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Carbon-Stabilized Interlayer-Expanded Few-Layer MoSe2 Nanosheets for Sodium Ion Batteries with Enhanced Rate Capability and Cycling Performance Yongchao Tang, Zongbin Zhao,* Yuwei Wang, Yanfeng Dong, Yang Liu, Xuzhen Wang, and Jieshan Qiu* State Key Lab of Fine Chemicals, Liaoning Key Lab for Energy Materials and Chemical Engineering, PSU-DUT Joint Center for Energy Research, School of Chemical Engineering, Dalian University of Technology, Dalian116024, P. R. China S Supporting Information *
ABSTRACT: Sodium ion batteries (SIBs) have been considered as a promising alternative to lithium ion batteries, owing to the abundant reserve and low-cost accessibility of the sodium source. To date, the pursuit of high-performance anode materials remains a great challenge for the SIBs. In this work, carbon-stabilized interlayer-expanded few-layer MoSe2 nanosheets (MoSe2@ C) have been fabricated by an oleic acid (OA) functionalized synthesis−polydopamine (PDA) stabilization−carbonization strategy, and their structural, morphological, and electrochemical properties have been carefully characterized and compared with the carbon-free MoSe2. When evaluated as anode for sodium ion half batteries, the MoSe2@C exhibits a remarkably enhanced rate capability of 367 mA h g−1 at 5 A g−1, a high reversible discharge capacity of 445 mA h g−1 at 1 A g−1, and a long-term cycling stability over 100 cycles. To further explore the potential applications, the MoSe2@C is assembled into sodium ion full batteries with Na3V2(PO4)3 (NVP) as cathode materials, showing an impressively high reversible capacity of 421 mA h g−1 at 0.2 A g−1 after 100 cycles. Such results are primarily attributed to the unique carbon-stabilized interlayer-expanded few-layer MoSe2 nanosheets structure, which facilitates the permeation of electrolyte into the inner of MoSe2 nanosheets, promoting charge transfer efficiency among MoSe2 nanosheets, and accommodating the volume change from discharge−charge cycling. KEYWORDS: oleic acid functionalization, carbon stabilization, expanded interlayer spacing, few-layer MoSe2 nanosheets, sodium ion batteries
1. INTRODUCTION Presently, lithium ion batteries (LIBs) have been applied widely in various electronic devices, such as laptops, cell phones, smart watches, and so forth, due to their excellent rate capability and cycling stability.1,2 However, the limited lithium source cannot afford ever-growing demands in the large-scale commercial application of LIBs. Therefore, seeking an alternative energy system has been put on the schedule. Most recently, sodium ion batteries (SIBs) have attracted rising research attention owing to abundant reserves and low-cost accessibility of the sodium source.3,4 Nonetheless, compared with lithium ions, sodium ions possess ca. 55% larger radius, which makes the sodiation−desodiation more difficult to be realized, resulting in a severly sluggish reaction dynamics.5,6 To date, the anode materials for the SIBs are still confronted with a series of challenges, mainly including lower reversible capacity, inferior rate capability, and cycling stability.3−6 © 2016 American Chemical Society
Proper host materials can effectively enhance the rate capability and cycling stability of the SIBs.3,7 As twodimentional graphene-like materials, layered transitional metal chalcogenides (LTMCs) such as MoS2 have been extensively investigated in many fields, such as electrochemical catalysis, dye-sensitized cell, LIBs, and SIBs, and so forth.8,9 However, when applied as anode materials for the SIBs, the MoS2 typically display a lower specific capacity and poor cycling stability.10,11 The main reasons for these can be attributed to the smaller pristine interlayer spacing (0.62 nm) and the lower conductivity of the MoS2, which are disadvantageous to the reversible sodiation−desodiation and high-efficient charge transfer.12,13 Last but not least, MoS2 nanosheets are usually Received: September 6, 2016 Accepted: November 7, 2016 Published: November 7, 2016 32324
DOI: 10.1021/acsami.6b11230 ACS Appl. Mater. Interfaces 2016, 8, 32324−32332
Research Article
ACS Applied Materials & Interfaces
capacity and robust cycling performance. Such properties are primarily attributed to the unique carbon-stabilized interlayerexpanded few-layer MoSe2 nanosheet structure, which facilitate the permeation of electrolyte into the inner of MoSe2 nanosheets, promotes charge transfer efficiency among MoSe2 nanosheets, and accommodates the volume change from discharge−charge cycling.
