Nb2O5-Decorated Nitrogen-Doped Carbon Nanotube Microspheres

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Materials and Interfaces

Nb2O5-decorated nitrogen-doped carbon nanotube microspheres for highly efficient sulfur confinement in lithium-sulfur batteries Xiaoyu Wen, Kaixiong Xiang, Yirong Zhu, Li Xiao, Haiyang Liao, Xianhong Chen, and Han Chen Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.9b00471 • Publication Date (Web): 03 May 2019 Downloaded from http://pubs.acs.org on May 3, 2019

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Industrial & Engineering Chemistry Research

Nb2O5-decorated nitrogen-doped carbon nanotube microspheres for highly efficient sulfur confinement in lithium-sulfur batteries Xiaoyu Wen, Kaixiong Xiang, Yirong Zhu, Li Xiao, Haiyang Liao, Xianhong Chen, Han Chen* School of Metallurgy and Materials Engineering, Hunan University of Technology, Zhuzhou Hunan 412007, PR China Corresponding author: Han Chen, [email protected] Tel: +8613786350206 Abstract To confine the intermediate polysulfides to obstruct their dissolution from the electrolyte and increase the utilization of sulfur, a novel sulfur host for decorated nitrogen-doped carbon nanotube (NCNTs) microspheres was produced by a terse ultrasonic spraying with succedent impregnation methods. Decoration effects for Nb2O5 are better than those for Sb. Nb2O5-NCNTs microspheres are woolen-like uniform balls which are twined with CNTs and infiltrated Nb2O5 nanoparticles. Then Nb2O5-NCNTs microspheres deliver the high initial charge and discharge capacities of 1369.5 and 1335.3 mAh g-1, good cycle stability of 1201.7 mAh g-1 at 0.1 C after 200 cycles, and outstanding long-term cycle life of 846.2 mAh g

−1

after 1000 cycles

at 2.0 C with a very low capacity decay of 0.016% per cycle. The Nb2O5-NCNTs microspheres with remarkable effects for sulfur confinement are a potential cathode material for lithium-sulfur batteries. Keyword: Li-S batteries; Composite materials; Carbon nanotubes; Nb2O5

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Industrial & Engineering Chemistry Research 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

nanoparticles; Electrochemical properties 1. Introduction The demands for alternative green and sustainable energy become more and more increasing with the excessive consumption of fossil fuel, serious environmental deterioration and the popularity of electric vehicles. Rechargeable batteries, such as lead-acid, Ni-MH, Ni-Cd and lithium-ion batteries, have occupied over the electronic markets for centuries. Lithium ion batteries have dominated energy storage equipment for hybrid-electric vehicles, electric vehicles and electric tools owing to their high energy density and long cycle life compared to other rechargeable batteries since their commercialization in the 1990s 1. Nonetheless, the energy density of traditional lithium ion batteries (less than 400 Wh kg-1 for LiCoO2/graphite cells) cannot satisfy the rapidly-growing requirement of portable electronics

2.

Therefore, the researchers have devoted themselves to

exploring new electrode materials which can provide high energy and power densities in the past decades. Meanwhile, lithium-sulfur batteries are considered as the most potential rechargeable lithium-ion batteries due to their high theoretical energy density of 2600 Wh kg-1 and theoretical capacity of 1675 mAh g-1. In addition, elemental sulfur has superiority such as natural abundance, low cost and non-pollution 3.

However, practical applications for Li-S batteries are still hindered by many

challenges: Firstly, sulfur and its discharge products (Li2Sx, x=1-8) show poor ionic and electronic conductivities, which increases the internal resistance of the batteries;

2


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Secondly, the dissolution of lithium polysulfides in the electrolytes during the charge-discharge process; Thirdly, the volume expansion of the S cathode upon cycling. These disadvantages lead to low sulfur utilization and Coulombic efficiency and inferior cycle life in Li-S batteries 4. To solve the above problem, many efforts have been explored to enhance the conductivity of sulfur, impede the dissolution of polysulfide into the electrolyte and relieve the volume expansion from the sulfur. The carbonaceous nanomaterials 5-13, 27, conductive polymers

14-16

and metal oxides

17-25

are involved to confine sulfur

cathodes and improve electronic conductivity of sulfur

26.

Among them, a large

number of carbonaceous nanomaterials, such as carbon nanotubes nanofibers

7, 8,

carbon spheres

9, 10

and 3D porous carbon

10-13,

5, 6,

carbon

have been generally

used as the sulfur hosts due to their high electrical conductivity, abundant porous structure and flexible elastic nature. A new cell configuration was reported using microporous carbon paper as an interlayer covered on the surface of a bare sulfur cathode, delivering an excellent capacity above 1000 mAh g-1 at 100 cycles at 1.0 C rate (1.0 C=1675 mA g-1)

27.

