Crystal and electronic structure of lithiated nanosized rutile TiO2 by
electron diffraction and electron energy-loss spectroscopy
C. M. Wang, Z. G. Yang, S. Thevuthasan, J. Liu, D. R. Baer et al.
Citation: Appl. Phys. Lett. 94, 233116 (2009); doi: 10.1063/1.3152783
View online: http://dx.doi.org/10.1063/1.3152783
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APPLIED PHYSICS LETTERS 94, 233116 共2009兲
Crystal and electronic structure of lithiated nanosized rutile TiO2
by electron diffraction and electron energy-loss spectroscopy
C. M. Wang,1,a兲 Z. G. Yang,2 S. Thevuthasan,1 J. Liu,3 D. R. Baer,1 D. Choi,2
D. H. Wang,3 J. G. Zhang,2 L. V. Saraf,1 and Z. M. Nie2
1
Environmental Molecular Sciences Laboratory, Pacific Northwest National Laboratory, Richland,
Washington 99352, USA
2
Energy and Environmental Directorate, Pacific Northwest National Laboratory, Richland,
Washington 99352, USA
3
Fundamental and Computational Science Directorate, Pacific Northwest National Laboratory, Richland,
Washington 99352, USA
共Received 14 November 2008; accepted 20 May 2009; published online 11 June 2009兲
The crystal and electronic structure of the lithiated nanosized rutile TiO2 were studied using electron
diffraction and electron energy-loss spectroscopy 共EELS兲 in a transmission electron microscopy.
EELS reveals the Li K-edge at the energy-loss position of ⬃61 eV. After lithiation, the t2g-eg
crystal-field splitting on both Ti L2,3-edge and O K-edge decreases, the O K-edge shifts toward a
higher energy-loss position and the separation between the pre-edge peak and main peak on the O
K-edge decreases, suggesting that the lithiation of rutile TiO2 was accompanied by the reduction in
Ti ion, indicating a charge transfer from Li to Ti. © 2009 American Institute of Physics.
关DOI: 10.1063/1.3152783兴
Much research has recently focused on exploring the use
of tailored nanostructured rutile,1 anatase,2 and B type TiO2
共Ref. 3兲 as potential anode materials for Li-ion batteries.
TiO2 offers several distinctive advantages that are not
matched by the traditional graphite anode materials. With a
redox potential of 1.5–1.8 V versus Li+ / Li, TiO2 used as
anode can avoid Li electroplating and therefore makes the
battery inherently safe. Nanostructured TiO2 have been demonstrated a higher capacity and charge/discharge rates than
conventional structures.4 Recently, Wang et al.5 have shown
that mesoporous crystalline TiO2 exhibits an excellent capacity retention, typically less than 10% capacity loss after more
than 100 cycles. It has been revealed that lithiation and
delithiation of nanostructured materials may indeed show
variations in both chemistry and structure on the scale of
several nanometers, as demonstrated recently over LixFePO4,
cathode materials.6–8 The microstructure and especially the
electronic structure are key physical parameters with respect
to the battery performance. X-ray diffraction,9 nuclear magnetic resonance, Mossbauer spectroscopy,9 and x-ray photoelectron spectroscopy10 are bulk or surface sensitive techniques and do not provide information of both the structural
and electronic properties on the nanometer scale. Electron
energy-loss spectroscopy 共EELS兲 in a TEM not only allows
the direct detection of Li following the lithiation, but also
provides information related to local electronic structure of
the materials with a spatial resolution from several nanometers to single atomic column.
A large body of literatures of both theoretical and experimental works exists with respect to the crystal and electronic
structures of TiO2.11,12 However, crystal and electronic structures of lithiated nanostructured TiO2 are lacking in the literatures. Electronic structure of lithiated TiO2, in principle,
can be calculated if the crystal structure is fully established.
a兲
Author to whom correspondence should be addressed. Electronic mail:
[email protected].
0003-6951/2009/94共23兲/233116/3/$25.00
However, no consistent picture has emerged regarding the
crystal structure of lithiated rutile TiO2, especially with a
particle size in the nanometer scale.13,14 Furthermore, for Li
insertion into the lattice of TiO2, detection of Li has been
normally carried out indirectly, mostly based on the phase
and structural analyses.5 Direct detection of Li in the lattice
of TiO2 and the associated electronic structure changes due
to the Li incorporation in TiO2 have not been carried out. In
this paper, we use TEM imaging, electron diffraction, and
EELS to probe the crystal and electronic structures of nanosized rutile TiO2 before and after the Li insertion by a mechanical activation method.
