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Nano-TiO2 decorated carbon coating on the separator to physically and chemically suppress the shuttle effect for lithium-sulfur battery
Journal of Power Sources 378 (2018) 537–545
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Nano-TiO2 decorated carbon coating on the separator to physically and chemically suppress the shuttle effect for lithium-sulfur battery
T
Hongyuan Shaoa, Weikun Wangb, Hao Zhangb, Anbang Wangb, Xiaonong Chena,∗∗, Yaqin Huanga,∗ a
State Key Laboratory of Chemical Resource Engineering, Beijing Laboratory of Biomedical Materials, Beijing University of Chemical Technology, Beijing 100029, PR China b Research Institute of Chemical Defense, Beijing 100191, PR China
H I G H L I G H T S
G RA P H I C A L AB S T R A C T
The nano-TiO /Carbon coating has a • synergistic effect to immobilize for 2
Illustration of the polysulfide-trapping and electron-transporting mechanisms of the 20%T-DCL separator.
LiPS.
The Li-S battery with nano-TiO / • Carbon coating shows high discharge 2
capacity.
The nano-TiO /Carbon coating can • protect lithium anode from corrosion. 2
A R T I C L E I N F O
A B S T R A C T
Keywords: Nano-TiO2 Conductive carbon layer Synergistic effect Shuttle effect Lithium-sulfur batteries
Despite recent progress in designing modified separators for lithium-sulfur (Li-S) batteries, detail in optimizing the synergistic effect between chemical and physical immobilization for lithium polysulfides (LiPS) in modified separator hasn't been investigated totally. Here, a nano-TiO2 decorated carbon layer (T-DCL) has been successfully applied to modify separator for the Li-S battery. The results indicate that appropriate weight percentage of nano-TiO2 uniformly distributed in conductive carbon layer is effective to chemically and physically immobilize for LiPS, and promote the electron transfer during discharge/charge process. The performance of the modified Li-S battery with T-DCL separator are significantly enhanced, with a specific capacity of 883 mAh g−1 retained after 180 cycles at 0.1 C and 762 mAh g−1 retained after 200 cycles at 0.5C, which are much higher than that of separators only coated with TiO2 layer or conductive carbon layer. Besides, the separator coated with T-DCL also shows low electrochemical impedance and good lithium anode protection. These results indicate that separator with T-DCL is promising to balance the physical and chemical LiPS trapping effect, and optimize the electrochemical performance for Li-S battery.
1. Introduction Fast developments of the global economy accelerate the consumption of fossil fuels and also cause severe environmental pollution.
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Therefore, sustainable energy storages are inspiring a rapid growth of attention during the last decade [1–3]. Because of the low theoretical capacity, the commonly used lithium-ion batteries can not satisfy the requirement for high energy density storage [4,5]. In the past twenty
Corresponding author. Corresponding author. E-mail addresses: [email protected] (X. Chen), [email protected] (Y. Huang).
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https://doi.org/10.1016/j.jpowsour.2017.12.067 Received 26 July 2017; Received in revised form 24 November 2017; Accepted 23 December 2017 0378-7753/ © 2017 Elsevier B.V. All rights reserved.
Journal of Power Sources 378 (2018) 537–545
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a specific capacity of 883 mAh g−1 retained after 180 cycles at 0.1 C and 762 mAh g−1 retained after 200 cycles at 0.5C. These results demonstrate the 20%T-DCL separator is promising ratio control strategy to balance the physical and chemical LiPS trapping effect and optimize the electrochemical performance for the Li-S battery.
years, lithium-sulfur (Li-S) batteries have been widely explored as a hopeful candidate for next generation of energy storage devices, because of the ultrahigh energy density (2600 Wh kg−1), abundant sources, and free of contamination [6,7]. Except for the advantages mentioned above, the poor electrical conductivity of sulfur, serious volume expansion rate (80%) during discharge/charge process, and shuttle effect of the lithium polysulfides (LiPS) (Sn2−, 4 ≤ n ≤ 8) intermediate result in a low utilization rate of sulfur and short lifetime of Li-S batteries, which makes them inappropriate for the commercial application [8–10]. To inhibit LiPS from shuttling in the liquid electrolyte, many methods had been presented during the past decades [11–15]. Among these methods, one effective strategy to prevent LiPS from shuttling is the modification of battery structure, for instance, building a physical barrier for LiPS between the sulfur cathode and separator. Several kinds of carbon-based interlayer [16–19] have been applied to enhance the electrochemical performance the Li-S battery. Although the interlayer provides an effective battery architecture, there is still a drawback that the extra weight of the interlayer will reduce the overall energy density. Recently, many groups focused on developing the modified separators with different carbon coatings layer on the surface of the Celgard separator [20–27]. Through placing the conductive carbon coating adjacent to the cathode, the carbon coating serves as an absorbent layer to restrain the soluble LiPS from shuttling. Besides, the carbon layer with high electrical conductivity also serves as an uppercurrent collector [28] to keep utilizing the trapped LiPS during charge/ discharge process. Although carbon materials have presented promise as sulfur-entrapping host structures, their weak interaction with the polar LiPS limits their application as LiPS traps [29–31]. Recently, metal oxides for LiPS entrapment also have aroused significant interest, such as TiO2 [32,33], Ti4O7 [34], MnO2 [35], NiFe2O4 [36], SiO2 [37], SnO2 [38], and MgO [39]. In particular, anatase TiO2 is a promising host to contain sulfur because of its good absorption ability. Previous works have shown the effective coupling of mesoporous TiO2 with a C-S composite to enhance the electrochemical performance [40]. These results demonstrated that the TiO2 in the carbon layers could stimulate the interaction between TiO2 and S, which was supposed to provide an electrostatic attraction [41] that enhanced the surface trapping of LiPS on the TiO2. Even though TiO2 has good LiPS trapping ability, the poor conductivity of TiO2 limits its direct application in the modified separator, because coating layer in modified separator should be conductive to serve as an upper collector and keep utilizing the trapped LiPS during long term cycling [42]. Improving the conductivity of TiO2 using conductive agent, such as carbon materials, is essential to obtain good electrochemical performance. Herein, we report a light-weight and efficient modified separator prepared by coating a nano-TiO2 decorated carbon layer (T-DCL) to the surface of a routine separator. The cooperative combination of nanoTiO2 and conductive carbon coating afforded a good construction strategy to chemically and physically trap LiPS. To further explore the synergistic effect between chemical and physical immobilization for LiPS, we gradually increased the weight percentage of nano-TiO2 in the coating layer from 0 wt% to 80 wt%. The results showed that compared with other carbon layers, carbon layer with 20 wt% of nano-TiO2 decorated (20%T-DCL separator) is effective in restraining the LiPS shuttling effect, protecting the lithium anode, reducing the electrochemical impedance and enhancing the performance of sulfur cathode. As illustrated in Scheme 1, compared with Carbon-coated and TiO2coated separator, the 20%T-DCL separator has three main advantages: 1) carbon layer can serve as the dispersants and conductive agent to increase the conductivity of nano-TiO2; 2) The 20%T-DCL has a good synergistic effect to chemically and physically immobilize for LiPS; 3) the conductive carbon layer served as an upper current collector can effectively enhance the batteries' reversibility and reduce the electrochemical impedance. The electrochemical performance of Li-S batteries with 20%T-DCL separator were also significantly enhanced, resulting in
