An ultrafine V₂O₃ modified hierarchical porous carbon microsphere as a high performance cathode matrix for lithium–sulfur batteries

Abstract

An ultrafine V₂O₃ modified carbon microsphere (VCM) was effectively prepared by a facile wet impregnation method and used as a cathode matrix for lithium–sulfur batteries. This resulting composite presents hierarchical porous frameworks of mesopores together with micropores, effectively restrains soluble polysulfides and enhances mass transport. Moreover, V₂O₃ modification in this mono-dispersed microsphere material can trap polysulfides and mitigate loss of active materials by chemical adsorption. The S/VCM nanocomposites show a high initial discharge capacity of 1177 mA h g⁻¹ at 0.5C and an excellent cycling performance with 921 mA h g⁻¹ after 100 cycles. And it still maintains higher rate capacities of 843 and 719 mA h g⁻¹ at 1C and 2C, respectively, after 100 cycles. These improved electrochemical performances are attributed to the synergistic effects of hierarchical pores and uniformly dispersed vanadium trioxide.

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1. Introduction

With the demand for energy storage devices with high energy, lithium–sulfur batteries have attracted increasing attention, as the sulfur electrode has a high specific capacity of 1675 mA h g⁻¹ and a high energy density of 2600 W h kg⁻¹.^1,2 Furthermore, sulfur has the advantages of low cost, natural abundance, and environmental friendliness.³ And lithium–sulfur batteries are considered as a promising candidate for next generation rechargeable batteries. However, there are some problems to hamper its rapid development, such as the insulation of sulfur, the solubility of lithium polysulfides (Li₂S_x, x > 4) and volume expansion of sulfur cathode.^4,5 Then, performances with low specific capacity, rapid capacity fading and poor cycling stability are commonly found in lithium–sulfur batteries.^6,7

Considerable effort has been devoted to the novel sulfur cathodes for lithium–sulfur batteries, various carbon materials (i.e., carbon nanotubes,⁸ nanofibers,⁹ carbon spheres¹⁰) and conductive polymers (i.e., polyaniline,¹¹ polythiophene,¹² polypyrrole¹³), to restrain diffusion of polysulfide and enhance electronic conductivity of the sulfur cathodes. Among these materials, the sulfur/carbon sphere composite has demonstrated high sulfur utilization and good electrochemical performance for 3D porous structure, high conductivity and abundant mesopores.^10,14,15 In fact, apart from the architectures, the chemical binding with polysulfides was found to be important for enhancing the sulfur cathode performance. For example, several metal oxide additive including TiO₂,¹⁶ La₂O₃,¹⁷ MnO₂,¹⁸ Ce₂O₃,¹⁹ and ZrO₂ ²⁰ can be favorable for inhibiting the dissolution of polysulfides by chemical binding. Hence, the incorporation of metal oxide within the carbon matrix provides a successful approach to modify the surface and to effectively adsorb sulfur species.

As an important transition metal oxide for modifying material, vanadium trioxide has been studied in energy storage fields due to its natural abundance,²¹ good electrical conductivity,²² and low toxicity.²³ And V₂O₃ may possess the adsorption capacity to inhibit the dissolution of polysulfides like TiO₂,¹⁶ La₂O₃.¹⁷ It could be a more promising candidate cathode additive for high performance lithium–sulfur battery. However, the works on V₂O₃ in energy storage are mainly limited to supercapacitors and lithium ion batteries.^24,25

Therefore, in this work, we report on the synthesis of carbon microsphere (CM) via a sol–gel method and the preparation of V₂O₃ modified carbon microsphere (VCM) by facile wet impregnation method. VCM microsphere encapsulates sulfur and traps the solute polysulfides by using the hierarchical porous structures and uniform dispersion of V₂O₃ in CM matrix. As shown in Fig. 1, CM with hierarchical pore presents many micropores and mesopores, where the micropores may exist in the outer shell and in the walls between the core mesopores. Sulfur is infiltrated into the mesopores and micropores, and the micropores around mesopores as “barricades” retards migration of the long-chain lithium polysulfides into electrolytes due to the mechanisms^26,27 associated with micropores. However, some mesopores in the outer shell could not house soluble lithium polysulfides, while V₂O₃ could effectively trap soluble polysulfides by its adsorption ability. The S/VCM composite as S electrode for lithium sulfur battery exhibits higher electrochemical performances than the S/CM composite. It can be found that VCM plays a role in promoting the adsorption of lithium polysulfides and enhancing the performance of cathode.

