Accepted Manuscript S-TiO2 composite cathode materials for lithium/sulfur batteries Xin Zhou Ma, Bo Jin, Hui Yuan Wang, Jia Zi Hou, Xiao Bin Zhong, Huan Huan Wang, Pei Ming Xin PII: DOI: Reference:
S1572-6657(14)00492-5 http://dx.doi.org/10.1016/j.jelechem.2014.11.007 JEAC 1883
To appear in:
Journal of Electroanalytical Chemistry
Received Date: Revised Date: Accepted Date:
23 July 2014 3 November 2014 6 November 2014
Please cite this article as: X.Z. Ma, B. Jin, H.Y. Wang, J.Z. Hou, X.B. Zhong, H.H. Wang, P.M. Xin, S-TiO2 composite cathode materials for lithium/sulfur batteries, Journal of Electroanalytical Chemistry (2014), doi: http:// dx.doi.org/10.1016/j.jelechem.2014.11.007
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S-TiO2 composite cathode materials for lithium/sulfur batteries Xin Zhou Ma, Bo Jin*, Hui Yuan Wang*, Jia Zi Hou, Xiao Bin Zhong, Huan Huan Wang, Pei Ming Xin Key Laboratory of Automobile Materials, Ministry of Education, and College of Materials Science and Engineering, Jilin University, Changchun 130025, China Author to whom correspondence should be addressed: Tel.: +86-431-85095170 E-mail: [email protected]
; [email protected]
Abstract TiO2 nanofibers were prepared by improved electrospinning technique and subsequent thermal treatment. The as-prepared TiO2 nanofibers possess anatase-rutile mischcrystal structure. Sulfur was mixed with the TiO2 nanofibers to form S-TiO2 composite by a melt diffusion process. The S-TiO2 composite displays more excellent discharge capacity retention of 58% after 50 cycles compared to pure S, and the discharge capacity is 530 mAh g-1 after 50 cycles, which is ascribed to the adsorption of lithium polysulfide by TiO2. Keywords: S-TiO2 composite; Cathode materials; Lithium/sulfur batteries
1. Introduction Recently, high-specific-energy rechargeable lithium-ion batteries (LIBs) and sodium-ion batteries have attracted ever-increasing attention due to the increasing energy demands and environmental crisis [1-3]. However, the current insertion oxide cathodes such as LiCoO2 and LiMn2O4 with low capacity (<300 mAh g-1) have limited the performance of LIBs, which forces material researchers to develop alternative high-capacity cathodes [4-6]. In the new cathode materials, sulfur is very attractive for LIBs due to its high theoretical specific capacity of 1675 mAh g-1, which is about five times higher than the current insertion oxide cathodes. In addition, sulfur is abundant, low-cost, non-toxic and environmental friendly [7-13]. Therefore, lithium/sulfur batteries have the potential to replace current LIBs in the near future [11,12]. However, there are many challenges remained for lithium/sulfur batteries before the commercialized application, that is as follows: (i) electric insulating of sulfur and the insoluble low-order lithium polysulfide; (ii) high dissolution of intermediate polysulfides, creating an internal ‘‘shuttle’’ phenomenon which causes an irreversible loss of sulfur and low coulombic efficiency; and (iii) large volumetric expansion of sulfur (80%) during the discharge/charge process [14-18]. To solve the above problems, extensive attempts have been made, including the use of various carbon [19-22] or conductive polymer substrates [23-27], the optimization of organic electrolyte [28-32] and the utilization of porous oxide additives [33-35]. Zheng et al. 
