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Phase Transition Mechanism and Electrochemical Properties of Nanocrystalline MoSe2 as Anode Materials for High Performance Lithium-Ion Battery Hui Wang, Xiaoyan Wang, Li Wang, Jin Wang, Danlu Jiang, Guopen Li, Yan Zhang, Honghai Zhong, and Yang Jiang J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b00353 • Publication Date (Web): 30 Mar 2015 Downloaded from http://pubs.acs.org on April 2, 2015

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The Journal of Physical Chemistry

Phase Transition Mechanism and Electrochemical Properties of Nanocrystalline MoSe2 as Anode Materials for High Performance Lithium-Ion Battery Ⅰ

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Hui Wang , Xiaoyan Wang , Li Wang , Jin Wang , Danlu Jiang , Guopen Li , Yan Zhang , Ⅰ Ⅰ Honghai Zhong , Yang Jiang ,* Ⅰ

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School of Materials Science and Engineering, School of Chemistry and Chemical Engineering, Hefei University of Technology，Hefei, Anhui 230009, P. R. China

Abstract: The real application of lithium-ion batteries in electric vehicles lacks the ideal anode materials. Herein, we report both experimental and theoretical study of MoSe2 nanocrystals as the anode materials. MoSe2 nanocrystals are successfully synthesized via a facile thermal-decomposition process. As the anode, the nanocrystalline MoSe2 yields the initial discharge and charge capacities of 782 and 600 mA h g−1 at the current of 0.1 C in a voltage of 0.1-3 V. First-principles simulation demonstrates that, during the initial discharge process, there is a Li atoms induced phase transition from 2H-MoSe2 to the O-MoSe2 phase at 0.9 V, and then Mo cluster occurs as more Li atoms intercalated into the MoSe2 lattice, which is associated with the formation of Mo and Li2Se. And the following charge/discharge processes are related to the conversion reaction between Mo and Li2Se. Meanwhile, the Li ion vacancy-hopping diffusion mechanism from octahedron to tetrahedron in MoSe2 lattice is proposed based on a quasi-2D energy favorable trajectory and the calculated diffusion constant is 1.31×10-13 cm2 s-1. For comparison, the amorphous MoSe2 demonstrates the

same

phase transition process after the initial

charge/discharge cycle. The results show that the nanocrystalline MoSe2 can be the very promising novel anode materials for high performance Li-ion batteries. 1

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Keywords: Lithium ion batteries, nanocrystalline MoSe2, first-principles simulation, phase transition, anode materials

1. Introduction Energy storage and conversion play the key role in our daily life.1,

2

Among the

various candidates, lithium-ion batteries (LIBs) are considered the most promising energy storage systems and have been used widely in portable electronic devices and electric vehicles (EVs), owing to higher energy density, long lifespan, no memory effect, and environmental benignity.3-6 As an anode material, graphitic material has achieved success in commercial LIBs due to their low cost and structural stability.7, 8 However, the low theoretical specific capacity of graphite (372 mA h g-1) cannot fully meet the demands associated with use in EVs, therefore, research to satisfy the everincreasing application requirements is still an imperious need.9 Similar to the graphite, two dimensional (2D) layered materials, such as MX2 (M=Mo, W; X=S, Se, Te ), are attracting expanding interests in lithium ion secondary batteries recently, due to their same open 2D channel crystal structures,10,

11

remarkable electronic properties,12-14 as well as the high theoretical specific capacity.15 Among these, molybdenum disulfide (MoS2) has attracted much attention. A variety of novel methods have been developed to synthesis MoS2 with different morphology, including MoS2 nanospheres,16,

17

nanoflake,21-23 MoS2 nanotubes and nanorods,24,

graphene-like MoS2,11, 25

18-20

MoS2

mesoporous MoS2,26 exfoliated

MoS2 composites27, 28 and amorphous MoS2. 29 At the same time, the phase transition 2


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of MoS2 monolayer from 2H-MoS2 to the 1T-MoS2 has been investigated by first principles calculation and in situ high resolution transmission electron microscopy.30, 31