Fig. 1 Schematic illustration of structure of the CM and VCM composites and electrochemical process.

2. Experimental

2.1. Material synthesis

2.1.1 Synthesis of CM. The carbon microspheres were synthesized by condensation reaction of resorcinol with formaldehyde using L-malic acid as catalyst and the procedure is as follows. Resorcinol (R), formaldehyde (F, 37 wt%), sodium dodecyl sulfate (SDS) and L-malic acid (L, a catalyst) were slowly added to deionized water (H₂O) under constant stirring at 85 °C, where molar ratios of R/F, R/L, R/SDS and R/H₂O were 1/2, 50/1, 500/1 and 1/50, respectively. After 30 min continuously stirring, the mixture was transferred to a beaker, and sealed at 85 °C for 12 h. Then the obtained hydrogel was directly dried at 120 °C at atmospheric pressure for 10 h and the intermediate product was heat-treated at 800 °C for 2 h under nitrogen gas atmosphere with a heating rate of 5 °C min⁻¹.

2.1.2 Synthesis of VCM and S/VCM composite. VCM was prepared by wet impregnation method. Typically, a certain amount of the ammonium vanadate (NH₄VO₃) was added into the solution containing carbon microspheres, sodium dodecyl sulfate, tartaric acid (T), and deionized water. After 24 h continuously stirring, the obtained product was directly dried at 120 °C at atmospheric pressure for 10 h and then it transformed into VCM by a pyrolysis route at 800 °C for 2 h with a heating rate of 5 °C min⁻¹ under a nitrogen gas atmosphere. In preparing VCM, the molar ratios of NH₄VO₃/CM, SDS/CM, T/CM, and H₂O/CM are 1 : 50, 1 : 100, 3 : 100, and 50 : 1. The S/VCM nanocomposite was acquired via a melt-diffusion route. Sublimed sulfur was mixed homogeneously with the as-prepared VCM composites in a 6 : 4 weight ratio. Then, the resulting S/VCM composites were annealed at 155 °C for 10 h in a sealed vessel to diffuse sublimed sulfur into the VCM.

2.2. Material characterizations

The morphology and microstructure of the samples were examined by field emission scanning electron microscopy (FESEM, SU8020) and field emission transmission electron microscopy (FETEM, JEM2010F). The specific surface area and the pore size distributions were calculated by the Brunauer–Emmett–Teller (BET) theory and Barrett–Joyner–Halenda (BJH) model. The crystalline structures of the synthesized samples were characterized by powder X-ray diffraction (XRD, X'Pert PRO MPD) using Cu-Kα radiation with a scan range from 10° to 90°. The sulfur contents in the sulfur-containing samples were obtained by thermogravimetry (TGA, STA449F3) with a heating rate of 10 °C min⁻¹. The surface chemical compositions of S/VCM were identified by X-ray photoelectron spectroscopy (XPS, ESCA LAB 250Xi).

2.3. Ex situ adsorption measurement

The adsorption ability of the CM and VCM on lithium polysulfide was examined by UV-Vis-NIR spectrophotometer (CARY 5000) at 415 nm.

The 2.5 mM lithium polysulfide solution with a molar ratio matching Li₂S₆ was prepared in THF according to the literature.^28–30 CM and VCM were dried for 12 h under vacuum and 50 mg of each sample was placed into 10 ml of the lithium polysulfide solution in an argon-filled glove box. After 24 h standing, the solutions were separated by a syringe and diluted by a factor of 5 to enable quantitative concentration measurements.