A novel hierarchical porous carbon-carbon nanotube

hybrid was prepared and delivered stabilized cycling performance in Li-S batteries with only 0.1% capacity decay per cycle up to 250 cycles at 1.0 C 5. A flexible and free-standing sulfur/porous carbon nanofibers-CNT composite (S@PCNFs-CNT) as cathode for Li-S batteries showed a reversible capacity of 637 mAh g-1 after 100 cycles at 50 mAh g -1 and a rate capability of 437 mAh g -1 at 1 Ag-1 7. In addition, Wu and co-workers designed polyelectrolyte multilayers (PEMs) and graphene sheets 3



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which are applied to sequentially coat on the surface of hollow carbon spheres/sulfur composite by a flexible layer-by-layer (LBL) self-assembly strategy. The fabricated composite exhibited very stable cycling stability for over 200 cycles at 1 A g -1, along with a high average coulombic efficiency of 99%

10.

Ding et al. reported that a

sulfur-carbon yolk-shell particles are based on 3D interconnected nanostructure as cathode material and exhibited a high initial capacity of 560 mAh g

-1

per gram

electrode with good cycling performance 13. Furthermore, Metal oxides, such as MnO2 17, 18, TiO2 19, 20, 37, Al2O3 21, MgO 22, 23 and SiO2

24, 25,

are widely investigated due to their intense chemical binding energy

with lithium polysulfides, which can avert the irreversible dissolution of polysulfides within the electrolyte during the charge and discharge processes

28.

An effective

nanometric MnO2 shell on sulfur particles were fabricated by an in-situ redox reaction between sulfur and KMnO4 under ambient conditions, the composite materials exhibited a very low capacity fade rate of 0.048% per cycle over 800 cycles at 2.0 C rate and provided a final reversible capacity of 480 mAh g-1

18.

A nanocomposite

cathode consisted of sulfur and hollow-mesoporous TiO2 embedded within CNTs, showed the initial capacity of 1113 mAh g-1 and the capacity retention of 93.5% after 100 cycles at 1.0 C rate

19.

In addition, Kim and co-workers designed the

lithium-sulfur cell with SMgO-10 composite which the hydrophilic MgO nanoparticles uniformly distributed on the surface of sulfur, demonstrating the best cycling performance with capacity retention of 83.8% after 100 cycles 23. Unlike other metal oxides, Nb2O5 is an electronic semi-conductor, which can provide the electrode 4


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with a more efficient charge transfer space due to the unique Nb-O crystalline structure. Therefore, Nb2O5 can as an active assistant material for affording extra capacity in discharge/charge reaction. Tao

4

prepared mesoporous carbon

microspheres/Nb2O5/S composites has a high intial capacity of 1289 mAh g-1 at 0.5 C with a reversible capacity of 913 mAh g-1 after 200 cycles. Herein,

a

novel

sulfur

host,

Nb2O5-decorated

woolen-like

NCNTs

(Nb2O5-NCNTs) microspheres were constructed by an ultrasonic spray method. Uniform and fine nanometer NCNTs powders can be obtained by ultrasonic spray method, which transfers the solution into uniform and tiny droplets with certain energy through the action of ultrasound. Nb2O5 nanoparticles are embedded within the abundant pores of the NCNTs microspheres, even penetrated into the inner NCNTs using an impregnation method with low concentration precursor solutions. Conductive NCNTs microspheres are self-woven under the effect of ultrasonic spraying to provide rich voids for the accommodation of sulfur and Nb2O5 nanoparticles. Moreover, Nb2O5 nanoparticles can provide strong chemical bonding with polysulfides to prevent the dissolution of soluble polysulfides during the electrochemical

reaction.

Therefore,

woolen-like

Nb2O5/carbon

nanotube

microspheres show a specific structure and effectively overcome the shortcoming of the electrochemical performance for Li-S battery.

5



2. Experimental 2.1 Materials Synthesis Schematic illustrations of the preparation process of the Nb2O5-NCNTs microspheres are given in Scheme 1. The NCNTs microspheres were made of amination carbon nanotubes (5 wt%, Aladdin) and sucrose in a weight ratio of 1:4 by the ultrasonic spraying. Then NCNTs microspheres were ultrasonically dispersed in 0.02 mol L-1 C10H5NbO20 solution and immersed at 25 oC for 6 days. The mixed solution was filtrated to acquire the sediment. The sedimentary product was placed in a tube furnace at 600 oC for 3 h under nitrogen gas flow and named Nb2O5-NCNTs microspheres. The Nb2O5-NCNTs/S microspheres were composed of Nb2O5-NCNTs microspheres and elemental sulfur powder in a weight ratio of 1:4 by fusion immersion method. Sb-NCNTs microspheres were produced according to the above experiment steps, with the equal amount of 0.02 mol L-1 SbCl3 solution instead of C10H5NbO20 solution.

Scheme 1. Schematic illustrations of the preparation process of the Nb2O5-NCNTs microspheres.

