studies of lithium insertion in ballmilled sugar carbons

62 J. Electrochem. Soc., Vol. 145, No. 1, January 1998 The Electrochemical Society, Inc.

laboratory to obtain more information concerning themechanism of Li-ion insertion into Sn02 anodes.

AcknowledgmentThis work is partly supported by NSFC No. 5967202?

and Beijing Zhongguancun Joint Center of Analysis andMeasurement, and we thank Professor Paul Hagenmullerfrom the Institute of Condensed Material Chemistry ofBordeaux, France, for helpful discussion.

Manuscript submitted May 14, 1997; revised manuscriptreceived July 23, 199?.

REFERENCES1. N. Nitta, S. Otani, and M. Haradome, J. Electron.

Mater., 9, 727 (1980).2. P Olivi, E. C. Pereira, E. Longo, J. A. Varella, and L. 0.

de S. Buihoes, This Journal, 140, L81 (1993).3. B. Orel, U. Lavrencic-Stangar and K. Kalcher ibid.,

141, L127 (1994).4. Fuji Photo Film Co., Ltd., Euro. Pat. 0,651,450, Al

(1995).

5. D. Wang, S. Wen, J. Chen, S. Zhang, and F Li, Phys.Rev. B, 49, 14282 (1994).

6. H. D. Tarey and T. A. Raju, Thin Solid Films, 128, 181(1995).

7. T. Minami, H. Nanto, and S. Takata, Jpn. J. Appl. Phys,27, L28? (1988).

8. V. Schlosser and G. Wind, in Proceedings of the 8th ECPhotovoltaic Solar Energy Conference, p. 998, Flo-rence, Italy (1988).

9. J. Lee and S. Park, J. Am. Ceram. Soc., 76, 77? (1993).10. B. D. Cullity, Elements of X-Ray Diffraction, 2nd ed.,

Addison-Wesley, Reading, MA (1978).11. H. S. Katiyar, P Dawson, M. M. Hargreave, and G. H.

Wilkinson, J. Phys. C: Solid State Phys., 4, 2421(1971).

12. J. Zuo, C. Xu, X. Liu, and C. Wang, J. Appl. Phys., 75,1835 (1994).

13. J. Isidorsson, C. G. Granqvist, L. Haggstrom, and E.Nordstrom, ibid., 80, 236? (1996).

14. J. Wang, I. D. Raistrick, and H. A. Huggins, This Jour-nal, 133, 45? (1986).

15. I. A. Courtney and J. H. Dahn, ibid., 144, 2045 (199?).

Studies of Lithium Insertion in Bailmilled Sugar Carbons

Weibing Xing,* R. A. Dunlap, and J. R. Dahnts'°

Department of Physics, Dalhousie University, Halifax, Nova Scotia B3H 3J5, Canada° Department of Physics and Department of Chemistry, Dalhousie University, Halifax, Nova Scotia B3H 3J5, Canada

ABSTRACT

Hard carbons were prepared by pyrolyzing sugar precursors at 1000°C. The sugar carbons have a microporous struc-ture and large specific capacity (�550 mAh/g) for lithium insertion in carbon/Li electrochemical test cells. Powders ofsugar carbon were then treated by high-impact bailmilling either in argon or air. These carbon samples were character-ized by x-ray diffraction, small-angle x-ray scattering, thermogravimetric analysis, chemical analysis, and Brunauer-Emmett-Teller surface area measurements. The structure of the balimilled powders was different from that of the origi-nal sugar carbons. As milling proceeds in argon or in air, the graphene layers initially become more stacked (as indicatedby changes in the 002 diffraction peak), the nanoscopic or microscopic pores are rapidly eliminated, and the number ofmacropores or mesopores increases. Upon further milling, the 002 diffraction peak weakens again, as the carbon struc-ture becomes more disordered. We explain these trends with a qualitative model. Thermogravimetric analysis and chem-ical composition analysis on the air-milled samples confirm that the materials contain substantial oxygen, suggest thatoxygen-containing surface functional groups are formed and show that the amount of the functional groups increaseswith milling time. Carbons ballmilled in argon atmosphere needed to be slowly exposed to air and kept cool or they burstinto flames when brought into contact with air. This implies that the milling created broken carbon-carbon bonds, whichare highly reactive, in the material. Studies of ballmilled carbon/Li coin cells showed that ballmilled carbons have largereversible specific capacities of more than 600 mAh/g for lithium insertion. However, the cells demonstrated large hys-teresis compared to that of unmilled sugar carbon/Li cells. We propose that the mechanism for quasi-reversible lithiuminsertion in ballmilled carbons may involve (i) reactions of Li atoms at the edge of small graphene sheets, (ii) intercala-tion in cases where stacked layers remain, and (iii) reactions with surface functional groups where they exist. It was foundthat hysteresis in the balimilled carbons is only weakly dependent on temperature and cycling rate.

IntroductionHigh-energy milling techniques have long been employed

in materials science for the production of, for example,composite metallic powders with a fine controlledmicrostructure.' Recently, this technique was used to studythe effect of mechanical grinding on lithium insertion ingraphitic and soft carbons by Tarascon's group.2 Theirmilled-carbon/Li cells have increased specific capacitiesof about 700 mAh/g and have large hysteresis behaviorcompared to those of the graphite/Li cells. They believethat the adsorption of lithium atoms on surfaces of single-layer carbons is responsible for the increased capacities inthese materials.2 We did not agree with their interpretationbecause the voltage profiles (voltage vs specific capacity)of their materials differed greatly from those of microp-orous carbons with single graphene sheets which havebeen previously studied extensively by us.38 Since thesecarbons appeared to be mysterious, we decided to studythe effect of milling on sugar-derived carbons ourselves.

