Transmission electron energy-loss spectrometry was used to investigate the electronic states of metallic Li and LiC6, which is the Li-intercalated graphite used in Li-ion batteries. The Li K edges of metallic Li and LiC6 were nearly identical, and the C K edges were only weakly affected by the presence of Li. These results suggest only a small charge transfer from Li to C in LiC6, contrary to prior results from surface spectra obtained by x-ray photoelectron spectroscopy. Effects of radiation damage and sample oxidation in the transmission electron microscopy are also reported

Ahn, C. C.

Fultz, B.

Hightower, A.

Rez, P.

English

Caltech Authors - Main

APPLIED PHYSICS LETTERS VOLUME 77, NUMBER 2 10 JULY 2000Electron energy-loss spectrometry on lithiated graphite
A. Hightower, C. C. Ahn,a) and B. Fultz
Division of Engineering and Applied Science, California Institute of Technology, Pasadena, California 91125
P. Rez
Department of Physics and Astronomy, Arizona State University, Tempe, Arizona 85287
~Received 29 November 1999; accepted for publication 23 May 2000!
Transmission electron energy-loss spectrometry was used to investigate the electronic states of
metallic Li and LiC6, which is the Li-intercalated graphite used in Li-ion batteries. The LiK edges
of metallic Li and LiC6 were nearly identical, and the CK edges were only weakly affected by the
presence of Li. These results suggest only a small charge transfer from Li to C in LiC6, contrary to
prior results from surface spectra obtained by x-ray photoelectron spectroscopy. Effects of radiation
damage and sample oxidation in the transmission electron microscopy are also reported. ©2000
American Institute of Physics.@S0003-6951~00!04428-4#Li
o
m
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inLithiated graphite is the standard anode material in
ion rechargeable batteries.1 Highly crystallized graphite can
intercalate Li atom to a maximum composition of LiC6. This
is equivalent to a specific charge of 372 Ah kg21,2 although
in practice graphite anodes have specific energies
320– 360 Ah kg21. Graphite anodes have high voltages
3–4 V versus the cathode, but the difference in electroche
cal potential between metallic Li and lithiated graphite
small, of order 0.01 V. The intercalation of Li into highl
crystallized graphite changes the stacking sequence of
hexagonal planes from an ABABAB to AAAAAA.3 This
change in stacking sequence and the high chemical pote
of Li in graphite suggest that a better understanding of
interlayer states of LiC6 may facilitate improvements to Li
ion electrochemical cells.
The results of numerous studies on the band structur
Li intercalated graphite demonstrate the difficulty in det
mining the degree of hybridization between Li atomic orb
als and graphite interlayer states. Early theoretical calc
tions of the LiC6 band structure began with the notion
complete charge transfer of Li valence electrons to
graphitep bands.4,5 This evolved into an elegant theory o
alkali-intercalated graphite interlayer states as interac
nonorthogonal hybrid states of Li 2s and graphite interlaye
states.6,7 This gives credence to x-ray photoelectron spectr
copy ~XPS! results by Momoseet al.8 and others9 claiming
Li to be intercalated into graphite as ionic Li1. Early experi-
mental work by Gruneset al.10 using electron energy-los
spectroscopy~EELS! demonstrated distortions of the grap
ite band structure upon intercalation of alkali meta
Hartwigsenet al.11 used a density functional theory, loc
density approximation to determine the degree of cha
transfer from Li to the intercalant host lattice to be 0.5e
LiC6 and 0.4e for LiC8. Further experiments using inelast
x-ray scattering spectroscopy by Schu¨lke12 were able to cor-
relate features of LiC6 spectra to band structure calculatio
by Holzwarthet al.13
The present letter reports transmission EELS meas
ments of the LiK edge in intercalated graphite and in m
a!Electronic mail: cca@caltech.edu2380003-6951/2000/77(2)/238/3/$17.00
Downloaded 12 Jan 2006 to 131.215.240.9. Redistribution subject to AIP-
of
f
i-
he
ial
e
of
-
a-
e
g
-
.
