Preparation and Electrochemical Performance of V2O5 @N-CNT/S Composite Cathode Materials

Lithium–sulfur battery hasreceived widespread attention because of its high energy density, low cost, environmental friendliness, and nontoxicity. However, the insulating properties of elemental sulfur, huge volume changes, and dissolution of polysulfides in electrolytes that result in the shuttle effect, low sulfur utilization, and low rate performance seriously hinder the commercialization of lithium–sulfur batteries. In this work, a composite material of nitrogen-doped multiwalled carbon nanotubes and V2O5 was designed and fabricated to serve as the positive electrode of lithium–sulfur battery via the hydrothermal method. The positive electrode of the V2O5@N-CNTs composite material could reach an initial discharge specific capacity of 1,453 mAh g−1at a rate of 0.1C. Moreover, the composite material could maintain a discharge ratio of 538 mAh g−1 at a rate of 0.5C even after 200 charge and discharge cycles. After 400 cycles, the composite had a specific discharge capacity of 439 mAh g−1 at a rate of 1.0C. The excellent electrochemical performance of the V2O5@N-CNT/S composite cathode material was due to the fact that V2O5 contains oxygen ions and has a strong polarized surface. Furthermore, nitrogen doping changed the hybrid structure of carbon atoms and provided additional active sites, thereby improving the conductivity of the material itself and effectively inhibiting the dissolution and diffusion of polysulfides.


INTRODUCTION
Lithium-ion batteries are widely used in energy storage and portable electronic device applications (Goodenough and Park, 2013;Wu et al., 2018;Yang et al., 2018a;Yang et al., 2018b;Zhang et al., 2018;Leng et al., 2019;Ye et al., 2019). However, the application of lithium-ion batteries in electric vehicles is limited because of their low theoretical energy density. Lithium-sulfur batteries have received increased attention because of their high energy density, low cost, environmental friendliness, and nontoxicity Wu et al., 2019a;Gu et al., 2020;Zhang et al., 2020). Nevertheless, the insulation of elemental sulfur, huge volume changes, and dissolution of polysulfides in electrolytesthat result in the shuttle effect, low sulfur utilization, and low rate performance seriously hinder the commercialization of lithium-sulfur batteries (Peng et al., 2017;Zhang et al., 2017;He et al., 2018;Gu and Lai, 2019;Zhu et al., 2019).
The electrochemical performance of lithium-sulfur batteries can be improved to solve polysulfide dissolution and diffusion. Various cathode materials, such as carbon nanotubes (CNTs), conductive polymers, and metal compounds, are widely used in lithium-sulfur batteries Guo et al., 2017;Zeng et al., 2017;Wu et al., 2019b). Carbon materials, carbon nanofibers, porous carbon, and grapheneare used and applied as sulfur host materials because of their large specific surface area, high conductivity, and suitable pore size (Balach et al., 2015;Kong et al., 2018;Yang et al., 2018c;Zhao et al., 2018;Yang et al., 2019). Nevertheless, the van der Waals interaction between carbon materials and polysulfides is a weak force and cannot effectively inhibit the shuttle effect. Polar materials can effectively increase polysulfide adsorption via strong chemical bonds, thereby reducing the dissolution and shuttle of polysulfides. Numerous studies on various polar materials, such as TiO 2 , MnO 2 , SiO 2 , and V 2 O 5 have been conducted Xue et al., 2017;Lin et al., 2018;Tang et al., 2018;An et al., 2019;Tong et al., 2019;Wang et al., 2019;Qin et al., 2020). Moreover, Yuan et al. found that transition metal phosphides can not only capture soluble polysulfides, but also effectively catalyze the decomposition of Li 2 S, which improve the utilization of active materials and cycle performance (Yuan et al., 2017). However, the conductivity of polar materials is generally lower than that of nanocarbon host materials, resulting in lower sulfur utilization and rate performance. Therefore, highly conductive and strongly adsorbing polar compound host materials combined with nanocarbon must be developed to increase electrical conductivity, improve electrolyte contact, and provide more effective chemical and physical constraints on polysulfides, thus improving battery performance (Hong et al., 2018;. Multiwalled CNTs (MWCNTs) are extensively utilized in energy storage