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Feather duster liked CeO2 as efficient adsorber host material for advanced lithium–sulfur batteries
Journal of Alloys and Compounds 823 (2020) 153743
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Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Feather duster liked CeO2 as efficient adsorber host material for advanced lithiumesulfur batteries Baoyun Ye a, *, Chenhe Feng a, Guohao Zhu a, Shuang Wang a, Ali Fakhri b a b
School of Environment and Safety Engineering, North University of China, Taiyuan, Shanxi, 030051, China Science and Research Branch, Islamic Azad University, Tehran, Iran
a r t i c l e i n f o
a b s t r a c t
Article history: Received 12 December 2019 Received in revised form 3 January 2020 Accepted 7 January 2020 Available online 8 January 2020
Many kinds of host materials have been investigated to deal with the issues of the LithiumeSulfur batteries, such as poor electronic conductivity and severe capacity fade during the electrochemical cycles. Among various host materials, metal oxides have become a promising host material due to their strong adsorption ability for the soluble polysulfide. Herein, feather duster liked CeO2 ceramics (FDCO) are synthesized via hydrothermal reaction with high surface area (198 m2 g1). While the sulfur is immersed into the feather duster liked CeO2, the as-prepared FDCO/S composites show superior capacity of 1436 mAh g1 at 0.1 C and a low capacity decay of 0.041% per cycle after 500 cycles at 2 C. The superior electrochemical performance is attributed to the strong chemical bond of polysulfide to CeO2, as well as the mesopores in the CeO2 matrix. © 2020 Elsevier B.V. All rights reserved.
Keywords: Lithium-sulfur batteries Polysulfide Shuttle effect Cycle performance
1. Introduction High energy density and long cycle life rechargeable batteries are greatly important for the electronic vehicles [1e3]. Lithiumsulfur batteries have drawn much attention because of their high specific capacity (1675 mAh g1) and energy density (2600 Wh Kg1), which are according to the redox reaction of sulfur to Li2S via various lithium polysulfides (Li2Sx, x ¼ 2e8) [4e8]. Besides, the sulfur is abundant, low cost and environment friendly [9]. However, the practical use for the lithium-sulfur batteries is hindered by the low sulfur utilization and poor cycle performance even at low current rates [10,11]. This is mainly caused by the low electronic conductivity and migration of soluble polysulfide, which is the socalled “shuttle effect” [12e15]. To deal with these problems, various host materials have been designed to limit the migration of the polysulfide. Especially, carbonaceous materials are mainly used to improve the electrochemical performance via enhancing the electronic conductivity. Whereas, the adsorption between the polar polysulfide and nonpolar carbonaceous materials is physical trapping and/or weak chemical bonding [16,17]. This will limit the adsorption ability between polysulfide and the host materials. Therefore, the
* Corresponding author. E-mail address: [email protected] (B. Ye). https://doi.org/10.1016/j.jallcom.2020.153743 0925-8388/© 2020 Elsevier B.V. All rights reserved.
carbonaceous materials are not likely to meet the requirements for the practical employment of the lithium-sulfur batteries [18]. Besides, great amounts of metal oxides are also attractive host materials due to their strong chemical bonding with polysulfide, such as TiO2 [19], Co3O4 [20], Al2O3 [21]. These metal oxides have been studied to improve the cycle stability via inhibiting the shuttle effect of the soluble polysulfide. Gao et al. firstly used yolk-shell S@TiO2 composites as cathode materials [22]. The as-prepared S@TiO2 yolk-shell structure could keep the structure stability. Moreover, the meatl oxide TiO2 could inhibit the migration of soluble polysulfide. As a result, the as-prepared S@TiO2 yolk-shell composites exhibited superior cycle stability and high specific capacities. Polar CeO2 materials have been widely used for in LieO2 batteries owing to its moderate band gap, multiple valence states [23]. It can still maintain its face cubic structure and the oxygen vacancy could also act as catalytic centers, which could improve the redox kinetics of polysulfide. In this work, feather duster liked CeO2 ceramics (FDCO) are synthesized via hydrothermal reaction with high surface area (198 m2 g1). While the sulfur is immersed into the feather duster liked CeO2, the as-prepared FDCO/S composites show superior capacity and excellent cycle stability. The superior electrochemical performance is attributed to the strong chemical bond of polysulfide to CeO2, as well as the mesopores in the CeO2 matrix.
