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First-principles study of molybdenum chalcogenide halide nanowires for Mg-ion battery cathode application Pei Shan Emmeline Yeo, and Man-Fai Ng Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.5b01231 • Publication Date (Web): 12 Aug 2015 Downloaded from http://pubs.acs.org on August 14, 2015

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Chemistry of Materials

First-principles study of molybdenum chalcogenide halide nanowires for Mg-ion battery cathode application Pei Shan Emmeline Yeo and Man-Fai Ng* Institute of High Performance Computing, Agency for Science, Technology and Research, 1 Fusionopolis Way, #16-16 Connexis, Singapore 138632

ABSTRACT: Two categories of molybdenum chalcogenide halide nanowires (NWs): (1) o

h

H , Ch = S and/or Se, Ha = I, where the unit cell consists of Mo octahedra

directly linked together; and (2)

o

h

H , where the unit cell consists of two Mo

octahedra linked by bridging atoms, were investigated for their performance as cathode materials for Mg-ion battery. Using density functional theory calculations, we found that Mg adsorbs most strongly on pure selenium-based NWs (i.e., adsorption sites around the bridging atoms of

o

e

o

e and

o

e ), with

having the most exothermic Mg

adsorption energies. We revealed that differences in Mg adsorption energy result from interactions between Mg and interactions between Mg and

o o

h h

I that are anti-bonding (unstable); whereas the are bonding (stable). In addition, we calculated

the Mg diffusion barriers and specific capacities of the selenium-based NWs and compared them with other state-of-the-art cathode materials. Our results indicate that

o

e NWs

could potentially be used as cathode materials for Mg-ion batteries due to their low Mg diffusion barrier.

ACS Paragon Plus Environment

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1. Introduction Portable electronic devices like laptops and smartphones require light-weight batteries with high energy densities to power them. Among the battery technologies currently in use,1 Liion intercalation batteries2 possess the highest specific capacity due to the low atomic weight and high standard potential of reduction of Li. Despite Li’s dv nt ges, researchers are developing batteries that use multivalent ions like Mg instead. Mg is more abundant than Li and is one of the top ten most common elements found in the e rth’s crust.3 Although the pure Mg anode has a lower theoretical specific capacity compared to Li, the theoretical volumetric capacity of Mg is actually higher than Li (3833 versus 2062 m h cm ) due to the bivalency and close-packed arrangement of Mg. Furthermore, Mg does not form dendrites like Li does, that could short-circuit the battery and cause fires.4,5 A good Mg-ion cathode material should possess (1) a highly exothermic reaction energy with Mg so that the battery voltage is large, (2) the ability to undergo charging and discharging with preferably few changes to its volume or structure, and (3) a low diffusion barrier for Mg. Cathode materials that have been explored for use in Mg-ion batteries are generally divided into two categories.6 Firstly, the intercalation-type: the Mg atom is hosted within interstitial or cavity sites of the cathode and the bivalent Mg donates electrons to the constituents of the cathode. Secondly, the conversion-type: Mg undergoes a chemical reaction with the cathode material to form a new compound. While attaining a higher battery voltage with conversiontype cathodes is more probable, thus far, the drastic volumetric and structural changes associated with the formation of the new compound have negatively affected the Mg diffusion rates and cycling capacities of conversion-type cathodes.


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Many intercalation-type compounds suitable for Li-ion batteries were found to be unsuitable for Mg-ion due to extremely slow Mg diffusion rates.7 The first intercalation-type Mg-ion cathode material to have moderately good performance is Chevrel Phase8–10 (CP) . CP materials possess channels to accommodate Mg and the Mo octahedral (

)

units that comprise the CP are able to quickly accommodate and redistribute the charges of Mg, thus allowing for faster diffusion of the Mg. However, it was later found that a percentage of Mg is trapped in the CP at room temperature and is not utilized during electrochemical cycling.11 This is due to Mg favoring circular diffusion within a channel of the CP rather than progressive diffusion along the channels.12,13 Besides the CP materials, other bulk crystalline compounds that have been investigated as Mg-ion cathodes are spinel14 , post-spinel15

, anatase14

, olivine14

, and tavorite16

.

