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C nanocomposite as an anode material for lithium-ion batteries
Journal of Alloys and Compounds 729 (2017) 583e589
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A novel MoS2/C nanocomposite as an anode material for lithium-ion batteries Yan Liu a, b, Daoping Tang a, Haoxiang Zhong a, Qianyu Zhang a, Jianwen Yang c, Lingzhi Zhang a, * a
Key Laboratory of Renewable Energy, Guangdong Key Laboratory of New and Renewable Energy Research and Development, Guangzhou Institute of Energy Conversion, Chinese Academy of Sciences, No. 2 Nengyuan Rd., Guangzhou, 510640, China University of Chinese Academy of Sciences, 19(A) Yu Quan Road, Beijing, 100049, China c College of Chemistry and Bioengineering, Guilin University of Technology, 12 Jiangan Road, Guilin, Guangxi, 541004, China b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 25 February 2016 Received in revised form 18 September 2017 Accepted 19 September 2017 Available online 21 September 2017
A novel molybdenum disulfide/carbon (MoS2/C) nanocomposite is synthesized by a simple hydrothermal method using glucose as a carbon source and Pluronic F127 as promoting agent in presence of MoS2 nanoparticles and followed by carbonization. Pluronic F127 is used as an essential agent which inhibits the spontaneous formation of carbon microspheres during the hydrothermal reaction. The composite electrode exhibits excellent cycling stability and rate capability, delivering a reversible capacity of 882.6 mA h g1 at a current density of 50 mA g1 and a capacity retention of 82.8% after 100 cycles at a current density of 100 mA g1. At a higher current density of 300/500 mA g1, it still retains a capacity of 603.6/461.6 mA h g1 respectively, as compared to 295.6/228.4 mA h g1 for the pristine MoS2 electrode. The composite shows favorable electrochemical kinetics compared with pristine MoS2 due to the incorporation of homogenous conductive carbon layer and its nanostructured morphology. © 2017 Elsevier B.V. All rights reserved.
Keywords: MoS2/C composite Pluronic Dispersing agent Nanocomposite Lithium-ion batteries
1. Introduction Taking the advantage of high energy density, light weight, ecofriendly features, and long cycle life, lithium-ion batteries (LIBs) have become the predominant power source for portable electronic devices and are expected to be one of the leading candidates for powering hybrid electric vehicles and electric vehicles [1e3]. Efforts in developing new electrode materials with higher capacity and longer cycle life have therefore step into an unprecedented level [4e8]. Recently, the molybdenum disulfide (MoS2) as an anode material for LIBs attracts particular attentions in the merit of its high theoretical capacity, structure stability, safety characteristics and inexpensive components of molybdenum and sulfur [9e11]. As a typical layered transition metal sulfide, MoS2 possesses a special SeMoeS layered structure and is characterized by the weak van der Waals forces between MoS2 layers and thus shows highly reversible Liþ ion intercalation and extraction properties, which makes it a promising electrode material in high energy
* Corresponding author. E-mail address: [email protected] (L. Zhang). https://doi.org/10.1016/j.jallcom.2017.09.201 0925-8388/© 2017 Elsevier B.V. All rights reserved.
density batteries [12]. However, the potential application of bulk MoS2 materials is seriously limited by its low reversible capacity and poor capacity retention, which are caused by poor electrical conductivity, massive structural reorganization and volumetric changes upon repetitive lithium insertion during alloying/de-alloying process as well as the loss of sulfur in the form of polysulfides [13]. One of the effective approaches to overcome these problems is to build nanostructured MoS2 with particular morphology. To date, nanostructured MoS2 with different morphologies including nanoparticles, nanoflakes, nanosheets, nanorods, nanowires, nanoribbons and mesoporous structure have been reported [14e26]. These materials showed improved lithium storage performance due to large surface area, short diffusion path, and controllable morphology. Another strategy to avoid capacity fading issue is to create nanocomposites with conductive hybrids based on MoS2 and a carbonaceous matrix. Xiao et al. reported that MoS2/PEO composite prepared by inserting PEO into the exfoliated MoS2 has a reversible capacity of >900 mA h g1 at a current density of 50 mA g1 [10]. Similarly, loosely packed MoS2 nanosheets prepared by a one-pot hydrothermal method with thin carbon coating exhibit a high reversible specific capacity of 1419 mA h g1 at 0.1 A g1, retaining 80% of the capacity after 50
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cycles [13]. Guo et al. developed a MoS2@CMK-3 (porous carbon) nanocomposite via a simple nanocasting technique which delivered a discharge capacity of 602 mA h g1 at 250 mA g1 after 100 cycles [27]. Ma et al. reported the synthesis of MoS2 nanosheets/ carbon microspheres prepared by a one-pot hydrothermal reaction using sodium molybdate and sulfocarbamide as starting materials and glucose as a carbon source. It is worth notice that the obtained product was a hybrid mixture of carbon microsphere/MoS2 nanosheet instead of MoS2/carbon composite with coating a layer of carbon on the surface of MoS2 [28]. We previously reported that a molybdenum disulfide/carbon (MoS2/C) submicrosphere could be prepared by hydrothermal method using Na2MoO4$2H2O and thiourea as starting materials and polyvinyl pyrrolidone as a dispersing agent and a carbon source [29]. Later, we developed MoS2/poly (3, 4-ethylenedioxythiophene): poly (styrenesulfonate) composite (MoS2/P) prepared through a facial and environmentally friendly dip-coating method [30]. MoS2/P composite showed an enhanced electrical conductivity and delivered a reversible capacity of 712 mA h g1 at a current density of 50 mA g1 and retained 81% capacity after 100 cycles. More recently, we synthesized a nanostructured silicon/porous carbon spherical composite by a hydrothermal reaction of glucose as a carbon source in presence of Pluronic F127 and nano-Si particles [31]. Herein, we report the synthesis of a novel nanostructured MoS2/ C composite prepared through a hydrothermal process using glucose as a carbon source and Pluronic F127 as a promoting agent in presence of MoS2 nanoparticles and followed by carbonization. The structural and electrochemical properties of MoS2/C composite have been investigated in details and compared with pristine MoS2. 