2.4. Electrochemical measurements

2.4.1 Cell assembly. Electrochemical performances of samples were conducted in CR2032-type coin cells. 80 wt% material (S/VCM or S/CM), 10 wt% conducting agent (acetylene black) and 10 wt% binder (polyvinylidene fluoride) were mixed with N-methyl-2-pyrrolidone (NMP). The slurry was coated onto an aluminum foil and dried at 60 °C for 12 h in a vacuum oven. And then the cells were assembled in Ar-filled glove box in which oxygen and water contents were less than 1 ppm, with a polypropylene film (Celgard 2400) as a separator, and lithium metal as counter electrode and reference electrode. The electrode was prepared into disk loading accurate mass of active materials in the range of 1.5–1.6 mg. Meanwhile, the electrolyte solution was prepared by dissolving LiTFSI (1 M) and LiNO₃ (0.1 M) into the mixture of dimethoxymethane (DME) and 1,3-dioxolane (DOL) (v/v, 1 : 1).

2.4.2 Cell measurements. The cyclic voltammetry (CV) measurements were conducted with an electrochemical workstation (CHI 660B), and the voltage range was controlled between 1.8 and 3.0 V at a scan rate of 0.1 mV s⁻¹. Galvanostatic charge/discharge tests were investigated by LAND CT2001A battery test system in the potential range from 1.8 to 3.0 V and specific capacities were calculated by net mass of sulfur. The batteries were first discharged to 1.8 V and then the cycle number was counted. The electrochemical impedance spectrometry (EIS) was measured on electrochemical workstation (CHI 660B) in the frequency range of 0.1 Hz to 100 kHz with amplitude of 5 mV. All electrochemical tests were performed at room temperature.

3. Results and discussion

3.1. Characterizations

Fig. 2a and b present the SEM images of CM and VCM. As shown in Fig. 2a, CM material is composed of microsized carbon spheres with smooth outer surfaces and uniform sizes. The VCM composite appears similar structure of CM, but with a little rough surfaces for V₂O₃ modification. Fig. 2c–e display the TEM images of VCM composite. From the Fig. 2c, it is obvious that these “dark dot” V₂O₃ nanoparticles with about 6 nm in size are evenly embedded in the composite. Fig. 2d exhibits the HRTEM image of the VCM composite and the lattice fringe with spacing of 0.25 nm, matching well with the (110) lattice plane of nanocrystal V₂O₃.^24,25 Fig. 2e1–e3 show the elemental mappings of vanadium, oxygen and carbon, respectively. The spatial distributions of V and O indicate nearly a uniform V₂O₃ modification within the carbon matrix. And there are no obviously aggregation of sulfur particles in the FE-SEM and EDS images of S/VCM (in Fig. S1†).

Fig. 2 SEM images of (a) CM and (b) VCM; (c) TEM image of VCM; (d) HRTEM image of VCM; (e) EDX mappings of VCM: (e1) V, (e2) O and (e3) C.

Fig. 3a shows the nitrogen adsorption–desorption isotherms of CM and VCM. These two samples exhibit a type isotherm between type II and IV, proving mesopores and micropores characteristic and suggesting that the pores in the spheres are accessible. As expected in the schematic illustration, CM and VCM display a hierarchical porous framework of mesopores together with micropores. The pore size distributions of CM and VCM in Fig. 3b reveal that the pores mainly lay within the range of 2–10 nm by BJH analysis. And their specific surface areas are close to 710 m² g⁻¹. More information about porosity properties of CM and VCM is presented in Table S1.† This hierarchical porosity can be beneficial for the mass transport of electrolyte by mesopores and the immobilization of polysulfides by micropores and small mesopores.³¹

Fig. 3 (a) N₂ adsorption/desorption isotherms and (b) pore size distributions of the CM and VCM composites.