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r
e-
tallic Li. After showing oxidation tendencies of the transmi
sion electron microscopy~TEM! samples and how this wa
controlled, we show that the LiK edge for Li in LiC6 re-
sembles the LiK edge of neutral metallic Li rather than o
Li1. We believe that the name ‘‘Li-ion’’ battery is therefor
misleading.
LiC6 samples were prepared by electrochemi
methods.14 Anodes were constructed from KS 44 graph
using 6 wt % poly~vinylidene fluoride! as a binder. A ternary
mixture of aykyl carbonates, i.e., 1:1:1 of ethylene carbona
diethylene carbonate, and dimethyl carbonate~DMC! with 1
M LiPF6 was used as the electrolyte. Carbon half-cells w
subjected to 40 electrochemical charge-discharge cycles.
carbon electrodes were left in a charged state before b
washed in DMC and vacuum dried.
Thin samples of metallic Li and LiF were prepared b
thermal evaporation onto amorphous holey carbon TE
grids. A glove bag filled with Ar gas was placed over th
evaporator to minimize atmospheric exposure during tra
fers. The evaporating chamber was backfilled with Ar whe
the samples were immersed in Flourinert® FC-43, gettered
with Li chips. The LiC6 samples were crushed with a mort
and pestle in an Ar-filled glove box. The powdered ano
material was placed onto holey carbon TEM grids. The TE
grids were immersed in Li-gettered Flourinert FC-43 befo
insertion into the microscope load lock, where the Flourin
evaporated. Transmission EELS spectra were acquire
room temperature using a Gatan 666 parallel detection m
netic prism spectrometer attached to a Philips EM 420 tra
mission electron microscope. Energy resolution of the sp
trometer was about 1.2 eV with a dispersion of 0.2 eV p
channel. Measurements were performed with 100 keV e
trons at a collection angle of either 11 or 50 mrad. EE
spectra of the evaporated samples were acquired with im
coupling. The LiC6 spectra were acquired with diffractio
coupling. The TEM beam current was approximately 7 n
X-ray diffractometry was performed with an Inel powd
diffractometer using MoKa radiation (l50.07092 nm). An-
ode materials were sealed under Ar with paraffin wax
Pyrex capillary tubes. X-ray diffraction patterns, as shown
Fig. 1, confirmed the presence of LiC6 and LiC12 phases in© 2000 American Institute of Physics
 license or copyright, see http://apl.aip.org/apl/copyright.jsp
st
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239Appl. Phys. Lett., Vol. 77, No. 2, 10 July 2000 Hightower et al.the graphite anode materials. These results were consi
with the color of lithiated graphite which depends on t
predominance of LiC6 ~green/gold! or LiC12 ~blue/black!.
15
The diffraction peaks were broader for LiC6 than for the
unlithiated KS 44 graphite samples.
The reactivity of reduced Li and its susceptibility to r
diation damage under an electron beam present a formid
challenge to EELS analysis of LiC6. The majority of radia-
tion damage is a result of the direct transfer of moment
from incident electrons to the Li nucleus, ejecting it from
lattice site. Average atomic displacement energies estim
from sublimation energies were found to be 6.1 eV for
and 31.2 eV for C in graphite.16 Assuming elastic collisions
the knock-on damage, threshold energies for incident e
trons are 19 keV for Li and 150 keV for C. Metallic L
samples appeared to ‘‘boil’’ during TEM observations at 2
keV. This was determined to be a consequence of knock
damage since the calculated temperature rise was neglig
approximately 1 K.17 We chose a microscope voltage of 10
keV as a reasonable compromise between knock-on dam
and instrument performance.
Beam damage of metallic Li was studied at 100 keV
concentrating the electron beam onto a thin area of the
sample. Figure 2 presents a time sequence of EELS sp
for this exposed region. Over a few minutes, the spectral a
of the low-loss region was reduced significantly, resulting
the suppression of Li plasmons at 7.5 and 15 eV. This
consistent with a thinning of the sample as Li atoms
ejected from the sample under the electron beam. In im
mode, regions of the specimen were observed to shrink
pecially under a focused incident beam.
