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Accepted Manuscript
Two-dimensional siligraphenes as cathode catalysts for nonaqueous lithium-oxygen
batteries
Huilong Dong, Yujin Ji, Tingjun Hou, Youyong Li
PII:
S0008-6223(17)31075-8
DOI:
10.1016/j.carbon.2017.10.077
Reference:
CARBON 12504
To appear in:
Carbon
Received Date: 9 August 2017
Revised Date:
3 October 2017
Accepted Date: 23 October 2017
Please cite this article as: H. Dong, Y. Ji, T. Hou, Y. Li, Two-dimensional siligraphenes as cathode
catalysts for nonaqueous lithium-oxygen batteries, Carbon (2017), doi: 10.1016/j.carbon.2017.10.077.
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ACCEPTED MANUSCRIPT
Two-dimensional siligraphenes as cathode catalysts for nonaqueous
lithium-oxygen batteries
Huilong Dong,* Yujin Ji, Tingjun Hou and Youyong Li*
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Institute of Functional Nano & Soft Materials (FUNSOM), Soochow University,
Suzhou, Jiangsu 215123, China.
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Abstract
The nonaqueous lithium-oxygen (Li-O2) battery is promising as a superior energy
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storage medium, whose practical applications are limited by the catalytic performance
of cathode catalysts. Using density functional theory calculations, we systematically
investigate the catalytic mechanism and catalytic performance of siligraphenes as
cathode catalysts in nonaqueous Li-O2 batteries. For the siligraphenes studied here,
Li2O is the final discharge product. Among them, the single-layered SiC (SL-SiC)
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exhibits potential as a suitable cathode catalyst due to the considerably low
overpotential values during discharge and charge process. That is, 0.73 V for oxygen
reduction reaction (ORR) and 1.87 V for oxygen evolution reaction (OER). The
quantitative analysis indicates that the ORR overpotential on siligraphenes is linearly
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correlated with the ∆Eads of the last added Li in Li4O2* (* denotes the adsorbed
surface), while the OER overpotential is linearly correlated with the Eads of LiO2 on
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C
surface. Our investigations elucidate quantitative correlations between the catalytic
performance of siligraphenes and the adsorption performance of Li-contained
intermediates on them, which provides promising approach to develop metal-free
cathode catalysts with high catalytic activity.
*
Corresponding authors.
E-mail: huilong_dong@126.com, Tel: (86)-512-65882037 (H. Dong);
E-mail: yyli@suda.edu.cn, Tel: (86)-512-65882037 (Y. Li).
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1. Introduction
Since its first proposal by Abraham and Jiang,[1] the lithium-oxygen (Li-O2)
battery has attracted tremendous scientific interest. Due to the extremely high
theoretical specific energy density [5–10 times higher than the traditional lithium ion
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batteries (LIBs)],[2, 3] nonaquous Li-O2 battery is viewed as competitive energy
storage medium for electric vehicles (EVs) or hybrid electric vehicles (HEVs).[4, 5]
In a Li-O2 battery, the pure lithium metal is employed as anode material, while
atmospheric oxygen is used as a reactant in cathode, which is stored externally and
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can be readily obtained from the surrounding air.[6, 7] Though it seems promising, the
practical application of Li-O2 battery still faces lots of challenge, such as poor
stability,
short
cycle
life,
low
discharge
capacity
and
high
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electrolyte
overpotential.[8-10] Most of the problems can be attributed to the slow kinetics of the
oxygen reduction reaction (ORR) during discharging process and the oxygen
evolution reaction (OER) during charging process.[6, 11] To effectively facilitate the
ORR and OER, the development of suitable cathode catalysts is of the great
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importance.[12]
Among the various cathode catalysts, carbon-based nanomaterials take
advantages in low cost, large specific surface area, high conductivity, and good
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stability,[13, 14] making them attractive in applications of Li-O2 battery. As the most
representative two-dimensional carbon nanomaterial, graphene was deeply researched
as the cathode catalyst for Li-O2 battery.[15-17] The experimental work by Qiao and
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coworkers had proven that the flat surface of two-dimensional carbon nanomaterial
could exhibit high catalytic activity.[18] Sun and coworkers[5] first reported the
application of graphene nanosheets (GNSs) as cathode active materials in a
nonaqueous lithium-oxygen battery, and found that the GNSs electrode exhibit
extremely high discharge capacity in comparison to carbon powders. Furthermore,
Jiang et al.[19] theoretically demonstrated that the existence of some specific point
defects (like single vacancy, DV5555-6-7777 defect) on graphene surface is the origin
of graphene’s excellent performance. However, the surface defects are difficult to
control, and are easy to migrate or be healed.[20] Doping is the effective way to
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overcome the drawbacks of point defects by introducing more stable and more active
sites into graphene. The hetero-atom doped graphenes (including N-doped,[21-24]
B-doped,[25] S-doped,[26] or N, B-codoped[14]) have been widely investigated as
the cathode catalysts in Li-O2 battery, both experimentally and theoretically.
