Neuron-Inspired Design of High-Performance Electrode Materials for

Oct 11, 2018 - ... for SIBs, the composite exhibits excellent electrochemical performance, including high specific capacity, excellent cycle stability...
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Neuron-Inspired Design of High Performance Electrode Materials for Sodium-Ion Batteries Yu-Lin Bai, Yu-Si Liu, Chao Ma, Kai-Xue Wang, and Jie-Sheng Chen ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.8b06585 • Publication Date (Web): 11 Oct 2018 Downloaded from http://pubs.acs.org on October 12, 2018

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Neuron-Inspired Design of High Performance Electrode Materials for Sodium-Ion Batteries Yu-Lin Bai, Yu-Si Liu, Chao Ma, Kai-Xue Wang,* Jie-Sheng Chen Shanghai Electrochemical Energy Devices Research Center, School of Chemistry and Chemical Engineering, Shanghai Jiao Tong University, Shanghai 200240, P. R. China *E-mail:

[email protected]

ABSTRACT Sodium-ion batteries (SIBs) are generally considered as promising cheap alternatives of lithium-ion batteries for stationary renewable energy storage and have received increasing attention in recent years. The exploration of anode materials with efficient electron transportation is essential for improving the performance of SIBs. Inspired by the signal transfer mode of a neuron, we designed a composite by stringing MoS2 nanoflower (soma) with multiwall carbon nanotubes (MWCNTs) (axons). HRTEM observation reveals a lattice matching growth mechanism of MoS2 nanosheets on the interface of MWCNTs and the lattice expansion of the (002) plane of MoS2. The lattice matching among the MoS2 nanosheet and MWCNT could facilitate electron transfer and structure maintenance upon cycling. The expanded distance of the (002) plane of MoS2 would also promote the sodium ion intercalation/deintercalation kinetics of the composite. Benefiting from the structural features, when used as an anode material for SIBs, the composite exhibits excellent electrochemical performance, including high specific capacity, excellent cycle stability, and superior rate capabilities. A stable capacity of 527.7 mAh g-1 can be achieved after 110 cycles at a current density of 100 mA g-1. The neuron-inspired design proposed is a promising and efficient strategy for the development of electrode materials for SIBs with high mass transport kinetics and structural stability.

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Keywords: MoS2/MWCNT, anode material, hierarchical structure, electrochemical performance, sodium ion batteries Sodium ion batteries (SIBs) have been considered as a promising cheap alternative energy storage system, due to the abundance and wide distribution of sodium in our planet compared with lithium. However, the large radius of sodium ion (Na+ 1.06 Å vs. Li+ 0.76 Å) will lead to slow diffusion kinetics and the structural collapse of the electrode materials in the charge/discharge process.1 Graphite is commercialized anode material in lithium ion batteries (LIBs). However, only a small amount of Na atoms can intercalate into graphite to form an intercalation compound.2,3 Thus, it is urgent and challenging to develop anode materials with high specific capacity for sodium ion insertion.4,5 Materials, such as alloys,6 metal oxide,7 transition metal sulfides,8 carbonaceous materials,9,10 and organic compounds11-13 have been investigated as anode materials for SIBs. Among them, MoS2 with a layered structure has received considerable attention.14,15 The large layer spacing of 0.615 nm and weak Van der Waals interaction among the layers are beneficial for Na+ diffusion and reversible intercalation/extraction. However, the poor electronic conductivity and low sodium diffusion kinetics hindered the further application of MoS2 as an anode material in SIBs. To address this issue, a variety of strategies, including the decrease of the particle size to nanoscale to shorten the diffusion distance of Na+ and the incorporation with conductive substrates to boost the electron conductivity have been proposed.16-19 The decrease in the particle size would also increase the specific surface area of the electrode materials, making the electrolyte sufficiently wet the electrode materials and consequently improve the ions transport kinetics of the electrodes.20-25 Nanostructured MoS2 with varying morphologies, such as nanoflowers,24 nanotubes,26 and nanospheres27 have been prepared and used as electrode materials for LIBs and SIBs. Carbon materials, such as carbon nanofibers (CNFs),28-30 carbon nanotubes (CNTs),31,32 carbon sphere,33 and graphene,34,35 can significantly improve the electronic conductivity and accommodate the volume 2 ACS Paragon Plus Environment