Over a slightly longer time scale under a focused bea
the Li plasmons evolved into distinct profiles commonly a
sociated with oxidized Li.18 During this time, as shown in
Fig. 2, the broad profile of the LiK edge evolves into two
sharp peaks at 59.5 and 64.5 eV, similar to peaks obse
from samples exposed to atmosphere.18 Exposure to the in-
tense electron beam stimulates Li reactions with moist
and oxygen present in the microscope, leading to the for
tion of LiOH and Li2O.
19 The Li K edge in our LiC6 samples
showed the same peaks at 59.5 and 64.5 eV after the sam
were exposed to atmosphere.
FIG. 1. X-ray diffraction patterns of KS 44 graphite and electrochemica
lithated graphite containing LiC6 and LiC12 phases.Downloaded 12 Jan 2006 to 131.215.240.9. Redistribution subject to AIPent
le
ed
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les
Figure 3 presents the CK edges of LiC6 and KS 44
graphite. A peak at 301 eV is found in LiC6 but not in the
graphitic carbon. Disko20 and others21 have demonstrated th
sensitivity of the CK edge to the direction of the momentu
transfer vectorq. We therefore do not expect the intensity
the 1s→p* peak to be reliable, since it is not an avera
over many crystal orientations. Nevertheless, the onset
ergy of the CK edge and the location of the 1s→p* peak
are essentially the same for graphitic carbon and LiC6.
Figure 4 displays the LiK edge obtained from interca
lated LiC6, LiF, and metallic Li. We identify the onset of th
Li K edge in the metallic sample at 55 eV, consistent w
results of Liu and Williams.18 The broad profile of the me
tallic Li K edge is consistent with the promotion of co
electrons into a continuum of free electron states. The LK
edge onset of LiC6 is shifted by only 0.2 eV from theK edge
of metallic Li and has a similarly broad profile. Our observ
Li K edge for LiC6 lacks the peaks at 59 and 63 eV prev
ously observed by Gruneset al.10 These peaks evidently
originate from oxidized Li—they are found in samples e
posed to atmosphere, or after long times in the microsc
~Fig. 2!. The Li K edge of LiF, similar to that reported b
FIG. 2. EELS spectra of beam-damaged Li. Spectra were acquired a
proximately equal intervals from nearly zero, to 4 min of electron be
exposure.
FIG. 3. CK edge for LiC6 and KS 44 graphite, normalized by the area
eV after edge onset.
 license or copyright, see http://apl.aip.org/apl/copyright.jsp
a
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.
240 Appl. Phys. Lett., Vol. 77, No. 2, 10 July 2000 Hightower et al.Chenet al.,22 shows a strong chemical shift and sharp fe
tures of well-defined unoccupied states. Such large dif
ences are expected with the large electron transfer from
to F.
Our EELS spectra of the LiK edge indicate that Li in
LiC6 has a local electronic structure more similar to Li me
than Li1, contrary to previous XPS results. Extensive XP
results confirm a LiK edge chemical shift of about 3 eV i
LiC6 with respect to metallic Li,
8,9 easily resolved from the
1.3 eV chemical shift of Li2O.
23 The mean free path of pho
toelectrons measured by XPS is on the order of 1 nm. Th
assuming that the surface Li remains unreacted, we ex
XPS to be more sensitive to Li surface states.
The similarities of LiK edges in LiC6 and metallic Li
are consistent with observed electrochemical potenti
Measurements of electrochemical potentials in half c
show LiC6 to be less than 0.01 V above Li metal.