bis(trifluoromethylsulfonyl)imide to modify the properties of the SEI layer formed on the Li metal surface in Li-S batteries. It was found that the IL-enhanced passivation film on the lithium anode surface exhibited very different morphology and chemical composition, effectively protecting lithium metal from continuous attack by soluble polysulfides. Recently, TiO2 as an additive or a substrate [14,16,36-39] for lithium/sulfur batteries has received much attention because its strong chemical adsorption of lithium polysulfide, which can significantly improve the cycle performance of lithium/sulfur batteries. Seh et al.  fabricated sulfur-TiO2 (amorphous) yolk-shell nanoarchitecture with an internal void space, which displayed a long cycling capability (over 1000 charge/discharge cycles) with a small decay rate of 0.033% per cycle. Li et al.  designed a sulfur-impregnated mesoporous hollow TiO2 (anatase) sphere cathode with intriguing capacity retention (71%) and high coulombic efficiency (93%) over 100 cycles at 1 C rate. To our knowledge, there is no paper related to TiO2 with anatase-rutile mischcrystal structure as the improved matrix for lithium/sulfur batteries. In this paper, TiO2 nanofibers were prepared by improved electrospinning technique and subsequent thermal treatment. Electrospinning can offer a simple and versatile route to the large-scale production of fibers from a variety of materials . Sulfur was mixed with the TiO2 nanofibers to form S-TiO2 composite by a melt diffusion process. The structural and morphological performance of the S-TiO2 composite was investigated by X-ray diffraction, transmission electron microscopy, scanning electron microcopy and field emission scanning electron microscopy, and
the electrochemical properties were analyzed by cyclic voltammograms and galvanostatic discharge/charge tests. 2. Experimental 2.1 Materials synthesis TiO2 nanofibers were prepared by improved electrospinning technique and subsequent thermal treatment as described by the previous reports [40,41]. In a typical synthesis, 0.35 g poly (vinyl pyrrolidone) was added to a mixture of 6.5 mL ethanol, 2 mL acetic acid and 1.5 mL tetrabutyl titanate, and then stirred for 10 h at room temperature in order to obtain a homogeneous precursor solution. The solution was driven from the syringe by a syringe pump (ALC-IP900, Shanghai Alcott Biotech. Co., Ltd.) at a constant rate of 1 mL h-1. TiO2 nanofibers were electrospun at 20 kV using a high voltage power supply (Tianjin Dongwen High Voltage Facility) with a 10 cm collection distance. TiO2 nanofibers were collected on a grounded aluminum (Al) foil collector and left overnight in air to fully hydrolyze, and then calcined in air at 500 °C for 6 h. As-prepared TiO2 nanofibers were ground for 15 min, and then mixed with sulfur at a weight ratio of 40:60. The mixture was ball-milled for 2 h at 200 rpm in ethanol. The obtained mixture was dried at 60 °C for 12 h to remove the solvent, and then heated to 155 °C for 12 h in a sealed 50 mL Teflon-lined stainless-steel autoclave. After cooling down to room temperature, the S-TiO2 composite was obtained. The synthesis route for the S-TiO2 composite is illustrated in Fig. 1. 2.2 Material characterization
The synthesized products were characterized by powder X-ray diffraction (XRD, Dmax/2500PC, Rigaku, Japan) with Cu Kα radiation (λ = 1.5406 Å), scanning electron microscopy (SEM, ZEISS EVO 18, Germany), field emission scanning electron microscopy (FESEM, JSM-6700F, Japan) and high-resolution transmission electron microscopy (HRTEM, FEI Tecnai G2 F20, operating at an accelerating voltage of 200 kV, USA). The elements on the surface of S-TiO2 were identified by energy-dispersive X-ray spectroscopy. The N2 adsorption/desorption tests were determined by Brunauer-Emmett-Teller (BET) measurements using an NOVA1000 surface area analyzer. Thermogravimetric analysis (TGA) was carried out to determine the weight content of sulfur in the composite at a heating rate of 10 °C min-1 under N2 atmosphere. 2.3 Electrochemical measurements The electrochemical tests were conducted by assembling coin-type batteries (CR2025) in an argon-filled glove box and lithium metal was used as both counter and reference electrode. The working electrodes were prepared by a slurry coating procedure. The slurry was made by mixing 70 wt% S-TiO2 composite, 20 wt% acetylene black and 10 wt% polyvinylidene fluoride in N-methyl-2-pyrrolidinone solvent. The slurry was uniformly spread onto Al foil and dried at 60 °C for 12 h in a vacuum oven. The used electrolyte was 1 M LiCF3SO3 in a mixed solvent of dimethoxyethane and dioxolane with a volume ratio of 50:50 containing 0.1 M LiNO3 as an electrolyte additive. The discharge and charge performance of half-cells was tested with LAND CT-2001A battery instrument at a current density of 335 mA g-1 in