materials because of their high structural stability and good electronic conductivity. Razzaq et al. (2019) prepared thin-film composites with CNTs as the main chain via electrospinning; these composites exhibited excellent cycle performance on the positive electrode of lithium-sulfur batteries. Other researchers adopted solidstate growth strategies for preparing highly nitrogen-doped graphene and carbon tube composite cathode materials with a 3D structure; Yuan et al. synthesized nitrogen-doped nanoparticles with a coordinated nitrogen structure and a controllable concentration. Pyridine nitrogen-doped carbon not only shows the strongest lithium polysulfide trapping ability, but also can accelerate the conversion of Li 2 S. The cathode based on pyridine nitrogen-doped carbon has higher capacity and better cycle stability (Yuan et al., 2018). Their studies proved that the introduction of nitrogen atoms can change the hybrid structure of carbon atoms to increase the conductivity of the composite material and the adsorption of polysulfides (Ding et al., 2016;Chen et al., 2018;Yin et al., 2018;Fan et al., 2019;Fang et al., 2019) . Several works fabricated CNTs whose surfaces are coated with V 2 O 5 of different thicknesses; their bonding point is located on the surface of the CNTs. Coating with V 2 O 5 effectively inhibits the shuttle of polysulfides and improves sulfurutilization (Carter et al., 2017). Given that MWCNTs have high conductivity and polar metal oxides have excellent adsorption and catalytic abilities, their interwoven conductive network structure can quickly transfer electrons, and the 3D network structure that they form provide greater adsorption for sulfur point .
In this study, a combination of a polar metal oxide, namely, V 2 O 5 , and nitrogen-doped MWCNTs was used as the sulfur host material. A nitrogen-doped 3D conductive network structure, namely, MWCNT/V 2 O 5 /S composite cathode, was prepared via simple melting and hydrothermal synthesis methods. The preparation process is uncomplicated, and the conductive network structure of MWCNTs can absorb V 2 O 5 nanoparticles well.

Material Preparation
Preparation of nitrogen-doped V 2 O 5 /MWCNTs: NH 3 VO 4 powder was calcined in air at 350°C for 2 h to obtain V 2 O 5 powder. Carbon tubes and urea were uniformly mixed at a ratio of 1:10 and then placed onto the lining of a 100 ml reactor. The mixture was hydrothermally reacted at 200°C for 12 h. The mixture was repeatedly centrifuged with deionized water and absolute ethanol to obtain nitrogen-doped MWCNTs (N-MWCNTs). Subsequently, 0.2 g of the V 2 O 5 powder was added to a suspension containing 0.3 g of N-MWCNTs, and then it was ultrasonicated for 30 min. The sonicated aqueous solution was vacuum-filtered with deionized water, dried in an oven at 60°C for 12 h, and completely and uniformly ground to obtain the composite material, namely, V 2 O 5 /N-MWCNT.
Preparation of V 2 O 5 /MWCNTs: V 2 O 5 /MWCNTs and sulfur were placed in an agate mortar at a ratio of 3:7 and evenly ground. The ground material was put into a glove box to remove oxygen, and then it was put onto the lining of a 25 ml reactor. Finally, it was melted in an oven at 155°C for 12 h to obtain a composite material loaded with sulfur. V 2 O 5 /MWCNT/S and MWCNT/S composite materials doped with nitrogen were obtained via the same method and at the same ratio.

Material Characterization and Analysis
The micro morphologies of the composite materials were examined via scanning electron microscopy (SEM, HITACHIS-4800). Excitation voltage was set to 5 kV, and relative pressure (P/P0) was set to 0.05-0.2. The crystal structures of the materials were analyzed via X-ray diffraction (XRD, MiniFlex-600) by using Cu-Ka as the radiation source at a scanning rate of 20 min −1 , cavity voltage of 40 kV, and tube current of 30 mA. The elemental compositions of the composite materialswere measured via X-ray energy spectrometry (EDS, S-00123). The electronic binding energies of the V 2 O 5 / N-MWCNT/S composite materials were measured via X-ray photoelectron spectroscopy (XPS, Escalab-250XI) to determine element valence distributions and bonding modes. Raman spectra under argon ion laser excitation were characterized using a Thermo Fisher-DXR system. The sulfur contents of the composite materials were detected via TGA analysis within the temperature range of 400-700°C at a heating rate of 20°C/min.