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change of MoS2, consequently improving the cycling stability and rate capability of the electrode. Although significant progress has been made, it is still challenging to construct stable MoS2–based hybrid electrode materials with an efficient conductive network for electrons transfer. Neuron typically composed of a soma and neuritis (dendrites or axon) is the basic structure and functional unit of the nervous system. With the help of axons, electrical and chemical signals can be transmitted efficiently from soma to other neurons or effector cells.36 Inspired by this signal transfer mode, we designed and prepared a MoS2/MWCNT composite by growing MoS2 nanoflower directly on MWCNTs. MWCNTs with high mechanical stability and superior electronic conductivity function as axons, stringing the MoS2 nanoflowers which act as soma to form an efficient nervous-system-like conductive network. The MoS2 nanosheets which are assembled into the hierarchical flower morphology could shorten the diffusion distance for sodium ions and provide a large active surface area for uptaking sodium ions. The hierarchical flower morphology together with MWCNTs could accommodate the volume change of MoS2 during charge/discharge process. The expanded interlayer spacing of MoS2 and lattice matching between MoS2 and MWCNT revealed by the TEM observation would facilitate the diffusion and reversible intercalation/extraction of sodium ions and reduce interface impedance, respectively. When used as anode materials of SIBs, the MoS2/MWCNT composites with a nervous-system-like conductive network exhibit excellent electrochemical performance, such as high specific capacity and improved cycling and structural stability.

RESULTS AND DISCUSSION The MoS2/MWCNT composites were synthesized through a simple one-pot hydrothermal method. In a typical synthesis procedure (Figure 1a), MWCNT was dispersed in 30 mL water by ultrasonic treatment. Then Na2MoO4·2H2O and thiourea were added to the above solution. After stirred for 20 min, the mixture was heated at 190 °C for 36 h. Finally, the black product was collected by 3 ACS Paragon Plus Environment

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centrifugation, washed with deionized water and alcohol for several times, and dried at 100 °C in a vacuum oven. For comparison, pure MoS2 composed of agglomerated nanosheets were prepared without the presence of MWCNTs (Figure S1). The crystalline structure and chemical composition of the as-prepared MoS2/MWCNT were investigated by X-ray diffraction (XRD), Raman, and thermogravimetric (TG) analyses. In the XRD pattern of MoS2/MWCNT (Figure 1b), the peaks at 13.8°, 32.7°, 39.5°, 49.8°, 58.3°, and 70.1° are attributed to (002), (100), (103), (105), (110), and (108) diffractions, respectively, of hexagonal MoS2 (JCPDS No. 37-1492). The (002) diffraction is observed shifting to the low-angle, indicating the expansion of the interlayer distance. As calculated by Bragg’s law, the interlayer distance of (002) plane of MoS2/MWCNT is 0.644 nm, larger than that of the bulk MoS2 (d002 = 0.615 nm). The enlarged interlayer distance is attributed to the intercalation of Na+ and NH4+ during the hydrothermal process.24,37 The expanded interlayer spacing can facilitate Na+ transfer and diffusion during cycling. The small peak at 25.9° is ascribed to the (002) diffraction of MWCNTs,24 indicating the presence of MWCNT in the composite. The detailed structural features of the composite were further characterized by Raman spectrum (Figure 1c). The Raman shifts at 376.7 cm-1 (E12g) and 404.8 cm-1 (A1g) indicate the formation of MoS2 with high crystallinity. The shifts at 1348 cm-1 and 1580 cm-1 are assigned to the D- and G-band of carbon, corresponding to the MWCNTs.34,38 As determined by the TG analysis (Figure S2), the contents of MoS2 and MWCNT in the composite are 76 and 24 wt%, respectively, assuming that the final product after TG analysis treatment in the air is pure MoO3.39 As revealed by the low magnification SEM image (Figure S3), the scanning electron microscopy (SEM), and transmission electron microscopy (TEM) observation (Figure 2a-c), the MoS2/MWCNT composites have a hierarchical structure composed of MoS2 nanoflowers which are strung together by carbon nanotubes. MoS2 nanosheets grown together form the hierarchically structured nanoflowers. High-resolution TEM (HRTEM) image (Figure 2d) show distinct lattice fringes with an interlayer 4 ACS Paragon Plus Environment

