Sodium storage and transport properties in pyrolysis synthesized MoSe2 nanoplates for high performance sodium-ion batteries

Accepted Manuscript Sodium Storage and Transport Properties in Pyrolysis Synthesized MoSe2 Nanoplates for High Performance Sodium-Ion Batteries Hui Wang, Xinzheng Lan, Danlu Jiang, Yan Zhang, Honghai Zhong, Zhongping Zhang, Yang Jiang PII:

S0378-7753(15)00330-4

DOI:

10.1016/j.jpowsour.2015.02.096

Reference:

POWER 20724

To appear in:

Journal of Power Sources

Received Date: 29 November 2014 Revised Date:

2 February 2015

Accepted Date: 17 February 2015

Please cite this article as: H. Wang, X. Lan, D. Jiang, Y. Zhang, H. Zhong, Z. Zhang, Y. Jiang, Sodium Storage and Transport Properties in Pyrolysis Synthesized MoSe2 Nanoplates for High Performance Sodium-Ion Batteries, Journal of Power Sources (2015), doi: 10.1016/j.jpowsour.2015.02.096. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

ACCEPTED MANUSCRIPT Wang Page1

Sodium Storage and Transport Properties in Pyrolysis Synthesized MoSe2

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Nanoplates for High Performance Sodium-Ion Batteries

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Hui Wang1,Xinzheng Lan1,Danlu Jiang1, Yan Zhang1,Honghai Zhong1, Zhongping Zhang1,2, Yang Jiang1,* 1

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School of Materials Science and Engineering, Hefei University of Technology Hefei, Anhui 230009, P. R. China 2 School of Chemistry, Chemical Engineering and Life Science, Chaohu University, Hefei, 238000, P.R. China. *Corresponding author: E-mail [email protected]; Tel.& Fax: +86551 62904358

HIGHLIGHTS

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MoSe2 nanoplates were first synthesized through pyrolysis process.

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MoSe2 demonstrates the first discharge and charge capacities of 513 and 440 mAh g−1.

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The sodium batteries exhibited good cycling stability and rate performance.

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The Na ion diffusion properties were investigated by first-principles calculation.

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The surface and interlayer diffusion barrier of Na ions are 1.36 and 0.344 eV.

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ABSTRACT

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The development of novel sodium-ion batteries has been hindered by the lack of ideal anode

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materials. Herein, we report both experimental and theoretical assessment of layered MoSe2

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nanoplates as the anode materials. The MoSe2 nanoplates are successfully synthesized by a facile

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thermal-decomposition process. As the anode, the MoSe2 nanoplates are capable of delivering

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the initial discharge and charge capacities of 513 and 440 mAh g−1 at the current of 0.1 C in a

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ACCEPTED MANUSCRIPT Wang Page2 voltage of 0.1-3 V, respectively. The analysis of Ex-situ XRD patterns reveals that there is no

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slippage between layers and the change of coordination of molybdenum when the MoSe2

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electrode is discharged to 0.6 V and conversion reactions during the following discharge/charge

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process are also demonstrated. In addition, the electronic structure, Na ions transport and

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conductivity are investigated by first-principles calculation. A quasi-2D energy favorable

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trajectory is proposed to illustrate the sodium ion vacancy-hopping migration mechanism form

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octahedron to tetrahedron in MoSe2 lattice. The results suggest great potential of MoSe2 as an

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anode material for Na ion batteries.

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Keywords: Sodium-ion batteries, MoSe2 nanoplates, First-principles calculation,

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materials, Ion diffusion barrier

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Anode

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1. Introduction Energy storage and conversion remain the key issues concerning our daily life [1]. Among

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various candidates, lithium-ion batteries (LIBs) are considered the most efficient energy

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storage systems and are used widely in portable electronic devices and electric vehicles,

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owing to high energy density, long lifespan, no memory effect, and environmental benignity

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[2-4]. However, due to shortage of lithium resources and increasing market demand, it is

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imperative to seek low-cost alternatives that are not resource-limited [5, 6].