In this paper, we report the effect of mechanical grind-ing on sugar-based hard carbons. Sugar carbons prepared

Electrochemical Society Student Member.* Electroebemical Society Active Member

under vacuum or inert gas38 contain a large number ofnanoscopic or microscopic pores and are highly disorderedwith a large fraction of single-layer graphene sheets. Thesesingle-layer graphene sheets are thought to be linked in arandom manner much like a "house of cards." Sugar car-bon/Li cells have demonstrated large reversible specificcapacities (�550 mAh/g) for lithium insertion, low voltageplateaus (V1 0 V), and small hysteresis (A170d 0.2 V)between charge and discharge cycles.3'8 LVCd is the differ-ence between the average charge and discharge voltages.The large capacity of sugar carbons is thought to be due tothe large content of single graphene sheets which mayadsorb lithium atoms on both sides.3'7 This is equivalent toadsorption on the interior surfaces of the nanoscopic ormicroscopic pores formed by the network of the graphenesheets as proposed by Sonobe et al.5

Here we show that after mechanical grinding, the numberof nanscopic pores in the carbons is dramatically reduced,as probed by small-angle x-ray scattering measurements. Alarge number of unsatisfied carbon bonds is created by ball-milling in argon, as evidenced by the violent reactions offreshly ground powders with oxygen in the air. Surfacefunctional groups are formed when sugar carbons are ball-milled in air. Our ballmilled sugar carbon/Li cells show spe-

) unless CC License in place (see abstract). ecsdl.org/site/terms_use address. Redistribution subject to ECS terms of use (see 134.208.103.160Downloaded on 2014-03-27 to IP

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J. Electrochem. Soc., Vol. 145, No. 1, January 1998 The Electrochemical Society, Inc. 63

cific capacities of more than 600 mAh/g with large hystere-sis, AVCd = 0.9 0.1 V. Our experimental data suggest thatthe mechanism for lithium insertion in balimilled sugar car-bons is very different from that of the starting material. Wepropose other mechanisms which could be responsible forthe large capacity in these materials.

ExperimentalSample preparation—Hard carbons were prepared by

pyrolyzing sugar precursors (table sugar) to 1000°C in anargon flow or under vacuum. Sugar precursors werecaramelized or dewatered at 185°C in air before high-tem-perature pyrolysis. Detailed procedures for sugar carbonpreparation are described in Ref. 8. Samples 51, S2, S3, andS5 were prepared by identical procedures and are expectedto have identical properties and compositions. Sample S4was allowed to oxidize slightly at 1000°C, which changed itsstructural properties slightly.8

A Spex 8000 mixer/mill was used for the mechanicalgrinding experiments. The grinding is in the shock mode.Powder grinding was done in argon with an 0-ringsealed vial or in air using similar vials which did nothave the 0-ring seal. The vials were made of hardenedsteel. For the grinding in argon, samples were loaded intothe vial and extracted from the vial in argon-filled gloveboxes. Hardened steel balls of 1/4 in. diam and 1.04 gmass were used for grinding. Different ball-to-sampleweight ratios, w = Wb/W,, where Wb and W are the ball andsample weights, respectively, were used for the bailmillingexperiments. The sample weights were generally about 1 to3 g, and the ball weight was adjusted accordingly. The vialsused for milling samples B1, B2, and B3 were smaller (short-er length) than those used for the other samples, so the veloc-ities of the balls and hence the milling intensity were lowerfor these samples compared to the other milled samples.

Material characterization.—Powder x-ray diffraction(XRD) measurements were made with a Siemens D5000 dif-fractometer equipped with Cu K.1 radiation and a diffract-ed-beam monochromator. Small angle x-ray scattering(SAXS) measurements were made in a transmission config-uration with the same instrument as described elsewhere.t°Single-point Brunauer-Emmett-Teller (BET) surface areameasurements (30% N2 in He) were made using aMicromeritics Flowsorb II 2300 surface area analyzer.

As a probe of the number of surface functional groups onthe carbon surface, a TA Instruments no. 51 thermal gravi-metric analyzer (TGA) was used to measure the relativeweight loss of samples vs temperature during heating.Powder samples were held in a Pt pan and heated to about1000°C at various heating rates in a flow of Ar. It is believedthat oxygen-containing functional groups react with carbonatoms in this temperature range and evolve as CO, C02, etc.To verify this, residual gas analysis (RGA) was carried outwith a Leybold Quadrex 200 residual gas analyzer (massspectrometer). The base pressure of the RGA is about 5 X108 Tore. During operation, a small stream of TGA exhaustgas was sampled by the mass spectrometer using a leakvalve. With the leak open, measurements were conducted ata chamber pressure of 4 >< 10 Tore. All gas flow lines wereheated above 100°C so that no water vapor or other gases ofinterest could condense in the lines before they reached themass spectrometer.

Chemical analyses for C, 0, H, and N were carried out atCanadian Microanalytical Service, Limited (Delta, B.C.,Canada). Some samples which were bailmilled in argonwere analyzed for C, H, N, and 0 (CHNO analysis), withoutexposure to air, using an inert gas load lock to transfer thesamples to the analyzer In order to understand the sourceof oxygen in these samples, one sample (sample B19) waspyrolyzed, transferred to the glove box without air exposureusing the apparatus described in Ref. 11 (before milling, thisis sample S5), loaded into the milling vial, milled, and thenanalyzed without ever exposing the sample to air

Electrochemical testing—Coin-type test cells were con-structed using sample materials as cathodes and lithium

metal foils as anodes as described by Xing et al.° Film elec-trodes were made by the doctor-blade spreading method. Inorder to test the electrochemical properties of ballmilledsample powders without air exposure, vials were broughtinto an argon-filled glove box and sample powders weredirectly spread into a cathode can. Cells constructed in thisway are refereed to as powder cells. For powder cells, theelectrode mass is not accurately known, because withoutbinder, some of the powder could move during cell assem-bly and not be positioned opposite to the Li metal anode;therefore, the measured capacity in such cells can be lessthan measured for cells with standard electrodes.Nevertheless, these cells still give good information aboutthe shape of the voltage profiles and differential capacities.

