Rational Design of Hierarchical Nanotubes through Encapsulating

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Rational Design of Hierarchical Nanotubes through Encapsulating CoSe Nanoparticles into MoSe/C Composite Shells with Enhanced Lithium and Sodium Storage Performance 2

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Jingyu Gao, Yapeng Li, Liang Shi, Jingjing Li, and Genqiang Zhang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 25 May 2018 Downloaded from http://pubs.acs.org on May 25, 2018

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ACS Applied Materials & Interfaces

Rational Design of Hierarchical Nanotubes through Encapsulating CoSe2 Nanoparticles into MoSe2/C Composite Shells with Enhanced Lithium and Sodium Storage Performance Jingyu Gao,† Yapeng Li, † Liang Shi,§ Jingjing Li§ and Genqiang Zhang*† †

Key Laboratory of Materials for Energy Conversion, Chinese Academy of Science;

Department of Materials Science and Engineering, University of Science and Technology of China, Hefei, Anhui 230026 China; §

Department of Chemistry, University of Science and Technology of China, Hefei, Anhui

230026 China *To whom the correspondence should be referred. Email:[email protected]

ABSTRACT Transition metal diselenides have been extensively studied as desirable anode candidates for both lithium-ion batteries (LIBs) and sodium-ion batteries (SIBs) due to their high theoretical capacities. However, it is of great challenge to achieve satisfactory cycling performance, especially for larger sodium ion storage, originated from electrode deterioration upon large volume change. Herein, we reported the construction of hierarchical tubular hybrid nanostructures through encapsulating CoSe2 nanoparticles into MoSe2/C composite shells via a simple two-step strategy including a hydrothermal method followed by vapor phase selenization process. The unique tubular structure enables the highly reversible Li/Na storage with high specific capacity, enhanced cycling stability and superior rate performance. It is indicated that the contribution of partial pseudocapacitive behavior greatly improves rate capability for SIBs, where a high capacity retention of 81.5% can be obtained when the 1

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current densities range from 0.1 to 3 A g−1 (460 mAh g-1 at 0.1 A g−1 vs. 379 mAh g-1 at 3 A g−1). This work provides an effective design rationale on transition metal diselenide based tubular nanostructures as superior host for both Li and Na ions, which could push forward the development of practical applications of transition metal diselenide based anode in LIBs and SIBs. KEYWORDS: hierarchical nanotube, transition metal diselenide, anode, sodium-ion battery, lithium-ion battery

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INTRODUCTION With the ever-increasing requirement of energy supply due to the rapid development of modern society, the aspiration on reducing the reliance on fossil fuels while turning to sustainable energy resources is urgent.1-2 Rechargeable batteries have been recognized as the promising approach to solve the intermittence shortage during the utilization of clean and renewable energy resources due to their high energy density and long cycle life.3-4 During the past decades, lithium-ion batteries (LIBs) have been broadly applied as power sources for portable electronics, and are being considered as the most promising candidate for electric vehicles.5 Currently, the central issue is to further improve the energy density of the LIBs in order to fulfill the comparable distance per charge to that of gasoline car. It is therefore the most imperative task to explore new electrode materials possessing higher capacity than currently used commercial graphite anode (~372 mAh g-1), which is the most critical factor deciding the energy density of the batteries.6 Besides, it is inevitable to face another tough issue of the increasing cost of the LIBs accompanying the rising market requirement, especially for the future grid-level energy storage systems because of the limited lithium sources in crust.7-9 In this context, sodium-ion batteries (SIBs) have recently been revived as an alternative candidate ascribed to the abundance and much lower price of sodium resources, which could be more appropriate for large-scale energy storage systems.10-12 Unfortunately, lacking of appropriate anode materials has become the bottleneck issue for the practical application of the SIBs due to the limited intercalation of sodium into graphite that is used as anode in commercial LIBs decided by the electrochemical kinetics between Na+ and C. Moreover, the larger radius of Na+ (1.02 Å) makes it more challenging to achieve satisfactory 3