Most bulk crystalline compounds face common problems of slow Mg diffusion rates, limited Mg intercalation levels,17 and pulverization due to severe volumetric changes on insertion/desertion of ions. Nanostructured or mesoporous materials with greater surface-tovolume ratios have more open voids to improve ion intercalation and diffusion, and to accommodate volumetric changes. For example, Yang et al.18 found that breaking bulk

into -D layers or -D nanoribbons lowers the Mg diffusion barrier and increases

the Mg adsorption energies on their surfaces. Liu et al.19 synthesized cathodes that had better cycling performance than bulk

nanowire

. However, an attempt to

nanosize CP materials by mechanical milling met with limited success: Lancry et al.20 found that CP materials milled in an Ar atmosphere for

minutes are reduced in size and the

average Mg diffusion time is shortened; but trying to obtain smaller particle sizes through longer milling times only results in partial amorphization of the CP materials, and a degradation in the discharge capacity of the battery made with these milled materials.


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Since scientists had successfully synthesized 1-D molybdenum chalcogenide halide nanowires (NWs) with the same Mo octahedral constituents as the CP materials,21–26 we surmised that these NWs are the perfect nanosize equivalents of the bulk crystalline CP materials: They have much higher surface-to-volume ratio than the bulk materials; yet they possess sufficient length in one dimension for efficient electron transport.27 We hypothesized that they might make good Mg-ion battery cathodes too — but minus the cation-trapping disadvantage of the bulk CP materials. To the best of our knowledge, while previous studies had investigated the adsorption of transition metal atoms28 or Li atoms29 on these NWs, no study has been done for Mg atoms. In this paper, we studied the suitability of 1-D molybdenum chalcogenide halide NWs — referred to subsequently as Mo-octahedral NWs — to be used as Mg-ion battery cathode materials, using density functional theory calculations. We first determined the stability and most-stable geometry of Mo-octahedral NWs with different elemental compositions, then calculated the Mg adsorption energy on the NWs. For the most promising NWs, the Mg diffusion energy and specific capacities were calculated. Finally, we compared the performance of our Mo-octahedral NWs with other state-of-the-art cathode materials.

2. Nanowire structures & models There are two types21–26 of 1-D Mo-octahedral NWs that have been experimentally synthesized so far:

and

, where Ch = chalcogen: e.g., S and/or Se

and x is an integer. The basic building unit of both types of NWs (Figure 1a) consists of two planar triangular Mo-Ch/I units offset by 60° and vertically stacked together; with the central Mo atoms forming an octahedron.30 In the

NWs (Figure 1b), the basic units are

directly linked together in the -direction; whereas the basic units are bridged by Ch or I atoms in the

NWs (Figure 1c).


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Figure 1. (a) The basic building unit of the Mo-octahedral NWs. (b) The o h I NW is constructed by directly linking multiples of the basic unit shown in (a) in the -direction. (c) The o h I NW is constructed by bridging two basic units in (a) with Ch or I atoms. Ch refers to chalcogen. The dashed boxes in (b) and (c) delineate the periodic unit cell of the NWs.

3. Methodology We performed spin-polarized calculations using the density functional theory (DFT) code31 VASP with the Perdew-Burke-Ernzerhof generalized gradient approximation (GGA).32 The ion-electron interactions were described using projector-augmented waves (PAW).33 We modeled the 1-D NWs with a periodic cell where the NW axis is oriented along the -axis and a vacuum space of at least 12 Å is included along the - and -axes (Figure 1b,c). For the total energy of our NWs to converge to within

eV, a kinetic energy cutoff of 450 eV for

the PAW, and a k-point sampling using the Monkhorst-Pack scheme of for the

and

and

single unit cells containing 12 and 30

atoms, respectively, were used. The Methfessel-Paxton smearing method34 with smearing width of 0.1 eV was used. Geometry Optimizations The -lattice parameter for the NWs of different compositions were first optimized. We started by initializing the lattice parameters of all NWs to the previously-reported lattice parameters28,35 of

and and

, respectively. The atoms in the NWs were geometry-optimized until the absolute


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value of the forces acting on each atom is less than energy was recorded. Next, we changed

by

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eV/Å and the value of the total