2. Experimental procedure 2.1. Materials Na2MoO4$2H2O (>99%) was purchased from Tianjin Chemicals Reagent Factory (China). Sublimed sulfur (99%) and Glucose (>99%) were purchased from Tianjin Fuchen Chemicals Regent Factory (China). Ethylene glycol (>98%) was purchased from Tianjin Fuyu Fine Chemicals (China). Pluronic F127 triblock copolymer (PEO100PPO65-PEO100, Mw ¼ 12600) was purchased from Sigma-Aldrich. Celgard 2400 was used as a separator. The electrolyte (1 M LiPF6 in ethyl carbonate/dimethyl carbonate (EC/DMC, v/v ¼ 1:1, water content < 10 ppm)) was purchased from Zhangjiagang GuotaiHuarong New Chemical Materials Co. (China). All reagents were of analytical grade and used without further purification. Deionized (DI) water was used throughout the study. 2.2. Hydrothermal synthesis of MoS2 and MoS2/C The pristine nano-MoS2 was prepared by previously reported method with minor modifications [29]. In a typical procedure, Na2MoO4$2H2O (0.36 g) and sublimed sulfur (0.14 g) were dispersed in deionized water (25 ml) and ethylene glycol (5 ml). Then the mixture was transferred into a 50 ml Teflon-lined stainless autoclave, heated at 190 C for 24 h, and then cooled to room temperature. Nano-MoS2 was obtained as a black powder after filtration, and washed 3 times with deionized water and once with absolute ethanol, then dried in a vacuum oven at 60 C for 24 h. MoS2/C nanocomposites were prepared by dispersing MoS2 nanoparticles in an aqueous solution of Pluronic F127 and glucose, followed by a hydrothermal process and a subsequent carbonization. In a typical synthesis, F127 (0.28 g) was dissolved in 20 mL of DI water. Glucose (3 g) in 10 mL of DI water was then added into the above solution. Nano-MoS2 (1 g) was then dispersed in the above solution by an ultrasonic cell disruption system (JY92-IIN ultrasonic
cell pulverizer) for 30 min. The suspension was then heated in a sealed steel autoclave at 180 C for 6 h. A precipitate was recovered, washed with water and ethanol, dried under vacuum at 60 C for 24 h. Then this precipitate was calcined at 700 C for 7 h under argon atmosphere at a heating rate of 2 C min1. 2.3. Characterization Scanning electron microscope (SEM) images were recorded on S4800 (Hitachi, Japan). The samples were prepared by sprinkling the powder materials onto double-sided carbon sticky tape which was mounted on a microscope stub. All samples were coated with a thin gold film under vacuum prior to microscopy. Transmission electron microscope (TEM) images were obtained on a transmission electron microscopy (JEOL JEM 2100F, Japan). Samples for TEM were prepared by dispersing the products in ethanol with 20 min ultrasonicating, and then dropping a few drops of the resulted suspension onto a copper grid precoated with amorphous carbon and allowing them to dry naturally. X-ray powder diffraction (XRD) data were collected on PANALYTICAL (Netherlands) with Cu Ka radiation (l ¼ 0.1540562 nm) from 20 to 80 (2q) at 40 kV and 40 mA. The BrunauerEmmett-Teller (BET) test was determined via an automated surface area and pore size analyzer (SIMP-10/PoreMaster 33, Quantachrome Instruments, USA). The thermogravimetric analysis (TGA) experiment was conducted on a NETZSCH STA 449C (NETZSCH-Gertebau GmbH, Germany) thermogravimetric analyzer in air atmosphere with a flow rate of 30 mL min1 from room temperature to 700 C at a heating rate of 10 C min1. Raman spectra were taken under ambient conditions using a HR800 Confocal Raman system (HORIBA Jobin Yvon, France) with a 532 nm laser radiation source. The electrical conductivity was measured by 2-wire Ohms measurement method with a CHI660D electrochemical workstation (China) [32,33]. 2.4. Electrochemical studies Coin cells (CR2025) were assembled to test the electrochemical performance of the obtained MoS2/C and MoS2 for comparison. The electrodes were prepared by mixing with acetylene black and polyvinylidene difluoride (PVDF) in a weight ratio of 8:1:1 in 1methyl-2-pyrrolidinone solvent to obtain homogeneous slurry. The slurry was coated onto a copper foil current collector and dried at 60 C for 12 h and then pressed to obtain the electrode sheets. The electrode sheets were further dried in a vacuum oven at 110 C for 24 h. The typical loading level of the active materials in MoS2 or MoS2/C cathode is 2e3 mg cm2. The half cells were assembled in an Ar filled glove-box, using 1 mol L1 LiPF6 solution in a 1:1 (V:V) mixture of ethylene carbonate (EC) and dimethyl carbonate (DMC) as electrolyte and Li foil as counter electrode. The cells were charged and discharged on a Shenzhen Neware battery cycler (China) in a voltage range of 0.01e3 V (vs. Liþ/Li) and the specific capacity was calculated on the basis of the whole mass of MoS2 and porous carbon. 3. Results and discussions 3.1. Characterization SEM and TEM were employed to investigate the morphology of the pristine MoS2 sample and MoS2/C composite (Fig. 1). MoS2 appeared as a rose-like particle (200e400 nm) with a 3D hierarchical structure composing of wrinkled nanosheets. After carbonization process, MoS2 particle was coated with a carbon layer with some tiny tumors on its surface. The high-resolution TEM image confirmed that the carbon layer was coated homogeneously on the
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Fig. 1. (a) SEM images and (d) TEM images of MoS2. (b) SEM images, (e) TEM images and (f) HRTEM image of MoS2/C. (c) MoS2/C prepared without using Pluronic F127.
surface of MoS2 particle with an average thickness of ca. 12 nm (Fig. 1def). A control experiment without addition of F127 in the hydrothermal reaction showed that carbon microsphere was generated together with MoS2/C particle (Fig. 1c). These results demonstrate that Pluronic F127 is an essential agent to promote the formation of MoS2/C composite without any carbonaceous impurity, probably due to the chelating effect on MoS2 surface and/or dispersing capability of F127 as a surfactant which inhibits the spontaneous formation of the carbon microspheres during the hydrothermal reaction. The specific surface area determined by BET measurement is 6.85 and 9.55 m2 g1 for MoS2 and MoS2/C, respectively. The slightly increased surface area can be attributed to the carbon tumors on the MoS2/C mentioned above. The crystal structures and crystallinity of as-synthesized MoS2 and MoS2/C were characterized by XRD (Fig. 2). The XRD patterns for both samples displayed the characteristic peaks for hexagonal2H MoS2 at 14.5 (002), 32.7 (100), 33.6 (101), 39.7 (103), and 58.4 (110), which are in agreement with the database in JCPDS file (JCPDS Card No. 37-1492). No obvious difference was observed from the diffraction patterns of MoS2 and MoS2/C, indicating that the hydrothermal process did not change the phase of MoS2 and the coated carbon possessed an amorphous character [13]. Raman spectra were used to study the structure of MoS2 and MoS2/C (Fig. 3a). Both MoS2 and MoS2/C exhibited two typical peaks at 379 and 403 cm1, corresponding to A1g and E2g mode of the hexagonal MoS2 crystal, respectively [34]. The E2g mode refers to the in-layer displacement of Mo and S atoms, whereas A1g mode
Fig. 2. XRD patterns of the MoS2 and MoS2/C.