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Particularly, Ren et al.[27] studied a series of hetero-atom doped graphene materials
(B, N, Al, Si and P) as OER catalysts to correlate catalytic activity with adsorption
structure and charge transfer properties. From their results it is found that Si-doped
graphene provides relatively favorable charging voltage as well as low barrier for
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oxygen evolution.
Actually, silicon always plays important role in energy storage materials. For
nanoparticles
for
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instance, Son et al.[28] reported the SiC-free graphene growth over silicon
high-performance
lithium-ion
battery
application,
which
demonstrates the synergistic effect of silicon and carbon in energy storage. Lots of
previous studies validated the great effect of Si-doped graphene or siligraphenes[29]
in oxygen dissociation[30] or ORR catalysis[31, 32]. Comparing with the Si-doped
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graphene, the siligraphene that formed by incorporating silicon into graphene
honey-comb structure benefits from more stable composite and richer reaction sites
for adsorption of oxygen molecules. Here we made comparative investigations on
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three kinds of siligraphenes (SL-SiC,[32] g-SiC2,[31, 33] and g-SiC3[34]) that show
high affinity to oxygen adsorption. By performing density functional theory (DFT)
calculations, we uncover the potential of siligraphenes (especially SL-SiC) as
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candidates of cathode catalysts for Li-O2 battery. The high Si : C ratio provides plenty
active sites for adsorption and dissociation of O2 molecules, and the strong binding
between the dissociated oxygen and Li promotes the four-electron pathway to form
Li2O as the final product. Our calculations also provide new insight into the
relationship between adsorption performance of LixO2 intermediates and the
ORR/OER overpotential, which is helpful to the material design on cathode of Li-O2
battery.
2. Computational details and modeling
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All the geometry optimization and energy calculations are carried out by
DMol3[35-37]
module
available
in
Materials
Studio
7.0.
The
Perdew-Burke-Ernzerhof (GGA-PBE) functional[38] is employed to describe the
electronic exchange and correlation effects. The double numerical atomic orbital
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including polarized p-function (DNP)[35] is used as the basis set, and orbital cutoff is
set as 4.6 Å. To correctly deal with the long-range weak interaction like van der Waals
forces, Grimme scheme is adopted for dispersion correction.[39] The geometry
optimization converges with energy, gradient and displacement reach the threshold
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values of 10−5 Ha, 0.001 Ha/Å, and 0.005 Å, respectively. Electron thermal smearing
effect is imposed by employing a smearing value (0.005 Ha) for all the calculations,
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which could effectively enhance the SCF convergence efficiency.