Fig. 4a presents XRD patterns for CM, V₂O₃, and VCM. The diffraction peaks of CM at 24° and 44° are mainly due to the amorphous carbon. And they correspond to the (002) and (100) planes of the graphite structure with irregularly packing. Compared with CM, the XRD pattern for VCM exhibits the major peaks at 2θ of 24.2°, 32.9°, 36.4° and 54.0° corresponding to (012), (104), (110), and (116) lattice planes of rhombohedra V₂O₃ (Fig. 2a), which match well to JCPDS no. 34-0187 for the V₂O₃. Fig. 4b displays XRD patterns of S, S/CM, and S/VCM composites. For sublimed sulfur, the peaks appeared at 2θ = 23.1° and 27.8° correspond to the (222) and (040) reflections of the Fddd orthorhombic structure (JCPDS no. 08-0247). And these sharp and strong diffractions suggest a well-defined crystal structure. The characteristic crystalline sulfur peaks of S/CM and S/VCM composites display fewer peaks of sulfur with lower intensity and exhibit a broad diffraction curve around 20–30°, demonstrating the presence of poor crystalline sulfur and the incorporation of sulfur into the CM and VCM.

Fig. 4 XRD patterns of (a) V₂O₃, VCM and CM composites and (b) S, S/VCM and S/CM composites.

Fig. 5a displays the TGA curves of S/CM and S/VCM and depicts weight losses of the two samples correlated to the content of sulfur. The two curves both show two-step feature for the sulfur loss, where the initial weight loss takes place in temperature range of 150–300 °C and the rest of sulfur releases at the temperate range from 300 to 600 °C. The first step is associated with the loss of sulfur located at the surface and mesopore of samples and the second one is mainly due to the evaporation of sulfur housed in the micropore of S/CM and S/VCM. Furthermore, there is a delay in removing sulfur for S/VCM as shown in Fig. S2,† indicating the interaction between sulfur and V₂O₃ nanoparticles.^10,32 According to TGA results, the sulfur content in S/CM and S/VCM is 56.62 wt% and 55.88 wt%, respectively.

Fig. 5 (a) TG curves for the S/CM and S/VCM composites under Ar atmosphere; (b) survey spectra of the S/CM and S/VCM samples; (c) O 1s spectra of the S/VCM sample; (d) V 2p spectra of the S/VCM sample; (e) C 1s spectra of the S/VCM sample and (f) S 2p spectra of the S/VCM sample.

Fig. 5b–f present the XPS characterization of S/CM and S/VCM and further describe chemical composition component and chemical bonding state. As shown in Fig. 5b, it can be found that S 2p, C 1s and O 1s peaks appeared at about 164.2 eV, 284.8 eV and 538.1 eV in the spectra, respectively. Compared with the spectra of S/CM, a strong V 2p peak is observed for S/VCM composite in Fig. 5b. Meanwhile, the results give the V/C molar ratio of 0.017.

Fig. 5c depicts the spectra of O 1s for S/VCM and the O 1s spectra present two peaks at 530.4 eV and 531.5 eV, corresponding to the V–O bonds and C–O bonds, respectively.^22,24 Fig. 5d shows the V 2p spectra for the S/VCM composite. There are two peaks centered at 516.9 and 524.3 eV which are attributed to the spin–orbit splitting of V 2p_3/2 and V 2p_1/2.²³ And the characteristic of vanadium in the +3 oxidation state indicates the formation of V₂O₃. Fig. 5e displays the C 1s spectra of the S/VCM composite and the peaks at 284.7 eV and 286.1 eV correspond to graphite C–C bonds and the formation of C–O species, respectively. Fig. 5f shows the XPS spectra of S 2p for S/VCM. The peaks situated at 165.4 and 164.2 eV are assigned to S 2p_1/2 and S 2p_3/2, matching well with the typical peaks of S₈.

In short, these physical–chemical characterizations indicate that the VCM composites have a 3D structure with the hierarchical porosity and possess uniformly dispersing V₂O₃. The unique structure and the V₂O₃ modification may make Li–S batteries having an effective improvement of the electrochemical performance.

