English

• In the past few decades, many significant advances have been made in the study of energy storage devices^[1]. In particular, rechargeable lithium-ion batteries have a dominant position in the power market of modern electronics due to their high energy density, long cycle life, and high charge and discharge efficiency. However, the performance of current lithium batteries cannot meet the power supply require-ments under the rapid advancement of technology. The emergence of lithium-sulfur batteries solved this problem due to its high theoretical capacity (1 675 mAh·g^-1) and an energy density (2 600 Wh·kg^-1)^[2-4]. And the sulfur has advantages of natural richness, low cost, and non-toxicity^[5]. It is possible to replace the lithium-ion batteries as the next generation battery. Despite the huge potential, lithium-sulfur batteries also have many shortcomings, such as poor conductivity of sulfur and solid polysufides produced by the reaction, that limit the utilization of sulfur during battery reactions. The polysulfides dissolved in the electrolyte will pass through the separator to the anode to affect the cycle performance of the batteries. And volume change occurs during charge and discharge process resulting in the unstable structure of sulfur cathode^[6]. These disadvantages result in poor cycling performance, which prevents the commercial application of lithium-sulfur batteries.

  In order to solve the above issues, many efforts have been made to explore novel structures and materials as the cathode, interlayer, electrolyte, or separator for lithium-sulfur batteries^[7]. Sulfur was combined with different kinds of host materials^[8], including carbon based materials, transition metal oxides, and transition metal sulfides, and so on. The carbon-based material mainly acts as a host and the baffle limit the sulfur by physical confinement^[9]. Transition metal oxides have drawn extensive attention recently in view of their strong polar-polar chemical interaction with polysulfides^[10]. This polar-polar chem-ical interaction has a great effect on suppressing the "shuttle effect" of lithium-sulfur batteries^[11]. At the same time, the anchoring ability of the sulfur host material for polysulfides is also the main research direction. Therefore, designing a sulfur host material for physical and chemical adsorption for polysulfides is still a significant challenge.

  Metal oxides, such as TiO₂, Ti₄O₇, and indium tin oxide can adsorb polysulfides on their inherently hydrophilic surfaces^[12]. Vanadium pentoxides (V₂O₅) is a promising candidate for sulfur host materials due to its unique layered structure, excellent chemical and physical properties, wide electrochemical window, high energy efficiency, and many oxidation states^[13]. Various nanostructured V₂O₅ have been used in the field of electrochemistry, such as nanowires^[14], multi-shelled nanospheres^[15], V₂O₅/graphene foam comp- osite^[16], nanofibers^[17], surface-uneven V₂O₅ nanopar-ticles^[18], nanosheets constructing 3D architectures^[19], and so on. Among them, 3D nanostructures with hollow interiors can buffer the volume expansion leading to an enhanced cycling stability^[20].

  In this work, the V₂O₅ hollow spheres were synthesized by hydrothermal synthesis and annealing. V₂O₅ has a relatively large specific surface area of 26 m²·g^-1. The hollow structure provides more space for storing sulfur, while V₂O₅ has a strong chemical adsorption of polysulfides, which can limit the shuttle effect of polysulfides. The hollow structure can also accommodate volumetric change of sulfur electrode. The prepared V₂O₅ electrode exhibited a good reversible capacity close to 600 mAh·g^-1 after 300 cycles at 1C, good rate capability, and stable cycling stability.

  1.   Experimental

  1.1   Synthesis of V₂O₅ hollow sphere

  All reagents and solvents were of analytical grade and used as received without further treatment. In a typical process, 0.117 g NH₄VO₃ was added into 25 mL deionized water water with stirring. Then, 0.5 mL of 1 mol·L^-1 HCl was added drop by drop, until the solution color turned orange. After that, 1.5 mL N₂H₂·H₂O was added into the solution, and it was stirred for 15 min at room temperature. Then, the mixture was transferred into a teflon lined stainless steel autoclave, sealed and maintained at 120 ℃ for 4 h. After the reaction, the product was washed several times using ethanol and water, and then dried at 80 ℃ for 8 h. Last, the above product was heated in a muffle furnace with heating rate of 5 ℃·min^-1 under air atmosphere at 400 ℃ for 2 h, and then cooled to room temperature naturally.