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spacing of 0.644 nm, ascribed to the expanded (002) plane of the MoS2 nanosheets. This result is consistent with the XRD analysis. Energy-dispersive X-ray spectroscopy (EDX) elemental mapping analyses show the distribution of Mo, S, and C in the MoS2/MWCNT composite (Figure 2e), further revealing the neuron-like structure of the composite. XPS analyses of the MoS2/MWCNT composite are shown in Figure 3. The survey spectrum demonstrates the presence of Mo, S, and C elements in the MoS2/MWCNT composite (Figure 3a). In the high-resolution Mo 3d spectrum (Figure 3b), peaks at 229 eV and 232.2 eV are assigned to Mo 3d5/2 and Mo 3d3/2, respectively. The peak at 226.2 eV is attributed to S 2s, while that at 236.0 eV to the Mo-O bond derived from the surface oxidized MoS2.40 In the S 2p spectrum (Figure 3c), peaks at 161.8 eV and 163.1 eV are ascribed to S 2p3/2 and 2p1/2, respectively. The extra deconvoluted peak at 164.4 eV might be due to the S-C bonds.39 In the C 1s spectrum (Figure 3d), the broad deconvoluted peak at 285.6 eV is attributed to C-S bonds. The formation of C-S bonds indicated by the XPS analyses might suggest that chemical bonds generate between MoS2 and MWCNTs. The integrity of the interface among the components plays an essential role for the properties of the composite. HRTEM observation and fast Fourier transform (FFT) were conducted on a selected area where thin MoS2 layer connected with MWCNT to investigate the interface between the MoS2 nanosheets and MWCNTs (Figure 4a). The FFT pattern of the selected area highlighted in Figure 4a shows two sets of diffraction rings attributable respectively to MoS2 and MWCNTs (Figure 4b), indicating the co-existence of MoS2 and MWCNTs. Distinct interface between the MoS2 nanosheet and the MWCNT is revealed by the HRTEM observation (Figure 4c). The obvious lattice spacing of 0.253 and 0.225 nm observed in the left part of the HRTEM image are assigned to the (100) and (103) planes of MoS2, while those at 0.344 and 0.215 nm in the right part are associated with the (002) and (100) planes, respectively, of MWCNTs. The FFT pattern of the left part shows distinct diffraction spots (inset in Figure 4c), indicating the single-crystal nature of the MoS2 nanosheets. The right part shows 5 ACS Paragon Plus Environment

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diffraction rings. The out diffraction ring can be indexed to the (100) plane of MWCNT, associated with the lattice spacing of 0.215 nm. As revealed by HRTEM observation, the (103) plane of MoS2 is well matched with the (100) plane of MWCNT with only 4.6 % lattice mismatch, indicating that MoS2 can epitaxially grow on the MWCNT structure.39 The good lattice match between MoS2 and MWCNT would facilitate the diffusion of Na+ ions and reduce the interfacial resistance, consequently improve the electrode kinetics. Employing this hydrothermal method developed in this work, MoS2 can also be epitaxially grown onto other types of carbon materials such as carbon cloth and even super P (Figure S4). Uniform MoS2 nanoflowers grow on the surface of carbon cloth (Figure S4 a-b). And when using the active carbon (Super P), nanoflowers formed (Figure S4 c-d). The XRD patterns of the MoS2carbon-based materials are shown in Figure S5. All the peaks can be well indexed to the hexagonally structured MoS2 (JCPDS No. 37-1492). The peak at 25.7° is originated from (002) plane of graphitized carbon materials. The electrochemical properties of the MoS2/MWCNT composite as an anode material for SIBs are evaluated by cyclic voltammetry (CV) and galvanostatic charge/discharge tests. Figure 5a shows the CV curves of the MoS2/MWCNT electrode at a scan rate of 0.15 mV s-1 in the voltage range of 0.01 – 3.0 V versus Na+/Na. In the first cathodic scan, one broad peak located at about 0.85 V is observed, corresponding to the intercalation of Na+ (MoS2 + x Na → NaxMoS2, x＜2) and the formation of solid electrolyte interface (SEI).14,29,33,41 The other peak at 0.05 V is attributed to the formation of Mo and Na2S (NaxMoS2 + (4-x) Na → Mo + 2Na2S).24 In the subsequent anodic scan, a broad peak at 1.65 V is detected, attributing to the decomposed of Na2S and formation of S (Na2S ↔ S + 2Na+ + 2e−).33,42 In the following cathodic scans, a new broad peak at 1.78 V is detected, resulted from the sodiation of S. In the anodic scans, two new small peaks at 0.5 V and 0.95 V present, associating with the formation of NaxMoS2 and MoS2, respectively. Figure 5b shows the galvanostatic discharge/charge voltage profiles of the MoS2/MWCNT composite at a current density of 50 mA g-1 within 0.01 and 3.0 V. The initial 6 ACS Paragon Plus Environment