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Sodium-ion batteries (NIBs) have attracted tremendous attention recently, because of the

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high availability of sodium, its low cost, and the similarity in the intercalation chemistry of

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sodium and lithium [7, 8]. However, due to larger ion radius and transport barrier compared

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to lithium, much effort has been taken to explore potential systems for sodium-ion battery.

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Many layered transition metal oxides, such as P2-NaxVO2 [9], Na0.71CoO2 [10], NaCrO2 [11],

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NaxMnO2

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Na[Ni1/3Fe1/3Mn1/3]O2 [13], and phosphates, NaMPO4, Na2MPO4F (M=Fe, Mn etc.) [14-16],

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Na4Fe3(PO4)2P2O7 [17], NaxM2(PO4) (M=Ti, V, etc.) [18-20] have been proposed as cathode

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materials for sodium-ion batteries, which have attracted extensive study from experiment to

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the theoretical calculation [14-16]. To our knowledge, only a few potential systems have

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been investigated as available anode materials for NIBs so far. Organic Na2C8H4O4 was

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reported to exhibit decent sodium storage capacity of ca. 250 mAhg−1, but with poor initial

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coulombic efficiency and electronic conductivity [21]. Hard carbon shows excellent Na

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storage properties in sodium-ion batteries, while its low sodium insertion voltage at 0.1 V

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may cause safety trouble in real operations [22]. Moreover, a novel variety of low cost

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P2-Na2/3[Ni1/3Mn2/3]O2

and

P2-Na0.875Li0.17Ni0.21Mn0.64O2

[7],

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ACCEPTED MANUSCRIPT Wang Page4 compounds reacting through conversion reactions and alloying reactions, whose gravimetric

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specific capacities that are two-to-five times larger than those attained with currently used

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materials, have been achieved, such as FeS2 [23, 24], Sb/C and SbSn/C [25, 26], but massive

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structural reorganization and volumetric changes prevented practical usage for sodium

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batteries [27]. Na2Ti3O7, which shows better sodium storage properties among the candidates

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for sodium batteries, but large band gap may result in poor rate performance [14]. MoS2, a

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typical layered transition metal sulfide, was used as anode material for NIBs and exhibited a

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decent performance [28, 29]. More recently, Shi-Xue Dou et al prepared the MoS2/graphene

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composites and obtained improved electrochemical performance for NIBs [30]. However, it

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is not very ideal to use MoS2 as anode material for sodium batteries in terms of the low

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specific capacity of such material, which may be owing to small layer distance and large

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structure resistance for Na ion insertion. It is therefore imperious to explore new potential

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material systems as anode materials for sodium batteries. MoSe2, similar to MoS2, with a

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novel sandwich structure, which is composed of stacked atom layers (Se-Mo-Se) held

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together by van der Waals force. Featuring larger space between adjacent layers and smaller

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band gap [31], which are expected to attribute to better coulombic efficiency and electronic

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conductivity, MoSe2 can be used as a very promising anode material for NIBs. Until now,

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little research has been done to the electrochemical performance, although Kang et al. has

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reported its excellent electrochemical properties with microsphere structure [32], most likely

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due to complicated synthetic method available. Therefore, the development of new but facile

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synthetic methods for MoSe2 should be of great technical significance, and more importantly,

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facilitate further assessment of their potential applications in NIBs.

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ACCEPTED MANUSCRIPT Wang Page5 In this manuscript, we first performed first principles calculations to assess the

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electrochemical performance of MoSe2. We found that the bulk MoSe2 can deliver a

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theoretical specific capacity as high as 422.28 mA h g-1. In the meantime, band structure and

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density of states analysis suggests that excellent electron conductivity can be expected when

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Na ions insert the lattice of MoSe2. Following the simulation work, we then developed a

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facile thermal-decomposition approach – one featuring ease of processing and low cost – for

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the synthesis of phase-pure MoSe2, by which were we able to experimentally assess further

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the application potential of MoSe2 as an anode material in NIBs. The results suggest that

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MoSe2 demonstrates the high first discharge/charge capacities of 513 and 440 mA h g-1 and

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the good cycling performance.