refers to the out-of-layer symmetric displacements of S atoms along the c axis [35]. The peak frequency difference between A1g and E2g mode (D) is often used to evaluate the number of layers in thin MoS2 crystals [18,36]. The value of D (24 cm1) for MoS2/C implies that the MoS2 nanosheet is very thin, composed of only a few layers of MoS2. There are two other Raman peaks at around 1352 and 1586 cm1 corresponding to the D and G band of carbon, respectively [37]. The actual carbon content in the MoS2/C composite was measured using TGA (Fig. 3b). The weight loss for the pristine MoS2 is about 12.6% over the temperature range from 30 C
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Fig. 3. (a) Raman spectra of MoS2 and MoS2/C; (b) TGA curves of MoS2 and MoS2/C.
to 680 C, while the weight loss of MoS2/C composite is 20.0%. There is nearly no carbon residue in MoS2/C at 680 C. Nevertheless, the retained weight of MoS2 still maintains 87.4%. Therefore, the amount of carbon in MoS2/C was estimated to be 8.5 wt %. 3.2. Electrochemical performances Fig. 4a shows the first three cyclic voltammetry (CV) curves of the MoS2/C composite between 0.01 V and 3.0 V (vs. Liþ/Li) at a scan rate of 0.3 mV s1. Two peaks at 1.05 V and 0.5 V appeared in the 1st cathodic sweep. The peak at ca. 1.05 V corresponds to the intercalation of lithium-ion on different defect sites of MoS2 lattice to form LixMoS2 according to Eq (1). The other peak at ca. 0.5 V corresponds to the conversion reaction process of deposition of Mo metal from LixMoS2 along with the Li2S [38,39]. The whole discharge process can be regarded as a two-step reaction based on the following
equations [9]:
MoS2 þ xLiþ þ xe ¼ Lix MoS2 ðx ¼ 3 4Þ
(1)
Lix MoS2 þ 4Li þ þ4e ¼ 2Li2 S þ Mo=Liy
(2)
In the reverse anodic sweep, the pronounced peak at ca. 2.3 V can be assigned to the oxidation of Li2S into sulfur (S) according to Eq (3) [13,40].
Li2 S 2e ¼ 2Liþ þ S
(3)
Therefore, after the first cycle, the electrode can be regarded actually as a mixture of S and Mo instead of the original MoS2 [9]. In the subsequent cycles, two new reduction peak at about 1.9 V and 1.13 V appeared, which can be explained by a multistep conversion of S and lithium-ion to Li2S [13,41,42]. The main oxidation peak at
Fig. 4. CV curves of MoS2/C composite electrodes at a scan rate of 0.3 mV s1 between 0.01 V and 3.0 V vs. Liþ/Li (a). Discharge/charge profiles of MoS2 and MoS2/C composite electrodes at the first (b) and tenth cycles (c) with a current density of 50 mA g1. 2-wire Ohms measurement of the electric conductivity of MoS2 and MoS2/C (d).
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2.3 V corresponds to the transformation of Li2S to S and/or polysulfides [18,30]. This value is consistent with the thermodynamic potential of S (2.24 V), which is calculated based on the Gibbs energy variation in the reaction of Eq (3) [40]. After the second cycle, the CV curves are almost overlapped, suggesting that the electrochemical properties are stabilized in the subsequent cycles after 2nd cycle. Fig. 4 (b and c) shows the galvanostatic charge-discharge voltage profiles of the half cells of MoS2 and MoS2/C electrode in a voltage range of 0.01e3.0 V at 50 mA g1. A sloping plateau starting from 2.2 to 1.0 V and a flat plateau at about 0.5 V were observed for MoS2/C in the first discharge process (Fig. 4b). This is consistent with the 1st cathodic sweep of CV curve in Fig. 4a. The potential variation in the lithium intercalation can be attributed to the insertion of additional Liþ ions into the expanded MoS2 structure or in the defect sites of MoS2. During the charge process, the defined plateau at about 2.3 V corresponds to the cathodic peak at 2.3 V of CV curves, which resulted from the oxidation of Li2S into sulfur (S) according to Eq (3). The MoS2 electrode showed a similar discharge-charge profile, which manifests the fact that the coated carbon layer did not change the lithium storage mechanism of MoS2. The MoS2/C electrode delivered a higher first discharge and charge capacity of 1181.7/882.6 mA h g1 respectively, as compared with 993.3/ 858.1 mA h g1 for MoS2, primarily due to the increased surface area of MoS2/C and thus more active sites for the insertion/ extraction of Liþ ions. In the first cycle, the irreversible capacity loss is unavoidable for the majority of transition metal sulfides, mainly resulting from the formation of the solid electrolyte interface (SEI), electrolyte degradation, and the structure reorganization of MoS2 [43,44]. A new discharge plateau at 1.5 V appeared in the 10th cycle while the potential plateau at 0.5 V in the first process disappeared (Fig. 4b), indicating that both MoS2 and MoS2/C experienced an irreversible phase transition. In the subsequent charge/discharge process, MoS2/C showed a smaller polarization than MoS2 due to the increased electrical conductivity of the coated carbon layer. Measurements confirmed the higher conductivity of 2.38 102 S cm1for MoS2/C as compared to 6.6 103 S cm1 for MoS2 (Fig. 4d). The cycling performances of MoS2 and MoS2/C were tested at a current density of 100 mA g1 (Fig. 5a, first 3 cycles at 50 mA g1). MoS2 delivered a first charge capacity of 808.7 mA h g1. The capacity quickly decreased to 479.1 mA h g1 after 100 cycles. As a comparison, MoS2/C showed a first charge capacity of 763.2 mA h g1 and a reversible capacity of 631.9 mA h g1 after 100 cycles, exhibiting a better capacity retention of 82.8% as compared with 59.2% for MoS2. The comparative rate performances between MoS2/C and MoS2 are presented in Fig. 5b. The electrodes were
587
Fig. 6. Nyquist plots of MoS2 and MoS2/C composite electrodes (a), and the equivalent circuit used to model the impedance spectra (b). The symbols are the experimental data whereas the continuous lines represent the fitted spectra.