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2. Experimental 2.1. Preparation of feather duster liked CeO2 nanofibers Typically, 0.2 g of methenamine and 0.8 ml of 0.1 M Ce(NO3)3$6H2O solutions were added into 30 ml ethanol with vigorously stirring for 30 min. Then, the mixture was transferred to a 50 ml sealed container for hydrothermal reaction at 180 C for 12 h. After that, the samples was collected and washed by deionized water for 3 times, and dried at 100 C for 24 h. Then, the obtained products were inserted into the tube furnace and heated at 800 C for 1 h under argon atmosphere with a heating rate of 5 C min1 to obtain the feather duster liked CeO2 materials. 2.2. Preparation of FDCO/S composites FDCO/S composites were obtained by a modified melt-diffusion method. The FDCO was mixed with sulfur with a weight ratio of 1:3. The mixture was heated at 155 C for 12 h in a sealed vial under Ar atmosphere.
a Bruker diffractometer at 40 kV and 40 mA with Cu Ka radiation. The thermogravimetric (TG) analyses were performed on a NETZSCH STA409PC instrument in a nitrogen atmosphere with a temperature ramp of 10 C min1. Scanning electron micrographs were acquired using a JSM-7001F field emission scanning electron microscope (FESEM) with pressure of 105-109. Transmission electron microscopy (TEM) analyses were carried out on a JEM2100F microscope operated at 200 kV coupled with an energy dispersive X-ray spectrometer (EDS). 2.4. Polysulfide adsorption test Li2S6 solution was prepared by mixing sublimed sulfur and lithium sulfidein a molar ratio of 5:1 in 1,3-dioxolane/1,2-dimethoxyethane (DOL/DME, 1:1 by volume and stirred overnight at 60 C. 20 mg FDCO and acetylene black were separately added to 10 ml of Li2S6 solution, respectively, and stirred for 24 h to facilitate adsorption. After that, 2 mL of each of the supernatant solutions was transferred to UVeVis analysis. 2.5. Electrochemical measurements
2.3. Materials characterization X-ray diffraction (XRD) measurements were carried out by using
Electrochemical performance was measured by using coin 2032 half batteries. The as-prepared FDCO/S composite was used as
Fig. 1. (a) and (b) SEM images of the feather duster liked CeO2 nanofibers. (c) SEM image of FDCO/S composites. (d), (e) and (f) Corresponding elemental mapping of O, Ce and S for the FDCO/S composites.
