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Journal of Electroanalytical Chemistry 516 (2001) 89–102

The effect of pressure on the electroanalytical response of graphiteanodes and LiCoO2 cathodes for Li-ion batteries

J.S. Gnanaraj, Yaron S. Cohen, M.D. Levi, D. Aurbach *Department of Chemistry, Bar-Ilan Uni�ersity, Ramat-Gan 52900, Israel

Received 12 May 2001; received in revised form 10 September 2001; accepted 11 September 2001

Abstract

The effect of the application of pressure during the preparation of composite flaky synthetic graphite anodes and LiCoO2

cathodes on their electrochemical behavior in Li insertion and de-insertion processes was studied using voltammetry, chronopoten-tiometry, electrochemical impedance spectroscopy (EIS), and ex situ AFM imaging. Unpressurized graphite electrodes reach ahigher capacity and have faster kinetics than the same electrodes compressed or rolled at 5×103 kg cm−3. In contrast, theperformance of rolled or compressed LiCoO2 electrodes in terms of capacity and kinetics was better than the performance of theunpressed electrodes. AFM imaging of pristine and cycled electrodes demonstrated a pronounced effect of pressure on themorphology of graphite electrodes, whereas the impact of pressure on the morphology of LiCoO2 electrodes was found to bemuch less pronounced. It was concluded that compressing graphite electrodes has an adverse effect on the contact between theactive mass and ions in solution, while compressing LiCoO2 electrodes does not adversely affect the contact between solutionspecies and the active mass, but rather, improves inter-particle electrical contact. © 2001 Elsevier Science B.V. All rights reserved.

Keywords: Li-ion batteries; Graphite; Transition metal oxide; Composite electrode; Li-ion conductivity; Impedance spectroscopy; Surface films

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1. Introduction

One of the most important successes of the electro-chemistry community in recent years is the commercial-ization of lithium-ion batteries in which the anode ismade of graphitic carbon and the cathode material isLiCoO2 [1]. Focusing attention on the anodes them-selves, there have been several kinds of graphitic mate-rials that were studied as Li insertion anodes forpractical battery systems. These include naturalgraphite (usually appearing as a flaky powder) [2],synthetic graphite flakes [3], mesocarbon microbeads(MCMB) [4], and carbon fibers [5]. Although disor-dered carbons may insert lithium at much higher capac-ities than graphitic carbons (i.e. �372 mA h g−1) [6,7],the latter materials seem to be more useful to date asanode materials in practical batteries due to factorssuch as cost, reproducible electrochemical behavior,

and the low, flat potential profile of Li intercalation–deintercalation in galvanostatic processes (which en-sures constant and high voltage of operation for Li-ionbatteries with graphite anodes) [8].

On the other hand, there are some disadvantages tothe use of graphite as an anode material for Li-ionbatteries. Since the forces between the graphene planes,from which graphite is composed, are relatively weak,cointercalation of solvent molecules, together withlithium ions, can lead to exfoliation of the graphiteparticles, and, hence, to their destruction [9]. In addi-tion, reduction of solvent molecules between grapheneplanes that form gas molecules (e.g. as in the case ofalkyl carbonate solvents) can easily crack the graphiteparticles as well [10]. Consequently, the stability andreversibility of graphite electrodes in repeated Li inser-tion–deinsertion cycling depend very strongly on passi-vation phenomena. When graphite (or any othercarbon or noble metal) electrode is polarized cathodi-cally in any polar aprotic solvent containing a Li salt,reduction of solvent molecules, trace O2 and H2O con-tamination and contamination by many commonly

* Corresponding author. Tel.: +972-3-532-6309; fax: +972-3-535-1250.

E-mail address: [email protected] (D. Aurbach).

0022-0728/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved.PII: S 0 0 2 2 -0728 (01 )00663 -5

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J.S. Gnanaraj et al. / Journal of Electroanalytical Chemistry 516 (2001) 89–10290

used salt anions are unavoidable [11]. Reduction ofmost of the solvent molecules, such as ethers, alkylcarbonates, esters, etc., H2O, O2, and salt anions suchas ClO4

−, AsF6−, PF6

−, etc., in the presence of Li ions,forms insoluble Li salts which precipitate on the elec-trode as surface films. Their adhesion to the electrodesurface and their cohesion determine the passivationproperties of these surface films. Li salts, both organicand inorganic, precipitated as thin surface films, canusually conduct Li-ions under an electrical field (ac-cording to the SEI model, demonstrated more thantwenty years ago by Peled [12]). Hence, when thin,compact, and passivating surface films are formed ongraphite electrodes at potentials higher than the poten-tials at which Li intercalation (and thus, solventcointercalation) takes place, the graphite particles maybe well protected from detrimental destructive pro-cesses, and hence behave reversibly in repeated charge–discharge cycling (Li insertion–deinsertion, respec-tively) [13]. For instance, it is commonly known that inelectrolyte solutions containing ethylene carbonate(EC), graphite electrodes behave highly reversibly be-cause of their efficient passivation due to the formationof surface films comprised of the EC reduction prod-ucts (e.g. (CH2OCO2Li)2) [14].

