Lithiation of silica through partial reduction
Chunmei Ban, Branden B. Kappes, Qiang Xu, Chaiwat Engtrakul, Cristian V. Ciobanu et al.
Citation: Appl. Phys. Lett. 100, 243905 (2012); doi: 10.1063/1.4729743
View online: http://dx.doi.org/10.1063/1.4729743
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Published by the American Institute of Physics.
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APPLIED PHYSICS LETTERS 100, 243905 (2012)
Lithiation of silica through partial reduction
Chunmei Ban,1,a) Branden B. Kappes,2,a) Qiang Xu,1 Chaiwat Engtrakul,1
Cristian V. Ciobanu,2 Anne C. Dillon,1 and Yufeng Zhao1,b)
1
National Renewable Energy Laboratory Golden, Colorado 80401, USA
Department of Mechanical Engineering and Materials Science Program Colorado School of Mines, Golden,
Colorado 80401, USA
2
(Received 27 April 2012; accepted 29 May 2012; published online 15 June 2012)
We demonstrate the reversible lithiation of SiO2 up to 2/3 Li per Si, and propose a mechanism for
it based on molecular dynamics and density functional theory simulations. Our calculations show
that neither interstitial Li (no reduction), nor the formation of Li2O clusters and Si–Si bonds (full
reduction) are energetically favorable. Rather, two Li effectively break a Si–O bond and become
stabilized by oxygen, thus partially reducing the SiO2 anode: this leads to increased anode
capacity when the reduction occurs at the Si/SiO2 interface. The resulting LixSiO2 (x < 2=3)
C 2012 American Institute of Physics.
compounds have band-gaps in the range of 2.0–3.4 eV. V
[http://dx.doi.org/10.1063/1.4729743]
Since the discovery of tin oxide as a potentially promising anode materials for Li-ion batteries,1 tremendous
research effort has been devoted to the development and
commercialization of anode materials based on the oxides of
tin and silicon.2–12 Pure silicon especially has been pursued
as a high-capacity anode material.13–15 However, widespread
manufacturing and commercialization of these anode materials are still hindered by poor cyclability due to the drastic
volume changes during charge-discharge processes. At the
same time, the occasional observation of SiO2 as a potential
anode material16–18 for reversible lithiation/delithiation is
important and surprising as a fundamental scientific problem.
It is well known that SiO2 is a wide band-gap material with a
very high conduction band minimum (CBM),19–22 which
means that SiO2 is an extremely poor electron acceptor. In
addition, the rigid and strong Si–O bonds in SiO2 are consistent with its chemical inactivity. On the other hand, the reducing nature of Li, with a much lower electronegativity than
Si, thermodynamically favors the lithiation of SiO2 if the
barrier to break the Si–O bonds can be overcome at reasonable temperatures.
In this study, we focus on identifying the low-energy Li
defect structures and the lithiation mechanism in SiO/SiO2
powder mixtures. We demonstrate experimentally a surprising increase in Li-ion capacity of SiO1.83 that occurs over
200 charge-discharge cycles. Using density functional
theory (DFT) and molecular dynamics (MD) simulations, we
have studied the Li defects in silica and will show that neither interstitial Li, nor the formation of Li2O clusters and
Si–Si bonds (i.e., full reduction) are energetically favorable.
Instead, two Li effectively break a Si–O bond and become
stabilized by oxygen, thus only partially reducing the SiO2
anode. We also propose a mechanism of lithiation in which
partial reduction at the Si/SiO2 interface enlarges the Si side
of the interface; since the Li capacity of Si is significantly
greater than that of SiO2, this mechanism provides an
explanation for the increased anode capacity observed in our
experiments.