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discharge and charge capacities are 846.3 and 571 mAh g-1, respectively, giving an initial coulombic efficiency (ICE) of 67.5 %. The initial irreversible capacity loss is primarily derived from the formation of SEI layer. The discharge and charge curves nearly overlapped after the first cycle, indicating the high reversibility and cycling stability of the MoS2/MWCNT composite. The cycling performances of the MoS2/MWCNT and MoS2 at a current density of 100 mA g-1 are shown in Figure 5c. The MoS2/MWCNT composite show much stable cycling performance. A high discharge capacity of 527.7 mAh g-1 is retained after 110 cycles with the capacity retention of approximately 100 % (compared with that of the second cycle), demonstrating the excellent cycle stability of the MoS2/MWCNT composite. However, for the electrode barely based on MoS2, the capacity retention is only approximately 70 % after 110 cycles. The excellent cycle stability and sodium storage performance of the MoS2/MWCNT composite are attributed to the involvement of the MWCNTs, which can enhance the electron conductivity and structural stability of the composite through the formation a threedimensional (3D) nervous-system-like hierarchical network. The formation of such a 3D hierarchical network can also provide a flexible structure to buffer the volume change and prevent the MoS2 nanosheets from aggregation during the charge/discharge process. The MoS2/MWCNT composite exhibits a superior rate capacity (Figure 5d). Reversible discharge capacities of 515.6 (10th), 484.6 (20th), 473.0 (30th), 427.7 (40th), and 411.0 mAh g-1 (50th) are obtained at current densities of 100, 200, 500, 1000, and 2000 mA g-1, respectively, which are much higher than those of pure MoS2 and MWCNTs tested under the same condition (Figure S6). For the pure MoS2, a specific discharge capacity of only 204 mAh g-1, almost half that of the composite is achieved at a current density of 2000 mA g-1. When the current density was set back to 100 mA g-1, a discharge capacity 496 mAh g-1 is recovered, indicating a high sustainability of the composite. Moreover, the cycle stability and rate capabilities of the MoS2/MWCNT composite are even superior to those of the MoS2-based electrode materials reported in the literature (listed in Table S1). 7 ACS Paragon Plus Environment

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To figure out the reasons of the superior electrochemical performance of the MoS2/MWCNT composite, the galvanostatic intermittent titration technique (GITT) was applied to analyse the overall potential response of MoS2/MWCNT and MoS2 anode for sodium storage during the third cycle. During the GITT measurement, a constant current of 100 mA g-1 was applied for 20 min and then the cells rest for 2 h at open circuit. As shown in Figure 6a, the dotted lines show the equilibrium opencircuit voltages (OCVs). The reaction resistances of MoS2/MWCNT and MoS2 at different discharge/charge states are calculated by dividing the over-potential by pulse current.43 Figure 6b exhibits the variation of the reaction resistances in the discharge process. The reaction resistances for both the MoS2/MWCNT and MoS2 electrodes are initially decreased slightly and then increased dramatically to maximums with the discharge capacity, forming volcano-shaped profiles. In the charging process (Figure 6c), the reaction resistances for both electrodes are relatively stable at the initial stage, increased dramatically at the end of the charge process. The distinct reaction resistances between discharge and charge processes are ascribed to the different electrochemical reaction mechanisms of the desodiation/sodiation process. Furthermore, the MoS2/MWCNT electrode shows much lower reaction resistance than that of the MoS2 electrode. Therefore, the superior electrochemical performance can be ascribed to the improved desodiation/sodiation kinetics. Moreover, the electrochemical impedance spectroscopies (EIS) of the MoS2/MWCNT and MoS2 electrodes before cycling were also measured from 0.01 Hz to 100 kHz (Figure 6d). The Nyquist plots for both electrodes are constituted of a depressed semicircle which is ascribed to the charge-transfer resistance (Rct) at the interfaces, and the slope line corresponding to the Warburg impedance (Zw) related to the sodium diffusion. The semicircle diameter of the MoS2/MWCNT electrode is smaller than that of MoS2 electrode, suggesting the lower interface Rct of the composite. The low interface Rct contributes to the fast interface kinetics and the superior rate capabilities of the MoS2/MWCNT composite. In addition,