cycling stability due to the more serious stress upon the sodiation/desodiation processes compared to that of Li+.13-16 Therefore, it is the common yet urgent task for both LIBs and SIBs to develop ideal Li+/Na+ host with high capacity, long cycle life and high rate performance. Over recent years, transition metal diselenides with lamellar structures analogous to that of graphite, have received tremendous attention due to their promising application as anode candidates in energy storage devices.17-18 The representative crystal structure is composed of metal layers sandwiched between two selenium layers via strong covalent bonding within the molecular layers, while the interlayers are stacked by weak van der Waals interactions, between which various guest ions are allowed to intercalate.19-22 However, when acted as anode materials, they generally suffer the severe capacity decay arising from the large volume expansion upon large capacity of lithium/sodium ion intercalation.23 Currently, there are multiple promising strategies that can effectively optimize the electrochemical behavior of transition metal diselenides as anodes for both LIBs and SIBs: i) Fabrication of various hollow micro-/nano-structures could use the interior voids to alleviate the volume change while the nanosized building blocks could enable the shorter ions transfer pathway, which can benefit the cycling capability and rate performance. However, the robustness of the hollow structure is unsatisfactory in most cases, especially for larger sodium ion intercalation, which still needs extra carbon coating process in order to achieve better cycling performance. Moreover, the large void portion will result in lower volumetric energy density, which is another disadvantage. ii) Construction of transition metal diselenide-carbon nanohybrid architectures have recently been the research focus since it can not only effectively inhibit the 4


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agglomeration and collapse of the electrodes benefitting from the excellent mechanic robustness of carbon, but also provide fast electron transfer pathway due to higher electrical conductivity of carbon.24-27 Unfortunately, the incorporation of carbon will increase the inactive weight which could reduce the gravimetric energy density. It is therefore highly desirable yet challenging to fabricate smart hybrid nanostructures which can possess the merits of carbon and interior voids while avoiding the accompanied disadvantages to the utmost extent.28-31 Herein, we developed a feasible two-step strategy for the fabrication of the hierarchical MoSe2/C composite nanotubes encapsulated with CoSe2 nanoparticles (denoted as CoSe2⊂MoSe2/C HNT) through hydrothermal coating of Mo/C precursor on the surface of the cobalt-nitrilotriacetic acid (denoted as Co-NTA) nanowires, followed by simple vapor

phase selenization process. Benefiting from the unique hybrid nanostructure, the CoSe2⊂MoSe2/C HNT exhibits outstanding electrochemical storage performance when acting as host for both Li+ and Na+ with highly reversible capacity and excellent rate capability.

Specifically, it can deliver a high reversible capacity of 1219 mAh g-1 at a current density of 0.1 A g-1 after about 100 cycles and 710 mAh g-1 at 1 A g-1after 200 cycles with a capacity retention of 92% for Li+ storage. Also, the hierarchical nanotube electrode exhibits a high specific capacity of 450 mAh g-1 at a current density of 0.1 A g-1 after 100 cycles for Na+ storage. Impressively, the rate capability of the CoSe2⊂MoSe2/C HNT for sodium storage is

excellent, which are 475 mAh g-1, 445 mAh g-1, 424 mAh g-1, 414 mAh g-1, 396 mAh g-1, 379

mAh g-1at the current densities of 0.1, 0.2, 0.5, 1, 2, 3 A g-1, respectively. This work sheds new light on the design principles for transition metal diselenide based tubular nanostructures 5



as highly efficient host for both Li and Na ions, which could contribute to their practical applications as electrode materials in energy storage systems. RESULTS AND DISCUSSION The fabrication process of CoSe2⊂MoSe2/C HNT is schematically illustrated in Figure

1. In brief, cobalt-nitrilotriacetic acid (Co-NTA) nanowires are firstly fabricated by a facile hydrothermal method with coordination between cobalt and nitrilotriacetic acid, which acts as the shape directed template and Co sources in the following step.25 Then, the Co-NTA@Mo/C

Figure 1. Schematic illustration of the formation of CoSe2⊂MoSe2/C HNT. (I) Mo/C coating

via a simple hydrothermal method, (II) Spontaneous formation of CoSe2⊂MoSe2/C HNT by vapor phase selenization process.

