Synthesis and electrochemical performance of TiO2–sulfur composite ...

J Solid State Electrochem (2013) 17:2959–2965 DOI 10.1007/s10008-013-2203-3

ORIGINAL PAPER

Synthesis and electrochemical performance of TiO2–sulfur composite cathode materials for lithium–sulfur batteries Qiang Li & Zhian Zhang & Kai Zhang & Lei Xu & Jing Fang & Yanqing Lai & Jie Li

Received: 24 June 2013 / Revised: 21 July 2013 / Accepted: 25 July 2013 / Published online: 16 August 2013 # Springer-Verlag Berlin Heidelberg 2013

Abstract Titania–sulfur (TiO2–S) composite cathode materials were synthesized for lithium–sulfur batteries. The composites were characterized and examined by X-ray diffraction, nitrogen adsorption/desorption measurements, scanning electron microscopy, and electrochemical methods, such as cyclic voltammetry, electrochemical impedance spectroscopy, and galvanostatic charge–discharge tests. It is found that the mesoporous TiO2 and sulfur particles are uniformly distributed in the composite after a melt-diffusion process. When evaluating the electrochemical properties of as-prepared TiO2–S composite as cathode materials in lithium–sulfur batteries, it exhibits much improved cyclical stability and high rate performance. The results showed that an initial discharge specific capacity of 1,460 mAh/g at 0.2 C and capacity retention ratio of 46.6 % over 100 cycles of composite cathode, which are higher than that of pristine sulfur. The improvements of electrochemical performances were due to the good dispersion of sulfur in the pores of TiO2 particles and the excellent adsorbing effect on polysulfides of TiO2. Keywords Lithium–sulfur batteries . Mesoporous TiO2 . TiO2–sulfur composite . Polysulfides

Introduction Lithium–sulfur (Li–S) batteries have been studied as one of the most promising systems for the next generation of highQ. Li : Z. Zhang (*) : K. Zhang : L. Xu : J. Fang : Y. Lai : J. Li School of Metallurgy and Environment, Central South University, Changsha 410083, China e-mail: [email protected] Z. Zhang : Y. Lai : J. Li Engineering Research Center of High Performance Battery Materials and Devices, Research Institute of Central South University in Shenzhen, Shenzhen 518057, China

energy rechargeable lithium batteries because of their high theoretical specific capacity (1,675 mAh/g) and energy density (2,600 Wh/kg) [1, 2]. As a cathode active material, sulfur also has advantages of nontoxicity and abundance in nature. However, there are still many problems hindering the practical application of Li–S batteries. Sulfur and its final discharge products (Li2S2, Li2S) are electrical insulators, which can cause poor electrochemical accessibility, leading to a low utilization of active materials [3–6]. In addition, polysulfides (Li2Sn, 4≤n≤8) produced in discharge/charge processes can dissolve into organic electrolyte and be reduced to lower-order polysulfides at the interface of the lithium anode. These reduced products will migrate back to the cathode in which they may be reoxidized. This process takes place repeatedly, creating polysulfide shuttle, which can cause loss of active materials and the low coulombic efficiency of Li–S batteries, eventually resulting in rapid capacity fading [7, 8]. To overcome the problem of polysulfide dissolution, the method of using adsorbing material to trap lithium polysulfides has been proposed in a number of publications. Such additives include carbon and oxide. Additives based on porous carbons have been used to improve the electrochemical performance of lithium–sulfur batteries [9, 10]. However, electronically conductive additives will facilitate the reduction of the polysulfides to insoluble sulfides onto their surface during the charge cycle. Once the adsorbents are covered, their function ceases [11]. The porous oxide additives are inert to the redox reaction, so the pores are less likely to be blocked by the reduced lithium sulfide deposits. Oxide additives, including Mg0.6Ni0.4O [12, 13], Mg0.8Cu0.2O [14], Al2O3 [15, 16], La2O3 [17], and titania (TiO2) [18] were employed as adsorbents in the sulfur cathode, and they showed some effect to increase cycle performance. In recent research, Nazar et al. [18] have reported the use of mesoporous titania as an additive to a sulfur/carbon composite. In this study, mesoporous carbon-additive material was first synthesized by mixing mesoporous carbon (50 mg) and

