NASA / TM--2001-210700

Effects of Surface Oxygen on the Performance

of Carbon as an Anode in Lithium-Ion Batteries

Ching-cheh Hung

Glenn Research Center, Cleveland, Ohio

Gregory W. Clark

Manchester College, North Manchester, Indiana

Prepared for the

198th Meeting

sponsored by The Electrochemical Society, Inc.

Phoenix, Arizona, October 22-27, 2000

National Aeronautics and

Space Administration

Glenn Research Center

October 2001
This report contains preliminary

findings, subject to revision as

analysis proceeds.

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EFFECTS OF SURFACE OXYGEN ON THE PERFORMANCE OF

CARBON AS AN ANODE IN LITHIUM-ION BATTERIES

Ching-cheh Hung

National Aeronautics and Space Administration

Glenn Research Center

Cleveland, Ohio 44135

Gregory W. Clark

Department of Physics

Manchester College

North Manchester, Indiana 46962

SUMMARY

Carbon materials with similar bulk structure but different surface oxygen were compared for their performance as anodes in lithium-ion battery. The bulk structure was such that the graphene planes were perpendicular

to the surface. Three types of surfaces were examined: surface containing C=O type oxygen, surface containing -O-C type oxygen, and surface containing high concentration of active sites. The test involved cycles of

lithium insertion into and release from the carbon materials, which was in the half cells of carbon/saturated

LiI-50/50 (vol %) EC and DMC/lithium. During the first cycle of lithium insertion, the presence of adsorbed

oxygen, -O-C type oxygen, active carbon sites, and C=O type oxygen resulted in the formation of solidelectrolyte interface (SEI) when the carbon's voltage relative to lithium metal was >1.35, 1 to 1.35, 0.5 to 1,

and 0.67 to 0.7 V, respectively. An optimum -O-C type oxygen and a minimum C=O type oxygen was found to

increase the reversible and decrease the irreversible capacity of carbon. Active sites on the carbon surface

result in a large irreversible capacity and a second lithium insertion-release mechanism. However, this new

mechanism has a short cycle life.

INTRODUCTION

In a previous study it was observed that the performance of an experimental carbon anode in a lithium-ion

half cell was affected significantly by the method of pretreatment of the anode immediately before the half

cell was assembled. The two pretreatment methods used in that study were 540 °C in 1 atm (1.01xl05 Pa)

nitrogen and 180 °C in 5 mm Hg (667 Pa) air. The later method results in an anode that appears to have a

longer cycle life and smaller capacity. It was suspected that the differences of these two anodes were resulted

from their differences in surface oxygen (ref. 1).

If this hypothesis is true, then the result suggests the possibility that carbon anodes in lithium-ion batteries

can be modified by a small amount of oxygen to better meet the needs of their particular applications.

The objective of this research was to confirm this hypothesis. Specifically, well-characterized experimental carbon fibers were pretreated differently to result in samples having similar bulk structure but different

surface oxygen. The surface oxygen on these samples was characterized, and the electrochemical properties

of these samples in lithium-ion half cells were examined. The possible cause-effect relations between surface

oxygen and performance as carbon anodes were than examined, and the possible implication of these results

discussed.

EXPERIMENTAL

Carbon

All experimental carbon used in this research was soft carbon obtained by chemical modification of a

commercially available graphitized carbon fiber P-100 (BP-Amoco). Physical and structural properties of these

NASA/TM--2001-210700 1
fibers were extensively characterized. Chemical properties of these fibers in the areas of intercalation and fluorination has also been studied extensively (refs. 2 to 7). The knowledge of the precursor as well as the history

of the chemical treatments is believed to be as important as the characterization of the experimental carbon in

understanding its electrochemical properties.

The graphitized carbon fibers P-100 were chemically modified to result in nongraphitized samples of soft

carbon (refs. 6 and 7) for electrochemical test. The chemical modification involved the sequence of

I 1) intercalation: exposing to vaporous bromine and iodine mixture at 110 °C and 1 atm (1.01×105 Pa)

total pressure for 3 days, where the partial pressures of bromine and iodine were approximately 1 atm

and 0, respectively, at the beginning of the reaction, and reached equilibrium with solid L and liquid

IBr at the end,

(2) fluorination: exposing to 3 cycles of pure nitrogen and pure fluorine at 330 to 370 °C and 1 atm

(1.01×105 Pa), 21 hr each half cycle, to form CF0.6s,

(3) defluorination: exposing to 1 atm (1.01×105 Pa) C4H4Br, vapor at 370 to 430 °C for 3 hr to remove

the majority of the fluorine atoms from fluorocarbon, and

(4) carbonization: heated to 1000 °C in nitrogen at 1 atm (1.01×105 Paj for 0.5 to 1 hr. This process

removes all but traces of noncarbon materials and resulted in many reactive carbon sites.