core shell nanowires are generated through the conformal coating of Mo/C composite layers on the surface of Co-NTA nanowires through a simple hydrothermal procedure utilizing ammonium molybdate and glucose as reaction agents. Finally, the CoSe2⊂MoSe2/C HNT can

be spontaneously formed during the vapor phase selenization conversion process at elevated

temperatures. The morphology evolution process could be reflected through the observation 6


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of the transmission electron microscope (TEM) images shown in Figure 2. As can be seen, the diameter of Co-NTA nanowires (Figure 2a) will increase while the morphology remains

Figure 2. TEM images of the morphology evolution process during the formation of CoSe2⊂MoSe2/C HNT: a) Co-NTA nanowires by coordination between cobalt and

nitrilotriacetic acid; b) Co-NTA@Mo/C core shell nanowires; c) CoSe2@MoSe2/C-350 porous nanowires; d) CoSe2⊂MoSe2/C HNT. similar to that of Co-NTA after the hydrothermal coating of Mo/C layer (denoted as Co-NTA@Mo/C) as shown in Figure 2b. Also, the X-ray diffraction (XRD) patterns of the Co-NTA and Co-NTA@Mo/C nanowires are the same (Figure S1a, Supporting Information), indicating the amorphous feature of the coating layer. After the selenization process at 350 oC (denoted as CoSe2@MoSe2/C-350), the morphology evolves from solid nanowire to porous

nanowires (Figure 2c) while the crystal structure changes from Co-NTA to hybrid phase of

MoSe2 and CoSe2 (Figure S1b, Supporting Information). The final CoSe2⊂MoSe2/C HNT can be formed as shown in Figure 2d when further annealed at 600 oC while the XRD pattern is similar to that of the CoSe2@MoSe2/C-350 (Figure S1b, Supporting Information). 7



Simultaneously, the typical field-emission scanning electron microscope (FESEM) images (Figure S2, Supporting Information) indicate that large-scale one-dimensional (1D) structure can be well retained after the hydrothermal coating and vapor phase selenization process, which elucidates the high yield of the products. Nitrogen adsorption/desorption isotherm (Figure S3, Supporting Information) confirms that CoSe2⊂MoSe2/C HNT has a mesoporous

structure with average pore size of 4 nm with a specific surface area of 86.14 m2 g−1, which is

higher than that of CoSe2@MoSe2/C-350 porous hybrid nanowires (~31.59 m2 g−1).

The detailed morphological and structural information of the CoSe2⊂MoSe2/C HNT are

further investigated with FESEM, TEM and high-resolution TEM (HRTEM) analysis, as

shown in Figure 3. The panoramic view of the products clearly indicates that large-scale 1D structure can be observed when the crystalline CoSe2⊂MoSe2/C HNTs are generated (Figure

3a, b) while the magnified FESEM image (Figure 3c) manifests the notable roughness of the surface, which could be derived from the formation layered MoSe2 in the composite shell. The hierarchical feature could be clearly observed through the TEM images (Figure 3d-f), which exhibit tubular structures with isolated nanoparticles encapsulated inside the nanotubes. The microstructural information is then analyzed through the HRTEM characterization at the selected areas on the shell part and encapsulated nanoparticles, as labelled in Figure 3g, respectively. It is evident that the shell of the hierarchical nanotube is composed of MoSe2 nanosheets and the lattice fringes with an interplanar spacing of 0.62 nm correspond to the d-spacing of the (002) planes (Figure 3h). As displayed in Figure 3i, the interplanar distance of 0.292 nm is indexed as the (200) crystal planes of cubic CoSe2 phase, which is consistent with that of XRD result. The formation of CoSe2⊂MoSe2/C hybrid structure is further 8


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Figure 3. Morphological and structural analysis of CoSe2⊂MoSe2/C HNT: a-c) SEM images

and d-f) TEM images with different magnifications, g-i) HRTEM analysis on the shell and inside nanoparticle, respectively.

demonstrated by elemental mapping investigation shown in Figure 4. The HAADF-STEM image (Figure 4a) gives the direct impression of the heterostructure of the nanotube. The elemental distributions of C and N are uniform throughout the nanotube (Figure 4b, c), indicating both the shell and the encapsulated nanoparticles are well protected with the N-doped C matrix. The distributions of Co, Mo and Se elements (Figure 4d) give the strong evidence that the encapsulated nanoparticles are CoSe2 while the outer shell is MoSe2/C composites, which confirms the formation of CoSe2⊂MoSe2/C HNT. The energy-dispersive