Electrochemical Performance Test
Theactive materials, namely, Super P (conductive agent) and polyvinylidene fluoride (battery-grade binder),were uniformly mixed in N-methylpyrrolidone ata mass ratio of 7:2:1. The active materials were uniformly coated onto an aluminum foil via the doctor blade coating technology, and sulfur loading was 1.1 mg cm −2 . Pole pieces were dried in an oven at 60°C for 12 h, cut into positive pole pieces with a diameter of 16 mm, and then assembled into a button cell (CR 2025). The contents of O 2 and H 2 O in the glove box were strictly controlled to less than 0.1 ppm, and the prepared electrode sheet was used as the positive electrode. A Celgard 2,400 polypropylene microporous membrane was used. A metal lithium sheet served as the negative electrode. The electrolyte type was LS-002 type. The main components were 1,3-dioxolane and 1,2dimethoxyethane (at a volume ratio of 1:1) containing a 1 wt% LiNO 3 additive. The top-down assembly sequence was completed in the order of positive electrode shell, positive electrode sheet, electrolyte, separator, metal lithium sheet, foamed nickel, and negative electrode shell. The assembled battery was left to stand overnight for 12 h, and a BTS battery test system (Neware BTS 7.0) was used for the relevant electrochemical performance tests at a constant temperature of 25°C. The ratio of electrolyte to sulfur is 20 µl mg −1 .

RESULTS AND DISCUSSION
The MWCNTs consisted of numerous closely intertwined CNTs that formed a 3D conductive network structure ( Figures 1A,B). The diameter of the carbon tubes was about 120 nm. This 3D network structure could tightly wrap the sulfur particles, and the internal 3D network space structure could hold large amounts of the active substances. Thus, these structures effectively alleviated the problem of sulfur volume expansion. The length of the MWCNTs could be extended from tens to hundreds of nanometers ( Figure 1C), thus effectively improving the overall conductivity of the composite materials. The V 2 O 5 particles loaded onto the surface of the MWCNTs not only promoted the rapid transmission of lithium ions but also exerted a certain catalytic effect, thereby accelerating the conversion process of the intermediate product lithium polysulfide. Simultaneously, the higher structural stability of the 3D network constructed by the MWCNTs effectively inhibited polysulfide dissolution and diffusion. The V 2 O 5 /MWCNT/S composite material after nitrogen doping is shown in Figure 1D. The morphology of the composite materials after nitrogen doping did not evidently change compared with that before nitrogen doping. Therefore, the composite materials were further analyzed using an elemental energy spectrometer. The EDS mapping of the samples corresponding to C, N, O, V, and S elements is shown in Figure 2. The overall   elemental distribution was relatively uniform. The presence of nitrogen could also be clearly observed, indicating that nitrogen was successfully doped into the V 2 O 5 /MWCNT/S samples via the hydrothermal method. Although the overall nitrogen content was relatively small, its distribution was relatively uniform. The introduction of nitrogen played a certain role in increasing the conductivity and adsorption of the composite materials.
The diffraction patterns of S, V 2 O 5 , and V 2 O 5 /MWCNT/S samples are depicted in Figure 3A, which shows that the composite sample retained the diffraction characteristic peaks of V 2 O 5 that corresponded to the (001), (110), (400), and (310) crystal planes. At 23.1°, the strongest characteristic peak of sulfur was still maintained in the composite materials, and this peak corresponded to the characteristic peak of the typical S 8 bond structure. The Raman spectra of the V 2 O 5 / MWCNT and V 2 O 5 /N-MWCNT composite materials are shown in Figure 3B. The D and G peaks of graphite could be observed at 1,350 and 1,590 cm −1 . Peak D indicated the presence of crystal defects in carbon atoms, whereas peak G denoted sp 3 hybridization. The values of the V 2 O 5 /MWCNT and V 2 O 5 /N-MWCNT composite materials were 1.00 and 1.02, respectively. Results demonstrated that the nitrogen-doped MWCNTs had a greater degree of defects and were more conducive to electron conduction, thus increasing the cycle efficiency of the lithium-sulfur battery Ma et al., 2017). The thermogravimetric diagrams of the samples are shown in Figure 4. The weight loss of the MWCNT/S, V 2 O 5 /MWCNT/S, and V 2 O 5 /N-MWCNT/S composite materials was 68.3, 68.2, and 68.5%, respectively, indicating that the MWCNTs supported a higher sulfur content while providing more active sites.