The presence of bromine and iodine during fluorination is needed in order to result in a complete carbonfluorine reaction without carbon loss in the form of light fluorocarbon. The presence of C4H4Br2 during fluorine

removal is important in preventing loss of or damage to carbon structure.

The electrical resistivity and x-ray diffraction data of both the experimental soft carbon thus obtained

(after more than 24 hrs of room temperature exposure) and its precursor are summarized in table I. Their

atomic force microscopic (AFM) images are shown in figure 1. The soft carbon and its precursor had very

different properties and appearance. However, the crystallite distribution and orientation were believed to be

similar. In the previous study, it was observed that heat treating both the soft carbon and its precursor to

2700 °C resulted in similar products (ref. 7).

Pretreatment

The purpose of the pretreatment is to let different amounts of oxygen be attached to the carbon surface

without making significant changes of the carbon's bulk properties. This was done by letting carbon samples be

exposed to air at certain temperature and pressure for a certain period of time immediately after the chemical

treatments described above.

The pretreatment of the carbon samples to be electrochemically tested was summarized in figure 2. The

pretreatments of their precursor samples were the same as Samples B and C in figure 2, except that the first

parts of the processes (intercalation, fluorination, and defluorination) were by-passed.

During room temperature air exposure, all samples were placed in a well-ventilated area. The samples that

were exposed to room temperature air for 16 rain to 2 days were complete strands of 2000 filaments. For the

samples that were exposed to room temperature air for longer than 2 days, every strand of sample was separated into about l0 substrands. The separation of the carbon fibers into small substrands increased the fiber's

accessibility to the surrounding air, and was found to significantly increase the rate of oxygen adsorption.

Surface Oxygen Analysis

The samples described in figure 2 were used for electrochemical experiments only. Their surfaces were not

characterized. Instead, surfaces of additional carbon samples were examined using x-ray photoelectron spectroscopy (XPS). These samples were treated according to the process described in figure 2, except that the

time and the sample's accessibility to the surrounding during room temperature air exposure was slightly different and will be described later in this report. The XPS data obtained from these samples were then used to

estimate the oxygen on the surface of the carbon samples that were electrochemically tested.

NASA/TM--2001-210700 2
TheXPSanalysisfocusedonamountof surfaceoxygenandwhetherit wasC-OHand/orC-O-Ctypeor

C=Otype.Thebindingenergyoftheoxygen'sIselectronpeaksat530.6+0.2eVif thesurfaceoxygenisC=O

type,and532.8+0.4eVif it isC-OHand/orC-O-Ctype(ref.8).

HalfCellTest

Theelectrochemical propertiesofthecarbonmaterialsweremeasuredinathree-electrode cellcontaininga

counterelectrodeandareferenceelectrodeoflithiummetal,andaworkingelectrodemadefromacarbonmaterial.

Bothlithiumandcarbonfiberswereconnectedtonickelrods,whichconnectedtotheelectrical instruments. The

electrolyteusedwassaturatedLiI ina50/50(vol%)mixtureofethylenecarbonateanddimethylcarbonate.

Thehalf-cellwaspreparedinaglovebagpurgedwithargon.

Duringtests,thecurrentwascontrolledat10mA/gmcarbon,andthevoltagemeasured. The"cut-offvoltage"

wassetat0Vforlithiuminsertion, and2.5Vforlithiumrelease. Inthisresearch, itisassumedthatthereversible

capacitywasapproximatelyqualtothecapacityofthelasthalfcycleoflithiumrelease. And,theirreversiblecapacitywasapproximatelyqualtothecapacitydifferencebetweenthefirsthalfcycleoflithiuminsertionandthelasthalf

cycleoflithiumrelease.

Forthepurposeofvisuallyobservingthepossibleinterferenceof lithiumsalt,atransparent pyrexglassweighingbottlewasusedasthecell,andLil wasusedasthelithiumsalt.Inthisarrangement, whenever andwherever12

isformedinthecellduringreaction,it canbeidentifiedbyitscolor.Forexample, thecoloroftheelectrolytedid

notchangeif thelithiumreleasewasstoppedat0.7V,butgraduallyturnedfromlightyellowtoorangeafterseveral

cyclesofcelltestif the"cut-offvoltage"was3.0V.