Li insertion processes into graphite electrodes havebeen rigorously studied in recent years [15]. The surfacechemistry of graphite electrodes has been extensivelystudied [10,11,13,14,16], and, in addition, the impact ofmany structural aspects such as the particle morphol-ogy [17], size [18], surface area [19], and phase composi-tion (e.g. ABAB phase vs. ABCABC phase [20]) on theperformance of graphite electrodes has been rigorouslystudied.

In parallel, we find in the literature many papersdealing with the performance of LiCoO2 electrodes interms of the synthetic sources [21–24] and structuralparameters such as morphology, particle size, surfacearea, etc. [25,26] We should also mention the rigorouselectroanalytical studies on both graphite and LiCoO2

electrodes that have revealed common features in themechanism of Li insertion into both materials (as theyare both layered compounds) [27,28]. It has also beenfound that, as in the case of graphite electrodes,LiCoO2 electrodes are also covered by surface filmsformed by interactions between the lithiated oxide andsolution species. Hence, in both cases (graphite,LiCoO2), Li insertion and deinsertion into the hostmaterials involve Li-ion migration into surface films[29].

In general, practical graphite anodes and LiCoO2

cathodes are composite electrodes in which the activemass particles are bound to a metallic current collectorwith a polymeric binder such as polyvinylidene difl-uoride (PVdF). In the case of graphite electrodes, thecurrent collector is a copper grid or foil. For LiCoO2

electrodes, the current collector is made of aluminumbecause of its natural passivation, which prevents itsdissolution at high potentials. In addition, the latterelectrodes have to contain a conductive additive, usu-ally carbon particles (e.g. carbon black, graphite, 5% byweight). These electrodes are usually prepared from aslurry of the particles and the binder in an organicsolvent, which is spread on the current collector, fol-lowed by drying. The final shape of the electrodes isobtained by applying some pressure to the electrode(either when it is dry, or to the slurry spread on thecurrent collector).

It should be emphasized that the electroanalyticalresponse of composite electrodes may be affected notjust by structural changes in the active mass duringpreparation. The orientation of the particle and inter-particle contact in composite electrodes may play amajor role in their electrochemical behavior.

This work is aimed at a study of the effect of pressureduring the preparation of graphite and LiCoO2 elec-trodes on their electrochemical behavior in Li insertionreactions. The electrolyte solution chosen was EC+DMC+LiAsF6, in which graphite and LiCoO2 elec-trodes behave highly reversibly [14], and the tools forthis study were fast and slow scan rate voltammetry,chronopotentiometry, impedance spectroscopy (EIS),and atomic force microscopy (AFM) for the morpho-logical studies. The effect of pressure on the active massstructure was studied by X-ray diffraction (XRD) ofboth electrodes and powders.

2. Experimental

The anodes were composed of graphite flakes (KS-6)from Timcal Inc. (average particle size ca. 6 �m, 90wt%), PVDF binder (10 wt%) from Solvey Inc., andcopper foil current collectors. The cathodes were com-prised of LiCoO2 powder from Merck KGaA (particlesize 5–10 �m, 80 wt%), 15 wt% of graphite powderKS-6 (Timcal) as a conductive additive, 5 wt% ofPVDF, and an aluminum foil current collector. Slurriescontaining the active mass and the binder were pre-pared using N-methyl pyrrolidone (Fluka Inc.). Theappropriate slurry was uniformly spread onto bothsides of the current collectors (1 cm2 surface area). Thefoils were preliminarily heated in an oven at 100 °C.After coating, the electrodes were dried in an oven at140 °C.

In order to obtain a high electroanalytical resolution,relatively thin electrodes (several micrometers thick)were prepared.

Three kinds of graphite and LiCoO2 electrodes weretested:1. As-prepared electrodes, which were not subjected to

any additional compression (which are termed ‘un-pressed electrodes’).