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2. Experimental Details

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2.1. Synthesis and characterization

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MoSe2 powders were synthesized by a simple thermal-decomposition method. For

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comparison, commercial MoSe2 (C-MoSe2) were also used as anode material for sodium

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batteries. The reaction sources of (NH4)6Mo7O24·4H2O (99.99%), Selenium (99.99%),

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ammonium hydroxide (NH3·H2O, 25%) and hydrazine hydrate (N2H4·H2O, 98%) solution

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were employed to prepare MoSe2. 3.531 g (NH4)6Mo7O24·4H2O was dispersed in 50 mL

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NH3·H2O under constant stirring to form a clear solution. In parallel, N2H4·H2O-Se solution

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was prepared by dissolving 6.3168 g Se powder in 40 mL N2H4·H2O in ambient air. The

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N2H4·H2O-Se was then added to the (NH4)2MoO4 solution slowly at room temperature,

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yielding a clear brown-red solution with a nominal Mo:Se mole ratio of 1:4. The solution was

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ACCEPTED MANUSCRIPT Wang Page6 then heated to 60°C and maintained at this temperature for 2 h under constant stirring. Next,

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the solution was heated at 200°C for 2 h, and the black crystal was obtained. Finally, the

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remaining precursor was transferred into a porcelain boat and calcined at the required

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temperature for 5 h under an Ar flow in a tube furnace. This yielded the final MoSe2

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nanoplates. MoSe2/graphene composite was prepared by combining MoSe2 active material and

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graphene in a 94:6 weight ratio. The mixture was dispersed in alcohol and ball milled for 10 h in a

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planetary mill and then dried at 80°C in an oven for 12 h. The phases and crystal structures of the

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synthesized materials were analyzed using a powder X-ray diffraction (XRD) analysis

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system (D/MAX2500V, Rigaku, Japan) equipped with a Cu Kα radiation source. The

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morphological features of the powder particles were analyzed by field-emission scanning

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electron microscopy (FESEM) (SU8020, Hitachi, Japan) and high-resolution field-emission

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transmission

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thermogravimetric (TG) analysis of the MoSe2/C sample was carried out on a diamond TG

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thermo-analyzer.

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(HRTEM)

(JEM-2100F,

JEOL,

Japan).

The

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2.2. Electrochemical performance tests and DFT calculations

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The working electrodes were prepared by casting the slurry of the active material (75

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wt%), acetylene black (15 wt%), polyvinylidene fluoride (PVDF) (10 wt%) dissolved in

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appropriate amount N-methyl-2-pyrrolidone (NMP) as the binder on a clean Al foil. The

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resultant electrochemical cells were assembled in a glove box filled with high-purity argon

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where pure sodium foil, 1 M NaClO4 in propylene carbonate (PC) and glass fiber were used

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as the counter electrode, the electrolyte and the separator, respectively. The discharge and

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ACCEPTED MANUSCRIPT Wang Page7 charge measurement were carried out on a NEWWARE battery test system (Shenzhen,

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China) in a voltage range of 0.1-3 V. After the first discharge test, the cells were

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disassembled in the Ar-filled glove box and the electrodes were rinsed by PC and dried in the

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vacuum chamber connected with glovebox until completely dried. Then the electrodes were

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subjected to XRD observation through a vacuum transfer box into XRD sample holder

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without air exposure. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy

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(EIS) were measured using a CHI600D (Shanghai, China) electrochemical workstation.

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A unit cell of 2H-MoSe2 was built to investigate the band structure and volumetric

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change during the charge/discharge process, while a triclinic MoSe2 bulk super-cell that

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contains 24 Mo and 48 Se atoms was constructed to explore the ion transport properties. The

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calculations were performed using density functional theory with the projector-augmented

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wave method [33]. The generalized gradient approximation(GGA) functional of Perdew and

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Wang (PW91) was adopted to treat the exchange and correlation potential in all calculations

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[34]. The effective corepotential was used for the electron-ion interactions, and the cut-off

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energy was set to 330 eV. The Brillouin zone sampling k-point set-mesh parameters were

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3×3×3, while the sampling k-point set-mesh parameters were 5×5×1 was used to study the

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Na ions diffusion on the MoSe2 surface. The following electronic states are treated as valence

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electrons: Mo, 4p65s14d5; Se, 4s24p4; and Na 3s13p0. The convergence error of the total

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energy is about 10−3eV. Atomic sites and lattice constants were relaxed until the residual

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forces were less than 0.01 eV Å−1 in each species.