The crystal structures of SL-SiC[32], g-SiC2[31] and g-SiC3[34] share graphene-like
hexagonal lattice, and their unit cells are derived from the recently reported work.[31,
32, 34] We expanded SL-SiC into 5×5×1 supercell (a = 15.42 Å), and expanded
g-SiC2 and g-SiC3 into 3×3×1 supercell (a = 15.00 Å and 16.84 Å for g-SiC2 and
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g-SiC3). The Brillouin zone is sampled by a 3×3×1 k-point Monkhorst-Pack grid,[40]
which is sufficient to reach convergence as tested [see Table S1 in Supplementary
material]. To determine the activation barriers (Eb) during the dissociation of
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adsorbed O2 molecule on siligraphene, transition state searches are conducted by
performing the complete linear synchronous transit/quadratic synchronous transit
(LST/QST) method.[41] All the obtained transition state (TS) structures show only
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one imaginary frequency, and are confirmed to directly connect corresponding
reactants and products by nudged-elastic band (NEB) algorithm.[42]
The adsorption energy (Eads) for Li atom or O2 molecule on siligraphene is
defined as
() = + ∗ − ∗
(1)
( ) = + ∗ − ∗
(2)
where the ∗ represents the energy of the siligraphene substrate (* denotes the
adsorbent), ∗ or ∗ represents the energy of optimized Li or O2 adsorbed
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siligraphene, and is the energy of O2 molecule (triplet as the ground state). It
should be pointed out that the used here is the energy per atom in bulk Li metal
(bcc structure), so Eads of Li indicates the energy difference between adsorbed Li atom
against Li atom in its bulk structure. For LixO2 adsorbed surface with several Li atoms,
as
∆ = ∗ + − ∗
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we defined the consecutive adsorption energy (∆Eads) of a newly added Li in LixO2*
(8)
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where the ∗ represents the energy of LixO2* and ∗ represents the
energy of Lix-1O2*. The ∆Eads of Li indicates the energy released when a new Li is
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added into Lix-1O2*, which can also be viewed as the insertion energy of Li atom.
The equilibrium potential can be obtained by the Nernst equation
= −
∆
(3)
In this equation n is the number of transferred electron and the standard formation
energy (∆ ) could be derived from
where () ,
()
− !() − "(#)
(4)
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∆ = () , and (#) are the calculated free energies of LixOy (denotes
the final product in crystal), Li (from bulk bcc structure) and O2 molecule (triplet as
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the ground state), respectively. Through above method we determined the equilibrium
potential of Li2O at 298.15 K and 1atm as 2.83 V, which agrees well with the
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experimental result of 2.91 V.[43]
The free energy change (∆G) is calculated following
∆G = ∆E + ∆ZPE − T∆S (5)
where ∆E is the change in total energy, ∆ZPE is the difference in zero point
vibrational energy, T = 298.15 K and ∆S is the change in entropy. The zero point
vibrational energy and entropy are obtained from frequency calculations on the
optimized structures.
The Bader charge analysis developed by Henkleman group[44] is applied for
quantitative charge analysis by using the Vienna Ab initio Simulation Package
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(VASP),[45, 46] due to its more reasonable results on positive charge assigned on Li.
More details on VASP calculations and the comparison on charge population can be
found in Supplementary material.
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3. Results and discussion
The geometric structures of the siligraphenes investigated here are shown in
Figure 1. Due to the larger electronegativity of C than Si, the electron tends to
accumulate around the C-rich zone. As displayed in Figure 1, the deformation
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electron density contours clearly depict that the electron density increases around the
C atoms in SL-SiC, the 4-C domain in g-SiC2 and the 6-C domain in g-SiC3. The
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accumulation of electron leads to the positively charged Si sites. According to our
charge analysis, the Si in SL-SiC, g-SiC2 and g-SiC3 carries positive charge of +1.40
|e|, +1.25 |e|, and +1.06 |e|, respectivey. Compared with the pristine graphene with
highly delocalized electronic distribution, the positively charged Si atoms could
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provide suitable adsorption sites for the nucleophilic O2 molecule.
Figure 1. The geometric structures and deformation electron density contours of (a)
SL-SiC, (b) g-SiC2 and (c) g-SiC3. The grey spheres denote C atoms and the yellow
spheres denote Si atoms. The “H” or “C” indicates different kinds of adsorption sites
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for Li atoms.
