Mesoporous TiO2 Nanofiber as Highly Efficient Sulfur Host for Advanced Lithium–Sulfur Batteries
Chinese Journal of Mechanical Engineering volume 32, Article number: 60 (2019)
Currently, lithium–sulfur batteries suffer from several critical limitations that hinder their practical application, such as the large volumetric expansion of electrode, poor conductivity and lower sulfur utilization. In this work, TiO2 nanofibers with mesoporous structure have been synthesized by electrospinning and heat treating. As the host material of cathode for Li–S battery, the as prepared samples with novelty structure could enhance the conductivity of cathode composite, promote the utilization of sulfur, and relieve volume expansion for improving the electrochemical property. The initial discharge capacity of TiO2/S composite cathode is 703 mAh/g and the capacity remained at 652 mAh/g after 200 cycles at 0.1 C, whose the capacity retention remains is at 92.7%, demonstrating great prospect for application in high-performance Li–S batteries.
With the rapid development of portable equipment, handheld electronic products and hybrid electric vehicles, problems related to energy storage and conversion devices have attracted more and more attention. Because of low cost of sulfur, no harmful to the environment and higher theoretical energy density, lithium–sulfur batteries are being expected to become the most potential next generation of batteries in the world [1, 2]. However, currently most traditional lithium–sulfur batteries suffer from several critical limitations that hinder their practical application on account of the large volumetric expansion of elemental sulfur during lithiation, poor conductivity of both the final products, and so on. In addition to this, in the discharge process, the major obstacle is that intermediate polysulfides are highly dissolved in organic electrolyte, which the so-called “shuttle effect” causes an irreversible loss of active sulfur, poor cycle stability and low coulombic efficiency during charge/discharge cycling [3,4,5].
To overcome these impediments for the development of Li–S batteries, several strategies have been developed. For example, carbon acted as the sulfur matrix (graphene, carbon nanotubes and porous carbon [6,7,8,9], which mainly encapsulated the sulfur and polysulfide species into porous conductive materials through physical interaction. Meanwhile, more and more metal oxides as host materials for Li–S batteries has been put forward, owing to their strong chemisorption effect towards polysulfides (LiPSs), including SiO2, MnO2, TiOx, Al2O3, etc [10,11,12,13,14]. Among them, titanium dioxide (TiO2) has drawn much attention due to its low cost, environmental protection and structural stability . An et al.  prepared TiO2@NC interlayer to absorpt polysulfide in Li–S batteries. Because of good electronic conductivity, the reversible capacity reached 1460 mAh/g at 0.2 C. She et al.  designed a sulphur-TiO2 yolk-shell nano-architecture to take in the large volumetric expansion of sulphur and minimize polysulphide dissolution, and the capacity decayed 0.033% every cycle after 1000 cycles. Li et al.  designed mesoporous hollow TiO2 spheres (HTSs) to solve the above problems, and the capacity retention of 71% at 1 C (1 C = 1672 mA/g). The mesoporous morphologies of host is important for the improving the electrochemical property of Li–S batteries, because of the higher utilization of sulfur and chemisorption of lithium polysulfides. Moreover, it could afford large surface to infuse sulfur and accommodate volume changes during the charge/discharge reactions. Therefore, it is important to prepare the TiO2 with the mesoporous structure by simple methods and research on charge and discharge mechanism of Li–S batteries deeply.
Herein, the mesoporous TiO2 nanofibers were synthesized through an electrospinning method and subsequent thermal treatment. The mesoporous structure could encapsulate sulfur in their pores to trap soluble polysulfides (S8 combining with the Li+ in the anode), and accommodate large volumetric expansion of sulphur during lithiation/delithiation. In addition, the cathode conductivity is improved by TiO2/S composite as electrodes. It results in excellent electrochemical performance and significantly improved cycle stability of the TiO2/S composite cathode for Li–S batteries.
2 Experimental Section
2.1 Reparation of the Porous Nanofiber TiO2 Membranes
The nanofiber TiO2 was prepared by electrospinning, the details as follows: 0.8 g Polyvinylpyrrolidone (PVP) was directly dissolved in 6 mL ethanol, and stirred for 3 h. Meanwhile, to obtain a solution of TiO2, 2.5 mL tetra-n-butyl titanate was dissolved in 2 mL acetic acid and 5 mL ethanol, and stirred for 3 h. After that above mixture was mixed with the PVP/ethanol solution together, and stirring for 12 h at room temperature. This solution was loaded into a perfusion tube equipped with pipette tip through syringe with silver-coated needle. The pipette tip was connected to a highvoltage power supply. The electric voltage was set as 15 kV. Gained nonwoven fabrics was consist of PVP/TiO2 nanofibers. After peeled off, nonwoven fabrics was calcined at 500 °C with heat-up speed of 5 °C/min for 2 h in air flow, following by being calcined at 700 °C for 5 h to yield the final products.
