Tunable Free-Standing Core-shell CNT@MoSe2 Anode for Lithium

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Tunable Free-Standing Core-shell CNT@MoSe2 Anode for Lithium Storage Muhammad Yousaf, Yunsong Wang, Yijun Chen, Zhipeng Wang, Waseem Aftab, Asif Mahmood, Wei Wang, Shaojun Guo, and Ray P. S. Han ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b19739 • Publication Date (Web): 13 Apr 2018 Downloaded from http://pubs.acs.org on April 14, 2018

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ACS Applied Materials & Interfaces

Tunable Free-Standing Core-shell CNT@MoSe2 Anode for Lithium Storage Muhammad Yousaf†, Yunsong Wang†, Yijun Chen†, Zhipeng Wang†, Waseem Aftab†, Asif Mahmood†, ‡, Wei Wang†, Shaojun Guo*,†, Ray P. S. Han*,† † ‡

Department of Material Science and Engineering, Peking University, Beijing 100871, China Department of Physics, South University of Sciences and Technology, Shenzhen 518000, China

Abstract: Heterogeneous nanostructuring of MoSe2 over carbon nanotube (CNT) sponge as a free standing electrode not only bring higher performance but also, eliminates the need for dead elements such as a binder, conductive carbon and supportive current collectors. Further, the porous CNT sponge can be easily compacted via an intense densification of the active material MoSe2 to produce an electrode with a high mass loading for a significantly improved areal capacity. In this work, we present a tunable coating of MoSe2 on a CNT sponge to fabricate a core-shell MoSe2@CNT anode. The three dimensional (3D) nanotubular sponge is synthesized via a solvothermal process followed by a thermal annealing to improve the crystallization. Structural and morphological studies revealed that MoSe2 grew as a layered structure (d = 0.66 nm), where numbers of layers can be controlled to yield optimized results for Li+ storage. We showed that the 10-layer core-shell CNT@MoSe2 hybrid sponge delivered a discharge capacity of 820.5 mAhg-1 after 100 cycles at 100 mAg-1 with a high cyclic stability and rate capability. Further, an exsitu structural and morphological analysis revealed that the ionic storage causes a phase change in MoSe2 from a crystalline to a partial amorphous state for a continuous increase in capacity with an extended cycling. We believe that the strategy developed here will assist users to tune the electrode materials for future energy storage devices, especially how the materials are changing with the passage of time and their effects on the device performance. KEYWORDS: Hetero-nanotubes, Core-shell structures, MoSe2, Free standing electrodes, LIBs.

1. INTRODUCTION Considerable attention has been focused on the development of new electrode materials for lithium ion battery (LIB) in order to meet the overwhelming demands of electric vehicles, small grid stations and portable electronic devices.17

_ENREF_1 Since the discovery of unusual

properties in graphene, 2-dimensional (2D) layered materials, especially transition metal dichalcogenides (TMDs) have received considerable attention for use in energy conversion and storage devices due to their superior physical, chemical and electronic properties.8-12 The layered structure

provides a short diffusion path for various guest species to readily intercalate/de-intercalate for energy drawdown and drawup.13-19 As a wellknown TMD, molybdenum diselenide (MoSe2) has a sandwich layered structure (Mo-Se-Mo) similar to MoS2.20-23 The MoSe2 offers a high theoretical capacity due to the conversion reaction along with the intercalation of Li+, better contact with electrolyte due to the high specific surface area and a relatively low operating voltage.24-27 However, MoSe2 electrodes have many inherent limitations such as low intrinsic conductivity, restacking and large volume expansions at high

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Tunable Core-shell CNT@MoSe2 Anode

current densities, which can affect the battery performance in terms of low Columbic efficiency, fast capacity decay, and inadequate rate capability.28-30 Controlling MoSe2 at the nanoscale can result in improved physical and chemical properties with enhanced Li+ storage capacity.9 Moreover, making 1D core-shell nanotubular structure by wrapping 2D TMD sheets over a 1D tubular substrate produces features such as radial heterojunctions and multi-functionalities.31-34 The presence of highly conductive and flexible substrate not only provides pathways for an efficient conductivity but also, provides structural stability during volume expansion with the shell (active material) offering active sites for charge storage that leads to an improved electrochemical performance and overall cyclic stability.35-37 However, coating a layer of TMDs onto a substrate is challenging due to rigid atomic layers of 2D TMDs and the absence of spatial confinement that can affect the conformal coating on the outer surface of the template. Lately, carbon nanotubes (CNTs) with unique properties (such as flexibility, high mechanical and electrical properties) have been used as a suitable template to grow different 2D materials including TMDs.38-41 Although, great effort has been devoted to fabricating CNT-based hybrid materials with TMDs for batteries,39 however, the electrochemical stability of TMDs@CNT is still poor, due mainly to random structures in the developed composites that seriously limit the mass diffusion and provide lesser space for volume expansion during a reversible Li storage.39-40 Moreover, structures such as “flower-like, hairy or curly” in TMD@CNT hybrids are more vulnerable to electrolyte etching and can detach during the reverse cycling.40,42-43 Hence, it is imperative to design a new topology where 3D interconnect-