2. Experimental 2.1. Synthesis of nano-TiO2 decorated carbon composite The acetylene black (AB) and anatase TiO2 nanopowder used were purchased from commercial corporations. The nano-TiO2 powder and AB were first mixed together with a specific weight ratio and then ultrasonically dispersed in gelatin (type B, derived from bovine bones) aqueous solution (2 wt% of gelation dissolved into the water) to form a homogeneous solution. The composites solution were finally operated by ball-milling for 5 h at 25 °C to form uniform slurry. 2.2. Preparation of the modified separators Four kinds of modified separators were fabricated by different slurries in this work. The first one (20%T-DCL separator) consisted of 20 wt% nano-TiO2, 60 wt% AB and 20 wt% gelatin; The second one (40%T-DCL separator) consisted of 40 wt% nano-TiO2, 40 wt% AB and 20 wt% gelatin; The third one (TiO2-coated separator) consisted of 80 wt% nano-TiO2 and 20 wt% gelatin; the fourth one (Carbon-coated separator) consisted of 80 wt% AB and 20 wt% gelatin. These four kinds of separators were modified by direct coating of the above mentioned slurries on the normal separator (Celgard 2400) with a spreader method. Then, the slurry-coated separators were parched in a vacuum oven at 60 °C for 24 h. Finally, the modified separators were cut into round pieces with a mass loading about 0.2 mg cm−2. 2.3. Preparation of the Li2S6-containing solution The Li2S6-containing solution was prepared in a pure Ar atmosphere through mixing the lithium and sublimed sulfur with a molar ratio of 2:6 in the tetrahydrofuran solvent. After 5 h, the Li2S6-containing solution was formed. 2.4. Battery assembly and characterization The sulfur electrodes were fabricated by a commonly used coating method by a spreader. Firstly, the sulfur cathode slurry containing 63 wt% sublimed sulfur (99.5%, Beijing Yili. Corp., China), 30 wt% AB and 7 wt% gelatin binder was prepared. Then the obtained slurry was coated onto the surface of aluminum foil and parched at 60 °C in vacuum for 24 h. Finally, the sulfur cathode was successfully prepared with a sulfur loading about 2.0 mg cm−2. The Coin-type (CR2025) batteries were assembled under an Ar-filled glove box (ULTRA S1) to keep the contents of oxygen and water lower than 1 ppm. A lithium metal was applied as the anode; Celgard 2400 separator, Carbon-coated separator, and the T-DCL separator were also used. The electrolyte consists of 1 M lithium bistriuoromethanesulfonylimide (LiTFSI, Beijing Chemical Reagent Research Institute) and 0.4 M lithium nitrate in the solvent of 1, 3-dioxolane (DOL) and 1,2-dimethoxyethane (DME) (1:1 V:V, Beijing Chemical Reagent Research Institute). The electrochemical tests were performed at the rates of 0.1, 0.5, 1, 1.5 and 2C between 1.7 and 2.8 V using a LAND-CT2001A instrument. The surface morphologies of the modified separator were detected by a scanning electron microscope (SEM, HITACHIS-4800). Transmission electron microscopy (TEM, HITACHI H-800) was used to characterize the microstructure of the samples. The X-ray photoelectron spectroscopy (XPS, Thermo Fisher Scientific USA ESCALAB-250) was performed to investigate the elemental composition of the materials. The thermogravimetric analyzer (TG, Hengjiu China) was applied to determine the 538
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Scheme 1. Illustration of the polysulfide-trapping and electron-transporting mechanisms of different modified separators.
and 40%T-DCL slurry. Besides, the EDS measurement in Figure S1d and S1e (Supporting information) also demonstrate the chemical composition of 20%T-DCL slurry and 40%T-DCL slurry, which is identical to the TG result. The TG and EDS measurements indicated the ratio of TiO2 and AB is approximately 20:60 and 40:40 in 20%T-DCL slurry and 40%T-DCL slurry, respectively. Fig. 2a, Figure S2a, S2b, S2c and S2d (Supporting information) display the discharge-charge voltage profiles cycled at different times (1st, 10th, 20th and 100th) at 0.1 C for Li-S batteries with 20%T-DCL separator, 40%T-DCL separator, TiO2-coated separator, Carbon-coated separator and routine separator, respectively. Throughout the redox reaction, the capacity-voltage profiles of battery with 20%T-DCL separator shows a two-step redox reaction. The first discharge plateau at 2.32 V correspond to the transformation of S8 to Li2Sn (4 < n ≤ 8), and the lower discharge plateau at 2.10 V is corresponded to the formation of Li2S2 and Li2S. The charge profiles show two plateaus at about 2.24 and 2.35 V, respectively, which is corresponded to oxidation reactions of Li2S2/Li2S to S8/Li2S8 [43]. The Li-S batteries with 20%T-DCL separator exhibit longer voltage plateaus and smaller polarization (ΔE) than that of Li-S batteries with other four kinds of separators. Moreover, the charge voltage also increases with the cycle number for the battery with 40%T-DCL separator, TiO2-coated separator and routine separator, however the cycle potentials of the batteries with 20%T-DCL separator remain almost constant. Fig. 2b, Fig. S2e, S2f, S2g and S2h (Supporting information) show a similar tendency for batteries with 20%T-DCL separator, 40%T-DCL separator, TiO2-coated separator, Carbon-coated separator and routine separator, when ΔE is examined at different rates of 0.1C, 0.5C, 1.0C and 2.0C. Even cycled at high rate (2.0C), batteries with 20%T-DCL separator still perform lower ΔE and higher capacity retention than that of other separators. These results evidently reveal that the 20%T-DCL separator is extremely beneficial for optimizing the charge/discharge kinetics and the system reversibility. The long-term cycle performances of Li-S batteries discharged at different rates are presented in Fig. 2c and 2d. The Li-S batteries were discharged at a rate of 0.05C at the initial 2 cycles for the cathode activating and then discharged at 0.5C in subsequent cycles. When discharged at 0.1C (Fig. 2c), the initial discharge capacity of the Li-S batteries with 20%T-DCL separators reaches up to 1227 mAh g−1 (approximately 73.3% of the theoretical capacity) and preserves a reversible capacity about 883 mAh g−1 after cycled for 180 times, which is much higher than that of batteries with other separators. When discharged at 0.5C, the special capacity of the battery with a 20%T-DCL separator, after 200 cycles, remains 762 mAh g−1, while the special capacities of batteries with 40%T-DCL separator, Carbon-coated separator and routine separator at 0.5C are only 589 mAh g−1, 529 mAh
content of composite slurry. The X-ray diffraction (XRD) patterns of the samples were recorded with Cu-Ka radiation at 40 kV by using an X-ray diffractometer (D/max-2500/PC, Rigaku). The Electrochemical impedance spectroscopy (EIS) was operated at an open-circuit voltage (OCV) from 100 mHz to 10 kHz at room temperature using the Solartron 1280Z. The Cyclic Voltammetry (CV) was also carried out using the same instrument, with a scanning rate of 0.1 mV/s. UV–visual spectroscopy was measured with TU-1810 to investigate the LiPS in tetrahydrofuran. 3. Results and discussion The scanning electron microscopy (SEM) image was used to investigate the morphology of the different modified separators. Fig. 1a, 1b, 1c, and 1d show the morphologies of the 20%T-DCL separator, 40%T-DCL separator, TiO2-coated separator, and Carbon-coated separator, respectively. The four modified separators are adhered well on the celgard separator by gelatin binder and show a homogeneously dispersed structure, which is contributed to increase the trapping sites for LiPS. The inset EDS mapping of elemental Ti and C in Fig. 1a and 1b displays that the nano-TiO2 is uniformly distributed on the coating surface and this distribution is beneficial for trapping LiPS. Figure S1 (Supporting information) presents the digital images and cross-sectional morphology of the 20%T-DCL separator, 40%T-DCL separator and TiO2-coated separator. The morphology of Carbon-coated separator had already reported in our previous work [31]. As shown in Fig. S1, the coating layers were deposited uniformly and approximately 4 μm thick on average. Fig. 1e and d shows the high magnification TEM image of 20%T-DCL separator. The high-resolution TEM image displays a lattice spacing of 0.35 nm corresponding to the (101) facet of anatase TiO2 and the nano-TiO2 are uniformly distributed on the AB layers without aggregation. The XRD pattern in Fig. 1g could be attributed to anatase TiO2 (JCPDS No. 21–1272), further suggesting the good combination of nano-TiO2 and carbon in the T-DCL. There are also two typical peaks marked in blue at about 24° and 44°, which can be related to diffraction from (002) and (100) of carbon material, respectively. The thermos-gravimetric (TG) trace in Fig. 1h shows the contents of 20%T-DCL slurry and 40%T-DCL slurry after being calcined from 25 °C to 900 °C with the heating rate of 10 °Cmin-1 in an air, respectively. The fast weight loss of the composites between 300 and 540 °C belongs to the removal of gelatin binder. The gelatin contents of 20%T-DCL slurry and 40%T-DCL slurry were approximately 20 wt%. The weight loss after 540 °C arose from the decomposition of AB. The continuous heating after 800 °C in air was intended to eliminate AB. The remaining 20 wt% and 40 wt% representing the TiO2 content in 20%T-DCL slurry 539
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Fig. 1. Morphology and composition of modified separators with different coating layers. The SEM images of the (a) 20%T-DCL separator, (b) 40%T-DCL separator, (c) TiO2-coated separator, and (d) Carbon-coated separator. The insert is corresponding elemental mapping for Ti and C. (e,f) TEM images of 20%T-DCL slurry. (g) XRD pattern of the 20%T-DCL separator. (h) TG image of 20%T-DCL and 40%T-DCL slurries calcined in air atmosphere.
