Oxygen vacancies enhance pseudocapacitive - · PDF fileIn the forat provided y the authors and unedited. 1 Supplementary Information Oxygen Vacancies Enhance Pseudocapacitive Charge

In the format provided by the authors and unedited.

1

Supplementary Information

Oxygen Vacancies Enhance Pseudocapacitive Charge Storage Properties of MoO3-x

Hyung-Seok Kim,1 John B. Cook,2,3 Hao Lin,1 Jesse S. Ko,1

Sarah H. Tolbert,1,2,3* Vidvuds Ozolins,1* and Bruce Dunn,1,3,*

1Department of Materials Science and Engineering, UCLA, Los Angeles, California 90095-1595,

United States

2Department of Chemistry and Biochemistry, UCLA, Los Angeles, California 90095-1569,

United States

3The California NanoSystems Institute, UCLA, Los Angeles, California 90095

*to whom correspondence should be addressed: [email protected]; [email protected];

[email protected]

Oxygen vacancies enhance pseudocapacitive charge storage properties of MoO3-x

© 2016 Macmillan Publishers Limited, part of Springer Nature. All rights reserved.

SUPPLEMENTARY INFORMATIONDOI: 10.1038/NMAT4810

NATURE MATERIALS | www.nature.com/naturematerials 1

http://dx.doi.org/10.1038/nmat4810

2

Figure S1. TEM images of (a) reduced MoO3-x, and (b) fully oxidized MoO3. (Inset of (a) shows

the blue color of as-prepared reduced MoO3-x)





3

Figure S2. AFM analysis of reduced MoO3-x. The images (top) and height profile (bottom) show

that the thickness of an individual nanobelts is about 15 nm.





4

Figure S3. TGA analysis of reduced MoO3-x in air and argon atmosphere. (Flow of 100 ml/min

and a ramping rate of 10 °C/min). The concentration of oxygen vacancies were calculated from

the difference in weight decrease between the two TGA traces.

50 100 150 200 250 300 350 40096.5

97.0

97.5

98.0

98.5

99.0

99.5

100.0

Wei

ght (

%)

Temperature (C)

400 C in Argon400 C in Air

1.36%





5

Table S1. Surface area measurement on reduced MoO3-x and fully oxidized MoO3.

Reduced MoO3-x Fully oxidized MoO3

BET surface area (m2g-1) 24 15





6

Discussions of HSE Vs. GGA+U in MoO3

It is widely known that local and semi-local xc functionals (such as LDA and GGA) fail

in systems with localized electrons electrons, such as transition metal compounds, due to

incomplete of cancellation of electron self-interaction effects1. The “+U” method is the simplest

approach to account for on-site Coulomb interactions in a mean-field manner; GGA+U has been

shown to calculate the redox potentials accurately2. HSE06 is arguably more sophisticated

because it is parameter-free and calculates the corrections self-consistently. Nevertheless, it still

a rather ad hoc mixture of DFT and Hartree-Fock with screening and has had mixed success in

transition metal compounds. For instance, Ong et al. showed that HSE is important when the

polaron lives in hybridized transition metal d - Oxygen p orbitals, in which case GGA+U cannot

localize the polaron because the +U is applied to the d orbital only3. However, in the same paper

it is shown that HSE predicts the wrong phase diagram for olivines, while GGA+U is better.

In MoO3, the polaron lives almost entirely in the d orbital. In our previous work4, we

calculated polaron energies and migration barriers in stoichiometric MoO3 using GGA+U and

HSE; the predictions are comparable and found to be in good agreement with experiment. We

expect that the same conclusion will hold for reduced MoO3-x because the localization of the

polaron is still confined to the Mo d orbitals. Therefore, we applied GGA+U method in the

research of this paper.





7

Discussions of polaron configurations, polaron binding energies and variance of bond

lengths in reduced MoO3-x.