J.S. Gnanaraj et al. / Journal of Electroanalytical Chemistry 516 (2001) 89–102 91

2. Electrodes compressed at 5×103 kg cm−2 using ahydraulic press, (which are termed ‘pressedelectrodes’).

3. Electrodes compressed by a rolling machine (be-tween iron wheels). The exact pressure applied tothe electrodes when they are rolled could not bemeasured precisely. The pressure is estimated to bein the order of a few tons per square centimeter, andis the same for both the anodes and cathodes thusprepared. The average active mass of the anodesand the cathodes was 2.8 mg cm−2, respectively.

For both anode and cathode testing, Li foil counterelectrodes and Li wire reference electrodes were used.The cells made of polyethylene frames and vesselsprovided a parallel plate configuration for the workingand counter electrodes. We used solutions of 1 MLiAsF6 (FMC Inc.) in a 1:1 (by volume) mixture of ECand DMC, Merck KGaA (Li battery grade). The watercontent in the solutions was around 20 ppm.

The cells were assembled in a glove box (M. BrownGmbH Inert Gas System) filled with highly pure argon(oxygen and water contamination did not exceed 1ppm). In order to ensure impregnation of the entireporous active mass of the electrodes with the solution,the electrodes were evacuated in a special glass vessel.The solution kept under atmospheric pressure was theninjected into the vessel, thus impregnating the evacu-ated electrode. The three-electrode cells were measuredelectrochemically out of the glove box using speciallydesigned hermetically sealed aluminum boxes, ther-mostated at 25�0.2 °C.

Freshly prepared graphite electrodes usually had anopen circuit potential of 3.3 V (vs. Li/Li+). They were

aged by voltammetric cycling between 3.3 and 0 V (vs.Li/Li+) at �=0.5 mV s−1 (three cycles). Li-ion interca-lation–deintercalation processes were then studied inthe potential range between 0.3 and 0 V (vs. Li/Li+) byslow scan rate voltammetry (SSCV) and impedancespectroscopy (EIS).

Freshly prepared thin LiCoO2 electrodes with opencircuit voltage (OCV) values of about 3.0 V (vs. Li/Li+)were initially cycled three times (voltammetry) between3.5 and 4.25 V (vs. Li/Li+) at �=0.5 mV s−1 beforethe electrochemical measurements.

Impedance spectra were measured within the wholerange of Li-intercalation under OCV conditions at pre-defined potentials, while the electrodes were at equi-librium. The potentials were changed in steps with apolarization time of about 2 h at each step (in order toreach an equilibrium within the thin electrode understudy). The alternating voltage amplitude was 3 mV,and the frequency range was between 100 000 Hz and 5mHz. In addition, the electrodes were tested by pro-longed galvanostatic cycling at C/10–C/20 rates.

For voltammetric and galvanostatic measurementsan Arbin computerized multichannel battery tester wasused. EIS was performed using a Solarton 1286 electro-chemical interface and a 1255 frequency response ana-lyzer driven by the CORRWARE software from ScribnerAssoc. (USA) with a Pentium PC.

AFM measurements of graphite and LiCoO2 elec-trodes were carried out ex situ under an argon atmo-sphere using standard equipment (microscope and cells)from Topometrix inc. (Discoverer Model 2010). XRDmeasurements of electrodes and powders were per-formed using the AD8 system from Bruker Inc.(Germany).

3. Results and discussion

3.1. Graphite electrodes

Fig. 1 shows the first and second (a and b, respec-tively) cyclic voltammograms (0.5 mV s−1) of the threetypes of graphite electrodes studied in this work: un-pressed, pressed, and rolled electrodes (solid, dashed,and dotted lines, respectively). The voltammograms ofthe three electrodes are generally similar:1. The first cathodic polarization (Fig. 1a) shows a

broad, irreversible cathodic wave around 1.5–0.8 V(Li/Li+), which relates to surface film formation onthe electrodes due to reduction processes of both thesolvent and the salt anion.

2. The cathodic currents at potentials below 0.35 Vrelate to Li insertion into graphite, to which theanodic peaks around 0.3 V correspond (Lideinsertion).

Fig. 1. First and second (a and b, respectively) cyclic voltammogramsmeasured at a scan rate of 0.5 mV s−1 with unpressed (solid line),pressed (dashed line), and rolled (dotted line) graphite electrodes. Theelectrode area was 1 cm2, 2.5 mg cm−2.