g−1 and 421 mAh g−1 remained. The high capacity retention after long cycles directly certifies the efficient inhibition of 20%T-DCL separator on the shuttle effect. Even though the Carbon-coated separator, 40%TDCL separator and 20%T-DCL separator have the same function to serve as an upper current collector and restrain the polysulfide shuttling effect, their electrochemical performances still show a lot of differences. Compared with the other two separators mentioned above, the notable improvement of long-term cycle performance and capacity retention attained by the Li-S battery with a 20%T-DCL separator can be corresponded to the uniformly dispersion of TiO2 in conductive carbon layer and the synergistic effect to chemically and physically immobilize for LiPS. 20 wt% TiO2 uniformly decorated in the carbon layer is effectively to balance the chemically and physically polysulfide trapping effect. Fig. 2 also displays that the electrochemical performance of the Li-S battery with TiO2-coated separator is significantly improved compared with that of the batteries with normal separator, which can be associated to the existence of chemical interaction between LiPS and the TiO2 layer. Besides, it is also clearly to see that the specific capacity of battery with 20%T-DCL separator is much higher than that of batteries with TiO2-coated separators at 0.5C and 1.0C. This is because in addition to the chemical trapping for LiPS, 20%T-DCL separator can also serve as an upper current collector to effectively enhance the batteries' reversibility. Table S1 (Supporting information) shows the cycle stability of other reported modified separators for the Li-S battery. The comparison indicates our present 20%T-DCL separator exhibits outstanding cycle stability.
The cycle stability was investigated for the batteries with five kinds of separators through discharged at diverse rates, as displayed in Fig. 2e. Batteries with the 20%T-DCL separator cycled at the rates of 0.1, 0.5, 1.0, and 2.0C exhibits reversible discharged capacities of 1130, 960, 850 and 710 mAh g−1. As the rate returned to 0.1C, the sulfur cathode almost recovers its initial capacity of about 1100 mAh g−1. This excellent reversibility indicated that 20%T-DCL separator could still effectively immobilize LiPS even at a high passing rate of ions. The desired outstanding rate capability of 20%T-DCL separator results from the superior ionic and electronic conductivity. Furthermore, the upper plateau discharge capacities of batteries with 20%T-DCL separator are better retained than those of other modified separator in Fig. 2f. This confirms that 20%T-DCL separator can successfully inhibit the shuttle effect of LiPS and reduce the loss of active materials during the redox reaction. After using a 20%T-DCL separator, the upper plateau discharge capacity in the first cycle is 363 mAh g−1, nearly 86.6% of the theoretical capacity (419 mAh g−1), demonstrating that the LiPS shuttling is inhibited. After 100 cycles, the cathode with 20%T-DCL separator remains at 69.7% of the theoretical value, while the other batteries with 40%T-DCL separator, TiO2-coated separator, Carboncoated separator and routine separator retain about 65.1%, 45.8%, 63% and 38.7%, respectively. The enhanced upper plateau capacity also demonstrates that 20%T-DCL separator is most effective to trap and reactivate LiPS among all the modified separators. To better know the enhanced electrochemical performances of batteries with different kinds of separators, the EIS measurements of 540
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Fig. 2. Electrochemical performance of Li-S batteries with different separators. (a) The discharge-charge voltage profiles of different cycles at a rate of 0.1C for batteries with 20%T-DCL separator; (b) Voltage profiles of 20%T-DCL separator cycled at various C-rates. The cycling performances of different batteries at (c) 0.1C and (d) 0.5C. (e) The performances of batteries cycled at different rates. (f) The upper plateau discharge capacity of different batteries cycled at 0.5C.
This could be due to the large amounts of nano-TiO2 cannot be uniformly dispersed by the carbon layer and this amounts of nano-TiO2 in the coating layer slightly inhibits the electron and ion movement during the charge/discharge process. Besides, it is also vital to see that the Rct of the battery with a TiO2-coated separator is highest in all separators. This is attributed to the TiO2 layer is not conductive, and the transformation of electron and ion is seriously inhibited during the redox reaction. Besides, the battery with the routine separator has only one semicircle, whereas the battery with TiO2-coated separator has another small semicircle at middle frequency region. This could be attributed to that battery with TiO2-coated separator has an interface contact resistance between TiO2 layer and cathode electrode, thus resulting in the second semicircle. As shown in Fig. 3b and Table S2b (Supporting information), after 5 cycles, the resistances of both batteries decline compared with that of the fresh batteries, demonstrating that the permanent deposition of non-conducting and insoluble Li2S and Li2S2 on the surface of coating layer and cathodes. The movements of Li+ become easier as the cycle number increasing, which improves the high
fresh batteries (Fig. 3a) and different cycled batteries (Fig. 3b) were examined. The EIS spectra contain an inclined line (related to Warburg impedance) in the low-frequency area and a medium-to-high frequency semicircle. The semicircle is corresponded to the charge-transfer process at the interface of the cathode and electrolyte. The Warburg impedance is related to semi-infinite diffusion of long-chain LiPS. The equivalent circuit models for investigating impedance spectra are presented in Fig. 3a and b. Re stands for the impedance donated by the electrolyte resistance, Rct represents the charge transfer resistance at the interface of electrode material, CPE is the constant phase that stands for capacitance. Wc represents the Warburg impedance related to the LiPS diffusion within the cathode. Rs stands for the deposit diffusion resistance of SEI film [44]. As presented in Fig. 3a and Table S2a (Supporting information), the notable reduction in the Rct of the batteries with 20%T-DCL separator and Carbon-coated separator are due to the good conductivity of the coating, thus decreasing the internal resistance of the Li-S batteries. While the Rct value of the battery with a 40%T-DCL separator is only little lower than that of normal battery. 541
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Fig. 3. The EIS curves of batteries with five types of separators before (a) and after (b) 5 cycles. The insert is the equivalent circuit models for analysing impedance spectra. The CV curves of batteries with (c) 20%T-DCL separator, (d) 40%T-DCL separator, (e) TiO2-coated separator and (f) Carbon-coated separator.