SiOx was synthesized from tetraethoxysilane (TEOS) as
the silicon source, polyvinylpyrrolidone (PVP) as a dispersant, and ethanol as a solvent. The precursor solution was solvothermally treated at 150 C for 24 h, followed by a rinse in
ethanol, and finally, dried in air. A binder-free fabrication
method23 was used to make the electrode for a 2032 cointype cell from the as-prepared SiOx. A Bio-Logic VMP3 was
used to cycle the cell between 0.005 and 3 V (vs. Li/Liþ) at
100 mA/g. Figure 1 shows an unexpected increase in the Liion capacity of the SiO1.83 anode material with the number
of charge-discharge cycles. This contravenes conventional
wisdom for two reasons. First, SiO2 is a well-known wide
band-gap insulator, and therefore, a poor electron acceptor.
Second, recent nuclear magnetic resonance (NMR)24 and
x-ray photoelectron spectroscopy (XPS)17 experiments indicate that the reaction with Li transforms SiO2 irreversibly
into Li2O and Li4SiO4. On the other hand, it is indeed promising to use SiO2 as an anode material because this material
is stable, light-weight, cheap, and environmentally benign.
a)
C. Ban and B. B. Kappes contributed equally to this work.
Author to whom correspondence should be addressed. Electronic mail:
Yufeng.Zhao@nrel.gov.
b)
0003-6951/2012/100(24)/243905/4/$30.00
FIG. 1. Measured specific capacity as a function of the number of charge/
discharge cycles for SiO1.83.
100, 243905-1
C 2012 American Institute of Physics
V
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243905-2
Ban et al.
To uncover the mechanism behind this experimental
result, we have performed four types of calculations: (a) bulk
structures of Li metal, diamond Si, and a-quartz (SiO2) as
reference calculations; (b) point defects that form in a-quartz
after incorporating Li atoms at a 1/8 Li-to-Si ratio to probe
the initial stages of Li reaction with SiO2; (c) crystal structures of partially reduced SiO2 at higher Li-to-Si ratio, ranging from 1/4 to 2/3, which are representative of the charging
process; and (d) structural optimizations of lithium ion insertion using molecular dynamics simulations.
For the first three types of calculations, we have used
the DFT method as implemented in the VASP package;25 for
the fourth type of simulations, we have used MD as implemented in the LAMMPS package.26 The DFT calculations have
been carried out in the local density approximation (LDA),
with projector-augmented wave pseudopotentials27 and the
Ceperley-Adler exchange-correlation functional.28 Supercells of 2 2 2 conventional a-quartz units were used,
with a total of 72 atoms; to these, we have added up to 16 Li
atoms spanning a range of the Li-to-Si ratios from 0 to 2/3.
The plane-wave cutoff was 400 eV, the Brillouin zone was
sampled with a 3 3 3 Monkhorst-Pack grid, and the
structures were relaxed until the residual force on any atom
became smaller than 0.01 eV/Å. To explore a wider range of
low-energy configurations, we have also performed MD simulations using the reactive force field (ReaxFF)29 parameterized for Li–Al silicates.30 A low-temperature annealing
schedule was carried out, in which the system was equilibrated at 50 K for 150 ps, and then cooled to 0.01 K over
100 ps.
We have performed DFT modeling of three types of
point defects by incorporating two Li atoms in the 2 2 2
supercell of a-quartz, corresponding to a 1/8 Li-to-Si ratio.
The first type of defect is the direct intercalation of Li atoms
into the interstitial sites in the bulk SiO2. With a formation
energy of 0.87 eV/Li (computed with respect to the Li metal
and the perfect SiO2 crystal), the intercalation of free Li
interstitials in a-quartz is energetically unfavorable. In the
second type of defect that we considered, two Li atoms
attack the oxygen atom shared by two SiO4 tetrahedra [refer
to Fig. 2(a)] and form a local Li2O cluster and a Si–Si bond
[Fig. 2(b)]. This represents the full reduction of silica by two
Li atoms, but also has a high (positive) formation energy,
0.28 eV/Li. The third type of defect shows the attack of a
Si–O bond by two Li atoms [Fig. 3(a)], resulting in the displacement of the Si atom outside of its original SiO4 tetrahedron. Both atoms from the attacked Si–O bond become
3-fold coordinated, Si(III) and O(III) [Fig. 3(a)]. As shown
in Fig. 3(b), the dangling bond of the Si(III) atom is saturated
through electronic charge transfer from a Li atom to the
Si(III) atom. This defect represents the partial reduction of
silica, and turns out to be thermodynamically favorable, i.e.,
it has a negative formation energy, 0.25 eV/Li.