The electrolyte used was 1 M LiPF6 dissolved in a 30/70v/v% mixture of ethylene carbonate (EC) and diethyl car-bonate (DEC). A microporous film (Celgard 2400) wettedwith the electrolyte was sandwiched between the activeelectrode and a Li metal foil anode. Electrochemical testingof the cells was performed using constant-current cyclerswhose currents are stable to 1%. The cells were placed inthermostats at 30 0.1°C. A specific current of 18.6 mA/gwas used for all charge-discharge cycles. Details of the elec-trochemical testing are described by Xing et al.°

Results and DiscussionFigure la shows the XRD pattem for a sugar carbon sam-

ple (Si) with no milling. Here, we designate S and B forsugar carbon and bailmilled carbon samples, respectively,

U

-4

1000

2008

1000

1008

500

0

400

200

00

Fig. 1. XRD profiles for samples SI, B I to B4 as indicated. Peaksarising from iron impurities are indicated in (ci).

3000

2000

1000

02000

10 20 30 40 50 60 70 80 90Scattering angle





CarbonSample source

Ball!sample (wt)

Millingtime H Hg(A)

SiBIB2B3B4B5B6S2B?S3B8B9B1OBilS4B12B13B14

SugarCSi515151Si51

Sugar CS2

Sugar CS3S3S3S3

Sugar CS4S4S4

—.444

101010—4

—10101010—

77.58

OhShinair11 hinair24 hin air25inArSOhinAr74 h mArOh25 hinairOhlOhinAr21.5 h in Ar3OhinArSOhinAr0 h10 h mAr20 h in Ar3ohinAr

1.701.7031.7391.8842.671.981.651.661.781.772.862.602.331.8752.583.0972.9152.761

5.01

5.16

B15 S4 8 S0hinAr 2.519B16 S4 8 76hinAr 2.199Bl7B1855B19

S4S4

SugarCS5

88

—10

l24hinAr2l4hinArOh24hinAr

1.8461.9411.6942.828

B20 S5 10 lghinAir 2.258

see Table I. The XRD pattern is typical of disordered car-bons, as indicated by broad Bragg peaks. Figure ib-d showsXRD patterns for samples Bi to B3 balimilled in air for 5,H, an 24 h, respectively The starting material for thesemillings was Si. The ball-to-sample weight ratio, w, wasfixed at 4 for these samples. The (002) Bragg peak for theballmilled samples remained broad. There are additionalpeaks appearing in the XRD patterns of the milled samplesfrom iron particles due to contamination from the balls andvial. For example, the iron (110) peak (20 = 44.67°) becamestronger as the milling time increased. Figure le shows theXRD pattern for sample powder B4 (sample Si was thestarting material) which was bailmilled for 25 h in anargon atmosphere. The ball-to-sample ratio for this sam-ple was w = 10.

To quantitatively analyze the XRD patterns, we use anempirical parameter, R, introduced by Liu at al.5 defined asthe ratio of the (002) peak intensity above zero to the back-ground above zero at the same angle. It was shown that thevalue of H is inversely related to the fraction of single-layerorganized regions in the sample, i.e., samples with more sin-gle layers have smaller values of R.5 Figure 2 shows H vsmilling time for samples ballmilled in air (starting materialSi, Bi to B3; starting material S5, B20 to B21) and for threeseries of samples milled in argon (starting material Si, B4 toB6; starting material S3, B8 to Bil; starting material 54,Bi2 to Bi8). Each experiment shows the same trend, eventhough the different starting materials have different initialvalues of H. That is, H initially increases and then decreas-es as milling proceeds. (Samples Bi to B3 milled in air hadw = 4 and used vials causing lower milling intensity sotheir structural changes proceeded on a slower time scale,12and the maximum in H was apparently not reached for thesesamples.)

The variation of H with milling time probably occursbecause the force of milling first "flattens" the graphenesheets (within the powder particles) into more parallelalignments. This causes a closure of many of the nanoscop-ic pores between the sheets, as discussed in a following sec-tion. Further milling presumably breaks up the particlesand sheets into very small fragments, causing a reduction inH. It was observed that the powders became "fluffier" andfluffier (their packing density decreased by up to a factor of4) with extended milling, in agreement with these ideas.Furthermore, portions of samples milled in argon whichwere about 0.2 g or larger were observed to burst into flameon rapid exposure to air. This occurs presumably when oxy-gen reacts with the numerous carbon atoms in reactive

0 50 100 150 200

Milling Time (h)

Fig. 2. Variation of R with milling time for five different experi-ments as indicated. The balko-sample weight ratio was 10, exceptfor the Si, Bi to B3 series and the 54, B12 to B18 series where theratio was 4 and 7.5, respectively. Lines are guides to the eye.

bonding arrangements at the edges of fractured carbonsheets, causing heat release and combustion. Samples ofthis size need to be slowly exposed to air so that the reac-tions proceed slowly without significant temperature rise,thereby preventing combustion. As shown in Ref. 2,graphite powders milled for extended times are alsoreduced to very small fragments, and the x-ray patterns ofthose materials show weak and broad (002) peaks. We sus-pect that our materials milled in argon for long times (e.g.,sample B4) are equivalent to those in Ref. 2, which have alsobeen milled by shock for extended times, even though thestarting materials were very different. Further evidence forthis is given later.