Fig. 2. Families of consecutive cyclic voltammetric curves measuredat a scan rate of: (a) 50 �V s−1; (b) 15 �V s−1; and (c) 5 �V s−1,with unpressed (solid line), pressed (dashed line), and rolled (dottedline) graphite electrodes. The electrode area was 1 cm2, 2.5 mg cm−2.

deinsertion processes in the following order:Cunpressed�Cpressed�Crolled.

Fig. 2 shows steady-state cyclic voltammograms ofthe three electrodes measured at slow scan rates: 60, 15,and 5 �V s−1, a–c respectively, in the 0.3–0 V (Li/Li+)range, which is the potential window in which Li inter-calation–deintercalation into graphite takes place[9,10]. As the potential scan rate applied is slower, theinitially featureless voltammograms (at high potentialscan rate) become resolved and include four sets ofpeaks centered at 0.22, 0.17 (small), 0.125, and 0.08 V,related to the stage transitions (diluted I�IV, IV�III,III�II, and II�I, respectively) which characterize Liintercalation into graphite up to a stoichiometry ofLiC6. Hence, each set of peaks in the SSCV reflects aphase transition, and each single peak relates to thecoexistence of two phases [30].

Fig. 2 clearly demonstrates that the voltammogramsof the unpressed electrodes are much more resolved(sharper, well distinguished peaks) as compared withthe other two electrodes. The difference in the chargeinvolved in the processes of the three electrodes showsthe same trend as seen in Fig. 1, namely, Cunpressed�Cpressed�Crolled. The difference in the voltammetric be-havior of the three electrodes seen in Fig. 2 reflects thepronounced difference in the kinetics of their electro-chemical processes. We should also take into accountthe possibility that the lower charge involved in theprocesses of the pressurized electrodes reflects somedestruction of their active mass, due to the applicationof pressure.

Fig. 3 shows the effect of scan rate on the reversiblecapacity of the graphite electrodes in repeated voltam-metric charge–discharge cycling. From the curves inFig. 3, it is clear that the electrodes differ from eachother not only in their kinetics, but also in the utility oftheir active mass. The unpressed electrodes reach theoptimal capacity (�370 mA h) at sufficiently slow scanrates (in the �V s−1 range), while the compressed elec-trodes reach lower capacities, even at very low potentialscan rates (Fig. 3).

Useful information about the kinetic behavior of thevarious electrodes can be obtained by impedance mea-surements. Fig. 4(I–III) shows typical Nyquist plotsmeasured at five selected equilibrium potentials with thethree electrodes. In the first potential, 300 mV (Li/Li+),the electrodes are at the foot of Li intercalation (dilutedstage 1�stage 4). The other four potentials of thespectra in Fig. 4, at 200, 100, 50, and 10 mV (Li/Li+)relate to the major Li intercalation stage transitions(Fig. 2). All of the spectra in Fig. 4 include highfrequency and medium-low frequency semicircles that,in some cases, may merge with each other, thus forminga flat, broad semicircle. At the low frequencies, thespectra appear as a straight line, initially at 45° (typicalof a ‘Warburg’-type element [31]), and at the very low

Fig. 3. The specific capacity calculated from voltammetric studies.Unpressed, pressed, and rolled graphite electrodes as indicated in thefigure of the graphite electrodes as a function of scan rate.

3. The first and second CVs are basically very similar,except that the irreversible cathodic wave around1.5–0.8 V appears only in the first CV (hence, in thesecond consecutive cycle, the electrodes are fullypassivated).

However, the three electrodes differ remarkably fromeach other in the charge involved in the Li insertion–


frequencies (�50 mHz) they become steeper. As al-ready discussed in detail [30], the impedance spectra oflithiated graphite electrodes reflect the serial nature ofthe Li insertion process: the high frequency semicirclerelates to Li-ion migration through the surface films,the medium– low frequency semicircle reflects chargetransfer, the ‘Warburg’-type element reflects sold statediffusion of Li into the graphite bulk, whereas at thevery low frequencies, the Nyquist plot appears as asteep line and reflects the accumulation of Li into thebulk, and, hence, the capacitive behavior of the elec-trode (i.e. its potential-dependent differential capaci-tance). At the lowest frequency, as ��0, Cint=1/Z��

(Z�= the imaginary part of the spectra). In fact,impedance spectra were measured in this work at morethan 20 different potentials in the Li intercalation range

250–10 mV (Li/Li+). With the three electrodes, theshape of 1/�Z� at ��0 (5 mHz in this work) vs. E wasvery similar to the shape of the slow scan rate CVs seenin Fig. 2c (5 �V s−1), as is indeed expected. As seen inFig. 4, while the impedance spectra of the three elec-trodes show some general similarities, there are cleardifferences in the behavior of the three electrodes:1. In general, the impedance of the unpressed electrode

is considerably lower than that of the compressedelectrodes.