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the straight line observed in the high frequency region indicates a capacitive behavior of the MoS2/MWCNT electrode, contributing to the sodium ion storage capability of the electrode. The reaction kinetics of the MoS2/MWCNT composite is further investigated by CV technique scanned at rates ranging from 0.15 to 1.0 mV s-1.44,45 The current response of an electrode at different potentials obeys the equation 1: i = a vb

(1)

where i is the current (mA), v is the scan rate, and a and b are parameters. The peak current is plotted against the scan rates logarithmically, and then the values of a and b can be calculated. When the value of b is approaching 1.0, the electrode is dominated by the capacitive process. When the value of b is approaching 0.5, the electrode is controlled by a diffusion-controlled intercalation process. As depicted in Figure 6e, there are two cathodic and three anodic peaks in the CV curves, denoted as C1-C2 (cathodic) and A1-A3 (anodic), respectively. Figure 6f shows the line of each peak plotted by log (peak current) as a function of log (scan rate). As revealed by the calculation, the b values of the redox peaks are approximately 1.0, indicating a surface-dominated capacitive process. The high capacitive contribution leads to an ultrafast sodium ion insertion/extraction, outstanding rate capability, and high cycling stability.

CONCLUSIONS Inspired by the signal transfer mode of a neuron, a MoS2/MWCNT composite with a nervous-systemlike structure has been successfully prepared. MWCNTs with high mechanical stability and superior electronic conductivity string the MoS2 nanoflowers into a hierarchically porous system, efficient for the electron and electrolyte transportation. The lattice matching between the MoS2 nanosheet and MWCNT revealed by TEM observation facilitates electron transfer and structure stability. The expanded distance of the (002) plane of MoS2 benefits the sodium ion intercalation/deintercalation 9 ACS Paragon Plus Environment

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kinetics of the composite. Employed as an anode material for SIBs, the MoS2/MWCNT composite exhibits excellent electrochemical performance, including high specific capacity, improved cycling stability, and rate performance, superior to that of pure MoS2 and MWCNT. The neuron-inspired strategy provides an avenue for the design of electrode materials for SIBs with high electrochemical performance.

METHODS Preparation of MoS2/MWCNT: MWCNT was supplied by Nanjing XFNANO Materials Tech Co., Ltd, Na2MoO4·2H2O was purchased from Sinopharm Chemical Reagent Co., Ltd and thiourea was purchased from Aladdin Industrial Corporation. All chemicals were used as received. Typically, 0.16 g of MWCNT was added to 30 mL of deionized water, followed by ultrasonic treatment for 10 min. Then, 1.0 mmol of Na2MoO4·2H2O and 4.0 mmol of thiourea were dissolved in the above solution with vigorous stirring for 20 min. After that, the mixture solution was transferred to a 50 mL autoclave and heated at 190 °C for 36 h. Finally, the black composite was collected by centrifugation, washed with deionized water and ethanol for three times and dried at 70 °C for 12 h in a vacuum oven. Characterization: The morphologies of the samples were observed by a scanning electron microscope (SEM, FEI Nova Nano SEM 230, USA), a transmission electron microscope (TEM, JEM2100F, JEOL Japan) and a high–resolution transmission electron microscope (FEI-TEM, TALOS F200X). X-ray diffraction (XRD) patterns were collected on an X-Ray diffractometer (Bruker D8 Advance Da Vinci) with Cu Kα radiation (   Å). Raman spectra were performed on an in Viareflex micro-Raman spectrometer (Renishaw, UK) with excitation laser beam wavelength of 532 nm. The X-ray photoelectron spectroscopy (XPS) measurements were conducted on an AXIS UltraDLD spectrometer. Thermal gravimetric (TG) analysis was performed on an SDT Q600 thermal analyzer in the air with a heating rate of 10 °C min-1. 10 ACS Paragon Plus Environment

