolyte. Theminimum and maximum values of the intensity ratioI(EC)/I(G) are 0.01 and 2.82, respectively, which meansthat large variation in electrolyte distribution exists at dif-ferent locations on the electrode, at least during the � rstfew hours after wetting of the electrode with the EC/DMC/LiClO4 solution. The medium surface roughness ofthe electrode as measured with a Dectac Pro� lometer is;6 6 2 mm. This roughness is comparable with the lat-eral resolution of 8 mm of the Raman spectra but is lowerthan the estimated axial resolution of .12 mm. However,it appears that the variations we see in the band ratioscorrelate with the electrode morphology (Fig. 2, top).Channels containing larger amounts of electrolyte can beidenti� ed in the video micrograph of the electrode sur-face. The Raman signals from the electrolyte in the cor-responding spectra are strong. The individual carbon par-ticles in the electrode typically have a size from 10 to 20

APPLIED SPECTROSCOPY 1133

FIG. 2. (Top) Video micrograph of the region investigated on the an-ode surface at open circuit. (Bottom) The corresponding Raman maprecorded at open circuit prior to the charge/discharge cycles with anexcitation power of 2.5 mW and 45 s integration time, and showing thedistribution of electrolyte over a surface of 80 mm 3 80 mm. Thisdistribution is described in terms of the ratio de� ned by dividing theintensity sampled in the wavenumber range 885–905 cm21 (bands as-signed to ethylene carbonate) by the intensity recorded for the interval1550–1610 cm21 (signals due to carbon). The minimum value of theratio is shown in black, the maximum in white. The letters indicate thelocations where the individual spectra shown in Fig. 3 were recorded.

FIG. 3. Individual spectra taken from the Raman map shown in Fig.2. Trace A features strong bands due to the carbon, while trace B isdominated by the signal of the electrolyte component. The inset in traceA shows the small signal of a Raman band assigned to a vibrationalmode of EC.

mm and are larger than the lateral resolution of the Ramanspectra. The surface of the carbon particles is normallycloser to the spectroscopic window and, therefore, lowervalues of the calculated intensity ratios are observed. Thedifference between these two extreme points can be il-lustrated by comparing the spectra recorded at the twopositions A and B, respectively (Fig. 3). In trace A, sig-nals due mainly to carbon are observed, while trace Bexhibits electrolyte and carbon Raman bands with com-parable intensities. To come to the point, we attribute theobserved spectral differences to morphology of the elec-trode; however, they could also arise from local variations

in porosity of the carbon materials used in that particularelectrode.

It must be emphasized that relative to our earlierwork,11 major progress was achieved in the signal-to-noise ratio of useful bands representing the surface of theelectrode. In the spectra reported in Ref. 11, the intensi-ties of solvent bands were always much larger than thoseof the bands due to carbon (cf. especially Fig. 3 in Ref.11). This is due to the fact that each point of the electrodesurface examined in the earlier in situ cell was observedthrough a column of excess electrolyte solution presentbetween the optical window and the electrode. This is adisadvantage because pertinent spectral informationcomes from the electrode/electrolyte interface. The Ra-man bands of the electrode materials (in the example dis-cussed, carbon) recorded with the in situ cell describedin this article have a much higher intensity because theamount of intervening electrolyte was minimized yet wasstill suf� cient to guarantee a good electrochemical per-formance.

The spectra obtained with the new cell for an arbitrari-ly selected particle of pyrolytic carbon were comparedwith earlier data obtained for a particle of graphite SFG44 (TIMCAL Group, Bodio)11 in order to compare thesignal-to-noise ratios (S/N) achieved with both cells. The

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TABLE I. Comparison of signal-to-noise ratios (S/N) achieved.

ParameterNew in situ

cellIn situ cell

from Ref. 11Improvement

factora

Laser power/mWIntegration time/sBand magnitude/a.u.Standard deviation of

off-band intensityS/NS/N/(mWs)0.5

0.430

245

5.842

3240230

13.517

(7.5)0.5

(8)0.5

2.519

a For shot-noise limitation of the detector used, both the laser powerand the integration time are compared as square roots.

FIG. 5. Sequence of Raman spectra recorded at one arbitrary locationon the anode during the � rst galvanostatic charge over the time rangefrom 0 to 7.5 h. The corresponding cell voltage is indicated in the � gure.The spectra were normalized to unit magnitude. Note the decrease inintensity of the bands assigned to the electrolyte that occurs as chargingproceeds. Acquisition time for each spectrum is 60 s.