The chemical composition of the structure of the V 2 O 5 / N-MWCNT/S composite material was analyzed via XPS. Figure 5A shows the C, N, O, V, and other elements in the V 2 O 5 /N-MWCNT/Scomposite material. N peak could be clearly observed, indicating that nitrogen was successfully doped into the CNTs. The peak positions of V2p and V2s corresponded to about 517 and 630 eV, respectively. A highresolution map of nitrogen in the material is displayed in Figure 5B. N 1S was fitted via the Gaussian peak splitting method. The four peaks clearly corresponded to pyridine N (398.5 eV), pyrrole N (399.6 eV), oxide N (400.9 eV), and graphite N (402.1 eV). Pyridine N and pyrrole N from urea can introduce more defects, thereby changing the electronic distribution state of charge, providing additional active adsorption sites for polysulfides, and effectively improving the cycle stability of lithium-sulfur batteries Jiang et al., 2018). The two peaks at 284.6 and 286.6 eV were C-C/C C and C-O, respectively ( Figure 5C) (Zheng et al., 2017). The XPS peak position of V2p 3/2 was located at 517.1 eV, corresponding to the V 5+ valence state of the V 2 O 5 compound ( Figure 5D). This result proved that the valence of V 2 O 5 did not change. The binding energies of 163.9 and 165.1 eV corresponded to the S2p 1/2 and S2p 3/2 orbitals, respectively ( Figure 5E), consistent with the position of the binding energy peak of elemental sulfur reported in the literature (Wang et al., 2014;Li et al., 2016). Whenthe V 2 O 5 @MWCNT/S composite material was charged and discharged at a rate of 0.2°C, the first discharge specific capacity could reach 1,257 mAh g −1 ( Figure 6A). Even after 200 cycles, the discharge specific capacity was still 518 mAh g −1 . Therefore, the cycle performance of the V 2 O 5 @MWCNTs/S composite material was better than that of the MWCNT/S composite material. Similarly, when charging and discharging at a rate of 0.5C, the specific capacity of the nitrogen-doped V 2 O 5 / MWCNT/S composite material could reach 538 mAh g −1 even after 200 cycles, and its overall cycle performance was also better than that of the V 2 O 5 /MWCNT/S (465 mAh g −1 ) and MWCNT/ S composite materials (395 mAh g −1 ) ( Figure 6B). In terms of rate performance, the overall performance of the V 2 O 5 / N-MWCNT/S composite material was also better than that of the MWCNT/S composite material ( Figure 6C). The excellent rate performance of the V 2 O 5 /N-MWCNT/S composite material was due to the 3D conductive network system constructed by V 2 O 5 and MWCNTs. Nitrogen doping effectively improved the conductivity of carbon and its chemical adsorption capacity for lithium polysulfide. Moreover, V 2 O 5 promoted the rapid transfer of electrons in the intermediate polysulfide. The charge-discharge cycle curves of different samples at a rate of 0.1C are exhibited in Figures 6D-F. The two obvious discharge platforms at 2.1 and 2.3 V were clearly similar to the multistep conversion process of sulfur. The charge-discharge platform at 2.3 V was relatively stable, transforming sulfur into long-chain polysulfides; at 2.1 V, the other stable segment belonged to shortchain polysulfides that further transformed into Li 2 S 2 /Li 2 S. The change in charge platform was actually the transition from Li 2 S 2 / Li 2 S to Li 2 S 8 /S 8 . Furthermore, the number of cycles increased, and the potential difference between the charge and discharge curves gradually increased. The polarization potential between the charge and discharge curves of the V 2 O 5 /MWCNT/S composite material after nitrogen doping was considerably smaller than that of the MWCNT/S and V 2 O 5 /MWCNT/S composite materials. This result also demonstrated that the positive electrode of the nitrogen-doped V 2 O 5 /MWCNT/S composite material had better cycle stability and played a certain role in inhibiting the shuttle effect of polysulfides.