2.2 Preparation of TiO2/S Cathode
As-prepared Nanofiber TiO2 and sulfur powder in a mass ratio of 3:7 were mixed and ground together for 30 min. Then the mixture of powder was converted to a sealed stainless steel vessel and calcined for 24 h at 155 °C. Finally, TiO2/S cathode was obtained, corresponding to a sulfur loading of 3.0 mg/cm2 and a sulfur content of 70 wt% in the whole cathode.
2.3 Materials Characterization
The microstructure of the samples were investigated by X-ray diffraction (XRD, D-MAX II A X-ray diffractometer). The micro morphological images were obtained by field emission scanning electron microscope (FE-SEM, S4800, Hitachi) and transmission electron microscopy (TEM, Tecnai F20). Energy-dispersive X-ray spectroscope (EDS) was measured to gain the elemental mapping results. Thermogravimetric (TG) analysis (Perkin-Elmer TGA 7 thermogravimetric analyzer) was used to evaluate the sulfur content of TiO2/S cathode at a heating rate of 5 °C/min from 40 °C to 600 °C in flowing N2. The pore size distributions and specific surface area of TiO2/S cathode were measured by TriStar II 3020 3.02 (Micromeritics Instrument Corporation, USA).
2.4 Electrochemical Performance Measurements
CR2025 coin cells were prepared in a glove box, which is filled with argon and conducted electrochemical testings. The working cathode was fabricated by coating a mixture containing 10 wt% polyvinylidene fluoride (PVDF) as a binder dissolved in NMP, 10 wt% carbon black and 80 wt% active material (nanofiber TiO2/S) on an aluminum foil following by drying at 60 °C for 12 h. Then the electrode disks were punched. Li metal was used as the anode. The electrolyte was a solution of 1.0 M LiTFSI dissolved in a mixture of dioxolane (DOL) (99.8%, Sigma-Aldrich) and 1,2-dimethoxyethane (DME) (99.5, Sigma-Aldrich) (1:1 by volume) with LiNO3 (1 wt%) additive. Separator (Celgard 2400) was bought from Celgard Company. CR2025 coin cells were assembled in an argon filled glovebox with water and oxygen content kept below 0.1 ppm and used for electrochemical tests. Galvanostatic charge/discharge cycling was carried out using a LAND-CT2001A instrument (Wuhan, China) in the voltage range 1.5 to 3 V.
3 Results and Discussion
Figure 1 shows the fabrication and electrochemical process of TiO2/S composite as cathode for Li–S battery. In step 1, the smooth of TiO2/PVP nanofibers was fabricated by electrospinning. Followed by pyrolysis of PVP, which is pyrolyzed into CO2 and H2O at high temperature calcination in Air-flow, the TiO2 nanofibers with mesoporous structure could be gained. In step 4, the TiO2 infused with sulfur forming the TiO2/S cathode. The sulfur elements are uniformly distributed in the TiO2 nanofibers because of the mesoporous structure and polar metal-O bond. After testing for 200 cycles, clear and smooth surface of TiO2/S composite electrode can be observed from the typical SEM images (in step 5), which improving the electrochemical performance.
The SEM images of the synthesis PVP/TiO2 composite nanofiber by electrospinning are shown in Figure 2. Figure 2(a) shows a typical low-magnification SEM image of PVP/TiO2 composite fibers (the average diameter is about 200 nm). The high-magnification SEM image of the composite nanofibers with smooth surface (Figure 2(b)). After electrospinning, the composite nanofibers were calcined to obtain pure TiO2 nanofibers. From the Figure 2(c) and (d), the TiO2 nanofibers with the average diameters about 200 nm were synthesized. Cross sections of enlarged TiO2 nanofibers in Figure 2(d) clearly shows that the fibers are very rough with many wormhole-like pores which could be used as good sulfur host to infuse high content of sulfur.