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TiO2 (5 mg) in DI H2O (5 mL) and sonicating for 1 h, then sulfur was impregnated into the mesoporous carbon-additive (TiO2) by heat treatment. The composite cathode material, ready for electrochemical studies, contained 48 wt.% of sulfur as active mass and 3.6 wt.% additive. This research demonstrated that cycleability in a Li–S cell can be improved with the use of a small amount of mesoporous titania in the sulfur/carbon cathode. In addition, Cui et al. [19] have demonstrated the design of sulfur–TiO2 yolk–shell nanoarchitecture with internal void space to accommodate the volume expansion of sulfur. In this study, sulfur nanoparticles were first prepared using the reaction of sodium thiosulfate with hydrochloric acid, then the sulfur nanoparticles were coated with TiO2 through controlled hydrolysis of a sol–gel precursor, titanium diisopropoxide bis(acetylacetonate), in an alkaline isopropanol/aqueous solution, resulting in the formation of sulfur–TiO2 core–shell nanoparticles. This research represents the best performance for long-cycle lithium–sulfur batteries so far with the capacity decay after 1,000 cycles, as small as 0.033 % per cycle. In this paper, we synthesized TiO2–sulfur composite cathode materials with a high sulfur content (>60 wt.%) via a simple melt-diffusion strategy for lithium–sulfur batteries. Mesoporous titania nanoparticles used as the adsorbing material for sulfur were prepared via sol–gel method. Using the mesoporous TiO2–sulfur composite, an initial discharge specific capacity of 1,460 mAh/g at 0.2 C and capacity retention ratio of 46.6 % over 100 cycles is achieved, which are higher than that of pristine sulfur.

Experimental Material preparation The TiO2 nanoparticles used as the adsorbing material for sulfur in this study were prepared via a sol–gel method according to previously described method [20]. First, titanium (IV) isopropoxide (Aladdin, 99.8 % purity) was dissolved in absolute ethanol, and distilled water was added to the solution in terms of a molar ratio of Ti/H2O=1:5. Then, the solution was vigorously stirred for 60 min in order to form sols. After aging for 24 h, the sols were transformed into gels. In order to obtain nanoparticles, the gels were dried under 120 °C for 2 h to evaporate the water and organic material. Then the dry gel was sintered at 500 °C for 4 h and was subsequently carried out to obtain the desired TiO2 nanoparticles. To synthesize TiO2–sulfur composite, sulfur (Sigma-Aldrich, 100 mesh particle size powder) and as-prepared TiO2 were mixed in the weight ratio of 5:3 and machine ball milled at 500 rpm for 2 h with ethanol as a dispersant. The precursor mixture was further dried in a vacuum oven at 60 °C for 5 h to remove the solvent and then heat treated at 155 °C for 12 h with heating rate of 5 °C/min in a tubular furnace under flowing argon