the voltage range of 1.5-3.0 V at ambient temperature, and rate performance was also tested at different current densities in the same voltage range. The specific capacity was calculated on the basis of the active sulfur material. Pure sulfur cathode was also prepared in the same way just in the absence of TiO2 to compare with the S-TiO2 composite. Pure TiO2 cathode was also prepared in the same way to indicate its role in the S-TiO2 composite. Cyclic voltammogram (CV) measurements were carried out on an electrochemical workstation (CHI650D, Shanghai Chenhua Instruments Ltd.) at a scan rate of 0.1 mV s-1 from 1.5 to 3.0 V at room temperature. 3. Results and Discussion XRD patterns of pure S, TiO2 and S-TiO2 are shown in Fig. 2. As shown in Fig. 2(a), all patterns of pure S can be indexed to a material having an orthorhombic structure, which is in good agreement with the one listed in the X-ray powder diffraction data file (JCPDS No. 24-0733) by the American Society for Testing Materials as standard. Fig. 2(c) demonstrates that TiO2 possesses anatase (JCPDS No. 01-0562)-rutile (JCPDS No. 65-0191) mischcrystal structure. As for S-TiO2, there are no any new phases in the final product except pure S and anatase-rutile mischcrystal, which could be an indication of the absence of chemical reaction between the composite components upon ball milling and the following heat treatment [38,42], only peak densities decrease compared to those of pure S and anatase-rutile mischcrystal, indicating the good dispersion of sulfur in the S-TiO2 composite . It is obvious that the added anatase-rutile mischcrystal does not change the crystal structure of pure S. 6
TGA curve of S-TiO2 under N2 atmosphere is shown in Fig. 3. As can be seen in Fig. 3, a weight loss of 57.5 wt% is found between 200 and 300 °C corresponding to the evaporation of sulfur. Therefore, the sulfur content is determined to be 57.5 wt% in the S-TiO2 composite. SEM image of pure S and FESEM images of TiO2 and S-TiO2 are shown in Fig. 4. TEM image of S-TiO2 and FESEM image of S-TiO2 and the corresponding element mapping of Ti and S is also displayed in Fig. 4. Pore size distribution curves and the specific surface areas of pure S and S-TiO2 are shown in Fig. 5. As shown in Fig. 4(a), the particle size of pure S is around 10-50 µm and in inhomogeneous distribution. As can be seen in Fig. 4(b), TiO2 nanofibers possess the length of 0.5-1 µm and the diameter of 50-200 nm. As for S-TiO2, sulfur particles do not change the morphology of the TiO2 nanofibers, and are mixed with the TiO2 nanofibers to form S-TiO2 composite by a melt diffusion process [37-39], and display good contact with the TiO2 nanofibers; and some sulfur coating on the surface might nucleate to form sulfur particles . Figure 4(d) further demonstrates that the TiO2 nanofibers are composed of nanoparticles with particle size of 20-50 nm and possess nanoarchitecture with internal void space. As shown in Fig. 4(e)-(g), the element mappings for S and Ti display a very similar intensity distribution, further demonstrating that S and TiO2 are well distributed in the S-TiO2 composite and also indicating that TiO2 matrix should have filled up of sulfur after a melt diffusion process. This can be further confirmed by the BET analysis and the pore size distribution that the specific surface area of S-TiO2 is reduced to 5.6 m2 g-1 from the 7
initial 12.1 m2 g-1 of TiO2 and S-TiO2 has no pore diameter compared to about 2 nm pore diameter of TiO2, as demonstrated in Fig. 5. Cyclic voltammograms of S-TiO2 and TiO2 at a scan rate of 0.1 mV s-1 are shown in Fig. 6. As for S-TiO2, during the first cycle, two reduction peaks appear at around 2.34 and 2.01 V, respectively, and one oxidation peak at about 2.48 V is observed. After five cycles, the oxidation peak decreases and shifts to low potential, the reduction peak at about 2.01 V also decreases and shifts to high potential, however, the reduction peak at about 2.34 V increases and shifts to high potential. This is due to an increase in the internal impedance of the battery upon charge/discharge cycling. The peak at around 2.34 V is ascribed to the reduction of sulfur to form the higher order lithium polysulfides (Li2Sn, n>>4), and the peak at about 2.01 V corresponds to further reduction of these lithium polysulfides to lower order lithium polysulfides (Li2Sn, n<4), even to Li2S; the oxidation peak at about 2.48 V can be attributed to the oxidation of lithium polysulfides (Li2Sn, n<4) and Li2S to Li2S8 [13,31,32]. In addition, the weaker oxidation peak at around 2.0 V corresponds to lithium extraction in TiO2, and the weaker reduction peak at about 1.73 V is ascribed to lithium insertion in TiO2, as described elsewhere [43,44]. Gao et al.  indicated that the lithiation started with the valence reduction of Ti4+ to Ti3+ leading to a LixTiO2 intercalation compound. In order to indicate the role of TiO2 in the S-TiO2 composite, we conducted CV test of TiO2, and the CV result is shown in the insertion of Fig. 6. It is demonstrated that TiO2 can still play a small role due to its weaker redox reaction compared to sulfur. 8