For cathode catalysis in nonaqueous Li-O2 battery, the ORR process have two
proposed mechanisms depending on the final product, that is, two-electron pathway or
reduced to Li2O2 following,
2( + + , - ) + → (6)
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four-electron pathway.[43] For two-electron pathway, the O2 molecule will be directly
While for four-electron pathway, the O2 molecule binds with four Li atoms and is
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reduced to Li2O as following,
4( + + , - ) + → 2 (7)
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During the four-electron pathway, the Li2O2 is formed as intermediate. For both
pathways the dissociation of O2 and the formation of LiO2 is the beginning step.
Before we consider the O2 dissociation, it is necessary to compare the adsorption
performance of Li and O2, which determines the initial growing process of LiO2 on
siligraphene. For the substrate, if the adsorption of O2 is more preferred than Li, the
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initial pathway should be O2* → LiO2* (* denotes the adsorbed substrate), and if the
adsorption of Li is more preferred than O2, the initial pathway will be Li* → O2Li*.
The adsorption of O2 on siligraphene has been deeply investigated in previous
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work.[31, 32, 47] It has been validated that the positively charged Si site possesses
high adsorption affinity for O2.[30] The moderate distance between the neighboring Si
atoms in our investigated g-SiC2 and g-SiC3 lead to stable bridged configuration of O2
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with the two neighboring Si atoms, while for SL-SiC the Si-C bridged site is more
energetically stable for adsorption of O2 (as shown in Figure S1). For adsorption of a
single Li atom on siligraphene, we systematically investigated all the possible
adsorption sites. Due to the symmetry restriction, for SL-SiC there is only one C top
site, one Si top site and one hollow (H) site for Li adsorption. For g-SiC2, all the
six-member rings are equal for Li adsorption (the hollow site), but there are two
different types of carbon atoms, the one directly bonds with Si is marked as C1, and
the one surrounded by three C1 atoms is marked as C2. For g-SiC3, all the C top sites
are equal to each other, so as the Si top site. But there are two different types of
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hollow sites, the one composed with 2 Si atoms and 4 C atoms is marked as H1, while
the one composed with 6 C atoms is marked as H2. The detailed location of (C1, C2)
or (H1, H2) sites could be found in Figure 1. As listed in Table 1, the O2 show very
stable adsorption with bridged configuration. However, for Li, only some of the sites
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can be adsorbed (the Li will automatically move from the initial site to another after
optimization). More importantly, only g-SiC2 could provide small positive adsorption
energy for Li, and the SL-SiC and g-SiC3 cannot prevent the clustering of individual
Li atoms. The comparison results demonstrate that O2 take dominant advantages in
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adsorption on the siligraphenes, so in the following discussion we will take O2* →
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LiO2* as the initial pathway.
Table 1. The adsorption site and corresponding adsorption energy (Eads) of Li and O2
on different siligraphenes. The H indicates hollow site. The detailed location of (C1,
C2) or (H1, H2) sites are marked in Figure 1.
Adsorption site
Adsorbate
Eads (eV)
Before optimization After optimization
Li
Li
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C
g-SiC2
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O2
Si top
Si top
-0.22
C top
C top
-0.72
H
Si top
-0.22
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SL-SiC
Li
O2
0.67
Si-Si bridge
0.66
Si top
H
0.22
C1 top
C2 top
0.15
C2 top
C2 top
0.15
H
H
0.22
O2
g-SiC3
Si-C bridge
Si-Si bridge
1.10
Si-C bridge
0.83
Si top
H1
-0.24
C top
H2
-0.16
H1
H1
-0.18
H2
H2
-0.16
Si-Si bridge
2.88
Si-C bridge
1.25
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To stimulate the formation of LiO2 on siligraphene, the dissociation of adsorbed
O2 molecule is the essential step. Here we performed transition state (TS) search to
simulate the reaction process, as displayed in Figure S1. Our calculation results
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indicate that the activation barriers (Eb) during the dissociation of adsorbed O2 are all
considerably low (0.30 eV on SL-SiC, 0.34 eV on g-SiC2 and 0.04 eV on g-SiC3) so
that the O2* is easy to dissociate on the siligraphenes under mild condition. To
exclude the influence from adsorption density, we also performed additional
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calculations on the same O2 dissociation reaction for different supercells. The
calculation results in Table S3 show that the enlarging of siligraphene supercell has
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merely influence on both Eads of O2 and the Eb (less than 0.1 eV).
