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Enhancing the cycling stability of Na-ion batteries by bonding MoS2 on assembled carbon-based materials
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Enhancing the cycling stability of Na-ion batteries by bonding MoS2 on assembled carbon-based materials Pin Song a, 1, Jun Di a, 1, Lixing Kang a, 1, Manzhang Xu a, Bijun Tang a, Jun Xiong b, Jiewu Cui c, Qingsheng Zeng a, Jiadong Zhou a, Yongmin He a, Qundong Fu a, Juan Peng d, Shasha Guo a, Bo Lin a, Jingyu Zhang a, Peng Meng a, Zheng Liu a, * a
School of Materials Science & Engineering, Nanyang Technological University, Singapore, 639798, Singapore Institute for Energy Research, Jiangsu University, 301 Xuefu Road, Zhenjiang, 212013, China c School of Materials Science and Engineering, Hefei University of Technology, Hefei, 230009, China d Stage Key Laboratory of High-efficiency Coal Utilization and Green Chemical Engineering, School of Chemistry and Chemical Engineering, Ningxia University, Yinchuan, 750021, China b
A R T I C L E I N F O
A B S T R A C T
Keywords: Na-ion batteries Carbon-based materials MoS2 Long cycle life
Room temperature Na-ion batteries (SIBs) show great potential for use as renewable energy storage systems. However, the large-scale application of SIBs has been hindered by the lack of an ideal SIBs anode material. We synthesized MoS2 on carbonized graphene-chitosan (G-C) using the hydrothermal method. The strong interaction between the MoS2 and the G-C greatly improved the electron transport rate and maintained the structural stability of the electrode, which lead to both an excellent rate capability and long cycle stability. The G-C monolith was proven to enhance the electrical conductivity of the composites and served as a matrix for uniformly dispersing active MoS2 nanosheets (NSs), as well as being a buffer material to adapt to changes in volume during the cycle. Serving as an anode material for SIBs, the MoS2-G-C electrode showed good cycling stability (527.3 mAh g1 at 100 mA g1 after 200 cycles), excellent rate capability, and a long cycle life (439.1 mAh g1 at 1A g1 after 200 cycles).
1. Introduction Lithium ions have been used for the past three decades. As an element of the same group, sodium offers a similar electrochemical performance [1–7]. Considering the abundant resources available and the low cost of sodium, SIBs have therefore attracted more significant research interest in recent years than commercial lithium ion batteries (LIBs) for practical applications [8,9]. However, many anode materials that are suitable for use with lithium ions lose their excellent electrochemical properties when applied to sodium ions. For example, when graphite is used as the anode material of SIBs [10–13], its capacity is lower than 35 mA h g1. Although metals (Sb, Sn) or metal oxides (Fe2O3, SnO2) [14–18] offer higher capacities, the resulting cycling performance was poor due to large volume variations in the charge and discharge processes. These electrochemical problems result from the larger radius of sodium ions (Naþ) as compared to the smaller lithium (Liþ). Therefore, developing a large number of anode materials that offer both high capacity and long
cycling performance for SIBs is of great importance and urgently required. Layered metal sulfides such as MoS2, WS2, SnS2 and SnS have been widely studied in various fields [19–23]. Due to their large interlayer spacing, they transport sodium ions more effectively than lithium ions [12,24–27]. However, the low conductivity of layered metal sulfides results in relatively poor electrochemical performances. In addition, although the structure of metal sulfides can compensate for the volume variations in sodium ion insertion-extraction processes [26], the electrodes will eventually be damaged because of the inevitable volume changes. The abovementioned phenomenon indicates that the use of layered metal sulfides as anodes for SIBs still remains challenging. Recently, work combining metal sulfides with carbon materials to improve electrochemical performance has been reported [3,7,28–34]. Carbon materials can be used as the matrix for layered materials due to carbon’s good electrical conductivity and outstanding flexibility, which can greatly enhance the layered materials’ electrical conductivity,
* Corresponding author. E-mail address: [email protected] (Z. Liu). 1 These authors contributed equally to this work. https://doi.org/10.1016/j.nanoms.2019.09.001 Received 1 July 2019; Accepted 23 August 2019 Available online xxxx 2589-9651/© 2019 Chongqing University. Production and hosting by Elsevier B.V. on behalf of KeAi. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
Please cite this article as: P. Song et al., Enhancing the cycling stability of Na-ion batteries by bonding MoS2 on assembled carbon-based materials, Nano Materials Science , https://doi.org/10.1016/j.nanoms.2019.09.001
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performance and cyclic voltammetry (CV) tests were conducted by an electrochemical workstation (PG302N) with the potential range of 0.01–3 V. The electrochemical impedance spectra of the synthesized samples were recorded with a frequency range of 0.1–105 Hz and an amplitude of 5 mV. All electrochemical measurements were taken at room temperature.
prevent agglomeration and cushion mechanical changes. MoS2 is a layered structure bonded by van der Waals forces. Its structure is favorable for the insertion-extraction of Naþ. Additionally, its fast charge transfer rate and its excellent structural stability as an electrode can be achieved by realizing a strong coupling between MoS2 and carbon materials. Therefore, developing a simple and reasonable synthetic route to bind MoS2 to carbon materials is highly desirable to create high-performance SIBs. To this end, a highly ordered hierarchical structure of MoS2-graphene-chitosan (MoS2-G-C) composites with excellent electrochemical performances has been obtained via a simple hydrothermal approach.