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2.2 Materials Characterization The scanning electron microscopy (SEM, JEOL, JSM-6360LV) and transmission electron microscopy (TEM, Tecnai G2 20ST) were represented to the morphology and microstructures of the electrode material. X-ray diffraction (XRD, Rigaku-TTRIII) were made with Cu Kα radiation at the 2 range of 5-80° to characterize the sample’s structures

29.

Sulfur content of the NCNTs/S and

decorated-NCNTs/S microspheres were determined by thermogravimetric analyzer (TGA, Metter Toledo-TGA/DSC), which with a heating rate of 5 oC min-1 from 20 to 800 oC under N2 atmosphere. The Brunauer-Emmett-Teller (BET) specific surface area and pore diameter distribution of the sample was estimated by Surface Area and Porosity Analyzer (ASAP 2020) under N2 adsorption-desorption isotherms. Raman spectra of the product was recorded by an Invia Raman microscope (Renishaw) in the range of 500-2200 cm-1 with a 532 nm excitation length. Surface functional groups and the elemental contents were established by using X-ray photoelectron spectroscopy (XPS, U1VAC-PHI PHI 5000 VersaProbe) and Elemental analysis (Elementar, vario ELIII), respectively 30. 2.3 Electrochemical Measurements For better testing the electrochemical of the NCNTs/S or decorated-NCNTs/S microspheres, the electrodes were prepared by mixing 80 wt% active materials (NCNTs/S or decorated-NCNTs/S microspheres), 10 wt% conductive acetylene black and 10 wt% poly (PVDF) binder in N-methyl pyrrolidinone (NMP) solvent. The 7



homogeneous slurry was spread with thickness of 20 mm onto aluminum foil. The sheets were cut into disks with diameters of 14 mm after dried at 60 oC overnight, the areal loading of sulfur was about 3.8-4.2 mg cm -2, and put the wafer in a vacuum oven dried at 50 oC for 24 h. CR2025 coin-type cells were assembled using lithium foil as the counter electrode and a Celgard 2400 as separators in an Ar-filled glove box. The liquid electrolyte used in the coin cells was 1 M bis (trifluoromethane) sulfonamidelitium salt (LiTFSI) in a mixed solvent of dimethoxyethane (DEM) and 1,3-dioxolane (DOL) at a volume ratio of 1:1. The coin cells galvanostatic charg /discharge tests were cycled from 1.7 to 2.8 V at various rates by NEWARE testing instruments (1 C=1675 mAh g-1 sulfur). Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) were performed on coin cells using a CHI660D electrochemical workstation. The CV measurement was tested in the voltage range of 1.7-2.8 V at different scan rates, and the EIS measurement was performed under open circuit conditions over a frequency range from 0.001 to 105 Hz with an alternating current amplitude of 5 mV 31. 3. Results and Discussion Scanning electron microscopy (SEM) images of the obtained NCNTs microspheres, Nb2O5-NCNTs microspheres and Sb-NCNTs microspheres are shown in Figure 1 a-f. The three composites display the similar morphology of the uniform diameter of about 1.5-2 um, are monodisperse and woolen-like microspheres. However, they show certain differences after the introducing of Nb2O5 and Sb. The

8


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woven textures in the crossing NCNTs can be distinguished and filled with abundant pores (Figure 1a, d). It seems that the surficial pores evidently decrease due to the infusion of Nb2O5 (Figure 1b, e) and Sb (Figure 1c, f) nanoparticles. Additionally, smooth and bright Sb particles emerge obviously, suggesting the incomplete infiltration of Sb into the NCNTs microspheres or NCNTs.

Figure 1 (a, d) SEM images of NCNTs microspheres; (b, e) SEM images of Nb2O5-NCNTs microspheres; (c, f) SEM images of Sb-NCNTs microspheres.

Typical transmission electron microscopy (TEM), high-magnification TEM (insets: corresponding fast Fourier transform diffraction patterns) and elemental mapping images are illustrated in Figure 2a-i, respectively. From the TEM images for Figure 2a-c, the three microspheres reveal the continuous 3D CNTs and the identical characteristic, and the NCNTs can be found surrounding the microspheres. As shown in the HRTEM images of Figure 2d-f, the NCNTs microstructure can be distinguished from the lattice fringes with FFT patterns. Moreover, the Nb2O5 nanoparticles are pregnant in the inner walls of the NCNTs (Figure 2e), and Sb particles are perceived at the outer (Figure 2f). N-rich regions can entirely overlap with the C-rich regions from the elemental mapping images (Figure g-i), demonstrating the perfect N-doping 9



for the CNTs. The Nb-rich region can also overlap with C-rich region in the circular shape but the Sb-rich image exposes the prominent circled regions compared to the C-rich regions, it shows the Nb2O5-NCNTs nanoparticles are existed in the inside of carbon nanotube and the aperture of NCNTs microspheres, but the Sb particles only are partially embedded into the hole of NCNTs microspheres. In addition, Sb content in the Sb-NCNTs microspheres is found to be about 2.57% (Table S1) based on EDS analysis, which is less than Nb content in the Nb2O5-NCNTs (3.16%), indicating easier infiltration for Nb2O5 nanoparticles than Sb due to the superior wettability between Nb2O5 and NCNTs. All results are suggesting the successful Nb2O5 modification for the NCNTs microspheres but the dissociative Sb particles.