cycled at a current density of 50 mA g1 for the initial 10 cycles. Then, the current density was increased gradually to 500 mA g1 and finally returned to 50 mA g1. The charge capacities of MoS2/C at 50, 100, 200, 300 and 500 mA g1 are 887.8, 793.8, 690.8, 603.6, and 461.6 mA h g1, respectively. MoS2 delivered lower capacities of 853.3, 716.7, 395.0, 295.6 and 228.4 mA h g1 at 50, 100, 200, 300 and 500 mA g1, respectively. Apparently, MoS2/C showed a better rate performance compared with MoS2. This improved cycling and rate performances can be ascribed to the unique rose-like morphology composed of the ultrathin MoS2 nano-sheets and interconnected conductive carbon network. The increased surface area of MoS2/C offers more active sites for electrochemical reactions and increases the contact area between the electrolyte and the electrode material, which is also beneficial for improving the electrochemical performances [45]. MoS2/C in this work exhibits better electrochemical performances our MoS2/Sdoped-carbon reported previously [30]. When comparing with those similar work in literature, our MoS2/C composite also shows better electrochemical performances than MoS2 nanoflake [46], MoS2/carbon nanotube core-shell nanocomposite [47], MoS2/carbon nanowire [48], but somewhat poorer than PEO/MoS2/graphene nanocomposite [9]. To better understand the electrochemical kinetics of MoS2 and MoS2/C, EIS measurements were carried out. Fig. 6 shows the Nyquist impedance plots of these samples and their equivalent circuit model. In the equivalent circuit, Re represents the internal resistance of the test battery, Rsf and CPEsf represend the resistance and constant phase element of the SEI film, respectively. Rct and
Fig. 5. Cyclic performance of MoS2 and MoS2/C composite electrodes at a current density of 100 mA g1 (a) and their rate performance (b).
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CPEct represend charge-transfer resistance and constant phase element of the electrode/electrolyte interface, respectively. Wdif and Cint correspond to the diffusion components of Warburg impedance and the intercalation capacitance, respectively [49]. As shown in Fig. 6a, the high-frequency semicircle is assigned to Rsf and CPEsf, and the semicircle in the medium frequency region corresponds to Rct and CPEct. The inclined line in low frequency range representing the Warburg resistance (Wdif) is assigned to the lithium-diffusion process within the bulk of the electrode material. MoS2/C electrode showed a smaller radius of semi-circle in the Nyquist plots, indicating a lower SEI film resistance and charge-transfer impedance. By fitting the impedance data based on the EIS equivalent circuit model in Fig. 6b, Rsf and Rct for MoS2/C are 4.78 U and 16.46 U respectively, smaller than 5.84 U and 39.76 U for MoS2. This fact confirms that both SEI film resistance and charge-transfer resistance of MoS2/C electrode are smaller than those of MoS2 electrode. Based on the above results, the carbon coating increases the conductivity of MoS2 and greatly enhances rapid electron transport during the lithium insertion/extraction reaction, thus resulting in huge improvement of electrochemical performances. 4. Conclusion Nanostructured MoS2/C composite with a rose-like morphology was synthesized through a Pluronic F127-promoted hydrothermal process using glucose as a carbon source in presence of MoS2 nanoparticles followed by a subsequent carbonization process. MoS2/C composite has a 3D hierarchical structure composed of wrinkled nanosheets with some tiny carbon tumors. Pluronic F127 inhibited the spontaneous formation of carbon microspheres in our two-step synthesis of MoS2/C composite. The MoS2/C composite showed favorable electrochemical kinetics due to the incorporation of conductive carbon, which suppressed the large volumetric change of MoS2 during charge/discharge process and increased the electrical conductivity of MoS2. Therefore, MoS2/C exhibited a remarkable improved cycling stability and rate capability, delivering a reversible capacity of 882.6/461.6 mA h g1 at a current density of 50/500 mA g1 and retaining a capacity of 82.8% at 100 mA g1 after 100 cycles. Acknowledgements This work was supported by National Natural Science Foundation of China (21573239), Guangdong Provincial Project for Science & Technology (2014TX01 N014/2014A050503050/2015B010135008), Guangzhou Municipal Project for Science & Technology (2014Y200219/201509010018) and Dongguan Municipal Project for Science & Technology (2013509104210). LZ thanks the Distinguished Scientist Program of Guangxi Province. References [1] P.G. Bruce, B. Scrosati, J.M. Tarascon, Nanomaterials for rechargeable lithium batteries, Angew. Chem. Int. Ed. 47 (2008) 2930e2946. [2] L.W. Ji, Z. Lin, M. Alcoutlabi, X.W. Zhang, Recent developments in nanostructured anode materials for rechargeable lithium-ion batteries, Energy Environ. Sci. 4 (2011) 2682e2699. [3] C.Z. Yuan, H.B. Wu, Y. Xie, X.W. Lou, Mixed transition-metal oxides: design, synthesis, and energy-related applications, Angew. Chem. Int. Ed. 53 (2014) 1488e1504. [4] D.L. Ma, Z.Y. Cao, A.M. Hu, Si-based anode materials for Li-Ion batteries: a mini review, Nano-Micro Lett. 6 (2014) 347e358. [5] J.B. Goodenough, K.S. Park, The Li-Ion rechargeable battery: a perspective, J. Am. Chem. Soc. 135 (2013) 1167e1176. [6] X.W. Wu, X.H. Li, Z.X. Wang, H.J. Guo, P. Yue, Y.H. Zhang, Improvement on the storage performance of LiMn2O4 with the mixed additives of ethanolamine and heptamethyldisilazane, Appl. Surf. Sci. 268 (2013) 349e354. [7] Q.S. Gao, L.C. Yang, X.C. Lu, J.J. Mao, Y.H. Zhang, Y.P. Wu, Y. Tang, Synthesis, characterization and lithium-storage performance of MoO2/carbon hybrid