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atmosphere. After cooling down to room temperature, the TiO2–sulfur composite was obtained. Material characterization Scanning electron microscopy (SEM, Sirion 200) was applied to characterize the materials. The elements on the surface of sample were identified by energy-dispersive X-ray spectroscopy (EDS). Powder X-ray diffraction (XRD, Rigaku3014) using Cu Кα radiation was employed to identify the crystalline phase of the prepared TiO2 and TiO2–sulfur composite. Thermogravimetric analysis (TGA, SDTQ600) was conducted in determining the sulfur content in the composite. N2 adsorption/desorption measurements were performed by using Quantachrome instrument (Quabrasorb SI-3MP) at 77 K. Electrochemical measurements The electrochemical performance was performed using a CR2025 coin-type cell. The working electrodes were prepared by a slurry coating procedure. The slurry consisted of 80 wt.% active material, 10 wt.% acetylene black, and 10 wt.% polyvinylidene fluoride (PVDF) dissolved in N-methyl pyrrolidinone. The slurries of the cathodes were pressed onto a conductive carbon fiber cloth (Hesen Electric, Shanghai), dried at 60 °C overnight, then the cathodes were cut into pellets with a diameter of 1.0 cm and dried for 12 h in a vacuum oven at 60 °C. The typical mass loading of active S was 1.0∼1.2 mg/cm2. CR2025-type coin cells were assembled in an argon-filled glove box to avoid contamination by moisture and oxygen. The electrolyte used was 1 M bis(trifluoromethane) sulfonamide lithium salt (Sigma Aldrich) in a mixed solvent of 1,3-dioxolane (Acros Organics) and 1,2-dimethoxyethane (Acros Organics) with a volume ratio of 1: 1, including 0.1 M LiNO3 as an electrolyte additive. Lithium metal was used as counter electrode and reference electrode, and Celgard 2400 was used as separator. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) measurements were conducted using PARSTAT 2273 electrochemical measurement system. CV tests were performed at a scan rate of 0.1 mV/s in the voltage range of 1.5 to 3.0 V. EIS measurements were carried out at open circuit potential in the frequency range between 100 kHz and 10 mHz, with a perturbation amplitude of 5 mV. Galvanostatic charge/discharge tests were performed in the potential range of 1.5 to 3.0 Vat 25 °C by using a LAND CT2001A battery-testing instrument.

Results and discussion Characterization of prepared TiO2 and TiO2–S composite Nitrogen adsorption–desorption isotherms and pore size distribution curves of prepared TiO2 derived from BET measurements

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are depicted in Fig. 1. BET specific surface area and pore volume of TiO2 is 93 m2/g and 0.26 cm3/g, respectively, which are similar to reference [18]. As shown in the inset of Fig. 1, nitrogen adsorption–desorption isotherms of TiO2 can be identified as type IV in the IUPAC classification with a typical mesopore hysteresis loop. As shown in the pore size distribution curves (Fig. 1), mesopores with diameters ranging from 6 to 16 nm exist, which can help to hold sulfur active material and absorb polysulfides efficiently. Figure 2 shows the SEM images of the synthesized TiO2 and TiO2–sulfur composites. The TiO2 particles’ average size is approximately 30 nm under Scherrer formula according to the XRD pattern of prepared TiO2, which is in agreement with reference [20]. However, from the observed SEM images (Fig. 2a), it can be seen that TiO2 is an aggregation of micron-sized sponge particles. A number of irregular pores were formed when the primary TiO2 particles aggregated into larger particles, which is consistent with BET measurement. It can be seen in Fig. 2b that the TiO2–sulfur composite material has a similar morphology and diameter, and that sulfur particles exhibit good contact with the TiO2 particles. Abundant irregular pores were also formed when the primary sulfur particles were embedded into TiO2 particles, which may positively affect the penetration of the electrolyte solution into the electrode and the necessary transport of Li ions [21, 22]. As a result of EDS mapping to TiO2–sulfur composite, nano-TiO2 particles and sulfur were well dispersed in the composite as shown in Fig. 2c–f, which also indicated that sulfur embedded into the pores of the TiO2 framework after the melt-diffusion process. This can be further confirmed by the BET analysis that the specific surface area of the TiO2/S composite is reduced to 20 m2/g from the initial 93 m2/g of TiO2. Therefore, it could be considered that nano-TiO2 effectively adsorbs lithium polysulfides by their abundant irregular pores and homogeneous distribution.