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cathode. The mass loading of sulfur in the electrode was about at 3.5 mg cm2. Lithium was used as anode. Celgard 2300 was as the separator, the electrolyte was composed of 1.0 M (LiTFSI) salt in DOL and DME (1:1 vol) with 1 wt% LiNO3 as an additive. Galvanostatic charge-discharge tests were performed at different scan rates in the voltage range from 1.7 to 2.8 V. EIS was conducted in the frequency range of 105e102 Hz with an amplitude of 5 mV characterized by using a CHI 660 E instrument. 3. Results and discussion The morphology of CeO2 nanofibers and FDCO/S composites were observed by using scanning electron microscopy (SEM) and corresponding elemental mapping. Fig. 1a shows the morphology of the CeO2 materials. It can be seen that the as-prepared CeO2 exhibits feather duster liked nanofiber structure. The dimeter of the FDCO is located at 80e90 nm. To clear observe the morphology, large magnification image was made for the FDCO. As shown in Fig. 1b, it can be observed that the surface of the FDCO consists of
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many feather liked fibers. Fig. 1c displays the SEM image of the FDCO/S composites. The as-prepared FDCO/S composites show similar morphology with that of the FDCO materials. This confirms that the element sulfur was immersed into the FDCO. Fig. 1def shows the corresponding elemental mapping of O, Ce and S for the FDCO/S composites. The elements O, Ce and S are uniformly distributed in the whole FDCO/S composites. To prove the crystal structure, XRD was conducted for the FDCO. As shown in Fig. 2a, the as-prepared FDCO exhibits a typical cubic fluorite structure with Fm3m space group [24]. All of these diffraction peaks could be attributed to related hkl planes. There are no impurity peaks, demonstrating superior crystallinity. To obtain the sulfur content of the FDCO/S composites, TG analysis was conducted for the FDCO/S composites. As shown in Fig. 2b, it can be calculated that the sulfur content in the FDCO/S composites ia about 74.8%. This is significant important for the calculation of the specific capacity of the lithium-sulfur battery. Fig. 3a shows the schematic illustration for the adsorption of polysulfide. The feather duster liked polar CeO2 nanofiber could
Fig. 2. (a) XRD pattern of FDCO. (b) TG curve of the FDCO/S composites.
Fig. 3. (a) Schematic illustration for the adsorption of polysulfide. (b) and (c) Photographs and UVevis spectra of Li2S6 solution with FDCO and AB. (d) N2 adsorption-desorption isotherm of FDCO.
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effectively adsorb the soluble polysulfide, and inhibit the shuttle effect. Besides, the mesoporous CeO2 also could provide weak physical adsorption for the polysulfide. To confirm this effect, FDCO and AB are added into Li2S6 solution to observe the color change with the increase of the time (Fig. 3b). It can be seen that the color of FDCO/Li2S6 mixture becomes colorless after 24 h, showing superior adsorption ability. While for the AB/Li2S6 mixture, it still remains weak yellow. After that, the UVevis spectra of Li2S6 solution with FDCO and AB were conducted. As shown in Fig. 3c, strong absorbance at about 400e500 cm1 could be observed for pure Li2S6 solution. Li2S6 solutions with the FDCO and AB show lower absorbance in 400e500 cm1. This peak is related to the presence of the Li2S6. What is more, the absorbance for the FDCO/Li2S6 mixture is much lower than AB/Li2S6 mixture, demonstrating stronger chemical absorption ability of FDCO. This is beneficial for improving the cycle stability and specific capacity [25]. Therefore, it can be concluded that the as-prepared FDCO has superior adsorption ability for the soluble polysulfide. N2 adsorptionedesorption analysis of FDCO was displayed in Fig. 3d. It indicates that the specific surface area of FDCO is 198 m2 g1. Therefore, the FDCO with high specific surface area could provide high active site and
fast electron transfer when used as cathode for LieS batteries. To further demonstrate the strong adsorption ability of polysulfide by using FDCO, S2p XPS spectra for the pristine Li2S6 and FDCO/Li2S6 mixture was conducted. As shown in Fig. 4a, for the pristine Li2S6, two S 2p3/2 cores at 160.8 and 162.3 eV can be observed, which are related to terminal sulfur (ST1) and bridging sulfur (S0B) respectively. After adding FDCO into the Li2S6 for 12 h, considerable shifting can be observed for the FDCO/Li2S6 mixture, which is corresponding to the reduction of electron cloud density in sulfur atoms as a result of strong chemical interactions between Li2S6 and FDCO [26]. This result is consistent with the previous report [27,28]. Fig. 5a exhibits the theory discharge transition, demonstrating chemical reduction and various products during the discharging process. Initially, the ring S8 molecules are transformed into Li2S8 and Li2S6 at 2.3 V. After that, long chain polysulfides were transformed into the insoluble Li2S2 and Li2S at 2.1 V [29]. Finally, all electrochemical reactions are completed and the theoretical specific capacity of 1675 mAh g1 could be achieved. Fig. 5b shows the initial discharge and charge profiles of the FDCO/S composites and pristine S electrode. It can be clearly seen that the initial capacity of
Fig. 4. High resolution S 2p XPS spectra of (a) pristine Li2S6 and (b) FDCO/Li2S6 mixture.