The as-synthesized MoS2 listed above has also demonstrated excellent

electrochemical performance when applied in lithium ion batteries. As one of the typical transition metal selenides, MoSe2 has the analogous structure of MoS2, this structure can be viewed as three stacked atom layers (Se-Mo-Se) held together by van der Waals forces.32 This special layered structure favors the intercalation/extraction of Li ions. Thus the excellent lithium storage performance of MoSe2 can be expected. Interestingly, compared to the comprehensive researches on MoS2, the metal selenide (MoSe2) analogues in the same graphite-type family are rarely studied including theoretical calculations and experiments for use as anode materials for LIBs, most likely due to complicated synthetic method available. Yang, Kong et al. synthesized MoSe2 nanosheets/films and mainly explored its catalytic properties.33, 34 Hu et al. prepared mesoporous MoSe2 through a nanocasting method by using a hard template, although the complicated synthesis process, the assynthesized material possesses the expected lithium storage performance.35 C. N. R. Rao et al synthesized MoSe2 nanotubes, which may yield the excellent electrochemical performance for lithium ion batteries, by decomposing the ammonium selenometallates(NH4)2MoSe4) in a flow of H2at 900 °C.36 Thus, the development of new but facile synthetic methods for MoSe2 should be of great technical significance. It is also important that the phase transition during the Li ions insertion/extraction into/from MoSe2 should be understood in discharge/charge 3



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process to facilitate further assessment of their potential applications in LIBs. In this manuscript, we first developed a facile thermal-decomposition approach – one featuring ease of processing and low cost–for the synthesis of MoSe2 nanoparticles, by which were we able to experimentally assess further the application potential of MoSe2 as anode materials in LIBs. We found that the MoSe2 nanocrystals are capable of delivering the higher initial discharge and charge capacities of 782 and 600 mAh g−1 at the current of 0.1 C in a voltage of 0.1-3 V. A reversible lithium storage capacity of 405 mAh g-1 for up to 50 cycles can be obtained. The first principles simulation was employed to understand the Li ions diffusion kinetics in MoSe2 electrode and the possible phase transition of MoSe2, which is important to fully understand the discharge/charge mechanism and harness the technological potential of MoSe2. In the meantime, the amorphous MoSe2 was also investigated for further understanding the phase transition mechanism as anode materials during charge/discharge. 2. Experimental section MoSe2 nanoparticles were synthesized by a simple thermal-decomposition method. In details,

the

chemicals

of

(NH4)6Mo7O24·4H2O(99.99%),

Selenium(99.99%),

ammonium hydroxide (NH3·H2O, 25%) and hydrazine hydrate (N2H4·H2O, 98%) solution were employed to prepare MoSe2. 0.00285 mol (NH4)6Mo7O24·4H2O and 0.1 g PVP was dispersed in 10 mL NH3·H2O under constant stirring to form a clear solution. In parallel, N2H4·H2O-Se solution was prepared by dissolving 0.04 mol Se powder in 20 mL N2H4·H2O in ambient air. The N2H4·H2O-Se solution was then 4


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added to the (NH4)6Mo7O24·4H2O solution slowly at room temperature, yielding a clear brown-red solution with a nominal Mo:Se mole ratio of 1:2. The reacted solution was then heated to 60°C and maintained at this temperature for 2 h under constant stirring, and heated at 100°C for 2 h, which forming the black crystals. Finally, the remaining precursor was transferred into a porcelain boat and calcined at the required temperature for 5 h under an Ar flow in a tube furnace. This yielded the final MoSe2 nanocrystals. Meanwhile, after several experiments, we found that amorphous MoSe2 was obtained when calcined at the lower temperature of 250°C. The phases and crystal structures of the synthesized materials were analyzed using a powder X-ray diffraction (XRD) analysis system (D/MAX2500V, Rigaku, Japan) equipped with a Cu Kα radiation source. The morphological features of the powder particles were analysed by field-emission scanning electron microscopy (FESEM) (SU8020, Hitachi, Japan) and high-resolution field-emission transmission electron microscopy (HRTEM) (JEM-2100F, JEOL, Japan). The working electrodes were prepared by casting the slurry of the active material (80 wt%), acetylene black (10 wt%), polyvinylidene fluoride (PVDF) (10 wt%) dissolved in appropriate amount N-methyl-2-pyrrolidone (NMP) as the binder on a clean Al foil. The resultant electrochemical cells were assembled in a glove box filled with highpurity argon where pure lithium foil, 1 M LiPF6 solution in a mixture of EC/DMC/EMC (1:1:1 in volume) and glass fiber were used as the counter electrode, the electrolyte and the separator, respectively. The discharge and charge measurements were carried out on a NEWWARE battery test system (Shenzhen, 5