The commercial TiO2 was lithiated through high energy
ball milling. The TiO2 powder was mixed with stoichiometric amount of Li metal foil 共99.9%, Sigma-Aldrich兲 that
gives an overall nominal composition of LiTiO2. The mixture was ball-milled in a Spex mill at room temperature for
1 h. The TEM and electron diffraction were carried out using
JEOL JEM-2010 microscope with a LaB6 filament and operated at 200 kV. The EELS spectra were acquired using Gatan
Image Filter 共GIF2000兲, which is postcolumn attached to the
microscope. Overall, the configuration of the system gives an
energy resolution of 1.2 eV as measured by the full width at
half magnitude of the zero-loss peak.
Figure 1共a兲 shows the as-received commercial TiO2
powder particles. This powder particles are the aggregates of
needlelike primary particles. Selected area electron diffraction indicates that these particles are dominated by rutile
structure as evidenced by the matching between the experimental electron diffraction pattern and the calculated pattern
based on the rutile structure 关Figs. 1共b兲 and 1共c兲兴. Mechanical milling of these powder particles with Li metal particles
induced significant morphological changes. After the milling,
the needle-shaped primary particles are no longer visible.
Instead, the lithiated powder aggregates as round-shaped particles, as illustrated by the bright-field TEM image shown in
Fig. 1共d兲. The electron diffraction pattern from the lithiated
powder particles is featured by two strong rings on the dif-
94, 233116-1
© 2009 American Institute of Physics
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233116-2
Wang et al.
Appl. Phys. Lett. 94, 233116 共2009兲
FIG. 2. 共Color online兲 EELS low-loss spectra of the as-received and lithiated TiO2 rutile.
FIG. 1. TEM images and electron diffraction analysis of the commercial
TiO2 particles before and after lithiation. 共a兲 Bright-field TEM image of the
as-received TiO2. 共b兲 Selected area electron diffraction pattern of the TiO2.
共c兲 Calculated electron diffraction ring pattern of TiO2 rutile. 共d兲 Bright-field
TEM image of the mechanically lithiated TiO2 powder particle shown in 共a兲.
共e兲 Selected area electron diffraction pattern of powder particles shown in
共d兲. 共f兲 Calculated electron diffraction ring pattern of cubic LiTiO2.
fraction pattern, as shown in Fig. 1共e兲, which does not match
with the diffraction pattern of rutile TiO2. The measured
d-space and relative intensity of these two strong lines match
with that of rock-salt structured LiTiO2, as shown in Figs.
1共e兲 and 1共f兲. This is consistent with the observation of Wang
et al.5 on TiO2 after electrochemical lithiation. They noticed
that following the initial charging and discharging, mesoporous rutile TiO2 irreversibly transformed to cubic structured
LiTiO2. The present observation clearly demonstrates that
mechanical lithiation leads to structural transformation from
TiO2 to LiTiO2, along with substantial morphological evolution of the particles.
The EELS spectra can be divided into two regions: the
low-loss region, which normally corresponds to an energyloss of up to 50 eV, and the core-loss region. The low-loss
region mostly originated from band gaps, interband transitions, excitations in insulator, and surface and bulk plasmon
excitations. Figure 2 is a comparison of the EELS low-loss
spectra between the as-received and lithiated TiO2. The Li
K-edge was located at the energy-loss position of the 61.6
eV, which is consistent with the observation of Koyama et
al.15 Because the Li K-edge sits on the tail of the plasmonloss peak, quantification of Li concentration will be affected
by the background subtraction parameters. Graetz et al.16
have attempted to use EELS to quantify the concentration of
Li inserted into their electrode materials and they noticed
that the quantification is not reliable due to the difficulty on
the background subtraction beneath the Li K-edge. Therefore, no attempt was made to quantify the concentration of Li
following the mechanical lithiation in this study.
