University of Groningen
Lithium trapping by excess oxygen in WO3
Wijs, G.A. de; Groot, R.A. de
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Physical Review B
DOI:
10.1103/PhysRevB.62.1508
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Wijs, G. A. D., & Groot, R. A. D. (2000). Lithium trapping by excess oxygen in WO3: A first-principles study.
Physical Review B, 62(3). DOI: 10.1103/PhysRevB.62.1508
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PHYSICAL REVIEW B
VOLUME 62, NUMBER 3
15 JULY 2000-I
Lithium trapping by excess oxygen in WO3 : A first-principles study
G. A. de Wijs1 and R. A. de Groot1,2
1
Electronic Structure of Materials, Research Institute of Materials, Faculty of Sciences,
Toernooiveld 1, NL-6525 ED Nijmegen, The Netherlands
2
Laboratory of Chemical Physics, Materials Science Center, Rijksuniversiteit Groningen,
Nijenborgh 4, NL-9747 AG Groningen, The Netherlands
共Received 9 March 2000兲
The process of lithium trapping by excess oxygen atoms in both crystalline and amorphous WO3 is studied
by first-principles calculations. In both materials, the excess oxygen is incorporated in the bonding network by
a peroxide-type bond. In both c-WO3 and a-WO3 , breaking of this bond makes oxygen states available for the
accommodation of the Li electrons. However, only in a-WO3 is this mechanism accompanied by a strong
reduction in total energy as this material has the flexibility to accommodate 共incorporate兲 the extra O2⫺ ion.
I. INTRODUCTION
In electrochromic devices, the optical properties are
modulated by a reversible insertion of small ions and chargecompensating electrons. Ideally, a change in, e.g., the optical
density, is apparent for 共and proportional to兲 any nonzero
fraction x of ions inserted. However, nonideal behavior is
known to occur frequently. An important aspect of this nonideal behavior is the irreversible uptake, i.e., trapping, of Li
共or H兲 atoms. Several experimental studies have reported on
this phenomenon in amorphous films of the ‘‘archetypical’’
electrochromic material WO3 .1 In these studies 共repeated兲
cycles of insertion and extraction of lithium 共or H兲 were
carried out. For sputter-deposited films, sizable amounts of
lithium2–5 or hydrogen6 were found to intercalate irreversibly, without electrochromic coloration.
The nature of the trapping center is still under debate, and
depends on the film deposition method. For sputter-deposited
films, a possible candidate has been strongly bonded water
共present in the film兲 that could possibly react with Li. However, this possibility has been ruled out on the basis of infrared measurements.2,4 Other suggestions are singly coordinated oxygen atoms,7 peroxotype links 共W-O-O-W兲,2 and the
presence of gaseous oxygen.2 In a recent study, Vink et al.
have determined experimentally that lithium trapping is associated with the excess oxygen content y of the film
(WO3⫹y ).5 In their films, all irreversibly intercalated lithium
was trapped in the first coloration/bleaching cycle. Moreover, they showed that the amount x of irreversibly inserted
lithium is twice the excess oxygen content.
In this paper, we study the potential of excess oxygen to
act as a trapping site for lithium, in both crystalline (c-WO3 )
and amorphous (a-WO3 ) tungsten trioxide, using firstprinciples simulation.8 The next section gives the computational details. Sections III A and III B present the results for
excess oxygen in c-WO3 and a-WO3 , respectively. Concluding remarks are in Sec. IV.
II. COMPUTATIONAL DETAILS
The calculations have been performed using the ab initio
total-energy and molecular-dynamics program VASP 共Vi0163-1829/2000/62共3兲/1508共4兲/$15.00
PRB 62
enna ab initio simulation program兲 developed at the Institut
für Theoretische Physik of the Technische Universität
Wien.9–11
Electron-ion interactions were described using Vanderbilt
ultrasoft pseudopotentials 共USPP兲 共Ref. 12兲 with a frozen
关 Xe兴 4 f 14 and 1s 2 core for W and O, respectively, and using
a norm-conserving pseudopotential with 关He兴 core for Li,13
including nonlinear core corrections for W and Li.14 For efficiency, the real-space projection scheme15 for the nonlocal
part of the pseudopotentials was used.
Calculations were done in the local-density approximation
共LDA兲 using the parametrization by Perdew and Zunger16 of
the Ceperley and Alder functional.17 The preference for LDA
over calculations in the generalized gradient approximation
共GGA兲 was motivated by the better agreement with the experimental structural parameters obtained with the former for
c-WO3 .19 Structural optimizations were carried out with a
2⫻2⫻2 Monkhorst-Pack mesh,18 while total energies and
densities of states were calculated with a 3⫻3⫻3 mesh.