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China) in a voltage range of 0.1-3 V. A triclinic MoSe2 bulk supercell that contains 32 Mo and 64 Se atoms was constructed to explore the Li ions transport properties as well as the phase transition of MoSe2. Periodic boundary condition is used to simulate 3D MoSe2, and all of the structures are relaxed without any symmetry constraint. The calculations were performed using density functional theory with the projector-augmented wave method.37 The generalized gradient approximation (GGA) functional of Perdew and Wang (PW91) was adopted to treat the exchange and correlation potential in all calculations.38 The effective core potential was used for the electron-ion interactions, and the cut-off energy was set to 330 eV. The Brillouin zone sampling k-point setmesh parameters were 3×3×3 for the triclinic MoSe2 bulk supercell. The following electronic states are treated as valence electrons: Mo, 4p65s14d5; Se, 4s24p4; and Na 3s13p0.

3. Results and discussion 3.1. Structural and morphological analysis Fig. 1a depicts the powder X-ray diffraction pattern (PXRD) of the as-synthesized MoSe2 nanoparticles at 400 and 250°C. The sample synthesized at 250°C exhibited the amorphous characteristics. On the other hand, all the diffraction peaks of the samples synthesized at 400°C were indexed to the hexagonal crystal phase with P63/mmc space group of MoSe2 (JCPDS: 77-1715, namely, 2H-MoSe2), which was consistent with prior reported in the literature for MoSe2.35 As illustrated in Fig. 1(b), MoSe2 can be viewed as strongly bonded two-dimensional Se-Mo-Se layers held together by relatively weak Vander Waals force. Within a single Se-Mo-Se sandwich, 6


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the Mo and Se atoms form two-dimensional hexagonal arrays.39 2H-MoSe2 has the symmetry of the space group Among them, the

,

,

and 12 phonon modes at Brillouin zone center. , and

are Raman active.33 In our work, we used

the excitation laser line at 532 nm to study the Raman spectrum of the MoSe2 nanocrystals, whose image of optical microscope was demonstrated in Fig. 1d. As also can be seen in Fig. 1c, the sharp Raman peaks at 166.88 cm-1, 239.74 cm-1, and 283.75 cm-1 are attributed to the

,

mode respectively, again implying the 2H-

MoSe2 has been synthesized successfully. 40 Morphology of the as-synthesized crystalline MoSe2 powder was characterized by electron microscopy. For the crystalline MoSe2, low magnification field emission scanning electron microscope (FESEM) images (Fig. 2a) presented a porous network of MoSe2 nanopartilces with an irregular morphology and various sizes ranging from 100-300 nm. It was noted that those nanoparticles are formed by the agglomeration of MoSe2 nano-crystals of 50-100 nm at higher magnification (Fig. 2b). TEM image shown in Fig.3a clearly revealed the nanocrytalline structure of the assynthesized MoSe2, which agrees well with that shown in Fig. 2b. Fig. 3(b,d) shows the HRTEM images of the MoSe2 nanocrystals, as labelled in dashed circle in Fig. 3a. Among them, the representative HRTEM image of MoSe2 in Fig. 3d illustrates the layered crystal lattice structure and the interlayer distance of (002) plane can be measured from HRTEM is 0.67 nm. The corresponding selected area electron diffraction (SAED) pattern in Fig. 3c demonstrates the polycrystalline structure of the 7



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aggregated MoSe2 powder. 3.2. Electrochemical performance The electrochemical properties of the nanocrystalline MoSe2 were carefully explored by cyclic voltammetry (CV), electrochemical impedance spectra (EIS) and galvanostatically discharge/charge at the current of 0.1 C, 0.2 C, 1 C, respectively, where, 0.1 C refers to 4 mol Li uptake into MoSe2 per formula unit in 10 h, as shown in Fig. 4. The first three CV curves of nanocrystalline MoSe2 are presented in Fig. 4a. In the first cycle, the reduction peak at 0.9 V is likely attributed to the phase change from 2H to 1T-MoSe2, which could be proved by our simulation later.18 The reduction peak at 0.68 and 0.35 V are associated with the formation of metal Mo and Li2Se, as well as the SEI layer, and the strong oxidation peak at 2.2 V can be ascribed to the oxidation of Li2Se to selenium (Se), which can be confirmed by ex-situ XRD patterns in Fig. S1.41 Thus, in the following cycles, the reduction peak at 1.74 and oxidation peak at 2.2 V are ascribed to the Li2Se associated with Mo. The Fig. 4b, 4c and 4d exhibit the first to the third discharge/charge profiles of crystalline MoSe2 electrode at the current of 0.1 C, 0.2 C, 1 C respectively. The plateaus in the voltage profiles are consistent well with the distinct peaks in the CV curves. The MoSe2 nanocrystals electrode exhibits the initial discharge/charge capacities of 782 mA h g-1 and 600 mA h g-1 at the current of 0.1 C respectively, and the corresponding low initial coulombic efficiency was 76.7%, which may be ascribed to the reactions of SEI and side-reactions of electrolyte reduction in the initial cycle.42 Meanwhile, MoSe2 nanocrystals are still capable of yielding the initial 8