Figure 3 shows SAXS intensity vs wave vector q (= 4irsin0/K, where K is the wavelength) for samples S2, 84, and B7(see Table I). Sample S2 is the original sugar carbon with nomilling; sample B4 was ballmilled in argon for 25 h withw = iO; and sample B7 was ballmilled in air for 25 h withto = 4. The effective milling time for sample B4 is longerthan that for sample B7, because w for B4 is larger than thatfor B?.'2 The intensities of the SAXS measurements for allsamples were normalized to that of sample S2 to correct for

Table I. Sfructural parameters of the samples studied. 3.2

2.8

2.4

2.0

1.6

5_ , I

xa S3,B8-Bl1

V 54,B12-818

'0' 55, B20-B21

I I

1000000

100000

10000

1000

l00

'B4

- - - - B4: milled in argon

--- B7: milled in air

L 52: no milling

Sample S2

0.1 1.0

q(A )

Fig. 3. SAXS results for samples 52, B4, and B7 as indicated.





U

CU

xC

0 h milling

Sugar carbonball milled in airsamples Si, Bl-B3

o 100 200 300 400 500 600 700 800 900Temperature (°C)

100

98

96

94

92

90

88 -

Fig. 4. Sample weight vs temperature for TGA measurements forsamples Si, Bi, B2, and B3 as indicated. Measurements made in Ar.

to air for different periods of time and at different temper-atures as indicated by the three curves A (2 h at room temp),B (14 days at room temp followed by 18 h at 110°C), and C(13 days at room temp). In contrast to the data in Fig. 4 forthe samples bailmilled in ah there is no significant weightloss vs temperature below about 500°C. Above 500°C thereis a rapid weight loss which extends up to about 700°C.

C

00I)

z

x-ray absorption by iron particles in the ballmilled samplesusing the methods described in Ref. 6. The circles are themeasured data points for sample S2. The solid line is a fit ofa model calculation, using Eq. 1. The measured data pointsfor samples B4 and B7 are not shown to preserve clarity.Instead, the SAXS data of samples B4 and B7 are repre-sented by the model calculations which describe them to thesame accuracy as the fit to sample S2 describes the S2 data.We used the Debye formula'3 subsequently adapted byKalliat et at.14 in our model calculations of the SAXS pro-files of these samples

A BR' CRI(q)=—+ +D [1]q° (1 + q2R)2 (1 + q2R)2

In Eq. 1, R and R2 are two characteristic correlation lengthsfor electron density variations in the solid. R, and R2 areused as fitting parameters. These can be associated withpore size by comparison to Guinier's formula,'5 see Table I.The radius of gyration, Rg, of the corresponding pores is ftimes either R1 or R2. A, B, and C in Eq. 1 are constants thatare proportional to the total surface areas and numbers ofmacropores, small micropores, and larger micropores,respectively. D is a constant used to model a constant back-ground. Porod's law15 gives n = 4, but we used a = 3.5 forslit-type (not pinhole) measuring systems in our fitting.6

The SAXS curves in Fig. 3 clearly demonstrate thechanges to the microporous structure in the sugar carbonswith milling time. The fitting of Eq. 1 to the experimentaldata showed that (i) the number of small micropores (—5 Ain dimension, from the parameter B) in sample B7 has beenreduced by a factor of three compared with that in sampleS2; (ii) there are no small micropores found in sample B4,because B 0 in the fit to the data; (iii) the total surfaceareas due to meso- and macropores (>50 A in dimension,from parameter A) for samples B4 and B7 are about 100 and50 times larger than that of sample S2, respectively; and (iv)the number of large micropores for samples S2 and B7 isvery small (C << B) compared with the number of smallmicropores. The SAXS data are consistent with the XRDdata. That is, when the carbon matrix, the house of cards, isinitially collapsing due to mechanical grinding, moregraphene sheets are stacked in parallel fashion (the part ofFig. 2 where R increases) and the number of small micro-pores decreases. As grinding proceeds the particles arereduced in size, the powder becomes more fluffy, and SAXSdetects only macropores between the small particles, oralternatively (and more likely) by Babinet's principle,16 thesmall particles themselves. (Babinet's principle states thatthe scattering from a small homogeneous object in a vacu-um is the same as the scattering from a void of the same sizeand shape in a homogeneous medium).

Figure 4 shows TGA data for samples Bi to B3 whichwere ballmilled in air and then stored in air for 2 monthsbefore the TGA measurements. The samples all show weightloss upon heating, but the amount of weight loss increaseswith milling time. The initial weight loss at temperaturesbelow 200°C is probably due to loss of water adsorbed onthe surfaces of the samples. The continuous weight loss athigher temperatures is very likely due to the decompositionof the surface functional groups, e.g., -COOH or -OH, whichare chemically bonded to the carbons. Figure 5a shows thenet weight loss at 700°C vs milling time, tm, for these sam-ples. The triangles are measured data and the line is a guideto the eye. Figure 5b shows the oxygen content of the sam-ples from chemical analysis [weight percent (w/o)J vs tm forthe same samples (B1 to B3). The circles are measured dataand the line is a guide to the eye. The oxygen contentincreases as a function of milling time, a trend similar tothat seen in Fig. 5a. Figure 5a-b suggests that certain oxy-gen-containing functional groups were formed during ball-milling in air and that the amount of functional groupsformed increases with milling time. The rate of increase inoxygen content slows as the milling time increases.

Figure 6 shows TGA data for sugar carbons ballmilled inargon (sample B4, tm = 25 h, and w = 10) and then exposed

0 5 10 15 20 25Milling time (h)

Fig. 5. Ia) Weight loss at 700°C vs milling time for the data in Fig.4. (b) Oxygen content (w/o) from chemical analysis for the samesamples ISI, Bi to B3).





V

100

95

90

85

80

0 100 200 300 400 500 600 700 800 900 1000

Temperature (°C)

Fig. 6. Sample weight vs temperature for IGA measurements (inargon) of sample 64 exposed to air for different times as indicated.

Above 700°C there is a second weight loss. It is interestingto compare the TGA measurements shown in Fig. 4 and 6with those on sugar carbon pellets pyrolyzed under vacuumand subsequently exposed to air for different periods oftime,11 In the latter case, the weight loss of the pellets wascontinuous above 2 00°C and was dependent on air exposuretime. This suggests that the surface functional groups whichform on the surfaces of the argon-milled carbons after airexposure are different from those which form on the

'CV

C

V

I,C..