Trialrunsweremadeusingcommerciallypurchasedcrystallinegraphitepowderas working electrodes,

where nickel screen was used as the substrate, and polyvinylidene fluoride (PVDF) was used as the binder for

the graphite powder. The capacity for the first half cycle of lithium insertion to 0 V was measured to be

380 mAhr/g of graphite, within experimental error of the literature value (390 to .420 mAhr/g).

RESULTS AND DISCUSSIONS

Surface Oxygen

Oxygen content at the surface of the carbon samples examined by XPS is summarized in table II. These samples

were treated according to the process described in figure 2, except that the time and the sample's accessibility to the

surrounding during room temperature air exposure was slightly different. The differences are also included in table I1.

Based on the characteristics of their surface oxygen, the samples were divided into

3 groups as follows:

A group (I) of three samples that had similar bulk structure but different chemical states of surface oxygen.

This includes sample A, B, and C in figure 2. They were found to have similar resistivity range (900 to

1050 la_-cm). Also, their (002) x-ray diffraction peaks were at similar location (3.38 A) and have similar

width (0.8°). Their surfaces were found to have low oxygen content, C=O type oxygen (binding energy of the

ls electron 531 eV), and C-OH and/or C-O-C type oxygen (binding energy of the ls electron 532.5 eV),

respectively. It is believed that this difference was a direct result the sample treatment method as described

in figure 2.

A group (II) of two precursor samples which was heated in 1000 °C nitrogen, exposed to room temperature air

for 16 rain, and, respectively, heated in 5 mm Hg air at 180 and 540 °C overnight. Data obtained from these two

precursor samples will be used to compare to those obtained from other samples made from them.

A group (III) of six samples that had similar bulk structure and similar surface HO-C and/or C-O-C type

oxygen, but different oxygen content. This includes sample C, D, E, F, G and H in figure 2. Again, it is

believed that their differences in surface oxygen were a direct result of different room temperature air exposure

time, which was unlikely to change their bulk structures significantly.

NASA/TM--2001-210700 3
FormationofSolid-Electrolyte Interface(SEI)

Surfaceoxygenoncarboncouldaffect the chemistry of SEI formation. The SE1 thus formed is critical in determining the performance of the carbon material as the anode in a lithium-ion battery (ref. 9). It is therefore essential

to study the effects of surface oxygen on the SEI formation, which occurs during the first half cycle of the electrochemical test.

The first half cycles of two typical samples (Samples C and precursor C) were shown on figure 3. From this

figure, dividing voltages were selected at the points where capacity-voltage curves began to change shape. In this

study, the entire first half cycles were divided into six periods (>2 V, 1.35 to 2 V, 1 to 1.35 V, 0.7 to 1 V, 0.5 to

0.7 V, and 0 to 0.5 V).

The capacities of every sample during every period was calculated and summarized in table III and will

be used to explain the SEI formation in the later sections of this report.

Surface Effects

Figure 4 shows the first cycle of lithium insertion and release of five samples (Groups 1 and II described above).

Again, sample in Group II were precursors of Samples in group l, and the major difference among the three chemically modified samples in Group I was the chemical state of their surface oxygen.

In the subsequent cycles of lithium insertion and release, the capacity of insertion and release were summarized

and shown in table IV. The surface oxygen contents of these samples were estimated based on the data in table II

and are included in table IV.

From the electrochemical data described in figure 4 and table IV, several phenomena were observed and were

described as follows:

Effects of C=O Type Oxvgen.--Two samples in figure 4 contain C=O type surface oxygen. One of them was

sample B in Group l, the other was the precursor(B). Both of them were treated at 540 °C in 5 mm Hg (667 Pa) air

overnight immediately before electrochemical tests or oxygen characterization using XPS. It can be seen in figure 4

that for both of them, immediately after the beginning of their first lithium insertion at 10 mA/g rate, the voltagecapacity curves dropped quickly from 3 to 0.68 V, and slowly rose to form a plateau at 0.7 to 0.75 V range. This

suggests the electrochemical reaction of lithium and oxygen (carbonate) at this voltage.