24 The
similar chemical potentials of Li in LiC6 and metallic Li
suggest similar environments for valence electrons. Elec
chemical potentials and EELS LiK edge profiles both sug
gest limited charge transfer from Li to surrounding carbo
The observed EELS CK edge is little affected by interca
lated Li, but effects should be diminished by the 6:1 ratio
C to Li, and could be diminished further if there were a lo
of Li from the TEM sample. The shape of the LiK edge, on
FIG. 4. Li K edge of metallic Li, LiC6, and LiF. The pre-edge backgroun
of the raw data was fit to an exponential and subtracted from the dataDownloaded 12 Jan 2006 to 131.215.240.9. Redistribution subject to AIP-
r-
Li
l
s,
ct
s.
s
o-
.
f
s
the other hand, would be unaffected by any possible los
Li, or perhaps would appear even more ionic, since the e
trochemical potential of Li would be lowered.
This work was supported by DOE through Basic Ener
Sciences Grant No. DE-FG03-94ER14493.
1A. Hérold, Bull. Soc. Chim. Fr.187, 999 ~1955!.
2M. S. Dresselhaus and G. Dresselhaus, Adv. Phys.30, 139 ~1980!.
3J. R. Dahn, A. K. Sleigh, H. Shi, B. M. Weydanz, J. N. Reimers,
Zhong, and U. von Sacken, inLithium Batteries: New Materials, Devel
opments and Perspectives, edited by G. Pistoia~Elsevier, Amsterdam,
1994!, p. 1.
4N. A. W. Holzwarth, S. G. Louie, and S. Rabii, Phys. Rev. B26, 5382
~1982!.
5J. J. Ritsko, Phys. Rev. B25, 6452~1982!.
6N. A. W. Holzwarth, S. G. Louie, and S. Rabii, Phys. Rev. B30, 2219
~1984!.
7M. Posternak, A. Baldereschi, A. J. Freeman, E. Wimmer, and M. We
ert, Phys. Rev. Lett.50, 761 ~1983!.
8H. Momose, H. Honbo, S. Takeuchi, K. Nishimura, T. Horiba, Y. M
ranaka, Y. Kozono, and H. Miyadera, J. Power Sources68, 208 ~1997!.
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Commun.33, 1127~1979!.
10L. A. Grunes, I. P. Gates, J. J. Ritsko, E. J. Mele, D. P. DiVincenzo, M
Preil, and J. E. Fische, Phys. Rev. B28, 6681~1983!.
11C. Hartwigsen, W. Witschel, and E. Spohr, Phys. Rev. B55, 4953~1997!.
12W. Shülke, A. Berthold, A. Kaprolat, and H. J. Guntherodt, Phys. Re
Lett. 60, 2217~1988!.
13N. A. W. Holzwarth, S. G. Louie, and S. Rabii, Phys. Rev. B28, 1013
~1983!.
14M. C. Smart, B. V. Ratnakumar, S. Surampudi, Y. Wang, X. Zhang, S.
Greenbaum, A. Hightower, C. C. Ahn, and B. Fultz, J. Electrochem. S
146, 3963~1999!.
15S. Basu, G. K. Wertheim, and S. B. Dicenzo, inLithium: Current Appli-
cations in Science, Medicine and Technology, edited by R. O. Bach
~Wiley, New York, 1985!, p. 187.
16S. Ono, K. Hojou, and K. Kanaya,Proc. Fifth International Conference
on High Voltage Electron Microscopy~Jap. Soc. Electron Microscopy
Kyoto, 1977!, p. 481.
17L. W. Hobbs, inQuantitative Electron Microscopy, Proceedings of th
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18D. R. Liu and D. B. Williams, Philos. Mag. B53, 123 ~1986!.
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Geiss~San Francisco Press, Inc., San Francisco, CA, 1981!, p. 218.
21W. Shülke, K.-J. Gabriel, A. Berthold, and H. Schulte-Schrepping, So
State Commun.79, 657 ~1991!.
22T.-C. Chen, M. Qian, and T. G. Stoebe, J. Phys.: Condens. Matter11, 341
~1999!.
23K. R. Zavadil and N. R. Armstrong, Surf. Sci.230, 47 ~1990!.