2. The impedance of the vertically pressed electrode isgenerally lower than that of the rolled electrode.

3. The medium frequency semicircle (attributed tocharge transfer) [30], is more distinctive and rela-tively more pronounced in the spectra of the com-pressed electrodes. There are also differences at the

Fig. 4. A family of Nyquist plots measured from (I) unpressed, (II) pressed, and (III) rolled graphite electrodes measured at five representativepotentials (a–e as indicated). The electrodes were equilibrated for at least 2 h at each potential before the measurements. The electrode area was1 cm2, 2.5 mg cm−2.

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Fig. 5. AFM pictures (measured ex situ) of unpressed, pressed and rolled graphite electrodes before the electrochemical treatment (a–c, respectively) and after prolonged charge–discharge cycling(d– f, respectively) in EC+DMC (1:1)+1 M LiAsF6 solutions. A height scale (by color) appears near each image. The average roughness factor (RF) is specified below each image.


Fig. 6. XRD patterns of the three types of graphite electrodes used in this study. Note the different scales in the panels.

very low frequencies between the spectra of theunpressed and compressed electrodes: the Z� vs. Z �curve, which is a straight line at the very lowfrequencies, is much steeper in the spectra of theunpressed electrodes as compared with the plotsobtained with the compressed ones. These differ-ences in the impedance response of these electrodesindicate that the application of pressure to them hasa detrimental impact on their kinetics, as also indi-cated in the voltammetric studies described above.

It should be noted that the low frequency behavior ofthe spectra in Fig. 4, namely, the differences in theslopes of the Z� vs. Z � curves at ��0, correlates wellwith the data provided in Fig. 3. The steep slope of theZ� vs. Z � plot of the unpressed electrode at ��0 andpotentials of full Li insertion (�50 mV vs. Li/Li+)indeed reflect a completed accumulation of lithium. Thevery low value of �Z�� at ��0 around 24 � for theunpressed electrode, as compared with 80 and 120 � forthe pressed and rolled electrodes, respectively, at lowpotentials (in which LiC6 should ideally be formed),clearly demonstrates the higher capacity of the un-pressed electrodes as compared with the compressedones (as Cint= (1/�)�Z��, ��0) [30].

Fig. 5 shows typical AFM images (measured ex situ)of the three electrodes in their pristine form (before theelectrochemical process) and after cycling. Image 5ademonstrates the disordered morphology of thepristine, unpressed electrode. Note that the dimensionsof the vertical (Z) axis are much smaller than thedimensions of the horizontal (X, Y) axes. Some bordersand gaps between micronic-size graphite particles are

clearly seen. The images of the pressed and rolledelectrodes (Fig. 5b and c) are quite different from thoseof the unpressed ones. The application of pressuredrastically changes the electrode morphology:1. It forces all of the particles to be oriented with the

basal planes parallel to the current collector.2. The particles are closely packed together, with no

visible gaps between them.3. Rolling the electrodes seems to force the graphite

flakes to slide on top of each other in a way thatmay close even further the gaps between the parti-cles in the composite electrode structure.

4. The application of pressure, either vertically with apress or with a rolling machine, also seems to influ-ence the 3D structure of the graphite particles, byintroducing defects which may have a detrimentaleffect on the capacity of the active mass (i.e. de-struction of sites for Li insertion).

5. As seen in the images of the cycled electrodes (Fig.5d– f), they are all covered by surface films. Thebasic morphology of the electrodes is retained aftercycling, as concluded by comparing images 5a–cwith 5d– f. In general, imaging cycled electrodesseems to indicate that their prolonged charge–dis-charge cycling in the EC+DMC solutions does notcause drastic visible damage to their active mass.

Fig. 6 shows XRD patterns of the three types ofgraphite electrodes used in this study. The patterns areidentical in the XRD peaks and their exact location,but differ in their intensity. The intensity of the XRDpeaks of the rolled electrode is the highest, the intensityof the unpressed electrode peaks is the lowest, and that


related to the pressed electrode in somewhere in be-tween. These data prove that the pressure has no effecton the particle structure, but rather, affects their orien-tation. As the particles are more oriented due to theeffect of pressure, with their basal planes parallel to thecurrent collector (and hence, to the plane of incidenceof the X-ray beam), their XRD peaks are indeed ex-pected to be more intense.