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Energy Mater. 2 (2012) 970e975. [26] X.P. Fang, X.Q. Yu, S.F. Liao, Y.F. Shi, Y.S. Hu, Z.X. Wang, G.D. Stucky, L.Q. Chen, Lithium storage performance in ordered mesoporous MoS2 electrode material, Micropor. Mesopor. Mat. 151 (2012) 418e423. [27] X.S. Zhou, L.J. Wan, Y.G. Guo, Facile synthesis of MoS2@CMK-3 nanocomposite as an improved anode material for lithium-ion batteries, Nanoscale 4 (2012) 5868e5871. [28] L. Ma, W.X. Chen, L.M. Xu, X.P. Zhou, B. Jin, One-pot hydrothermal synthesis of MoS2 nanosheets/C hybrid microspheres, Ceram. Int. 38 (2012) 229e234. [29] H. Luo, L.Z. Zhang, L. Yue, Synthesis of MoS2/C submicrosphere by PVPassisted hydrothermal method for lithium ion battery, Adv. Mater. Res. 531 (2012) 471e477. [30] X.L. Zhao, Y.J. Mai, H. Luo, D.P. Tang, B. Lee, C.U. Huang, L.Z. Zhang, Nano-MoS2/ poly (3,4-ethylenedioxythiophene): poly(styrenesulfonate) composite prepared by a facial dip-coating process for Li-ion battery anode, Appl. Surf. Sci. 288 (2014) 736e741. [31] D. 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Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
A novel MoS2/C nanocomposite as an anode material for lithium-ion batteries Yan Liu a, b, Daoping Tang a, Haoxiang Zhong a, Qianyu Zhang a, Jianwen Yang c, Lingzhi Zhang a, * a
Key Laboratory of Renewable Energy, Guangdong Key Laboratory of New and Renewable Energy Research and Development, Guangzhou Institute of Energy Conversion, Chinese Academy of Sciences, No. 2 Nengyuan Rd., Guangzhou, 510640, China University of Chinese Academy of Sciences, 19(A) Yu Quan Road, Beijing, 100049, China c College of Chemistry and Bioengineering, Guilin University of Technology, 12 Jiangan Road, Guilin, Guangxi, 541004, China b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 25 February 2016 Received in revised form 18 September 2017 Accepted 19 September 2017 Available online 21 September 2017
A novel molybdenum disulfide/carbon (MoS2/C) nanocomposite is synthesized by a simple hydrothermal method using glucose as a carbon source and Pluronic F127 as promoting agent in presence of MoS2 nanoparticles and followed by carbonization. Pluronic F127 is used as an essential agent which inhibits the spontaneous formation of carbon microspheres during the hydrothermal reaction. The composite electrode exhibits excellent cycling stability and rate capability, delivering a reversible capacity of 882.6 mA h g1 at a current density of 50 mA g1 and a capacity retention of 82.8% after 100 cycles at a current density of 100 mA g1. At a higher current density of 300/500 mA g1, it still retains a capacity of 603.6/461.6 mA h g1 respectively, as compared to 295.6/228.4 mA h g1 for the pristine MoS2 electrode. The composite shows favorable electrochemical kinetics compared with pristine MoS2 due to the incorporation of homogenous conductive carbon layer and its nanostructured morphology. © 2017 Elsevier B.V. All rights reserved.
Keywords: MoS2/C composite Pluronic Dispersing agent Nanocomposite Lithium-ion batteries
1. Introduction Taking the advantage of high energy density, light weight, ecofriendly features, and long cycle life, lithium-ion batteries (LIBs) have become the predominant power source for portable electronic devices and are expected to be one of the leading candidates for powering hybrid electric vehicles and electric vehicles [1e3]. Efforts in developing new electrode materials with higher capacity and longer cycle life have therefore step into an unprecedented level [4e8]. Recently, the molybdenum disulfide (MoS2) as an anode material for LIBs attracts particular attentions in the merit of its high theoretical capacity, structure stability, safety characteristics and inexpensive components of molybdenum and sulfur [9e11]. As a typical layered transition metal sulfide, MoS2 possesses a special SeMoeS layered structure and is characterized by the weak van der Waals forces between MoS2 layers and thus shows highly reversible Liþ ion intercalation and extraction properties, which makes it a promising electrode material in high energy
* Corresponding author. E-mail address: [email protected] (L. Zhang). https://doi.org/10.1016/j.jallcom.2017.09.201 0925-8388/© 2017 Elsevier B.V. All rights reserved.
density batteries [12]. However, the potential application of bulk MoS2 materials is seriously limited by its low reversible capacity and poor capacity retention, which are caused by poor electrical conductivity, massive structural reorganization and volumetric changes upon repetitive lithium insertion during alloying/de-alloying process as well as the loss of sulfur in the form of polysulfides [13]. One of the effective approaches to overcome these problems is to build nanostructured MoS2 with particular morphology. To date, nanostructured MoS2 with different morphologies including nanoparticles, nanoflakes, nanosheets, nanorods, nanowires, nanoribbons and mesoporous structure have been reported [14e26]. These materials showed improved lithium storage performance due to large surface area, short diffusion path, and controllable morphology. Another strategy to avoid capacity fading issue is to create nanocomposites with conductive hybrids based on MoS2 and a carbonaceous matrix. Xiao et al. reported that MoS2/PEO composite prepared by inserting PEO into the exfoliated MoS2 has a reversible capacity of >900 mA h g1 at a current density of 50 mA g1 [10]. Similarly, loosely packed MoS2 nanosheets prepared by a one-pot hydrothermal method with thin carbon coating exhibit a high reversible specific capacity of 1419 mA h g1 at 0.1 A g1, retaining 80% of the capacity after 50
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cycles [13]. Guo et al. developed a MoS2@CMK-3 (porous carbon) nanocomposite via a simple nanocasting technique which delivered a discharge capacity of 602 mA h g1 at 250 mA g1 after 100 cycles [27]. Ma et al. reported the synthesis of MoS2 nanosheets/ carbon microspheres prepared by a one-pot hydrothermal reaction using sodium molybdate and sulfocarbamide as starting materials and glucose as a carbon source. It is worth notice that the obtained product was a hybrid mixture of carbon microsphere/MoS2 nanosheet instead of MoS2/carbon composite with coating a layer of carbon on the surface of MoS2 [28]. We previously reported that a molybdenum disulfide/carbon (MoS2/C) submicrosphere could be prepared by hydrothermal method using Na2MoO4$2H2O and thiourea as starting materials and polyvinyl pyrrolidone as a dispersing agent and a carbon source [29]. Later, we developed MoS2/poly (3, 4-ethylenedioxythiophene): poly (styrenesulfonate) composite (MoS2/P) prepared through a facial and environmentally friendly dip-coating method [30]. MoS2/P composite showed an enhanced electrical conductivity and delivered a reversible capacity of 712 mA h g1 at a current density of 50 mA g1 and retained 81% capacity after 100 cycles. More recently, we synthesized a nanostructured silicon/porous carbon spherical composite by a hydrothermal reaction of glucose as a carbon source in presence of Pluronic F127 and nano-Si particles [31]. Herein, we report the synthesis of a novel nanostructured MoS2/ C composite prepared through a hydrothermal process using glucose as a carbon source and Pluronic F127 as a promoting agent in presence of MoS2 nanoparticles and followed by carbonization. The structural and electrochemical properties of MoS2/C composite have been investigated in details and compared with pristine MoS2. 