% and rescaled the atomic positions

accordingly. The atomic positions were re-optimized and the total energy was again recorded. The optimized was the minimum point of the quadratic curve obtained by fitting the data points of total energies against different values. Mg adsorption energies We placed a single Mg atom on various possible adsorption sites on the

supercell for

larger supercell for

and a single unit cell for

; the

was to prevent the Mg atom from interacting with its

periodic image. The details on the adsorption sites that we investigated for the NWs are shown in the Supporting Information. The forces on the atoms were relaxed until the absolute value of the forces acting on each atom is less than

eV/Å.

Analysis To analyze the interactions between Mg and the NWs, projected density of states (PDOS) calculations were performed on the self-consistent charge densities from the DFT calculations, but we used a smaller smearing width of 0.01 eV and denser k-point grids of and

for

and

, respectively. To classify the

interactions between the Mg and the NW (e.g., bonding, non-bonding, or anti-bonding), we processed the self-consistent wavefunctions from the DFT calculations using the LOBSTER program,36 to obtain the Crystal Orbital Hamilton Populations (COHP). The Bader method37 was used to quantify the charge transfer from Mg to the NWs. The diffusion barriers for Mg to diffuse from one low-energy adsorption site to another on the NWs, were calculated using the Climbing Image Nudged Elastic Band (CI-NEB) method.38 First, linear interpolation was applied between the initial and final states of the Mg diffusion path to obtain 3 (5) intermediate states — or images — for

(

) NWs. Second, the

geometries of the images were optimized with a spring constant of

eV/Å2 applied

between adjacent images to prevent them from collapsing back to the initial- or final-state


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geometry. At the conclusion of the optimization, the image highest in energy was allowed to further maximize its energy along the diffusion pathway, so that it ended up on the saddle point. The energy difference between this image and the initial image was the diffusion barrier. The force convergence criterion for the optimization was set to

eV/Å.

4. Results & discussion 4.1 Lattice parameters & stability of Mo-octahedral NWs of different compositions We first determined the stability of NWs with different compositions by calculating their cohesive energy

, which for [

where [ o

h

is given by ]

[

]

(

I ] is the total energy of

o

) [ h

]

[]

I , and [ o]

(Eq ) [ h]

[I] are the

energies of the respective atoms in vacuum. Table 1 shows the compositions of the NWs that we studied, as well as their corresponding . All the NWs investigated in this paper are thermodynamically stable because their values are negative. Experimentally, the reaction conditions to synthesize NWs are similar. However, note that because stable, the synthesis reaction will not favor the production of

and NWs are more NWs unless the

stoichiometric ratios of the reacting elements are chosen carefully.39


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Table 1. The composition, lattice parameter, and cohesive energy and NWs that were studied in this work.

of the

4.2 Performance of Mo-octahedral NWs as Mg-ion battery cathode materials Mg adsorption energies Next, we investigated the adsorption energy of an Mg atom on the NWs, since the more exothermic the adsorption energy, the higher the attainable battery voltage. As expected, the lowest-energy site is the one where Mg interacts with the most number of atoms on the NW. The adsorption energy of Mg on the nanowire (abbreviated in the rest of the paper as

) is calculated as follows:

( [ where [ atom adsorbed on it, [ [

]

[

]

[

])

(

)

] is the energy of the geometry-optimized nanowire with the Mg ] is the energy of the pure nanowire without the Mg, and

] is the energy of an Mg atom in vacuum. The higher

is, the more exothermic is the

adsorption of Mg on the nanowire.