X-ray spectroscopy (EDS) analysis indicates that the atomic ratio between CoSe2 and MoSe2 is around 1.16:1 (Figure S4， Supporting Information). As a comparison, the morphological and structural analysis of the CoSe2@MoSe2/C-350 (Figure S5, Supporting Information) is

also investigated. It can be clearly observed that low temperature selenization process can 9



Figure 4. a) HAADF-STEM image and b-f) Corresponding elemental mapping results for the CoSe2⊂MoSe2/C HNT, showing the homogeneous distribution of C, N, Co, Mo, and Se elements.

induce the formation of porous nanowires with interconnected small CoSe2 nanoparticles uniformly sheathed by MoSe2/C composite shell, instead of hierarchical CoSe2⊂MoSe2/C tubular structures generated after further thermal annealing treatment at 600 oC.

The X-ray photoelectron spectroscopy (XPS) analysis is then performed to characterize the surface chemical composition and valence state of CoSe2⊂MoSe2/C HNTs. The survey

spectrum (Figure 5a) indicates the presence of five elements, i. e. C, N, Co, Mo, Se. In the C

Figure 5. XPS spectra of CoSe2⊂MoSe2/C HNT: a) Survey spectrum, b) C 1s, c) N 1s, d) Co 2p, e) Mo 3d, and f) Se 3d.

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1s spectrum (Figure 5b), peaks at 282.8, 284.7 and 286.2 eV correspond to Mo-C, sp2 C and C=N, respectively.32-33 As seen in Figure 5c, Mo-N and pyrrolic N are located at 395.2 and 400.1 eV, respectively.34 The binding energies at 778.1 and 793.5 eV in the high-resolution spectrum (Figure 5d) coincide with of Co 2p3/2 and Co 2p1/2 of Co2+ cations in CoSe2.35-36 The peaks at 780.5 and 796.3 eV correspond to the Co 2p3/2 and Co 2p1/2 of the Co–O bonding structures of native (amorphous) oxide layers at the surface.37 Besides, two pronounced shake-up satellites were found at the higher energy end of each Co 2p signal, which reveals the antibonding orbital between the Co atom and Se atom.38 Further, the Co 3d electron configuration prefers to be in the form of t2g6eg1, exhibiting a metallic conductor.39 Figure 5e shows the high resolution XPS spectra of Mo 3d, the primary doublet peaks at 229.1 and 232.3 eV can be assigned to the Mo 3d5/2 and Mo 3d3/2 peaks, indicating the existence of Mo4+ in MoSe2.40 As to Se 3d (Figure 5f), the peaks at 55.3 eV and 54.5 eV are assigned to the Se 3d3/2 and Se 3d5/2, where the lower binding energy is corresponding to metal–selenium of CoSe2 and MoSe2.41 Signals were also examined in the spectra of Se 3d at binding energies of 56.5 eV, owning to the deposition of a bit of metalloid Se on the carbon through final annealing treatment. Simultaneously, the XPS spectra of CoSe2@MoSe2/C-350 (Figure S6, Supporting Information) was as well measured to verify the consistence of elements: C, N, Co, Mo, Se, and the analytical results are in common with CoSe2⊂MoSe2/C-600 HNT with the sole exception of Se 3d signal where the intensity of Se

3d5/2 untwined with the metal–selenium bond increases gradually as annealing temperature

grows from 350 oC to 600 oC, which reflects better crystallinity of metal selenides. The content of the carbon in the CoSe2⊂MoSe2/C HNT is calculated to be about 21.54 % by the 11



thermogravimetric analysis (TGA) result (Figure S7). The electrochemical performance of CoSe2⊂MoSe2/C HNT is first investigated as anode

material for LIBs, as shown in Figure 6. The charge/discharge profiles for first, second, fifth and tenth cycles at a current density of 0.1 A g-1 are indicated in Figure 6a, where an initial