Fig. 4. The digital images of (a) fresh normal separator and other kinds of separators: (b) 20%T-DCL separator, (c) 40%T-DCL separator, (d) TiO2-coated separator, (e) Carbon-coated separator and (f) normal separator towards anode side after 5 cycles. The digital and SEM images of (a1, a2) fresh lithium, cycled lithium anodes with (b1, b2) 20%T-DCL separator, (c1, c2) 40%T-DCL separator, (d1, d2) TiO2-coated separator, (e1, e2) Carbon-coated separator and (f1, f2) routine separator after 5 cycles.
operated with a scan rate of 0.1 mV/s between 1.7 and 2.8 V vs Li/Li+. In the cathodic scan, two reduction peaks at about 2.33 and 2.03 V were displayed for the battery with a 20%T-DCL separator. The peak located at about 2.35 V could be attributed to the reduction of sulfur to longchain LiPS (Li2Sn, 4 ≤ n ≤ 8). And the peak located at about 2.05 V relates to the reduction of long-chain LiPS to Li2S2 and Li2S. In the following anodic scan, the main peak located at about 2.43 V is assigned to the transformation of Li2S and LiPS into elemental S [45].
rate capability and cycle stability of the sulfur cathode. Besides, the 20%T-DCL separator, exhibit lower charge transfer resistance than those of 40%T-DCL separator, Carbon-coated separator, TiO2-coated separator and routine separator in 5th cycle, which is due to the higher utilization of active material and less shuttle effect. To explore the detail redox reactions of the modified Li-S battery, Cyclic Voltammetry (CV) measurements were presented. Fig. 3 exhibits the CV results of the batteries with four separators. The CV test was 542
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Fig. 5. Cross-sectional images of (a) 20%T-DCL separator, (b) TiO2-coated separator, (c) Carbon-coated separator after 5 cycles at 0.5C. (d) Digital images and (e) UV–vis spectra of Li2S6containing solution with nano-TiO2. (f) XPS spectra of the Ti 2p peaks of the TiO2 power and TiO2/Li2S6, together with their respective fitted peaks. (g) Illustration of the polysulfidetrapping and electron-transporting mechanisms of the 20%T-DCL separator.
Moreover, after the first cycle, the areas and positions of the main peaks almost keep stable as the cycle number increasing, suggesting enhanced cycling stability and good reversibility of the batteries with 20%T-DCL separators. Besides, compared with the batteries with other kinds of separators, the CV results of batteries with 20%T-DCL separator displays smaller voltage gaps, indicating little polarization. These are the same with the results got from Fig. 2. After completing the cycling shown in Fig. 2 and Fig. 3, the batteries were disassembled in order to look for any physical evidence for sulfur crossover through the nano-TiO2/AB layer. Fig. 4 shows the images of different separators (towards anode side) after 5 cycles and their corresponding images of cycled lithium anodes. Compared with the TiO2coated separator, 40%T-DCL separator and routine separator after 5 cycles, the 20%T-DCL separator and Carbon-coated separator show a darker yellow color in the cathode side, which means more amounts of LiPS is inhibited and deposited in the coating layer. Fig. 4a1-4f1 and
4a2-4f2 further display the digital and SEM images of cycled anodes with different separators after 5 cycles. It is visibly that compared with the cycled anode with other modified separators, the Li anodes with a 20%T-DCL separator shows the smoothest surface and the best surfaceprotecting benefits against erosion, which demonstrates the 20%T-DCL layer can largely prevent the shuttle effect. Besides, the Li anodes with TiO2-coated separator and routine separator show the most serious corrosion than that of other modified separators, which indicated the coating layer only contains TiO2 is not efficient enough to protect Li anode from corrosion. Figure S3a, S3b and S3c (Supporting information) show the SEM images of the 20%T-DCL separator, TiO2-coated separator and Carboncoated separator after being discharged for 5 times. Compared with the cycled TiO2-coated separator and Carbon-coated separator, the surface of cycled 20%T-DCL separator is uniformly covered by a layer of aggregates, indicating the migrating active material is absorbed and 543
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Acknowledgements
captured during the electrochemical cycling. Fig. 5a–c shows the crosssectional images of cycled 20%T-DCL separator, TiO2-coated separator and Carbon-coated separator, respectively. After cycled for 5 times, an additional active material layer (about 1.3 μm thick) was deposited uniformly on the 20%T-DCL separator. This layer is due to the coating layer servers as an upper-current collector to continuously recycle the trapped LiPS during long term cycling. The additional active material layers in cycled TiO2-coated separator and Carbon-coated separator were 0.5 μm and 1 μm, respectively. These layers are thinner than that of cycled 20%T-DCL separator, which means more amounts of active materials were blocked by 20%T-DCL separator during discharge/ charge process. To observe the existence of interactions between LiPS and TiO2, the yellow color Li2S6-containing solution in vial A was divided equally into two vials. As shown in Fig. 5d, when the nano-TiO2 powders were introduced to vials B, the solution got transparent immediately. This result encourages us to explore the detail by UV–visual spectroscopy. Fig. 5e shows that the solution in vial A show a UV absorption band in the 400-500 cm−1 region, which could be ascribed to existence of Li2S6. The UV absorption curve of solution in bottle B shows no peaks in the 400-500 cm−1 region, which means the existence of strong interaction exists between nano-TiO2 and Li2S6 [46,47]. To verify the presence of interactions between TiO2 and Li2S6, we presented the XPS measurements (Fig. 5f). Compared with TiO2, the Ti 2p spectrum of TiO2/Li2S6 shows a 0.3 eV shift to lower binding energy, which indicates the donation of electron from the sulfide to the Ti element. The XPS measurement results further demonstrated the interaction between TiO2 and Li2S6. These findings are identical with the LiPS adsorption results mentioned above. All the results indicate 20%T-DCL separator is a promising modified separator for the Li-S battery, which is benefited from its uniformly distributed structure and composition as shown in Fig. 5g. Firstly, the 20 wt% of nano-TiO2 can be uniformly dispersed by the conductive carbon layer. This 20%T-DCL has a good synergistic effect to chemically and physically immobilize for LiPS, thereby mitigating the shuttle effect and protecting the lithium anode from corrosion; Secondly, the continuous phase of conductive carbon layer can still serve as an upper current collector to keep utilizing the trapped LiPS during long term cycling and enhance the batteries' reversibility.
Financial supported from the National Natural Science Foundation of China (No. 51432003 and No. 51672020) is gratefully appreciated. Appendix A. Supplementary data Supplementary data related to this article can be found at http://dx. doi.org/10.1016/j.jpowsour.2017.12.067. References [1] Y. Sun, N. Liu, Y. Cui, Promises and challenges of nanomaterials for lithium-based rechargeable batteries, Nature Energy (2016) 16071. [2] B. Scrosati, J. Garche, Lithium batteries: status, prospects and future, J. Power Sources 195 (2010) 2419–2430. [3] P.G. Bruce, S.A. Freunberger, L.J. Hardwick, J.M. Tarascon, Li-O2 and Li-S batteries with high energy storage, Nat. Mater. 11 (2012) 19–29. [4] J.B. Goodenough, K.S. Park, The Li-ion rechargeable battery: a perspective, J. Am. Chem. Soc. 135 (2013) 1167–1176. [5] N.S. Choi, Z. Chen, S.A. Freunberger, X. Ji, Y.K. Sun, K. Amine, G. Yushin, L.F. Nazar, J. Cho, P.G. Bruce, Challenges facing lithium batteries and electrical double-layer capacitors, Angew. 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4. Conclusions In summary, a promising separator coated with nano-TiO2 decorated carbon layer was successfully used for Li-S batteries. The uniformly dispersion of nano-TiO2 in the conductive carbon layer (continuous phase) brings three main advantages: (1) carbon layer can serve as a dispersant and conductive agent to increase the conductivity of nano-TiO2; (2) The 20%T-DCL has a good synergistic effect to chemically and physically immobilize for LiPS; 3) the conductive carbon layer served as an upper current collector can effectively enhance the batteries' reversibility. The EIS measurement indicated the good conductivity of 20%T-DCL separator. The cyclic performance of the Li-S battery with 20%T-DCL separator are significantly improved, resulting in a specific capacity of 883 mAh g−1 after 180 cycles at 0.1 C and 762 mAh g−1 after 200 cycles at 0.5C, which is much higher than that of separators only coated with Carbon layer or TiO2 layer. The CV and EIS measurements indicated that Li-S battery with 20%T-DCL separator shows small voltage gap, good cycle reversibility and low electrochemical impedance. SEM measurement of the cycled anode, Li2S6 absorption experiments, UV spectrum and XPS measurement of nanoTiO2/Li2S6 demonstrated the strong interactions between nano-TiO2 and LiPS. All the results indicated that 20%T-DCL separator could largely prevent the shuttle effect of LiPS and enhance the cyclic performance of the Li-S battery. Our findings are also a good guidance to develop good-performance separators for the Li-S battery for the future. 544
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Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour
Nano-TiO2 decorated carbon coating on the separator to physically and chemically suppress the shuttle effect for lithium-sulfur battery
T
Hongyuan Shaoa, Weikun Wangb, Hao Zhangb, Anbang Wangb, Xiaonong Chena,∗∗, Yaqin Huanga,∗ a
State Key Laboratory of Chemical Resource Engineering, Beijing Laboratory of Biomedical Materials, Beijing University of Chemical Technology, Beijing 100029, PR China b Research Institute of Chemical Defense, Beijing 100191, PR China
H I G H L I G H T S
G RA P H I C A L AB S T R A C T
The nano-TiO /Carbon coating has a • synergistic effect to immobilize for 2
Illustration of the polysulfide-trapping and electron-transporting mechanisms of the 20%T-DCL separator.