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3. Results and discussions

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3.1.

DFT calculations

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ACCEPTED MANUSCRIPT Wang Page8 First-principles were first performed to assess the electrochemical properties of such

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material. The electronic conductivity of electrode materials has an important influence on the

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electrochemical properties, the intrinsic characterization of which can be initially described

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by band structure from first-principles predications [35]. As shown in figure 1c, the Fermi

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level is located above the top of the valence band. The conduction band is mainly made up of

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Mo 3d orbit, while the valence band is mainly associated with Se 2p orbit. The indirect gap

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of bulk MoSe2 is 0.974 eV, which is comparable to 1.1 eV [31], and this is smaller than

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MoS2 of 1.29 eV [36]. When Na ions are inserted into MoSe2, an equal amount of electrons

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will also be injected into the system to keep electricneutrality. These electrons will occupy

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the t2g orbits of Mo3d in the MoSe2 conduction band, which will induce the transition from

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semiconductor to metal. The Na ion induced transition may endow MoSe2 with excellent

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conductivity. This has been further proved by the analysis of density of states in figure 7b.

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Where the electronic conductivity is not a concern, ion transport properties may play the

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key role in the electrochemical performance of such material for rechargeable batteries [37].

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Therefore, sodium diffusion trajectories in MoSe2 were carefully investigated by first-

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principles calculation. Figure 1a shows two possile Na sites; the first site is on top of a Mo

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atom, and the other is in a hollow in the middle of a Mo triangle. The top site is a tetrahedral

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configuration while the hollow site gives anoctahedral environment. The diffusion path in the

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interlayer is investigated between two adjacent O-sites, due to the energetic favorability to

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the O-sites (the energy of O-sites is 13 meV higher than the T-sites), while the diffusion path

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on the surface is investigated betweentwo adjacent T-sites, due to the energetic favorability to

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the T-sites (the energy of T-sites is 20 meV higher than the O-sites), by assuming dilute Na

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concentrations and no constraints from electronic mobility. In the interlayer of MoSe2, the

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ACCEPTED MANUSCRIPT Wang Page9 specific path by which Na migrates between two adjacent O sites is through a nearest-

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neighbor T site in a zigzagway; on the surface of MoSe2, the specific path by which Na

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migrates between twoadjacent T sites is through a nearest-neighbor O site in a zigzagway,

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which is similar to Mg diffusion process in bulk MoS2 [38]. The LST method was used to

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calculate the energy barrier. The corresponding energy along such two pathesis plotted in

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figure 1b, and theactivation energy is identified to be 1.36 and 0.344 eV respectively.

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Furthermore, according to

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= 26 meV) [35], the diffusion coefficients DNa+ at room temperature can be calculated to be

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2.1×10-25 cm2 s-1 and 1.94×10-8cm2 s-1. It can be concluded that the Na ion diffusion between

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layers of bulk MoSe2 is much slower than the diffusion on the surface, which offers guidance

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for the design of such material. Therefore, we can expect the good electrochemical

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performance of the synthesized MoSe2 nanoplates, and this can be confirmed by our later

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experiments.

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(a ≈ 3.289Å, υ = 1011 Hz, and kBT

The MoSe2 powders were first synthesized by decomposing (NH4)2MoSe4 at 800°C.

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The four reaction steps involved are listed as following:

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(4)

Figure 2a depicts the powder X-ray diffraction pattern (PXRD) of the as-synthesized MoSe2.