Fig. 5. (a) The theoretical specific capacity obtained by the discharge curve. (b) The constant discharge and charge profiles of the pristine sulfur and FDCO/S composites electrode at 0.1 C. (c) Rate capability of the pristine sulfur and FDCO/S composite electrode at various current densities from 0.1 C to 2 C. (d) EIS of the pristine sulfur and FDCO/S composite electrode. (e) Long cycle performance of the FDCO/S electrode at 2 C for 500 cycles.
B. Ye et al. / Journal of Alloys and Compounds 823 (2020) 153743
the FDCO/S composites is as high as 1436 mAh g1 at 0.1 C, which is much higher than the pristine S electrode. Fig. S1 shows the CV curves of the FDCO/S composites. It can be seen that there are two reduction peaks and one oxidation peak at 2.3 V, 2.1 V, 2.4 V, respectively. The two reduction peaks are corresponding to the chemical reaction from sulfur to polysulfide and Li2S. The oxidation peak is related to the inverse reaction from Li2S to pristine sulfur. Fig. 5c displays the rate performance of the FDCO/S and pristine S electrode. The FDCO/S composites show excellent rate capability at various current densities. Even at the high current densities of 2 C, the specific capacity could achieve 965mAh g1, demonstrating superior rate performance. To investigate the electrochemical impedance, EIS was tested for the FDCO/S composite and pristine S electrode [30]. It can be seen that the FDCO/S composites exhibit smaller resistance than the pristine S electrode. Finally, long cycle performance for the FDCO/S composites was tested at 2 C for 500 cycles. The FDCO/S composites exhibits a high initial capacity of 1050 mA h g1 at 2 C and retains about 836 mA h g1 at 2 C after 500 cycles corresponding with a capacity decay of 0.041% per cycle. 4. Conclusions In summary, feather duster liked CeO2 nanofiber ceramics are successfully prepared via hydrothermal reaction with high surface area (198 m2 g1). While the sulfur is immersed into the feather duster liked CeO2, the as-prepared FDCO/S composites show superior capacity of 1436 mAh g1 at 0.1 C and a low capacity decay of 0.041% per cycle after 500 cycles at 2 C. The superior electrochemical performance is attributed to the strong chemical bond of polysulfide to CeO2, as well as the mesopores in the CeO2 matrix. CRediT authorship contribution statement Baoyun Ye: Writing - original draft, Writing - review & editing. Chenhe Feng: Data curation. Guohao Zhu: Formal analysis. Ali Fakhri: Writing - review & editing. Acknowledgement We thank the financial support from the North University of China. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.jallcom.2020.153743.
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Declaration of competing InterestCOI We declare that we have no interest conflict.