3.2. Adsorption ability

The adsorption abilities of CM and VCM on lithium polysulfides were examined by UV-Vis-NIR spectrophotometer and the adsorption quantities of polysulfides for CM and VCM are presented in Fig. 6. The polysulfide solutions after exposure to both the V₂O₃ modified and unmodified CM show lower the amounts of remaining polysulfides due to many micropores and mesopores of the CM materials, indicating the adsorption ability of polysulfides by the hierarchical porous structure. But the amounts of remaining polysulfides for the solution exposed to VCM is significantly lower than that for the solution exposed to unmodified CM (Fig. 6). This suggests that the V₂O₃-modifying can remarkably enhance the adsorption ability of soluble polysulfides. It is also confirmed by the much lighter color of the soluble polysulfides solution after exposure to VCM (inset in Fig. 6). These results imply that VCM could effectively immobilize soluble polysulfides during cycling and mitigate loss of active materials. In addition, the chemical adsorption maybe promote more even deposition of solid sulfur and prevent active material loss by formation of large, inactive particles.³³

Fig. 6 Comparison of the amount of remaining polysulfides for the CM and VCM (the inset picture is the photograph of a polysulfide solution before and after exposure to CM and VCM).

3.3. Electrochemical properties

Fig. 7a shows the CV curves of the S/CM and S/VCM composite cathodes within a potential range of 1.8–3.0 V at initial second cycle. There are one oxidation peak and two remarkable reduction peaks for the S/CM and S/VCM composite cathodes. For the S/VCM composite cathode, the two sulfur peaks of reduction reaction at 2.32 and 2.06 V relate to the reduction of the common cyclo-S₈ molecule to high-order lithium polysulfides (Li₂S_x, 4 ≤ x < 8) and further the reduction of soluble polysulfide to lower order Li₂S₂/Li₂S, respectively.⁶ During the charging process, only one oxidation peak at 2.38 V is associated with a conversion of Li₂S₂/Li₂S to Li₂S₈.³³ Compared with S/VCM, S/CM presents lower potential of reduction peaks and a higher one of oxidation peak, indicating less serious polarization of S/VCM electrode. Meanwhile, the results are consistent with the discharge/charge curves of the two electrodes at the second cycle in Fig. S3.†

Fig. 7 (a) Cyclic voltammogram of the cell with the S/CM and S/VCM composites at a scan rate of 0.1 mV s⁻¹ in the voltage range from 1.8 to 3.0 V (Li/Li⁺); (b) EIS of the cell with the S/CM and S/VCM composites.

Fig. 7b shows the variation of the EIS for the initial two cycles of the S/CM and S/VCM composite cathodes, further exhibiting the effect of V₂O₃ modification on the resistance in the cells. And two equivalent circuits³⁴ in the inset are used to model the measured EIS data, where circuit 1 fits the data before cycling and circuit 2 suits the one during charge/discharge process. R_s represents the solution resistance, R is the SEI film resistance, R_ct is the charge-transfer resistance, and W related to the Li⁺ ion diffusion is the Warburg impedance.³⁵ As shown in Fig. 7b, for each curve of the S/CM and S/VCM before cycling, a semicircle at the high and middle frequency region relates to the charge transfer process at electrode/electrolyte interface, and an oblique line at the low-frequency corresponds to region due to Li⁺ ion diffusion/transport.⁹ R_ct and R_s of S/CM are higher than that of S/VCM, indicating the improved charge transfer process due to the V₂O₃ modified carbon sphere. After cycling, they present two depressed semicircle in each curve, implying that the solid sulfur is converted to polysulfides.³⁶ And the two semicircles of S/VCM are still smaller than those of S/CM, indicating the faster reaction kinetics and better cycling stability. Meanwhile, compared with S/CM, S/VCM displays an obvious character shown in Table 1 that R_ct, R and R_s have decreased and it suggests that S/VCM presents better electrolyte infiltration and electrical conductivity. This feature of S/VCM is beneficial for achieving the high capacity reversibility and rate performance.

Table 1 Impedance parameters of S/CM and S/VCM

	State	Rs (Ω)	R (Ω)	Rct (Ω)
S/CM	Before cycling	7.1		76.7
	After cycling	4.7	32.4	34.9
S/VCM	Before cycling	6.5		69.8
	After cycling	4.1	18.8	20.5