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All recorded diffraction peaks can be indexed to the hexagonal crystal phase with P63/mmc

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space group of MoSe2 (JCPDS: 77-1715, namely, 2H-MoSe2). The cell parameters [a=3.2880

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Å, b=3.2880 Å, c=12.930 Å, α=90°, β=90°, γ=120°] were obtained,‐ these values are

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consistent with prior reported in the literature for MoSe2 [39]. As illustrated in figure 2b,

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MoSe2 can be regarded as strongly bonded two-dimensional Se-Mo-Se layers which are

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loosely coupled to each other by relatively weak Vander Waals force. Within a single Se-

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Mo-Se sandwich, the Mo and Se atoms form two-dimensional hexagonal arrays [40]. Na ions

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can occupy the Na1 (octahedral sites) and Na2 (tetrahedral sites) sites, as is shown in figure

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2b. In the presence of these vacancy clusters, groupings of vacancies within the layers,

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provide a common atomic hop mechanism that mediates Na diffusion in MoSe2 [41].

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According to the simulation analysis, four Na ions can insert into one MoSe2 formula unit in

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terms of this special structure, thus leading to a capacity of 422.28 mA h g-1.

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Morphology of the as-synthesized MoSe2 nanoplates was characterized by electron

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microscopy. Figure 3a and 3b are high magnification field emission scanning electron

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microscope (FESEM) images of the MoSe2. The microstructure consists of plate like

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agglomerates in the range of 50-300 nm. A higher magnification image reveals that the

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thickness of each plate is about 10-50 nm.

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TEM image of the as-synthesized MoSe2 is given in figure 3c. Clear nanoplate structure is

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revealed, which agrees well with that shown in figure 3b. Figure 3(d-e) shows the HRTEM

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images of one MoSe2 nanoplate, as labeled in dashed circle in figure 3c. The two[Insert Running title of <72 characters]

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meantime, the two well-defined interfringe distances of 0.278 nm can be indexed to crystal

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plane of hexagonal MoSe2. The corresponding selected area electron diffraction (SAED)

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patterns, as shown in figure 3f, indicate the single crystal characteristic of the MoSe2

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nanoplate. In addition, the quasi-hexagonal diffraction spots, associated with {101} and {

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crystal planes, can be indexed as the

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3.3. Electrochemical performance

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Figure 4b exhibits the first to the third discharge/charge profiles of MoSe2 electrode at the

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current rate of 0.1 C (0.1 C refers to 4 mol Na uptake into MoSe2 per formula unit in 10 h, 1

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C=422.28 mA g-1) between 0.1 and 3 V. Figure 4b illustrates that two potential plateaus, at

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0.65 and 0.44 V, are observed for the MoSe2 electrode in the first discharge (sodium insertion

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process). The plateaus at 0.65 V is indicative of the formation of NaxMoSe2, and the plateau

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variation in the sodium intercalation is attributed to the different defect sites of MoSe2, while

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the plateaus at 0.44 V may be related to the reduction of Mo4+ to Mo metal accompanied by

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the formation of Na2Se as is shown in figure 6a [32]. After the first cycle, the MoSe2

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electrode displays one plateau at 1.37 V, and the potential plateau at 0.65 and 0.44 V in the

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first discharge disappear. In the charge (sodium extraction process), the MoSe2 electrode

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exhibits inconspicuous potential plateaus at 1.8 and 2.1 V because of the existence of an

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intermediate phase, and this may be associated with the partial oxidation of Mo and Na2Se

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[42]. The initial discharge and charge capacities of the MoSe2 electrode are 513 and 440 mAh

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g−1 with a rather low efficiency of 85.7%. This may be owing to the side-reactions of the

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capacity is larger than the theoretical specific capacity of bulk MoSe2, this may be attributed

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to the increased structure disorder of MoSe2 nanoplates compared with bulk materials, thus

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leading to more Na ions reversibly interact with the expanded MoSe2 structure [43]. From the

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second to fifth cycle, the coulombic efficiency gradually increases, which is similar to MoS2

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[28]. Figure 4c shows the cycling performance of MoSe2. As can be seen in figure 4c, the

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discharge capacity decreased from 513 to 369 mA h g-1 after 50th cycles. This may be not

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only associated with lager electrode polarization, which can be attributed to large activation

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barriers of Na motion in the interlayer of bulk as discussed in the simulation part, but also

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with some other reasons as discussed later. The rate capability of the MoSe2 electrode was

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also measured in figure 4d. It demonstrates that the charge capacity decreases from 440 mAh

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g-1 at 0.1 C rate to about 250 mAh g-1 at 10 C rate, again implying electrode polarization and

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reduced sodium insertion/extraction into/from MoSe2. Upon decreasing the current rate from

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10 C to 0.1 C, 360 mAh g-1 was obtained. This demonstrates good capacity retention when

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operating conditions switch from high to low rate.

