Electrochimica Acta 115 (2014) 165–169 Contents lists available at ScienceDirect Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta Construction of 3D flower-like MoS2 spheres with nanosheets as anode materials for high-performance lithium ion batteries Ting Yang, Yuejiao Chen, Baihua Qu, Lin Mei, Danni Lei, Haonan Zhang, Qiuhong Li ∗ , Taihong Wang Key Laboratory for Micro-Nano Optoelectronic Devices of Ministry of Education, State Key Laboratory for Chemo/Biosensing and Chemometrics, Hunan University, Changsha, PR China a r t i c l e i n f o Article history: Received 18 September 2013 Received in revised form 16 October 2013 Accepted 17 October 2013 Available online November 2013 Keywords: MoS2 3D flower-like Nanosheets Alcohol-assisted Lithium ion battery a b s t r a c t In this work, we constructed 3D flower-like MoS2 spheres with nanosheets (less than 10 nm) by a simple alcohol-assisted solvothermal route It was found that the presence of alcohol enhanced the dispersity of MoS2 samples, and the distilled water facilitated the formation of nanosheets through varying the volume ratio of alcohol and distilled water The prepared MoS2 samples delivered high initial discharge capacity (1346 mA h g−1 at a current density of 100 mA g−1 ), good coulombic efficiency (77.49% retention for the first cycle and ∼100% for the subsequent cycles), excellent cycling performance (947 mA h g−1 at 100 mA g−1 after 50 cycles) and remarkable rate behavior as anode materials for lithium ion batteries The superior behavior of MoS2 samples for lithium ion batteries can be ascribed to the thin nanosheets, high specific surface area and their unique layered structure © 2013 Elsevier Ltd All rights reserved Introduction Lithium-ion batteries (LIBs) [1–3] have been extensively used in portable electronic devices and even electronic vehicles to tackle energy and environmental problems owing to their high capacity, no memory effect and environmental friendliness, etc Up to now, Graphite materials are most widely applied for anode materials in commercial LIBs with good cycling performance because of their stable structure However, their low theoretical capacity (372 mA h g−1 ), in some degrees, could not fulfill the gradually increased demands for high capacity under the rapid development of electronic technology [4,5] Transition metal oxides and sulfides [6–10] have been paid close attention as anode materials for LIBs because they own high theoretical capacities However, some of them suffer from volume expansion, safety, and resource limited issues, which restrict their use for next-generation battery applications Nowadays, molybdenum disulfide (MoS2 ) has been extensively studied in different fields, such as, lubricants [11–13], transistors [14–16] and supercapacitors [17,18], etc, owing to its special layered molecule structures in which Mo and S atoms are firstly bounded by strong ionic/covalent forces to form two-dimensional layers, and then these individual layers are further stacked by weak van der Waals interaction [19,20] The unique structure can permit ∗ Corresponding author E-mail addresses: yt-29@163.com (T Yang), liqiuhong2004@hotmail.com (Q Li) 0013-4686/$ – see front matter © 2013 Elsevier Ltd All rights reserved http://dx.doi.org/10.1016/j.electacta.2013.10.098 Li ions to insert and extract without remarkable volume damage because of the weak van der Waals interaction between MoS2 layers On the other hand, it is quite significant to develop a rational design to maximize their electrochemically active sites for redox reactions through obtaining “opened” structures to further increase their energy storage density [21–23] In this paper, dispersive 3D flower-like MoS2 spheres with nanosheets (MoS2 -1) were synthesized through a facile alcoholassisted solvothermal method The function of alcohol and distilled water were discussed and a possible growth mechanism was also proposed according to the time-dependent experiments We examine the electrochemical properties of MoS2 -1 as anode materials for LIBs and the results revealed that it delivered a high initial capacities of 1346 mA h g−1 at 100 mA g−1 and the corresponding initial coulombic efficiencies is 77.49% The capacity of MoS2 -1 can still remain 947 mA h g−1 after 50 cycles and it also exhibited excellent rate behaviors All the remarkable results could be contributed to large specific area, nano-sized structure and the unique layered structure of MoS2 It also demonstrated that MoS2 is a very promising candidate for next generation high performance LIBs Experimental 2.1 Synthesis of MoS2 Sodium molybdate (Na2 MoO4 ·2H2 O) and thiourea (NH2 CSNH2 ) were used as the reagents to synthesize MoS2 without any further purification In a typical synthesis of MoS2 -1, mmol of 166 T Yang et al / Electrochimica Acta 115 (2014) 165–169 Fig SEM images of (a, b) MoS2 -1and (c, d) MoS2 -0 Na2 MoO4 ·2H2 O was dissolved in 15 ml distil water, and then 15 ml ethanol was added After stirring for several minutes, mmol of NH2 CSNH2 was mixed The resulting solution was transferred into a 50 ml Teflon-lined stainlesss autoclave until it became transparent The autoclave was heated at 200 ◦ C for 24 h in a furnace and then cooled to room temperature naturally Black products were