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Supporting Information

Exploration of the Sodium Ion Ordered Transfer Mechanism in MoSe2@Graphene Composite for Superior Rate and Lifespan Performance

Hai-Ning Fana, Qi Zhanga, Qin-Feng Gub*, Yang Lia, Wen-Bin Luoa*, Hua-Kun Liua

aInstitute for Superconducting and Electronic Materials, University of Wollongong, Wollongong, NSW 2522 Australia

bAustralian Synchrotron (ANSTO), 800 Blackburn Rd, Clayton, Victoria 3168, Australia.

E-mail: qinfeng@ansto.gov.au; luow@uow.edu.au

Keywords: sodium-ion batteries, graphene, MoSe2, diffusion pathway, two-dimensional material.

Experimental section

Preparation of Graphite oxide: All chemical reagents included in this research work were

purchased from Sigma-Aldrich Company and directly used without any further purification.

Graphite oxide was synthesized by the typical Hummers' process. 24 Briefly, in a 0 °C ice bath,

graphite flakes (2 g) and NaNO3 (1 g) were thoroughly mixed with 60 mL concentrated H2SO4

in a 250 mL flask and then stirred for 30 min, after which 7 g solid KMnO4 was slowly added

into above suspension. In order to proceed with the oxidation process, 25 mL of 30 % H2O2

was added dropwise until gas evolution ceased. The as-obtained graphite oxide was thoroughly

washed with deionized (DI) water and 10 % dilute HCl, and finally, collected by drying in a 50

°C vacuum oven.

Synthesis of B, N-Co-doped Graphite Oxide: To synthesize B, N-co-doped graphite oxide, 30

mg graphite oxide was dissolved in 100 mL deionized water containing 1 ml borane-

tetrahydrofuran (THF) adduct in a round-bottom flask to form an aqueous suspension. Then,

the flask was immersed in an oil bath set at 120℃ with continuous stirring for 4 days, yielding

Electronic Supplementary Material (ESI) for Journal of Materials Chemistry A.This journal is © The Royal Society of Chemistry 2019

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a dark brown suspension. While the color turned gray, 2 mg hydrazine hydrate was added, and

the suspension was kept at 80℃ for another 6 hours. After that, B, N-co-doped graphite oxide

was obtained by washing with DI water and centrifuging at 13000 rpm for 15 min.

Preparing strategy for MoSe2@CGO: In order to exfoliate B, N doped graphite oxide, 1 mg

graphite oxide was dispersed in 50 ml distilled water with thorough ultra sonication, followed

by freeze-drying to avoid restacking of its layered structure restack. For the preparation of

MoSe2@CGO, 0.206 g Na2MoO4∙2H2O and 0.028 g CGO were mixed in 15 mL

dimethylformamide (DMF) supported by ultrasonic dispersion. To prepare the hydrated-Se

solution, 78.96 mg stoichiometric Se powder was dissolved into 10 mL of 80% N2H4·H2O at

80 °C under stirring for about 1 hour. Subsequently, the hydrated-Se solution was slowly added

into Na2MoO4 slowly, and then transferred to a 50 mL autoclave and kept in an electrical oven

at 200 °C for 24 hours. Finally, the precipitate was collected by centrifugation and DI water

washing. To obtain the final product, the precipitate was annealed at 500 °C for 4 h under argon

atmosphere. The synthesis procedures for the bulk MoSe2 are similar to those for MoSe2@CGO

hybrids, but without the addition of CGO solution in the solvothermal process.

Electrochemical measurements: The corresponding electrochemical performance was

evaluated by a CR2032 coin-type cell assembled in an Ar-filled glovebox (Mbraun, Unilab,

Germany), where both O2 and H2O levels were less than 0.1 ppm. The test electrode was

produced by a slurry-coating procedure. To be specific, 78 wt. % active material, 10 wt. %

acetylene black, and 12 wt. % polyvinylidene fluoride (PVDF) were dissolved in N-methyl-2-

pyrrolidone (NMP) to form a coating slurry, which was then coated on aluminum foil and dried

under vacuum at 60 °C to produce the working electrode. And then the coin-type cells were

assembled by employing sodium metal as the counter electrode, Celgard 2300 as separator and

1 M NaClO4 in ethylene carbonate (EC) / diethyl carbonate (DEC) ( 1 : 1 v/v) mixed solvent

with fluoroethylene carbonate (FEC, 5 wt.%) additive as the electrolyte. After resting for one

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day, charge-discharge tests were performed between 0.1 V and 3.0 V on a Land CT2001A

battery test system (Wuhan, China). Electrochemical impedance spectroscopy (EIS) and cyclic

voltammetry (CV) were conducted on an electrochemical workstation (Biologic VMP3). CV

was carried out with a scanning rate of 0.1 mV s-1 between 0.1 V and 3.0 V (vs. Na/Na+). In

this work, the electrode loadings (total mass of acetylene black, PVDF and MoSe2@CGO) is

3.78 mg, and the specific capacity is calculated based on the mass of MoSe2@CGO.

Materials characterizations: The crystalline phase of the as prepared samples was characterized

by powder X-ray diffraction (XRD) using a Bruker D8 Advance diffractometer with Cu Kα

radiation from 5° to 80°. Phase identifications were carried out by laser Raman confocal

spectroscopy, using a HOBIBA Lab RAM HR 800 spectrometer with a 532 nm solid-state 50

mW laser source. Morphological characterizations were carried out on a Hitachi-S4800

(Hitachi, Japan) field emission scanning electron microscope (SEM) at 5 kV and a JEM-2100F

TEM (JEOL, Japan) transmission electron microscope (TEM) at 200 kV. Scanning

transmission electron microscopy (STEM) and energy dispersive X-ray spectroscopy (EDX)

were performed using a 200 kV JEOL ARM-200F instrument. For the TEM/STEM studies, the

as-prepared powder samples were dispersed in an isopropyl alcohol (IPA) solution and then

bath-sonicated for 15 – 20 minutes. Thermogravimetric analysis (TGA) was conducted on a

TG-DTA7300 thermogravimetric analyzer under N2 atmosphere at the heating rate of 5 °C min-

1 from room temperature to 600°C.

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Figure S1. a) Scanning electron microscope (SEM) image of pure B, N co-doped reduced graphene oxide (CGO). b) and c) SEM images of MoSe2@CGO. d), e), and f) SEM images of MoSe2 nanoflowers at different magnifications.

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Figure S2. a) and b) High resolution transmission electron microscope (HRTEM) images of MoSe2@CGO. c) and d) Elemental mapping of MoSe2@CGO. e) Energy dispersive spectroscopy (EDS) spectrum of MoSe2@CGO.

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Figure S3 TGA curves for MoSe2@CGO composite and its individual components for comparison.

Figure S4 XPS survey spectrum, providing evidence of the chemical composition of MoSe2@CGO, with the B 1s spectrum enlarged in the inset.

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Figure S5 Cycle life of MoSe2@CGO at 0.2 A g-1 with different adhesives. PVDF:

polyvinylidene difluoride; PAA: polyacrylic acid; SA: sodium alginate.

Figure S6. Rate performance of MoSe2@CGO for the first 40 cycles with different adhesives;

charge-discharge curves for the first and thirtieth cycles of rate cycling with different adhesives.

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Figure S7. HRTEM comparison of MoSe2@CGO in the original phase (left), fully discharged (middle) as well as fully charged (right). TEM images (top row) for the interlayer spacing of MoSe2@CGO at charge-discharge voltage of 0, 3.0, and 0 V with the specific layer spacings

shown in the middle row.

Figure S8. The electrochemical behaviors of the doped graphene electrodes.