Based on the fact that the adsorption of O2 is superior to Li atom and O2 is easily
dissociated on the siligraphene surface, we propose that the ORR processed on the
siligraphenes follows the four-electron pathway as silicene does.[48] The detailed
reaction steps should be (a) (Li++e-)+2O*
Li3O2*, and (d) (Li++e-)+Li3O2*
Li2O2*, (c)
Li4O2* (the * denotes the
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(Li++e-)+Li2O2*
LiO2*, (b) (Li++e-)+LiO2*
siligraphene surface). To make sure if a second O2 molecule will adsorb onto the
Li2O2* intermediate, we constructed the Li2O4* structures and performed calculations,
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the optimized structures of Li2O4* are shown in Figure S2. As results, the second O2
molecule shows repulsive interaction against the Li2O2 on SL-SiC and g-SiC2, and
weak physisorption on Li2O2-SiC3 (0.30 eV). The O-O bond of the second O2
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molecule merely elongates (1.282 Å in SL-SiC, 1.289 Å in g-SiC2, and 1.307 Å in
g-SiC3), very close to the O-O bond length obtained from the isolated O2 molecule
(1.232 Å), indicating weak electronic interaction with the Li2O2*. Moreover, previous
experimental results also indicate that Li2O is preferred to form on catalysts with high
oxygen catalyst bond strength.[49] Based on these, we consider that the O2 molecule
is hardly reduced by the the Li2O2* intermediate and Li4O2* is more likely to be the
final product on siligraphenes.
In Figure 2, we depicted the most stable structures of the optimized intermediates
LiO2*, Li2O2*, Li3O2* and Li4O2*. It is worth noting that the possible adsorption site
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of the third Li on the top of Li2O2* is also considered (see Figure S3). The adsorption
of Li on the top site of Li2O2* exhibits significantly weaker consecutive adsorption
energy (∆Eads = 1.30, 1.20 and -0.10 eV for SL-SiC, g-SiC2 and g-SiC3, respectively)
than that on the corresponding side site (the ∆Eads values for the third Li in Li3O2* is
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listed in Table 1). That’s why we chose the structures of Li3O2* in Figure 2 as the
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most stable ones.
Figure 2. Schematics of the growing pathways of Li4O2 on (a) SL-SiC, (b) g-SiC2 and
(c) g-SiC3. The grey, yellow, red and purple spheres represent C, Si, O and Li atoms,
respectively.
It should be noted that the final product Li4O2 is taken as a representative of the
bulk Li2O. As we can see in Figure 3, the structures of Li4O2 unit formed on different
siligraphene are highly similar with the Li4O2 units in the (110) facet of Li2O.
Considering that there are plenty of Si sites, it is expected the Li2O (110) facet could
form with the siligraphene surface is fully covered by Li4O2 units, as shown in Figure
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3(b-d).
Figure 3. (a) The (110) facet cutting from bulk Li2O, the Li4O2 units in the surface are
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displayed as ball and stick model for clarity. (b-d) The simulated siligraphene surfaces
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(SL-SiC, g-SiC2 and g-SiC3) that are fully covered by Li4O2 units.
Figure 4. The calculated free energy diagrams for ORR/OER catalyzed by (a) SL-SiC,
(b) g-SiC2 and (c) g-SiC3 at zero potential (U = 0 V), equilibrium potential (Ueq),
discharge potential (UDC) and charge potential (UC). (d) The correlation between ORR
overpotential and ∆Eads of last adsorbed Li in Li4O2* (the black line), as well as the
correlation between OER overpotential and Eads of LiO2 on different siligraphenes
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(the blue line).
