Lithiation of silica through partial reductionChunmei 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 View Table of Contents: http://apl.aip.org/resource/1/APPLAB/v100/i24 Published by the American Institute of Physics. Related ArticlesThe role of upstream distal electrodes in mitigating electrochemical degradation of ionic liquid ion sources Appl. Phys. Lett. 101, 193504 (2012) Coordination state probabilities and the solvation free energy of Zn2+ in aqueous methanol solutions J. Chem. Phys. 137, 164504 (2012) Varying the charge of small cations in liquid water: Structural, transport, and thermodynamical properties J. Chem. Phys. 137, 164501 (2012) The spontaneous curvature of the water-hydrophobe interface JCP: BioChem. Phys. 6, 10B603 (2012) The spontaneous curvature of the water-hydrophobe interface J. Chem. Phys. 137, 135102 (2012) Additional information on Appl. Phys. Lett.Journal Homepage: http://apl.aip.org/ Journal Information: http://apl.aip.org/about/about_the_journal Top downloads: http://apl.aip.org/features/most_downloaded Information for Authors: http://apl.aip.org/authors

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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)

1National Renewable Energy Laboratory Golden, Colorado 80401, USA2Department of Mechanical Engineering and Materials Science Program Colorado School of Mines, Golden,Colorado 80401, USA

(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)

compounds have band-gaps in the range of 2.0–3.4 eV. VC 2012 American Institute of Physics.

[http://dx.doi.org/10.1063/1.4729743]

Since the discovery of tin oxide as a potentially promis-

ing 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 materi-

als 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 consist-

ent with its chemical inactivity. On the other hand, the reduc-

ing 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 reasona-

ble 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 surpris-

ing 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 nei-

ther 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 dispers-

ant, and ethanol as a solvent. The precursor solution was sol-

vothermally 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 coin-

type 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 Li-

ion 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 indi-

cate that the reaction with Li transforms SiO2 irreversibly

into Li2O and Li4SiO4. On the other hand, it is indeed prom-

ising to use SiO2 as an anode material because this material

is stable, light-weight, cheap, and environmentally benign.

FIG. 1. Measured specific capacity as a function of the number of charge/

discharge cycles for SiO1.83.

a)C. Ban and B. B. Kappes contributed equally to this work.b)Author to whom correspondence should be addressed. Electronic mail:

Yufeng.Zhao@nrel.gov.

0003-6951/2012/100(24)/243905/4/$30.00 VC 2012 American Institute of Physics100, 243905-1

APPLIED PHYSICS LETTERS 100, 243905 (2012)

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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 struc-

tures of partially reduced SiO2 at higher Li-to-Si ratio, rang-

ing from 1/4 to 2/3, which are representative of the charging

process; and (d) structural optimizations of lithium ion inser-

tion 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 imple-

mented 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 Super-

cells 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/A. To explore a wider range of

low-energy configurations, we have also performed MD sim-

ulations using the reactive force field (ReaxFF)29 parameter-

ized for Li–Al silicates.30 A low-temperature annealing

schedule was carried out, in which the system was equili-

brated 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 dis-

placement of the Si atom outside of its original SiO4 tetrahe-

dron. 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/

Li, refer to Fig. 4(a)] corresponds to x¼ 2/3 and has a band-

gap of 2.4 eV. In this structure, 1/3 of all Si atoms are par-

tially reduced through the mechanism shown in Fig. 3(a).

The dangling bonds of Si(III) atoms are saturated by lone-

electron 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 signifi-

cant 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 sta-

ble (lowest energy). To verify this, we turn to the cooperative

use of molecular statics (MS), MD, and DFT. MS calcula-

tions based on the ReaxFF formalism29,30 and DFT show re-

markable 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

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.

243905-2 Ban et al. Appl. Phys. Lett. 100, 243905 (2012)

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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 fol-

lows 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 for-

mation. These pathways reveal that the partial reduction of

Si forms a Li–O bond and a three-fold coordinated Si dis-

placed outside of the original (unreacted) SiO4 tetrahedron

[refer to Fig. 3(a)]. Using DFT, we have found that the re-

moval 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 re-

moval 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 read-

ily accept another electron upon recombination with the sec-

ond Liþ/e� pair, leaving the Liþ stabilized by hydrogen

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 Li4SiO4-

like environment (see Fig. 5), which acts as a lithium trap

and reduces the reversibility of lithium insertion. The

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 three-

fold 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 satu-

rated by a lone electron pair due to charge transfer from the Li atom to the

Si(III) atom.

FIG. 4. The identified lowest-energy Li2/3SiO2 crystal: (a) The atomic struc-

ture; (b) the partial charge density near the Fermi level, showing lone elec-

tron 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).

243905-3 Ban et al. Appl. Phys. Lett. 100, 243905 (2012)

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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 sub-

stantially 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 struc-

tures 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 para-

sitic loses) over 200 charge/discharge cycles. Combined

DFT and MD simulations reveal that partial reduction of a-

quartz leads to the formation of Li-containing defects and an

�4 eV decrease in the band-gap. While earlier interpreta-

tions24 attribute the Li4SiO4 that persists through charge/dis-

charge 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

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 per-

formed on resources provided by the Golden Energy Com-

puting Organization.

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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 indi-

cated by the arrow. This silicon bonds with the silicon nanoparticle, as

shown. Two lithium atoms, which adopt a Li4SiO4-like structure, are con-

sumed irreversibly during charging.

243905-4 Ban et al. Appl. Phys. Lett. 100, 243905 (2012)

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