ed sponge made of 1D CNTs with sufficient spaces for volume expansion and mass diffusion can be utilized for the loading of active materials. This 3D CNT sponge not only provides a 3D conductive pathway but eliminates dead elements such as a current collector, polymer binder and conductive agent for it to be used as a free standing electrode. Hence, developing new freestanding electrode chemistries where a 3D CNT sponge can be loaded with electrochemically active material MoSe2 for enhanced Li+ storage with high cyclic stability is critical. In this work, tunable MoSe2 nanosheets have been coated homogeneously over CNTs via a simple solvothermal-calcination process, using CNT sponge as a template. The highly crystalline, continuous and multi-walled MoSe2 layers over CNTs are tuned by simple varying the quantity of the precursors. The resulting hybridized inorganic core-shell CNT@MoSe2 sponge not only possesses a 3D porous network with a high surface area and a high electrical conductivity but it also, buffers large volume changes in the active material (MoSe2) to prevent restacking and agglomeration during the charge/discharge process. Therefore, the 3D hybrid nanotubular sponge with optimized MoSe2 layers can be used as a freestanding electrode without the need for binder elements. The hybrid sponge exhibits a good performance with a high cyclic stability for Li+ storage. Further, we discussed the factors for the high electrochemical performance by exploring the underlying mechanism that involves a phase transition from a highly crystalline to a partial amorphous state. 2. EXPERIMENTAL SECTION 2.1. Materials. Sodium Molybdate dihydrate (Na2MoO4.2H2O), Se powder and hydrazine hy-

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drate (N2H4.H2O, 85%) were purchased from Beijing Tongguang Fine Chemical Co and the ethanol (CH3CH2OH, ≥ 99.7%) supplied by Beijing Chemical Works. All chemicals were of the analytical grade and used without further purification. 2.2. Fabrication of core-shell CNT@MoSe2 sponge. A CNT sponge was synthesized by chemical vapor deposition (CVD) using ferrocene and 1,2-dichlorobenzene as a catalyst and carbon precursor as described in our previous report.44 To fabricate the core-shell CNT@MoSe2 tubular structure, the as-grown CNT sponge was cut into blocks (15 × 8 × 3 mm3) and then, subjected to a solvothermal-calcination process. Typically, a 1 mmol of Na2MoO4.2H2O was dissolved in deionized (DI) water (12.5 mL), with magnetic stirring at room temperature to form the Mo-precursor. In a separate flask, 2 mmol Se powder was dissolved in 5 mL hydrazine hydrate (N2H4.H2O, 85%) with magnetic stirring at room temperature in air to form the Se-precursor. Instantly, the color of the mixture solution changed from colorless to red and remains unchanged under air conditions for about one day. Prior to the addition of precursors into the CNT sponge (40 mg), several drops of ethanol were added to improve its hydrophilicity. Two precursors were loaded one after the other into the CNT sponge with a micropipette. First, the Mo-precursor was loaded into CNT sponge and dried at 70 °C, subsequently, the Se-precursor was loaded into the Mo precursor embedded sponge at a ratio of Mo to Se of 1:2. The sponge was then transferred into a Teflon-lined stainless steel autoclave, which was sealed and baked in an oven at 200 °C for 24 h. The sponge was then taken out and washed several times with a mixture of ethanol and DI water (1:1) and freezedried to maintain the 3D porous structure. Finally, the sponge was annealed at 800 oC for 3 h in an