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discharge/charge capacities of 701 mA h g-1/559 mA h g-1 at 0.2 C and 580 mA h g1

/447 mA h g-1 at 1 C, implying the excellent capacity retention ability of the materials.

The cycling performance and coulombic efficiencies of crystalline MoSe2 tested at the current of 0.1 C, are shown in Fig. 5a. It can be seen that the capacity of crystalline MoSe2 declines relative rapidly at 0.1 C after several cycles. After the 50th cycles, the capacity decreases to 405 mA h g-1. Fig. 5b demonstrates that the charge capacity of the crystalline MoSe2 electrode decreases from 600 mA h g-1 at 0.1 C rate to about 322 mA h g-1 at 10 C rate, which implying electrode polarization and reduced Li ions insertion/extraction into/from MoSe2 at high current rates. Upon decreasing the current rate from 10 C to 0.1 C, 405 mA h g-1 was obtained. This demonstrates good capacity retention when operating conditions switch from high to low rate. Another appealing property of the MoSe2 nanocrystal electrode is its superior rate performance comparing to its analogue micro-sized MoSe2 and nano-sized MoS2 material. Fig. 5b shows the rate performance of the MoSe2 nanocrystal electrode between 0.1 and 3.00 V. When the discharge/charge rate increases from 0.1 C to 0.2 and 1 C, the charge capacity slowly decreases from 600 to 590 and 500 mAh g− 1, which is much better than those values reported for micro-sized MoSe2 and mesoporous MoS2 electrode as shown Fig. S2, respectively.35 In the case of micro-sized MoSe2, when the discharge/charge rate increases from 0.1C to 0.2 and 1 C, the capacity decreases from 590 to 495 and 240 mAh g−1.35 The enhanced rate performance may be attributed to the electrochemical nature of MoSe2 and better kinetics. Interestingly, the synthesized MoSe2 nanocrystal electrodes deliver the similar excellent electrochemical 9



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performance compared with mesoporous MoSe2 prepared though a complicated synthesis method, again implying the MoSe2 nanocrystals synthesized by thermaldecomposition process can be a very potential anode material for LIBs.35 To further understand the interface structure and the electrode reaction kinetics of such material, EIS measurements were performed before the 1st and after the 1st cycle for the frequencies ranging from 0.01 Hz to 100 kHz during the discharge state. Fig. 5c and 5d exhibit the Nyquist plots of amorphous and crystalline MoSe2 respectively. The straight line in the low frequency region represents typical Warburg behavior, while the depressed semicircles in the moderate frequency region are ascribed to the charge transfer process. It is clear that the semicircle of crystalline MoSe2 is smaller than that of the amorphous materials, indicating a lower charge transfer resistance compared with the amorphous MoSe2. 3.3. DFT calculations First-principles calculations were performed to understand the lithium ions diffusion kinetics and the possible phase change of 2H-MoSe2. As indicated in Fig. 6a, Li ions can occupy octahedral sites (O-sites) and tetrahedral sites (T-sites). The diffusion path is investigated between two adjacent O-sites, due to the energetic favorability to the O-sites, by assuming dilute Na concentrations and no constraints from electronic mobility.43 Using the complete LST/QST method, we find the specific path by which Li migrates between two adjacent O-sites is in a curved way. The corresponding energy along this path is plotted in Fig. 6b, and the activation energy is identified to (a ≈ 3.29 Å, υ = 1011

be 0.542 eV. Furthermore, according to 10


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Hz, and kBT = 26 meV),44 the diffusion coefficients DLi+ at room temperature can be calculated to be 1.31× 10-13 cm2 s-1. Hence, the low lithium ion diffusion constant may be attributed to the capacity decrease. In the following, we introduced Li atoms on the lattice of the MoSe2 to induce phase transition. We assumed that O-MoSe2 has the same structure with O-MoS2.31 For 2H-MoSe2, lithium ions can occupy two type sites: (1) top of Mo atoms (T-sites); and (2) hollow (O-sites) above the center of hexagon. For O-MoS2, lithium ions can also occupy two type sites: (1) top of Mo atoms (T´-sites); (2) hollow (O´-sites) above Se-Se triangle but not on top of a Mo atom. To check the most stable lithium atoms adsorption sites on the 2H-MoSe2 and O-MoSe2, we calculated the binding energy (Eb), which is defined as