inclined to agglomerate together, especially under annealing, due to their higher surface energy. Such properties markedly lower the application efficiency of MoS2 nanosheets. To improve the electrochemical performance of MoS2-based anode materials, researchers have made enormous effort and achieved some meaningful progress.13−17 Hydrothermal synthesis and ultrasonic exfoliation have been employed for the production of MoS2 nanosheets with expanded interlayer spacing for better rate capability and higher reversible capacity.11,13,17,18 In addition, combining the MoS2 with nanocarbon (e.g., graphene, carbon nanotubes, and so forth) is an effective strategy adopted generally to improve the sodium storage performance of the composites.14,16,19,20 However, in most cases, the MoS2 nanosheets are in situ grown on the surface of carbon matrix, which improves the electronic conductivity of anode material, but fails to efficiently accommodate the great volume change (up to 300%) from discharge−charge cycling, thus usually exhibiting an unsatisfied sodium storage performance. As an analogue to the MoS2, the MoSe2 has attracted recent research attentions in the SIBs, owing to their similar physicochemical properties.21−25 Compared with the MoS2, the MoSe2 possess larger interlayer (0.64 nm) and higher conductivity resulted from smaller band gap.26 Such properties endow the MoSe2 with considerable potential as anode for superior SIBs with better Coulombic efficiency and rate capability.23,27 In addition to the intrinsic properties of MoSe2, the sodium storage performance is mainly determined by the unique structures of the materials resulted from novel synthesis routes. Most recently, there have been some explorations concerning MoSe2-based anodes, such as layered MoSe2 nanoplates for the SIBs, the porous hollow carbon sphere decorated with MoSe2 nanosheets for the LIBs/SIBs, and sheet-like MoSe2/C composite for the LIBs.21−23 However, these anode materials reported exhibits unsatisfied reversible discharge capacities (∼400 mA h g−1 at 1.5 A g−1) and mediocre rate capability, especially at larger current density (>2 A g−1).23,28 In many cases, such MoSe2 based materials are synthesized by hydrothermal method or other solid chemistry routes. Same as the MoS2, due to the absence of stabilizer, the as-synthesized MoSe2 nanosheets usually also demonstrates a severe agglomeration upon experiencing high-temperature annealing. Until now, employing proper strategy to bypass the shortcomings of MoSe2 (such as inadequate conductivity, easy agglomeration, and so forth) for superior sodium storage remains an open question. Herein, via an OA fuctionalized synthesis−PDA stabilization−carbonization strategy, carbon-stabilized interlayer-expanded few-layer MoSe2 nanosheets (MoSe2@C) have been successfully acquired, wherein the few-layer MoSe2 nanosheets have been separated and stabilized by the OA-PDA-derived carbon layers. The formation mechanism of the MoSe2@C has been prudently discussed. Other than the LTMCs-based electrode materials with the growth of multilayer LTMC nanosheets on the surface nanocarbon,20,21,23,24,29 the MoSe2@ C is composed of many interlayer-expanded few-layer nanosheets separated and stabilized by amorphous carbon layers, which effectively prohibit the agglomeration of MoSe2 nanosheets. When applied as anode for sodium ion half batteries, the MoSe2@C exhibits an excellent sodium storage perfromance, especially, encompassing impressively high reversible specific capacity and outstanding rate capability. Moreover, when assembled in a full cell with NVP as cathode, the MoSe2@C anode also displays a higher reversible specific
2. EXPERIMENTAL SECTION 2.1. Synthesis of Oleic Acid-Functionalized MoSe2 (MoSe2@ OA). The MoSe2@OA was synthesized by a general solvothermal method, oleic acid was employed as a solvent and capping agent.30 2 mmol (0.158 g) selenium powders were first ultrasonically dissolved in 5 mL hydrazine hydrate solution in an ambient environment. Subsequently, the hydrazine hydrate−selenium solution was added into a separate 100 mL beaker, in which 1 mmol (0.243 g) Na2MoO4· 2H2O has been added beforehand, and stirred for at least 10 min. Finally, 20 mL oleic acid was dropwise added into the abovementioned beaker under rigorous stirring, the mixture gradually becomes a uniform gray paste. Then the paste was put into a 50 mL autoclave and treated at 220 °C for 12 h, the obtained black precipitate was washed by absolute ethanol thoroughly for three times. For comparison, the MoSe2 was also synthesized with the help of ethanol by a similar route. 2.2. Synthesis of Polydopamine-Stabilized MoSe2 (MoSe2@ OA-PDA). Typically, the as-synthesized MoSe2@OA (containing 1 mmol MoSe2) was dispersed into a buffer solution containing 61 mg 2amino-2-hydroxymethylpropane-1, 3-dio (Tris). Subsequently, 50 mg dopamine was added into the above solution, and keeping stirring for 24 h in air. During the synthesis of MoSe2@OA-PDA, in addition to the self-polymerization, the dopamine can react with the oleic acid through amidation reaction.12 The obtained product was carefully centrifuged for three times, and freeze-dried overnight. For comparison, the MoSe2@PDA with different PDA contents was synthesized by tuning the dosages (such as 25, 50, 75, and 100 mg) of dopamine as well. 2.3. Synthesis of Carbon-Stabilized MoSe2 (MoSe2@C). The as-synthesized MoSe2@OA-PDA was annealed at different temperatures (such as 700, 800, and 900 °C) under N2 atmosphere for 2 h. The heating rate was controlled to 2 °C min−1. Herein, the carbon content can be tailored by using various dosages of dopamine. 