Based on the optimized structures, we further calculated their free energy changes
to elucidate the ORR and OER performance of the siligraphenes in nonaqueous Li-O2
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batteries. In Figure 4 (a-c), the calculated free energy diagram for ORR/OER
catalyzed by the three siligraphenes at zero potential (U = 0 V), equilibrium potential
(Ueq), discharge potential (UDC) and charge potential (UC) are depicted. Here, the UDC
is the highest voltage needed to make the free energies of ORR steps downhill, while
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the UC is the lowest voltage needed to make the free energies of OER steps downhill.
Due to the difference between practical potential and equilibrium potential, the
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overpotential arises, which is useful to evaluate the catalytic performance by
calculating 011 = − 23 , and 041 = 5 − . According to the definition,
011 is the discharge overpotential and 041 is the charge overpotential. Obviously,
the low overpotential value means high catalytic activity for the catalyst.
From the free energy diagram we can see that the discharge potential of
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siligraphene catalyzed ORR follows the sequence of SL-SiC (2.10 V) < g-SiC2 (1.98
V) < g-SiC3 (0.97 V). As comparison, SL-SiC and g-SiC2 possess higher discharge
potential than that of graphene (calculated as 1.21 - 1.35 V[14, 19]), showing the
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potential as candidates for high density energy storage. The charge potential of
siligraphene catalyzed ORR also follows the sequence of SL-SiC (4.70 V) < g-SiC2
(5.03 V) < g-SiC3 (7.44 V). The results indicate that among the three siligraphenes the
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SL-SiC has the best performance due to its high discharge voltage as well as low
charge voltage, while g-SiC3 is unsuitable to be used as cathode catalytic material.
Concluded from Figure 4 (a-c), another finding is that for siligraphenes, the rate
determining step (RDS) during ORR process is the formation of Li4O2*, and the RDS
during OER process is the formation of LiO2*.
To reveal the possible factor that affects ORR overpotential, we calculated all the
∆Eads values of the newly inserted Li in LixO2*. As listed in Table 2, all of the ∆Eads
values are positive, indicating that the insertion of Li atom is exothermic process.
Considering that the formation of Li4O2* is the RDS for ORR process, we found that
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the ∆Eads of Li in Li4O2* shows strong correlation with the ORR overpotential, that is,
the higher ∆Eads of Li in Li4O2* (the last adsorbed Li) is, the lower ORR overpotential
is needed. This correlation is linear and is depicted in Figure 4(d) (the solid black
triangle), with a fitting coefficient (R2) value as high as 0.995, which can be expressed
011 = −0.97∆ + 2.73
(9)
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as
Similarly, to find out the factor that directly determines the OER overpotential,
we calculated the adsorption energies (Eads) of LixO2 on siligraphenes following
(10)
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= + ∗ − ∗
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where the represents the energy of isolated LixO2 cluster. As listed in Table 2,
we also disclose that the Eads of LiO2 is strongly related with the OER overpotential
since the formation of LiO2 is the RDS of OER process. The linear correlation
depicted in Figure 4(d) (the hollow blue triangle) shows a fitting coefficient (R2)
value almost equal to 1. This correlation can be described as
(11)
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041 = 1.03 − 3.32
It is not accidental that there is such quantitative correlation between the
adsorption performance of LixO2 intermediates and the catalytic performance of
cathode catalysts. Similar correlation is also reported when the N or B doped
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graphene is used as the cathode catalysts.[14] From the microscopic view, it is found
that the ∆Eads of Li in LixO2 gradually decreases as the number of Li increases, which
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makes the binding of the last Li atom the most difficult. The energy released when the
last Li is inserted could directly determine the discharge voltage. Here the ORR
overpotential actually means the potential needed to modulate the difference between
surface of catalyst and bulk Li2O. That’s why during ORR process, the overpotential
highly depends on the ∆Eads of last Li in Li4O2. For the OER process, the strongest
adsorption affinity of LiO2 among the intermediates also makes its removal the most
difficult. Combining with the fact that the charge process aims at oxidize all the
formed reduced products into mobile Li atoms and O2 molecules again, the OER
overpotential could help to provide the extra energy to desorb the LiO2 from the
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surface of catalyst.