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The phenomena of the rapid capacity decay during initial 10 cycles and low Coulombic

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efficiency over about 30 cycles may be attributed to the following reasons. First, the SEI

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formation and deformation are ongoing throughout the initial 30 cycles; however, compared

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to the initial 10 cycles, to a lower extent and at lower rate , as can be partly reflected by the

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Nyquist plots in figure 7a [44]. Second, the volumetric changes and structural reorganization

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associated with the conversion reaction as discussed later [27]. And finally, the incomplete

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conversion reaction also contributes to the capacity decay [45]. Some efforts in optimizing

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carbon coating to protect the anode [30]. MoSe2/graphene composite was also prepared by

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combining MoSe2 active material and graphene in a 93:7 weight ratio. The final carbon content

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of the sample is 6.5% obtained from the TG curve in an air atmosphere as shown in Figure 5d. We

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find that the MoSe2/C composites indeed yield the better capacity retention and Coulombic

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efficiency as indicated in figure 5a 5b and 5c, which agrees well with the above analysis.

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For comparison, we also explored the electrochemical performance of commercial MoSe2

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particles with average diameter of several micrometers (C-MoSe2) as indicated in figure S1.

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As shown in figure S1c and S1d, commercial MoSe2 presented the similar electrochemical

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behavior but with much poorer electrochemical performance, which imply that the

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synthesized MoSe2 nanoplates are more suitable for NIBs than commercial MoSe2.

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To investigate the discharge mechanism, we performed the related experiments to check

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whether the structure of electrode material is changed during Na intercalation process. Ex-

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situ XRD patterns of MoSe2 electrodes were carried out in the first discharge/charge cycle

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are shown in figure 6a. Comparing with the pristine MoSe2, there was no XRD pattern

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change of this material discharged to 0.6 V, which indicated that no structure change

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occurred during this discharge process. The plateaus at 0.65 V can be ascribed to the rich Na

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and poor Na phase of NaxMoSe2. According to the XRD analysis, we then performed the

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simulation to investigate the density of states of MoSe2 and NaMoSe2. The results suggest

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that the Na ions insertion induce the transition from semiconductor to the metal as shown in

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figure 6b. The peaks at 44, 65 degrees in the XRD pattern are attributed to the aluminum

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ACCEPTED MANUSCRIPT Wang Page14 current collector. After discharged to 0.1 V, the MoSe2 converts to Mo and Na2Se. The

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existence of Na2Se was revealed by its characteristic peak at 26 degree (JCPDS No. 47-1699).

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As is indicated in figure 6a, no Mo crystal can be observed in the fully discharged MoSe2

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electrode, which may be partly due to the formation of amorphous Mo [46]. After fully

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charged to 3.0 V, the crystalline MoSe2 was observed as evidenced by the characteristic (103)

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peak at 37 degree. It is worth noticing that a small peak of Na2Se was also observed in fully

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charged electrode, which is due to the incomplete conversion reaction of Na2Se during

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charge process. Meanwhile, the crystalline selenium is not detected, suggesting the complete

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reaction from Na2Se to MoSe2 rather than Se during desodiation. Compared with MoS2, the

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MoSe2 exhibited a very different charge/discharge mechanism [30].