2. Experimental procedure 2.1. Materials Na2MoO4$2H2O (>99%) was purchased from Tianjin Chemicals Reagent Factory (China). Sublimed sulfur (99%) and Glucose (>99%) were purchased from Tianjin Fuchen Chemicals Regent Factory (China). Ethylene glycol (>98%) was purchased from Tianjin Fuyu Fine Chemicals (China). Pluronic F127 triblock copolymer (PEO100PPO65-PEO100, Mw ¼ 12600) was purchased from Sigma-Aldrich. Celgard 2400 was used as a separator. The electrolyte (1 M LiPF6 in ethyl carbonate/dimethyl carbonate (EC/DMC, v/v ¼ 1:1, water content < 10 ppm)) was purchased from Zhangjiagang GuotaiHuarong New Chemical Materials Co. (China). All reagents were of analytical grade and used without further purification. Deionized (DI) water was used throughout the study. 2.2. Hydrothermal synthesis of MoS2 and MoS2/C The pristine nano-MoS2 was prepared by previously reported method with minor modifications [29]. In a typical procedure, Na2MoO4$2H2O (0.36 g) and sublimed sulfur (0.14 g) were dispersed in deionized water (25 ml) and ethylene glycol (5 ml). Then the mixture was transferred into a 50 ml Teflon-lined stainless autoclave, heated at 190 C for 24 h, and then cooled to room temperature. Nano-MoS2 was obtained as a black powder after filtration, and washed 3 times with deionized water and once with absolute ethanol, then dried in a vacuum oven at 60 C for 24 h. MoS2/C nanocomposites were prepared by dispersing MoS2 nanoparticles in an aqueous solution of Pluronic F127 and glucose, followed by a hydrothermal process and a subsequent carbonization. In a typical synthesis, F127 (0.28 g) was dissolved in 20 mL of DI water. Glucose (3 g) in 10 mL of DI water was then added into the above solution. Nano-MoS2 (1 g) was then dispersed in the above solution by an ultrasonic cell disruption system (JY92-IIN ultrasonic
cell pulverizer) for 30 min. The suspension was then heated in a sealed steel autoclave at 180 C for 6 h. A precipitate was recovered, washed with water and ethanol, dried under vacuum at 60 C for 24 h. Then this precipitate was calcined at 700 C for 7 h under argon atmosphere at a heating rate of 2 C min1. 2.3. Characterization Scanning electron microscope (SEM) images were recorded on S4800 (Hitachi, Japan). The samples were prepared by sprinkling the powder materials onto double-sided carbon sticky tape which was mounted on a microscope stub. All samples were coated with a thin gold film under vacuum prior to microscopy. Transmission electron microscope (TEM) images were obtained on a transmission electron microscopy (JEOL JEM 2100F, Japan). Samples for TEM were prepared by dispersing the products in ethanol with 20 min ultrasonicating, and then dropping a few drops of the resulted suspension onto a copper grid precoated with amorphous carbon and allowing them to dry naturally. X-ray powder diffraction (XRD) data were collected on PANALYTICAL (Netherlands) with Cu Ka radiation (l ¼ 0.1540562 nm) from 20 to 80 (2q) at 40 kV and 40 mA. The BrunauerEmmett-Teller (BET) test was determined via an automated surface area and pore size analyzer (SIMP-10/PoreMaster 33, Quantachrome Instruments, USA). The thermogravimetric analysis (TGA) experiment was conducted on a NETZSCH STA 449C (NETZSCH-Gertebau GmbH, Germany) thermogravimetric analyzer in air atmosphere with a flow rate of 30 mL min1 from room temperature to 700 C at a heating rate of 10 C min1. Raman spectra were taken under ambient conditions using a HR800 Confocal Raman system (HORIBA Jobin Yvon, France) with a 532 nm laser radiation source. The electrical conductivity was measured by 2-wire Ohms measurement method with a CHI660D electrochemical workstation (China) [32,33]. 2.4. Electrochemical studies Coin cells (CR2025) were assembled to test the electrochemical performance of the obtained MoS2/C and MoS2 for comparison. The electrodes were prepared by mixing with acetylene black and polyvinylidene difluoride (PVDF) in a weight ratio of 8:1:1 in 1methyl-2-pyrrolidinone solvent to obtain homogeneous slurry. The slurry was coated onto a copper foil current collector and dried at 60 C for 12 h and then pressed to obtain the electrode sheets. The electrode sheets were further dried in a vacuum oven at 110 C for 24 h. The typical loading level of the active materials in MoS2 or MoS2/C cathode is 2e3 mg cm2. The half cells were assembled in an Ar filled glove-box, using 1 mol L1 LiPF6 solution in a 1:1 (V:V) mixture of ethylene carbonate (EC) and dimethyl carbonate (DMC) as electrolyte and Li foil as counter electrode. The cells were charged and discharged on a Shenzhen Neware battery cycler (China) in a voltage range of 0.01e3 V (vs. Liþ/Li) and the specific capacity was calculated on the basis of the whole mass of MoS2 and porous carbon. 3. Results and discussions 3.1. Characterization SEM and TEM were employed to investigate the morphology of the pristine MoS2 sample and MoS2/C composite (Fig. 1). MoS2 appeared as a rose-like particle (200e400 nm) with a 3D hierarchical structure composing of wrinkled nanosheets. After carbonization process, MoS2 particle was coated with a carbon layer with some tiny tumors on its surface. The high-resolution TEM image confirmed that the carbon layer was coated homogeneously on the
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Fig. 1. (a) SEM images and (d) TEM images of MoS2. (b) SEM images, (e) TEM images and (f) HRTEM image of MoS2/C. (c) MoS2/C prepared without using Pluronic F127.