  1.2   Synthesis of V₂O₅/S composites

  The mixtures of the prepared V₂O₅ hollow spheres and sulfur were sealed and heated at 155 ℃ for 12 h. Then, the mixtures were heated to 250 ℃ under argon flow for 30 min in tube furnace to eliminate the sulfur on the outside surface of the V₂O₅ hollow spheres. The resulting V₂O₅/S composites with the sulfur content of 70%(w/w) were obtained, according to thermogravi-metric analysis (TGA).

  1.3   Characterization

  XRD measurements were carried out on a Philip XRD X′PERT PRO X-ray diffractometer using Cu Kα radiation (λ=0.154 18 nm). The working current was 40 mA and working voltage was 40 kV with 2θ range of 15°~70°. The structure and morphology were characterized by SEM (Hitachi S-4800, 10 kV) and TEM (Hitachi 7700, 100 kV). High-resolution TEM (HRTEM) images were recorded on FEI Talos F200X field-emission transmission electron microscope operated at 200 kV. UV-Vis adsorption spectra were recorded on a PerkinElmer UV-Vis spectrometer Lambda 650S. The valence states of elements were analyzed by X-ray photoelectron spectroscopy (PHI 5000 VersaProbe). Thermogravimetric analysis (TGA) was used to determine the sulfur content of the materials on a TGA instrument (NETZSCH STA-449 C) employing a heating rate of 10 ℃·min^-1 from room temperature to 700 ℃ under a nitrogen flow.

  CR2032-type coin cells were assembled in a glovebox filled with argon. The working electrodes were prepared by mixing 80%(w/w) active materials, 10%(w/w) acetylene black and 10%(w/w) polyvinyli-dene fluoride (PVDF) binder in N-methyl pyrrolid-inone (NMP). The slurries were homogeneously coated onto aluminum foil current collectors. The electrodes were dried at 60 ℃ for 12 h under vacuum. Subsequently, the electrodes were cut into disks with a diameter of 13 mm. A piece of lithium foil was used for the combined counter and reference electrodes. 1.0 mol·L^-1 lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) in 1, 3-dioxolane and 1, 2-dimethoxyethane (volume ratio, 1:1) with 1% (w/w) LiNO₃ as an additive was used as the electrolyte. The LiNO₃ was added to help passivate the surface of the lithium anode and reduce the "shuttle effect". Celgard 2400 was used as a separator film. The cycle performances, rate capa-bility and galvanostatic charge/discharge tests were carried out on LAND CT2001A in a potential range of 1.5~2.8 V (vs Li/Li⁺). The specific capacity was calcu-lated based on the weight of sulfur. Cyclic voltam-metry (CV) (scan rate: 0.2 mV·s^-1, cut-off voltage: 1.5~2.8 V) and electrochemical impedance spectra (frequency range from 100 kHz to 0.01 Hz) were measured with an electrochemical workstation VMP3.

  2.   Results and discussion

  The precursor of V₂O₅ was fabricated using one-step hydrothermal method. The obtained precursor was heated under air atmosphere. The final porous V₂O₅ spheres could be formed after annealing. Scanning electron microscopy (SEM) images (Fig. 1(a, b)) show that the shape of precursor was uniform spheres with the average diameter around 500 nm. The spherical structure can be well maintained after annealing, as shown in Fig. 1(d, e). It can be seen from the TEM images of the precursor and the obtained V₂O₅ exhibited a hollow structure with the spherical shell thickness was about ~80 nm. Compared with the precursor, the surface of the V₂O₅ hollow spheres became smooth. Fig. 1(g~j) show the energy dispersive X-ray spectroscopy (EDS) elemental mappings which demonstrated the homogeneous distribution of V and O as well as sulfur inside the V₂O₅ nanospheres.

  Figure 1

  

  Figure 1.  SEM (a, b, d, e) and TEM (c, f) images of precursor (a, b, c) and V₂O₅ (d, e, f); (g~j) Scanning transmission electron microscopy image and corresponding EDS elemental mapping images of V₂O₅/S composites