2.4. Synthesis of Carbon-Coated Na3V2(PO4)3 (NVP). The synthesis of carbon-coated Na3V2(PO4)3 was referred to the method reported elsewhere.31 V2O5 (1.32 g), NH4H2PO4 (2.5 g), Na2CO3 (1.41 g), and acetylene black powder (0.15 g) were dissolved to deionized water (50 mL) and vigorously stirred for 4 h to obtain a mixture solution. This mixture was then dried at 50 °C and preheated at 350 °C for 4 h. After being reground, the mixture was annealed in a quartz tube under the protection of argon atmosphere at 650 °C for 8 h. 2.5. Characterization of Materials. The crystal structure of the samples was characterized by X-ray diffraction (Rigaku D/Max 2400 type X-ray spectrometer with Cu Ka radiation (11/41.5406 Å)). The morphology was analyzed by field emission scanning electron microscopy (FE-SEM). The structure details were further characterized by transmission electron microscopy (TEM, FEI Tecnai G2 Spirit, operated at 120 kV) and high-resolution transmission electron microscopy (HR-TEM, FEI Tecnai G2 F30S-Twin, operated at 300 kV). Selected area electron diffraction (SAED) patterns were collected with a Gatan charge-coupled device (CCD) camera in a digital format. The valence states of the samples were analyzed by an X-ray photoelectron spectrometer (XPS, Thermo ESCALAB 250). Surface functional groups were studied using a Nicolet-20DXB Fourier transform infrared spectrometer (FT-IR). The Raman characterization was performed using a Raman microscope (DXR). The thermogravimetric analysis was conducted by a thermogravimetric analyzer (TGA, STA 449 F3 Jupiter). 2.6. Electrochemical Test. For the electrochemical test, the electrodes were fabricated by mixing the samples (MoSe2@C, MoSe2, 32325
DOI: 10.1021/acsami.6b11230 ACS Appl. Mater. Interfaces 2016, 8, 32324−32332
Research Article
ACS Applied Materials & Interfaces
Scheme 1. (a) Synthesis Procedure of the MoSe2@C. SL = Single Layer. (b) Amidation Reaction Happened between Oleic Acid and Dopamine
Figure 1. Crystallinity phase, chemical compositions, and morphological features of the samples. (a) XRD patterns, (b) Raman spectra, and (c, d) FE-SEM images of the annealed MoSe2 and MoSe2@C. recorded with a scan rate of 0.1 mV s−1 in the cutoff voltage range of 0.01−3 V. All the cycling performance tests were conducted in the cutoff voltage range of 0.01−3 V at different current densities. For the sodium ion full cell, MoSe2@C and NVP were used as anode and cathode, respectively. The mass ratio of cathode to anode was controlled to be ca. 6:1 in the full cell. Before the assembly of full cell, the anode was presodiated for two cycles with sodium platelet as counter electrode. The specific capacities of full cells are calculated on the basis of anode mass.
etc.), acetylene carbon black, and binder (poly(vinylidene fluoride), PVDF) with the weight ratio of 7:2:1 in the n-methyl-2-pyrrolidone (NMP) solvent to form a slurry. Then, the slurry was pasted onto copper foil to make electrodes. After dried in a vacuum oven at 120 °C overnight, the electrodes were pressed and weighted. CR2016 coin cells were assembled in an argon-filled glovebox (UniLab, Mbraun, Germany). Na metal platelet (Sigma-Aldrich, 99%) and glass microfiber (Whatman) were used as the counter electrode and separator, respectively. The electrolyte was 1 M NaClO4 (SigmaAldrich, 99%) in ethylene carbonate (EC, Sigma-Aldrich, 99%) and diethyl carbonate (DEC, Sigma-Aldrich, 99%) with a volume ratio of 1:1. Cyclic voltammetry (CV) and discharge−charge measurements were carried out on a BioLogic electrochemical workstation (VMP 3B5) and Land battery testing system, respectively. The CV curves were
3. RESULTS AND DISCUSSION The synthesis procedure of carbon-stabilized interlayerexpanded few-layer MoSe2 nanosheets (MoSe2@C) is illus32326
DOI: 10.1021/acsami.6b11230 ACS Appl. Mater. Interfaces 2016, 8, 32324−32332
Research Article
ACS Applied Materials & Interfaces
The MoSe2 and MoSe2@C were further characterized by TEM and HR-TEM. As shown in Figure 2a, the MoSe2@C is
trated in Scheme 1a. The experimental details are shown in Experimental Section. First, the OA-functionalized few-layer MoSe2 (MoSe2@OA) is synthesized with OA as capping agent, since the OA can coordinate with the Mo atoms, prohibiting the formation of MoSe2 along (002) crystal plane.30 As shown in Figure S1a of the Supporting Information, X-ray diffraction (XRD) pattern of as-synthesized MoSe2@OA exhibits an inferior crystallinity with the absence of (002) diffraction peak at ca. 13.0 o (JCPDS 29−0914), indicating the presence of fewlayer structure in the MoSe2@OA. Moreover, the MoSe2@OA displays an obvious agglomeration owing to the adhesive attraction of OA (Figure S1b). Moreover, HR-TEM image of MoSe2@OA possessed obvious few-layer structure (