Figure 2 (a) TEM, (d) HRTEM and (g) HAADF and elemental mapping images of NCNTs microspheres (insets: FFT patterns); (b) TEM, (e) HRTEM and (h) HAADF and elemental mapping 10


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images of Nb2O5-NCNTs microspheres (insets: FFT patterns); (c) TEM, (f) HRTEM and (i) HAADF and elemental mapping images of Sb-NCNTs microspheres (insets: FFT patterns).

Raman spectra of the NCNTs microspheres, Nb2O5-NCNTs microspheres and Sb-NCNTs microspheres are shown in Figure 3a. There is hardly difference for D-band (amorphous carbon) and G-band (graphitic carbon), identified at 1326.1 and 1599.5 cm-1, respectively 32. The intension ratio of the D-band and the G-band (ID/IG) is in the order: Nb2O5-NCNTs microspheres > NCNTs microspheres > Sb-NCNTs microspheres, indicating more defect states and enhanced reduction in the Nb2O5-NCNTs microspheres due to Nb2O5 nanoparticles impregnated in the NCNTs. But there is inadequate combination because of the feeble wettability between Sb and NCNTs, the ID/IG ratio of Sb-NCNTs and NCNTs microspheres has little difference (Sb-NCNTs microspheres is smaller one). The X-ray diffraction (XRD) patterns (Figure 3b) indicate that two wide diffraction peaks for the NCNTs can be clearly watched about 25° (002) and 42° (110), and Nb2O5-NCNTs microspheres can be confirmed by additional peaks at 28.6° for (180) lattice planes of Nb2O5 (PDF#32-0710). The typical diffraction peaks for Sb-NCNTs microspheres are located at 28.7°, 40.1°, 42.1°, 65.9° and 68.5°, which corresponded to the (012), (104), (110), (116) and (122) planes for Sb (PDF#35-0732) 33.

Sb2O3 is easily reduced to elemental Sb under the reducing atmosphere of carbon

and can be decomposed at the high temperature because of its nature of amphoteric oxide. The pore structures and pore-size distribution of the NCNTs, Nb2O5-NCNTs and 11



Sb-NCNTs microspheres are showed by nitrogen absorption-desorption isotherms as shown in Figure 3c and d. The specific parameters are tabulated in Table 1. As shown in Figure 3c, the NCNTs microspheres exhibit very semblable type IV isotherm and H 4 hysteresis, the Nb2O5-NCNTs and Sb-NCNTs microspheres present very similar type IV isotherm and H 1 hysteresis, suggesting that NCNTs microspheres present a composite hierarchical pore structure while Nb2O5-NCNTs and Sb-NCNTs microspheres exhibit abundant mesopores as the adhesion of Nb2O5 or Sb nanocrystals

34.

The corresponding BET specific surface areas of the NCNTs,

Nb2O5-NCNTs and Sb-NCNTs microspheres are respectively 567, 579 and 113 m2g-1. Pore volumes of the NCNTs, Nb2O5-NCNTs and Sb-NCNTs microspheres are 0.5614, 0.54467 and 0.1334 cm3g-1, respectively. Compared with NCNTs microspheres, the intercalation and distribution of Nb2O5 nanoparticles are effective between NCNTs, which provide the increased BET surface area and the decreased pore size of Nb2O5-NCNTs microspheres. But the Sb nanocrystals are attached to the outside of the NCNTs, the BET surface area and the pore size of Sb-NCNTs microspheres are both decreased.

12


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Figure 3 (a) Raman spectra and (b) XRD patterns of NCNTs, Nb2O5-NCNTs and Sb-NCNTs microspheres; (c) N2 adsorption-desorption isotherms and (d) corresponding pore size distributions of NCNTs, Nb2O5-NCNTs and Sb-NCNTs microspheres