Figure 6. Lithium storage performance of the CoSe2⊂MoSe2/C HNT anode in half-cell

configuration: a) Discharge/charge profiles under the current density of 0.1 A g−1; b) Rate capability at current rates ranging from 0.1 to 3 A g-1; and c) Cycling stability evaluation with 0.1 and 1 A g-1, respectively.

discharge and charge capacity of around 1141 mAh g-1 and 911 mAh g-1 can be achieved, respectively, leading to a relatively high initial coulombic efficiency of about 79.8%. The irreversible capacity loss in the first cycle is principally derived from the formation of the solid electrolyte interface (SEI) layer and other side reactions, which is commonly observed 12


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in nanostructured conversion-type anode.32, 42-45 The charge/discharge curves for the following cycles are well coincident, indicating the high reversibility for Li+ storage. The CoSe2 ⊂MoSe2/C HNT electrode also exhibits excellent rate performance, as depicted in Figure 6b.

Specifically, the average specific capacities are 860, 855, 842, 820, 742 and 677 mAh g-1

corresponding to the current densities of 0.1, 0.2, 0.5, 1.0, 2.0, and 3 A g−1, respectively. Importantly, a high specific capacity of 1085 mAh g-1 can be recovered when the current density returns to 0.1 A g-1, confirming the highly reversible feature of the electrode. The cycling stability of the CoSe2⊂MoSe2/C HNT is also examined with different current

densities as shown in Figure 6c. When cycled at a current density of 0.1 A g-1, the specific capacity gradually increases from 933 mAh g-1 to around 1219 mAh g-1 after around 100 cycles. The increasing capacity trend could be ascribed to the activation process of the active

materials in the carbon matrix and gradual formation of a polymeric gel-like SEI layer, which has been broadly observed in nanostructured conversion-type electrode materials.42,

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Impressively, CoSe2⊂MoSe2/C HNT could achieve a superior capacity retention rate up to

95 % after 200 cycles under high charge/discharge current density of 1 A g-1, indicating the promising application of this unique hybrid nanostructure electrode in LIBs.

The potential application of the CoSe2⊂MoSe2/C HNT as anode candidate for SIBs is

then evaluated in a half-cell configuration. Figure 7a shows the charge/discharge profiles for

first, second, fifth and tenth cycles at a current density of 0.1 A g-1. The initial coulombic efficiency reaches as high as 76.3% (590 mAh g-1 upon discharge vs. 450 mAh g-1 upon charge), which is superior compared with those of previously reported values in conversion-type materials.47-50 It can be seen that the charge/discharge curves are 13



Figure 7. Sodium storage performance of the CoSe2⊂MoSe2/C HNT anode in half-cell configuration: a) Discharge/charge profiles at the current density of 0.1 A g−1; b) Rate

capability under current densities ranging from 0.1 to 3 A g-1; c) Cycling stability at 0.1 and 1 A g-1, respectively; d) CV curves at various scan rates, e) Capacitive contribution (shaded region) in CV curves (black line) under scan rate of 0.5 mV s-1, f) The percentage of capacitance contribution at different scan rates ranging from 0.5 to 10 mV s-1.

superimposed and there are notable plateau at around 1.5 V (vs. Na/Na+) in the following cycles, which is beneficial to the promising application in devices. Figure 7b exhibits prominent rate capability with current densities ranging from 0.1 to 3 A g−1, where 81.5% capacity retention can be achieved (460 mAh g-1at 0.1 A g−1 vs. 379 mAh g-1 at 3 A g−1). Moreover, the specific capacity can almost 100 % recovered when the current density reduces back to 0.1 A g−1 after high rate cycling test. The cycling stability of the CoSe2⊂MoSe2/C HNT electrode is evaluated with different current densities, as shown in Figure 7c. With a

current density of 0.1 A g−1, the specific capacity remains very stable at about 450 mAh g-1 after around 100 cycles, leading to the superior capacity retention of about 98% as compared to the discharge capacity at the second cycle. In addition, the CoSe2⊂MoSe2/C HNT could 14