Oxygen vacancy was modeled by removing one oxygen atom in a 3×1×3 supercell

containing 36 formula units of stoichiometric MoO3. The preference site of oxygen vacancy in

our results is consistent with previous GGA+U calculations, but the most stable polaron state is

different. Coquet et al.5 found that the structure with a terminating oxygen vacancy and with the

reduced MoO3-x center in the Mo4+ state is the most stable structure, which is the structure with

the second lowest formation energy in our calculation, while in another study6 the polaron state

was not mentioned. We argue that our results should be more reliable because at least two

polaron configurations were considered in each type of oxygen vacancy structure and previous

calculations only showed one polaron configuration. Moreover, our prediction for polaron states

is in good agreement with our experimental observation (Observation of Mo5+ states) and our

prediction for the polaron orbital (dxz) in the six-fold coordinated Mo6+ion is consistent with our

previous study of fully oxidizedα-MoO37. Since GGA methods cannot describe the localized d

electrons in the transition metals correctly, it is not surprising that the preferred site of oxygen

vacancy by GGA methods is the Os site, which contradicts the results from GGA+U methods5.

Furthermore, binding energies of the polarons were investigated by comparing the total

energy difference of structures with various polaron configurations. If the bipolaron structure

with an Ot vacancy is treated as a reference structure (see the polaron orbitals in Fig. 2c and the

structure a in Fig. S4), the energies of the structures with a Mo4+ cation at the defect center

(structure b in Fig. S4) and with two adjacent Mo5+ cations within the same bilayer are about

0.20 eV higher (structure c in Fig. S4). However, the separation of the bipolaron requires at least

0.37 eV. As shown in Structure d to f in Fig. S4, isolation of two polarons within the same





8

bilayer needs 0.37 eV, while separation of two polarons into two different bilayers demands 0.47

eV. Bipolaron structures are favored in reduced MoO3-x, because adjacent two polarons can

minimize the total energy by relaxing the lattice distortion locally and this was confirmed by the

analyzing the variance of bond lengths. Compared to the same kind of Mo-O bond lengths in

stoichiometric MoO3, only 5 Mo-O bonds lengths in the reference structure are changed about

0.1 Å, while in structure 3 to 6, at least 8 Mo-O bond lengths are changed approximately 0.1 Å

(See Fig. S5). As for structure c, although the distance of the two polarons is close, the two

MoO6 octahedra containing the two polarons are edge sharing in the different layers, which is

believed to be less flexible than the two polarons in the two adjacent corner-sharing octahedra in

reference structure a. We point out that in the Mo4+ structure, only one Mo-O bond length is

changed more than 0.1 Å, but the repulsion between the two polarons in the same Mo cation

center might lead to the increase of energy.





9

Figure S4. Binding energy of polarons with different polarons configuration (a-f).The

configuration (a) used as the reference. Mo4+, Mo5+, Mo6+ ions are highlighted in green, blue and

white, respectively.





10

Figure S5. The difference of corresponding bond lengths between the reduced MoO3-x structures

(a to f in Figure S5) and the stoichiometric MoO3. (Unit: Å)





11





12

Figure S6. Long-term cycling of R-MoO3-x. Long-term cycling measurements were performed

using a three-electrode configuration. The working electrode was prepared as a nanoparticle thin-

film containing roughly 40 μg of active material (R-MoO3-x) with a coverage of 1 cm2 onto an O2

plasma treated stainless steel current collector. The counter and reference electrodes were lithium

metal foil and the electrolyte was 1M LiClO4 in propylene carbonate. The R-MoO3-x electrodes

contained no carbon or binder additives so that the fundamental long-term cycling behavior of

the material was evaluated. The electrodes were cycled between cut-off voltages of 2 – 2.8 V vs.