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The thermal decomposition characteristic of the TiO2–sulfur composite material was investigated under a nitrogen atmosphere by means of TGA. TGA of the TiO2–sulfur composite in nitrogen flow showed a weight loss of approximately 60.2 wt.% between 200 and 300 °C (Fig. 3), which corresponds to the evaporation of sublimed sulfur [23]. Hence, the sulfur content in the TiO2–sulfur sample was determined to be a higher sulfur content of 60.2 wt.% compared to the references [18, 24]. The XRD analysis was used to investigate potential structural changes due to the starting material interactions including possible reactions of TiO2 with sublimed sulfur. Figure 4 shows the XRD patterns of the starting components sulfur, the as-prepared TiO2, and the TiO2–sulfur composite. From the X-ray diffraction pattern of TiO2, it can be seen that the X-ray diffraction peaks around 25.6, 37.9, 48.0, 54.7, 63.1, 70.0, and 75.5° are in good agreement with the standard spectrum of anatase TiO2 (JCPDS, card no: 21–1272). One can see that the XRD patterns of the TiO2–sulfur composite have sharp peaks of an anatase phase with reduced peaks intensity compared with those of the initial TiO2. The XRD data do not show any new phases in the final product, which could be an indication of the absence of chemical reaction between the composite components upon ball milling and the following heat treatment [13]. Cyclic voltammetry of lithium–sulfur batteries Figure 5 shows the CV curves of the TiO2–sulfur composite cathode. During the first cathodic scan, there are two remarkable reduction peaks at 2.05 and 2.30 V. The peak at 2.30 V associates with the conversion of elemental sulfur to soluble lithium polysulfide (Li2Sn, 4≤n≤8), and the peak at 2.05 V is related to the reduction of lithium polysulfides to insoluble Li2S2 and Li2S [22, 25–27]. In the anodic scan, only one sharp oxidation peak is observed in the potential of 2.45 V, which corresponds to the conversion of Li2S into high-order soluble polysulfides [4, 28]. The inset of Fig. 5 presents the comparison of initial CV curves of Li/S cells with the TiO2–sulfur composite cathode and a sublimed sulfur cathode. Note that the voltage gap (ΔE) between oxidation and reduction peaks for the TiO2–sulfur composite cathode is smaller than that of the sublimed sulfur cathode, and the peak of curve of TiO2– sulfur composite cathode is much sharper than that of the sublimed sulfur cathode. Charge/discharge characteristics of lithium–sulfur batteries

Fig. 1 Nitrogen adsorption–desorption isotherms and pore size distribution curves of TiO2

Figure 6 shows the initial discharge/charge voltage profiles of Li/S cells with the TiO2–sulfur composite cathode and the sublimed sulfur cathode at 0.2 C (335 mA/g). Two typical discharge potential plateaus are observed for both cathodes, in accordance with the CV curves. Note that the difference

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Fig. 2 SEM images of a TiO2 and b the TiO2–sulfur composite; EDS mapping showing the distribution of S, Ti, and O elements (c–f)

between charge and discharge voltages of the TiO2–sulfur composite cathode is much smaller than that for the sublimed sulfur cathode. The smaller electrochemical polarization indicates a better conductivity of the TiO2–sulfur composite cathode because of the good dispersion of sulfur in the pores of TiO2 particles.

Fig. 3 TGA curves of TiO2–sulfur composite under N2 atmosphere

Cycling performances of the TiO2–sulfur composite cathode and the sublimed sulfur cathode are presented in Fig. 7. All capacity values in this study were calculated based on sulfur mass. As shown in Fig. 7a, the initial discharge capacity

Fig. 4 XRD patterns of sublimed sulfur, TiO2, and TiO2–sulfur composite

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Fig. 5 Cyclic voltammogram profiles of the TiO2–sulfur composite cathode and the sublimed sulfur cathode at a scan rate of 0.1 mV/s

and charge capacity of the sublimed sulfur cathode are 1,050 and 915 mAh/g at a current density of 335 mA/g, respectively. After 100 cycles, the reversible capacity of the sublimed sulfur cathode decreases to 400 mAh/g. For the TiO2–sulfur composite cathode (Fig. 7b), it displays a higher utilization of active materials than the sublimed sulfur cathode with the initial discharge capacity and charge capacity of 1,460 and 1,180 mAh/g, respectively. Moreover, the reversible capacity of the TiO2–sulfur composite cathode still retain 680 mAh/g after 100 cycles, which can be attributed to the good dispersion of sulfur in the pores of TiO2 particles and absorption of lithium polysulfide by TiO2 nanoparticles [29]. We have also conducted an experiment with prepared TiO2 cathode (consisting of 80 wt.% prepared TiO2, 10 wt.% acetylene black, and 10 wt.% PVDF) in our electrolyte system. The result shows that the initial discharge capacity of prepared TiO2 cathode is only 56 mAh/g at 335 mA/g and decreases to