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  The XRD patterns of precursor and V₂O₅ are shown in Fig. 2a. From the XRD pattern of precursor, it can be seen that there were no obvious diffraction peaks due to its very low crystallinity. Line b in Fig. 2a is the XRD pattern of V₂O₅ hollow spheres. All of the main diffraction peaks can be indexed and assigned to the orthogonal V₂O₅ phase (PDF No.41-1426)^[21]. Raman spectrum displays the characteristic peaks of V₂O₅, as shown in Fig. 2b. These vibration modes could be described to the characteristic of orthorhombic V₂O₅^[22-23]. The low frequency modes at 105, 141, and 195 cm^-1 can be attributed to external modes corresponding to the displacements of [VO₅] units with respect to each other^[24]. A sharp Raman band appeared at 995 cm^-1 was assigned to the vanadyl stretch (V-O₁) within the VO₅ square pyramids^[25]. The mode at 698 cm^-1 was ascribed to an anti-phase stretching vibration of the V-O₂ bond. Another mode at 524 cm^-1 was due to the stretching of inter-chain V-O₂ bonds. Two other modes at 403 and 294 cm^-1 can be assigned to bond rocking oscillations involving the apical oxygen (O₁) atoms^[26].

  Figure 2

  

  Figure 2.  (a) XRD patterns of precursor and V₂O₅; (b) and (c) Raman spectrum and N₂ adsorption-desorption isotherms of V₂O₅, respcetively; (d) TGA curve of V₂O₅/S composites

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  The specific surface area and pore volume of the V₂O₅ hollow spheres can be known by the N₂ adsorption-desorption isotherm. Fig. 2c showed a sharp capillary condensation step at high relative pressure which belonged to type Ⅳ isotherm, characteristic of the presence of mesoporous material according to IUPAC classification. Brunauer-Emmett-Teller (BET) specific surface area of the as-obtained V₂O₅ hollow spheres is 26 m²·g^-1, and the most probable distribution pore size of V₂O₅ was around 30 nm. TGA (Fig. 2d) shows the sulfur content in V₂O₅/S. The TGA curve showed a weight loss between 150 and 300 ℃, which was due to the evaporation of elemental sulfur in V₂O₅/S composite materials. The mass fraction of sulfur content in the composite was about 70%(w/w).

  To verify the physical trapping and strong chemical anchoring of V₂O₅ for polysulfides, an ex situ experiment of the absorption of Li₂S₆ by V₂O₅ was carried out. The orange polysulfides solution turned to colorless after adding V₂O₅ (inset in Fig. 3a). As shown in Fig. 3a, the peak at 300~350 nm was contributed to S₆^2-. It became weak after adding V₂O₅, demonstrating a good adsorption of V₂O₅ for polysulfides. In order to prove the interaction of polysulfides with V₂O₅, the chemical states of V₂O₅ and V₂O₅/Li₂S₆ were investigated by XPS, as shown in Fig. 3(b~f). In Fig. 3b, the V2p spectrum showed two deconvoluted peaks near 524 and 517 eV, corresponding to V⁴⁺ and V⁵⁺ of the original V₂O₅, respectively^[27-28]. It can be seen from Fig. 3e that the two peaks of V2p were displaced after combining with Li₂S₆, indicating an increase in electron density at the metal element and formation of chemical bond between V₂O₅ and Li₂S₆^[29]. On the one hand, the peaks of 524.4 and 516.9 eV exhibited a 0.3 and 0.7 eV shift compared to that of V₂O₅, respectively. On the other hand, the formation of the S-V bond^[30] can be clearly recognized by the strong broad peaks in the range of 516~518 and 523~525 eV. Fig. 3d shows the S2p spectrum of the V₂O₅/Li₂S₆. The peak located at 162 and 163.1 eV can be attributed to terminal (S_T^-1) and bridging sulfur (S_B⁰), respectively^[31]. In addition, a new characteristic peak at 162.5 eV emerge in S2pspectrum, corresponding to V-S bonds. The peak at 168.4 eV revealed V-O interaction in the V₂O₅/Li₂S₆, and the peak around 164 eV can be attributed to the S-S bond of Li₂S₆ species^[32]. Two characteristic peaks of O1s can be observed in Fig. 3c, the peak at 529.7 eV was attributed to V-O corresponding to oxidation of V based material. Besides, the peak at 531 eV corresponded to absorption of H₂O^[33]. After adsorption of Li₂S₆, the total peak of O1s in V₂O₅/Li₂S₆ showed a large shift of about 0.4 eV to higher energy in Fig. 3f, the formation of S-O bond can be clearly identified by the strong wide peak in the range of 528.5~530 eV^[34]. Such results reveal a strong interaction between polysulfides species and V₂O₅^[35].

