Ultrathin Moo2 Nanosheets Encapsulated in Carbon Matrix for Superior Lithium Storage

Supporting Information

Ultrathin MoO2 Nanosheets Encapsulated in Carbon Matrix for Superior Lithium Storage

Jiangfeng Nia, Yang Zhaoa, Liang Lia, *, Liqiang Maib, * a College of Physics, Optoelectronics and Energy & Collaborative Innovation Center of Suzhou Nano Science and Technology, Soochow University, Suzhou 215006, China b State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, WUT-Harvard Joint Nano Key Laboratory, Wuhan University of Technology, Wuhan 430070, China

* Corresponding authors. Tel: +86-87467595, Fax: +86-27-87644867 E-mail addresses: [email protected] (L. Li), [email protected] (L. Mai)

1 Table S1 Li storage capability of the MoO2/C nanosheets in comparison with other MoO2 nanomaterials.

a MoO2 Sample Retention / Cycle Rate capability Reference −1 −1 −1 Mesporous MoO2 670 mAh g / 30 cycles 450 mAh g at 0.08 A g [11] −1 b −1 MoO2/C nanosphere 570 mAh g / 30 cycles 410 mAh g at 3 C [12]

−1 −1 −1 MoO2/C nanowire 467 mAh g / 30 cycles 356 mAh g at 1.0 A g - [7]

−1 −1 −1 MoO2/C nanocrystal 554 mAh g / 70 cycles 575 mAh g at 0.4 A g [5]

−1 −1 −1 MoO2/rGO 598 mAh g / 70 cycles 408 mAh g at 2 A g [14]

−1 −1 −1 MoO2 nanorod 830 mAh g / 30 cycles 80 mAh g at 0.8A g [9]

−1 −1 −1 MoO2/rGO 640 mAh g / 50 cycles 380mAh g at 2 A g [18]

−1 −1 −1 MoO2/C nanobelt 617 mAh g / 30 cycles 434 mAh g at 1 A g [10]

−1 −1 −1 MoO2/OMC 668 mAh g / 50 cycles 401 mAh g at 2 A g [13]

−1 c −1 −1 Amorphous MoO2 800 mAh g / 50 cycles 705 mAh g at 5 A g [20]

d −1 −1 −1 MoO3/C 521 mAh g / 300 cycles 483 mAh g at 3 A g [35]

−1 −1 −1 MoS2/CNT 698 mAh g / 60 cycles 369 mAh g at 1 A g [36] FeS/C nanosheet 615 mAh g−1 / 100 cycles 349 mAh g−1 at 3 A g−1 [37]

−1 −1 −1 MoO2/C nanosheet 1051 mAh g / 100 cycles 719 mAh g at 5 A g This work a The current rate for this work is 0.5 A g−1, while it is no greater than 0.5 A g−1 for others; b The exact value for C is unknown; c The lithiation (discharge) is restricted to a much lower rate of 0.1 A g−1. d The capacity was calculated based on the total mass of MoO3 and C as it does in our work.

Figure S1 TEM images of hydrothermally prepared MoO3 samples for various reaction times

(a) 3 h, (b) 6 h, (c) 12 h, and (d) 24 h. The images show that MoO3 nanosheets are formed and developed when the reaction time extends to 12 h and beyond.

2 Figure S2 TEM image of MoO2/C showing sheet-like morphology.

Mo Cu: sample holder y t i s n

0 2 4 6 8 10 Energy / keV

Figure S3 EDS of MoO2/C product. The diagram confirms that the product is composed of elements Mo, O, and C.

120 150

) 100 100 V %  (

339 C

T 90 50 T D 80

70 0 100 200 300 400 500 600 Temperature / C

Figure S4 TG-DTA curves of MoO2/C nanosheets in flowing air. The mass gain before 250

°C can be ascribed to MoO2 oxidation to MoO3, while the mass loss between 300 to 400 °C is due to burning out of amorphous carbon.

(a) (b) 0.4 3.0 1 )  i 1st g L

/ 2.5 2nd 0.2 + i A 3rd

2.0 y . t i 0.0 s s v

1 V e

Current rate 100 mA g / d

l t 1.0 a n i t

e 1st n r -0.4 r

2nd e 0.5 t u

3rd o C -0.6 P 0.0 0.0 1.0 2.0 3.0 0 200 400 600 800 1000 1200 1400 +  Potential / V vs. (Li /Li) Specific capacity / mAh g

Figure S5 (a) Cyclic voltammograms and (b) charge and discharge profiles of MoO2 nanosheet electrodes during initial cycles. The drastic variation in the CV and discharge curves during initial cycles suggests a poor reversibility for the pure MoO2 nanosheets. The initial discharge and charge capacity are 1387.5 and 972.4 mAh g −1, resulting in a CE of ~70% compared to 78% of carbon encapsulated counterpart.

60 Before cycle After 100 cycles 45 

Z  15

0 0 15 30 45 60 Z / real 

Figure S6 Impedance spectra of MoO2/C nanosheet electrodes before cycle and after 100 cycles at 0.5 A g−1. The spectra were collected at charged state in the frequency range of 0.1– 100k Hz using an AC signal of 5 mV. No apparent change is observed in the semicircles at high and middle frequencies, suggesting that neither Li diffusion across passivation film nor subsequent charge transfer process is interfered. Meanwhile, the reduced length of spike line in the low frequency indicates improved diffusion capability of Li within the electrode.

−1 Figure S7 Post TEM image of MoO2/C nanosheet after 100 cycles at 0.5 A g . Surface amorphous layer out of the crystalline core is clearly visible. The interplanar spacing of 0.24 nm corresponds to (200) plane of monoclinic MoO2 crystal.

3.0 )  i 2.5 L / i L (

. 1

s 0.25 A g

v 1 1.5 0.5 A g

V 1 1 A g /

l 1.0 5 A g1 a i 1 t 10 A g n 0.5 e t o

P 0.0 0 200 400 600 800 1000 1 Specific capacity / mAh g

Figure S8 Charge and discharge curves of MoO2/C nanosheet electrodes at varying current rates. The profiles are primarily preserved when the current rate increases from 0.25 to 10 A g−1.

3.0 )  i 2.5 L / i L (

2.0 . 0.25 A g1 s

v 1 1.5 0.5 A g

V 1

1 A g /

1 l 1.0 5 A g a

i 1 t 10 A g n 0.5 e t o

P 0.0 0 200 400 600 800 1000 1 Specific capacity / mAh g

Figure S9 Charge and discharge curves of MoO3 nanosheet electrodes at varying current rates.

6 (a) 3.0 (b) -80 )  i

L 2.5 / e i

e -60 L r (

2.0 g . e

s 45 d v

The electrode was held / -40

at 1.2 V for 6h for EIS test e /

a 1.0 i h t -20 n P

e 0.5 t o

P 0.0 0 0 200 400 600 800 1000 1200 10-3 10-2 10-1 100 101 102 103 104 105 Specific capacity / mAh g1 Frequency / Hz

Figure S10 (a) Discahrge curve and (b) EIS spectrum of the MoO2/C nanosheet electrode. The electrode was discharged to 1.2 V and then held at this potential for 6 h before impedance test. The phase >45 ° at low frequency region indicates a capacitive behavior of the MoO2/C electrode.