Table 2. The calculated consecutive adsorption energies (∆Eads, eV) of Li in LixO2
adsorbed siligraphenes, and the corresponding adsorption energies (Eads, eV) of LixO2
LiO2*
Li2O2*
∆Eads
2.70
2.46
3.71
∆Eads
2.29
2.57
2.29
Eads
6.00
6.45
8.59
∆Eads
2.18
2.21
1.02
Eads
6.44
6.92
7.87
Li4O2*
∆Eads
2.03
1.95
0.89
Eads
6.51
6.92
6.81
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SL-SiC
g-SiC2
g-SiC3
Eads
5.11
5.28
7.70
Li3O2*
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on siligraphenes.
To find out how the difference on adsorption performance origins, the Bader
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charge analysis is performed. For Li3O2 adsorbed siligraphenes, we revealed that the
charge distributed on Li3O2 is negative and the charge value decreases following the
sequence of SL-SiC > g-SiC2 > g-SiC3 for different surface, as listed in Table 3. This
is consistent with the insertion energy of Li into Li3O2* [the ∆Eads (Li) in Table 3],
due to the fact that larger negative charge has stronger attraction to the inserted Li
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atom. For adsorption of LiO2 on surface of siligraphenes, we used electrostatic energy
term as a descriptor. The electrostatic energy term indicates the weak electrostatic
interaction between the adsorbate and substrate, which can be expressed as
< <
1
. In this expression, the Q1 and Q2 stands for the charge on interacting
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ES = −
particle 1 and 2, respectively, and R is the distance between the interacting particles.
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In Table 3, we listed the charge distributed on LiO2 and siligraphene surface for
LiO2* and the distance between LiO2 and surface (R). The calculated electrostatic
energy terms (ES) in Table 3 show positive correlation with the corresponding Eads of
LiO2. This finding reveals that weaker electrostatic energy between the adsorbed LiO2
and the surface is favorable to weaken the binding of LiO2, thus helps to its desorption
during charge process and leads to a lower OER overpotential. Another important
conclusion on the charge population is that to obtain lower ORR and OER
overpotential, smaller negative charge on LiO2 and larger negative charge on Li3O2 is
recommended, which is directly related with the catalytic surface.
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Table 3. The charge population Q (LiO2) and Q (surface) on LiO2 and siligraphene
surface in LiO2*, the distance between LiO2 and surface (R), as well as the calculated
values for electrostatic energy terms (ES), the charge population Q (Li3O2) on the
also listed for comparison.
Q (LiO2)
Q (surface) R (Å)
ES
Eads (LiO2) Q (Li3O2) ∆Eads (Li)
SL-SiC
-1.80
1.80
1.60
2.04
5.11
g-SiC2
-1.83
1.83
1.57
2.15
5.28
g-SiC3
-2.09
2.09
1.60
2.75
7.70
-0.53
2.03
-0.51
1.98
-0.50
0.89
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Surface
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adsorbed Li3O2 for different siligraphenes. The Eads (LiO2) and ∆Eads (Li) in Li4O2* is
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Additionally, we can summarize from Eq (9) and Eq (11) that when the final
discharging product is Li2O, the ideal cathode catalyst should have proper Eads of LiO2
as well as ∆Eads of the last Li atom in Li4O2*. The obtained slope and intercept values
indicate that the ideal ∆Eads of Li inserted into Li3O2* should be close to 2.81 eV
(2.73/0.97) and ideal Eads value of LiO2 should be close to 3.22 eV (3.32/1.03). 2D
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materials like g-SiC3 is unsuitable as cathode catalysts because that the extremely
strong adsorption of O2 will result in exceedingly high Eads of the initially formed
LiO2, and the strong binding between dissociated O and Si atom leads to relatively
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low ∆Eads for the following adsorbed Li atoms. As an example, the previously
reported silicene cathode also yields very high OER overpotential (4.09 V), due to the
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extremely strong binding of LiO2 on silicene (6.17 eV),[48] which is consistent with
our findings. Thus our investigations elucidate that the siligraphene can be used as
good model system to quantitatively correlate the adsorption performance of Li-O
intermediates with the catalytic performance of cathode catalyst in Li-O2 battery.