collected after thoroughly washed by distilled water and dried in a vacuum oven To make a contrast, a gathered MoS2 sample with nanosheets (MoS2 -0) and scattered MoS2 spheres (MoS2 -2) were obtained by using only distilled water and ethanol as solution and keeping other reaction conditions unchanged, respectively 2.2 Characterization The morphology and microstructure of the as-synthesized products were characterized by scanning electron microscope (SEM, Hitachi S4800) and transmission electron microscope (TEM, JEOL 2010) operated at an accelerating voltage of 200 kV Their crystal structures were examined by powder X-ray diffraction (XRD, Siemens D-5000 diffractometer with Cu-Ka irradiation ˚ Their surface areas were tested by nitrogen ( = 1.5406 A) adsorption/desorption analysis (Automated Physisorption and Chemisorption Analyzer, micromeritics ASAP 2020) 2.3 Electrochemical measurements The electrochemical measurements were done using CR2025type coin cells: the electrode materials were prepared by mixing active materials (80 wt%), conductivity agent (10 wt%, carbon black, Super-P-Li) and binder (10 wt%, carboxyl methyl cellulose (CMC), Aldrich) in distilled water and absolute alcohol mixture and stirred at a constant speed for 12 h in order to form a homogeneous slurry The well-mixed slurry was then spread onto a copper foil and dried at 80 ◦ C in a vacuum oven for 12 h A Celgard 2400 microporous polypropylene membrane was used as a separator The electrolyte contained a solution of M LiPF6 in ethylene carbonate/dimethyl carbonate/diethyl carbonate (1:1:1, in wt%) These cells were assembled in the glovebox (Super 1220/750, Switzerland) filled with highly pure argon gas (O2 and H2 O levels less than ppm) The cells were aged for 12 h before the measurements to ensure percolation of the electrolyte to the electrodes The discharge and charge measurements were carried out on an Arbin Fig The schematic of function of alcohol when preparing MoS2 T Yang et al / Electrochimica Acta 115 (2014) 165–169 167 Fig The TEM images of MoS2 -1 Fig The possible growth mechanism of MoS2 -1 (all scale were 500 nm) The SEM images were obtained with the reaction time (b) 2, (c) and (d) 24 h, respectively BT2000 system with the potential window of 0.01–3 V at current density of 100 mA g−1 (S), so the dispersive MoS2 spheres were formed The relationship between them could be described as follows: S= Results and discussion Fig shows the typical SEM images of prepared MoS2 samples Large quantities of MoS2 spheres with the diameter of about 700 nm are clearly observed in Figs 1a and S1a, while the MoS2 -0 is agglomerated seriously (Fig 1c) The formation of dispersed sphere structures can be attributed to the use of ethanol and the possible mechanism is proposed (Fig 2) On one hand, according to the thermodynamic principle, the presence of alcohol can reduce the solvation power of solvent because the alcohol possesses a lower permittivity compared to distilled water, which result in a lower solubility of product (C1 ) and a higher supersaturation of solvent Fig The XRD spectrums of both MoS2 samples r= C C1 (1) s−1 M vRT s lnS (2) On the other hand, large numbers of ethanol molecules exist because of the low ionization degree of ethanol, so during the formation of MoS2 , ethanol molecules can partially insert the interval of MoS2 molecules for their similar size [24], and thus dispersive MoS2 spheres formed We also obtained MoS2 spheres with nanosheets by changing the volume ratio of the mixed solvent, as depicted in Fig S2 In order to further identify the mechanism, other types of alcohols including ethylene glycol and glycerol, as the similar size of ethanol, were also used as the solvent in place of ethanol to synthesize MoS2 , and the results showed that similar dispersive MoS2 spheres with nanosheets were obtained as well (Fig S3a and b) The enlarged SEM images (Fig 1b and d) demonstrate that MoS2 -1 and MoS2 -0 are comprised of very thin nanosheets with the thickness about 7–10 nm (the insets of Fig 1b and d) Meanwhile, the nanosheets of MoS2 -1 are much looser than MoS2 -0, which is beneficial for obtaining large surface areas (the surface area of MoS2 -1 is 23.34 m2 g−1 , while the MoS2 -0 is 12.86 m2 g−1 ) The formation of nanosheets could be contributed to the utilization of distilled water, as depicted in Fig S1b With the absence of distilled water, there are only dispersed nanoparticles with some neglectable small nanosheets TEM was also carried out to characterize the structure of MoS2 1, as depicted in Fig The results are coincident with SEM (The TEM of MoS2 -0 is shown in Fig S4.) In order to study the growth process of 3D flower-like MoS2 spheres with nanosheets in detail, time-dependent experiments 168 T Yang et al / Electrochimica Acta 115 (2014) 165–169 Fig CVs of (a) MoS2 -1 and (c) MoS2 -0 electrodes at a scan rate of 0.25 mV s−1 Charge and discharge curves of (b) MoS2 -1 and (d) MoS2 -0 at a current density of 100 mA g−1 were carried out and a possible growth mechanism is proposed, as depicted in Fig During the first h (Fig 4b), MoS2 spherical structure composed of nanoparticles could be obtained When the reaction time reached h (Fig 4c), there were some small nanosheets scattered on the spherical