  Figure 3

  

  Figure 3.  (a) UV-Vis spectra of Li₂S₆ and V₂O₅/Li₂S₆; High-resolution XPS spectra for (b) V2p and (c) O1s of V₂O₅; High-resolution XPS spectra for (d) S2p, (e) V2p and (f) O1s of V₂O₅/Li₂S₆

  Inset in a: Digital images of Li₂S₆ and V₂O₅/Li₂S₆

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  The electrochemical performances of the V₂O₅/S cathode were evaluated by lithium-sulfur batteries. As shown in Fig. 4a, the CV profiles of V₂O₅/S electrode was performed at a scan rate of 0.1 mV·s^-1 in the potential range between 1.7 and 2.8 V vs Li⁺/Li. The two prominent reduction peaks located at 2.27 and 2.03 V corresponded to the formation of soluble lithium polysulfides (Li₂S_n, 4≤n≤8) and further reducing to short-chain insoluble lithium sulfides (Li₂S₂/Li₂S). The oxidation peak around 2.39 V can be ascribed to the conversion from Li₂S/Li₂S₂ to Li₂S_n and eventually S₈. The electrochemical impedance spectroscopy (EIS) curves of V₂O₅/S before and after CV were shown in Fig. 4b. The charge-transfer resistance (R_ct) and the adsorption impedance (W_s) of V₂O₅/S electrode decreased sharply at 24 Ω after CV, confirming the outstanding electrocatalytic activity and the strong adsorption to the intermediate polysulfides. Importantly, the small value and stable variation of the Warburg impedance (Z_W) indicated the good diffusion of soluble species in the 3D framework of V₂O₅ hollow spheres in the V₂O₅/S electrode, which is indispensable and essential for the subsequent adsorp-tion and electrocatalysis processes of soluble intermed-iate polysulfides^[36]. As shown in Fig. 4c, the reversible capacities of the V₂O₅/S electrode decreased gradually from 1 439 to 1 107, 885, 800, 654 and 507 mAh·g^-1 with the current densities increasing from 0.1C to 3C. When the current returned back to 0.2C, the capacity can be restored to an excellent value about 950 mAh·g^-1, showing good stability at different current dens-ities. As shown in Fig. 4d, the well-defined charge-discharge plateaus maintained well for V₂O₅/S even at high current densities. The cathode showed a sloping charge/discharge process only when the current density reached 3C. At 0.1C, the voltage platform difference was 118 mV, that was smaller compared with other electrodes, such as reduced graphene oxide nanotubes (183 mV)^[37] and Fe₂O₃ (130 mV)^[38]. At 1C, the potential difference was 230 mV which was less than MnO₂ (400 mV)^[39], suggesting that the electro-chemical reaction reversibility of the V₂O₅/S was high. There was a relatively small polarization phenomenon of the V₂O₅/S cathode in different current densities, suggesting that the spherical V₂O₅ was benefit for improving the electrochemical kinetics of the sulfur cathode.

  Figure 4

  

  Figure 4.  (a) CV curves of the first three cycles and (b) Nyquist plots of the V₂O₅/S electrode before and after CV; (c) Rate capabilities from 0.1C to 3C of V₂O₅/S electrode; (d) Discharge/charge curves during cycling at different rates

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  As shown in Fig. 5a, CV curves at different scan rates from 0.1 to 0.4 mV·s^-1 were further recorded to investigate the reaction kinetics and lithium diffusion properties of V₂O₅/S cathode. With the increase of the scan rate, the peak separation began to increase due to overpotential^[40]. However, the peaks retained the well-defined shape even when the scan rate increased to 0.5 mV·s^-1. All the cathodic and anodic peak currents for the measured cathode displayed a linear relationship with the square root of scanning rates. The classical Randles-Sevcik equation can be applied to describe the Li⁺ ion diffusion process: I_p=(2.69×105)n^1.5AD^0.5C_Liν^0.5, where I_p is the peak current, n is the charge transfer number, A is the area of the active electrode, D is the lithium-ion diffusion coefficient, C_Li is the concentration of lithium ions in the cathode, and ν is the potential scan rate^[41]. The slopes of curves in Fig. 5b were positively correlated to the lithium-ion diffusion rate. Accordingly, by Randles-Sevcik equa-tion, D of V₂O₅/S cathode for redox peak A1, C1 and C2 were calculated to be 5.42×10^-9, 3.37×10^-9 and 3.29×10^-9 cm²·s^-1, respectively, implying that the presence of V₂O₅ can enhance the polysulfides redox kinetics in terms of sulfur reduction and oxidation reactions.