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Table 2 shows the

values for the

NWs at the lowest-energy adsorption

site (site 4). Although we tested four different sites, site 4 proved overwhelmingly the most stable: Mg atom located at site 3 ended up drifting far away from the NW, whereas Mg atoms on site 1 and 2 frequently converged to site 4 after the geometry optimization (please see Supporting Information for details on the sites that we tested). We see that replacing S with Se increases the

slightly, while substituting the chalcogen

with halogen atoms (e.g., substituting S with I) reduces the highest

in the

by almost 4 times.

has

family of NWs.

Table 2. Mg adsorption energy site.

on

NWs for the lowest-energy adsorption

Note that we had commenced our study with I as the substituent instead of F or Cl, because nanowires synthesized by experimentalists thus far had contained S and I only.21–26 When NWs with

demonstrated low

values, we calculated

for the Cl-

substituted NW as well to prove that halogen-substitution is detrimental to Supporting Information). To explain why

(see

NWs containing halogens have low

values, we calculated the projected density of states (PDOS) of Mg and its nearest-


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neighbor atoms for

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. The PDOS of the same atoms for the pure

were also calculated for comparison. Panels (i) in Figure 2 shows the PDOS of pure

,

and

. Clearly,

when the S and Se atoms are replaced by I atoms, there are fewer electronic states present close to the Fermi level. For

(Figure 2a ii), the majority of the Mg states is

located within a narrow energy range of

eV, and are charge-depleted since they are

located above the Fermi level. When the S atoms are substituted with Se atoms, the PDOS for (Figure 2b ii) show that the Mg states are pushed even higher above the Fermi level (

eV) as compared to

. Conversely, the majority of the Mg states

as they are located below the Fermi level from -

are filled for

-

eV

(Figure 2c ii). From our Crystal Orbital Hamilton Population (COHP) analysis of , and

,

as shown in panels (iii) in Figure 2, respectively, Mg

interacts with the NWs mainly through anti-bonding interactions. While are able to deplete these anti-bonding orbitals of charges, why the bonding between Mg and

is unstable and

and

is not, and this explains is low. This is corroborated

by our Bader analysis showing that the Bader charge of Mg when it is adsorbed on and

is only

charge of

and

, respectively (a drop from the initial valence

); while Mg adsorbed on

has a much higher Bader charge of

.


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Figure 2. Panels (i): Projected density of states (PDOS) of (a) pure , (b) and (c) . Panels (ii): PDOS of (a) + Mg, (b) + Mg and (c) + Mg. Panels (iii): Crystal Orbital Hamilton Population (COHP) of (a) + Mg, (b) + Mg and (c) + Mg. For simplicity, we only show interactions between orbitals with the greatest COHP values. [Following convention, we multiplied the COHP values by - so that bonding (anti-bonding) interactions are positive (negative)]. For all graphs: the Fermi level has been offset to eV and only one spin channel is shown for simplicity. ( ) means that the data has been scaled up by times.

Next, we look at the lower symmetry than the

of

NWs. Since the NWs, we report the lowest

NWs have a found for four general


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locations on

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NWs: around the bridging atoms of the nanowire (Bridge 1 and

Bridge 2), and around the Mo octahedra (Mo ohd 1 and Mo ohd 2) (Figure 3 inset). Among the

NWs,

has either the highest or one of the highest

four locations (Figure 3). The

values of the

NWs are also higher than

NWs of the same composition; for example, up to 2.5 times higher than

for all

has

values that are

. Substituting chalcogen with halogen atoms on the

nanowire does not decrease the

to the extent seen in

.

Figure 3. Mg adsorption energy on NWs for the lowest-energy sites on four different general locations of the NWs. (Inset) Di gr m showing the ―Bridge 1‖ ―Bridge 2‖ ― o ohd 1‖ nd ― o ohd 2‖ loc tions on the nanowire.