We believe that the morphological and structuralstudies described above may explain the difference inthe electrochemical behavior of the three types of

graphite electrodes. Application of pressure on graphiteelectrodes may have two adverse impacts:1. Compact packing of the graphite flakes with an

orientation of the basal planes in parallel to thecurrent collector causes problems of accessibility ofthe active mass to the solution. Li insertion intographite takes place in the particle facets, which areperpendicular to the basal planes (i.e. the edgefacets). Hence, when the particles are compactlypacked with very narrow gaps between them, thesolution cannot percolate between the particles andreach the entire active mass. Surface film formationon the closely packed particles may further preventgood interaction between the active mass and solu-tion species.

2. Compressing graphite particles may damage them,increasing a concentration of defects that may blockdiffusion of lithium into the active mass. Hence, thesoft, fragile structure of graphite particles should betaken into account whenever pressure has to beapplied in order to produce compact graphiteelectrodes.

It appears that application of pressure by rolling maybe more detrimental to the performance of graphiteelectrodes for Li insertion than vertical compression,because rolling the electrodes forces the particles toslide onto each other, thus efficiently closing possiblegaps between the particles. In addition, the possibledamage to the structure of the graphite particles byrolling them under pressure seems to be more pro-nounced than in the application of vertical pressure.

3.2. LiCoO2 electrodes

Fig. 7 shows selected steady state cyclic voltam-mograms of unpressed, pressed, and rolled LiCoO2

electrodes at three potential scan rates. As expected[27], when the potential scan rate is sufficiently low, Liinsertion–deinsertion into LiCoO2 appears as a pair ofcorresponding CV peaks that become narrower as thepotential scan rate decreases. As already explained[27,32], these peaks reflect Li accumulation and extrac-tion, which is accompanied by attractive interactionsbetween the Li insertion sites in the host materials. It isclearly demonstrated that the pressed and rolled elec-trodes behave similarly and show very good voltammet-ric resolution, while the resolution of thereduction–oxidation peaks of the unpressed electrode isvery poor and thus reflects very sluggish kinetics.

Fig. 8 shows the variation of the electrode capacity asa function of the potential scan rates in repeatedvoltammetric charge-discharge cycling. From the curvesin Fig. 8 it is clear that the pressed LiCoO2 electrodesalmost reach the ideal capacity of the active mass, andtheir kinetics are relatively fast, as can be concludedfrom the flat C vs. � curve (i.e. most of the ideal

Fig. 7. Families of consecutive voltammograms measured by cyclingat scan rates of: (a) 250 �V s−1; (b) 25 �V s−1; and (c) 10 �V s−1,with unpressed (solid line), pressed (dashed line), and rolled (dottedline) LiCoO2 electrodes. The electrode area was 1 cm2, 5 mg cm−2.

Fig. 8. The specific capacity calculated as a function of scan rate fromvoltammetric studies for the unpressed, pressed, and rolled LiCoO2

electrodes (indicated in the figure).


capacity can be reached even at relatively fast potentialscanning rates). As is seen in Fig. 8, the rolled elec-trode, at very slow potential scanning rates, reaches a

lower capacity of 125 mA g−1 as compared with 135mA g−1 for the pressed electrode). However, the effectof the scan rate is similar, to that for the pressed

Fig. 9. (A) A family of Nyquist plots measured from a pressed LiCoO2 electrode measured at five representative potentials (a–e as indicated). Theelectrode was equilibrated at each potential for at least 2 h before the measurements. The electrode area was 1 cm2, 5 mg cm−2. The inserts in(a) and (b) relate to the high frequency domain. (B) Same as (A), a rolled LiCoO2 electrode.


Fig. 10. The radii of the high and low frequency semicircles (denotedas Rfilm and RCT, respectively) as a function of potential, calculatedfor the Nyquist plots related to the pressed and rolled LiCoO2

electrodes (as indicated).

indicated. The impedance response of these two typesof electrodes as a function of potential and Li contentis very similar. In its lithiated state (�3.8 V), theNyquist plots show a small, high frequency semicircleand a large, low frequency arc. As delithiation pro-ceeds, a medium semicircle appears at potentials above3.8 V (Li/Li+) and becomes smaller as delithiationprogresses, reaching a steady size at potentials above 4V. In addition, a ‘Warburg’-type element characterizesthe spectra at low frequencies.