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Electrochemical Measurements: The working electrodes were made by mixing 80 wt % active material, 10 wt % Super P, and 10 wt % carboxymethyl cellulose (CMC) in water. The mixture was ground for half an hour to make a slurry, which was then spread onto a copper foil and dried at 100 °C for 24 h in a vacuum drying oven to fabricate the electrode. The electrode was pressed at 6 MPa and then cut into discs of 12 mm in diameter. The mass loading of the active material was about 1.2 mg cm2.

The CR2016 coin cells were assembled in a glovebox filled with Ar by using 1.0 M NaClO4 in a

mixture of ethylene carbonate (EC) and propylene carbonate (PC) (1:1 in volume) with 5 wt % fluoroethylene carbonate (FEC) as the electrolyte, glass-fiber (GF) as the separator, and sodium metal as the counter electrode. Galvanostatic charge/discharge experiment was performed with a cell test system (LANDCT2001A) in the voltage range of 0.01-3.0 V. The cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) measurement were conducted on an electrochemical workstation (Metrohm Autolab PGSTAT302N).

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Associated contend This work was financially supported by the National Natural Science Foundation of China (21871177, 51472158, 21673140, 21720102002) and the National Basic Research Program (2014CB932102). Supporting Information Supporting Information is available free of charge on the ACS Publications website. SEM images and XRD patterns of MWCNT and pure MoS2; TG analysis of the MoS2/MWCNT composite; The low magnification SEM image of the MoS2/MWCNT composite; Morphology and phase characterization of MoS2 growth on carbon cloth and Super P. Electrochemical performance of the MWCNT electrode. AUTHOR INFORMATION Corresponding Author *E-mail:

[email protected]

ORCID Kai-Xue Wang: 0000-0002-2076-5487

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Figures and Captions:

Figure 1. (a) Schematic illustration of the preparation of the neuron-inspired nanostructure of MoS2/MWCNT. (b) XRD pattern, (c) Raman spectrum of the MoS2/MWCNT composite.

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Figure 2. (a,b) SEM, (c) TEM, (d) high-resolution TEM images, and (e) EDX elemental mapping of Mo, S, and C of the MoS2/MWCNT composite.

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Figure 3. (a) XPS spectrum of the MoS2/MWCNT composite, and high-resolution spectra of (b) Mo 3d, (c) S 2p, and (d) C 1s.

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Figure 4. (a) HRTEM image and (b) FFT pattern of the MoS2/MWCNT composite. (c) HRTEM image of the selected area in (a). Insets in (c) are the corresponding FFT image of MoS2 and MWCNT, respectively.

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Figure 5. (a) CV curves of at a scan rate of 0.15 mV s-1 and (b) the selected charge/discharge profiles at a current density of 50 mA g-1 of the MoS2/MWCNT composite, (c) cycling, and (d) rate performance of the MoS2/MWCNT and MoS2 electrodes.

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Figure 6. (a) GITT profiles and (b,c) reaction resistance of discharge and charge of the MoS2/MWCNT and MoS2 electrodes. (d) Nyquist plots of the MoS2/MWCNT and MoS2 electrodes before cycling in the frequency range from 0.01 Hz to 100 kHz. (e) CV curves of the MoS2/MWCNT electrode at elevated scan rates. (f) The value of b plotted by log (scan rate) verse log (peak current).

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The table of contents entry MoS2/MWCNT composites with a 3D nervous-system-like hierarchical network were prepared through a simple hydrothermal method. The composites exhibiting superior sodium ion storage capability are promising anode materials for advanced SIB system.

Keyword: MoS2/MWCNT, anode material, hierarchical structure, electrochemical performance, sodium ion batteries Yu-Lin Bai, Yu-Si Liu, Chao Ma, Kai-Xue Wang,* Jie-Sheng Chen Neuron-Inspired Design of High Performance Electrode Materials for Sodium-Ion Batteries

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