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deliver a satisfactory cycling performance at high current density of 1 A g−1, where a reversible specific capacity of 305 mAh g-1 can be retained after 200 cycles. Furthermore, we investigated the Na-ion storage behavior and reaction kinetics of the CoSe2 ⊂MoSe2/C HNT by the analysis on the cyclic voltammetry (CV) curves under different

scan rates ranging from 0.5 to 10 mV s-1, as shown in Figure 7d. There are a couple of reduction/oxidation peaks in each curve, agreeing with the discharge/charge profiles. Notably,

the peak current (i) is not perfectly proportional to the square root of the scan rate (v), revealing that the charge and discharge processes are consisted of both non-Faradaic and Faradaic behavior. It has been demonstrated that the relationship between peak current (i) and scan rate (v) could be described as the following equations:51 (1) (2) where a and b are the adjustable parameters.51 When b equals to 1, the electrochemical process is mainly pseudocapacitance-controlled while it is dominated by ionic diffusion if b value is 0.5. In our case, the log (i) versus log (v) plots (Figure S8, Supporting Information) at reduction and oxidation peaks give the b values of 0.85 and 0.82, respectively, suggesting that the redox processes of the CoSe2⊂MoSe2/C HNT include partial pseudocapacitive

contribution. This could contribute to the fast Na+ interaction/extraction process, leading to enhanced

cycling

stability

and

rate

performance.

The

relative

calculation

of

pseudocapacitance contribution at fixed potential could be described by the equation as follows. (3) 15



where k1v refers to pseudocapacitance contribution and k2v0.5 represents diffusion contribution.52 Accordingly, around 48.1% of the specific capacity is contributed by pseudocapacitance at a scan rate of 0.5 mV s-1 (Figure 7e). The pseudocapacitance/diffusion capacity ratios at different scan rates are calculated as shown in Figure 7f, where it is found that the pseudocapacitive contribution slightly increases with the increasing of scan rates. The capacitive contribution may profit from the shorter ion diffusion and faster electron transfer pathways leading to the high-rate capability of CoSe2⊂MoSe2/C HNT electrode. 51, 53

The enhanced Li/Na storage performance of the CoSe2⊂MoSe2/C HNT could be derived

from the hierarchical tubular structures in the following aspects: First, the tubular structure with mesopores and hollow interior could largely reduce the Li/Na diffusion pathway54 and facilitate the penetration and contact area of the electrolyte, which promotes the electrochemical kinetics of Li/Na intercalation with better cycling stability. More importantly, the composite shell containing both electroactive MoSe2 and C could simultaneously facilitate the ion and electron transport, which could contribute to the high specific energy and better cycling stability and rate performance.

55-56

Moreover, the carbon could also effectively

alleviate the volume charge upon Li/Na insertion/extraction process, benefitting the cycling capability. In addition, the encapsulation electroactive CoSe2 nanoparticle inside the nanotubes could provide extra support to the shell of the nanotubes, which could further optimize the cycling stability, besides the contribution to the specific capacity. CONCLUSION In summary, we have developed a feasible two-step method including a facile hydrothermal coating process followed by vapor-phase selenization conversion treatment, for 16


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the synthesis of hierarchical nanotubes with CoSe2 nanoparticles encapsulated inside MoSe2/C composite shells (CoSe2⊂MoSe2/C HNT). Owing to its distinct architecture, the CoSe2⊂MoSe2/C HNT electrode exhibit highly reversible Li and Na storage performance

with high specific capacity, enhanced cycling capacity and superior rate performance as anode for LIBs and SIBs, respectively. The enhanced Li/Na storage performance could be originated from the synergistic effects of the shorter ion diffusion pathway from the hollow interior of the tubular structure, simultaneous faster ion and electron transport pathway and better structural robustness from the MoSe2/C composite shell and extra contribution of capacity and