argon flow to improve the crystallinity. The mass loading of MoSe2 in the core-shell CNT@MoSe2 could be controlled by simply adjusting the quantity of both precursors loaded into the bulk sponge using a micropipette. The percentage of mass loading of MoSe2 was determined by measuring the mass of sponge with a microbalance before and after the solvothermal-calcination process. 2.3. Characterization. X-Ray diffraction (XRD) was executed using a Bruker D8 Focus Xray diffractometer having a graphite monochromatized Cu Kα radiation (λ = 1.54178 Å). Scanning electron microscopy (SEM), Transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) imaging with energy-dispersive spectroscopy (EDS) spectra were acquired on Hitachi S-4800 and FEI Tecnai F20, respectively. Raman spectroscopy was performed on a Renishaw by Raman Microscope using laser excitation at 532 nm. X-ray photoelectron spectroscopy (XPS) spectra were collected on an AXIS-Ultra instrument from Kratos Analytical with a monochromatic Al Kα radiation (225 W, 15 mA, 15 kV) and low-energy electron flooding for charge reimbursement. The thermo-gravimetric analysis (TGA) was carried out from 30 oC to 700 oC under the flow of air with a temperature ramp of 10 o C min-1. The N2 adsorption-desorption isotherms were provided by an automated gas sorption analyzer (Autosorb-iQ-2MP, Quantachrome, USA) at 77 K and pore size distribution was measured by non-linear density functional theory (NL-DFT). 2.4. Electrochemical testing. The core-shell CNT@MoSe2 sponges were used as the positive electrodes (without dead elements) and Li metal acting as the negative electrodes whereas, a polypropylene film (Celgard 2400) as separators. One molar (1M) LiPF6 dissolved in a mixed solution of ethylene carbonate (EC), dimethyl car-

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bonate (DMC), and ethyl methylcarbonate (EMC) with a volume ratio of 1:1:1 was used as the electrolyte. Coin type (CR 2032) half-cells were manufactured in an argon (Ar)-filled glove box with oxygen (O2) and water (H2O) content of 10 times higher than that of the pure MoSe2 (7.46×10-12 cm2/s). This can be attributed to the presence of the highly conductive CNT network in the hybrid sponge. Although some composite electrodes of MoSe2 with CNT or amorphous carbon have been reported39-40,54 our 3D core-shell hybrid sponge is unique in that it consisted of continuously coated CNTs with MoSe2 nanosheets for its simple and controllable microstructure with superior electrochemical properties over traditional powdered hybrids. Foremost, the sponge is flexible and can be used as a free standing electrode with high active mass loading and without dead elements (current collector, binder, conductive agent), which makes it a viable applicant for a flexible and compressible electrochemical energy storage and conversion device. Secondly, the loading of the MoSe2 can be controlled for optimized performance simply by adjusting the number of MoSe2 layers in the core-shell nanotube sponge for a selected application. Thirdly, the porosity of the 3D CNT sponge is able to handle any volume expansion from the reversible Li+ storage on the electrode surface. Observe that the coaxial CNT@MoSe2 sponge depicts a high capacity retention with a continuous increase for several cycles. To explore this intriguing phenomenon we examined the TEM, HRTEM and XRD of dismantled electrodes after

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200 cycles. As shown in Figure 7a,b the MoSe2 nanosheets remain firmly coated on individual CNTs and the original core-shell hetero tubular structure is well maintained. The HRTEM analysis further revealed that a structural transition from continuous highly crystalline shells of MoSe2 to a partial amorphous state has occurred (Figure 7c). This structural transition was further evaluated after a battery test by a SAED pattern (Figure S10) and XRD on the hybrid CNT@MoSe2-10 sponge (Figure 7d). As observed in the XRD pattern, there were no obvious MoSe2 peaks observed. The structural evolution from a continuous crystalline shells into a partial amorphous state is induced by a repetitive intercalation/de-intercalation of Li+ through the active material MoSe2. During a reversible Li+ storage, the crystalline MoSe2 is gradually transformed into a more amorphous state, which can be proved by a post battery testing analysis with HRTEM and the absence of any significant crystalline peaks in the XRD analysis. Due to the presence of the large compressible spaces in the 3D freestanding porous electrode the gradual conversion of the crystalline MoSe2 to amorphous MoSe2 leads to more charged storage sites in the electrode material without damaging the internal structure of the electrode. Hence, the fabricated electrode materials exhibited good charge storage capacity with a long cyclic stability.