For Sample B, after the voltage-capacity curve passed the plateaus, the voltage bypassed the range of

intercalation-deintercalation (between 0 to 0.3V) and quickly reached zero. This result suggests that, if the carbon

surface contains C=O type oxygen, its ability to intercalate-deintercalate lithium could be greatly reduced to near

zero.

The phenomenon that surface carbonate oxygen causes a reduction of intercalation capacity was not observed for

the precursor (P- 100). It is not clear why the carbonate oxygen (6.3 percent) at carbon surface could prevent lithium

intercalation completely for one sample (Sample B), but not for another (precursor of sample B). One possible

explanation is that intercalation into disordered soft carbon is more difficult than intercalation into the more ordered

graphitized precursor, and therefore easier to be blocked by the carbonate oxygen surface.

Effects of Reactive Carbon Sites Without Surface Oxygen.--Sample A in figure 2 has low surface oxygen. Also,

since it was heated at 540 °C, 1 atm (l.01xl05 Pa) nitrogen after room temperature exposure, it is believed to have

carbon sites that became chemically reactive when oxygen was removed during the heating process. Sample C, on

the other hand, had higher surface oxygen (hydroxide) and lower reactive carbon sites because it was treated at lower

temperature and pressure in air. Comparing the electrochemical data of these two samples in figure 4 and table IV, it

was found that the carbon sample with low surface oxygen and high reactive sites has a larger irreversible capacity. It

also had an extra reversible capacity during the half cycles of lithium release when the voltage was larger than 0.3 V.

The extra reversible capacity during the half cycle of lithium release when the voltage was larger than 0.3 V,

however, decreased cycle after cycle, as described in figure 5. It can be seen that the parts of capacities that were

resulted from intercalation (<0.3 V) remain unchanged during these cycles, but the parts from other mechanism

(>0.3 V) decreased. As a result, the total reversible capacity of this sample decreased cycle after cycle, as described

in table IV.

To explain the above phenomena, it is suggested that the active carbon sites were the locations where reactions

involving lithium ions and carbon occur. During the reactions, some of such sites produced strong lithium-carbon

NASA/TM--2001-210700 4
bondsthatdidnotdissociateduringtheprocessof lithium release. Some other sites, however, produced weak lithium-carbon bonds that did dissociate during the process of lithium release. The former resulted in extra irreversible

capacity, and the later resulted in extra reversible capacity when the voltage is greater than 0.3 V. In addition, the

dissociation of lithium-carbon bonds may result in carbon's structural and SEI damage, and therefore the decreasing

reversible capacities after cycles of lithium insertion and release.

Based on the above hypothesis, the first half cycle data for sample A described in figure 2 and table II1 indicates

that the reaction involving lithium and carbon active sites during SEI formation happened at about 0.7 V, where

large quantity of lithium ion influx was indicated.

Effects of Surface HO-C and/or C-O-C Type Oxyg¢ll.--Again, based on the above hypothesis, it is further suggested that when the active carbon sites were exposed to 1 atm (1.01xl0 s Pa) air at room temperature or 5 mm Hg

(667 Pa) air at 180 °C, carbon-oxygen bonds formed at the active sites, where oxygen was of C-OH and/or C-O-C

type. In this situation the carbon sites become inert to lithium during the half cycles of lithium insertion, and the

large irreversible capacity as well as extra reversible capacity when voltage was above 0.3 V disappeared.

In addition to the above results, the effects of surface HO-C and/or C-O-C type oxygen were further studied from

the electrochemical test on the samples in group IIl, figure 2 (Samples C, D, E, F, G, and H). These were similar

samples, except that the concentrations of their surface HO-C and/or C-O-C type oxygen were different. The curves of

carbon voltage versus capacity lbr the first cycles were summarized in figure 6. The capacity data for the first several

cycles were summarized in table V. The surface oxygen contents of these samples were estimated based on the data

in table II and are included in table V.

Data in table V indicate that there is an optimum oxygen (hydroxide) content where the reversible capacity of the carbon anode reaches maximum. Also, the higher the surface oxygen lhydroxide) content, the lower

the cycle numbers needed before the charge and discharge capacities approach to a constant value.

Again, some hypothesis was made to explain the above-described effects by surface oxygen (hydroxide) cannot

be found. It appears that the surface oxygen (hydroxide) could contribute positively in the process of SEI formation.

The more the surface oxygen (hydroxide), the faster the effective SEI could be formed, and the less the solvent could

attack the carbon structure. However, too much oxygen (hydroxide) could result in a SEI layer that is too thick for a

complete lithium intercalation.