Li/Li+ using galvanostatic charging-discharging (at 30C) and cyclic voltammetry (at 10 mVs-1)

for a total of 10000 cycles. The reduced MoO3-x exhibited excellent cyclability. At 30C,

galvanostatic (a, b) charge-discharge curves indicate 70% capacity retention, decreasing to a very

respectable value of 90 mAh g-1 at 10,000 cycles. Cyclic voltammetry (CV) experiments (c, d)

indicate capacity retention of 76% over 10,000 cycles. These scans also show that the capacity

loss with cycling is associated with a decrease in the diffusion limited current peaks in the CV

(compare Fig. S6c with Fig. 3e). This suggests that the capacitor-like currents in the CV cycle

well while the diffusion-controlled currents cannot ‘keep up’ with high rate cycling (30C or

10mVs-1). For both the potentiostatic and galvanostatic measurements, the coulombic efficiency

was over 99%.





13

Electrochemical analysis of thin film

MoO3 electrodes. To complement the

results shown in Figure 3b, which

considers the sweep rate dependence

on capacity, we have plotted here the

specific capacitance (Fg-1) as a

function of sweep rate. The outcome is

the same as we observe the R-MoO3-x

(red) having a much greater specific capacitance than that of F-MoO3 (black)

0 2000 4000 6000 8000 100000

100

200

300

400

500

600

700

800

Galvanostatic Cycling Charge Discharge

Cycle Number (n)

Cap

acity

(C/g

)

0

20

40

60

80

100C

oulombic Efficiency (%

)

b

0 2000 4000 6000 8000 100000

100

200

300

400

500

600

700

Charge Discharge

Cycle Number (n)

Cap

acity

(C/g

)

Cyclic Voltammetry0

20

40

60

80

100

Coulom

bic Efficiency (%)

d

0 20 40 60 80 100 120 140

2.0

2.2

2.4

2.6

2.8 a

Cycle 10 Cycle 100 Cycle 1000 Cycle 10000

Galvanostatic Cycling30C; 2 - 2.8 V vs. Li/Li+

Volta

ge (V

vs.

Li/L

i+ )

Capacity (mAh/g)

1.8 2.0 2.2 2.4 2.6 2.8 3.0

-0.3

-0.2

-0.1

0.0

0.1

0.2

0.3

0.4

Cyclic Voltammetry10 mV/s; 2 - 2.8 V vs. Li/Li+

Cycle 10 Cycle 100 Cycle 1000 Cycle 10000

Cur

rent

(mA)

Voltage (V vs. Li/Li+)

c Cycle 100.22 V

Cycle 100000.03 V

0.05 VCycle 100

0 20 40 60 80 1000

100

200

300

400

500

Sweep rate (mVs-1)

Cap

acita

nce

(Fg-1

)

Reduced MoO3-x

Fully Oxidized MoO3





14

Figure S7. Kinetic analysis of reduced MoO3-x and fully oxidized MoO3 using the power law

relationship. The b-values of reduced

MoO3-x (red) and fully oxidized MoO3

(black) are calculated from the slope of

the log (i) vs. log () plot using the

anodic current response at ~ 2.5 V (vs.

Li/Li+).

-1.0 -0.5 0.0 0.5 1.0 1.5 2.0-2.5

-2.0

-1.5

-1.0

-0.5

0.0

0.5

log (sweep rate) (mVs-1

)lo

g (p

eak

curr

ent)

(mA

) Anodic peak

b = 0.67

b = 0.85

Reduced MoO3-x

Fully oxidized MoO3





15

Figure S8.The intercalation energies of all symmetry-distinct lithium insertion sites were

calculated using repeated random initialization of the polaron states. (a) Calculated lithium

intercalation voltages at interlayer sites (cross) and intralyer sites (triangle) of reduced MoO3-x.

The polarons introduced by an oxygen vacancy are shown in blue. A, B, C, D sites indicate the

positions of an additional polaron introduced by lithium intercalation. For example, the A site

means that the additional polaron is located at the A planes shown in (b).

(a)

(b)





16

Discussions regarding polaron configurations after lithium intercalation.

As shown in Fig. S8, our results indicate that the additional polaron introduced by lithium

intercalation does not favor the B plane, where the oxygen defect induced polarons are located.