Fig. 7. TGA/RGA results For sample 62: (a) sample weight vstemperature and (b) partial pressures of mass 28 and 44 in the mass

spectrometer sampling the evolved gas from the TGA experiment.

unmilled material. This, in tum, suggests the sites where thegroups form are different, which is not surprising given theseverity of the milling process.

We carried out RGA measurements in conjunction withthe TGA measurements on our ballmilled samples. Figure7a shows the TGA data for powders ballmilled in air (sam-ple B2: tm = 11 h and w = 4). Figure 7b shows the RGA datafor the same sample. A noticeable peak for mass 44 or CO2around 350°C was observed in the mass spectrum. A grad-ual increase in the signal for mass 28 (CD) as a function oftemperature is also observed, followed by a rapid rise above800°C. Other mass numbers were basically found to be fea-tureless. The TGA and RGA measurements may provideinformation about oxygen functional groups formed in theballmilled samples. It is known that carboxylic groups areleast stable among oxygen-containing functional groups,'7and they decompose with formation of CO2. It is thereforevery likely that -COOH groups were formed when carbonswere ballmilled in air.

Figure Ba-b shows the TGA and RGA measurements forpowders of sample B4 bailmilled in argon for 25 h (w = 10)and then exposed to air for 19 days. The mass spectrumshows a peak for mass 28 or CO around 700°C. Mass 44showed little or no signal in the mass spectrum. By compar-ison with Fig. 7, the surface groups on this sample are verydifferent than those on the samples ballmilled in air.

Table II shows the results of chemical analysis of the sam-ples. A sample of meso carbon microbeads (MCIV1B) graphi-tized at 2800°C was used as a control to establish the relia-bility of the CHNO analysis. The MCMB sample shows 99%carbon and little else, as expected. Sample 51, which wasused as a starting material for some of the milled samples(Bi to B6), contained oxygen initially. This oxygen mayhave slowly accumulated in the sample over its long storagetime. During milling in air (samples Bi to B3) the oxygencontent increased dramatically (Fig. Sb). The carbon con-tents apparently drop because of contamination of the pow-der by iron from the bailmilling process. After milling inargon, the oxygen content also increases, as seen by com-paring sample B4 (never exposed to air) to Si. The cause forthis increase is unknown, but it could involve transfer ofoxygen from the steel balls to the carbon or a small leak of

.V

C

V

'I'VC.,

Fig. 8. TGA/RGA results for sample 64 exposed to air for 19days before the measurement: (a) sample weight vs temperatureand (b) partial pressures of mass 28 and 44 in the mass spectrom-eter sampling the evolved gas from the TGA experiment.

88

SE-IC

4E-l0

0 100 200 300 400 500 600 700 800 900 1000

Temperature (°C)

mass 44

0 100 200 300 400 500 600 700 800 900 1000

Temperature (°C)




J. Electrochem. Soc., Vol. 145, No. 1, January 1998 @ The Electrochemical Society, Inc.

Table \I. Chemical concentrations of C, H, N, and 0 for selected samples.

Ball/ Time Original sample Milling for air C H N 0 sample (wt) time exposurea (w/o) (w/o) (w/o)(w/o)

B19 10 MCMB - .

0 h 1.5 years 93.7 0.2 0.7 2.2 5 h in air 5 h 83.8 0.4 0.2 7.0 l lh ina i r l l h 81.9 0.4 0.1 7.5 24 h in air 24 h 79.2 0.6 0.2 9.8 25hinAr Oh 81.3 0.3 0.8 4.8 25 h in Ar 14 days 80.5 0.5 0.8 6.4 25 h in Ar 0.5 h 79.2 0.4 0.8 6.8

12 hat 110°C 24hinAr Oh 90.4 0.3 <0.1 1.4 0 h - 98.9 <0.1 <0.1 <0.1

" At room temperature otherwise stated.

air into the vial. When sample B4 was exposed to air, its oxygen content increased further (see Table 11). One expects that the materials reported in Ref. 2 have similar oxygen contents (about 6 w/o), since they were prepared in a manner similar to B4 and then exposed to air. Finally we measured the oxygen content of a sample which was pyrolyzed under vacuum, transferred to the glove box, sealed in the milling vial under argon, milled, packed in a sealed argon- filled vial, and analyzed without air exposure (sample B19). This sample had 1.4 w/o oxygen, again consistent with some small transfer of oxygen to the sample during the milling process.

Figure 9a shows the voltage vs specific capacity for a sugar carbon/Li cell. The electrode film of the sugar carbon powder (Sl) was prepared by doctor-blade spreading. The data plotted are for the first and second discharges (ID and 2D) and the first and second charges (1C and 2C). The dif- ference in specific capacity between the first discharge and the first charge is taken to be the irreversible capacity, Q,,, which is clearly indicated by the space between the 1D and 2D curves near 3 V. The reversible capacity, Q,,, is calculat- ed as the average of the capacities of the second discharge and charge. The sugar carbon/Li cell has Q, = 215 mAh/g and Q,, = 575 rnAh/g. Notice that more than half the capacity is in the low-voltage range, close to 0 V. Electrochemical results are given in Table 111.

Figure 9b shows the voltage profile of a cell made from a powder electrode of sample B4 (never exposed to air) vs lithium metal. This type of cell is referred to as a powder cell as described in the Experimental section. For comparison, some powder of sample B4 was also exposed to air for 12 h and then used to make a powder electrode/Li cell. The voltage profile for one of the cells is shown in Fig. 9c. Because of the uncertainties in active powder weight, the specific capacities of the powder cells are not known, and they were scaled to have the same first discharge capacity as a film cell (Fig. 9d). Figure 9d shows the voltage profile of a cell with a film electrode of sample B4 vs Li metal. The voltage profiles of the B4/Li cells (Fig. 9b-d) all showed no low-voltage plateaus and large hysteresis between charge and discharge cycles compared to that of the original sugar carbon/Li cell (Fig. 9a). The data in Fig. 9b-d are similar to

(a) 0 h milling film cell sample S 1

I . l . I . I , I . I ,

3 - (b) 25 h (in Ar)

2 -

1 -

0 -

(c) 25 h (in Ar) powder cell 12 h air exp.