It is worth noting that there has been theoretical investigation reported that the
binding of Li and LiO2 are the key descriptors of catalytic activity of Pt-based
bimetallic catalysts in nonaqueous Li-O2 batteries, and low ORR and OER
overpotentials are closely related to strong binding of Li and weak adsorption of LiO2
on catalytic surface, respectively.[11] Our results demonstrate that the metal-free
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two-dimensional siligraphene also shows similar rules and we further provide
quantitative formulas to correlate the adsorption of Li and LiO2 on catalytic surface
with ORR and OER overpotentials for the first time.
Since the exploit on better cathode catalysts for nonaqueous Li-O2 batteries is still
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in its preliminary stage, the SL-SiC investigated here is promising candidate as the
cathode catalyst. Comparing with many other graphene-based cathode catalysts, the
SL-SiC exhibits fairly high discharge potential and acceptable charge potential. As
comparison, the well-known N doped graphene exhibits discharge overpotential
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ranges from 0.89-1.66 V with the different doping configurations (Li4O4 as the
discharge product).[22] Moreover, the high concentration of Si and great structural
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stability will also be helpful to suppress the formation of side product Li2CO3, which
could significantly limits the performance of carbon-based nanomaterial. It has been
demonstrated that the C-O radical arisen in defective graphene or N-doped graphene
may account for the formation of Li2CO3.[19, 24] However, in siligraphene the C-O
radical won’t generate because O2 molecule is tightly captured by Si sites and results
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in the formation of Si-O bonds. Besides, the experimental realization of SiC
nanosheets has been reported by solution exfoliation or reaction between graphene
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and silicon source,[50, 51] making it practical in applications.
4. Conclusion
In this article, we investigated the potential application of siligraphenes
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(including SL-SiC, g-SiC2 and g-SiC3) as cathode catalysts in nonaqueous Li-O2
batteries by DFT calculations. The O2 molecule is easily dissociated on the surface of
siligraphene, which provides adsorption sites for the binding of Li atoms to form the
final product of Li4O2. Through DFT calculations, we established the reaction
pathway for the discharge process and calculated the free energy diagrams for
ORR/OER catalyzed by siligraphenes. Our results reveal that the SL-SiC exhibits
considerably low ORR overpotential (0.73 V) and OER overpotential (1.87 V),
showing the potential as suitable cathode catalyst for nonaqueous Li-O2 battery. The
quantitative analysis indicates that the ORR overpotential on siligraphenes is linearly
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correlated with the ∆Eads of the last added Li in Li4O2*, while the OER overpotential
is linearly correlated with the Eads of LiO2 in LiO2*. The charge population is used to
find out the influencing factor on the adsorption performance. Moreover, our research
summarized quantitative correlations between the catalytic performance of
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siligraphenes and the adsorption performance of LixO2 intermediates on them, which
provides promising approach to develop metal-free cathode catalysts with high
catalytic activity.
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Acknowledgments
This work is supported by the Ministry of Science and Technology of China
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(Grants No. 2017YFA0204802 and 2017YFB0701601), the National Natural Science
Foundation of China (Grants No. 51761145013, 21673149 and 21703145), China
Postdoctoral Science Foundation (Grant No. 2017M611892), and a Project Funded by
the Priority Academic Program Development of Jiangsu Higher Education Institutions
(PAPD). This is also a project supported by the Fund for Innovative Research Teams
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of Jiangsu Higher Education Institutions, Jiangsu Key Laboratory for Carbon Based
Functional Materials and Devices, Collaborative Innovation Center of Suzhou Nano
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Science and Technology.
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