24J. R. Dahn, Phys. Rev. B44, 9170~1991!. license or copyright, see http://apl.aip.org/apl/copyright.jsp


Electron energy-loss spectrometry on lithiated graphite

APPLIED PHYSICS LETTERS VOLUME 77, NUMBER 2 10 JULY 2000Electron energy-loss spectrometry on lithiated graphite
A. Hightower, C. C. Ahn,a) and B. Fultz
Division of Engineering and Applied Science, California Institute of Technology, Pasadena, California 91125
P. Rez
Department of Physics and Astronomy, Arizona State University, Tempe, Arizona 85287
~Received 29 November 1999; accepted for publication 23 May 2000!
Transmission electron energy-loss spectrometry was used to investigate the electronic states of
metallic Li and LiC6, which is the Li-intercalated graphite used in Li-ion batteries. The Li K edges
of metallic Li and LiC6 were nearly identical, and the C K edges were only weakly affected by the
presence of Li. These results suggest only a small charge transfer from Li to C in LiC6, contrary to
prior results from surface spectra obtained by x-ray photoelectron spectroscopy. Effects of radiation
damage and sample oxidation in the transmission electron microscopy are also reported. © 2000
American Institute of Physics. @S0003-6951~00!04428-4#Lithiated graphite is the standard anode material in Li-
ion rechargeable batteries.1 Highly crystallized graphite can
intercalate Li atom to a maximum composition of LiC6. This
is equivalent to a specific charge of 372 Ah kg21,2 although
in practice graphite anodes have specific energies of
320– 360 Ah kg21. Graphite anodes have high voltages of
3–4 V versus the cathode, but the difference in electrochemi-
cal potential between metallic Li and lithiated graphite is
small, of order 0.01 V. The intercalation of Li into highly
crystallized graphite changes the stacking sequence of the
hexagonal planes from an ABABAB to AAAAAA.3 This
change in stacking sequence and the high chemical potential
of Li in graphite suggest that a better understanding of the
interlayer states of LiC6 may facilitate improvements to Li-
ion electrochemical cells.
The results of numerous studies on the band structure of
Li intercalated graphite demonstrate the difficulty in deter-
mining the degree of hybridization between Li atomic orbit-
als and graphite interlayer states. Early theoretical calcula-
tions of the LiC6 band structure began with the notion of
complete charge transfer of Li valence electrons to the
graphite p bands.4,5 This evolved into an elegant theory of
alkali-intercalated graphite interlayer states as interacting
nonorthogonal hybrid states of Li 2s and graphite interlayer
states.6,7 This gives credence to x-ray photoelectron spectros-
copy ~XPS! results by Momose et al.8 and others9 claiming
Li to be intercalated into graphite as ionic Li1. Early experi-
mental work by Grunes et al.10 using electron energy-loss
spectroscopy ~EELS! demonstrated distortions of the graph-
ite band structure upon intercalation of alkali metals.
Hartwigsen et al.11 used a density functional theory, local
density approximation to determine the degree of charge
transfer from Li to the intercalant host lattice to be 0.5e for
LiC6 and 0.4e for LiC8. Further experiments using inelastic
x-ray scattering spectroscopy by Schu¨lke12 were able to cor-
relate features of LiC6 spectra to band structure calculations
by Holzwarth et al.13
The present letter reports transmission EELS measure-
ments of the Li K edge in intercalated graphite and in me-
a!Electronic mail: cca@caltech.edu2380003-6951/2000/77(2)/238/3/$17.00
Downloaded 12 Jan 2006 to 131.215.240.9. Redistribution subject totallic Li. After showing oxidation tendencies of the transmis-
sion electron microscopy ~TEM! samples and how this was
controlled, we show that the Li K edge for Li in LiC6 re-
sembles the Li K edge of neutral metallic Li rather than of
Li1. We believe that the name ‘‘Li-ion’’ battery is therefore
misleading.