3. Results and discussions The morphology of the synthesized MoS2 is shown in Fig. S1. Pristine MoS2 exhibits a large number of units (Supporting Information, Fig. S1a). The structural characteristic of MoS2 was further observed by TEM (Supporting Information, Fig. S1b). Ultra-small sized nanosheets could greatly shorten the diffusion distance of Naþ, and further enhance the material/electrolyte interface reaction activity during charge/discharge processes [36–39]. Therefore, inspired by the elastic structure of macroscale, we have created elastic carbonized graphene-chitosan (G-C) composite, which is composed of microscale structures and arranged in a three-dimensional hierarchical structure. To obtain the ideal hierarchical structure, we have adopted the unidirectional freezing method to prepare the layered GO-CS scaffold with the uniformly dispersed GO and CS suspension as raw materials (Supporting Information, Fig. S4, Fig. S5). It can be observed that the size of the GO sheets was reduced significantly after ultrasonic treatment (Supporting Information, Figs. S4a–b), and then dispersed uniformly in the CS solution (Supporting Information, Figs. S4c–d). The FTIR spectra shows the interaction between the GO and the CS after mixing (Supporting Information, Fig. S4e). The characteristic peaks located at 1730 – O stretching of and 1630 cm1 in the GO spectrum correspond to the C– –COOH– and the deformation of the O–H bond in water. The characteristic peaks presented at 1651 and 1596 cm1 in the spectrum of CS are – O stretching vibration of –NHCO– and the N–H attributed to the C– bending of –NH2. It is worth noting that in the CS-GO spectrum, the – O stretch at 1730 cm1 was not observed. In characteristic peak of C– – O stretching vibration at addition, both characteristic peaks of C– 1651 cm1 and N–H bending at 1596 cm1 are shifted to the lower wavenumbers. These results suggest that there is hydrogen bonding interaction between GO and CS [40]. Through this procedure (Fig. 1a), the three-dimensional GO-CS scaffold, G-C monolith and MoS2-G-C composite composed in parallel can be easily obtained (Fig. 1b-g, Supporting Information, Fig. S2). The GOCS scaffold was assembled into a three-dimensional structure by using the unidirectional freezing method (Fig. 1b, e, Supporting Information, Fig. S2a, Fig. S2d) [41,42]. After annealing, the three-dimensional structure can be preserved (Fig. 1c, f, Supporting Information, Fig. S2b, Fig. S2e). The SEM images (Fig. 1d, g, Supporting Information, Fig. S2c, Fig. S2f) show that MoS2 NSs can be uniformly distributed on the G-C monoliths. The energydispersive X-ray spectroscopy (EDX) analysis revealed the element composition of the MoS2-G-C composite (Supporting Information, Fig. S3). The C, O, Mo, and S elements were identified in the targeted region. These results indicate that the atomic ratio of S and Mo is approximately 2:1, which is in good agreement with the expected value (Supporting Information, Table S1). In previous works, GO has been used as the substrate on which MoS2 NSs can generally uniformly grow onto the surface [43–47]. Chen et al. [47] have reported that the carbon atom in GO co-shares the electron cloud with the sulfur atom in MoS2. Herein, our results have demonstrated that MoS2 NSs can grow on the G-C surface. During the annealing process, the volume shrinkage of CS is much larger than the shrinkage of GO; this is due to the loss of a large amount of relative material in the CS matrix (Supporting Information, Fig. S5, Fig. S6). At the same time, GO is reduced during annealing, and CS is carbonized and becomes amorphous carbon [48,49]. The result of the elemental analysis shows that the total carbon content of G-C monolith is 76.87 wt% (Supporting Information, Fig. S7). Considering that the initial
2. Experimental details Material sources: All reagents used in this work were procured from Sinopharm Chemical Reagent Company and Shanghai Aladdin Bio-Chem Technology Co.,LTD. Fabrication of the GO-CS scaffolds: Graphene oxide (GO) was obtained by an improved Hummers’ method of oxidizing natural graphite powder [35]. The obtained GO was further processed with ultrasonic waves for 10 min (500 W, JY92-IID). A chitosan (CS) solution with a deacetylation degree of 95% was obtained by dissolving chitosan powder into an acetic aqueous solution (2 wt%). Next, GO and CS were mixed together and underwent ultrasonic treatment for 30 min to ensure the GO sheets were sufficiently dispersed in the CS matrix. The GO–CS suspension was then poured into a rectangular silicone mold on a steel plate, and the steel plate was inserted into liquid nitrogen to create the temperature gradient on its surface. As a result, the ice nucleus grew along the temperature gradient and formed parallel ice columns with a highly ordered alignment. After the sample was completely frozen, it was further freeze dried. G–C monoliths: The GO–CS scaffolds were annealed in a furnace with argon, heated from 25 C to 500 C with a heating rate of 2 C min1 and kept at 500 C for 1 h, and then tuned from 500 C to 800 C with a heating rate of 5 C min1 and kept at 800 C for 2 h. Finally, G–C monoliths were obtained. MoS2-G-C composite: First, 1g of ammonium molybdate, 1.8g of thiourea and 30 ml of water were added to a Teflon-lined autoclave (50 ml) and stirred sufficiently on a magnetic agitator. Second, the prepared G–C monoliths were added to the mixture and soaked for 24 h, and reacted at 180 C for 48 h. After cooling naturally, the sample was washed several times with deionized water and ethanol, and then dried at 60 C for 8 h. Finally, the MoS2-G-C composite was obtained. Characterization: The morphology of the samples was characterized using a scanning electron microscope (SEM) (Zeiss Supra 40, Germany) at an accelerating voltage of 10 kV. A TEM image was obtained using a Hitachi H7700 TEM. Fourier transform infrared spectroscopy (FT-IR) was carried out from 4000 to 400 cm1 wavenumber at 25 C (Nicolet 6700). The chemical composition of samples was determined by X-ray photoelectron spectroscopy (XPS) (ESCALAB MKII). Compression tests were conducted on an Instron 5565A at the speed of 10 mm/min. The electrical conductivity of samples was measured using a two-probe method. Electrochemical performance measurements: The electrochemical performance was evaluated with half cells. The working electrodes were mixed with an as-synthesized sample (active material), acetylene black (conductive agent) and polyvinylidene fluoride (binder) at a weight ratio of 8:1:1 to form a slurry. Then the slurry was coated uniformly onto a copper foil. After being vacuum dried at 60 C overnight, the electrodes were punched into circular discs. The electrode had a diameter of 12 mm and an active material loading of ~2 mg cm2. Swagelok-type batteries were assembled in an argon-filled glovebox by pairing the abovementioned working electrode with sodium metal as the counter electrode. A carbonate electrolyte (160 μl) consisting of 1 M NaClO4 in ethylene carbonate/dimethyl carbonate (1:1 v/v) (Zhangjiagang Guotai Huarong New Chemical Materials Co., Ltd.) was used. Glass fibers (GF/D) from Whatman were used as the separator. Galvanostatic cycle 2
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Fig. 1. Hierarchical structure and morphology characterization of MoS2-G-C composite. (a) Schematic diagram for the preparation of MoS2-G-C composite. Side-view SEM images of (b) GO-CS, (c) G-C, (d) MoS2-G-C. (e–g) High magnification SEM images of (b–d).