After preparing of TiO2/S, the morphology of the TiO2 nanofibers as the host could not be changed after the infusion of sulfur (as shown in Figure 3(a) and (b)). The TEM images of TiO2 nanofibers before and after infusion of sulfur are shown in Figure 3(c), (d). The mesoporous structure can be clearly seen in the TEM image of TiO2 nanofibers, these porous structure was disappeared in the TEM image of TiO2/S nanofibers, reflecting the good infusion of sulfur by mesoporous structure in TiO2 nanofibers. The HRTEM image (Figure 3(e)) reveals the clear lattice fringe spacing is 0.35 nm, which is consistent with the (101) crystalline interplanar spacing of TiO2 structure. Additionally EDS area mapping showing in Figure 3(f), and can see the elemental of Ti, O, and S homogeneous distribution in all over the fiber.
The XRD patterns of TiO2 and TiO2/S nanofibers are shown in Figure 4 to confirm the crystal phase formation about synthesized TiO2 nanofibers and the existence of sulfur about TiO2/S nanofibers. The XRD pattern of TiO2 nanofibers demonstrates that TiO2 possesses almost pure rutile (JCPDS No. 65-0191) structure [19, 20]. Then, the XRD pattern of TiO2/S composites was well defined for orthorhombic structure of crystalline sulfur (JCPDS Card No. 08-0247), which is identical to the element sulfur powder [5, 21]. It reveals the TiO2 nanofibers with sulfur loading have been synthesized successfully.
The nitrogen adsorption and desorption isotherms of the TiO2 and TiO2/S composite were obtained, which correspond to type IV isotherms in the IUPAC classification with a typical mesopore hysteresis loop, from Figure 5(a) [22, 23]. Notably, compared with TiO2/S composite, TiO2 has larger pore volume and specific surface area. The pore size distribution of TiO2 fibers is in the range of 20‒30 nm by Barrett-Joyner-Halenda as shown in Figure 5(a) inset. After sulfur incorporation, the fibers pore distribution decrease to 10‒20 nm, because S particles is covered on or embedded into the mesopores of TiO2 fibers. To evaluate the content of sulfur in TiO2/S composite, thermo gravimetric (TGA) was performed from room temperature to 600 °C at a heating rate of 10 °C/min under N2 atmosphere. The sulfur content in TiO2/S nanocomposite (Figure 5(b)) is estimated as high as 70 wt%, which demonstrates such higher sulfur loading.
Figure 6(a) and (b) display the galvanostatic the discharging-charging curves of the TiO2/S composite electrode at a current rate of 0.2 and 0.5 C (1 C = 1672 mA/g). The profile apparently shows the two plateaus in the discharging curves, which could be assigned to the two-step reaction of sulfur with lithium. The first plateau, at 2.35 V, was due to the reduction of sulfur to higher polysulfide. The low voltage plateau, at 2.1 V, showed the reactions of the higher polysulfides (Li2Sn, 4 ≤ n ≤ 8) finally the further to the lower polysulfides (Li2Sn, n ≤ 3) [24,25,26,27]. And the initial discharge capacity and charge capacity of as prepared electrode were 763 mAh/g and 827 mAh/g at 0.2 C, respectively. And at 0.5 C as shown in Figure 6(b), the electrode discharge capacities was found to be 423 mAh/g, and charge capacities was 405 mAh/g.
The excellently stable cycling performance of TiO2/S composite electrodes at different current densities have been shown in Figure 6(c). The TiO2/S electrodes exhibit the excellent capacity retention at 0.1, 0.2 and 0.5 C. The initial discharge capacity was 703 mAh/g and the capacity remained at 652 mAh/g after 200 cycles at 0.1 C. Figure 6(d) is the discharge rate capability performance with 100 cycles at different current densities. When the current density is increased to 0.2, 0.5 and 1 C, the discharge capacities are 498, 402 and 298 mAh/g. When the current density returns to 0.1 C, the reversible capacity was recovered to 613 mAh/g, indicating the reliability and stability of the TiO2/S composite electrode.