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Fig. 8a and b depict the initial discharge/charge potential capacity curves of the cells with S/CM and S/VCM composite cathode at different current rates (0.2C, 0.5C, 1C and 2C). Two plateaus are also observed during the discharge process and are consistent with the CV tests. And compared with discharge/charge curves of S/CM, those of S/VCM present the smaller voltage hysteresis between charge and discharge plateaus, suggesting a low resistance and high electrochemical redox reaction¹⁶ for the S/VCM cathode. In other words, these results imply that the V₂O₃ modification could chemically take effects in electrochemical reaction mechanism for the sulfur cathode and enhance electrochemical performance. And with the increase of the current density, a decrease about the voltage of discharge plateaus is found in each sample, especially at 2C and it can be ascribed to the polarization at high rates. In addition, the S/VCM composite exhibits higher discharge voltage plateaus and capacities than the S/CM composite during discharge. Specific discharge capacities of 1286, 1177, 1006 and 878 mA h g⁻¹ are obtained at 0.2, 0.5, 1 and 2C, respectively, and these values are all higher to those of S/CM. These results indicate that the polysulfides are stably restricted in the VCM confinement structure leading to an excellent rate performance for the Li–S battery.

Fig. 8 The initial discharge/charge curves at different current rates from 0.2 to 2C with the (a) S/CM and (b) S/VCM composite cathode; (c) cycling performances of the cell with the S/CM and S/VCM composite cathode at 0.5C. (d) Rate performances of the S/CM and S/VCM composite cathode.

Fig. 8c presents the cyclic performances of the S/CM and S/VCM composite cathodes at 0.5C. Compared with the cathode without V₂O₃ modification, the S/VCM composite cathode delivers a better cyclic stability and higher capacity. A high initial specific discharge capacity of 1177 mA h g⁻¹ is achieved, which is still maintained at 921 mA h g⁻¹ after 100 cycles with a high capacity retention of 78% of the initial value. While for the cell the S/CM composite cathode, it also presents a high initial specific discharge capacity of 1069 mA h g⁻¹ due to the hierarchical porous structure, but the 100th reversible capacity is only 701 mA h g⁻¹ with capacity retention of 65%. Therefore, V₂O₃ plays a key role in enhancing the utilization of active materials, and the improved electrochemical performance may benefit from its adsorption ability to polysulfides.

Fig. 8d displays the rate performances of the cells with S/CM and S/VCM composite cathodes at different current rates from 0.2 to 2C. During the process, the discharge capacities of the cell with S/VCM composite cathode exhibit a trend similar to that of S/CM composite cathode. At the current rate of 0.2C, S/CM and S/VCM deliver the reversible specific capacities of 1226 mA h g⁻¹ and 1286 mA h g⁻¹, respectively. With rate increasing to 0.5C, the discharge capacity of S/CM decreases to 980 mA h g⁻¹, only having 86% of the S/VCM capacity. A high capacity of 869 mA h g⁻¹ for S/VCM is retained even at a high current rate of 2C, with only 9% decrease compared with that at 1C, indicating remarkable performance at high rates. When the current rate reduces back to 0.2C, a higher capacity recovers to 1141 mA h g⁻¹ for the cathode with V₂O₃ modification and imply suppression of “shuttle effect”. These results reveal that a nice rate performance is achieved by intimate contact of sulfur between the matrix CM and V₂O₃, and the effective V₂O₃ modification.

To better understand the electrochemical performance of the S/VCM composite cathode in the Li–S batteries, we also further conduct a series of electrochemical tests of the S/VCM electrode. Fig. 9a depicts a long cycling performance of S/VCM at 0.2C. The composite displays a stable cycle performance during this entire charge/discharge process, furthermore, even after 250 cycles, a high reversible capacity of 912 mA h g⁻¹ is retained, indicating capacity decay of 0.5% per cycle. The ultrafine V₂O₃ and the 3D structure play important effects on retarding polysulfide dissolution. Naturally the S/VCM system retards the fast capacity decay and shows high capacity retention.

Fig. 9 (a) Long cycling performance of the cell with the S/VCM composite at 0.2C; the charge/discharge profiles of Li–S cell with the S/VCM composite cathode at (b) 1C and (c) 2C.