From table III, it can be seen that the higher the surface oxygen content, the more the contribution to SEI during the 1.35 to 1.0 V period.

Effects of Adsorbed Oxygen.--In table IIl, it is observed that at the beginning of the first cycle, the voltages of

some samples dropped to <1.35 V much quickly than others. The former included soft carbon de-gased at 180 °C,

5 mm Hg air (Group II) and their all samples (precursor and soft carbon} de-gased at 540 °C, 5 mm Hg (667 Pa) air.

The later included soft carbon "de-gased" at 540 °C, 1 atm (1.01×105 Pa) nitrogen and its precursor de-gased at

180 °C, 5 mm Hg air. There is a general trend that a stronger de-gas condition lead to a shorter period of>1.35 V.

Assumining "de-gas" removes adsorbed oxygen, it is believed that the adsorbed oxygen on the carbon surface was a

reason for the irreversible capacity in the form of extra SEI formation during the time when the voltage was >1.35 V.

Perhaps because the concentration of such oxygen in this study is low, the effects of such SEI formation on the carbon's performance as anode in lithium-ion battery can not be identified from the data presented here.

CONCLUSIONS

The formation and the performance of the SEI, and therefore the performance of the carbon materials as

the anode, in lithium-ion battery are affected by different types and concentrations of surface oxygen on

carbon.

If the surface oxygen is of C=O type, it causes SEI formation when the carbon voltage was 0.68 V relative

to lithium metal. The SEI thus formed had only negative observable effect: It could greatly lower the capacity

for the carbon anode to be intercalated with lithium.

If the surface oxygen is of C-OH and/or C-O-C type, the SEI formation at 1 to 1.35 V becomes important.

The oxygen content need to be optimal in order to produce an SEI that has optimum thickness: Thick enough

to effectively protect the lithium which is subsequently intercalated into carbon, but thin enough to not

become a barrier on the path of lithium ion transport.

NASA/TM--2001-210700 5
If thesurfaceoxygenisinthestateof loose adsorption, it would cause SEI formation at >1.35 V. The

effects of such SEI formation on the carbon's performance as anode cannot be determined in this study

because of its low concentration.

On the other hand, if the carbon surface contains low oxygen but high concentration of active sites, it

would cause a large amount of SEI formation at about 0.7 V. The SEI thus formed contains some lithium atoms

that are tightly bonded, and some lithium atoms that are loosely bonded. The former cannot be removed from

SEI during the half cycle of lithium release, and therefore become the source of high irreversible capacity. The

later can be removed from SEI during the half cycle of lithium release, and therefore become a new mechanism of reversible capacity. This new mechanism, however, can result in damaged SEI, and therefore short

cycle life for the carbon anode.

REFERENCES

1. Hung, C., NASA TM-209437, 1999.

2. Singer, L., Fuel, 70, 1981, pp. 839-847.

3. Hoffman, W., Carbon, 30 (3), 1992, pp. 315-331.

4. Jaworske, D., Gaier, J., Hung, C., and Banks, B., SAMPE Quarterly, 18, 1, 1986, pp. 9-14

5. Hung,C. and Kucera, D., Carbon, 32 (8), 1994, pp. 1441-1448

6. Hung, C., U.S. Patent 5,662,683, 1997.

7. Hung, C., NASA TM-106398, 1994.

8. Harvey, J.. Kozlowski, C., and Sherwood, P., Journal of Material Science. 22, 1987, pp. 1585-1596.

9. Dahn, J., Sleigh, A., Shi, H., Way, B., Weydanz, W., Reimers, J., Zhong, Q., and yon Saken, U., Carbon

and Graphite as Substitutes for the Lithium Anode in Lithium Batteries, G. Pistoia (ed.) pp. 1-47, Elsevier,

Amsterdam, 1994.