The SEM images of the TiO2/S cathode after cycled 200 times are displayed in Figure 7(a). The TiO2/S fibers are still maintains the original fiber structure, although the some nanowire structure was destroyed. From Figure 7(b)‒(d), the cathode elemental of Ti, O, and S still displays homogeneous distribution. It reveals that sulfur could be trapped in the nanofibers after 200 cycles, and further shows the advantages of mesoporous nanofibers in repeated cycling, which accordingly enhances the cyclic stability and rate capability. Figure 7(e) is corresponding reaction mechanism of the TiO2/S cathode during cycling. In the discharge process, S8 (elemental sulfur) in the nanofibers is combined with the Li+ from the anode, and reduced to form soluble Li2Sn (2 ≤ n < 8), which is also trapped in the mesoporous structure without diffused into the electrolyte and transferred to anode . Then the soluble Li2Sn is further reduced to insoluble Li2S. Because the process of elemental sulfur reacted to form lithium polysulfide and eventually to lithium sulfide, which is always carried out in TiO2 nanofibers, it could prevent the soluble lithium sulfide from being dissolved in the electrolyte, indicating the remission of the “shuttle effects”. Meanwhile, the volume expansion caused by the conversion of sulfur into lithium sulfide could be alleviated, because of TiO2 nanofibers with mesoporous structure. The electrochemical performance comparison of the TiO2–S composite between the current work and previously reported in Table 1. It shows that the TiO2 nanofibers with mesoporous structure in our work improved cycle stability of cathode for Li–S battery.
Therefore, it is owing to the following reasons of TiO2 nanofiber with mesoporous structure as highly efficient sulfur host for improving cycle stability and high efficiency of battery. Firstly, TiO2 possesses the excellent catalytic dissociation ability of lithium polysulfides. And rutile phase TiO2 can in-situ adsorb the lithium polysulfides by stable chemical bonding force leading to further trapping of polysulfide anions . Secondly, the interconnected fiber architecture provided fast pathways for electron/ion transfer and the mesoporous structure provide enough large surface to infuse sulfur while physically absorbing soluble lithium polysulfides and accommodating volume changes by the charge/discharge reactions.
TiO2 nanofibers with mesoporous structure were prepared by post thermal-treatment of electrospun. The fibers as the superior host material to load sulfur up to the 70 wt % for Li–S batteries. The TiO2/S cathode demonstrated cycle stability and high efficiency. The TiO2/S cathode maintains a capacity of 652 mAh/g at 0.1 C after 200 cycles, corresponding to a capacity retention of 92.7%. The mesoporous TiO2 fibers enhance the conductivity of sulfur, promote the utilization of sulfur and provide large active sits to absorb soluble lithium polysulfides. The mesoporous TiO2 nanofibers as cathode host material has great potential in high-performance lithium–sulfur batteries.
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LD and ZG was in charge of the whole trial; XS wrote the manuscript; XZ and JY assisted with sampling and laboratory analyses. All authors read and approved the final manuscript.
Xinyu Shan, born in 1996, is currently a master candidate at Key Laboratory of Automobile Materials, School of Materials Science and Engineering, Jilin University, China. She received her bachelor degree from Jilin Jianzhu University, China, in 2018. Her research interests include solar energy and energy storage materials.
Zuoxing Guo, born in 1963, is currently a professor at Key Laboratory of Automobile Materials, School of Materials Science and Engineering, Jilin University, China. University, China, in 2006.
Xu Zhang, born in 1996, is currently a master candidate at Advanced Institute of Materials Science & Department of Materials Science and Engineering, Changchun University of Technology, China. She received her bachelor degree from Changchun University of Technology, China, in 2017.
Jie Yang, born in 1996, is currently a master candidate at Advanced Institute of Materials Science & Department of Materials Science and Engineering, Changchun University of Technology, China. She received her bachelor degree from Liaocheng University, China, in 2017.
Lianfeng Duan, born in 1981, is currently a professor at Advanced Institute of Materials Science & Department of Materials Science and Engineering, Changchun University of Technology, China, in 2018. He received his PhD degree from Jilin University, China, in 2011. His research interests include new energy materials and devices.
The authors sincerely thanks to Feifei Zhang of National University of Singapore and Junkai Wang of University of Chinese Academy of Sciences for their critical discussion and reading during manuscript preparation.
The authors declare that they have no competing interests.
Supported by National Nature Science Foundation of China (Grant No. 61774022) and Education Department of Jilin Province of China (Grant No. JJKH20181030KJ).
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Shan, X., Guo, Z., Zhang, X. et al. Mesoporous TiO2 Nanofiber as Highly Efficient Sulfur Host for Advanced Lithium–Sulfur Batteries. Chin. J. Mech. Eng. 32, 60 (2019). https://doi.org/10.1186/s10033-019-0374-2
- TiO2 nanofibers
- Mesoporous structure
- Lithium–sulfur batteries
- Electrochemical property