LiPS.
The Li-S battery with nano-TiO / • Carbon coating shows high discharge 2
capacity.
The nano-TiO /Carbon coating can • protect lithium anode from corrosion. 2
A R T I C L E I N F O
A B S T R A C T
Keywords: Nano-TiO2 Conductive carbon layer Synergistic effect Shuttle effect Lithium-sulfur batteries
Despite recent progress in designing modified separators for lithium-sulfur (Li-S) batteries, detail in optimizing the synergistic effect between chemical and physical immobilization for lithium polysulfides (LiPS) in modified separator hasn't been investigated totally. Here, a nano-TiO2 decorated carbon layer (T-DCL) has been successfully applied to modify separator for the Li-S battery. The results indicate that appropriate weight percentage of nano-TiO2 uniformly distributed in conductive carbon layer is effective to chemically and physically immobilize for LiPS, and promote the electron transfer during discharge/charge process. The performance of the modified Li-S battery with T-DCL separator are significantly enhanced, with a specific capacity of 883 mAh g−1 retained after 180 cycles at 0.1 C and 762 mAh g−1 retained after 200 cycles at 0.5C, which are much higher than that of separators only coated with TiO2 layer or conductive carbon layer. Besides, the separator coated with T-DCL also shows low electrochemical impedance and good lithium anode protection. These results indicate that separator with T-DCL is promising to balance the physical and chemical LiPS trapping effect, and optimize the electrochemical performance for Li-S battery.
1. Introduction Fast developments of the global economy accelerate the consumption of fossil fuels and also cause severe environmental pollution.
∗
Therefore, sustainable energy storages are inspiring a rapid growth of attention during the last decade [1–3]. Because of the low theoretical capacity, the commonly used lithium-ion batteries can not satisfy the requirement for high energy density storage [4,5]. In the past twenty
Corresponding author. Corresponding author. E-mail addresses: [email protected] (X. Chen), [email protected] (Y. Huang).
∗∗
https://doi.org/10.1016/j.jpowsour.2017.12.067 Received 26 July 2017; Received in revised form 24 November 2017; Accepted 23 December 2017 0378-7753/ © 2017 Elsevier B.V. All rights reserved.
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a specific capacity of 883 mAh g−1 retained after 180 cycles at 0.1 C and 762 mAh g−1 retained after 200 cycles at 0.5C. These results demonstrate the 20%T-DCL separator is promising ratio control strategy to balance the physical and chemical LiPS trapping effect and optimize the electrochemical performance for the Li-S battery.
years, lithium-sulfur (Li-S) batteries have been widely explored as a hopeful candidate for next generation of energy storage devices, because of the ultrahigh energy density (2600 Wh kg−1), abundant sources, and free of contamination [6,7]. Except for the advantages mentioned above, the poor electrical conductivity of sulfur, serious volume expansion rate (80%) during discharge/charge process, and shuttle effect of the lithium polysulfides (LiPS) (Sn2−, 4 ≤ n ≤ 8) intermediate result in a low utilization rate of sulfur and short lifetime of Li-S batteries, which makes them inappropriate for the commercial application [8–10]. To inhibit LiPS from shuttling in the liquid electrolyte, many methods had been presented during the past decades [11–15]. Among these methods, one effective strategy to prevent LiPS from shuttling is the modification of battery structure, for instance, building a physical barrier for LiPS between the sulfur cathode and separator. Several kinds of carbon-based interlayer [16–19] have been applied to enhance the electrochemical performance the Li-S battery. Although the interlayer provides an effective battery architecture, there is still a drawback that the extra weight of the interlayer will reduce the overall energy density. Recently, many groups focused on developing the modified separators with different carbon coatings layer on the surface of the Celgard separator [20–27]. Through placing the conductive carbon coating adjacent to the cathode, the carbon coating serves as an absorbent layer to restrain the soluble LiPS from shuttling. Besides, the carbon layer with high electrical conductivity also serves as an uppercurrent collector [28] to keep utilizing the trapped LiPS during charge/ discharge process. Although carbon materials have presented promise as sulfur-entrapping host structures, their weak interaction with the polar LiPS limits their application as LiPS traps [29–31]. Recently, metal oxides for LiPS entrapment also have aroused significant interest, such as TiO2 [32,33], Ti4O7 [34], MnO2 [35], NiFe2O4 [36], SiO2 [37], SnO2 [38], and MgO [39]. In particular, anatase TiO2 is a promising host to contain sulfur because of its good absorption ability. Previous works have shown the effective coupling of mesoporous TiO2 with a C-S composite to enhance the electrochemical performance [40]. These results demonstrated that the TiO2 in the carbon layers could stimulate the interaction between TiO2 and S, which was supposed to provide an electrostatic attraction [41] that enhanced the surface trapping of LiPS on the TiO2. Even though TiO2 has good LiPS trapping ability, the poor conductivity of TiO2 limits its direct application in the modified separator, because coating layer in modified separator should be conductive to serve as an upper collector and keep utilizing the trapped LiPS during long term cycling [42]. Improving the conductivity of TiO2 using conductive agent, such as carbon materials, is essential to obtain good electrochemical performance. Herein, we report a light-weight and efficient modified separator prepared by coating a nano-TiO2 decorated carbon layer (T-DCL) to the surface of a routine separator. The cooperative combination of nanoTiO2 and conductive carbon coating afforded a good construction strategy to chemically and physically trap LiPS. To further explore the synergistic effect between chemical and physical immobilization for LiPS, we gradually increased the weight percentage of nano-TiO2 in the coating layer from 0 wt% to 80 wt%. The results showed that compared with other carbon layers, carbon layer with 20 wt% of nano-TiO2 decorated (20%T-DCL separator) is effective in restraining the LiPS shuttling effect, protecting the lithium anode, reducing the electrochemical impedance and enhancing the performance of sulfur cathode. As illustrated in Scheme 1, compared with Carbon-coated and TiO2coated separator, the 20%T-DCL separator has three main advantages: 1) carbon layer can serve as the dispersants and conductive agent to increase the conductivity of nano-TiO2; 2) The 20%T-DCL has a good synergistic effect to chemically and physically immobilize for LiPS; 3) the conductive carbon layer served as an upper current collector can effectively enhance the batteries' reversibility and reduce the electrochemical impedance. The electrochemical performance of Li-S batteries with 20%T-DCL separator were also significantly enhanced, resulting in
2. Experimental 2.1. Synthesis of nano-TiO2 decorated carbon composite The acetylene black (AB) and anatase TiO2 nanopowder used were purchased from commercial corporations. The nano-TiO2 powder and AB were first mixed together with a specific weight ratio and then ultrasonically dispersed in gelatin (type B, derived from bovine bones) aqueous solution (2 wt% of gelation dissolved into the water) to form a homogeneous solution. The composites solution were finally operated by ball-milling for 5 h at 25 °C to form uniform slurry. 2.2. Preparation of the modified separators Four kinds of modified separators were fabricated by different slurries in this work. The first one (20%T-DCL separator) consisted of 20 wt% nano-TiO2, 60 wt% AB and 20 wt% gelatin; The second one (40%T-DCL separator) consisted of 40 wt% nano-TiO2, 40 wt% AB and 20 wt% gelatin; The third one (TiO2-coated separator) consisted of 80 wt% nano-TiO2 and 20 wt% gelatin; the fourth one (Carbon-coated separator) consisted of 80 wt% AB and 20 wt% gelatin. These four kinds of separators were modified by direct coating of the above mentioned slurries on the normal separator (Celgard 2400) with a spreader method. Then, the slurry-coated separators were parched in a vacuum oven at 60 °C for 24 h. Finally, the modified separators were cut into round pieces with a mass loading about 0.2 mg cm−2. 