process (Supporting Information, Fig. S9a). However, it changed into G-C monolith after annealing, displaying excellent elasticity. When the compression strain is 50%, G-C monolith has shown strong resilience, and can completely recover to its original height without plastic deformation, yielding or lateral displacement after the compression process (Supporting Information, Fig. S9b). Note that even the mechanical properties of MoS2-G-C composites are not significantly changed compared to the G-C monolith (Supporting Information, Figs. S9c–d). Based on the above characteristics, we have obtained a threedimensional hierarchical MoS2-G-C composite by a simple
CS content in GO-CS scaffold is 50 wt% (Supporting Information, Fig. S7), and based on the thermos gravimetric analysis results (Supporting Information, Fig. S6b), we believe that the carbon constituent is mainly amorphous carbon from carbonized CS. It is worth noting that G-C monoliths have shown different electrical conductivities due to their anisotropic structure, where the value along the ice crystal’s growth direction is 8.9 S m1 and the value perpendicular to the ice crystal’s growth direction is 2.1 S m1 (Supporting Information, Fig. S8). As expected, the GO-CS has shown weak resilience, and thus could not completely recover to its original height after the compression 3
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Fig. 2. Schematic illustration of the synthesis of MoS2-G-C composite.
CV measurements at a scan rate of 0.1 mV/s for the initial five cycles were conducted to evaluate the electrochemical performance of the MoS2-G-C composite as an electrode (Fig. 4a). During the discharge process, the reduction peak was observed at 0.88 V, which is due to the Naþ ions inserted into 2H–MoS2 crystals. Another reduction peak appears at 0.6 V, which corresponds to the conversion reaction between Naþ ions and 2H-NaxMoS2. During the charging process, the oxidation peak was detected at 1.6 V, which is related to the reverse conversion reaction between the metallic Mo and Na2S matrix. Fig. 4b shows the static charge/discharge curves of the MoS2-G-C electrode at 100 mA g1 with different cycle numbers, the MoS2-G-C electrode after 100th cycles still displays a similar charge/discharge curve to that at the 2nd cycle. For comparison, the CV measurement and charge/discharge curves of the original MoS2 electrode were performed and analyzed (Fig. S10). The charge/discharge platform of the MoS2 electrode diminished significantly along with the increasing number of cycles. However, except for the initial cycle, the MoS2-G-C electrode demonstrates nearly overlapping charge/discharge curves, indicating that it has both an excellent electrochemical cycle stability and reversibility. All of the above results suggest that the MoS2-G-C electrode offers an excellent electrochemical performance attributed to its highly conductive carbon network and stable hierarchical structure. As shown in Fig. 5a, we evaluated the cycle performance of the MoS2G-C electrode. The respective cycle performance data of the MoS2-G, MoS2–C and MoS2 electrodes have been included for comparison. The initial charge/discharge capacities of the four electrodes are 536.9/ 998.9, 435.2/850, 408.7/849.3 and 314.6/736.4 mAh g1 under 100 mAg1 current density, respectively, and correspond to coulombic efficiencies of 53.7, 51.2, 48.1 and 42.7%. The improvement of the coulombic efficiency and the increase of electrode capacity are attributed to the 3D conductive network of G-C and the interfacial interaction of the C–O–Mo bonds between MoS2 and G-C. It has been reported that the irreversible capacity loss of the first cycle is inevitable due to the formation of the SEI layer [44]. After 200 cycles, the capacity of MoS2-G, MoS2–C and MoS2 electrodes declined rapidly to 310.9, 184.4 and 135.9
hydrothermal method, as shown in Fig. 2. During the initial reaction, G-C monolith surface formed MoOx crystal nuclei, and further reacted with CH3CSNH2 (S source and reductant), and MoS2 nanocrystals were eventually obtained [50]. Here, the G-C monolith serves as a substrate, and MoS2 can nucleate and grow on the surface during the hydrothermal process. MoS2 consists of an Mo layer and two S layers, and the Mo atoms are completely exposed at the end of the MoS2 [51,52]. Therefore, considering the layered structure of MoS2, it is reasonable to assume that the C–O–Mo bonds can be formed between the Mo atom and carbon materials. Moreover, it can also be seen that MoS2 NSs are easier to grow on the carbon materials ascribed to the influence of C–O–Mo bonds. The interface between the MoS2 and carbon materials was examined by XPS. The composite consisted of Mo, S, C and O elements (Fig. 3a). Fig. 3b shows two characteristic peaks at 228.6 eV and 231.9 eV, which correspond to the binding energy of Mo 3d5/2 and Mo 3d3/2, respectively [53,54], from the Mo4þ in MoS2. In addition, a small characteristic peak at 225.8 eV can be observed, corresponding to the S 2s component. The characteristic peak at 162.5 eV and 161.3 eV (Fig. 3c) are ascribed to S 2p1/2 and S 2p3/2 [53]. As shown in Fig. 3d, the C 1s characteristic peak – C/C–C, while another characteristic peak at 285.1 eV corresponds to C– of the oxygen-containing functional group occurring at 286.8 eV corresponds to the C–O [55]. Relatively weak C(O)–O characteristic peaks indicate that GO sheets were reduced to graphene. It is worth noting that the spectrum of O 1s shows a new characteristic peak appearing at 531.9 eV, which was different from the peak at 533.3 eV for the C–OH (Fig. 3e). It has been reported that the O 1s characteristic peak of the C–O-metal bond is approximately 530–533 eV [55–58]. Therefore, from the above results, we could conclude that the C–O–Mo bond is presented in the MoS2-G-C composite. Fig. 2 has demonstrated the illustration of a successful synthesis process of MoS2-G-C composite. It has been reported that the covalent C–O-M bonding between MxOy (M ¼ Fe, Co) and GO can greatly enhance the electron transport rate and maintain the structural stability of the MxOy/graphene electrode [55–58]. Therefore, considering the strong interfacial interaction of the C–O–Mo bonding between the MoS2 and carbon materials, it is promising to design the MoS2-G-C composite as an anode material for SIBs.