FIG. 4. Comparison of in situ Raman spectra measured at the surfaceof a graphite electrode at an electrode potential of ;0.2 V vs. Li/Li1.(Top) new in situ cell; (bottom) old in situ cell described in Ref. 11.

results are summarized in Table I. The improvement isvisualized in Fig. 4 where in situ Raman spectra for par-tially lithiated graphite, LixC6, obtained with both cellsare compared. Since the spectra were recorded with thesame Raman microscope and with identical settings oflaser wavelength and spectrograph position, we may sup-pose that the characteristics of the instrument are com-parable for both setups. Based on the de� nition of the S/N ratio as the peak height divided by the standard devi-ation of the background, it turns out that with the newdesign of the in situ cell, a substantial improvement insignal-to-noise ratio (a factor of 19) was achieved.

The traces shown in Fig. 5A–5F, illustrate the changesobserved in the in situ Raman spectra upon lithium in-sertion into the anode material. The spectra were recordedat the same location on the electrode surface during the� rst charging half-cycle. To facilitate their comparison,they were normalized with respect to the maximum Ra-man signal intensity. In the following, both the timeelapsed since the start of the charge and the cell voltageat which the spectrum was recorded are given for eachspectrum shown, along with a description of the salientfeatures.

During charging of a fresh cell, the voltage rises rathersteeply until a plateau is reached at about 3800 mV. Therecording of spectrum A was started 5 s after switchingon the charging current, and covers the cell voltage in-terval from 0 to 1800 mV.§ In this spectrum, both the D-band (at ;1300 cm21) and the G-band (at ;1600 cm21)of the carbon material are clearly visible. From the in-tensity ratio of these bands, we can estimate the corre-lation length La at this particular point on the electrodeby use of the Tuinstra–Koenig relation.3 A value of 38nm is obtained, implying the presence of a well-graphi-tized carbon material. In addition, bands that originatefrom the electrolyte compounds are clearly discernible.

The spectrum shown in Fig. 5B, recorded after 3400 sat a cell voltage of 3200 mV, is very similar to the oneshown in Fig. 5A, but changes in the intensity ratio R 5I(EC, free)/I(EC, coord.) are noted. This intensity ratio iscalculated using the magnitudes of bands attributed to then7-mode of free EC (891 cm21) and EC coordinated withLi1 (902 cm21).15 In the spectrum shown in Fig. 5C, re-

§ The cell voltage increased rapidly from 0 to 1800 mV during thecollection of the spectrum. The rapid change in the cell voltage meansthat no considerable electrochemical reactions proceed in this voltageinterval.

APPLIED SPECTROSCOPY 1135

TABLE II. Band positions and linewidths (both in cm21) calculat-ed for the spectra included in Fig. 5. Only data for the G-band(E2g

2-mode of graphite) are reported. E2g2(b) and E2g

2(i) denote theE2g

2-modes observed in graphene layers of intercalated graphite forgraphene layers adjacent to the intercalated species (E2g

2(b)) and forgraphene layers adjacent to other graphene sheets (E2g

2(i)). Line-widths are given in square brackets.

Spectrum E2g2 E2g

2(i) E2g2(b)

A

B

C

D

E

F

1574.0[17.4]1574.1[17.6]1577.7[11.7]1580.3[12.4]

1566.2[n.a.]

1567.7[11.4]

1586.0[31.2]1592.8[30.7]

n.a.: the linewidth could not be determined.

FIG. 6. (Top) Intensity (band area) calculated for the vibrational modes assigned to free ethylene carbonate (891 cm21), open circles, and thevibrational mode assigned to ethylene carbonate coordinated with lithium ion (905 cm21), open squares. (Bottom) Intensity ratio R 5 I(EC, free)/I(EC, coord.) of the bands recorded for ethylene carbonate during charging, plotted against time elapsed. The time scale is the same in the twographs.

corded after 8700 s at a cell voltage of 3490 mV, it isobserved that the ratios of intensities of the Raman bandsof the electrolyte to the intensities of the bands due tothe carbon material decrease. Also, we note that the in-

tensity of the D-band decreases. The same observationsapply to the spectra D, E, and F recorded subsequently.After 11 730 s, at a voltage of 3590 mV, a sloping back-ground is observed and the D-band of carbon can nolonger be detected (cf. Fig. 5D). At a voltage of 3730mV, which is attained after 18 800 s, the G-band pro� lechanges. A shoulder at a lower wavenumber is observed,in agreement with the observations previously report-ed.9,11 In the last spectrum (Fig. 5F) included in this set,which was recorded at a cell voltage of 3765 mV after26 500 s, both components of the G-band are clearly dis-cernible.