Fig. 9b and c depict the 1st, 5th, 20th, 50th and 100th charge/discharge potential capacity profiles of the S/VCM cathode at 1C and 2C, respectively. It is also obvious that the charge/discharge plateaus are accord with the redox peaks of the CV tests. At 1C, the initial specific capacity of the S/VCM composite cathode is 1006 mA h g⁻¹, showing a high sulfur utilization of 60%. The capacity retentions for the 5th and 20th cycles reach 97% and 93%, respectively, and after 100 cycles it remains a high value of 843 mA h g⁻¹. The high electrochemical performance of S/VCM could be attributed to the ultrafine V₂O₃ uniformly dispersed in the hierarchical porous CM that enables sufficient electrolyte infiltration, enough electrochemical reaction sites and efficient polysulfide immobilization. Furthermore, at 2C, S/VCM still remains a high discharge capacity of 719 mA h g⁻¹ after 100 cycles, with a high capacity retention of 82% compared to the initial capacity. And from the 5th to 100th cycle, the capacity fades by about 16%, corresponding to a low capacity fading of 1.49 mA h g⁻¹ per cycle and indicating a stable cycling performance. These results illustrate that at high current density the processes such as the ‘shuttle effect’ caused by the dissolved polysulfides are minimized by the V₂O₃ modification.

4. Conclusions

In summary, the spherical carbon with V₂O₃ modification was successfully prepared via the facile wet impregnation method. As cathode for Li–S batteries, S/VCM composite delivers a high initial capacity of 1177 mA h g⁻¹ at 0.5C and a stable cycling performance with 921 mA h g⁻¹ after 100 cycles. Meanwhile, the ultrafine V₂O₃ dispersed in hierarchical porous CM can effectively retard active material loss, trap dissolved polysulfides and significantly enhance electronic conductivity. These features also ensure a good rate performance of S/VCM, and it still retains high reversible capacities with 843 and 719 mA h g⁻¹, respectively, even after 100 cycles at 1C and 2C. Therefore, VCM would be a promising cathode matrix for high performance lithium–sulfur batteries.

Acknowledgements

This work was financially supported by the Science and Technology Project of Anhui Province (1301022077).