Next, we focus on analyzing the electronic structure of a
few hypothetical LixSiO2 crystal structures that are derived
through the partial reduction of SiO2, with 1=4 x 2=3.
For these structures, the band-gaps decrease significantly
with respect to the pure a-quartz value, ranging between 2.0
and 3.4 eV, depending on the structure. The structure with
the lowest formation energy that we have found [0.68 eV/
Appl. Phys. Lett. 100, 243905 (2012)
FIG. 2. Local reduction of an O atom (red) by two Li atoms (green). (a) The
two Li atoms react with a nearby O to form a local Li2O cluster. The two Si
atoms (yellow), originally bound to the O atom, now form a Si–Si bond. (b)
Partial charge density near the Fermi level shows the electronic charge of a
Si–Si bond.
Li, refer to Fig. 4(a)] corresponds to x ¼ 2/3 and has a bandgap of 2.4 eV. In this structure, 1/3 of all Si atoms are partially reduced through the mechanism shown in Fig. 3(a).
The dangling bonds of Si(III) atoms are saturated by loneelectron pairs from the Li atoms, as suggested by the partial
electronic charge density near the Fermi level [Fig. 4(b)].
The site-projected density of states [Fig. 4(c)] shows significant overlap between the states associated with the two Li
atoms and those of the Si(III) and O(III) atoms, both below
and above the band-gap; this is consistent with the partial
charge densities showing saturation of Si(III) dangling bonds
[Fig. 4(b)].
Several configurations of Li in SiO2 have been predicted
heuristically and their structures optimized using DFT; of
these, the structure of Fig. 3(a) was found to be the most stable (lowest energy). To verify this, we turn to the cooperative
use of molecular statics (MS), MD, and DFT. MS calculations based on the ReaxFF formalism29,30 and DFT show remarkable agreement (< 3 meV/atom) between the relative
energies of the unstable interstitial lithium and the partially
reduced Li1/6SiO2 of Fig. 3(a), validating the cooperative use
of the DFT and ReaxFF formalisms. A lithium ion placed at
a site known from DFT to be at a local energy minimum, but
far from a lowest-energy configuration, is used as the starting
point in a more deterministic approach to finding the final,
low-energy structure. With a Li-ion placed at an interstitial
site, a low temperature, MD anneal and subsequent DFT
relaxation recovers the same structure as that seen in
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243905-3
Ban et al.
Appl. Phys. Lett. 100, 243905 (2012)
FIG. 3. Intercalation of Li atoms in SiO2 through partial reduction. (a) The
two inserted Li atoms cleave a Si–O bond and push the Si atom away from
its original position so that both the Si atom and the O atom become threefold coordinated [denoted by Si(III) and O(III)]. (b) Partial charge density
near the Fermi level shows that the dangling bond of the Si(III) atom is saturated by a lone electron pair due to charge transfer from the Li atom to the
Si(III) atom.
Fig. 3(a). This methodology reveals the benefit of exploring
the configurational space accessible through atomic-level
simulations coupled with the accuracy of DFT calculations.
As a bond order potential, the reactive force field follows empirically established “bonding rules” throughout the
MD simulations. Using a low temperature MD annealing
schedule, this potential correctly predicts the formation of
both stable (Li4SiO4) and transitional (Li2O) type of
defects,17,24 and gives insight into the pathway of their formation. These pathways reveal that the partial reduction of
Si forms a Li–O bond and a three-fold coordinated Si displaced outside of the original (unreacted) SiO4 tetrahedron
[refer to Fig. 3(a)]. Using DFT, we have found that the removal of the Li (e.g., during discharge) leaves a zero-barrier
path for the Si(III) atom to form the previously-broken Si–O
bond and recover the ideal a-quartz structure.