Figure 4a,b shows the PDOS for

and

before [panels (i)] and after

[panels (ii)] Mg adsorption. For both NWs, the Mg states are well-distributed within the energy range shown, suggesting good hybridization between Mg and the NWs. This contrasts greatly with the PDOS of the analysis of

NWs shown in Figure 2. Furthermore, the COHP and

shown in panels (iii) in Figure 4 shows

that Mg interacts with the NWs mainly through bonding interactions. The stable, bonding


12

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interactions (in contrast to the unstable, anti-bonding interactions seen in the NWs) explain why

is higher for the

charge for Mg adsorbed on

than

and

— slightly lower than for Mg adsorbed on

NWs. The Bader

is and

and

, respectively

.

Figure 4. Panels (i): Projected density of states (PDOS) of (a) pure and (b) . Panels (ii): PDOS of (a) + Mg and (b) + Mg. Panels (iii): Crystal Orbital Hamilton Population (COHP) of (a) + Mg and (b) + Mg. For simplicity, we only show interactions between orbitals with the greatest COHP values. [Following convention, we multiplied the COHP values by - so that bonding (antibonding) interactions are positive (negative)]. ( ) and ( ) means that the data has been scaled up by and times, respectively.


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Mg diffusion barriers Since good cathode materials should preferably have low Mg diffusion barriers to facilitate fast battery charging/discharging, we investigated the Mg diffusion barrier on For

and

.

, we calculated the diffusion barrier for the Mg atom to move from one lowest-

energy adsorption site to another (see insets of Figure 5). The diffusion barrier is the case of the

eV. In

nanowire, we investigated the shorter diffusion pathway involving

Mg moving from the lowest-energy adsorption site to the adsorption site that is secondlowest in energy (see insets of Figure 6). The diffusion barrier is

eV.

Figure 5. Energy barrier that an Mg atom has to overcome to diffuse from one lowest-energy adsorption site [initial state (1)] to another [final state (5)] on the nanowire.


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Figure 6. (a) Energy barrier that an Mg atom has to overcome to diffuse from one lowestenergy adsorption site to another, on . (b) Configurations (upper: side view; lower: top view) of the initial (1), intermediate (3 and 5), and final (7) states of Mg diffusion on .

Specific capacities of the NWs Finally, we increased the number of Mg atoms adsorbed on and

to find the specific capacities of these NWs. The initial adsorption of

a single Mg atom on the pure NWs is when deposited on the NWs, consider how

is at the highest; as more Mg atoms are

for each additional Mg atom should drop. We find it useful to

per Mg atom changes as the number of Mg atoms is increased from

to

on the NWs, [

where [

]

( )

atoms adsorbed on it, and [

[

]

(

) [

]

(

)

] is the energy of the geometry-optimized NW with ( ) Mg ] is the energy of the Mg atom in vacuum.

indication of how the driving power of the

and

gives an

cathode would change as

they are discharged. For

, we increased the Mg coverage on

until all the possible lowest-energy

adsorption sites were occupied. The final product is

. For

, we found

that it is energetically more favorable for Mg atoms to first occupy sites on Bridge 1 and Bridge 2 (see inset of Figure 3 or Figure S3 in Supporting Information), followed by Mo ohd


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Page 16 of 25

1 and Mo ohd 2. In fact, some of the Mg atoms that were initially placed around Mo ohd 1 and Mo ohd 2 moved to Bridge 1 and Bridge 2 after the optimization. The final product is ; we stopped adding Mg atoms after the 14th Mg atom, since

became

negative on adding the 15th Mg atom. The specific capacity

of

(or

) for different

is calculated by

the F r d y’s equ tion ( where

is the valency of Mg, (or

Figure 7. The variation of and and .

Figure 7 shows how

)

(

is F r d y’s const nt

) nd

is the molecular weight of

).

(see ) versus specific capacity for . (Insets) Geometry-optimized structures of

changes with the specific capacity of the material. Realistically,

the full specific capacity of the NWs will not be realized because

becomes too small to

do useful work. More realistic estimates of the specific capacities is where where

mAh/g and

mAh/g for

and

eV,

, respectively.