2.3. Preparation of the Li2S6-containing solution The Li2S6-containing solution was prepared in a pure Ar atmosphere through mixing the lithium and sublimed sulfur with a molar ratio of 2:6 in the tetrahydrofuran solvent. After 5 h, the Li2S6-containing solution was formed. 2.4. Battery assembly and characterization The sulfur electrodes were fabricated by a commonly used coating method by a spreader. Firstly, the sulfur cathode slurry containing 63 wt% sublimed sulfur (99.5%, Beijing Yili. Corp., China), 30 wt% AB and 7 wt% gelatin binder was prepared. Then the obtained slurry was coated onto the surface of aluminum foil and parched at 60 °C in vacuum for 24 h. Finally, the sulfur cathode was successfully prepared with a sulfur loading about 2.0 mg cm−2. The Coin-type (CR2025) batteries were assembled under an Ar-filled glove box (ULTRA S1) to keep the contents of oxygen and water lower than 1 ppm. A lithium metal was applied as the anode; Celgard 2400 separator, Carbon-coated separator, and the T-DCL separator were also used. The electrolyte consists of 1 M lithium bistriuoromethanesulfonylimide (LiTFSI, Beijing Chemical Reagent Research Institute) and 0.4 M lithium nitrate in the solvent of 1, 3-dioxolane (DOL) and 1,2-dimethoxyethane (DME) (1:1 V:V, Beijing Chemical Reagent Research Institute). The electrochemical tests were performed at the rates of 0.1, 0.5, 1, 1.5 and 2C between 1.7 and 2.8 V using a LAND-CT2001A instrument. The surface morphologies of the modified separator were detected by a scanning electron microscope (SEM, HITACHIS-4800). Transmission electron microscopy (TEM, HITACHI H-800) was used to characterize the microstructure of the samples. The X-ray photoelectron spectroscopy (XPS, Thermo Fisher Scientific USA ESCALAB-250) was performed to investigate the elemental composition of the materials. The thermogravimetric analyzer (TG, Hengjiu China) was applied to determine the 538
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Scheme 1. Illustration of the polysulfide-trapping and electron-transporting mechanisms of different modified separators.
and 40%T-DCL slurry. Besides, the EDS measurement in Figure S1d and S1e (Supporting information) also demonstrate the chemical composition of 20%T-DCL slurry and 40%T-DCL slurry, which is identical to the TG result. The TG and EDS measurements indicated the ratio of TiO2 and AB is approximately 20:60 and 40:40 in 20%T-DCL slurry and 40%T-DCL slurry, respectively. Fig. 2a, Figure S2a, S2b, S2c and S2d (Supporting information) display the discharge-charge voltage profiles cycled at different times (1st, 10th, 20th and 100th) at 0.1 C for Li-S batteries with 20%T-DCL separator, 40%T-DCL separator, TiO2-coated separator, Carbon-coated separator and routine separator, respectively. Throughout the redox reaction, the capacity-voltage profiles of battery with 20%T-DCL separator shows a two-step redox reaction. The first discharge plateau at 2.32 V correspond to the transformation of S8 to Li2Sn (4 < n ≤ 8), and the lower discharge plateau at 2.10 V is corresponded to the formation of Li2S2 and Li2S. The charge profiles show two plateaus at about 2.24 and 2.35 V, respectively, which is corresponded to oxidation reactions of Li2S2/Li2S to S8/Li2S8 [43]. The Li-S batteries with 20%T-DCL separator exhibit longer voltage plateaus and smaller polarization (ΔE) than that of Li-S batteries with other four kinds of separators. Moreover, the charge voltage also increases with the cycle number for the battery with 40%T-DCL separator, TiO2-coated separator and routine separator, however the cycle potentials of the batteries with 20%T-DCL separator remain almost constant. Fig. 2b, Fig. S2e, S2f, S2g and S2h (Supporting information) show a similar tendency for batteries with 20%T-DCL separator, 40%T-DCL separator, TiO2-coated separator, Carbon-coated separator and routine separator, when ΔE is examined at different rates of 0.1C, 0.5C, 1.0C and 2.0C. Even cycled at high rate (2.0C), batteries with 20%T-DCL separator still perform lower ΔE and higher capacity retention than that of other separators. These results evidently reveal that the 20%T-DCL separator is extremely beneficial for optimizing the charge/discharge kinetics and the system reversibility. The long-term cycle performances of Li-S batteries discharged at different rates are presented in Fig. 2c and 2d. The Li-S batteries were discharged at a rate of 0.05C at the initial 2 cycles for the cathode activating and then discharged at 0.5C in subsequent cycles. When discharged at 0.1C (Fig. 2c), the initial discharge capacity of the Li-S batteries with 20%T-DCL separators reaches up to 1227 mAh g−1 (approximately 73.3% of the theoretical capacity) and preserves a reversible capacity about 883 mAh g−1 after cycled for 180 times, which is much higher than that of batteries with other separators. When discharged at 0.5C, the special capacity of the battery with a 20%T-DCL separator, after 200 cycles, remains 762 mAh g−1, while the special capacities of batteries with 40%T-DCL separator, Carbon-coated separator and routine separator at 0.5C are only 589 mAh g−1, 529 mAh
content of composite slurry. The X-ray diffraction (XRD) patterns of the samples were recorded with Cu-Ka radiation at 40 kV by using an X-ray diffractometer (D/max-2500/PC, Rigaku). The Electrochemical impedance spectroscopy (EIS) was operated at an open-circuit voltage (OCV) from 100 mHz to 10 kHz at room temperature using the Solartron 1280Z. The Cyclic Voltammetry (CV) was also carried out using the same instrument, with a scanning rate of 0.1 mV/s. UV–visual spectroscopy was measured with TU-1810 to investigate the LiPS in tetrahydrofuran. 3. Results and discussion The scanning electron microscopy (SEM) image was used to investigate the morphology of the different modified separators. Fig. 1a, 1b, 1c, and 1d show the morphologies of the 20%T-DCL separator, 40%T-DCL separator, TiO2-coated separator, and Carbon-coated separator, respectively. The four modified separators are adhered well on the celgard separator by gelatin binder and show a homogeneously dispersed structure, which is contributed to increase the trapping sites for LiPS. The inset EDS mapping of elemental Ti and C in Fig. 1a and 1b displays that the nano-TiO2 is uniformly distributed on the coating surface and this distribution is beneficial for trapping LiPS. Figure S1 (Supporting information) presents the digital images and cross-sectional morphology of the 20%T-DCL separator, 40%T-DCL separator and TiO2-coated separator. The morphology of Carbon-coated separator had already reported in our previous work [31]. As shown in Fig. S1, the coating layers were deposited uniformly and approximately 4 μm thick on average. Fig. 1e and d shows the high magnification TEM image of 20%T-DCL separator. The high-resolution TEM image displays a lattice spacing of 0.35 nm corresponding to the (101) facet of anatase TiO2 and the nano-TiO2 are uniformly distributed on the AB layers without aggregation. The XRD pattern in Fig. 1g could be attributed to anatase TiO2 (JCPDS No. 21–1272), further suggesting the good combination of nano-TiO2 and carbon in the T-DCL. There are also two typical peaks marked in blue at about 24° and 44°, which can be related to diffraction from (002) and (100) of carbon material, respectively. The thermos-gravimetric (TG) trace in Fig. 1h shows the contents of 20%T-DCL slurry and 40%T-DCL slurry after being calcined from 25 °C to 900 °C with the heating rate of 10 °Cmin-1 in an air, respectively. The fast weight loss of the composites between 300 and 540 °C belongs to the removal of gelatin binder. The gelatin contents of 20%T-DCL slurry and 40%T-DCL slurry were approximately 20 wt%. The weight loss after 540 °C arose from the decomposition of AB. The continuous heating after 800 °C in air was intended to eliminate AB. The remaining 20 wt% and 40 wt% representing the TiO2 content in 20%T-DCL slurry 539
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Fig. 1. Morphology and composition of modified separators with different coating layers. The SEM images of the (a) 20%T-DCL separator, (b) 40%T-DCL separator, (c) TiO2-coated separator, and (d) Carbon-coated separator. The insert is corresponding elemental mapping for Ti and C. (e,f) TEM images of 20%T-DCL slurry. (g) XRD pattern of the 20%T-DCL separator. (h) TG image of 20%T-DCL and 40%T-DCL slurries calcined in air atmosphere.