NASA/TM---2001-210700 6
TABLE I. RESISTIVITY AND X-RAY DIFFRACTION DATA

OF GRAPHITIZED CARBON FIBERS P-100 AND A SOFT

CARBON MADE FROM P-100 USING THE SEQUENTIAL

PROCESSES OF INTERCALATION WITH BROMINE

AND IODINE, FLUORINATION 1N FLUORINE,

DEFLUORINATION IN BrtI,.C-CH-CII-CHzBr,

AND CARBONATION AT 1000 °C IN N,

P- 100 Soft carbon

Diameter, t_m 10.0 12.5

! Resistivity. ttD-cm 390 1020

X-ray diffraction, 002

Peak position, A 3.38 3.37

Peak width at half max position, deg 0.45 1.0

Relative peak height 5 1

TABLE 11. TIlE TYPE AND TIlE CONTENT (%) OF THE OXYGEN ON THE SURFACE OF

THE CARBON SAMPLES OBTAINED FROM TIlE PROCESS DESCRIBED IN FIGURE 2,

EXCEPT TIlE SLIGttT DIFFERENCES IN ROOM TEMPERATURE EXPOSURE

CONDITIONS AS DESCRIBED IN TIlE TABLE

Sample"

C

C

C

C

C

C

C

C

G

G

Room temperature

air exposure, 1_ step

Time Fiber compacmess

during this step

16 min single strand

16 min single strand

16 min single strand

22 hr sin,gte strand

32 hr single strand

30 hr separated strand

72 hrs separated strand

150 hr ti,ght strand

16 min single strand

16 min single strand

Overnight heating,

2 "d step

Temp, Pressure. Type of gas

°C Pa

540 1.01xl0 _ Nz

540 667 Air

180 667 Air

180 667 Air

180 133 Air

t 80 667 Air

180 667 Air

180 667 Air

540 667 Air

180 667 Air

Oxygen

Content, Type

at %

1.4 -O-C h

6.3 O-C

4.6 -O-C

5.0 -O-C

4.7 -O-C

8.2 -O-C

8.5 -O-C

5.1 -O-C

6.8 O-C

0.9 -O-C

"Sample G is graphitized carbon fiber P-100 heated to 1000 °C in Nz. Sample C is the soft carbon

made from P-100 using the sequential processes of intercalation with bromine and iodine,

fluorination in fluorine, defluorination in BrHeC-CH-CH-CHzBr, and carbonation at 1000 °C in N_,.

hThis sample has high concentration of active sites and low oxygen concentration.

TABLE Ill. TftE CAPACITIES OF THE SAMPLES STUDIED IN THIS REPORT DURING DIFFERENT PERIODS 1N TIlE

FIRST HALF CYCLE OF THE LITHIUM INSERTION

Sample _

Label Group b

A 1

B I

C I & II1

D IIl

E III

F Ill

G IIl

H Ill

Precursor (B) II

Precursor (C) [I

SurNce oxygen

Content, Type

at %"

1.5 -O-C _

6 O=C

4.5 -O-C

4.5 -O-C

5 -O-C

5 -O-C

9 -O-C

9 -O-C

7 O=C

1 -O-C

Capacities during the periods of different voltage ranges in the 1" half cycle,

mAh/gm

>2 V

1.4

0

0.04

0.11

0.2

0.08

0.3

0.3

0.4

4.1

1.35 to 2 V 1 to t.35 V 0.7to 1 V 0.5 to 0.7V 0 to 0.5V

4.6 2.9 82.6 71.8 303.1

0 0.05 1.0 130.8 '_ 29.9

0.86 2.6 55.2 22.4 255.7

2.1 4.2 57.1 25.9 269.3

1.1 7.8 53 21.9 285.6

1 6.9 56 23.4 274.7

2.1 8.3 44.5 19.3 260.2

1.5 8.2 44.5 19.9 26l .5

0.8 0.9 3.7 44.2 250.3

0.5 2.5 4.9 6.7 238.3

_Samples A to H were obtained from the process described in figure 2. Precursor (C) and _recursor (B) were obtained in the same way

as sample C and sample B, respectively, except the first 3 steps (intercalation, fluorination, and defluorination) ",','ere by-passed

_The samples were divided into 3 groups to facilitate data comparison.

'Surface oxygen was estimated based on the result described in table Il.

dln this particular sample, the voltage quickly dropped to 0.68 V, then increased slowly to a maximum of 0.78 V, and then slowly

decrease to 0.5 V. The capacities were 0.7, 33.8 and 96.3 mAh/g during the sub-divisions of 0.7 to 0.68 V, 0.68 to 0.78 V, and 0.78 to

0.5 V, respectively.

"This sample has high concentration of active sites and low oxygen concentration.