Fig. 10 shows the variations in the diameters of thetwo semicircles appearing in the Nyquist plots of thepressed and rolled electrodes, as a function of theirequilibrium potentials. As was already shown and dis-cussed in detail [32], the impedance behavior of thesetwo electrodes as presented in Figs. 9 and 10 is ex-pected, and reflects the serial nature of electrochemicalLi insertion–deinsertion processes into many host ma-terials (including graphite, as discussed above, andother LixMOy compounds [27]). The high frequencysemicircle is attributed to Li migration into surfacefilms that are formed in an alkyl carbonate solution onseveral LixMOy compounds [27], coupled with filmcapacitance. The fact that this semicircle is invariantwith potential correlates with these assignments. Thepotential dependent, medium frequency semicircle isattributed to the charge transfer through the surface

electrode. In contrast to the pressed and rolled elec-trodes, the kinetics of the unpressed electrode are verysluggish (as can be seen from the pronounced depen-dence of the capacity obtained, on the potential scan-ning rate). Its capacity is also lower.

Fig. 9 shows typical Nyquist plots of the pressed androlled LiCoO2 electrodes (A and B, respectively) mea-sured at five selected potentials (under equilibrium con-ditions), 3.5, 3.75, 3.85, 4 and 4.25 V vs. Li/Li+, as

Fig. 11. Same as Fig. 9A and B, an unpressed LiCoO2 electrode.

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Fig. 12. AFM pictures (measured ex situ) of unpressed, pressed, and rolled LiCoO2 electrodes before the electrochemical treatment (a–c, respectively), and after prolonged charge–dischargecycling (d– f, respectively) in EC+1:1 DMC+1 M LiAsF6 solutions. A height scale (by color) appears near each image. The average roughness factor (RF) is specified below each image.


Fig. 13. XRD patterns of the three types of LiCoO2 electrodes used in this study. Note the different scales in the panels.

film active–mass interface (coupled with interfacial ca-pacitance) and the low frequency, ‘Warburg’-type ele-ment in the spectra (Fig. 9) is attributed to solid state,Li-ion diffusion into the active mass [27,32]. The diame-ters of the two semicircles are denoted in Fig. 10 asRfilm and RCT.

Fig. 11 is similar to Fig. 9, presenting Nyquist plotsof an unpressed electrode at the same potentials as inFig. 9. The impedance spectra of this electrode arequite different from those of the pressed and rolledelectrodes, showing only one large, high frequencysemicircle coupled with a sloping line at low frequen-cies. In general, the impedance of the unpressed elec-trodes is much higher (at least five times) than that ofthe pressed electrodes. This higher impedance correlateswell with the sluggish kinetics of these electrodes, as isdemonstrated in Figs. 7 and 8.

Fig. 12 shows AFM images of the three types ofLiCoO2 electrodes in their pristine state (before theelectrochemical treatment) and after charge–dischargecycling (a–c and d– f, respectively). Fig. 12a shows animage of the unpressed electrode. Some typical particlesof irregular micronic size, as well as the boundariesbetween them, are clearly seen. As is seen in image 12b,related to the pressed electrode, the application ofvertical pressure does not drastically change the elec-trode morphology. While the image may reflect distor-tion in the pyramidal shape of some particles, theinter-particle boundaries are clearly seen. Image 12creflects a drastic change in the electrode morphologydue to the application of pressure by rolling. It appearsthat the particles were cracked and that the electrodesurface was flattened (also indicated by reduction of theroughness factor), due to the application of pressure byrolling. Cycling definitely causes some morphologicalchanges, as are seen in images 12d– f. These changes,

which basically retain the initial electrode morphology,correlate with our previous findings that LixMOy elec-trodes (M=Co, Ni, Mn, etc.) develop surface films inalkyl carbonate solutions [26]. Fig. 13 shows XRDpatterns of the three types of LiCoO2 electrodes used inthis study. The patterns are identical in the peak loca-tion, but differ markedly in their peak intensity. Thetwo pressurized electrodes have much more intenseXRD peaks than the compressed electrode. These pat-terns show that the pressure has no effect on theintrinsic structure of the particles, but rather affectstheir orientation, as in the case of the graphite elec-trodes dealt with previously. The pronounced impact ofpressure on the particle orientation is understood be-cause LiCoO2 particles basically have a layered struc-ture. In contrast to the case of graphite electrodes (Fig.6d above), there was no difference between the rolledand the pressed LiCoO2 electrodes in their XRD peakintensity. This is because the latter particles do not havethe structure of flakes that can slide on each other, thusforming highly compact, oriented electrodes, as is thecase for the graphite material that we used. The irregu-lar shape of the LiCoO2 particles leads to the similarXRD response of the pressurized electrodes, no matterhow the pressure was applied. The difference in theelectroanalytical behavior of the three types of LiCoO2

electrodes can be better understood in the light of thesestructural and morphological studies.