In other words polarons are unlikely to remain together as the polaron concentration increases

from 0.22 to 0.33 within the same plane. The highest intercalation energy corresponds to the

structure in which the lithium ion at the interlayer site is far from the oxygen vacancy center (6.3

Å) and the additional polaron introduced along with the lithium ion is not in the same layer as

those polarons introduced by an oxygen vacancy (see Fig. S9).

An explanation regarding the additional peak during the cathodic sweep

The calculated voltage difference between reduced MoO3-x and fully oxidized MoO3

explain the new peak at 3.0 V in the CV of reduced MoO3-x (see Fig. 3e). Compared to lithium

intercalation in fully oxidized MoO3, oxygen vacancies in the reduced MoO3-x might release the

structural strain energy during the lithium intercalation and lead to higher voltages. Therefore,

the new peak is only observed in the reduced MoO3-x. In the discharge process, this new peak is

less distinctive than that in the charge process, because lithium ions might not migrate directly

from the electrolyte to the high voltage sites in reduced MoO3-x.Instead, the weak signal of the

new peak in the discharge process is contributed by the diffusion of lithium from the electrolyte

to the high voltage sites existing near the interface between electrode and electrolyte.





17

Figure S9. The lithium intercalation sites for the highest lithium intercalation voltage.





18

Table S2. Powder XRD derived d-spacing values for the (020), (040), and (060) reflections. A

silicon internal standard was used to calibrate the position of the peaks.

Table S3. Lattice parameters of reduced MoO3-x and fully oxidized MoO3 calculated by DFT

when one oxygen vacancy is introduced.

The deviation between the “b” lattice parameter used in the DFT calculations from the measured

value for the fully reduced sample (b=0.13Å out of b=14Å, or less than 1%) is small and will

not affect the calculated intercalation energies significantly. At any rate, this difference is smaller

than the uncertainty in the calculated van der Waals gap for stoichiometric MoO3 using different

exchange-correlation functionals8.

Sample (020) d (Å)

(040) d (Å)

(060) d (Å)

Reduced MoO3-x 7.031 3.505 2.335

Fully Oxidized MoO3 6.943 3.462 2.307

a(Å) b(Å) c(Å)

Fully oxidized MoO3 11.686 13.904 11.357

ReducedMoO2.972 11.650 13.930 11.393





19

Electrical conductivity measurements of reduced MoO3-x and fully oxidized MoO3.

Improved electrical conductivity in the reduced MoO3-x also contributes to the fast kinetic

of reduced MoO3-x. The bulk electrical conductivity measurement is illustrated in Fig. S9.From

this setup the resistivity was measured and the electrical conductivity of each sample was

calculated from following equation:

� � ��(1)

‘k’ is electrical conductivity of material, ‘t’ is sample thickness, ‘A’ is contact area, and

‘R’ is resistivity of sample. Table S4 shows that electrical conductivity of reduced MoO3-x was

an order of magnitude higher than fully oxidized MoO3. These measured values are in reasonable

agreement with the reported electrical conductivity of a single MoO3 nanobelt (10-4 S/cm for

non-lithiated MoO3 nanobelt9). The conductivity of our sample was measured as a thick film with

thickness of 10 μm and a 1 cm2 area, so these values represent the upper bound of the intrinsic

conductivity due to contact resistance.





20

Figure S10. Experimental setup for bulk electrical conductivity measurement of reduced MoO3-

x, partly reduced, and fully oxidized MoO3.

Table S4. Calculated bulk electrical conductivity using equation (S1) in each sample.

Samples Electrical conductivity

Reduced MoO3-x 10-4 S/cm

Fully oxidized MoO3 10-5 S/cm

MoO3 film by drop casting (thickness: ~10 μm)

Silver paste (contact area: 0.5 cm2)

ITO coated glass

Etched area





21

Figure S11. Ex-situ XPS spectra at specific potentials after electrochemical cycling (at 5 mVs-1).