LiC6 samples were prepared by electrochemical
methods.14 Anodes were constructed from KS 44 graphite
using 6 wt % poly~vinylidene fluoride! as a binder. A ternary
mixture of aykyl carbonates, i.e., 1:1:1 of ethylene carbonate,
diethylene carbonate, and dimethyl carbonate ~DMC! with 1
M LiPF6 was used as the electrolyte. Carbon half-cells were
subjected to 40 electrochemical charge-discharge cycles. The
carbon electrodes were left in a charged state before being
washed in DMC and vacuum dried.
Thin samples of metallic Li and LiF were prepared by
thermal evaporation onto amorphous holey carbon TEM
grids. A glove bag filled with Ar gas was placed over the
evaporator to minimize atmospheric exposure during trans-
fers. The evaporating chamber was backfilled with Ar where
the samples were immersed in Flourinert® FC-43, gettered
with Li chips. The LiC6 samples were crushed with a mortar
and pestle in an Ar-filled glove box. The powdered anode
material was placed onto holey carbon TEM grids. The TEM
grids were immersed in Li-gettered Flourinert FC-43 before
insertion into the microscope load lock, where the Flourinert
evaporated. Transmission EELS spectra were acquired at
room temperature using a Gatan 666 parallel detection mag-
netic prism spectrometer attached to a Philips EM 420 trans-
mission electron microscope. Energy resolution of the spec-
trometer was about 1.2 eV with a dispersion of 0.2 eV per
channel. Measurements were performed with 100 keV elec-
trons at a collection angle of either 11 or 50 mrad. EELS
spectra of the evaporated samples were acquired with image
coupling. The LiC6 spectra were acquired with diffraction
coupling. The TEM beam current was approximately 7 nA.
X-ray diffractometry was performed with an Inel powder
diffractometer using Mo Ka radiation (l50.07092 nm). An-
ode materials were sealed under Ar with paraffin wax in
Pyrex capillary tubes. X-ray diffraction patterns, as shown in
Fig. 1, confirmed the presence of LiC6 and LiC12 phases in© 2000 American Institute of Physics
 AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp
239Appl. Phys. Lett., Vol. 77, No. 2, 10 July 2000 Hightower et al.the graphite anode materials. These results were consistent
with the color of lithiated graphite which depends on the
predominance of LiC6 ~green/gold! or LiC12 ~blue/black!.15
The diffraction peaks were broader for LiC6 than for the
unlithiated KS 44 graphite samples.
The reactivity of reduced Li and its susceptibility to ra-
diation damage under an electron beam present a formidable
challenge to EELS analysis of LiC6. The majority of radia-
tion damage is a result of the direct transfer of momentum
from incident electrons to the Li nucleus, ejecting it from its
lattice site. Average atomic displacement energies estimated
from sublimation energies were found to be 6.1 eV for Li
and 31.2 eV for C in graphite.16 Assuming elastic collisions,
the knock-on damage, threshold energies for incident elec-
trons are 19 keV for Li and 150 keV for C. Metallic Li
samples appeared to ‘‘boil’’ during TEM observations at 200
keV. This was determined to be a consequence of knock-on
damage since the calculated temperature rise was negligible,
approximately 1 K.17 We chose a microscope voltage of 100
keV as a reasonable compromise between knock-on damage
and instrument performance.
Beam damage of metallic Li was studied at 100 keV by
concentrating the electron beam onto a thin area of the Li
sample. Figure 2 presents a time sequence of EELS spectra
for this exposed region. Over a few minutes, the spectral area
of the low-loss region was reduced significantly, resulting in
the suppression of Li plasmons at 7.5 and 15 eV. This is
consistent with a thinning of the sample as Li atoms are
ejected from the sample under the electron beam. In image
mode, regions of the specimen were observed to shrink, es-
pecially under a focused incident beam.
Over a slightly longer time scale under a focused beam,
the Li plasmons evolved into distinct profiles commonly as-
sociated with oxidized Li.18 During this time, as shown in
Fig. 2, the broad profile of the Li K edge evolves into two
sharp peaks at 59.5 and 64.5 eV, similar to peaks observed
from samples exposed to atmosphere.18 Exposure to the in-
tense electron beam stimulates Li reactions with moisture
and oxygen present in the microscope, leading to the forma-
tion of LiOH and Li2O.19 The Li K edge in our LiC6 samples
showed the same peaks at 59.5 and 64.5 eV after the samples
were exposed to atmosphere.