It appears that, in contrast to the case of graphiteelectrodes, where application of pressure may be detri-mental to their performance, in the case of LiCoO2

electrodes the application of pressure during the pro-duction of electrodes is very important. We assume thatthe application of pressure to LiCoO2 improves theinter-particle electrical contact, as well as the electroniccontact between the active mass and the current collec-


tor. Since LiCoO2 particles are not good electronicconductors, when the active mass is not properly com-pressed, part of it is not accessible to electron transferfrom the current collector. The irregular shape of theparticles ensures that compression does not block thepercolation of solution through the active mass of theelectrode, in contrast to what was found for thegraphite electrodes, which are comprised of flat flakes.We found that application of pressure to LiCoO2 elec-trodes by rolling may be somewhat detrimental to theactive mass, as is reflected by the AFM images and thelower capacity obtainable, as compared with verticallypressed electrodes (Fig. 8), probably due to some me-chanical damage to the particles (e.g. cracking). How-ever, pressurizing by rolling definitely does not changethe basic lattice structure of the LiCoO2 particles(XRD, Fig. 13). We therefore attribute the slowerkinetics (Fig. 7), lower capacity (Fig. 8), and higherimpedance (Fig. 11) of unpressed LiCoO2 electrodes toa poor electronic contact between the particles. Thismeans that only part of the LiCoO2 particles are in-volved in the electrochemical process.

4. Conclusions

In this work we demonstrated that the application ofpressure during the preparation of graphite and LiCoO2

has opposite effects on their performance. Syntheticgraphite particles, which are often used in Li insertionanodes, have a morphology of soft, flat flakes. Thus,application of pressure to composite graphite electrodescompresses the active mass, orienting the graphiteflakes so that their wide dimension (the basal planes) isparallel to the current collector. The gaps between theparticles are thus closed, and, hence, the solution pathstowards inner particles are blocked. The formation ofsurface films on the compressed active mass at lowpotentials due to reduction of the solution species fur-ther blocks the contact between Li ions in the solutionphase and a considerable part of the inner graphiteparticles. This situation leads to high impedance and tothe sluggish kinetics of compressed graphite electrodes.Application of too high a pressure may also damage the3D structure of the soft graphite particles, thus reduc-ing the overall electrode capacity. However, structuralanalysis of the electrodes by XRD clearly showed thatthe pressure applied to the electrodes in this work hadno effect on the structure of the graphite particles, butrather the application of pressure affects only the elec-trode morphology, i.e. the particle orientation on thecomposite electrodes. Application of pressure by rollingis more detrimental to the electrode performance thanvertical compression, because rolling forces the particlesto slide on each other, thus improving the compressionof the active mass, which increases the blockage of

inner graphite particles towards ions in solution phase.Since graphite particles are highly electronically con-ductive, compression is not needed to enhance theelectrical contact within the active mass. In contrast,application of pressure to LiCoO2 electrodes duringtheir preparation is critical for obtaining a high elec-trode performance. Compression improves the electricalcontact between the particles and the current collector,with no adverse effect on the capacity because LiCoO2

particles are sufficiently hard. Thus, application ofmoderate pressure is not expected to destroy their basic3D structure at all, as is clearly demonstrated by theXRD analysis. The usual irregular shape of LiCoO2

particles, commonly used as the active mass of Li-ionbattery cathodes, ensures that compression does notblock the contact between particles and Li ions in thesolution phase. Therefore, compressed LiCoO2 elec-trodes exhibit faster kinetics, lower impedance, andhigher capacity, as compared with unpressed electrodes.

It should be noted that mass production of com-posite electrodes for Li-ion batteries always requires theapplication of some pressure on the electrodes byrolling, due to the lamination processes by which theseelectrodes are produced. The present work clearlyshows that application of pressure to graphite elec-trodes during their production has to be carefully car-ried out and well adjusted, in order to prevent thedetrimental effect of compression on the kinetics andcapacity of graphite anodes.

Acknowledgements

Partial support for this work was obtained from theBMBF, the German Ministry of Science, in the frame-work of the DIP program for Collaboration betweenIsraeli and German Scientists, and from the NationalScience Foundation of the Israeli Academy of Science.

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