Cyclic voltammetry (CV) is a widely accepted method not only to investigate electrode

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process in a large range of potential, but also to estimate electrode reaction parameters by

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analyzing the shape of curves. Figure 4a shows the CV curves corresponding to the first two

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cycles of the MoSe2 for potentials ranging from 0 to 3V (vs. Na+/Na), the scan rate was 0.08

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mV s-1. In the first cathodic sweep, three pronounced reductive peaks at around 0.65, 0.44,

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and 0.05 V could be assigned to the Na ion insertion into MoSe2 and the side-reactions of the

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electrolyte reduction, respectively [35]. In the second cathodic sweep, a new peak centered at

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1.37 V appeared, as described above, which may be related to the reduction of Mo4+ to Mo

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metal accompanied by the formation of Na2Se. However, the peaks located at 0.65 and 0.44

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V disappeared, which agrees with the previous discharge curves. During the first and second

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anodic sweep, the oxidative peak with shoulder peaks at 1.8 and 2.1 V was observed, which

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could be caused by the existence of an intermediate phase.

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such material, EIS measurements were performed before the 1st and after the 1st cycle after

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12th and 13th cycles for the frequencies ranging from 0.01 Hz to 100 kHz during the

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discharge state. Figure 7a shows the Nyquist plots of the MoSe2 synthesized at an appropriate

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temperature. The equivalent circuit model of the studied system is also shown in Figure 7b,

6

which is similar to grapheme nanosheets [47]. Re represents the internal resistance of the test

7

battery, Rf and Q1 are related to the resistance and constant phase element of the SEI film, Rct

8

and Q2 are associated with the charge-transfer resistance and double-layer capacitance at the

9

electrode-electrolyte interface, and Zw is related to the Warburg impedance corresponding to

10

the sodium-diffusion process. As is demonstrated in figure 7a, the high-frequency semicircle

11

corresponds to the resistance Rf and Q1 of the SEI film, and the semicircle in the medium-

12

frequency region is ascribed to the charge-transfer resistance and Q2 of the electrode-

13

electrolyte interface. The sloping line in the low-frequency range is associated with the

14

diffusion of Na ions in the bulk of the electrode materials. The fitting parameters are shown

15

in Table 1. All the errors were within bounds. It can be seen that the SEI film resistance Rf

16

and the charge-transfer resistance Rct after the 1st cycle was larger than that before the 1st

17

cycle. This is mainly because the continuous deposition of Na ions in the SEI film increases

18

its thickness. During the discharge-charge process, the lager particle crush into smaller one

19

through electrochemical grinding, and this increase the interface resistance, thus may also

20

result in the increase of Rct [48]. The constant phase element Q1 of the SEI film and the

21

double-layer capacitance Q2 was also lager, thus leading to more extra energy consumption

22

spent on the non-faradic current charge/discharge. All above may be associated with the low

23

initial coulombic efficiency of MoSe2. After 12th cycles, the SEI become stable as indicated

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2

cycles. According to the above statement, there is a need to modify the interface structure of

3

bulk MoSe2.

4

4. Conclusion

5

In summary, we have demonstrated a facile method for the synthesis of MoSe2 nanoplates.

6

Electrochemical tests show that MoSe2 may serve as a good anode material, which exhibited

7

initial discharge and charge capacities of 513 and 440 mAh g-1 for potentials ranging from

8

0.1 to 3 V at the current rate of 0.1 C. First-principles simulation suggests that there is a Na-

9

induced transition from semiconductor to metal in MoSe2, thus endowing MoSe2 with good

10

electronic conductivity. Furthermore, the reversibility of the conversion reaction between

11

MoSe2 and Na ions was demonstrated by Ex-situ XRD. The Na ion vacancy-hopping

12

diffusion mechanism form octahedron to tetrahedron in MoSe2 lattice and from tetrahedron to

13

octahedron on the MoSe2 surface were also proposed based on a quasi-2D energy favorable

14

trajectory. Therefore, MoSe2 nanoplates hold great potential as anode material for NIBs.

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Acknowledgements

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This work was supported by grants from the National High Technology Research and

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Development Program of China (No. 2007AA03Z301), the National Natural Science

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Foundation of China (No. 61076040), and the Specialized Research Fund for the Doctoral

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Program of Higher Education of China (No.2012011111006).

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Figures and tables

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Figure 1. (a) Na migration path in the interlayer of MoSe2 bulk and on the surface of MoSe2. (b) Energy

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curve along the Na diffusion path. Einterlayer barrier and Esurface barrier indicate the activation barrier of

15

such two processes. (c) Calculated band structure along some high symmetric wave vectors in the

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irreducible brillourin zone.