$ 1 sample B4

1 , 1 . 1 . 1 . 1 , 1 .

(d) 25 h (in Ar) film cell sample B4

1

0

(e) 24 h (in air)

0

0 200 400 600 800 1000 1200 Capacity (mAhIg)

Fig. 9. Voltage vs capacity for samples: (a) S1; (b) 84, never exposed to air (powder cell); (c) 84, exposed to air for 12 h (powder cell); (d) 84, made into a film cell; and (e) 83. Ca acities for (b) and (c) were normalized to ha t of panel (d). The Rrst discharge (ID), first charge (lC), and second cycle (20 and 2C) are indicated.

those for milled samples in Ref. 2. The B4/Li film cell has an average charge voltage of 1.27 V (about 0.9 V higher than that of the Sl/Li film cell). Both Q,, (620 mAh/g) and Q,,, (250 mAh/g) of the B4/Li film cell increased by

Table Ill. Electrochemical properties of selected samples.

Ball/ BET Average Average Carbon sample Milling Q,, Q,, Q,dQRv surface area charge discharge

Sample source (wt) time (mAh/g) (mAh/g) (%) R R,(& (m2/g) (v) (V)

Sugar C S 1 S 1 S 1 S1 S 1 S1

Sugar C S2

0 h 5 h in air 11 h in air 24 h in air 25 h in Ar 50 h in Ar 74 h in Ar 0 h 25 h in air





about 40 mAh/g compared to those of the SI/Li cell (seeTable III). Figure 9e shows the voltage profile of a cell witha film cathode made from sample powder B3 (ball-milled inair for 24 h, w = 4) vs Li. The B3/Li cell showed even larg-er hysteresis compared to that of the B4/Li cells (Fig. 9b-c).

Figure 10 shows differential capacities for the first chargecycle of the same cells shown in Fig. 9. It can be seen fromFig. 10 that the bailmilled samples have much larger capac-ities near 1 V than the original sugar carbon sample (Fig.lOa). Samples balimilled in air (Fig. lOe) also show addi-tional capacity in the range between 2 and 3 V We believethis excess capacity above 2 V is associated with lithiumatoms being withdrawn from bonding environments nearsurface functional groups. The RGA data suggest that theseare COOH groups.

These cell data strongly suggest that the mechanism forlithium insertion in balimilled carbons is entirely differentthan that in sugar carbons without milling. This is becausethe original microporous structure in sugar carbon has beenchanged by the mechanical grinding, as shown by our XRD

Fig. 10. Differential capacity (do/dy) for the same samples as inFig. 9. The data for the first charge (1 C) is shown. The verticalscales ore in arbitrary units for the powder cells shown in panels(b) and (c).

and SAXS measurements. The impact of the balls breakscarbon-carbon bonds and therefore creates unsatisfiedbonding (carbon radicals) in the material. When a largeamount of powder is bailmilled in Ar and then rapidlyexposed to air, it immediately bursts into flames, indicatinga strong reaction of the unsatisfied carbon atoms with oxy-gen in the air. These unsatisfied carbon bonds are probablycreated at the edges of the graphene sheets during ball-milling and probably can react with lithium atoms in anelectrochemical cell. For sugar carbons balimilled in an Aratmosphere, the mechanism for the quasi-reversible reac-tion of Li, may involve (i) reaction with carbon radicals atthe edges of graphene sheets, (ii) intercalation of Li betweenthe few stacked layers which exist, and (ih) reactions withthe small amount of surface functional groups. For sugarcarbons bailmilled in air, more surface groups were formedduring mechanical grinding, as evidenced by the TGAmeasurements and chemical analysis. Therefore, reactionswith the surface groups are more important for carbonsballmilled in air, leading to the large capacity above 2 Vduring charge. We do not believe there is adsorption of Li onmicropore walls occurring, as implied in Ref. 2, because themicropores in these milled materials are destroyed and thevoltage profiles of milled carbons do not resemble those ofthe microporous materials.

To study the nature of the hysteresis in the bailmilledsugar carbons, we cycled some of the cells at higher tem-perature and at a lower rate. Figure 11 compares the volt-age profiles of the powder cell of B4/Li cycled using cur-rents of 744 and 74 mA, respectively (about 6 and 60 hdischarges, respectively). Figure 12 compares the voltageprofiles of the film cell of B4/Li cycled at 30°C (Ct10) and at55°C (C/40), respectively. In both these figures the curveshave been shifted horizontally to align, at 3 V, the point ofthe irreversible capacity on the first cycle. This was donebecause the curves were not measured on sequential cycles.Figures 11 and 12 both show (1) the overall capacity for thereaction of lithium increases due to the slow cycling rate, (ii)a plateau near 2.5 V in the charging voltage profile appearsfor the slow-rate cycling, corresponding to extra capacitynear 0 V on discharge, and (iii) the hysteresis is not reducedeven when the cells were cycled at 55°C and/or at muchlower cycling rates.

The hysteresis in the bailmilled sugar carbons is differentfrom that in hydrogen-containing carbons.18 Zheng et al.81

Sugar carbon ball-milledinargonfor2Sh, l2hairexp.powder cell, sample B4

1= 744 IA

1= 14

200 400 600 800 1000

Capacity (mAhlg)

Fig. 11. Voltage vs capacity for two cycles (the third and fourth)of a Li/84 U2 h air exposure) powder cell. The cycle taken at 744pA (6 h discharge) was normalized to have the same capacity as acorresponding film cell. The cycle taken at 74 pA used the samenormalization factor. The data was shifted by the irreversiblecapacity measured in the first cycle.