surfaces As the reaction time was prolonged to 24 h (Fig 4d), 3D flower-like MoS2 spheres with nanosheets were obtained finally by using mixed solvent of ethanol and distilled water Fig shows the XRD patterns of both MoS2 samples in different solvents Though both samples exhibit a broadened feature in XRD patterns due to low crystallinity, all the diffraction peaks are in good agreement with the standard data for the pure phase of MoS2 (JCPDS Card No 37-1492) According to the previous studies [25–27], the reaction consists of three steps: (a) the formation of H2 S through the hydrolysis of thiourea; (b) the production of Mo (IV) by reduction of Mo (VI); (c) the formation of MoS2 The process could be expressed as follows: CS(NH2 )2 + 2H2 O → 2NH3 + CO2 + H2 S (3) 4Na2 MoO4 + 15CS(NH2 )2 + 6H2 O → MoS2 + Na2 SO4 + 6NaSCN + 24NH3 + 9CO2 Fig (a) Typical cycling behaviors and coulombic efficiency of both MoS2 samples (b) Rate capability of both MoS2 samples at different current densities (4) The electrochemical property of prepared MoS2 -1 and MoS2 -0 were tested as anode materials of LIBs with the potential window of 0.01–3 V at the current density of 100 mA g−1 and the scan rate of CVs is 0.25 mV s−1 Fig 6b and d shows the charge and discharge curves of initial three cycles of MoS2 -1 and MoS2 -0, respectively In the first discharge process, two redox plateaus were observed at ∼1.3 V and ∼0.6 V, which were coincident with the first cathodic peaks in the CV sweeps, respectively The plateau at 1.3 V resulted from the formation of Lix MoS2 , and the plateau at 0.6 V can be attributed to a conversion reaction process, which first entails the in situ decomposition of MoS2 into Mo particles embedded into a Li2 S matrix and then the formation of a gel-like polymeric layer resulting from electrochemically driven electrolyte degradation [28] In the subsequent cycles, both products reveal T Yang et al / Electrochimica Acta 115 (2014) 165–169 two potential plateaus at 1.9 V and 1.2 V, and the potential plateau at 0.6 V disappears In the charge process, remarkable potential plateaus are observed at 2.2 V of both samples These data of charge and discharge curves agreed with the CV analysis MoS2 -1 delivers a high initial capacity of 1346 mA h g−1 , and it remains a reversible capacity of 1043 mA h g−1 , while MoS2 -0 reveals a discharge and charge capacities of 1325 mA h g−1 and 863 mA h g−1 , respectively The cycling performances and rate behaviors of both MoS2 samples are presented in Fig 7a and b MoS2 -1 and MoS2 -0 still exhibit high capacities of 947 and 710 mA h g−1 after 50 cycles at the current density of 100 mA h g−1 (Fig 7a) As shown in Fig 7b, when the current density increase to 200, 500, 1000 and 2000 mA g−1 , MoS2 and MoS2 -0 remain 906, 818, 707, 544 mA h g−1 and 698, 647, 554, 475 mA h g−1 , respectively Even when the current density up to 5000 mA g−1 , the prepared MoS2 -1 and MoS2 -0 still deliver 211 and 165 mA h g−1 After the current density return to 100 mA g−1 , the two electrodes perform 931 and 715 mA h g−1 Both MoS2 samples deliver excellent cycling and rate performance owing to its unique layered structure, which can provide a relative stable environment for Li+ insertion and extraction when testing Moreover, MoS2 -1 shows a relative higher capacity than MoS2 -0, which can be attributed to the formed dispersive 3D flower-like spheres with larger specific surface area The excellent electrochemical performance of 3D flower-like MoS2 spheres with nanosheets can be attributed to the following factors: (1) Dispersive 3D flower-like spheres with nanosheets structure owns lots of open spaces and large specific surface area, which can afford a large number of reaction sites, so 3D flower-like MoS2 spheres with nanosheets can deliver a high specific capacitance (2) The nanometer size (7–10 nm) of MoS2 sheet and its distinct layered structure can reduce the diffusion length of ions within MoS2 and afford a stable environment during the charge and discharge process, so we can obtain excellent cycling and rate performance Acknowledgements We acknowledge the financial support of the National Natural Science Foundation of China (Grant No 21003041), the Specialized Research Fund for the Doctoral Program of Higher Education of China (20120161110016), the Hunan Provincial Natural Science Foundation of China (Grant No 11JJ7004), and the Hunan Provincial Major Project of Science and Technology Department (Grant No 2012TT1004) Appendix A Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.electacta 2013.10.098 References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] Conclusions We have successfully synthesized 3D flower-like MoS2 spheres with nanosheets by a facile alcohol-assisted solvothermal method The tests show the unique MoS2 structure own high capacity and excellent cycling performance as anode materials for LIBs The MoS2 electrodes provided a high initial capacity of 1346 mA h g−1 at a current density of 100 mA g−1 and the corresponding charge capacity is 1043 mA h g−1 Even after 50 cycles, MoS2 electrodes still exhibited a superior capacity of 947 mA h g−1 and excellent cycling stability The excellent electrochemical performance of 3D 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