, where x indicates the

number of lithium atoms. The results suggest that a Li atom on O-site is about 0.016 Ha and 0.034 Ha lower than that of T-site for 2H-MoSe2/O-MoSe2 respectively. Thus, we only consider the O-sites and O´-sites for inserting Li atoms in the following. Once the Li site distribution with the lowest energy has been found, we define this as well as the second lowest energy configuration as two initial structures for inserting another Li atom. This process is repeated until the number of Li atoms increases to 22 in a (4 × 4) supercell. We systematically examined the most stable configurations of LixMoSe2, as shown in Fig. 7 and 8 for 1-22 atoms. There are 18 O-sites/64 T-sites and 18 O´-sites/64 T´sites for Li atoms adsorption in unit 2H-MoSe2/O-MoSe2 supercell. As indicated in Fig. 7a、7b, Fig. 8a、 8b, and Fig. 9, the 2H-MoSe2 is still the most stable phase 11



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when 1-18 atoms are added to the system. That is to say, when all the O-sites and O´sites are occupied by Li atoms, the 2H-MoSe2 is same stable as O-MoSe2. As more Li atoms added into the MoSe2 lattice, Mo clusters are formed in both phases as shown in Fig. 7c, 7d and 8c, 8d. However, it can be noted that the O-MoSe2 phase becomes more stable when more Li atoms added on the MoSe2 T- sites/T´-sites in Fig. 9, which means Li atoms–induced phase transition indeed exist and this may be attributed to the initial discharge potential plateau at 0.9 V. Meanwhile, as more Li atoms added, more Mo clusters are formed as shown in Fig. 7c, 7d and Fig. 8c, 8d, this may be associated with the formation of Mo and Li2Se. In order to further assess the MoSe2 as a potential anode material, we explored the electrochemical performance difference between amorphous and nanocrystalline MoSe2. TEM image of the MoSe2 synthesized at 250°C is given in Fig. 10a. Upon inspection of HRTEM images shown in Fig. 10b and c, no lattice fringes can be detected, implying its amorphous characteristics, which can be furthered confirmed by SAED pattern in Fig. 10d. As to the CV curves of amorphous MoSe2 in Fig. 11a, it can be observed that the first cathodic sweep displays three distinct reduction peaks at 1.68 V, 1.1 V and 0.35 V, which does not match with the crystalline MoSe2, and this may be due to the absence of long-range order in amorphous MoSe2 lattice.29 The reduction peak at 1.68 V and 45

1.1 V mat be ascribed to the

Similar to the

crystalline MoSe2, the reduction peak at 0.35 V could be associated with the formation SEI layer. The strong oxidation peak at 2.28 V can be ascribed to the 12


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oxidation of Li2Se to selenium (Se). After the first cycle, both amorphous and crystalline MoSe2 present the similar CV curves, which may be most likely due to the same conversion reaction as discussed above. As presented in Fig. 11b, it was interested to note that amorphous MoSe2 was also able to deliver the relative high initial discharge/charge capacity of 660 mA h g-1/492 mA h g-1 at 0.1 C, but the initial coulombic efficiency of 74.5% is lower than that of crystalline MoSe2 (76.7%), and this may be associated with the larger electrode interface impendence as shown in Fig. 5 c and 5d. The cycling performance and coulombic efficiencies of amorphous MoSe2 tested at the current of 0.1 C, are shown in Fig. 11c. It was clear that the capacity of amorphous MoSe2 declines slower than that of the crystalline MoSe2. It may be attributed to the little structure deformation caused by strong coulombic repulsions between Li ions.46 Though the little structure deformations of amorphous MoSe2 as discussed above are expected to lead to better rate performance, however, taking the relative poor electronic conduction into account,46 the amorphous MoSe2 presents the similar rate properties as shown in Fig. 5b and 11d. Given above results, it is clear the naocarytalline MoSe2 presents better electrochemical performance.