surface of MoS2 particle with an average thickness of ca. 12 nm (Fig. 1def). A control experiment without addition of F127 in the hydrothermal reaction showed that carbon microsphere was generated together with MoS2/C particle (Fig. 1c). These results demonstrate that Pluronic F127 is an essential agent to promote the formation of MoS2/C composite without any carbonaceous impurity, probably due to the chelating effect on MoS2 surface and/or dispersing capability of F127 as a surfactant which inhibits the spontaneous formation of the carbon microspheres during the hydrothermal reaction. The specific surface area determined by BET measurement is 6.85 and 9.55 m2 g1 for MoS2 and MoS2/C, respectively. The slightly increased surface area can be attributed to the carbon tumors on the MoS2/C mentioned above. The crystal structures and crystallinity of as-synthesized MoS2 and MoS2/C were characterized by XRD (Fig. 2). The XRD patterns for both samples displayed the characteristic peaks for hexagonal2H MoS2 at 14.5 (002), 32.7 (100), 33.6 (101), 39.7 (103), and 58.4 (110), which are in agreement with the database in JCPDS file (JCPDS Card No. 37-1492). No obvious difference was observed from the diffraction patterns of MoS2 and MoS2/C, indicating that the hydrothermal process did not change the phase of MoS2 and the coated carbon possessed an amorphous character [13]. Raman spectra were used to study the structure of MoS2 and MoS2/C (Fig. 3a). Both MoS2 and MoS2/C exhibited two typical peaks at 379 and 403 cm1, corresponding to A1g and E2g mode of the hexagonal MoS2 crystal, respectively [34]. The E2g mode refers to the in-layer displacement of Mo and S atoms, whereas A1g mode
Fig. 2. XRD patterns of the MoS2 and MoS2/C.
refers to the out-of-layer symmetric displacements of S atoms along the c axis [35]. The peak frequency difference between A1g and E2g mode (D) is often used to evaluate the number of layers in thin MoS2 crystals [18,36]. The value of D (24 cm1) for MoS2/C implies that the MoS2 nanosheet is very thin, composed of only a few layers of MoS2. There are two other Raman peaks at around 1352 and 1586 cm1 corresponding to the D and G band of carbon, respectively [37]. The actual carbon content in the MoS2/C composite was measured using TGA (Fig. 3b). The weight loss for the pristine MoS2 is about 12.6% over the temperature range from 30 C
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Fig. 3. (a) Raman spectra of MoS2 and MoS2/C; (b) TGA curves of MoS2 and MoS2/C.
to 680 C, while the weight loss of MoS2/C composite is 20.0%. There is nearly no carbon residue in MoS2/C at 680 C. Nevertheless, the retained weight of MoS2 still maintains 87.4%. Therefore, the amount of carbon in MoS2/C was estimated to be 8.5 wt %. 3.2. Electrochemical performances Fig. 4a shows the first three cyclic voltammetry (CV) curves of the MoS2/C composite between 0.01 V and 3.0 V (vs. Liþ/Li) at a scan rate of 0.3 mV s1. Two peaks at 1.05 V and 0.5 V appeared in the 1st cathodic sweep. The peak at ca. 1.05 V corresponds to the intercalation of lithium-ion on different defect sites of MoS2 lattice to form LixMoS2 according to Eq (1). The other peak at ca. 0.5 V corresponds to the conversion reaction process of deposition of Mo metal from LixMoS2 along with the Li2S [38,39]. The whole discharge process can be regarded as a two-step reaction based on the following
equations [9]:
MoS2 þ xLiþ þ xe ¼ Lix MoS2 ðx ¼ 3 4Þ
(1)
Lix MoS2 þ 4Li þ þ4e ¼ 2Li2 S þ Mo=Liy
(2)
In the reverse anodic sweep, the pronounced peak at ca. 2.3 V can be assigned to the oxidation of Li2S into sulfur (S) according to Eq (3) [13,40].
Li2 S 2e ¼ 2Liþ þ S
(3)
Therefore, after the first cycle, the electrode can be regarded actually as a mixture of S and Mo instead of the original MoS2 [9]. In the subsequent cycles, two new reduction peak at about 1.9 V and 1.13 V appeared, which can be explained by a multistep conversion of S and lithium-ion to Li2S [13,41,42]. The main oxidation peak at
Fig. 4. CV curves of MoS2/C composite electrodes at a scan rate of 0.3 mV s1 between 0.01 V and 3.0 V vs. Liþ/Li (a). Discharge/charge profiles of MoS2 and MoS2/C composite electrodes at the first (b) and tenth cycles (c) with a current density of 50 mA g1. 2-wire Ohms measurement of the electric conductivity of MoS2 and MoS2/C (d).
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2.3 V corresponds to the transformation of Li2S to S and/or polysulfides [18,30]. This value is consistent with the thermodynamic potential of S (2.24 V), which is calculated based on the Gibbs energy variation in the reaction of Eq (3) [40]. After the second cycle, the CV curves are almost overlapped, suggesting that the electrochemical properties are stabilized in the subsequent cycles after 2nd cycle. Fig. 4 (b and c) shows the galvanostatic charge-discharge voltage profiles of the half cells of MoS2 and MoS2/C electrode in a voltage range of 0.01e3.0 V at 50 mA g1. A sloping plateau starting from 2.2 to 1.0 V and a flat plateau at about 0.5 V were observed for MoS2/C in the first discharge process (Fig. 4b). This is consistent with the 1st cathodic sweep of CV curve in Fig. 4a. The potential variation in the lithium intercalation can be attributed to the insertion of additional Liþ ions into the expanded MoS2 structure or in the defect sites of MoS2. During the charge process, the defined plateau at about 2.3 V corresponds to the cathodic peak at 2.3 V of CV curves, which resulted from the oxidation of Li2S into sulfur (S) according to Eq (3). The MoS2 electrode showed a similar discharge-charge profile, which manifests the fact that the coated carbon layer did not change the lithium storage mechanism of MoS2. The MoS2/C electrode delivered a higher first discharge and charge capacity of 1181.7/882.6 mA h g1 respectively, as compared with 993.3/ 858.1 mA h g1 for MoS2, primarily due to the increased surface area of MoS2/C and thus more active sites for the insertion/ extraction of Liþ ions. In the first cycle, the irreversible capacity loss is unavoidable for the majority of transition metal sulfides, mainly resulting from the formation of the solid electrolyte interface (SEI), electrolyte degradation, and the structure reorganization of MoS2 [43,44]. A new discharge plateau at 1.5 V appeared in the 10th cycle while the potential plateau at 0.5 V in the first process disappeared (Fig. 4b), indicating that both MoS2 and MoS2/C experienced an irreversible phase transition. In the subsequent charge/discharge process, MoS2/C showed a smaller polarization than MoS2 due to the increased electrical conductivity of the coated carbon layer. Measurements confirmed the higher conductivity of 2.38 102 S cm1for MoS2/C as compared to 6.6 103 S cm1 for MoS2 (Fig. 4d). The cycling performances of MoS2 and MoS2/C were tested at a current density of 100 mA g1 (Fig. 5a, first 3 cycles at 50 mA g1). MoS2 delivered a first charge capacity of 808.7 mA h g1. The capacity quickly decreased to 479.1 mA h g1 after 100 cycles. As a comparison, MoS2/C showed a first charge capacity of 763.2 mA h g1 and a reversible capacity of 631.9 mA h g1 after 100 cycles, exhibiting a better capacity retention of 82.8% as compared with 59.2% for MoS2. The comparative rate performances between MoS2/C and MoS2 are presented in Fig. 5b. The electrodes were
587
Fig. 6. Nyquist plots of MoS2 and MoS2/C composite electrodes (a), and the equivalent circuit used to model the impedance spectra (b). The symbols are the experimental data whereas the continuous lines represent the fitted spectra.