Further details concerning the electronic structure part of the
calculations can be found in Refs. 19 and 20.
As a starting point for the calculations on c-WO3 , the
optimized LDA structure from Ref. 19 was used. For
a-WO3 , the starting point was an amorphous LDA structure
from Ref. 20. This stoichiometric, amorphous structure contains eight formula units in a cubic, periodically repeated,
supercell. A sample with excess oxygen was made by adding
an extra oxygen to the amorphous structure from Ref. 20
共fixed volume兲, followed by molecular dynamics at very high
temperature, and an abrupt quench 共a procedure similar to
that of Ref. 20兲. For further details, we refer to Ref. 20.
III. RESULTS
A. c-WO3
As a starting point, we take the optimized structure of the
monoclinic room-temperature modification of c-WO3 at the
LDA equilibrium volume.19 Its density of states 共DOS兲 is
shown in Fig. 1共a兲. It is a semiconductor with a LDA gap of
1.1 eV. Monoclinic c-WO3 共Ref. 21兲 has a distorted perov1508
©2000 The American Physical Society
PRB 62
BRIEF REPORTS
FIG. 1. Total density of states for c-WO3 and c-WO3 with
extra atoms added. 共a兲 WO3 , 共b兲 W8 O25 , 共c兲 Li2 •W8 O25 , and
共d兲 Li2 O•W8 O24 .
skite structure ABO3 with W at the A position inside the
octahedron and the B site left empty. The unit cell contains
eight oxygen octahedra, which are linked at the corners, in a
2⫻2⫻2 arrangement. The tungsten atoms are located
slightly off center in the octahedra. The shape of the octahedra deviates a bit from the perfect octahedron, and they are
somewhat tilted relative to each other. One unit cell (W8 O24)
is taken as the periodically repeated unit. In all subsequent
calculations, the positional parameters are allowed to relax,
whereas the cell shape is fixed.
Oxygen excess is mimicked by the addition of an extra
oxygen atom (W8 O25). The extra atom was introduced in the
center of one of the voids in between the octahedra 共a B site兲
and left to find a favorable binding site. The relaxed structure
is shown in Fig. 2共a兲. The extra oxygen has moved close to
one of the tungsten atoms (d W-O⫽1.93 Å兲. It has formed a
bond with one of the oxygens from the surrounding octahedron (d O-O⫽1.44 Å兲. This situation is quite comparable with
peroxide bonds as they occur in other materials 共e.g., in crystalline LiO, the O-O distance is 1.51 Å兲.22 The DOS pertaining to this structure is shown in Fig. 1共b兲. An occupied state
of oxygen O-p character is found at the top of the valence
band, the 共empty兲 antibonding state mixes with the conduction band. Thus, by forming the peroxidelike bond, the oxygen atom prevents hole formation in the valence band. Also
note the 7 eV bonding–antibonding splitting of the oxygen
2s states 共for the LiO crystal, we obtain a comparable large
splitting of 5.5 eV兲.
FIG. 2. Crystalline structures with extra atoms added. Tungsten,
oxygen, and lithium are white, light gray, and dark spheres, respectively. 共a兲 W8 O25 , 共b兲 Li2 •W8 O25 , the Li ions are approximately at
two neighboring C sites, 共c兲 Li2 O•W8 O24 .