SEM images of the NCNTs/S and decorated NCNTs/S microspheres are shown in Figure 4a-c, respectively. Compared to before loading sulfur, the three microspheres after loading sulfur only exhibit the surface and pore difference, the smooth surfaces and the disappeared pores indicate the loading enough sulfur in the NCNTs microspheres. The amount of loading sulfur nanoparticles is in the order: Nb2O5-NCNTs microspheres > NCNTs microspheres > Sb-NCNTs microspheres. Moreover, Sb particles can also be found and Sb-NCNTs microspheres are concealed by the unloading sulfur (Figure 4c). TEM images of NCNTs/S and decorated NCNTs/S microspheres are described in Figure 4d-f. Sulfur distribution in the NCNTs/S and Nb2O5-NCNTs/S microspheres 13



is more uniform than those in the Sb-NCNTs/S microspheres. As shown in HRTEM images of NCNTs/S and Nb2O5-NCNTs/S (Figure 4g, h), The sulfur nanoparticles can be clearly discovered in the intraluminal space of NCNTs, and sulfur coating cannot be found on the surface of NCNTs. But the sulfur coater can be covered on the surface of NCNTs and the sulfur nanoparticles cannot be observed in the inner NCNTs viewed from the HRTEM image of Sb-NCNTs/S microspheres (Figure 4i). To better demonstrate the distribution of sulfur nanoparticles in the three samples, elemental mapping of the product is checked out by the HAADF scanning TEM mode. The S-rich and N-rich adequately overlap with C-rich regions in the circled shapes, but the S-rich region for the Sb-NCNTs/S microspheres can also be distinguished beyond the central circle region for the concentrated sulfur distribution (Figure S1).

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Figure 4 (a) SEM, (d) TEM and (g) HRTEM images of NCNTs/S microspheres (insets: FFT patterns); (b) SEM, (e) TEM and (h) HRTEM images of Nb2O5-NCNTs/S microspheres (insets: FFT patterns); (c) SEM, (f) TEM and (i) HRTEM images of Sb-NCNTs/S microspheres (insets: FFT patterns).

X-ray diffraction (XRD) analysis is carried out and presented in Figure 5a. The representative diffraction peaks for NCNTs /S and decorated NCNTs/S microspheres are situated at 15.5°, 23°, 25.8°, 26.3°, 26.7° and 34.1°, which corresponds to the (022), (222), (026), (224), (311) and (137) planes for sulfur nanoparticles. It is noteworthy that Sb-NCNTs/S microspheres are confirmed by additional peaks at 31.6 (400), 37.1(212), and 42.8 (412) planes for Sb2S3 (PDF#83-2283), suggesting the chemical reaction between sulfur and Sb nanoparticles during the loading processes. The amounts of loading sulfur in NCNTs/S and Nb2O5-NCNTs/S microspheres respectively are determined to be 58.3% and 76.6% according to TG analysis (Figure 5b). The TGA curve of Sb-NCNTs/S microspheres shows the sulfur evaporated region with the loss weight of 30.2%, and an evident region for loss weight around 300

oC

and 600

oC

which demonstrates Sb2S3 nanoparticles are existed in

Sb-NCNTs/S microspheres according to the evaporation of Sb2S3 at 600 oC. These results are good agreement with the XRD analysis of Sb-NCNTs/S microspheres. For the sake of analysis of the the surface functional groups and the elemental contents, X-ray photoelectron spectroscopy (XPS) measurements are tried out and the consequences are exhibited in Figure 5 c-h. In the total XPS spectrum of NCNTs/S, Nb2O5-NCNTs/S and Sb-NCNTs/S microspheres (Figure 5c), four common peaks can 15



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be obviously found around 165, 284, 400 and 530 eV are equivalent to S 2p, C 1s, N 1s and O 1s, respectively. In particular, Nb2O5-NCNTs/S microspheres have the typical peak of Nb 3d (212.27 eV), and Sb-NCNTs/S microspheres show the typical peaks of Sb 4d (234.82 eV), Sb 3d (541.67 eV) and Sb 3p (794.15 eV). In addition, XPS tests of the NCNTs, Nb2O5-NCNTs and Sb-NCNTs microspheres are shown in Figure S2, and the elemental composition quality percentages of three specimens are aggregated from the XPS survey spectra in Table 1. Nb and Sb loading content in NCNTs microspheres is respectively 2.64 wt% and 2.29 wt%. The high-resolution C 1s, N 1s, O 1s, Nb 3d and S 2p spectra for Nb2O5-NCNTs/S microspheres are display in Figure 5d-h. In the C 1s spectrum (Figure 5d), the binding peaks of 284.73 and 285.51 eV are corresponded to C-C/C=C and C-O/C-S/C-N, respectively. It illustrated that there are some heteroatom groups on the surface of the Nb2O5-NCNTs/S microspheres to enhance the binding ability of LiPSs, effectively 35. In Figure 5e, the N 1s spectrum presents the peak of 399.61 and 400.76 eV, respectively demonstrating the existence of pyridinic N and pyrrolic N, which is helpful to constitute more active dots and provides intense chemisorption of polysulfides. The O 1s XPS spectrum is embodied in Figure 5f. The chemical components of C=O and O 1s of Nb2O5 can be found at 532.83 and 530.43 eV, respectively. Figure 5g shows the XPS spectrum of Nb 3d, it shows that the peaks located at 207.57 and 210.32 eV can be considered to 3d

5/2

and 3d

3/2,

which are

consistent with the binding energy for Nb 3d (Nb5+) in Nb2O5. Both of them confirm the presence of the Nb2O5 phase. The S 2p spectrum of Nb2O5-NCNTs/S 16


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microspheres (Figure 5h) exhibits two suited peaks centered at 165.12 and 164.34 eV, which can be attributed to the S-C/S-S and S 2p 1/2, respectively 36. In addition, based on EDS analysis, sulfur content in the Nb2O5-NCNTs/S microspheres is found to be about 7.22% (Table S2). The above results show that the presence of the N element and Nb2O5 nanoparticles can improve the conductivity incorporated with Nb2O5-NCNTs microspheres to adsorb more sulfur.