cycled at a current density of 50 mA g1 for the initial 10 cycles. Then, the current density was increased gradually to 500 mA g1 and finally returned to 50 mA g1. The charge capacities of MoS2/C at 50, 100, 200, 300 and 500 mA g1 are 887.8, 793.8, 690.8, 603.6, and 461.6 mA h g1, respectively. MoS2 delivered lower capacities of 853.3, 716.7, 395.0, 295.6 and 228.4 mA h g1 at 50, 100, 200, 300 and 500 mA g1, respectively. Apparently, MoS2/C showed a better rate performance compared with MoS2. This improved cycling and rate performances can be ascribed to the unique rose-like morphology composed of the ultrathin MoS2 nano-sheets and interconnected conductive carbon network. The increased surface area of MoS2/C offers more active sites for electrochemical reactions and increases the contact area between the electrolyte and the electrode material, which is also beneficial for improving the electrochemical performances [45]. MoS2/C in this work exhibits better electrochemical performances our MoS2/Sdoped-carbon reported previously [30]. When comparing with those similar work in literature, our MoS2/C composite also shows better electrochemical performances than MoS2 nanoflake [46], MoS2/carbon nanotube core-shell nanocomposite [47], MoS2/carbon nanowire [48], but somewhat poorer than PEO/MoS2/graphene nanocomposite [9]. To better understand the electrochemical kinetics of MoS2 and MoS2/C, EIS measurements were carried out. Fig. 6 shows the Nyquist impedance plots of these samples and their equivalent circuit model. In the equivalent circuit, Re represents the internal resistance of the test battery, Rsf and CPEsf represend the resistance and constant phase element of the SEI film, respectively. Rct and
Fig. 5. Cyclic performance of MoS2 and MoS2/C composite electrodes at a current density of 100 mA g1 (a) and their rate performance (b).
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CPEct represend charge-transfer resistance and constant phase element of the electrode/electrolyte interface, respectively. Wdif and Cint correspond to the diffusion components of Warburg impedance and the intercalation capacitance, respectively [49]. As shown in Fig. 6a, the high-frequency semicircle is assigned to Rsf and CPEsf, and the semicircle in the medium frequency region corresponds to Rct and CPEct. The inclined line in low frequency range representing the Warburg resistance (Wdif) is assigned to the lithium-diffusion process within the bulk of the electrode material. MoS2/C electrode showed a smaller radius of semi-circle in the Nyquist plots, indicating a lower SEI film resistance and charge-transfer impedance. By fitting the impedance data based on the EIS equivalent circuit model in Fig. 6b, Rsf and Rct for MoS2/C are 4.78 U and 16.46 U respectively, smaller than 5.84 U and 39.76 U for MoS2. This fact confirms that both SEI film resistance and charge-transfer resistance of MoS2/C electrode are smaller than those of MoS2 electrode. Based on the above results, the carbon coating increases the conductivity of MoS2 and greatly enhances rapid electron transport during the lithium insertion/extraction reaction, thus resulting in huge improvement of electrochemical performances. 4. Conclusion Nanostructured MoS2/C composite with a rose-like morphology was synthesized through a Pluronic F127-promoted hydrothermal process using glucose as a carbon source in presence of MoS2 nanoparticles followed by a subsequent carbonization process. MoS2/C composite has a 3D hierarchical structure composed of wrinkled nanosheets with some tiny carbon tumors. Pluronic F127 inhibited the spontaneous formation of carbon microspheres in our two-step synthesis of MoS2/C composite. The MoS2/C composite showed favorable electrochemical kinetics due to the incorporation of conductive carbon, which suppressed the large volumetric change of MoS2 during charge/discharge process and increased the electrical conductivity of MoS2. Therefore, MoS2/C exhibited a remarkable improved cycling stability and rate capability, delivering a reversible capacity of 882.6/461.6 mA h g1 at a current density of 50/500 mA g1 and retaining a capacity of 82.8% at 100 mA g1 after 100 cycles. Acknowledgements This work was supported by National Natural Science Foundation of China (21573239), Guangdong Provincial Project for Science & Technology (2014TX01 N014/2014A050503050/2015B010135008), Guangzhou Municipal Project for Science & Technology (2014Y200219/201509010018) and Dongguan Municipal Project for Science & Technology (2013509104210). LZ thanks the Distinguished Scientist Program of Guangxi Province. References [1] P.G. Bruce, B. Scrosati, J.M. Tarascon, Nanomaterials for rechargeable lithium batteries, Angew. Chem. Int. Ed. 47 (2008) 2930e2946. [2] L.W. Ji, Z. Lin, M. Alcoutlabi, X.W. Zhang, Recent developments in nanostructured anode materials for rechargeable lithium-ion batteries, Energy Environ. Sci. 4 (2011) 2682e2699. [3] C.Z. Yuan, H.B. Wu, Y. Xie, X.W. Lou, Mixed transition-metal oxides: design, synthesis, and energy-related applications, Angew. Chem. Int. Ed. 53 (2014) 1488e1504. [4] D.L. Ma, Z.Y. Cao, A.M. Hu, Si-based anode materials for Li-Ion batteries: a mini review, Nano-Micro Lett. 6 (2014) 347e358. [5] J.B. Goodenough, K.S. Park, The Li-Ion rechargeable battery: a perspective, J. Am. Chem. Soc. 135 (2013) 1167e1176. [6] X.W. Wu, X.H. Li, Z.X. Wang, H.J. Guo, P. Yue, Y.H. Zhang, Improvement on the storage performance of LiMn2O4 with the mixed additives of ethanolamine and heptamethyldisilazane, Appl. Surf. Sci. 268 (2013) 349e354. [7] Q.S. Gao, L.C. Yang, X.C. Lu, J.J. Mao, Y.H. Zhang, Y.P. Wu, Y. Tang, Synthesis, characterization and lithium-storage performance of MoO2/carbon hybrid
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