300

200

100

I

0

40

20

0

40

20

0400

200

0

3-

2

200

100

00.0 0.5 1.0 1.5

Voltage2.0

(V)2.5 3.0





200 400 600 800Capacity (mAhlg)

Fig. 2. Voltage vs capacity for the third (Cl 10, 30°C) and fifth(C/40, 55°C) cycles of a film cell of sample B4. The data was shift-ed by the irreversible capacity measured in the first cycle.

proposed that the bonding sites near hydrogen atoms all havethe same energy level and these sites can be accessed byovercoming an energy barrier. Both the model and experi-ment showed that the hysteresis behavior in the hydrogen-containing carbons is associated with a thermally activatedprocess!8 The hysteresis in the samples described in Ref. 18showed strong temperature and rate dependence, unlikethat observed here. More complex modeling beyond thescope of this paper is required to explain the hysteresis inthe balimilled carbons. For example, the bonding sites forLi (the carbon radicals at the edges of the broken graphenesheets) in these materials may involve a distribution of dif-ferent energy levels, and this leads to a sloping voltage pro-file, but not necessarily to hysteresis.

Figure 13 shows the effect of different voltage ranges usedin the cycling for the B3/Li cell. When the voltage limits arewide (0 to 3.0 V) the hysteresis is large. This shows thatsome Li atoms inserted near 0 V can be removed only near3.0 V. We believe that these are the Li atoms bonded to sur-face functional groups or to carbon radicals, but we can-not explain the origin of the hysteresis. The 2.5 V plateauin Fig. 11 and 12 grows as the charge-discharge rate isslowed or as the temperature is raised. This shows that eventhe materials which are bailmilled in argon also containsites for Li which can be filled near 0 V and emptied onlyslowly near 2.5 V. The hysteresis in Fig. 13 is reduced as thevoltage limits are narrowed. For example, when the limitsare constrained to 0 to 0.6 V, there is almost no hysteresis.We believer that Li is being inserted and removed from tra-ditional intercalation sites between graphene sheets whenthe cycling is restricted to this range. We emphasize thatthis behavior is very different from that of the microporouscarbons before milling (Fig. 9a).

ConclusionsThe microporous structure of sugar carbons is destroyed

by extended bailmilling in air or in argon. The structuralchanges take place in an interesting way: first the graphenelayers become better stacked as the pores are flattened, andthen the stacking becomes poor again as the particles arereduced to smaller and smaller fragments. Milling in argoncreates large numbers of carbon radicals at the edges offractured graphene sheets, which react with the compo-nents of air upon air exposure. Milling in air causes largeamounts of oxygen to be incorporated in the carbons, pre-sumably within surface functional groups.

U

0

Fig. 13. Voltage vs capacity for the third (0.0 — 3.0 -+ 0.0 V),tenth (0.0 — 1.4 - 0.0 V) and fifteenth (0.0 - 0.6 —'0.0 V) cycles ofa Li/B3 film cell. For the third cycle the data was shifted by the irre-versible capacity measured in the first cycle, and latter cycles werealigned approximately with the third cycle.

The electrochemical reaction of lithium with milled car-bons is entirely different from the reaction with microporouscarbons, in contrast to the model presented in Ref. 2. The cellsdemonstrate large hysteresis compared to that of the originalsugar carbons. We propose that the mechanism for quasi-reversible reaction of lithium in balimilled carbons involves(i) reaction of lithium with carbon radicals at the edges offractured graphene sheets, (ii) the intercalation processwherein stacked graphene sheets remain, and (iii) reactionswith surface functional groups. The cell data show that thehysteresis in the ballmified samples is weakly dependent ontemperature and cycling rate (Fig. 11 and 12). We do notunderstand the origin of the hysteresis in these samples.

AcknowledgmentThis work was supported by the Natural Sciences and

Engineering Research Council and by 3M Canada Companyunder the NSERC/3M Canada Company Industrial ResearchChair in Materials for Advanced Batteries.

Manuscript submitted May 5, 1997; revised manuscriptreceived July 27, 1997.

Daihousie University assisted in meeting the publicationcosts of this article.

REFERENCES1. J. S. Benjamin, Met. Powder Rep., 45, 122 (1990).2. F. Disma, L. Aymard, L. Dupont, and J-M. Tarascon,

This Journal, 143, 3959 (1997).3. W. Xing, J. S. Xue, T Zheng, A. Gibaud, and J. R. Dahn,

ibid., 143, 3482 (1996).4. J. R. Dahn, T Zheng, Y. Liu, and J. S. Xue, Science, 270,

5236 (1995).5. Y. Liu, J. S. Xue, T. Zheng, and J. R. Dahn, Carbon, 34,

193 (1996).6. T. Zheng, W Xing, and J. R. Dahn, ibid., 34, 193 (1996).7. J. R. Dahn, W. Xing, and V. Gao, ibid., 35, 825 (1997).8. W Xing, J. S. Xue, and J. R. Dahn, This Journal, 143,

3046 (1996).9. N. Sonobe, M. Ishikawa, and T. Iwasaki, Paper 2B10

presented at the 35th Battery Meeting in Nagoya,Japan, Nov. 14-16, 1994, Extended Abstracts p. 49.

10. A. Gibaud, J. S. Xue, and J. R. Dahn, Carbon, 34, 499(1996).

11. W.XingandJ.R.Dahn, This Journal, 144, 1195 (1997).12. D. A. Eelman, R. A. Dunlap, and V. Srinivas, Pro-

ceedings of the International Conference on Physics

3

2

Sugar carbon ball-milledin argon for 25 h; film cellsample B4

C/b, 30°C- - - - C/40, 55°C

0

1000 300 400 500 600 700 800 900Capacity (mAh/g)





and Disordered Materials, Submitted (1997); X. X.Yan, N. Bois, and G. Cizeron, J. Phys. III, 4, 1913(1994).