The Ti L2,3-edge represents the transition of electron
from 2p63dn to 2p53dn+1 and the O K-edge corresponds to
the electron transition from 1s to O 2p states hybridized with
the Ti 3d states localized at the Ti sites.17 The EELS of Ti
L2,3-edge and O K-edge for the TiO2 before and after the
lithiation were shown in Fig. 3. Because of the uncertainty
for the accurate determination of the absolute scale of the
energy-loss position, the spectra were aligned at the Ti
L3-edge of 458 eV. With this alignment, any chemical shift
due to the lithiation can be measured from the O K-edge. In
addition, any structural change will also be reflected in the
fine structural features of both Ti L2,3-edge and O K-edges as
discussed below. Figure 3 reveals that both Ti L2,3-edge and
O K-edge show some degree of changes following the mechanical lithiation. With the Ti L2,3-edge aligned, we noticed
that the O K-edge energy-loss position of the lithiated sample
shifted 1.6 eV to a higher energy-loss direction as compared
with that of the sample before the lithiation. Both experiment
and DFT calculation have previously demonstrated that the
O K-edge of rock-salt structured TiOx 共x ⬍ 2兲 will shift toward a higher energy-loss position as compared with that of
TiO2.12,18 Therefore, the observed shifting in the O K-edge
toward the higher energy is related to the reduction of the Ti
ions following the insertion of Li into the lattice, indicating
charge transfer from Li to Ti ions. This conclusion is further
supported by the fine structural features on Ti L2,3-edge and
O K-edge as discussed bellow.
A detailed comparison of the Ti L2,3-edge collected from
the samples before and after the lithiation is shown in Fig. 4.
The Ti 3d character is separated into two groups: the threefold t2g and the twofold eg which are known as the crystal-
FIG. 3. 共Color online兲 EELS core-loss spectra of Ti L2,3-edge and O K-edge
for the samples before and after the lithiation. The spectra are aligned at the
Ti L3-edge.
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233116-3
Appl. Phys. Lett. 94, 233116 共2009兲
Wang et al.
sistently support the notion that during the lithiation of TiO2,
the Ti was reduced and the electron from the Li was transferred to Ti. Similar behavior is also observed on TiO2 rutile
following a chemical lithiation. The conclusion derived from
the EELS fine structural analysis is also consistent with the
electron diffraction analysis of the mechanical lithiated
sample in which the majority of the phase was transformed
to rock-salt structured LiTiO2. The present observation on
the electron transfer from Li to Ti is contrast with the case of
Li inserted into the graphite. Based on EELS studies, Hightower et al.20 have concluded that only a small charge transfers from Li to C in LiC6.
Lithiation of nanosized TiO2 via mechanical activation
induced significant changes on crystal and electronic structure. EELS measurements reveal the lithiated TiO2 shows a
characteristic Li K-edge at the energy-loss position of 61 eV
and the fine structural features of both Ti L2,3-edge and O
K-edge suggest charge transfer from Li to Ti following the
lithiation.
FIG. 4. 共Color online兲 Comparison of fine structural features of the EELS
spectra on the samples before and after the lithiation. 共a兲 Ti L2,3-edge and 共b兲
O K-edge, note the 0.8 eV redshift of peak b following the lithiation 共The
spectra are aligned at the onset energy position of the pre-edge兲.
field splitting. The t2g-eg crystal-field splitting is very well
resolved for rutile TiO2. However, following the lithiation,
the t2g-eg peak appears to be smeared out. Consistent with
above observation is the feature shown on the O K-edge, as
shown in Fig. 4. The O K-edge is featured by two main
peaks labeled as a and b in Fig. 4共b兲. The pre-edge peak a on
the O K-edge for the TiO2 rutile also splits into a doublet,
which is the result of the hybridization of the oxygen 2p
orbitals with the Ti 3d orbitals, because the Ti 3d orbitals are
affected by the octahedral ligand-field and split into t2g-eg.
The main peak b on the O K-edge corresponds to electron
transition from 1s to oxygen 2p states hybridized with titanium 4s and 4p states.
Two signification features can be seen on the O K-edge
as comparing with the samples before and after the lithiation.
First, the separation of the pre-edge doublet decreases following the lithiation. Second, the separation of peaks a and b
is decreased. Based on DFT calculation, Yoshiya et al.12 have
systematically calculated the EELS of both TiO2 rutile and
the rock-salt structured TiOx 共x ⬍ 2兲. They noticed that with
the decreases of the x value, the t2g-eg splitting on both Ti
L2,3-edge and O K-edge decreases and this splitting cannot
be resolved when x is less than a value between 1.5 and 1.2.