g−1 and 421 mAh g−1 remained. The high capacity retention after long cycles directly certifies the efficient inhibition of 20%T-DCL separator on the shuttle effect. Even though the Carbon-coated separator, 40%TDCL separator and 20%T-DCL separator have the same function to serve as an upper current collector and restrain the polysulfide shuttling effect, their electrochemical performances still show a lot of differences. Compared with the other two separators mentioned above, the notable improvement of long-term cycle performance and capacity retention attained by the Li-S battery with a 20%T-DCL separator can be corresponded to the uniformly dispersion of TiO2 in conductive carbon layer and the synergistic effect to chemically and physically immobilize for LiPS. 20 wt% TiO2 uniformly decorated in the carbon layer is effectively to balance the chemically and physically polysulfide trapping effect. Fig. 2 also displays that the electrochemical performance of the Li-S battery with TiO2-coated separator is significantly improved compared with that of the batteries with normal separator, which can be associated to the existence of chemical interaction between LiPS and the TiO2 layer. Besides, it is also clearly to see that the specific capacity of battery with 20%T-DCL separator is much higher than that of batteries with TiO2-coated separators at 0.5C and 1.0C. This is because in addition to the chemical trapping for LiPS, 20%T-DCL separator can also serve as an upper current collector to effectively enhance the batteries' reversibility. Table S1 (Supporting information) shows the cycle stability of other reported modified separators for the Li-S battery. The comparison indicates our present 20%T-DCL separator exhibits outstanding cycle stability.
The cycle stability was investigated for the batteries with five kinds of separators through discharged at diverse rates, as displayed in Fig. 2e. Batteries with the 20%T-DCL separator cycled at the rates of 0.1, 0.5, 1.0, and 2.0C exhibits reversible discharged capacities of 1130, 960, 850 and 710 mAh g−1. As the rate returned to 0.1C, the sulfur cathode almost recovers its initial capacity of about 1100 mAh g−1. This excellent reversibility indicated that 20%T-DCL separator could still effectively immobilize LiPS even at a high passing rate of ions. The desired outstanding rate capability of 20%T-DCL separator results from the superior ionic and electronic conductivity. Furthermore, the upper plateau discharge capacities of batteries with 20%T-DCL separator are better retained than those of other modified separator in Fig. 2f. This confirms that 20%T-DCL separator can successfully inhibit the shuttle effect of LiPS and reduce the loss of active materials during the redox reaction. After using a 20%T-DCL separator, the upper plateau discharge capacity in the first cycle is 363 mAh g−1, nearly 86.6% of the theoretical capacity (419 mAh g−1), demonstrating that the LiPS shuttling is inhibited. After 100 cycles, the cathode with 20%T-DCL separator remains at 69.7% of the theoretical value, while the other batteries with 40%T-DCL separator, TiO2-coated separator, Carboncoated separator and routine separator retain about 65.1%, 45.8%, 63% and 38.7%, respectively. The enhanced upper plateau capacity also demonstrates that 20%T-DCL separator is most effective to trap and reactivate LiPS among all the modified separators. To better know the enhanced electrochemical performances of batteries with different kinds of separators, the EIS measurements of 540
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Fig. 2. Electrochemical performance of Li-S batteries with different separators. (a) The discharge-charge voltage profiles of different cycles at a rate of 0.1C for batteries with 20%T-DCL separator; (b) Voltage profiles of 20%T-DCL separator cycled at various C-rates. The cycling performances of different batteries at (c) 0.1C and (d) 0.5C. (e) The performances of batteries cycled at different rates. (f) The upper plateau discharge capacity of different batteries cycled at 0.5C.
This could be due to the large amounts of nano-TiO2 cannot be uniformly dispersed by the carbon layer and this amounts of nano-TiO2 in the coating layer slightly inhibits the electron and ion movement during the charge/discharge process. Besides, it is also vital to see that the Rct of the battery with a TiO2-coated separator is highest in all separators. This is attributed to the TiO2 layer is not conductive, and the transformation of electron and ion is seriously inhibited during the redox reaction. Besides, the battery with the routine separator has only one semicircle, whereas the battery with TiO2-coated separator has another small semicircle at middle frequency region. This could be attributed to that battery with TiO2-coated separator has an interface contact resistance between TiO2 layer and cathode electrode, thus resulting in the second semicircle. As shown in Fig. 3b and Table S2b (Supporting information), after 5 cycles, the resistances of both batteries decline compared with that of the fresh batteries, demonstrating that the permanent deposition of non-conducting and insoluble Li2S and Li2S2 on the surface of coating layer and cathodes. The movements of Li+ become easier as the cycle number increasing, which improves the high
fresh batteries (Fig. 3a) and different cycled batteries (Fig. 3b) were examined. The EIS spectra contain an inclined line (related to Warburg impedance) in the low-frequency area and a medium-to-high frequency semicircle. The semicircle is corresponded to the charge-transfer process at the interface of the cathode and electrolyte. The Warburg impedance is related to semi-infinite diffusion of long-chain LiPS. The equivalent circuit models for investigating impedance spectra are presented in Fig. 3a and b. Re stands for the impedance donated by the electrolyte resistance, Rct represents the charge transfer resistance at the interface of electrode material, CPE is the constant phase that stands for capacitance. Wc represents the Warburg impedance related to the LiPS diffusion within the cathode. Rs stands for the deposit diffusion resistance of SEI film [44]. As presented in Fig. 3a and Table S2a (Supporting information), the notable reduction in the Rct of the batteries with 20%T-DCL separator and Carbon-coated separator are due to the good conductivity of the coating, thus decreasing the internal resistance of the Li-S batteries. While the Rct value of the battery with a 40%T-DCL separator is only little lower than that of normal battery. 541
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Fig. 3. The EIS curves of batteries with five types of separators before (a) and after (b) 5 cycles. The insert is the equivalent circuit models for analysing impedance spectra. The CV curves of batteries with (c) 20%T-DCL separator, (d) 40%T-DCL separator, (e) TiO2-coated separator and (f) Carbon-coated separator.