NASA/TM--2001-210700 7
TABLEIV. THECAPACITIES/mAh/g) OFELECTROCHEMC1AL INSERTION ANDRELEASE OF

LITHIUM IONS AT 10 mA/g RATE FOR SAMPLES A, B, C, PRECURSOR (B) AND

PRECURSOR (C) DESCRIBED IN TABLE Ill

Sample (b) Cycle number

Oxygen" 1 2 3 4 5 6

A I 466 315 309 303 301 296

1.5% -O-C R 304 301 295 295 292 289

B I 161 95

6% -O R 95 86

C I 337 25l 251 249 247 248

4.5% -O-C R 252 249 249 247 246 247

Precursor (C) 1 257 242 246

1% -O-C R 245 241 244

Precursor (B) I 298 274 274 272

7% =O R 272 271 271 268

"Oxygen type and at %.

7 8 9

249 248 249

247 247 246

Irreversible

capacities J

177

75

91

13

30

hEqual to the difference between the capacity of the first lithium insertion and the capacity of the last lithium

release.

TABLE V.--THE CAPACITIES (mAh/g) OF ELECTROCHEMICAL INSERTION AND RELEASE OF

LITHIUM IONS AT 10 mA/g RATE FOR SAMPLES D, E, F, G, AND H DESCRIBED IN

FIGURE 4 AND TABLE 3.

Sample (b) Cycle number l_eversible

capacities"

Oxygen at%" 1 2 3 4 5 6 7 8 9

C 1 337 251 251 249 247 248 249 248 249 91

4.5% R 252 249 249 247 246 247 247 247 246

D I 359 260 258 254 254 254 254 106

4.5% R 259 256 254 254 254 254 253

E I 370 283 279 277 276 275 95

5% R 280 276 277 276 275 275

F 1 362 268 266 99

5% R 267 265 263

G 1 335 252 250 249 247 247 246 88

9% R 250 249 249 249 248 248 247

H 1 334 252 85

9% R 250 249

"The surface oxygen on the samples described in this table had the oxidation state of hydroxide.

bEqoal to the difference between the capacity of the first lithium insertion and the capacity of the last

ithium release.

NASA/TM--2001-210700 8
1.75

E 0.88

(a) 0

ixm

4

1.25

E 0.64

_m

4

0 ....... 2

(b) 0 Fm

Figure 1(a).--Atomic force microscopy of surfaces

of the soft carbon electrochemically tested in this

study. (b). Atomic force microscopy of surfaces of

the precursor electrochemically tested in this study.

Data taken after the samples were exposed to

ambient air for more than 1 week.

I p. 001

Intercalation

Fluorination

Defluorination

Carbonization

v

Soft carbon with active surface sites

l Room temperature air exposure

116 130128 2 J dTa_

minutes minutes hours days

latm 5mmHg 5mmHg I

nitrogen air air /

A B C DEFGH

Figure 2.mPretreatments of the carbon samples electrochemically tested in this

research.

16

days

NASA/TM--2001-210700 9
3.0

2.5

2.0

v 1.5

1.0

0.5

0.0

--C

....... Precursor (C)

i

The first 7 mAhr/g

3

0 2 4 6

mAhr/g

0 50 100 150 200 250 300 350 40O

Capacity (mAhr/g)

Figure 3.---Carbon electrode voltage during the first half cycle of lithium

insertion for sample C and precursor (C).

NASA/TM--2001-210700 10
3.5

3.0

>

2.5

E

.E 2.0

1.5

O_

>

G)

m 1.0

0.5

0.0

: A /.r--

C_.

IP (B)

....... A (low oxygen)

....... B (O = C)

is _-- P (C) -- CPrecursor (-O-C) (C)

Precursor (B)

i-_-B

-500 -400 -300 -200 -100 0

(a) Capacity (mAh/g)

i

2.5 ,_ / ...........A (ow oxygen) JC P_ (B).'

2.0 l D -'_'v"_'/ / ........... _ B(=O, C (-O-C) I /

| / -- Precursor(C) _i ,;' A

i 1 / ..............Precursor(B) j l /

"-z

1.5 / /

l.0 ' ,

g

)...

0.0

0 50 100 150 200 250 300 ;50

(b) Capacity (mAh/g)

Figure 4.mVoltage of anodes made from carbon with different surfaces during

first half cycles of lithium. (a) Insertion. (b) Release.

NASA/TM--2001-210700 11
E

E

._=

¢-

u_

O

3.0

3"--_ r-2

Cycle _ /

2.5 _ /

I st

2nd

2.0 3rd

6th 6 -J" 1

1.5

1.0

0.5

0.0 .......

0 50 100 150 200 250 300

Capacity (mAh/g)

Figure 5.---Capacities of lithium release from an anode made from carbon

containing active sites.