Cyclic voltammetry in (a) reduced MoO3-xand (b) fully oxidized MoO3and high resolution XPS

spectra of Mo 3d region at specific potentials (i – v).

1.5 2.0 2.5 3.0 3.5

-6-4-202468

105 mVs-1Reduced MoO

3-x

Cur

rent

(mA

)

Potential (V vs Li/Li+)

(iv) 2.8 V

(v) 3.5 V

(iii) 1.5 V

(ii) 2.5 V

(i) 3.5 V

238 236 234 232 230 228

5+

6+

5+

6+

Binding energy (eV) In

tens

ity (a

.u.)

238 236 234 232 230 228Binding energy (eV)

4+4+

5+5+

6+

6+

238 236 234 232 230 228

4+4+

Binding energy (eV)

5+

6+5+

6+


4+4+

5+

6+

5+6+


5+

6+

5+6+

(i) 3.5 V (ii) 2.5 V (iii) 1.5 V

(iv) 2.8 V (v) 3.5 V

(a)

238 236 234 232 230 228 Binding energy (eV)

In

tens

ity (a

.u.) 4+4+

5+

6+5+

6+

(iii) 1.5 V

1.5 2.0 2.5 3.0 3.5

-4

-2

0

2

4

6

Potential (V vs Li/Li+)

Cur

rent

(mA

)

Fully oxidized MoO

3

5 mVs-1

(v) 3.5 V(iv) 2.8 V

(iii) 1.5 V(ii) 2.5 V

(i) 3.5 V


Inte

nsity

(a.u

.)

4+4+

5+

6+

5+

6+


4+4+

5+

6+

5+

6+


4+4+

5+

6+

5+

6+


4+4+

5+

6+

5+

6+

(i) 3.5 V (ii) 2.5 V (iii) 1.5 V

(iv) 2.8 V (v) 3.5 V

(b)

(iii) 1.5 V


4+4+ 5+

6+

5+6+

238 236 234 232 230 228

Inte

nsity

(a.u

.)

Binding energy (eV)

4+4+ 5+

6+

5+6+





22

References

1. Cheng, J., Sulpizi, M., VandeVondele, J. & Sprik, M. Hole localization and

thermochemistry of oxidative dehydrogenation of aqueous rutile TiO2(110).

Chemcatchem 4, 636-640 (2012).

2. Zhou, F. et al. First-principles prediction of redox potentials in transition-metal

compounds with LDA + U. Phys. Rev. B 70, (2004).

3. Ong, S. P., Chevrier, V. L. & Ceder, G. Comparison of small polaron migration and

phase separation in olivine LiMnPO4 and LiFePO4 using hybrid density functional

theory. Phys. Rev. B 83, (2011).

4. Ding, H. et al. Computational investigation of electron small polarons in alpha-MoO3. J.

Phys. Chem. C 118, 15565-15572 (2014).

5. Coquet, R. & Willock, D. J. The (010) surface of α-MoO3, a DFT + U study. Phys. Chem.

Chem. Phys. 7, 3819-3828 (2005).

6. Lei, Y.-H. & Chen, Z.-X. DFT+U study of properties of MoO3 and hydrogen adsorption

on MoO3(010). J. Phys. Chem. C 116, 25757-25764 (2012).

7. Ding, H. et al. Computational investigation of electron small polarons in alpha-MoO3. J.

Phys. Chem. C 118, 15565-15572 (2014).

8. Ding, H., Ray, K. G., Ozolins, V. & Asta, M. Structural and vibrational properties of

alpha-MoO3 from van der Waals corrected density functional theory calculations. Phys.

Rev. B 85, (2012).

9. Mai, L. et al. Lithiated MoO3 nanobelts with greatly improved performance for lithium

batteries. Adv. Mater. 19, 3712-3716 (2007).





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Oxygen vacancies enhance pseudocapacitive - · PDF fileIn the forat provided y the authors and unedited. 1 Supplementary Information Oxygen Vacancies Enhance Pseudocapacitive Charge