After 400 charge and discharge cycles at a rate of 1C, the discharge specific capacity was 439 mAh g −1 ( Figure 7A). The nitrogen-doped V 2 O 5 /MWCNT/S composite material maintained an excellent cycle performance. The charge and discharge performance curves of the battery at different rates are illustrated in Figure 7B. As current density increased, the overall discharge specific capacity of the battery gradually decreased, and the polarization phenomenon between the charge and discharge curves became increasingly serious. Nevertheless, the lithium-sulfur battery still maintained a  Figure 7C, which shows one oxidation peak and two reduction peaks. The reduction peaks at 2.3 and 2.0 V corresponded to the conversion of S 8 to Li 2 S X (4 ≤ x ≤ 8) and the conversion of Li 2 S X (4 ≤ x ≤ 8) to Li 2 S 2 /Li 2 S, respectively. The CV curve also had an oxidation peak at 2.5 V, which corresponded to the conversion reaction of Li 2 S 2 /Li 2 S to Li 2 S X (4 ≤ x ≤ 8) and S8. The oxidation peak potential of the V 2 O 5 / MWCNT/S composite material was lower and its reduction peak potential was higher than that of the MWCNT/S composite material. This observation also indicated that the V 2 O 5 / MWCNT/S composite material had a lower oxidation and a higher reduction potential than the other composite materials.
In addition, the polar metal oxide (V 2 O 5 ) accelerated the kinetic conversion process, and the synergy of V 2 O 5 and MWCNTs also played a certain role in improving the cycle stability of the lithium-sulfur battery. After the Z-view software was fitted, the charge transfer resistance of the nitrogen-doped V 2 O 5 / MWCNT/S became smaller after the cycle, also indicating that the charge transfer ability of the composite sample was stronger ( Figure 7D).

CONCLUSION
In this work, MWCNTs were doped with nitrogen via the hydrothermal reduction method. V 2 O 5 nanoparticles were attached onto the surface of the MWCNTs and combined with sulfur (i.e., MWCNT/S) to prepare battery cathode materials. The 3D conductive network structure constructed by the MWCNTs accelerated electron transfer. The polar metal oxide, i.e., V 2 O 5 , had greater chemical adsorption and catalytic abilities that allowed it to accelerate the entire kinetic reaction process. The hybrid structure of carbon atoms changed, polysulfide adsorption improved, and the dissolution and diffusion of polysulfides were effectively inhibited by using the nitrogen source provided by urea to introduce more defects. Under the synergistic effects of V 2 O 5 and MWCNTs, the overall electrochemical performance of the composite cathode materials was effectively enhanced. The initial discharge specific capacity could reach 1,453 mAh g −1 at a rate of 0.1C, and a high discharge specific capacity of 538 mAh g −1 was maintained even after 200 charge and discharge cycles at a rate of 0.5C. The capacity retention rate was 60%. At a rate of 1.0C, the composite material still had a specific discharge capacity of 492 mAh g −1 even after 400 charge and discharge cycles. The excellent cycle performance of the composite material was mainly due to the effective binding of V 2 O 5 to polysulfides that reduced the further dissolution of the active substances. The improvement in the rate performance of the V 2 O 5 /N-MWCNT composite material was due to the stable and efficient 3D conductive network system built by the nitrogen-doped MWCNTs. This structure provided numerous active adsorption sites for sulfur and effectively improved the utilization of the active materials. The conductive 3D network structure that formed exhibited good electrochemical performance. This result offers a promising prospect for developing lithium-sulfur batteries.

DATA AVAILABILITY STATEMENT
The original contributions presented in the study are included in the article/Supplementary Material, further inquiries can be directed to the corresponding authors.