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Fig. 3. XPS spectra for MoS2-G-C composite (a) survey spectra, (b) Mo 3d spectra, (c) S 2p spectra, (d) C 1s spectra, (e) O 1s spectra.
mAh g1, respectively. This is primarily due to their volume changes, which lead to severe structure damage and the pulverization of their active material. On the contrary, the MoS2-G-C electrode provided a very stable and highly reversible capacity of 527.3 mAh g1 after 200 cycles, with negligible capacity loss. This value is much higher than those reported in the literature (Supporting Information, Table S2) and suggests that this method is very feasible for practical applications. These results indicate that the sodium storage capacity of the MoS2-G-C electrode has been significantly enhanced as a result of the synergetic effect among MoS2, G and C. High ratio performance is also an important aspect for the high power type of SIBs. Compared with G-MoS2, C–MoS2 and MoS2, the MoS2-G-C electrode showed a superior rate capability (Fig. 5b). We have measured
the charge capacity of the MoS2-G-C electrode, which is 981.5, 489.5, 446.5, 391.4, 356.5, 316.5 and 262.1 mAh g1 at 100, 200, 500, 1000, 2000, 5000 and 10000 mA g1, respectively. In contrast, the capacities of the MoS2-G, MoS2–C and MoS2 electrodes decline rapidly with increasing rate. Fig. 5c shows that MoS2-G-C electrode exhibits an excellent longterm cycling performance; with the charge/discharge at 1 A g1 for 100 cycles, the MoS2-G-C electrode still retained a reversible capacity of 439.1 mAh g1. Ascribed to its highly ordered three-dimensional conductive network of carbon material and strong interfacial interaction of C–O–Mo bonds, the MoS2-G-C electrode has an excellent rate performance. To further understand the electrochemical kinetics of the MoS2-G-C electrode, we measured the electrochemical impedance by spectroscopy. 5
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Fig. 4. (a) CV curves of MoS2-G-C electrode for the initial 5 cycles, (b) Charge/discharge curves of MoS2-G-C electrode at 100 mA g1 for different cycles.
Fig. 5. (a) Cycling performance at 100 mA g1 for MoS2-G-C, MoS2-G, MoS2–C and MoS2 electrodes. (b) Rate capability for MoS2-G-C, MoS2-G, MoS2–C and MoS2 electrodes. (c) Cycling performance at 1 A g1 for MoS2-G-C, MoS2-G, MoS2–C and MoS2 electrodes. (d) Nyquist plots of MoS2-G-C, MoS2-G, MoS2–C and MoS2 electrodes.
Fig. 5d shows the Nyquist plots of the MoS2-G-C, MoS2-G, MoS2–C and MoS2 electrodes at 100 mA g1 after 200 cycles. Both Nyquist plots consist of a semicircle in the high-frequency region and a sloping line in the low-frequency region. The resistance (R) of the MoS2-G-C electrode is significantly smaller than those of the MoS2-G, MoS2–C and MoS2 electrodes (Fig. 5d), indicating that the incorporation of graphene and chitosan can greatly promote charge transfer in the electrochemical reaction, thus leading to excellent rate performance.
materials, which is attributed to the direct coupling of the edge Mo of the MoS2 with oxygen from the functional groups of carbon materials (C–O–Mo bonds). The hierarchical structure of our MoS2-G-C composite can effectively prevent the agglomeration of MoS2 and the restacking of graphene during the charge/discharge processes. The MoS2-G-C electrode has a high reversible capacity, excellent rate capability and long cycle life when used as SIBs anode material. Specifically, the MoS2-G-C electrode showed an outstanding reversible capacity of 527.3 mAh g1 at 100 mA g1, retained 52.7% of its initial reversible capacity, and demonstrated a long, stable cycle life (439.1 mAh g1) at 1 A g1 after 200 cycles. Ascribed to the presence of a large number of active sites, MoS2 NSs can grow on graphene sheets, and the MoS2-G-C electrode exhibited an excellent electrochemical performance. As an active
4. Conclusions In conclusion, we have successfully designed a hierarchically structured MoS2-G-C composite. MoS2 NSs are apt to grow on carbon
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material, the nanoscale sized MoS2 NSs ensured rapid electrolyte penetration and shortened the sodium ions’ diffusion distance. In addition, the formation of C–O–Mo bonds promoted rapid electron propagation from carbon to MoS2 NSs, resulting in a high reversible capacity and an excellent rate performance of the electrode materials. Therefore, this MoS2-G-C composite with its hierarchical structure is a very promising SIBs anode material with high electrochemical performance.
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Acknowledgements This work was financially supported by the Singapore National Research Foundation under NRF RF Award No. MOE2016-T2-1-131, Tier 1 2017-T1-001-075, MOE2018-T3-1-002. Appendix A. Supplementary data Supplementary data to this article can be found online at https ://doi.org/10.1016/j.nanoms.2019.09.001. References [1] C. Luo, Y. Zhu, Y. Xu, et al., Graphene oxide wrapped croconic acid disodium salt for sodium ion battery electrodes, J. Power Sources 250 (2014) 372–378. [2] D. Sun, D. Ye, P. Liu, et al., MoS2/Graphene nanosheets from commercial bulky MoS2 and graphite as anode materials for high rate sodium-ion batteries, Adv. Energy Mater. 8 (2018) 1702383–1702393. [3] X. Xie, T. Makaryan, M. Zhao, et al., MoS2 nanosheets vertically aligned on carbon paper: a freestanding electrode for highly reversible sodium-ion batteries, Adv. Energy Mater. 6 (2016) 1502161–1502167. [4] J. Li, W. Qin, J. Xie, et al., Rational design of MoS2-reduced graphene oxide sponges as free-standing anodes for sodium-ion batteries, Chem. Eng. J. 332 (2018) 260–266. [5] Y. Jiang, Y. Guo, W. Lu, et al., Rationally incorporated MoS2/SnS2 nanoparticles on graphene sheets for lithium-ion and sodium-ion batteries, ACS Appl. Mater. Interfaces 9 (2017) 27697–27706. [6] P. Li, J.Y. Jeong, B. Jin, et al., Vertically oriented MoS2 with spatially controlled geometry on nitrogenous graphene sheets for high-performance sodium-ion batteries, Adv. Energy Mater. 8 (2018) 1703300–1703307. [7] Changbao Zhu, Xiaoke Mu, A. Peter, van Aken, et al., Single-layered ultrasmall nanoplates of MoS2 embedded in carbon nanofibers with excellent electrochemical performance for lithium and sodium storage, Angew. Chem. Int. Ed. 53 (2014) 2152–2156. [8] P. Barpanda, G. Liu, C.D. Ling, et al., Na2FeP2O7: a safe cathode for rechargeable sodium-ion batteries, Chem. Mater. 25 (2013) 3480–3487. [9] Y. Zhang, P. Zhu, L. Huang, et al., Few-layered SnS2 on few-layered reduced graphene oxide as Na-ion battery anode with ultralong cycle life and superior rate capability, Adv. Funct. Mater. 25 (2015) 481–489. [10] Y. Wen, K. He, Y. Zhu, et al., Expanded graphite as superior anode for sodium-ion batteries, Nat. Commun. 5 (2014) 4033–4042. [11] P. GE, M. Fouletier, Electrochemical intercalation of sodium in graphite, Solid State lonics, Diffusion & Reactions 28 (1988) 1172–1175. [12] X. Xie, Z. Ao, D. Su, et al., MoS2/Graphene composite anodes with enhanced performance for sodium-ion batteries: the role of the two-dimensional heterointerface, Adv. Funct. Mater. 