tuned simply by adjusting the active material loading. The hybrid sponge can be used as a free standing electrode for LIB and it delivered a discharge capacity of 820.5 mAhg-1 after 100 cycles at 100 mAg-1 with a good cyclic stability and rate capability. The high electrochemical performance can be attributed to the structural evolution from a continuous long crystalline MoSe2 shell into an amorphous state as evident by ex-situ investigations. The 3D core-shell coaxial nanotubular sponge design has the advantage of being flexible, easily tunable together with an intense compaction of the active material to yield a significantly improved areal capacity, and possesses a large number of active sites for reversible charge storage with a large number of pathways via CNTs for a fast charge transport. The resulting electrode can be potentially used in applications for flexible and compressible electrochemical energy storage and conversion devices. ASSOCIATED CONTENT The supporting information provides insights over EDX, TEM, HRTEM, XPS analysis and battery test of several products presented in this manuscript. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Authors * E-mail: [email protected] (R.P.S.H.) * E-mail: [email protected] (S.G.) Note: The authors declare no competing interest.

ACKNOWLEDGMENT 4. CONCLUSION In summary, we have fabricated a 3D interconnected core-shell CNT@MoSe2 sponge by a facile solvothermal method and a calcination process. The hybrid sponge has a core-shell nanotubular structure with a uniform MoSe2 nanosheets layered coating over the CNTs. The number of MoSe2 layers in the core-shell structure can be

This work was financially supported by the National Nature Science Foundation of China (Grant No. 51325202). Further, the researchers would like to acknowledge the provision of bulk CNT sponges from Professor AY Cao’s lab.

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Figure 1. Fabrication process of the core-shell CNT@MoSe2 sponge: (a) A schematic sketch of a synthesized CNT@MoSe2 sponge. (b) The original pure CNT sponge (15 × 8 × 3 mm3) and the CNT@MoSe2 sponge with its bended state.

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Figure 2. Crystal structure and morphology of CNT@MoSe2 sponge: (a) XRD pattern of the pure CNT and CNT@MoSe2 sponges. (b) SEM image of a pure CNT sponge. (c, d) SEM images, (e) TEM image, (f, g) HRTEM images, and (h) SAED pattern of CNT@MoSe2 sponge, respectively. (i) HAADF-STEM and EDS elemental maps of a single CNT@MoSe2 tube depict core shell structure. (j) EDS line scan measured along the diameter of the nanotube (shown by the yellow line in g) of a core-shell CNT@MoSe2 tube.

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Figure 3. Tuning the MoSe2 layers over CNTs: (a) Schematic representation of different layers of MoSe2 over CNT. (b-d) Various layers of MoSe2 nanotubes restrictively and directionally growing on CNT templates by adjusting the mass ratios of MoSe2 at about ≈45, ≈60, and ≈75 wt% , which corresponds to ≈5, ≈10 and ≈15 layers, respectively.

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Figure 4. Structural analysis of a core-shell CNT@MoSe2 sponge: (a) Raman spectra of pure CNT and hybrid CNT@MoSe2 sponges, respectively. (b-d) XPS survey spectra for the as-obtained CNT@MoSe2 sponge: high resolution spectra of (b) C 1s, (c) Mo 3d, and (d) Se 3d, respectively.

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Figure 5. TGA curves of (a) Pure CNT sponge and (b) CNT@MoSe2 sponge with ≈5, ≈10 and ≈15 layers, respectively. (c) Nitrogen adsorption and desorption isotherms of CNT sponge and CNT@MoSe2 sponge. (d) Pore size distributions for respective samples.

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Figure 6. Electrochemical performances of CNT@MoSe2 sponge: (a) The first five CV curves at a scan rate of 0.3 mVs-1. (b) Galvanostatic charge/discharge voltage profiles of 1st, 2nd, 3rd and 100th cycles at 100 mAg-1. (c) Comparison of cycling performance of pure MoSe2, CNT@MoSe2-U and CNT@MoSe2 sponges with various MoSe2 layers at 100 mAg-1. (d) Cycling performance of CNT@MoSe2-10 sponge at current density of 100 mAg-1 after 200 cycles. (e) Rate capability of CNT@MoSe2-10 sponge under shifty charge/discharge rates between 100 and 1600 mAg-1, respectively. (f) Nyquist plots of CNT@MoSe2-10 sponge and pure MoSe2 powder after 20 and 200 cycles.

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Figure 7. Ex-situ structural and morphological analysis of CNT@MoSe2-10 sponge after battery test: (a) TEM image, (b) HRTEM image and (c) zoomed view of the yellow rectangular box of the sponge under a charged state after 200 cycles. (d) XRD patterns of the pure CNT sponge and CNT@MoSe2 sponge after 200 cycles.

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Tunable Free-Standing Core-shell CNT@MoSe2 Anode for Lithium

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