1509
To investigate the possibility of Li trapping, first one, and
then a second Li were introduced at B-type positions near the
extra O. The relaxed structure is shown in Fig. 2共b兲. The
peroxide-type bond has not been broken (d O-O⫽1.45 Å兲. The
Li⫹ have moved closer to several of the oxygen ions, but
have failed to establish a strong bond: The distances to the
extra oxygen are 1.95 and 2.37 Å , to be compared with 1.64
Å for the calculated Li-O distance in the gas-phase Li2 O
molecule. Li-O bond formation is also not apparent from the
electronic structure: The DOS 关Fig. 1共c兲兴 shows that the Li
electrons just serve to fill the lowest conduction band 共tungsten兲 states, i.e., no oxygen states have been made available
as the peroxide-type bond has remained intact. From this
information, the excess O does not seem a suitable Li trapping center. This is confirmed by energetic considerations. If
we compare a situation with two Li atoms at large distance
and far from the excess oxygen 共modeled with two
Li•W8 O24 and W8 O25) with the present cluster with all three
extra atoms 共modeled by Li2 •W8 O25 and two W8 O24), the
present cluster is unstable by 0.7 eV. The above estimate is a
bit crude, as it comes from 共a兲 a comparison of rather small
periodically repeated configurations, that 共b兲 are all infinite
crystals with bandlike electronic states. Point 共a兲 might cause
an underestimation of lattice relaxation effects. As the configurations are neutral themselves, we do not expect large
contributions from artificial electrostatic effects. Point 共b兲
is particularly troublesome for the occupancy of the
tungsten-derived conduction band states. Indeed, taking a
Li2 •W8 O24 共and W8 O25) as a starting point reduces the destabilization to about 0.2 eV. Part of this problem is overcome considering the Li trapping relative to an external reservoir 共res兲 with chemical potential ␮ Li2 .23 For the trapping
reaction:
Li2 共 res兲 ⫹O•W8 O24→Li2 •O•W8 O24 ,
共1兲
we find a formation energy of ⫺9.9 eV⫺␮ Li2 .
The addition of Li did not cause a breaking of the oxygenoxygen bond, and therefore no holes were present in the
valence band that could be filled by the extra electrons donated by Li. This mechanism, which might lead to Li trapping, could possibly have been prevented by a barrier for
breaking the oxygen-oxygen bond. To test for this possibility, we carried out a minimization starting from a situation
without an O-O bond where a Li2 O molecule was placed
inside the WO3 crystal. The final geometry 关Fig. 2共c兲兴 does
not contain an O-O bond, but the extra oxygen ion has
formed a bond with one of the tungsten atoms (d W-O⫽1.75
Å兲. The Li2 O has partly decomposed and the oxygen remains
close to only one Li (d Li-O⫽1.75 Å兲. The excess electrons
from the Li are trapped in a dispersionless, i.e., localized,
in-gap state 关Fig. 1共d兲兴. Thus, the extra oxygen has formed
an O2⫺ by trapping the excess Li electrons in the in-gap
state. Energetically, this configuration is about 0.2 eV more
favorable, i.e., the formation energy of the final configuration
from O•W8 O24 and a reservoir of Li2 is ⫺10.1 eV⫺ ␮ Li2 . It
is still unstable against decomposition in the solid by ⬃0.5
eV.
BRIEF REPORTS
1510
FIG. 3. Amorphous structures with extra atoms added. Atom
symbols as in Fig. 2. 共a兲 W8 O25 , the distance from O2 to the
tungsten atom with the dot is 2.31 Å , 共b兲 Li2 •W8 O25 , and
共c兲 Li2 O•W8 O24 .
B. a-WO3
In the model of the amorphous structure, the extra oxygen
also gives rise to a peroxide bond 关 d O-O⫽1.43 Å, see Fig.
3共a兲兴. The two oxygen atoms involved share the same,
single, tungsten neighbor (d O1⫺W⫽1.89 Å, d O2⫺W⫽2.00 Å兲
and one of them comes close to another tungsten ion
关 d O2⫺W⫽2.31 Å, see Fig. 3共a兲兴. Thus, in a fashion similar to
c-WO3 , hole formation in the valence band is again prevented 关see Fig. 4共a兲兴.
Li intercalation was modeled, like for c-WO3 , by addition
of one 共followed by a second兲 Li atom close to the peroxide
species, and a relaxation of the atomic coordinates. The
bonding network was not much affected, the major change
being the breaking of an O-W bond and formation of another
共with the same W兲. However, none of these atoms are participating in 共or very close to兲 the peroxidelike species. The
peroxide-type bond itself was not broken 关Fig. 3共b兲兴 and the
Li valence electrons were just donated into a conduction
band state 关Fig. 4共b兲兴. The energy released by formation of
the complex 共with lithium taken from a Li2 reservoir兲 is
E form⫽⫺9.5 eV⫺ ␮ Li2 . A rough estimate gives ␮ Li2
⫽⫺10.1 eV,24 and thus a positive formation energy
E form⫽0.6 eV.
Just as for the crystal, the possible presence of a barrier
for peroxide decomposition, or a possible unfavorable start-
FIG. 4. Total density of states for a-WO3 with extra atoms
added. 共a兲 W8 O25 , 共b兲 Li2 •W8 O25 , and 共c兲 Li2 O•W8 O24 .