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Figure 2. (a) XRD pattern of the MoSe2 nanoplates synthesized by the thermal-decomposition method. (b)

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Schematic of MoSe2 structure.

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Figure 3. (a) and (b) indicates the FESEM images of the MoSe2 nanoplates. HRTEM images of MoSe2

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under (c) low magnification and (d and e) high magnification. (f) The corresponding SAED pattern.

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Figure 4. Electrochemical Na-ion insertion and extraction behavior of MoSe2 nanoplates: (a) cyclic

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voltammograms and (b) discharge/charge profiles (at the current rate of 0.1 C) of MoSe2 nanoplates, (c)

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cycling performance of the MoSe2 electrode at the current of 0.1 C and (d) rate performance of the MoSe2

15

nanoplates.

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Figure 5. Electrochemical Na-ion insertion and extraction behavior of MoSe2/C: (a) discharge/charge

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profiles (at the current rate of 0.1 C) and (b) discharge/charge profiles (at the current rate of 1 C). (c)

13

Cycling performance of the MoSe2/C electrode at the current of 0.1 C and (d) TG curve of the MoSe2/C

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sample in an air atmosphere.

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Figure 6. (a) XRD patterns of pristine MoSe2 electrode and MoSe2 electrode discharged to 0.6 V 0.1 V

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and charged to 3.0 V. (b) Density of states of MoSe2 and NaMoSe2. The blue dash line represents the

16

fermi level.

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Figure 7. (a) Nyquist plots of the MoSe2 composite electrodes before and after the 1st cycle after 12th

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13th cycles and (b) equivalent circuit model of the studied system. Q represents the constant phase element.

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2

Impedance parameters derived from using equivalent circuit model for MoSe2 electrode. Rf

Q1

Rct

Q2

(Ω)

(µF)

(Ω)

(µF)

Before 1st cycle

9.22

162.5

147.2

76.68

51.77

After 1st cycle

9.37

183.3

191.1

84.6

64.74

After 12th cycle

10.10

285.2

293.4

210.2

190.4

After 13th cycle

10.35

288.9

297.6

233.4

Electrode

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In this manuscript, we developed a facile thermal-decomposition method to synthesis MoSe2

3

nanoplates. Electrochemical tests show thatMoSe2 may serve as a good anode material,

4

which exhibited initial discharge and charge capacities of 513 and 440 mAh g-1 at the current

5

rate of 0.1 C. First-principle simulation is also employed to understand the electronic

6

structure and Na ions diffusion kinetics in MoSe2 electrode.

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Keywords: sodium ion batteries; MoSe2 nanoplates; first-principle simulation; anode

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materials; sodium ion diffusion mechanism

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By Hui Wang,Xinzheng Lan,Danlu Jiang, Yan Zhang, HonghaiZhong, Zhongping

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Zhang,Yang Jiang*

Sodium Storage and Transport Properties in Pyrolysis Synthesized MoSe2 Nanoplates

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for High Performance Sodium-Ion Batteries

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HIGHLIGHTS MoSe2 nanoplates were first synthesized through pyrolysis process.




MoSe2 demonstrates the first discharge and charge capacities of 513 and 440

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mAh g−1.

The sodium batteries exhibited good cycling stability and rate performance.




The Na ion diffusion properties were investigated by first-principles

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The surface and interlayer diffusion barrier of Na ions are 1.36 and 0.344 eV.

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calculation.

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JOURNAL OF POWER SOURCES

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Supporting information Sodium Storage and Transport Properties in Pyrolysis Synthesized

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MoSe2 Nanoplates for High Performance Sodium-Ion Batteries

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Hui Wang, Xinzheng Lan, Danlu Jiang, Yan Zhang, Honghai Zhong, Zhongping Zhang,Yang Jiang1,*

Figure S1. (a) HRTEM images of commercial MoSe2 under high magnification. (b) The corresponding SAED pattern. (c) discharge/charge profiles (at the current rate of 0.1 C) and (d) cycling performance of the C-MoSe2 electrode at the current of 0.1 C.