So far, we have discussed lithiation in ideal a-quartz.
Below, we propose a mechanism for lithium insertion and removal in random mixtures of Si and SiO2 that form SiOy,
where y < 2.24 The recombination of the first Li ion and an
electron (first Liþ/e pair) in the anode leads to the partial
reduction of an SiO2 bond and the formation of a three-fold
coordinated silicon atom with a single unpaired electron
[Fig. 3(a)]. The Si(III) atom is highly reactive and will readily accept another electron upon recombination with the second Liþ/e pair, leaving the Liþ stabilized by hydrogen
FIG. 4. The identified lowest-energy Li2/3SiO2 crystal: (a) The atomic structure; (b) the partial charge density near the Fermi level, showing lone electron pairs localized on the three-fold coordinated Si; (c) the site-projected
density of states (DOS) shows that the highest occupied states are localized
mainly on the three-fold coordinated Si, consistent with the partial charge
density shown in panel (b).
bonding to the four oxygen atoms neighboring the interstitial
site. The resulting structure is very similar to Li4SiO4, which
is consistent with recent NMR measurements.24
Subsequent removal of lithium from LixSiO2 fully
recovers the bulk quartz structure (as discussed above, there
is zero barrier for the Si(III) to reform the Si–O bond upon
delithiation), except at the Si/SiO2 interfaces (Fig. 5). As
show in Fig. 5, rather than becoming three-fold coordinated,
as it would in quartz, the Si atom that is displaced outside its
original SiO4 tetrahedron enlarges the adjacent Si particle,
since the Si side of the Si/SiO2 interface gains that displaced
silicon atom. Insertion of the second Li creates a Li4SiO4like environment (see Fig. 5), which acts as a lithium trap
and reduces the reversibility of lithium insertion. The
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243905-4
Ban et al.
Appl. Phys. Lett. 100, 243905 (2012)
observed cycling behavior and the Li–Si–O phases identified
both here and elsewhere.17,24
This work was funded by the U.S. Department of
Energy under Subcontract No. DE-AC36-08GO28308
through the Office of Energy Efficiency and Renewable
Energy, the Office of the Vehicle Technologies Program,
and by the National Science Foundation through Award Nos.
OCI-1048586 and CMMI-0846858. Computations were performed on resources provided by the Golden Energy Computing Organization.
1
FIG. 5. Schematic showing the growth of the Si phase following formation
of a three-fold coordinated Si [Si(III)] defect (Fig. 3) at a Si/SiO2 boundary.
As described in the text, lithium breaks a Si–O bond (marked with an “X”)
and that Si reflects through the basal plane of the SiO4 tetrahedron, as indicated by the arrow. This silicon bonds with the silicon nanoparticle, as
shown. Two lithium atoms, which adopt a Li4SiO4-like structure, are consumed irreversibly during charging.
observed increase in global capacity upon further cycling
[Fig. 1] is explained by the growth of the Si phase: the Si
volume gained, capable of holding 4 Li per Si, has a substantially larger capacity than the SiO2 volume lost. While
this process is naturally limited by the constraint that the
Si:O ratio remains constant, we have not considered structures that may form when this limitation prevents further
Li4SiO4-mediated Si growth.
In conclusion, we have shown that the Li storage
capacity of a SiO1.83 anode increases (after some initial parasitic loses) over 200 charge/discharge cycles. Combined
DFT and MD simulations reveal that partial reduction of aquartz leads to the formation of Li-containing defects and an
4 eV decrease in the band-gap. While earlier interpretations24 attribute the Li4SiO4 that persists through charge/discharge cycling to the irreversible reduction of Si in SiO2, we
find that the insertion of Li forms Li4SiO4-like defects that
are reversible within bulk SiO2, but irreversible at Si/SiO2
interfaces. When formed at an interface, these structures
increase the volume fractions of Si and Li4SiO4, and increase
the lithium storage capacity, which explains both our
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