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Research Article www.acsami.org

Tunable Free-Standing Core−Shell CNT@MoSe2 Anode for Lithium 2 Storage 1

Muhammad Yousaf,† Yunsong Wang,† Yijun Chen,† Zhipeng Wang,† Waseem Aftab,† Asif Mahmood,†,‡ † ,† 4 Wei Wang, Shaojun Guo,* and Ray P. S. Han*,† 3

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

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S Supporting Information *

ABSTRACT: Heterogeneous nanostructuring of MoSe2 over a carbon nanotube (CNT) sponge as a free-standing electrode not only brings 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 nanotubular sponge is synthesized via a solvothermal process, followed by thermal annealing to improve 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 mAh g−1 after 100 cycles at 100 mA g−1 with a high cyclic stability and rate capability. Further, an ex situ structural and morphological analysis revealed that ionic storage causes a phase change in MoSe2 from a crystalline to a partial amorphous state for a continuous increase in the capacity with 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: heteronanotubes, 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 batteries (LIBs) in order to meet the overwhelming demands of electric vehicles, small grid stations, and portable electronic devices.1−7 Since the discovery of unusual properties in graphene, two-dimensional (2D)-layered materials, especially transition-metal dichalcogenides (TMDs), have received considerable attention for use in energy conversion and storage devices because of their superior physical, chemical, and electronic properties.8−12 The layered structure provides a short diffusion path for various guest species to readily intercalate/deintercalate for energy drawdown and drawup.13−19 As a well-known TMD, molybdenum diselenide (MoSe2) has a sandwich layered structure (Mo−Se− Mo) similar to MoS2.20−23 MoSe2 offers a high theoretical capacity due to the conversion reaction along with the intercalation of Li+, better contact with the 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 current densities, which can © XXXX American Chemical Society

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 a one-dimensional (1D) core−shell nanotubular structure by wrapping 2D TMD sheets over a 1D tubular substrate produces features such as radial heterojunctions and multifunctionalities.31−34 The presence of a 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 because of the 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. Received: December 28, 2017 Accepted: April 13, 2018 Published: April 13, 2018 A

DOI: 10.1021/acsami.7b19739 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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was added to improve its hydrophilicity. Two precursors were loaded one after the other into the CNT sponge with a micropipette. First, the molybdenum precursor was loaded into the CNT sponge and dried at 70 °C; subsequently, the selenium precursor was loaded into the molybdenum-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 freeze-dried to maintain the 3D porous structure. Finally, the sponge was annealed at 800 °C 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 X-ray diffractometer having a graphitemonochromatized Cu Kα radiation (λ = 1.54178 Å). Scanning electron microscopy (SEM), transmission electron microscopy (TEM), and high-resolution TEM (HRTEM) imaging with energydispersive X-ray spectroscopy (EDX) spectra were acquired on Hitachi S-4800 and FEI Tecnai F20 microscopes, 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 monochromatic Al Kα radiation (225 W, 15 mA, and 15 kV) and low-energy electron flooding for charge reimbursement. Thermogravimetric analysis (TGA) was carried out from 30 to 700 °C under the flow of air with a temperature ramp of 10 °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 the pore-size distribution was measured by nonlinear density functional theory (NL-DFT). 2.4. Electrochemical Testing. The core−shell CNT@MoSe2 sponges were used as the positive electrodes (without dead elements), with lithium metal acting as the negative electrode and a polypropylene film (Celgard 2400) as a separator. LiPF6 (1 M) dissolved in a mixed solution of ethylene carbonate, dimethyl carbonate (DMC), and ethyl methylcarbonate with a volume ratio of 1:1:1 was used as the electrolyte. Coin-type (CR 2032) half-cells were manufactured in an argon-filled glovebox with oxygen and water contents of 10 times higher than that of the pure MoSe2 (7.46 × 10−12 cm2 s−1). 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 reported,39,40,54 our 3D core− shell hybrid sponge is unique in that it consists of continuously coated CNTs with MoSe2 nanosheets for its simple and controllable microstructure with superior electrochemical properties over traditional powder hybrids. First, 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, and conductive agent), which makes it a viable applicant for a flexible and compressible electrochemical energy storage and conversion device. Second, the loading of 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. Third, the porosity G


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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b19739. Insight into EDX, TEM, HRTEM, XPS analysis, and battery tests of several products presented in this manuscript (PDF)

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AUTHOR INFORMATION

Corresponding Authors

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*E-mail: [email protected] (S.G.). *E-mail: [email protected] (R.P.S.H.).

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ORCID

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Shaojun Guo: 0000-0003-4427-6837 Ray P. S. Han: 0000-0002-3551-4795

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Notes

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The authors declare no competing financial interest.

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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-nanosheet-layered coating over the CNTs. The number of MoSe2 layers in the core−shell structure can be tuned simply by adjusting the active material loading. The hybrid sponge can be used as a freestanding electrode for LIBs, and it delivered a discharge capacity of 820.5 mAh g−1 after 100 cycles at 100 mA g−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 is 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 fast charge transport. The resulting electrode can be potentially used in applications for flexible and compressible electrochemical energy storage and conversion devices.

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ACKNOWLEDGMENTS This work was financially supported by the National Nature Science Foundation of China (Grant 51325202). Further, researchers acknowledge the provision of bulk CNT sponges from Professor A. Y. Cao’s laboratory.

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REFERENCES

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