  Figure 5

  

  Figure 5.  (a) CV curves of V₂O₅/S electrode at various scan rates and (b) the corresponding relationship between the square root of the scan rate v and peak current I_p of the V₂O₅/S electrode in a voltage range of 1.7~2.8 V vs Li⁺/Li

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  Fig. 6a displays the cycling performance of V₂O₅/S cathode at different current densities. The beginning capacities of V₂O₅/S at 0.2C, 0.5C and 1C are 1 429, 1 014 and 805 mAh·g^-1. After 100 cycles, it can still remain 890, 754 and 682 mAh·g^-1, respectively. During cycling, the coulomb efficiencies have no obvious change, indicating that the batteries have excellent cycle performance. As shown in Fig. 6c, the V₂O₅/S cathode delivered a capacity of 805 mAh·g^-1 after activation at 0.1C for five cycles. After 300 cycles at 1C, the V₂O₅/S cathode still remains a high capacity of 584 mAh·g^-1, corresponding to an ultralow capacity decay of 0.24% per cycle. Due to the influence of temperature, the battery capacity fluctuates slightly, and finally returned to the normal value, which proved that the battery has good stability. And the charge/discharge plateaus were maintained well after 300 cycles, as can be seen from Fig. 6b, indicating an excellent cycling stability.

  Figure 6

  

  Figure 6.  (a) Cycling performance of the V₂O₅/S cathode at the charge/discharge rate of 0.2C, 0.5C and 1C; (b) Galvanostatic charge-discharge voltage curves of V₂O₅/S electrode at different cycles at 1C; (c) Prolonged cycling stability of the V2O₅/S electrode over 300 cycles at 1C

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  3.   Conclusions

  In summary, we have designed and synthesized an efficient sulfur host based on V₂O₅ hollow spheres. Due to this hollow spherical structure, this novel host not only improves the sulfur content, but also limits the expansion of the sulfur electrode. Meantime, the V₂O₅ spheres have strongly chemical adsorption for the polysulfides to improve the sulfur utilization and the cycling stability of batteries. V-S bond played critical role in effectively encapsulating of V₂O₅ on polysul-fides and improve the electrochemical performances. Benefiting from the unique structure, sulfur content was up to 70% (w/w) and remained a high capacity of 584 mAh·g^-1 over 300 cycles at the current density of 1C.

  
  
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• 

  Figure 1  SEM (a, b, d, e) and TEM (c, f) images of precursor (a, b, c) and V₂O₅ (d, e, f); (g~j) Scanning transmission electron microscopy image and corresponding EDS elemental mapping images of V₂O₅/S composites

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  Figure 2  (a) XRD patterns of precursor and V₂O₅; (b) and (c) Raman spectrum and N₂ adsorption-desorption isotherms of V₂O₅, respcetively; (d) TGA curve of V₂O₅/S composites

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  Figure 3  (a) UV-Vis spectra of Li₂S₆ and V₂O₅/Li₂S₆; High-resolution XPS spectra for (b) V2p and (c) O1s of V₂O₅; High-resolution XPS spectra for (d) S2p, (e) V2p and (f) O1s of V₂O₅/Li₂S₆

  Inset in a: Digital images of Li₂S₆ and V₂O₅/Li₂S₆

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  Figure 4  (a) CV curves of the first three cycles and (b) Nyquist plots of the V₂O₅/S electrode before and after CV; (c) Rate capabilities from 0.1C to 3C of V₂O₅/S electrode; (d) Discharge/charge curves during cycling at different rates

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  Figure 5  (a) CV curves of V₂O₅/S electrode at various scan rates and (b) the corresponding relationship between the square root of the scan rate v and peak current I_p of the V₂O₅/S electrode in a voltage range of 1.7~2.8 V vs Li⁺/Li

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  Figure 6  (a) Cycling performance of the V₂O₅/S cathode at the charge/discharge rate of 0.2C, 0.5C and 1C; (b) Galvanostatic charge-discharge voltage curves of V₂O₅/S electrode at different cycles at 1C; (c) Prolonged cycling stability of the V2O₅/S electrode over 300 cycles at 1C

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