By non-linear least-squares analysis, one can obtain theband positions and the linewidths of the components ofthe band pro� le. These data are compiled in Table II.From an inspection of the values reported in Table II, we� nd that the results obtained are in good agreement withthe values reported in the literature. We note that the G-band shifts to higher wavenumbers upon lithium inter-calation and that at cell voltages of about 3700 mV andhigher, a split in two components is observed, namely theE2g

2(b) and E2g2(i) modes. It is interesting to note that the

G-band pro� le shown in Fig. 5E, cannot be approximatedsatisfactorily using the Voigt pro� les employed in the

1136 Volume 55, Number 9, 2001

FIG. 7. Video micrograph of the region investigated on the anode surface while charging in the plateau region with almost constant voltage (3840mV). Typical individual spectra are shown on the right-hand side, with arrows pointing to the location from which the data were obtained.

standard analysis method. The line pro� le is V-shaped;hence only the band position and an estimate of linewidthof its major component have been included in Table II.

Figure 6 presents a detailed view of the changes ob-served upon charging in the band pro� le assigned to theelectrolyte. In the top graph, the intensities recorded forthe two bands assigned to the EC component are givenfor the data shown in Fig. 5. It can be seen that both theband of free EC and that of EC coordinated with lithiumions decrease in intensity but subtle differences betweenthe two components are present. These differences arevisualized in the bottom graph, which shows the intensityratio R 5 I(EC, free)/I(EC, coord.) on the same timescale. It turns out that after the start of the charging, theintensity ratio R rapidly increases to a constant value ofabout 1.0, but it later decreases. Unfortunately, calcula-tions of the intensity ratios R become ambiguous fortimes . 30 000 s because in our experiments the intensityof the electrolyte bands then decreases so much that areliable calculation is no longer possible. Therefore, aninterpretation is attempted only for the data points pre-sented in Fig. 6. Under our experimental conditions, onlya limited amount of electrolyte solution is available.Thus, when the current has been switched on, it seemsthat an equilibrium concentration of lithium ions at theelectrode/electrolyte interface develops on a timescale ofa few minutes. Later, with the start of bulk lithium in-sertion at cell voltages higher than 3600 mV and after

the formation of the solid electrolyte interphase (SEI), theintensity ratio R drops, possibly because of kinetic ef-fects. These observations demonstrate that Raman mi-croscopy is able to provide very detailed information onthe processes involved in practical electrode materials.

So far we have discussed observations concerning theelectrolyte at the interface. The Raman technique is alsoable to identify the different types of carbon materialsused, as demonstrated in the following for the exampleof a Raman map (5 3 5 points) measured at a cell voltageof 3840 mV. Figure 7 shows a video micrograph recordedat the area under investigation, together with two spectraselected from the map obtained at the locations indicatedby the arrows. The bright carbon particle marked by ar-row A features a Raman spectrum where the split of theG-band is clearly observable. However, only a singleband is observed in the spectrum recorded from the dark-er particle indicated by arrow B. This is in agreementwith previous reports, according to which the split of theG-band upon lithium insertion is only observable for wellgraphitized materials.9 From the video micrograph, weinfer that spectrum A refers to a particle of pyrolytic car-bon, and spectrum B is that associated with an MCMBparticle.

CONCLUSIONThe spectro-electrochemical setup described in this ar-

ticle constitutes a signi� cant improvement in optical ef-

APPLIED SPECTROSCOPY 1137

� ciency of the in situ Raman sampling process from lith-ium-ion battery electrodes. A direct comparison of per-formance of the new cell with that of the design reportedpreviously from our laboratory shows that the signal-to-noise ratio has been raised substantially. The electro-chemical properties of the present design admit continu-ous observations over a period of at least several days,which is suf� cient for most spectroscopic experiments.In addition, the design of the new cell can be used as abasis for standardizing the method, which is required inorder to make Raman microscopy a standard character-ization tool in the � eld of rechargeable lithium-ion bat-teries.

ACKNOWLEDGMENTS

We are grateful for some experiments performed by Martin Weber(PSI), for the technical support of Christian Marmy (PSI), and for help-ful comments made by Dr. Felix Joho, Dietrich Goers (both PSI), andDr. Ivan Exnar (Renata AG, Itingen). The valuable suggestions of bothreferees are gratefully appreciated. The � nancial support of the KTIorganization (Bern, Switzerland) is gratefully acknowledged.

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