Fig. 4. The digital images of (a) fresh normal separator and other kinds of separators: (b) 20%T-DCL separator, (c) 40%T-DCL separator, (d) TiO2-coated separator, (e) Carbon-coated separator and (f) normal separator towards anode side after 5 cycles. The digital and SEM images of (a1, a2) fresh lithium, cycled lithium anodes with (b1, b2) 20%T-DCL separator, (c1, c2) 40%T-DCL separator, (d1, d2) TiO2-coated separator, (e1, e2) Carbon-coated separator and (f1, f2) routine separator after 5 cycles.
operated with a scan rate of 0.1 mV/s between 1.7 and 2.8 V vs Li/Li+. In the cathodic scan, two reduction peaks at about 2.33 and 2.03 V were displayed for the battery with a 20%T-DCL separator. The peak located at about 2.35 V could be attributed to the reduction of sulfur to longchain LiPS (Li2Sn, 4 ≤ n ≤ 8). And the peak located at about 2.05 V relates to the reduction of long-chain LiPS to Li2S2 and Li2S. In the following anodic scan, the main peak located at about 2.43 V is assigned to the transformation of Li2S and LiPS into elemental S [45].
rate capability and cycle stability of the sulfur cathode. Besides, the 20%T-DCL separator, exhibit lower charge transfer resistance than those of 40%T-DCL separator, Carbon-coated separator, TiO2-coated separator and routine separator in 5th cycle, which is due to the higher utilization of active material and less shuttle effect. To explore the detail redox reactions of the modified Li-S battery, Cyclic Voltammetry (CV) measurements were presented. Fig. 3 exhibits the CV results of the batteries with four separators. The CV test was 542
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Fig. 5. Cross-sectional images of (a) 20%T-DCL separator, (b) TiO2-coated separator, (c) Carbon-coated separator after 5 cycles at 0.5C. (d) Digital images and (e) UV–vis spectra of Li2S6containing solution with nano-TiO2. (f) XPS spectra of the Ti 2p peaks of the TiO2 power and TiO2/Li2S6, together with their respective fitted peaks. (g) Illustration of the polysulfidetrapping and electron-transporting mechanisms of the 20%T-DCL separator.
Moreover, after the first cycle, the areas and positions of the main peaks almost keep stable as the cycle number increasing, suggesting enhanced cycling stability and good reversibility of the batteries with 20%T-DCL separators. Besides, compared with the batteries with other kinds of separators, the CV results of batteries with 20%T-DCL separator displays smaller voltage gaps, indicating little polarization. These are the same with the results got from Fig. 2. After completing the cycling shown in Fig. 2 and Fig. 3, the batteries were disassembled in order to look for any physical evidence for sulfur crossover through the nano-TiO2/AB layer. Fig. 4 shows the images of different separators (towards anode side) after 5 cycles and their corresponding images of cycled lithium anodes. Compared with the TiO2coated separator, 40%T-DCL separator and routine separator after 5 cycles, the 20%T-DCL separator and Carbon-coated separator show a darker yellow color in the cathode side, which means more amounts of LiPS is inhibited and deposited in the coating layer. Fig. 4a1-4f1 and
4a2-4f2 further display the digital and SEM images of cycled anodes with different separators after 5 cycles. It is visibly that compared with the cycled anode with other modified separators, the Li anodes with a 20%T-DCL separator shows the smoothest surface and the best surfaceprotecting benefits against erosion, which demonstrates the 20%T-DCL layer can largely prevent the shuttle effect. Besides, the Li anodes with TiO2-coated separator and routine separator show the most serious corrosion than that of other modified separators, which indicated the coating layer only contains TiO2 is not efficient enough to protect Li anode from corrosion. Figure S3a, S3b and S3c (Supporting information) show the SEM images of the 20%T-DCL separator, TiO2-coated separator and Carboncoated separator after being discharged for 5 times. Compared with the cycled TiO2-coated separator and Carbon-coated separator, the surface of cycled 20%T-DCL separator is uniformly covered by a layer of aggregates, indicating the migrating active material is absorbed and 543
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Acknowledgements
captured during the electrochemical cycling. Fig. 5a–c shows the crosssectional images of cycled 20%T-DCL separator, TiO2-coated separator and Carbon-coated separator, respectively. After cycled for 5 times, an additional active material layer (about 1.3 μm thick) was deposited uniformly on the 20%T-DCL separator. This layer is due to the coating layer servers as an upper-current collector to continuously recycle the trapped LiPS during long term cycling. The additional active material layers in cycled TiO2-coated separator and Carbon-coated separator were 0.5 μm and 1 μm, respectively. These layers are thinner than that of cycled 20%T-DCL separator, which means more amounts of active materials were blocked by 20%T-DCL separator during discharge/ charge process. To observe the existence of interactions between LiPS and TiO2, the yellow color Li2S6-containing solution in vial A was divided equally into two vials. As shown in Fig. 5d, when the nano-TiO2 powders were introduced to vials B, the solution got transparent immediately. This result encourages us to explore the detail by UV–visual spectroscopy. Fig. 5e shows that the solution in vial A show a UV absorption band in the 400-500 cm−1 region, which could be ascribed to existence of Li2S6. The UV absorption curve of solution in bottle B shows no peaks in the 400-500 cm−1 region, which means the existence of strong interaction exists between nano-TiO2 and Li2S6 [46,47]. To verify the presence of interactions between TiO2 and Li2S6, we presented the XPS measurements (Fig. 5f). Compared with TiO2, the Ti 2p spectrum of TiO2/Li2S6 shows a 0.3 eV shift to lower binding energy, which indicates the donation of electron from the sulfide to the Ti element. The XPS measurement results further demonstrated the interaction between TiO2 and Li2S6. These findings are identical with the LiPS adsorption results mentioned above. All the results indicate 20%T-DCL separator is a promising modified separator for the Li-S battery, which is benefited from its uniformly distributed structure and composition as shown in Fig. 5g. Firstly, the 20 wt% of nano-TiO2 can be uniformly dispersed by the conductive carbon layer. This 20%T-DCL has a good synergistic effect to chemically and physically immobilize for LiPS, thereby mitigating the shuttle effect and protecting the lithium anode from corrosion; Secondly, the continuous phase of conductive carbon layer can still serve as an upper current collector to keep utilizing the trapped LiPS during long term cycling and enhance the batteries' reversibility.
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4. Conclusions In summary, a promising separator coated with nano-TiO2 decorated carbon layer was successfully used for Li-S batteries. The uniformly dispersion of nano-TiO2 in the conductive carbon layer (continuous phase) brings three main advantages: (1) carbon layer can serve as a dispersant and conductive agent to increase the conductivity of nano-TiO2; (2) The 20%T-DCL has a good synergistic effect to chemically and physically immobilize for LiPS; 3) the conductive carbon layer served as an upper current collector can effectively enhance the batteries' reversibility. The EIS measurement indicated the good conductivity of 20%T-DCL separator. The cyclic performance of the Li-S battery with 20%T-DCL separator are significantly improved, resulting in a specific capacity of 883 mAh g−1 after 180 cycles at 0.1 C and 762 mAh g−1 after 200 cycles at 0.5C, which is much higher than that of separators only coated with Carbon layer or TiO2 layer. The CV and EIS measurements indicated that Li-S battery with 20%T-DCL separator shows small voltage gap, good cycle reversibility and low electrochemical impedance. SEM measurement of the cycled anode, Li2S6 absorption experiments, UV spectrum and XPS measurement of nanoTiO2/Li2S6 demonstrated the strong interactions between nano-TiO2 and LiPS. All the results indicated that 20%T-DCL separator could largely prevent the shuttle effect of LiPS and enhance the cyclic performance of the Li-S battery. Our findings are also a good guidance to develop good-performance separators for the Li-S battery for the future. 544
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