References

1. Z. Li, Y. Huang, L. Yuan, Z. Hao and Y. Huang, Carbon, 2015, 92, 41–63.
2. R. J. Chen, T. Zhao and F. Wu, Chem. Commun., 2015, 51, 18–33.
3. X. Fang and H. Peng, Small, 2015, 11, 1488–1511.
4. L. Chen and L. L. Shaw, J. Power Sources, 2014, 267, 770–783.
5. A. Manthiram, S.-H. Chung and C. Zu, Adv. Mater., 2015, 27, 1980–2006.
6. G. Zhou, F. Li and H.-M. Cheng, Energy Environ. Sci., 2014, 7, 1307–1338.
7. P. G. Bruce, S. A. Freunberger, L. J. Hardwick and J.-M. Tarascon, Nat. Mater., 2012, 11, 19–29.
8. L. Sun, D. Wang, Y. Luo, W. Kong, Y. Wu, L. Zhang, K. Jiang, Q. Li, Y. Zhang, J. Wang and S. Fan, ACS Nano, 2016, 10, 1300–1308.
9. Q. Li, Z. A. Zhang, K. Zhang, J. Fang, Y. Q. Lai and J. Li, J. Power Sources, 2014, 256, 137–144.
10. X. Li, L. Chu, Y. Wang and L. Pan, J. Mater. Sci. Eng. B, 2016, 205, 46–54.
11. P. Wei, M. Q. Fan, H. C. Chen, X. R. Yang, H. M. Wu, J. Chen, T. Li, L. W. Zeng, C. M. Li, Q. J. Ju, D. Chen, G. L. Tian and C. J. Lv, Renewable Energy, 2016, 86, 148–153.
12. C. Erhardt, S. Soergel, S. Meinhard and T. Soergel, J. Power Sources, 2015, 296, 70–77.
13. N. Nakamura, T. Yokoshima, H. Nara, T. Momma and T. Osaka, J. Power Sources, 2015, 274, 1263–1266.
14. T. Xu, J. Song, M. L. Gordin, H. Sohn, Z. Yu, S. Chen and D. Wang, ACS Appl. Mater. Interfaces, 2013, 5, 11355–11362.
15. Z. Li and L. Yin, ACS Appl. Mater. Interfaces, 2015, 7, 4029–4038.
16. Z. Xiao, Z. Yang, L. Wang, H. Nie, M. E. Zhong, Q. Lai, X. Xu, L. Zhang and S. Huang, Adv. Mater., 2015, 27, 2891–2898.
17. F. G. Sun, J. T. Wang, D. H. Long, W. M. Qiao, L. C. Ling, C. X. Lv and R. Cai, J. Mater. Chem. A, 2013, 1, 13283–13289.
18. X. Liang, C. Hart, Q. Pang, A. Garsuch, T. Weiss and L. F. Nazar, Nat. Commun., 2015, 6, 5682.
19. X. Li, L. Pan, Y. Wang and C. Xu, Electrochim. Acta, 2016, 190, 548–555.
20. C. Y. Wan, W. L. Wu, C. X. Wu, J. X. Xu and L. H. Guan, RSC Adv., 2015, 5, 5102–5106.
21. A. C. Santulli, W. Xu, J. B. Parise, L. Wu, M. C. Aronson, F. Zhang, C.-Y. Nam, C. T. Black, A. L. Tiano and S. S. Wong, Phys. Chem. Chem. Phys., 2009, 11, 3718–3726.
22. L. Jiang, Y. Qu, Z. Ren, P. Yu, D. Zhao, W. Zhou, L. Wang and H. Fu, ACS Appl. Mater. Interfaces, 2015, 7, 1595–1601.
23. G. Xu, X. Wang, X. Chen and L. Jiao, RSC Adv., 2015, 5, 17782–17785.
24. H. Jiang, G. Jia, Y. Hu, Q. Cheng, Y. Fu and C. Li, Ind. Eng. Chem. Res., 2015, 54, 2960–2965.
25. H.-Y. Li, K. Jiao, L. Wang, C. Wei, X. Li and B. Xie, J. Mater. Chem. A, 2014, 2, 18806–18815.
26. S. Urbonaite, T. Poux and P. Novák, Adv. Energy Mater., 2015, 5, 1500118.
27. J. Liu, W. Li, L. Duan, X. Li, L. Ji, Z. Geng, K. Huang, L. Lu, L. Zhou, Z. Liu, W. Chen, L. Liu, S. Feng and Y. Zhang, Nano Lett., 2015, 15, 5137–5142.
28. A. Jozwiuk, H. Sommer, J. Janek and T. Brezesinski, J. Power Sources, 2015, 296, 454–461.
29. J. Song, M. L. Gordin, T. Xu, S. Chen, Z. Yu, H. Sohn, J. Lu, Y. Ren, Y. Duan and D. Wang, Angew. Chem., Int. Ed., 2015, 54, 4325–4329.
30. D. S. Jung, T. H. Hwang, J. H. Lee, H. Y. Koo, R. A. Shakoor, R. Kahraman, Y. N. Jo, M.-S. Park and J. W. Choi, Nano Lett., 2014, 14, 4418–4425.
31. G. Y. Xu, B. Ding, P. Nie, L. F. Shen, H. Dou and X. G. Zhang, ACS Appl. Mater. Interfaces, 2014, 6, 194–199.
32. J. Yang, J. Xie, X. Zhou, Y. Zou, J. Tang, S. Wang, F. Chen and L. Wang, J. Phys. Chem. C, 2014, 118, 1800–1807.
33. J. Song, Z. Yu, M. L. Gordin and D. Wang, Nano Lett., 2016, 16, 864–870.
34. C.-P. Yang, Y.-X. Yin, H. Ye, K.-C. Jiang, J. Zhang and Y.-G. Guo, ACS Appl. Mater. Interfaces, 2014, 6, 8789–8795.
35. L. Yuan, X. Qiu, L. Chen and W. Zhu, J. Power Sources, 2009, 189, 127–132.
36. X. Zhao, M. Liu, Y. Chen, B. Hou, N. Zhang, B. Chen, N. Yang, K. Chen, J. Li and L. An, J. Mater. Chem. A, 2015, 3, 7870–7876.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra12687a

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