4. Conclusions In summary, we have demonstrated a facile method for the synthesis of MoSe2 nanoparticles. Electrochemical tests show that MoSe2 nanocrystals may serve as a good anode material, which exhibited initial discharge and charge capacities of 782 and 600 mAh g-1 for potentials ranging from 0.1 to 3 V at the current rate of 0.1 C. 13



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First-principles simulation suggests that there is a phase transition from 2H-MoSe2 to the O-MoSe2 in the initial discharge process at 0.9 V and Mo cluster occurs as more Li atoms intercalated into the MoSe2 lattice, which is associated with the formation of Mo and Li2Se. The Li ion vacancy-hopping diffusion mechanism form octahedron to tetrahedron in MoSe2 lattice was proposed based on a quasi-2D energy favorable trajectory and the calculated diffusion constant is 1.31× 10-13 cm2 s-1. Meanwhile, we observed that the amorphous MoSe2 presents the same phase transition process after the initial discharge/charge process but with poor electrochemical performance compared with the nanocrystalline MoSe2. Therefore, nanocrystalline MoSe2 holds great potential as anode materials for LIBs.

Author Information Corresponding author *Corresponding author: E-mail [email protected]; Tel.& Fax: +86551 62904358

Acknowledgements This work was supported by grants from the National High Technology Research and Development Program of China (No. 2007AA03Z301), the National Natural Science Foundation of China (No. 61076040), and the Specialized Research Fund for the Doctoral Program of Higher Education of China (No.2012011111006).

Supporting Information The ex-situ XRD patterns of MoSe2 electrode discharged to 0.1 V and charged to 3 V. 14


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And the comparison of the rate performance among the nanocrystalline MoSe2 、 micro-sized MoSe2 、 mesoporous MoSe2 and mesoporous MoS2 electrodes. This information is available free of charge via the Internet at http://pubs.acs.org.

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Fig. 1. (a) PXRD pattern of the synthesized MoSe2 powder. (b) Schematic of MoSe2 structure. (c) Raman spectrum of the powder. (Black line is the simulated Raman spectrum.) (d) Image of MoSe2 powder under optical microscope.

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Fig. 2. FESEM images of crystalline MoSe2 at (a) low magnification and (b) high magnification.

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Fig. 3. HRTEM images of MoSe2 nanocrystals under (a) low magnification and (b and d) high magnification. (c) The corresponding SAED pattern.

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Fig. 4. Electrochemical Li-ion insertion and extraction behaviors of nanocrystalline MoSe2: (a) Cyclic voltammograms and (b, c, d) discharge/charge profiles of nanocrystalline MoSe2 at the current of 0.1 C, 0.2 C, 1 C.

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Fig. 5. (a) Cycling performances of the nanocrystalline MoSe2 electrode (at the current of 0.1 C). (b) Rate performances of the nanocrystalline MoSe2 electrode. Nyquist plots of (c) MoSe2 electrode and (d) amorphous MoSe2 electrode before and after the 1st cycle.

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Fig. 6. (a) Li ions migration path in the lattice of MoSe2 bulk. (b) Energy curve along the plotted Li ions diffusion path. Ebarrier indicates the activation barrier of this process.

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Fig. 7. The optimized structures of the most stable (a) 2H-MoSe2, (b) Li18Mo32Se64with all the 18 Li atoms occupied O-sites, (c) Li20Mo32Se64 with extra 2 Li atoms occupied T-sites and (d) Li22Mo32Se64 with extra 4 Li atoms occupied T-sites.

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Fig. 8. The optimized structures of the most stable (a) O-MoSe2, (b) Li18Mo32Se64with all the 18 Li atoms occupied O´-sites, (c) Li20Mo32Se64 with extra 2 Li atoms occupied T´-sites and (d) Li22Mo32Se64 with extra 4 Li atoms occupied T´-sites.

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Fig. 9. Variation of the energy for the 2H-MoSe2 and O-MoSe2 as the number of Li ion changes.

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Fig. 10. HRTEM images of amorphous MoSe2 under (a) low magnification and (b and c) high magnification. (d) The corresponding SAED pattern.

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Fig. 11. Electrochemical Li-ion insertion and extraction behaviors of amorphous MoSe2: (a) Cyclic voltammograms and (b) discharge/charge profiles of amorphous MoSe2 at the current of 0.1 C. (c)

Cycling performances of the amorphous MoSe2 electrode (at the current of 0.1 C). (d) Rate performances of the amorphous MoSe2 electrode.

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Table of contents:

Li-ions diffusion properties, the initial discharge/charge profile as well as the lithium atoms induced phase transition for MoSe2 electrode.

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Phase Transition Mechanism and Electrochemical Properties of

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