Discharge and charge curves of S-TiO2 after different cycles at a current density of 335 mA g-1 are shown in Fig. 7. S-TiO2 presents the discharge capacities of 914, 807, 669, 530 mAh g-1 in the first cycle, the second cycle, the tenth cycle and fiftieth cycle, respectively. As shown in Fig. 7, all the discharge curves show two typical plateaus corresponding to the two-step reaction of sulfur with lithium during the discharge process, which is consistent with the result of CV measurement in Fig. 6. The upper plateau is assigned to the change of sulfur to lithium polysulfides (Li2Sn, n>>4), and the lower plateau can be attributed to further reduction of these lithium polysulfides to lithium polysulfides (Li2Sn, n<4), even to Li2S. Cycling performance of pure S, TiO2 and S-TiO2 at a current density of 335 mA g-1 is shown in Fig. 8. Rate performance of pure S and S-TiO2 at different current densities is shown in Fig. 9. As shown in Fig. 8(a), the discharge capacity of pure S is 630 mAh g-1 in the first cycle, and decreases to 222 mAh g-1 after 18 cycles; and pure S retains stable cycle performance from 18 to 50 cycles with capacity retention of 35% after 50 cycles. However, S-TiO2 presents more excellent discharge capacity retention of 58% after 50 cycles compared to pure S, and the discharge capacity is 530 mAh g-1 after 50 cycles, which is ascribed to the adsorption of lithium polysulfide by TiO2 . TiO2 has hydrophilic Ti-O groups and surface hydroxyl groups, which can bind profitably with polysulphide anions, hence further restricting the extent of polysulphide dissolution, and consequently improving the cycle performance of lithium/sulfur batteries [36,46]. In order to indicate the contribution of TiO2 to the total capacity, we also carried out a discharge/charge experiment of TiO2. Figure 8(b) 9
shows that TiO2 possesses the discharge capacity of 36 mAh g-1 in the first cycle and 10 mAh g-1 after 50 cycles, respectively. It demonstrates that the role of TiO2 in the total capacity is small in the voltage range of 1.5-3.0 V, which is consistent with the previous literature . Although the role of TiO2 in the total capacity is small, TiO2 can still act as a valuable second active component, which is consistent with the CV result in Fig. 6. Figure 9 obviously demonstrates that the rate performance of S-TiO2 is better than that of pure S. S-TiO2 can deliver a discharge capacity of 357 mAh g-1 at a current density of 1000 mA g-1, however, only 125 mAh g-1 for pure S at the same current density. 4. Conclusions TiO2 nanofibers with anatase-rutile mischcrystal structure have been successfully prepared by improved electrospinning technique and subsequent thermal treatment. Sulfur was mixed with the TiO2 nanofibers to form S-TiO2 composite by a melt diffusion process. The results demonstrate that the S-TiO2 composite displays more excellent discharge capacity retention of 58% after 50 cycles compared to pure S due to the adsorption of lithium polysulfide by TiO2 and the discharge capacity is 530 mAh g-1 after 50 cycles.
Acknowledgment The authors acknowledge the financial supports from The Project Sponsored by the Scientific Research Foundation for the Returned Overseas Chinese Scholars, State Education Ministry (Grant No. 609); International Science and Technology Cooperation Plan, Science and Technology Bureau of Changchun City (Grant No. 11GH05).
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Captions Fig. 1. The synthesis schematic diagram of S-TiO2. Fig. 2. XRD patterns of (a) Pure S, (b) S-TiO2 and (c) TiO2. Fig. 3. TGA curve of S-TiO2 under N2 atmosphere. Fig. 4. SEM image of (a) pure S and FESEM images of (b) TiO2 and (c) S-TiO2; TEM image of (d) S-TiO2; FESEM image of (e) S-TiO2 and the corresponding element mapping of (f) Ti and (g) S. Fig. 5. Pore size distribution curves of pure S and S-TiO2; the inset table displays the specific surface areas of pure S and S-TiO2. Fig. 6. Cyclic voltammogram of S-TiO2 at a scan rate of 0.1 mV s-1, the insertion is cyclic voltammogram of TiO2 at a scan rate of 0.1 mV s-1. Fig. 7. Discharge and charge curves of S-TiO2 after different cycles at a current density of 335 mA g-1. Fig. 8. Cycling performance of (a) pure S and S-TiO2, and (b) TiO2 at a current density of 335 mA g-1. Fig. 9. Rate performance of (a) pure S and (b) S-TiO2 at different current densities.
Highlights ► TiO2 was prepared by improved electrospinning technique and subsequent thermal treatment. ► TiO2 nanofibers possess anatase-rutile mischcrystal structure. ► Sulfur was mixed with TiO2 to form S-TiO2 by a melt diffusion process. ► The results demonstrate that S-TiO2 displays excellent cycle stability and rate capability.