further support of shell from the encapsulated CoSe2 nanoparticles inside the nanotubes. This work provides a new rationale on designing hierarchical hybrid hollow nanostructures as an advanced architecture for high performance electrode for both LIBs and SIBs, which could contribute to the advance of next-generation high energy storage devices. EXPERIMENTAL SECTION Synthesis of Co-NTA nanowires: In a typical synthesis, 0.77 g of CoCl2 is dissolved in 30 mL of deionized water followed by the addition of 0.6 g of nitrilotriacetic acid (NTA) and 10 mL of 2-propanol. After stirring for 15 mins, the solution is transferred to 50 mL Teflon-lined autoclave and treated at 180 oC for 6 h in the electric oven. Finally, the Co-NTA nanowires are collected after washing with DI water and ethanol several times. Synthesis of Co-NTA@Mo/C core/shell nanowires: Typically, 0.18 g of Co-NTA nanowires are dispersed into a solution of 10 mL deionized water and 20 mL ethylene glycol by sonication, and then 0.18 g ammonium molybdate was added into the above solution. After the addition of 0.36 g glucose, the mixed solution was further transferred into a 50 mL 17



Teflon-lined autoclave for heating at 200 oC for 10h. The products are collected by centrifugation after washing with deionized water and ethanol for several times. Synthesis of CoSe2⊂MoSe2/C HNT: 25 mg of Co-NTA@Mo/C product and 50 mg Se

powders were separately put into both ends of a quartz boat with the distance of around 8 mm. The quartz boat was further transferred into a tube furnace. Then CoSe2⊂MoSe2/C HNT is

obtained by annealed at 350 oC for 2h and up to 600 oC for 2h in argon with the Se powders

upriver of carrier gas. Also, CoSe2@MoSe2/C-350 porous nanowires could be obtained by

selenization process at 350 oC for 6h under the same conditions.

Materials Characterization: The crystal structure of samples was analyzed by X-ray diffraction (XRD, TTR-III, Japan) using Cu Ka radiation. The morphology and structure of the synthesized materials were investigated by field-emission scanning electron microscopy (FESEM, JSM-6700F, Japan), transmission electron microscopy (TEM, H-7650, Japan; Talos F200X, USA). Nitrogen sorption isotherms were characterized by the Surface Area and Porosity Analyzer (ASAP 2020) X-ray photoelectron spectroscopy (XPS, ESCALAB 250, USA) was used to evaluate the valence of Mo, Co and Se. Electrochemical Measurements: The electrochemical cycling tests were carried out in CR2016 coin-type cells. Working electrode was prepared by a homogeneous slurry composed of CoSe2⊂MoSe2/C HNT, carbon black (Super-P), and sodium carboxymethylcellulose

(CMC) in a weight ratio of 7:2:1 on a copper foil. The loading mass of active materials is

around 1.2-1.5 mg cm-2. For lithium-ion batteries, Li foil was used as both reference and counter electrode, and the electrolyte was 1.0 M LiPF6 in ethyl carbonate/diethyl carbonate (1:1 v/v ratio). For sodium-ion batteries, Na foil was used as both reference and counter 18


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electrode, and 1.0 M NaClO4 in ethyl carbonate/diethyl carbonate (1:1 v/v ratio) containing 5 wt% fluoroethylene carbonate was used as the electrolyte. Cyclic voltammetry (CV) measurements was performed on a CHI660E electrochemical workstation. Galvanostatic discharge/charge tests were conducted on a NEWARE battery tester between 0.01 and 3V.

ASSOCIATED CONTENT

Supporting Information Available: XRD patterns of Co-NTA, Co-NTA@Mo/C, CoSe2@MoSe2/C-350 and CoSe2⊂MoSe2/C

HNT;

SEM

images

of

Co-NTA,

Co-NTA@Mo/C,

CoSe2@MoSe2/C-350

and

CoSe2⊂MoSe2/C HNT; Nitrogen absorption/desorption isotherms and pore size distributions of

CoSe2 ⊂MoSe2/C

HNT

and

CoSe2@MoSe2/C-350

HNT;

EDS

spectrum

of

CoSe2⊂MoSe2/C HNT; SEM images and HRTEM images of CoSe2@MoSe2/C-350; XPS spectra of CoSe2@MoSe2/C-350 porous nanowires; TG curves of a) CoSe2⊂MoSe2/C HNT

and b) CoSe2@MoSe2/C-350 porous nanowires. Plots of log (scan rate) versus log (peak current) of CoSe2⊂MoSe2/C HNT. Acknowledgements

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For Table of Contents Only

27


Rational Design of Hierarchical Nanotubes through Encapsulating

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