25 (2015) 1393–1403. [13] D.A. Stevens, J.R. Dahn, The mechanisms of lithium and sodium insertion in carbon materials, J. Electron. Soc. 148 (2001) A803–A811. [14] L. Liang, Y. Xu, C. Wang, et al., Large-scale highly ordered Sb nanorod array anodes with high capacity and rate capability for sodium-ion batteries, Energy Environ. Sci. 8 (2015) 2954–2962. [15] Yihang Liu, Yunhua Xu, Yujie Zhu, et al., Tin-coated viral nanoforests as sodium-ion battery anodes, ACS Nano 7 (2013) 3627–3634. [16] Y.-X. Wang, Y.-G. Lim, M.-S. Park, et al., Ultrafine SnO2 nanoparticle loading onto reduced graphene oxide as anodes for sodium-ion batteries with superior rate and cycling performances, J. Mater. Chem. A 2 (2014) 529–534. [17] Z. Jian, B. Zhao, P. Liu, et al., Fe2O3 nanocrystals anchored onto graphene nanosheets as the anode material for low-cost sodium-ion batteries, Chem. Commun. 50 (2014) 1215–1217. [18] L. Wang, K. Zhang, Z. Hu, et al., Porous CuO nanowires as the anode of rechargeable Na-ion batteries, Nano Research 7 (2014) 199–208. [19] C. Zhu, X. Mu, P.A. van Aken, et al., Single-layered ultrasmall nanoplates of MoS2 embedded in carbon nanofibers with excellent electrochemical performance for lithium and sodium storage, Angew. Chem. Int. Ed. 53 (2014) 2152–2156. [20] L. Ma, Y. Hu, G. Zhu, et al., In situ thermal synthesis of inlaid ultrathin MoS2/ graphene nanosheets as electrocatalysts for the hydrogen evolution reaction, Chem. Mater. 28 (2016) 5733–5742. [21] J. Liang, G. Zhu, C. Wang, et al., MoS2-Based all-purpose fibrous electrode and selfpowering energy fiber for efficient energy harvesting and storage, Adv. Energy Mater. 7 (2017) 1601208–1601216.
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Enhancing the cycling stability of Na-ion batteries by bonding MoS2 on assembled carbon-based materials Pin Song a, 1, Jun Di a, 1, Lixing Kang a, 1, Manzhang Xu a, Bijun Tang a, Jun Xiong b, Jiewu Cui c, Qingsheng Zeng a, Jiadong Zhou a, Yongmin He a, Qundong Fu a, Juan Peng d, Shasha Guo a, Bo Lin a, Jingyu Zhang a, Peng Meng a, Zheng Liu a, * a
School of Materials Science & Engineering, Nanyang Technological University, Singapore, 639798, Singapore Institute for Energy Research, Jiangsu University, 301 Xuefu Road, Zhenjiang, 212013, China c School of Materials Science and Engineering, Hefei University of Technology, Hefei, 230009, China d Stage Key Laboratory of High-efficiency Coal Utilization and Green Chemical Engineering, School of Chemistry and Chemical Engineering, Ningxia University, Yinchuan, 750021, China b
A R T I C L E I N F O
A B S T R A C T
Keywords: Na-ion batteries Carbon-based materials MoS2 Long cycle life
Room temperature Na-ion batteries (SIBs) show great potential for use as renewable energy storage systems. However, the large-scale application of SIBs has been hindered by the lack of an ideal SIBs anode material. We synthesized MoS2 on carbonized graphene-chitosan (G-C) using the hydrothermal method. The strong interaction between the MoS2 and the G-C greatly improved the electron transport rate and maintained the structural stability of the electrode, which lead to both an excellent rate capability and long cycle stability. The G-C monolith was proven to enhance the electrical conductivity of the composites and served as a matrix for uniformly dispersing active MoS2 nanosheets (NSs), as well as being a buffer material to adapt to changes in volume during the cycle. Serving as an anode material for SIBs, the MoS2-G-C electrode showed good cycling stability (527.3 mAh g1 at 100 mA g1 after 200 cycles), excellent rate capability, and a long cycle life (439.1 mAh g1 at 1A g1 after 200 cycles).
1. Introduction Lithium ions have been used for the past three decades. As an element of the same group, sodium offers a similar electrochemical performance [1–7]. Considering the abundant resources available and the low cost of sodium, SIBs have therefore attracted more significant research interest in recent years than commercial lithium ion batteries (LIBs) for practical applications [8,9]. However, many anode materials that are suitable for use with lithium ions lose their excellent electrochemical properties when applied to sodium ions. For example, when graphite is used as the anode material of SIBs [10–13], its capacity is lower than 35 mA h g1. Although metals (Sb, Sn) or metal oxides (Fe2O3, SnO2) [14–18] offer higher capacities, the resulting cycling performance was poor due to large volume variations in the charge and discharge processes. These electrochemical problems result from the larger radius of sodium ions (Naþ) as compared to the smaller lithium (Liþ). Therefore, developing a large number of anode materials that offer both high capacity and long
cycling performance for SIBs is of great importance and urgently required. Layered metal sulfides such as MoS2, WS2, SnS2 and SnS have been widely studied in various fields [19–23]. Due to their large interlayer spacing, they transport sodium ions more effectively than lithium ions [12,24–27]. However, the low conductivity of layered metal sulfides results in relatively poor electrochemical performances. In addition, although the structure of metal sulfides can compensate for the volume variations in sodium ion insertion-extraction processes [26], the electrodes will eventually be damaged because of the inevitable volume changes. The abovementioned phenomenon indicates that the use of layered metal sulfides as anodes for SIBs still remains challenging. Recently, work combining metal sulfides with carbon materials to improve electrochemical performance has been reported [3,7,28–34]. Carbon materials can be used as the matrix for layered materials due to carbon’s good electrical conductivity and outstanding flexibility, which can greatly enhance the layered materials’ electrical conductivity,
* Corresponding author. E-mail address: [email protected] (Z. Liu). 1 These authors contributed equally to this work. https://doi.org/10.1016/j.nanoms.2019.09.001 Received 1 July 2019; Accepted 23 August 2019 Available online xxxx 2589-9651/© 2019 Chongqing University. Production and hosting by Elsevier B.V. on behalf of KeAi. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
Please cite this article as: P. Song et al., Enhancing the cycling stability of Na-ion batteries by bonding MoS2 on assembled carbon-based materials, Nano Materials Science , https://doi.org/10.1016/j.nanoms.2019.09.001
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performance and cyclic voltammetry (CV) tests were conducted by an electrochemical workstation (PG302N) with the potential range of 0.01–3 V. The electrochemical impedance spectra of the synthesized samples were recorded with a frequency range of 0.1–105 Hz and an amplitude of 5 mV. All electrochemical measurements were taken at room temperature.