350

NASA/TM--2001-210700 12
REPORT DOCUMENTATION PAGE FormApprove_

OMB No, 0704-0188

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1. AGENCY USE ONLY (Leave b/ank) 2. REPORT DATE 3. REPORT TYPE AND DATES COVERED

October 2001 Technical Memorandum

4. TITLE AND SUBTITLE 5. FUNDING NUMBERS

Effects of Surface Oxygen on the Performance of Carbon as an Anode

in Lithium-Ion Batteries

6. AUTHOR(S)

Ching-cheh Hung and Gregory W. Clark

7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES)

National Aeronautics and Space Administration

John H. Glenn Research Center at Lewis Field

Cleveland, Ohio 44135- 3191

9. SPONSORING/MONITORING AGENCY NAME(S) AND ADDRESS(ES)

National Aeronautics and Space Administration

Washington, DC 20546-0001

WU-755-A4-13--00

8, PERFORMING ORGANIZATION

REPORT NUMBER

E-12631

10, SPONSORING/MONITORING

AGENCY REPORT NUMBER

NASA TM--2001-210700

11. SUPPLEMENTARY NOTES

Prepared for the 198th Meeting sponsored by The Electrochemical Society, Inc., Phoenix, Arizona, October 22-27, 2000.

Ching-cheh Hung, NASA Glenn Research Center; Gregory W. Clark, Department of Physics, Manchester College,

North Manchester, Indiana 46962. Responsible person, Ching-cheh Hung, organization code 5480, 216-433-2302.

12a. DISTRIBUTION/AVAILABILITY STATEMENT 12b. DISTRIBUTION CODE

Unclassified- Unlimited

Subject Categories: 23 and 25 Distribution: Nonstandard

Available electronically at htto://_ltrs._rc.nasa.2ov/GLTRS

This publication is available from the NASA Center for ,aeroSpace Information. 301_21_)390.

13. ABSTRACT (Maximum 200 words)

Carbon materials with similar bulk structure but different surface oxygen were compared for their performance as

anodes in lithium-ion battery. The bulk structure was such that the graphene planes were perpendicular to the surface.

Three types of surfaces were examined: surface containing C=O type oxygen, surface containing -O-C type oxygen, and

surface containing high concentration of active sites. The test involved cycles of lithium insertion into and release from

the carbon materials, which was in the half cells of carbon/saturated Lii-50/50 (vol %) EC and DMC/lithium. During the

first cycle of lithium insertion the presence of adsorbed oxygen, -O-C type oxygen, active carbon sites, and C=O type

oxygen resulted in the formation of solid-electrolyte interface (SEI) when the carbon's voltage relative to lithium metal

was >1.35, 1 to 1.35, 0.5 to 1, and 0.67 to 0.7 V, respectively. An optimum -O-C type oxygen and a minimum C=O type

oxygen was found to increase the reversible and decrease the irreversible capacity of carbon. Active sites on the carbon

surface result in a large irreversible capacity and a second lithium insertion-release mechanism. However, this new

mechanism has a short cycle life.

14. SUBJECT TERMS

Lithium-ion battery: Carbon anode; Surface oxygen; Active carbon sites; SEI formation

17. SECURITY CLASSIFICATION

OF REPORT

Unclassified

NSN 7540-01-280-5500

18. SECURITY CLASSIFICATION

OF THIS PAGE

Unclassified

15. NUMBER OF PAGES

19

16. PRICE CODE

19. SECURITY CLASSIFICATION 20. LIMITATION OF ABSTRACT

OF ABSTRACT

Unclassified

Standard Form 298 (Roy. 2-89)

Prescribed by ANSI Std. Z39-18

298-102
3.0

E _

2.5-

2.0

v 1.5

1.0

0.5

0.0

-400

(a)

C

D

--- G and H

I i I

-350 -300 -250 -200

(mAh/g)

-- C 4.5%

D 4.5% +

-- E5%

--F5%+

-- G9%

H9%

-150 -100 -50

2.5

2.0

1.5

v

1.0

0.5

-- C 4.5%

D 4.5% +

E5%

-- F5%+

-- G9%

--H9%

0.0

0 50

(b)

I . I I

100 150 200 250 300

(mAh/g)

Figure 6.--Voltage of anodes made from carbon with different concentration

of --O-C type surface oxygen during the first half cycles of lithium. (a) Insertion. (b) Release.

NASA/TM--2001-210700 13