PRB 62
ing configuration for the lithium atoms, may have prevented
trapping of the extra 共Li兲 electrons by the oxygens. To test
for this possibility, we 共a兲 removed one of the peroxide oxygens 共O1兲, 共b兲 relaxed the resulting geometry, 共c兲 added a
Li2 O species in the void near the original peroxide location,
and 共d兲 relaxed this structure. The last step was carried out in
two small steps. First only the positions of the added Li2 O
were relaxed, later all atoms were allowed to relax. The fully
relaxed result is shown in Fig. 3共c兲. Relaxation in step 共b兲 did
not much affect the geometry. However, in step 共d兲 the peroxide bond had not restored again. The oxygen of the added
Li2 O is accommodated as a terminal oxygen 共single W-O
bond, to another W as initially, d O2⫺W⫽1.78 Å兲 and its distance to the Li atoms is increased (d O-Li⫽1.92, 1.95 Å,
d Li-Li⫽3.09 Å兲. The other oxygen from the peroxide has remained in almost the same bonding configuration (d W⫺O1
⫽1.80, 1.99 Å兲. Energetically, a much different picture
emerges 共as for c-WO3 ). At the intermediate stage of only
relaxing the Li2 O in the frozen host material, the formation
energy is still quite high: E form⫽⫺8.6 eV⫺ ␮ Li2 . However,
here the extra oxygen 共O2兲 has not found its way yet to a
tungsten neighbor. Its 2p states lie above and in the gap and
its 2s state more than 2 eV above the other oxygens’ 2s
states. Full relaxation 关Fig. 3共c兲兴, including the host material,
brings the formation energy down dramatically: E form⫽
⫺14.1 eV⫺ ␮ Li2 , i.e., relative to the Li2 reservoir the complex is stable by 4 eV. Compared to the initial configuration
W8 O25 , several W-O bonds have been broken and others
formed. The electronic structure 关Fig. 4共c兲兴 shows no special
features related to any special oxygen atom, contrary to the
behavior of the crystal.
IV. CONCLUSIONS
The study of an extra oxygen atom in c-WO3 did not
provide a plausible mechanism of lithium trapping. Our calculations provide a strong indication that a Li2 •O complex is
energetically not favored inside the crystalline host material.
Breaking of the O-O bond leads to a donation of the lithium
electrons in oxygen-derived states. However, this mechanism
does not provide a sufficiently strong driving force for trapping as the total energy reduction is too small. Moreover, the
electrons are not trapped in a conduction band state, but remain in an in-gap state.
In a-WO3 lithium trapping at the extra oxygen atom
seems very well possible. If the peroxide-type bond is broken, the system falls into a well of ⬃4 eV, i.e., this price has
to be paid in order to move the Li⫹ ions and their valence
electrons away from the extra oxygen into a stoichiometric
part of a-WO3 . We found that relaxations of the host material are very important: if only the added Li2 O is allowed to
relax inside the frozen host material, the well seems absent.
Moreover, the relaxation of the host material allows for a
complete incorporation of the extra oxygen 共dressed with the
Li valence electrons兲 into the amorphous host: it cannot be
distinguished from the other oxygen ions. In c-WO3 , on the
contrary, the features of the extra oxygen still stand out
clearly in, e.g., the DOS.
In conclusion, comparing the behavior of the extra oxy-
PRB 62
BRIEF REPORTS
gen atom in c-WO3 and a-WO3 , we think that the key element of the Li trapping mechanism is the adaptability of the
amorphous host, which allows for a ‘‘natural’’ incorporation
of the extra oxygen ion. As a result, we expect that Li trapping in crystals occurs at defect structures, such as, e.g.,
shear planes 共than a different mechanism applies, see, e.g.,
Ref. 25兲.
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1511
ACKNOWLEDGMENTS
Stimulating discussions with Dr. E.P. Boonekamp, Dr.
T.J. Vink, Mr. J.C.L. Hageman, and Professor L.F. Feiner
are acknowledged. This work was part of the research program of the Stichting for Fundamenteel Onderzoek der Materie 共FOM兲 with financial support from the Nederlandse Organisatie voor Wetenschappelijk Onderzoek 共NWO兲.
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If the reservoir is in equilibrium with the Li-loaded solid, the
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equilibrium chemical potential ␮ Li2 , however, depends on the
共local兲 Li concentration.
24
␮ Li2 is obtained by assuming equilibrium between Li2 in the bath
and in a-WO3 (Li2 W8 O24). a-WO3 was modeled by an amorphous configuration from Ref. 20. a-WO3 with Li intercalated,
was modeled by the result of a relaxation of that configuration
with 2 Li atoms added.
25
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