V2O5 Hollow Spheres as High Efficient Sulfur Host for Li-S Batteries
In the past few decades, many significant advances have been made in the study of energy storage devices. 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. 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. 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. Sulfur was combined with different kinds of host materials, 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. Transition metal oxides have drawn extensive attention recently in view of their strong polar-polar chemical interaction with polysulfides. This polar-polar chem-ical interaction has a great effect on suppressing the "shuttle effect" of lithium-sulfur batteries. 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. 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. Various nanostructured V2O5 have been used in the field of electrochemistry, such as nanowires, multi-shelled nanospheres, V2O5/graphene foam comp- osite, nanofibers, surface-uneven V2O5 nanopar-ticles, nanosheets constructing 3D architectures, and so on. Among them, 3D nanostructures with hollow interiors can buffer the volume expansion leading to an enhanced cycling stability.
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.
1.1 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).
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.
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). 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. A sharp Raman band appeared at 995 cm-1 was assigned to the vanadyl stretch (V-O1) within the VO5 square pyramids. 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.
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 S62-. 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. 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 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 (ST-1) and bridging sulfur (SB0), respectively. 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. 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. 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. Such results reveal a strong interaction between polysulfides species and V2O5.
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 (Li2Sn, 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 Li2Sn 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 (Rct) and the adsorption impedance (Ws) 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 (ZW) 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. 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) and Fe2O3 (130 mV). At 1C, the potential difference was 230 mV which was less than MnO2 (400 mV), 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.
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. 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: Ip=(2.69×105)n1.5AD0.5CLiν0.5, where Ip 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, CLi is the concentration of lithium ions in the cathode, and ν is the potential scan rate. 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.
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.
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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