17



Figure 5 (a) XRD patterns and (b) TGA curve of NCNTs/S, Nb2O5-NCNTs/S and Sb-NCNTs/S microspheres; (c) XPS survey spectra of the Nb2O5-CNTs/S, Sb-CNTs/S and NCNTs/S microspheres; XPS survey spectra of Nb2O5-NCNTs/S microspheres: (d) C 1s, (e) N 1s, (f) O 1s, (g) Nb 3d and (h) S 2p. 18


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Sample

SBET

V tatal

Elemental composition (wt %)

(m2g-1)

(cm3g-1)

C

N

O

Nb

Sb

NCNTs

567

0.5614

86.01

5.65

8.34

-

-

Nb2O5-NCNTs

579

0.5467

82.48

4.3

10.58

2.64

-

Sb-NCNTs

113

0.1334

85.93

4.3

7.48

-

2.29

Table 1 Textural properties obtained from the N2 adsorption/desorption isotherm and composition content estimated form XPS spectra.

First charge-discharge profiles of the NCNTs/S and decorated NCNTs/S microspheres in the voltage range from 1.7-2.8 V vs. Li/Li+ at 0.1 C rate (1 C=1675 mA g-1) is shown in Figure 6a. There are two obvious potential discharge plateaus located around 2.05 and 2.35 V, while one potential charge plateau is observed around 2.4 V. Nb2O5-NCNTs/S microspheres deliver the highest initial discharge capacity of 1335.3 mAh g-1 with the Coulombic efficiency of 97.5 %, while NCNTs/S and Sb-NCNTs/S microspheres reveal respectively the initial discharge capacity of 998.6 and 838.1 mAh g-1 with the Coulombic efficiency of 95.8 and 94.1%. After 200 cycles, they obtain the reversible capacities of 723.1 mAh g-1 for NCNTs/S, 1201.7 mAh g-1 for Nb2O5-NCNTs/S and 464.5 mAh g-1 for Sb-NCNTs/S, corresponding to 73%, 90% and 55% of their initial capacities. The Nb2O5-NCNTs/S microspheres exhibited a considerably higher initial discharge than the results reported by Cui and co-workers

37,

a novel reduce TiO2 inverse opal structure cathode which initial

specific capacity of 1100 mAh g-1 and obtained the discharge capacities of 890 mAh g-1 after 200 cycles. The rate capabilities for the NCNTs/S and decorated NCNTs/S microspheres are evaluated at various current rates from 0.05 to 2.0 C rates. The discharge capacities of 19



Nb2O5-NCNTs/S microspheres are respectively 1394.9, 1334.9, 1259.1, 1200.9, 1108.7 and 1012.1 mAh g-1 cycled at 0.05, 0.1, 0.2, 0.5, 1.0 and 2.0 C rates (Figure 6c). As shown in Figure 6d, Nb2O5-NCNTs/S microspheres exhibit more excellent rate capability than NCNTs/S microspheres and Sb-NCNTs/S microspheres at any rates. After the current rate returns back to 0.1 C rate, the specific capacity of Nb2O5-NCNTs/S cathode can recover to 1300.2 mAh g-1. To further testify the stability of the Nb2O5-NCNTs/S cathode at a high rate, the continuous cycling performance is evaluated at 2.0 C (Figure 6e). The Nb2O5-NCNTs/S cathode delivers a discharge capacity of 1009.7 mAh g-1 at 2.0 C after activation at 0.1 C for three cycles. After 1000 cycles at 2.0 C, the cathode obtains reversible capacities of 846.2 mAh g-1 for Nb2O5-NCNTs/S. The capacity retention is calculated to be 84%, corresponding to a low capacity decay rate (0.016%). As shown in Figure 6f, a comparison of continuous cycling performance at 2.0 C of the Nb2O5-NCNTs/S microspheres to others reported. It's very clear that the capacity and stability for the Nb2O5-NCNTs/S microspheres is better than the rinsed NS-core/MnO2 and Ti4O7/S cathodes which reported by ref. (18) and (39), respectively. While Nb2O5-NCNTs/S cathode shows lower initial discharge capacity than that MCM/Nb2O5/S composite which reported by ref. (4) and similar to Co9S8 @CNTs/S and S@G/G-V2O3 which reported by ref. (38) and (31), respectively. But the Nb2O5-NCNTs/S cathode exhibits the best stability of long-term cycling rate than three cathodes at 2.0 C. The results indicate that the Nb2O5 modification can 20