They also noticed that with the decrease of the x value, the
separation between peaks a and b on the O K-edge also decreases. These conclusions derived by Yoshiya et al.12 are
fully consistent with the EELS experimental results carried
out by Mitterbauer et al.18 and Weng et al.19 for the system
of TiOx. Therefore, the characteristic features observed on
the Ti L2,3-edge and the O K-edge as described above con-
This work was supported US Department of Energy
共DOE兲, Office of Science, Offices of Basic Energy Sciences
and Biological and Environmental Research. The work was
conducted in the William R. Wiley Environmental Molecular
Sciences Laboratory 共EMSL兲, a national scientific user facility sponsored by DOE’s Office of Biological and Environmental Research and located at Pacific Northwest National
Laboratory 共PNNL兲. PNNL is operated for the DOE under
Contract No. DE-AC06-76RLO 1830.
1
C. H. Jiang, I. Honma, T. Kudo, and H. S. Zhou, Electrochem. Solid-State
Lett. 10, A127 共2007兲.
2
C. H. Jiang, M. D. Wei, Z. M. Qi, T. Kudo, I. Honma, and H. S. Zhou, J.
Power Sources 166, 239 共2007兲.
3
G. Armstrong, A. R. Armstrong, P. G. Bruce, P. Reale, and B. Scrosati,
Adv. Mater. 共Weinheim, Ger.兲 18, 2597 共2006兲.
4
S. Y. Chung, J. T. Bloking, and Y. M. Chiang, Nature Mater. 1, 123
共2002兲.
5
D. Wang, D. Choi, Z. G. Yang, V. V. Viswanathan, Z. M. Nie, C. M. Wang,
Y. J. Song, J. G. Zhang, and J. Liu, Chem. Mater. 20, 3435 共2008兲.
6
P. Gibot, M. Casas-Cabanas, L. Laffont, S. Levasseur, P. Carlach, S. P.
Hamelet, J. M. Tarascon, and C. Masqulier, Nature Mater. 7, 741 共2008兲.
7
C. Delmas, M. Maccario, L. Croguennec, F. Le Cras, and F. Weill, Nature
Mater. 7, 665 共2008兲.
8
S. I. Nishimura, G. Kobayashi, K. J. Ohoyama, R. J. Kanno, M. Yashima,
and A. Yamada, Nature Mater. 7, 707 共2008兲.
9
A. S. Andersson, B. Kalska, L. Haggstrom, and J. O. Thomas, Solid State
Ionics 130, 41 共2000兲.
10
H. Momose, H. Honbo, S. Tkeuchi, K. Nishimura, T. Horiba, Y. Muranaka, and H. M. Y. Kozono, J. Power Sources 68, 208 共1997兲.
11
R. Brydson, H. Sauer, W. Engel, J. M. Thomas, E. Zeitler, N. Kosugill,
and H. Kurodall, J. Phys.: Condens. Matter 1, 797 共1989兲.
12
M. Yoshiya, I. Tanaka, K. Kaneko, and H. Adachi, J. Phys.: Condens.
Matter 11, 3217 共1999兲.
13
Y. S. Hu, L. Kienle, Y. G. Guo, and J. Maier, Adv. Mater. 共Weinheim,
Ger.兲 18, 1421 共2006兲.
14
E. Baudrin, S. Cassaignon, M. Koesch, J. P. Jolivet, L. Dupont, and J. M.
Tarascon, Electrochem. Commun. 9, 337 共2007兲.
15
Y. Koyama, T. Mizoguchi, H. Ikeno, and I. Tanaka, J. Phys. Chem. B 109,
10749 共2005兲.
16
J. Graetz, C. C. Ahn, R. Yazami, and B. Fultz, J. Phys. Chem. B 107,
2887 共2003兲.
17
K. Lie, R. Brydson, and H. Davock, Phys. Rev. B 59, 5361 共1999兲.
18
C. Mitterbauer, G. Kothleitner, and F. Hofer, Microsc. Microanal. 9, 834
共2003兲.
19
X. Weng, P. Fisher, M. Skowronski, P. A. Salvador, and O. Maksimov, J.
Cryst. Growth 310, 545 共2008兲.
20
A. Hightower, C. C. Ahn, and B. Fultz, Appl. Phys. Lett. 77, 238 共2000兲.
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