prevent agglomeration and cushion mechanical changes. MoS2 is a layered structure bonded by van der Waals forces. Its structure is favorable for the insertion-extraction of Naþ. Additionally, its fast charge transfer rate and its excellent structural stability as an electrode can be achieved by realizing a strong coupling between MoS2 and carbon materials. Therefore, developing a simple and reasonable synthetic route to bind MoS2 to carbon materials is highly desirable to create high-performance SIBs. To this end, a highly ordered hierarchical structure of MoS2-graphene-chitosan (MoS2-G-C) composites with excellent electrochemical performances has been obtained via a simple hydrothermal approach.
3. Results and discussions The morphology of the synthesized MoS2 is shown in Fig. S1. Pristine MoS2 exhibits a large number of units (Supporting Information, Fig. S1a). The structural characteristic of MoS2 was further observed by TEM (Supporting Information, Fig. S1b). Ultra-small sized nanosheets could greatly shorten the diffusion distance of Naþ, and further enhance the material/electrolyte interface reaction activity during charge/discharge processes [36–39]. Therefore, inspired by the elastic structure of macroscale, we have created elastic carbonized graphene-chitosan (G-C) composite, which is composed of microscale structures and arranged in a three-dimensional hierarchical structure. To obtain the ideal hierarchical structure, we have adopted the unidirectional freezing method to prepare the layered GO-CS scaffold with the uniformly dispersed GO and CS suspension as raw materials (Supporting Information, Fig. S4, Fig. S5). It can be observed that the size of the GO sheets was reduced significantly after ultrasonic treatment (Supporting Information, Figs. S4a–b), and then dispersed uniformly in the CS solution (Supporting Information, Figs. S4c–d). The FTIR spectra shows the interaction between the GO and the CS after mixing (Supporting Information, Fig. S4e). The characteristic peaks located at 1730 – O stretching of and 1630 cm1 in the GO spectrum correspond to the C– –COOH– and the deformation of the O–H bond in water. The characteristic peaks presented at 1651 and 1596 cm1 in the spectrum of CS are – O stretching vibration of –NHCO– and the N–H attributed to the C– bending of –NH2. It is worth noting that in the CS-GO spectrum, the – O stretch at 1730 cm1 was not observed. In characteristic peak of C– – O stretching vibration at addition, both characteristic peaks of C– 1651 cm1 and N–H bending at 1596 cm1 are shifted to the lower wavenumbers. These results suggest that there is hydrogen bonding interaction between GO and CS [40]. Through this procedure (Fig. 1a), the three-dimensional GO-CS scaffold, G-C monolith and MoS2-G-C composite composed in parallel can be easily obtained (Fig. 1b-g, Supporting Information, Fig. S2). The GOCS scaffold was assembled into a three-dimensional structure by using the unidirectional freezing method (Fig. 1b, e, Supporting Information, Fig. S2a, Fig. S2d) [41,42]. After annealing, the three-dimensional structure can be preserved (Fig. 1c, f, Supporting Information, Fig. S2b, Fig. S2e). The SEM images (Fig. 1d, g, Supporting Information, Fig. S2c, Fig. S2f) show that MoS2 NSs can be uniformly distributed on the G-C monoliths. The energydispersive X-ray spectroscopy (EDX) analysis revealed the element composition of the MoS2-G-C composite (Supporting Information, Fig. S3). The C, O, Mo, and S elements were identified in the targeted region. These results indicate that the atomic ratio of S and Mo is approximately 2:1, which is in good agreement with the expected value (Supporting Information, Table S1). In previous works, GO has been used as the substrate on which MoS2 NSs can generally uniformly grow onto the surface [43–47]. Chen et al. [47] have reported that the carbon atom in GO co-shares the electron cloud with the sulfur atom in MoS2. Herein, our results have demonstrated that MoS2 NSs can grow on the G-C surface. During the annealing process, the volume shrinkage of CS is much larger than the shrinkage of GO; this is due to the loss of a large amount of relative material in the CS matrix (Supporting Information, Fig. S5, Fig. S6). At the same time, GO is reduced during annealing, and CS is carbonized and becomes amorphous carbon [48,49]. The result of the elemental analysis shows that the total carbon content of G-C monolith is 76.87 wt% (Supporting Information, Fig. S7). Considering that the initial
2. Experimental details Material sources: All reagents used in this work were procured from Sinopharm Chemical Reagent Company and Shanghai Aladdin Bio-Chem Technology Co.,LTD. Fabrication of the GO-CS scaffolds: Graphene oxide (GO) was obtained by an improved Hummers’ method of oxidizing natural graphite powder [35]. The obtained GO was further processed with ultrasonic waves for 10 min (500 W, JY92-IID). A chitosan (CS) solution with a deacetylation degree of 95% was obtained by dissolving chitosan powder into an acetic aqueous solution (2 wt%). Next, GO and CS were mixed together and underwent ultrasonic treatment for 30 min to ensure the GO sheets were sufficiently dispersed in the CS matrix. The GO–CS suspension was then poured into a rectangular silicone mold on a steel plate, and the steel plate was inserted into liquid nitrogen to create the temperature gradient on its surface. As a result, the ice nucleus grew along the temperature gradient and formed parallel ice columns with a highly ordered alignment. After the sample was completely frozen, it was further freeze dried. G–C monoliths: The GO–CS scaffolds were annealed in a furnace with argon, heated from 25 C to 500 C with a heating rate of 2 C min1 and kept at 500 C for 1 h, and then tuned from 500 C to 800 C with a heating rate of 5 C min1 and kept at 800 C for 2 h. Finally, G–C monoliths were obtained. MoS2-G-C composite: First, 1g of ammonium molybdate, 1.8g of thiourea and 30 ml of water were added to a Teflon-lined autoclave (50 ml) and stirred sufficiently on a magnetic agitator. Second, the prepared G–C monoliths were added to the mixture and soaked for 24 h, and reacted at 180 C for 48 h. After cooling naturally, the sample was washed several times with deionized water and ethanol, and then dried at 60 C for 8 h. Finally, the MoS2-G-C composite was obtained. Characterization: The morphology of the samples was characterized using a scanning electron microscope (SEM) (Zeiss Supra 40, Germany) at an accelerating voltage of 10 kV. A TEM image was obtained using a Hitachi H7700 TEM. Fourier transform infrared spectroscopy (FT-IR) was carried out from 4000 to 400 cm1 wavenumber at 25 C (Nicolet 6700). The chemical composition of samples was determined by X-ray photoelectron spectroscopy (XPS) (ESCALAB MKII). Compression tests were conducted on an Instron 5565A at the speed of 10 mm/min. The electrical conductivity of samples was measured using a two-probe method. Electrochemical performance measurements: The electrochemical performance was evaluated with half cells. The working electrodes were mixed with an as-synthesized sample (active material), acetylene black (conductive agent) and polyvinylidene fluoride (binder) at a weight ratio of 8:1:1 to form a slurry. Then the slurry was coated uniformly onto a copper foil. After being vacuum dried at 60 C overnight, the electrodes were punched into circular discs. The electrode had a diameter of 12 mm and an active material loading of ~2 mg cm2. Swagelok-type batteries were assembled in an argon-filled glovebox by pairing the abovementioned working electrode with sodium metal as the counter electrode. A carbonate electrolyte (160 μl) consisting of 1 M NaClO4 in ethylene carbonate/dimethyl carbonate (1:1 v/v) (Zhangjiagang Guotai Huarong New Chemical Materials Co., Ltd.) was used. Glass fibers (GF/D) from Whatman were used as the separator. Galvanostatic cycle 2