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effectively enhance the utilization of active substance and significantly relieve the polysulfide shuttle effect. Electrochemical impedance spectroscopy (EIS) measurements are further carried out to assess the conductivity of the NCNTs/S, Nb2O5-NCNTs/S and Sb-NCNTs/S microspheres. As shown in Figure 6g, h, the Nyquist plots and the equivalent circuit models of the Li-S cells before cycling and fully charges after 200 cycles at 0.1 C rate. Nyquist plots of all samples are composed of a semicircle at high-to-medium frequency and an oblique line in the low frequency area before cycling 40. Figure 6h shows two semicircles can be observed from the Nyquist plots of all samples after 200 cycles at 0.1 C rate. The emerged new semicircle in the higher frequency region is related to the solid-electrolyte interface film that is contributed by the Li2S (or Li2S2) layers on the surface of the electrode (Rs). The corresponding impedance parameters which are simulated from the equivalent circuits are listed in Table 2. It illustrates that the electrical conductivity of three cathodes decrease in the order: Nb2O5-NCNTs/S microspheres > NCNTs/S microspheres > Sb-NCNTs/S microspheres, which is well consistent with their rate performance 41.

21



Figure 6 (a) Galvanostatic charge/discharge profiles at first cycle and (b) cycling performances and coulombic efficiency at 0.1 C for the NCNTs/S, Nb2O5-NCNTs/S and Sb-decorated NCNTs/S microspheres; (c) Galvanostatic charge/discharge profiles at different rates for the Nb2O5-NCNTs/S microspheres; (d)Rate capability of NCNTs/S, Nb2O5-NCNTs/S and Sb-NCNTs/S microspheres; (e) Long term cycling performances and coulombic efficiency at 2.0 C for the Nb2O5-NCNTs/S microspheres; (f) A comparison of continuous cycling performance at 2.0 C of the Nb2O5-NCNTs/S 22


Page 22 of 35

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microspheres to that of materials reported in the literature; (g) Nyquist plots after 1st and (h) 200th cycles (inset: equivalent circuit models) for the NCNTs/S, Nb2O5-NCNTs/S and Sb-NCNTs/S microspheres. Sample

Re [][a]

Rct [][b]

Rs [][c]

after

after

after

after

after

1 cycle

200 cycles

1 cycle

200 cycles

200 cycles

Sb-NCNTs/S

22.4

33.5

398.5

354.7

436.3

Nb2O5-NCNTs/S

25.5

26.3

141.8

171.3

267.3

NCNTs/S

8.9

31.4

184.7

106.6

314.1

[a] Resistance of electrolyte; [b] Charge-transfer resistance; [c] Deposit diffusion resistance of SEI film. Table 2 Impedance parameters simulated form the equivalent circuits.

To further understand Li+ diffusion properties, cyclic voltammetry (CV) measurements at different scanning rates are investigated. It is easy to find that the peak current of three cathodic and anodic is a linear relationship to the square root of scan rates in the Figure 7a-c. The Li+ diffusion coefficient can be calculated by the classical Randles Sevcik equation

42,

and the results are tabulated in the Table 3. It

can be obviously observed that the Li Nb2O5-NCNTs/S

microspheres

>

+

diffusion velocity decreases in the order:

NCNTs/S

microspheres

>

Sb-NCNTs/S

microspheres, which in agreement with the electrical performance of three cathodes.

23



Page 24 of 35

Figure 7 CVs at different scan rates of Li-S cells: (a) Sb-NCNTs/S microspheres, (b) Nb2O5-NCNTs/S microspheres, (c) NCNTs/S microspheres and (d) linear fits of the peak currents for the cells. Sample

D Li+ [cm2s-1] A (anodic peak at

B (cathodic peak at

C (cathodic peak at

2.4V)

2.3V)

2.0V)

Sb-NCNTs/S

2.258×10-13

6.787×10-15

1.823×10-14

Nb2O5-NCNTs/S

2.891×10-13

6.173×10-14

8.583×10-14

NCNTs/S

2.765×10-13

1.953×10-14

6.416×10-14

Table 3 List of calculated diffusion coefficient of lithium ions.