Fig. 7 Cycling performance and coulombic efficiency of a sublimed sulfur cathode and b TiO2–sulfur composite cathode at 335 mA/g

approximately zero after 100 cycles. It indicates that the contribution of TiO2 to the total capacity is very small in the voltage range, which is consistent with the recent report by Cui et al. [19]. Hence, the TiO2–sulfur composite can be considered as a promising cathode material for lithium–sulfur batteries. Meanwhile, the rate capacity of cell with TiO2–S composite cathode is shown in Fig. 8. During the first 6 cycles, the discharge capacity faded gradually at 0.1 C rate and remained at a capacity of 890 mAh/g. When the cell operated at 0.2, 0.5, and 1 C rate, the TiO2–S composite electrode delivered a capacity of 700, 601, and 538 mAh/g, respectively. And then the rate was reset back to 0.1 C, the electrode resumes the capacity of 850 mAh/g, without abrupt capacity fade. The result indicates good robustness and stability of the cathode material. Electrochemical impedance spectra

Fig. 6 Initial discharge/charge curves of the sublimed sulfur cathode and the TiO2–sulfur composite cathode at 335 mA/g

To get further insight into the improved electrochemical performance with the use of mesoporous TiO2, the electrochemical

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Fig. 8 Rate capability of TiO2–sulfur composite cathode

impedance spectroscopies before and after 100 cycles were measured for the Li/S cells with the TiO2–sulfur composite cathode and the pristine sulfur cathode. As shown in Fig. 9a, all of the impedance spectra have similar feature: a medium-tohigh frequency depressed semicircle and an inclined lowfrequency line. The intercept at real axis Zim corresponds to the combination resistance, which represents the ionic resistance of the electrolyte, the contact resistance at the active material/current collector interface, and the intrinsic resistance of the active materials [30, 31]. The sloping line corresponds to the diffusion of ions within the cathode. It is observed that the semicircle magnitude of the TiO2–sulfur composite cathode is a little smaller than that of the pristine sulfur cathode. The result confirms that the conductivity of the TiO2–sulfur composite cathode is enhanced because of the good dispersion of sulfur into a porous TiO2 matrix. After 100 cycles, a new semicircle appears in the middle frequency range, which corresponds to charge–transfer resistance [30, 31]. As seen from Fig. 9b, the sizes of two semicircles of the TiO2–sulfur composite cathode are smaller than the pristine sulfur cathode. It could be attributed to the good ionic conductivity of TiO2 and the sponge-like structure of the TiO2–sulfur composite cathode, which helps to absorb lithium polysulfide and reduce the negative impact of insulating precipitation on the cathode [32].

Fig. 9 Impedance plots for the sublimed sulfur cathode and the TiO2– sulfur composite cathode a before cycling and b after 100 cycles

sulfur cathode were 1,460 and 1,050 mAh/g, respectively. Furthermore, the composite electrode exhibited better cycling performance with a reversible specific capacity of 680 mAh/g after 100 cycles, which was more than 400 mAh/g of sublimed sulfur cathode. The enhanced performance of TiO2–sulfur composite cathode material may be attributed to the good dispersion of sulfur in the pore structure of TiO2 particles and adsorption of lithium polysulfides by TiO2. We propose that the mesoporous TiO2–sulfur composites described here are promising electrode material for lithium–sulfur batteries with good cycling performance.

Conclusions In this paper, mesoporous TiO2–sulfur composite materials were successfully synthesized by a simple melt-diffusion strategy. The TiO2/S composite showed high specific capacities and good cycling stabilities. At a current rate of 335 mA/g, the discharge capacities of composite cathode and sublimed

Acknowledgments The authors thank the financial support of the Strategic Emerging Industries Program of Shenzhen, China (JCYJ20120618164543322) and the Science and technology project of Hunan Province (2011FJ3151). We also thank the support of the Engineering Research Center of Advanced Battery Materials, the Ministry of Education, China.

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