13. P. Debye, H. H. Anderson, and H. Brumberger, J. AppI.Phys., 28, 659 (1957).

14. M. Kalliat, C. Y. Kwak, and P. W. Schmidt, in NewApproaches in Coal Chemistry, B. D. Blaustein, B. C.Bockrath, and S. Freidman, p. 3, American ChemicalSociety, Washington, DC (1981).

15. A. Guinier and G. Fournet, in Small Angle Scatteringof X-Rays, translated by C. B. Walker and K. L.Yudowich, John Wiley & Sons, New York (1955).

16. H. P. Boehm, Carbon, 32, 759 (1994).17. S. G. Lipson, H. Lipson, and D. S. Tannhauser, Optical

Physics, p. 189, Cambridge University Press, Cam-bridge (1995), or any elementary book on optics.

18. T. Zheng, W. R. McKinnon, and J. R. Dahn, This Jour-nal, 143, 2137 (1996).

Temperature Dependence of the Oxygen Evolution Reactionon the Pb/Pb02 Electrode

D. Pavlov* and B. Monahov

Central Laboratory of Electro chemical Power Sources, Bulgarian Academy of Sciences, 1113 Sofia, Bulgaria

ABSTRACT

Upon temperature increase the Tafel dependence of the evolution of oxygen on a PbO, layer formed anodically on Pbshifts toward higher curents. The dependence of the activation energy of the elementary processes involved in the oxygenevolution reaction on electrode potential is determined from Arrhenius curves. Upon increase in potential the activationenergy increases, passing through a maximum, and decreases thereafter. This change in activation energy is explained bythe mechanism of the elementary processes of the oxygen evolution reaction in which the hydrated layer of the lead diox-ide is also involved.

InfroductionWhen a Pb/Pb02 electrode immersed in HQSO4 solution

is anodically polarized H,0 is decomposed, releasing oxy-gen. The oxygen evolution proceeds at the end of charge,during overcharge, and on self-discharge of the lead-acidbattery PbO,, plates. Hence, the battery cells must be peri-odically topped-up with water. In order to eliminate theneed for maintenance, conditions were created in lead-acid cells for a closed oxygen cycle to operate. These bat-teries appeared on the market under the name valve-regu-lated lead-acid batteries (VRLAB). 1-3

The oxygen evolution reaction has long been subject toscientific investigation.''2 Recently, the expanding pro-duction of VRLA batteries has increased interest in thisreaction.""

Practice has shown that the rate of the closed oxygencycle in VRLA batteries depends strongly on temperature.Data published so far on the effect of temperature uponthe current/potential relationship in the oxygen evolutionreaction are controversial. Thus Puzey and Taylor9 haveestablished a dependence of the Tafel slope on tempera-ture, whereas other authors'°"8 have not observed such arelationship. All researchers of the oxygen evolution reac-tion have found that the origin of lead dioxide influencesthe behavior of the oxygen electrode.

Last year we proposed a mechanism of the elementaryreactions involved in the process of oxygen evolution2'based on the gel-crystal structure of lead dioxide.22" It hasbeen experimentally established that the structure of thelead dioxide layer consists of crystal (PbO,) and hydrated(gel) zones [PbO(OH),]." In an earlier paper on the evo-lution of oxygen" we reported experimental results whichled us to the conclusion that the oxygen evolution reactionproceeds at a certain number of active centers located inthe hydrated (gel) zones. It is assumed that the gel zonesare built of a polymer network of hydrated chains." Basedon this structure of the gel zones we have proposed the fol-lowing mechanism of the reactions of oxygen evolution(including two electrochemical and one chemical reac-tion)." An electron from one OW group of the polymerchain (active center) jumps into the polymer network,overcoming a certain potential barrier (B,,, activation

* Electrochemical Society Active Member.

energy); thus, the electron becomes common for the entirepolymer network and moves along it. As a result of thisprocess the active center is charged positively

PbO*(OH), -* PbO*(OH)...(OH)° + e [1']

PbO*(OH), is an active center. The (OH)' radicalsformed remain bound to the active center, and the bond isdenoted by (...). Pb0*(OHY...(0H)* is electroneutralizedthrough an interaction with a H,0 molecule in the hydrat-ed layer. A hydrogen ion released from the H,0 moleculemoves out of the gel zone and thus the positive chargepasses into the bulk solution

PbO*(OH)+...(OH)* + H,0 -> PbO*(OH),...(OH)* + W [1"]

These processes represent the first anodic electrochemicalreaction (FAER), which can be expressed by the followinggeneral equationPbO*(OH), + H,O -, PbO*(OH),...(OH)° + H' + e (pr) [1]

When this reaction proceeds the active centers areblocked by (OH)' radicals and the electrode is passivated.Upon increase of the potential above a value p5, the secondanodic electrochemical reaction (SAER) begins

PbO5(OH), ... (OH)' PbO*(OH), + 0 + W + e (Ps) [2]

Electrons overcome the potential barrier (B,,, activationenergy) and enter the polymer network. Hydrogen ionsmigrate into the solution. Oxygen atoms leave the activecenters and the latter are unblocked. Reaction 1 proceedsagain in the vacant active centers. The oxygen atoms accu-mulate in the hydrated zones and recombine according toreaction 3

20 - 0, [3]

0, leaves the lead dioxide layer when its pressure becomesequal to atmospheric.

The effect of temperature on this mechanism is an inter-esting problem. Hence, further research focused on deter-mining the mechanism of the influence of temperature onthe behavior of oxygen evolution reactions.

The aim of the present investigation is to determine thetemperature dependence of the rate of oxygen evolutionand from it to calculate the activation energy at various




studies of lithium insertion in ballmilled sugar carbons

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