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Fig. 1. Hierarchical structure and morphology characterization of MoS2-G-C composite. (a) Schematic diagram for the preparation of MoS2-G-C composite. Side-view SEM images of (b) GO-CS, (c) G-C, (d) MoS2-G-C. (e–g) High magnification SEM images of (b–d).
process (Supporting Information, Fig. S9a). However, it changed into G-C monolith after annealing, displaying excellent elasticity. When the compression strain is 50%, G-C monolith has shown strong resilience, and can completely recover to its original height without plastic deformation, yielding or lateral displacement after the compression process (Supporting Information, Fig. S9b). Note that even the mechanical properties of MoS2-G-C composites are not significantly changed compared to the G-C monolith (Supporting Information, Figs. S9c–d). Based on the above characteristics, we have obtained a threedimensional hierarchical MoS2-G-C composite by a simple
CS content in GO-CS scaffold is 50 wt% (Supporting Information, Fig. S7), and based on the thermos gravimetric analysis results (Supporting Information, Fig. S6b), we believe that the carbon constituent is mainly amorphous carbon from carbonized CS. It is worth noting that G-C monoliths have shown different electrical conductivities due to their anisotropic structure, where the value along the ice crystal’s growth direction is 8.9 S m1 and the value perpendicular to the ice crystal’s growth direction is 2.1 S m1 (Supporting Information, Fig. S8). As expected, the GO-CS has shown weak resilience, and thus could not completely recover to its original height after the compression 3
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Fig. 2. Schematic illustration of the synthesis of MoS2-G-C composite.
CV measurements at a scan rate of 0.1 mV/s for the initial five cycles were conducted to evaluate the electrochemical performance of the MoS2-G-C composite as an electrode (Fig. 4a). During the discharge process, the reduction peak was observed at 0.88 V, which is due to the Naþ ions inserted into 2H–MoS2 crystals. Another reduction peak appears at 0.6 V, which corresponds to the conversion reaction between Naþ ions and 2H-NaxMoS2. During the charging process, the oxidation peak was detected at 1.6 V, which is related to the reverse conversion reaction between the metallic Mo and Na2S matrix. Fig. 4b shows the static charge/discharge curves of the MoS2-G-C electrode at 100 mA g1 with different cycle numbers, the MoS2-G-C electrode after 100th cycles still displays a similar charge/discharge curve to that at the 2nd cycle. For comparison, the CV measurement and charge/discharge curves of the original MoS2 electrode were performed and analyzed (Fig. S10). The charge/discharge platform of the MoS2 electrode diminished significantly along with the increasing number of cycles. However, except for the initial cycle, the MoS2-G-C electrode demonstrates nearly overlapping charge/discharge curves, indicating that it has both an excellent electrochemical cycle stability and reversibility. All of the above results suggest that the MoS2-G-C electrode offers an excellent electrochemical performance attributed to its highly conductive carbon network and stable hierarchical structure. As shown in Fig. 5a, we evaluated the cycle performance of the MoS2G-C electrode. The respective cycle performance data of the MoS2-G, MoS2–C and MoS2 electrodes have been included for comparison. The initial charge/discharge capacities of the four electrodes are 536.9/ 998.9, 435.2/850, 408.7/849.3 and 314.6/736.4 mAh g1 under 100 mAg1 current density, respectively, and correspond to coulombic efficiencies of 53.7, 51.2, 48.1 and 42.7%. The improvement of the coulombic efficiency and the increase of electrode capacity are attributed to the 3D conductive network of G-C and the interfacial interaction of the C–O–Mo bonds between MoS2 and G-C. It has been reported that the irreversible capacity loss of the first cycle is inevitable due to the formation of the SEI layer [44]. After 200 cycles, the capacity of MoS2-G, MoS2–C and MoS2 electrodes declined rapidly to 310.9, 184.4 and 135.9
hydrothermal method, as shown in Fig. 2. During the initial reaction, G-C monolith surface formed MoOx crystal nuclei, and further reacted with CH3CSNH2 (S source and reductant), and MoS2 nanocrystals were eventually obtained [50]. Here, the G-C monolith serves as a substrate, and MoS2 can nucleate and grow on the surface during the hydrothermal process. MoS2 consists of an Mo layer and two S layers, and the Mo atoms are completely exposed at the end of the MoS2 [51,52]. Therefore, considering the layered structure of MoS2, it is reasonable to assume that the C–O–Mo bonds can be formed between the Mo atom and carbon materials. Moreover, it can also be seen that MoS2 NSs are easier to grow on the carbon materials ascribed to the influence of C–O–Mo bonds. The interface between the MoS2 and carbon materials was examined by XPS. The composite consisted of Mo, S, C and O elements (Fig. 3a). Fig. 3b shows two characteristic peaks at 228.6 eV and 231.9 eV, which correspond to the binding energy of Mo 3d5/2 and Mo 3d3/2, respectively [53,54], from the Mo4þ in MoS2. In addition, a small characteristic peak at 225.8 eV can be observed, corresponding to the S 2s component. The characteristic peak at 162.5 eV and 161.3 eV (Fig. 3c) are ascribed to S 2p1/2 and S 2p3/2 [53]. As shown in Fig. 3d, the C 1s characteristic peak – C/C–C, while another characteristic peak at 285.1 eV corresponds to C– of the oxygen-containing functional group occurring at 286.8 eV corresponds to the C–O [55]. Relatively weak C(O)–O characteristic peaks indicate that GO sheets were reduced to graphene. It is worth noting that the spectrum of O 1s shows a new characteristic peak appearing at 531.9 eV, which was different from the peak at 533.3 eV for the C–OH (Fig. 3e). It has been reported that the O 1s characteristic peak of the C–O-metal bond is approximately 530–533 eV [55–58]. Therefore, from the above results, we could conclude that the C–O–Mo bond is presented in the MoS2-G-C composite. Fig. 2 has demonstrated the illustration of a successful synthesis process of MoS2-G-C composite. It has been reported that the covalent C–O-M bonding between MxOy (M ¼ Fe, Co) and GO can greatly enhance the electron transport rate and maintain the structural stability of the MxOy/graphene electrode [55–58]. Therefore, considering the strong interfacial interaction of the C–O–Mo bonding between the MoS2 and carbon materials, it is promising to design the MoS2-G-C composite as an anode material for SIBs.