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4.3 Comparison of the performance of Mo-octahedral NWs with other state-of-the-art cathode materials In Table 3, we compared the performance of the Mo-octahedral NWs with other state-ofthe-art Mg-ion cathode materials.

Crystalline materials

Species

Voltage (V)

Chevrel Phase Mo6S8

0.78, 1.22, 1.28 1.0-1.2

Nanostructured materials

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Spinel Ni2O4

3.52

Post-spinel Mn2O4

2.7

Anatase TiO2

Spec. cap. (mAhg-1)

Diffusion Ref. barrier (eV) 8 10

122

*

0.626

14

0.40

15

0.67

0.547

14

Olivine FePO4

2.66

0.608

14

Tavorite VPO4F

2.6

156

0.7

16

Nanowire Mo6Se6

1.0 (max)

60

0.29

Nanowire Mo12Se18

1.83 (max)

220

1.21

270.1

This paper This paper

Layered CoO2

3.18

0.647

14

Layered V18O44

2.37

0.156

14

0.48

18

Nanoribbon MoS2 Nanowire WSe2

223.2 203

19

*

Layered MnO2

109

40

*

Tunnel MnO2

310

40

*

1.6

Table 3. Performance of some current state-of-the-art Mg cathode materials. References marked with asterisks are experimental work.

Note that since the open-circuit voltage of a battery is given by ( where

)

( ) is the chemical potential of the working ion in the cathode (anode), and

valency of the working ion. Depending on the anode used, the maximum possible V and

V for

and

is the will be

, respectively.


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Between

and

, as both

, and

Page 18 of 25

is a closer 1-D structural analog to bulk CP do not have atoms bridging the Mo-octahedra. Thus, we

can compare the performance of

and bulk CP

to see the effect of nanosizing

CP materials. We see that the theoretical voltage and specific capacity of than bulk CP

; this is likely due to the channels in bulk CP

are lower offering 3-D

interactions with Mg,12,13 as compared to the fewer interactions offered by the open surface of the

NWs. However, the sacrifice in theoretical voltage and specific capacity is

compensated for by a highly favorable Mg diffusion energy that facilitates the charging and discharging process of the battery, and is competitive with the other materials listed in Table 3. , with bridging atoms between the Mo-octahedra, has theoretical voltage and specific capacity that are almost double that of bulk CP

. Unfortunately, its Mg

diffusion barrier is too high for it to be feasibly used as a battery cathode material.

5. Conclusion Using density functional theory calculations, we investigated two types of molybdenum chalcogenide halide NWs (or Mo-octahedral NWs) with general formulas

and

, where Ch = S or Se, as cathode materials for Mg-ion batteries. We showed )

that

NWs consistently have more exothermic Mg adsorption energies (

than

NWs because of the bonding interactions with Mg. In contrast, NWs have anti-bonding interactions with Mg, and an exothermic

depends

on whether the NW can push these anti-bonding orbitals above the Fermi level — which does not happen for highest

. The pure selenium-based NWs (i.e.,

and

) have the

in each category, and we compared their Mg diffusion barriers and specific

capacities with other potential cathode materials. Our comparisons show that the theoretical


18

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voltages and specific capacities for

and

are modest. However, while

has a high Mg diffusion barrier that makes it unfeasible to be used as a battery cathode material, the low Mg diffusion barrier of

should ensure facile

charging/discharging of the battery, and could potentially be used as a cathode material for Mg-ion batteries.

ASSOCIATED CONTENT Supporting Information. The optimized structures of all the o

h

I

o

h

I

and

nanowires; the details of the Mg adsorption sites on these nanowires; Mg

adsorption energy on

. This material is available free of charge via the Internet at

http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT We acknowledge the A*STAR Computational Resource Centre (A*CRC) of Singapore through the use of its high performance computing facilities.


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