The effects of Sb and Nb2O5 nanoparticles of the electrochemical stability for the two electrode materials are illustrated comparatively (Figure 8). The woven textures for the crossing NCNTs can be distinguished and the tow microspheres are full of the abundant pores. In the Sb-NCNTs microspheres electrode (Figure 8A), some Sb nanoparticles stuffed the opening at the end of the NCNTs, others remained in the 24


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pores by the crossing NCNTs instead of the surface of NCNTs because of the weak wettability between Sb and NCNTs. It is difficult for Sb-NCNTs microspheres to prevent most the soluble long-chain lithium polysulfides from the migration into electrolytes due to hardly chemical bonds between Sb and sulfur element, the insoluble short-chain lithium polysulfides can fully fill in the pores for Sb-NCNTs microspheres during the lithiation process. By contrast, Nb2O5 nanoparticles were tightly adhered in the inner and outer walls of NCNTs in the Nb2O5-NCNTs microspheres due to the favorable wettability between Nb2O5 and NCNTs. The sulfur nanoparticles were tightly immobilized by the mutual effect for the pores of microspheres and Nb2O5 nanoparticles due to the superior chemisorption between Nb2O5 and sulfur. The soluble long-chain lithium polysulfides were suppressed in the inner walls of NCNTs and macro-mesopores, and the contents of short-chain lithium polysulfides are less than Sb-NCNTs microspheres. Nb2O5 nanoparticles are uniformly distributed in the NCNTs microspheres due to the excellent wettability of Nb2O5, and provide more adhesion point to polysulfide owe to powerful chemisorption between Nb2O5 nanoparticles and sulfur. Besides, the physical confinement of woolen-like structure to prevent the dissolution of polysulfides. Nb2O5-NCNTs microspheres can increase the sulfur loading and intrinsic effect on suppressing

shuttling

of

polysulfides,

and

the

chemical

Nb2O5-NCNTs microspheres is better than Sb-NCNTs microspheres.

25


performance

of


Figure 8 Schematic illustration of the electrode structures and their electrochemical processes.

In order to further verify the rationality of the schematic illustration of the electrode structures, scanning electron microscopy of the electrodes after 200 cycles are detected. The insulating Li2S particles can be easily found on the surface of the Sb-NCNTs/S microspheres, and the diameter of microspheres barely changed (Figure 9 a, c). Otherwise, the contents of sulfur and the diameter of Nb2O5-NCNTs/S microspheres are both decreased (Figure 9 b, d). It illustrated that a few Nb2O5 nanoparticles was adhered in the inner walls of NCNTs, which provided the nucleation sites for Li2S. Then the soluble long-chain lithium polysulfides were restricted in the inner walls of NCNTs and decreased electrode passivation by Li2S during charge-discharge processes which owe to the strong interaction between Nb2O5 nanoparticles and Li2S. While the electrode passivation of Sb-NCNTs/S microspheres is obvious ascribe to poor wettability and interaction between Sb nanoparticles and NCNTs, sulfur 43. Those results are consistent with the above electrochemical test.

26


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Figure 9 SEM images of cathodes based on (a) Sb-NCNTs/S microspheres and (b) Nb2O5-NCNTs/S microspheres; SEM images of (c) Sb-NCNTs/S microspheres and (d) Nb2O5-NCNTs/S microspheres cathodes after 200 cycles.

4. Conclusions In summary, Nb2O5/Sb-NCNTs microspheres were obtained by a terse ultrasonic spraying with succedent impregnation methods. NCNTs fabricate the porous woolen-like microspheres with abundant networks by self-woven under the effect of ultrasonic spraying. But decorative effects for Nb2O5 are better than those for Sb. Nb2O5-NCNTs microspheres can effectively enhance the utilization of active substance sulfur and restrain the polysulfide dissolved in the electrolyte by chemisorption

of

Nb2O5

nanocrystals

to

the

polysulfide.

The

excellent

electrochemical performance is mainly shown in high capacity (the initial discharge capacity of the Nb2O5-NCNTs/S microspheres is 1335.3 mAh g-1), good rate capability and eminent cycle life (reversible capacities of 1201.7 mAh g-1 after 200 27



Page 28 of 35

cycles, accounting for 90% of initial capacities), a very low capacity decay (0.016% per cycle), and an outstanding cycling stability (846.2 mAh g

−1

after 1000 cycles at

2.0 C). It is attributed to the physical confinement of woolen-like structure to immobilize more sulfur in pores of NCNTs microspheres, and Nb2O5 nanoparticles provides strong chemical bonding with polysulfides to prevent the dissolution of soluble polysulfides during the electrochemical reaction. Therefore, the unique structure of woolen-like Nb2O5-NCNTs microspheres provides an effective exploration for production of hopeful cathode candidates for high energy density Li-S batteries. Supporting Information: HAADF and element mapping images of NCNTs/S and Nb2O5-NCNTs/S and Sb-NCNTs/S microspheres (insets: FFT patterns); XPS survey spectra of the Sb-NCNTs, Nb2O5-NCNTs and NCNTs microspheres; Element contents of NCNT, Nb2O5-NCNTs and Sb-NCNTs microspheres from EDS results; EDS results of Nb2O5-NCNTs/S microspheres. Acknowledgments This work is supported by the National Nature Science Foundation of China (51572079, 51772090) and the Natural Science Foundation of Hunan Province (2016JJ5008, 2016JJ5041) and Hunan Provincial Education Department (16A055).

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