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Fig. 3. XPS spectra for MoS2-G-C composite (a) survey spectra, (b) Mo 3d spectra, (c) S 2p spectra, (d) C 1s spectra, (e) O 1s spectra.
mAh g1, respectively. This is primarily due to their volume changes, which lead to severe structure damage and the pulverization of their active material. On the contrary, the MoS2-G-C electrode provided a very stable and highly reversible capacity of 527.3 mAh g1 after 200 cycles, with negligible capacity loss. This value is much higher than those reported in the literature (Supporting Information, Table S2) and suggests that this method is very feasible for practical applications. These results indicate that the sodium storage capacity of the MoS2-G-C electrode has been significantly enhanced as a result of the synergetic effect among MoS2, G and C. High ratio performance is also an important aspect for the high power type of SIBs. Compared with G-MoS2, C–MoS2 and MoS2, the MoS2-G-C electrode showed a superior rate capability (Fig. 5b). We have measured
the charge capacity of the MoS2-G-C electrode, which is 981.5, 489.5, 446.5, 391.4, 356.5, 316.5 and 262.1 mAh g1 at 100, 200, 500, 1000, 2000, 5000 and 10000 mA g1, respectively. In contrast, the capacities of the MoS2-G, MoS2–C and MoS2 electrodes decline rapidly with increasing rate. Fig. 5c shows that MoS2-G-C electrode exhibits an excellent longterm cycling performance; with the charge/discharge at 1 A g1 for 100 cycles, the MoS2-G-C electrode still retained a reversible capacity of 439.1 mAh g1. Ascribed to its highly ordered three-dimensional conductive network of carbon material and strong interfacial interaction of C–O–Mo bonds, the MoS2-G-C electrode has an excellent rate performance. To further understand the electrochemical kinetics of the MoS2-G-C electrode, we measured the electrochemical impedance by spectroscopy. 5
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Fig. 4. (a) CV curves of MoS2-G-C electrode for the initial 5 cycles, (b) Charge/discharge curves of MoS2-G-C electrode at 100 mA g1 for different cycles.
Fig. 5. (a) Cycling performance at 100 mA g1 for MoS2-G-C, MoS2-G, MoS2–C and MoS2 electrodes. (b) Rate capability for MoS2-G-C, MoS2-G, MoS2–C and MoS2 electrodes. (c) Cycling performance at 1 A g1 for MoS2-G-C, MoS2-G, MoS2–C and MoS2 electrodes. (d) Nyquist plots of MoS2-G-C, MoS2-G, MoS2–C and MoS2 electrodes.
Fig. 5d shows the Nyquist plots of the MoS2-G-C, MoS2-G, MoS2–C and MoS2 electrodes at 100 mA g1 after 200 cycles. Both Nyquist plots consist of a semicircle in the high-frequency region and a sloping line in the low-frequency region. The resistance (R) of the MoS2-G-C electrode is significantly smaller than those of the MoS2-G, MoS2–C and MoS2 electrodes (Fig. 5d), indicating that the incorporation of graphene and chitosan can greatly promote charge transfer in the electrochemical reaction, thus leading to excellent rate performance.
materials, which is attributed to the direct coupling of the edge Mo of the MoS2 with oxygen from the functional groups of carbon materials (C–O–Mo bonds). The hierarchical structure of our MoS2-G-C composite can effectively prevent the agglomeration of MoS2 and the restacking of graphene during the charge/discharge processes. The MoS2-G-C electrode has a high reversible capacity, excellent rate capability and long cycle life when used as SIBs anode material. Specifically, the MoS2-G-C electrode showed an outstanding reversible capacity of 527.3 mAh g1 at 100 mA g1, retained 52.7% of its initial reversible capacity, and demonstrated a long, stable cycle life (439.1 mAh g1) at 1 A g1 after 200 cycles. Ascribed to the presence of a large number of active sites, MoS2 NSs can grow on graphene sheets, and the MoS2-G-C electrode exhibited an excellent electrochemical performance. As an active
4. Conclusions In conclusion, we have successfully designed a hierarchically structured MoS2-G-C composite. MoS2 NSs are apt to grow on carbon
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material, the nanoscale sized MoS2 NSs ensured rapid electrolyte penetration and shortened the sodium ions’ diffusion distance. In addition, the formation of C–O–Mo bonds promoted rapid electron propagation from carbon to MoS2 NSs, resulting in a high reversible capacity and an excellent rate performance of the electrode materials. Therefore, this MoS2-G-C composite with its hierarchical structure is a very promising SIBs anode material with high electrochemical performance.
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