Mesoporous TiO₂ Nanofiber as Highly Efficient Sulfur Host for Advanced Lithium–Sulfur 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 [3‐5].

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 [6‐9], 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, TiO_x, Al₂O₃, etc [10‐14]. Among them, titanium dioxide (TiO₂) has drawn much attention due to its low cost, environmental protection and structural stability [15]. An et al. [16] 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. [17] 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. [18] 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.

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 (Li₂S_n, 4 ≤ n ≤ 8) finally the further to the lower polysulfides (Li₂S_n, n ≤ 3) [24‐27]. 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 Li₂S_n (2 ≤ n < 8), which is also trapped in the mesoporous structure without diffused into the electrolyte and transferred to anode [28]. Then the soluble Li₂S_n 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 [29]. 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.