FIG. 1. X-ray diffraction patterns of KS 44 graphite and electrochemically
lithated graphite containing LiC6 and LiC12 phases.Downloaded 12 Jan 2006 to 131.215.240.9. Redistribution subject toFigure 3 presents the C K edges of LiC6 and KS 44
graphite. A peak at 301 eV is found in LiC6 but not in the
graphitic carbon. Disko20 and others21 have demonstrated the
sensitivity of the C K edge to the direction of the momentum
transfer vector q. We therefore do not expect the intensity of
the 1s→p* peak to be reliable, since it is not an average
over many crystal orientations. Nevertheless, the onset en-
ergy of the C K edge and the location of the 1s→p* peak
are essentially the same for graphitic carbon and LiC6.
Figure 4 displays the Li K edge obtained from interca-
lated LiC6, LiF, and metallic Li. We identify the onset of the
Li K edge in the metallic sample at 55 eV, consistent with
results of Liu and Williams.18 The broad profile of the me-
tallic Li K edge is consistent with the promotion of core
electrons into a continuum of free electron states. The Li K
edge onset of LiC6 is shifted by only 0.2 eV from the K edge
of metallic Li and has a similarly broad profile. Our observed
Li K edge for LiC6 lacks the peaks at 59 and 63 eV previ-
ously observed by Grunes et al.10 These peaks evidently
originate from oxidized Li—they are found in samples ex-
posed to atmosphere, or after long times in the microscope
~Fig. 2!. The Li K edge of LiF, similar to that reported by
FIG. 2. EELS spectra of beam-damaged Li. Spectra were acquired at ap-
proximately equal intervals from nearly zero, to 4 min of electron beam
exposure.
FIG. 3. C K edge for LiC6 and KS 44 graphite, normalized by the area 50
eV after edge onset.
 AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp
240 Appl. Phys. Lett., Vol. 77, No. 2, 10 July 2000 Hightower et al.Chen et al.,22 shows a strong chemical shift and sharp fea-
tures of well-defined unoccupied states. Such large differ-
ences are expected with the large electron transfer from Li
to F.
Our EELS spectra of the Li K edge indicate that Li in
LiC6 has a local electronic structure more similar to Li metal
than Li1, contrary to previous XPS results. Extensive XPS
results confirm a Li K edge chemical shift of about 3 eV in
LiC6 with respect to metallic Li,8,9 easily resolved from the
1.3 eV chemical shift of Li2O.23 The mean free path of pho-
toelectrons measured by XPS is on the order of 1 nm. Thus,
assuming that the surface Li remains unreacted, we expect
XPS to be more sensitive to Li surface states.
The similarities of Li K edges in LiC6 and metallic Li
are consistent with observed electrochemical potentials.
Measurements of electrochemical potentials in half cells
show LiC6 to be less than 0.01 V above Li metal.24 The
similar chemical potentials of Li in LiC6 and metallic Li
suggest similar environments for valence electrons. Electro-
chemical potentials and EELS Li K edge profiles both sug-
gest limited charge transfer from Li to surrounding carbon.
The observed EELS C K edge is little affected by interca-
lated Li, but effects should be diminished by the 6:1 ratio of
C to Li, and could be diminished further if there were a loss
of Li from the TEM sample. The shape of the Li K edge, on
FIG. 4. Li K edge of metallic Li, LiC6, and LiF. The pre-edge background
of the raw data was fit to an exponential and subtracted from the data.Downloaded 12 Jan 2006 to 131.215.240.9. Redistribution subject tothe other hand, would be unaffected by any possible loss of
Li, or perhaps would appear even more ionic, since the elec-
trochemical potential of Li would be lowered.
This work was supported by DOE through Basic Energy
Sciences Grant No. DE-FG03-94ER14493.
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Electron energy-loss spectrometry on lithiated graphite

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