References [1] Z.Q. Ye, Y. Jiang, J. Qian, W.L. Li, T. Feng, L. Li, F. Wu, R.J. Chen, Nano Energy 64 (2019) 103965. [2] N. Li, K.H. Chen, S.Y. Chen, F. Wang, D.D. Wang, F.Y. Gan, X. He, Y.C. Huang, Carbon 149 (2019) 564e571. [3] R. Xu, J. Lu, K. Amine, Adv. Energy Mater. 5 (2015) 150048. [4] Y.P. Li, T.Y. Jiang, H. Yang, D. Lei, X.Y. Deng, C. Hao, F.X. Zhang, J.L. Guo, Electrochim. Acta 330 (2020) 135311. [5] J. Li, L. Zhang, F.R. Qin, B. Hong, Q. Xiang, K. Zhang, J. Fang, Y.Q. Lai, J. Power Sources 442 (2019) 227232. [6] M.D. Walle, M.Y. Zhang, L. Zeng, Y.J. Li, Y.N. Liu, Appl. Surf. Sci. 497 (2019) 143773. [7] J. Fang, W.J. Shen, S.H. Cheng, S. Ghashghaie, H.K. Shahzad, C.Y. Chung, J. Power Sources 441 (2019) 227202. [8] L.L. Fan, M. Li, X.F. Li, W. Xiao, Z.W. Chen, J. Lu, Joule 3 (2019) 361e386. [9] H. Cheng, N. Cai, M. Wang, Solid State Ion. 337 (2019) 12e18. [10] L. Wu, Z.F. Wang, C.H. An, G. He, J. Alloy. Comp. 806 (2019) 881e888. [11] B.E. Li, Z.H. Sun, Y. Zhao, Z.S. Zhang, Mater. Lett. 255 (2019) 126529. [12] J.L. Cheong, A.A. Hamid, J.Y. Ying, Nano Energy 66 (2019) 104114. [13] C.L. Wang, L.S. Sun, X.X. Wang, L.M. Wang, Carbon 153 (2019) 691e698. [14] C.Y. Zha, D.H. Wu, T.K. Zhang, J.H. Wu, H.Y. Chen, Energy Storage Mater 17 (2019) 118e125. [15] W. Zhang, J.F. Zhang, Y. Zhao, X. Wang, Mater. Lett. 255 (2019) 126595. [16] B.Y. Hao, H. Li, W. Lv, Y.B. Zhang, S.Z. Niu, Q. Qi, S.J. Xiao, J. Li, F.Y. Kang, Q.H. Yang, Nano Energy 60 (2019) 305e311. [17] J.M. Chabu, K. Zeng, W.S. Chen, A. Mustapha, Y.J. Li, Y.N. Liu, Appl. Surf. Sci. 493 (2019) 533e540. [18] S.P. Li, X. Chen, F. Hu, R. Zeng, Y.H. Huang, L.X. Yuan, J. Xie, Electrochim. Acta 304 (2019) 11e19. [19] H.F. Xu, Y.Z. Shi, S.B. Yang, B. Li, J. Power Sources 430 (2019) 210e217. [20] S. Lin, Y.R. Cai, F.X. Ruan, J. Wu, B.D. Sireesh, X. Yao, J.K. Gao, J.M. Yao, J. Alloy. Comp. 779 (2019) 412e419. [21] J.H. Kim, D.J. Byun, H.S. Kim, W.C. Choi, S.O. Kim, J. Power Sources 427 (2019) 165e173. [22] X.N. Gao, X.Y. Wang, Z. Yang, Y.H. Shen, A.J. Xie, Appl. Surf. Sci. 475 (2019) 135. [23] W.T. Qi, W. Jiang, F. Xu, J.B. Jia, C. Yang, B.Q. Cao, Chem. Eng. J. 382 (2020) 122852. [24] Z.S. Han, F.J. Kong, X.L. He, S. Tao, X.F. Jiang, B. Qian, J. Phys. Chem. Solids 134 (2019) 187e192. [25] X.L. Lu, Q.F. Zhang, J. Wang, S.H. Chen, J.M. Ge, Z.M. Liu, L.L. Wang, H.B. Ding, D.C. Gong, H.G. Yang, X.Z. Yu, J. Zhu, B.G. Lu, Chem. Eng. J. 358 (2019) 955e961. [26] Z.K. Kong, Y. Li, Y.L. Wang, Y.Z. Zhang, K.L. Shen, X. Chu, H.C. Wang, J.Y. Wang, L. Zhan, Chem. Eng. J. 4 (2019) 123697. [27] Y. Zheng, S.S. Zheng, H.G. Xue, H. Pang, J. Mater. Chem. A. 7 (2019) 3469. [28] L. Y. Du, X. Y. Cheng, F. J. Gao, Y. B. Li, Y. F. Bu, Z. Q. Zhang, Q. Wu, L. J. Yang, X. Z. Wang and Z. Hu, 55 (2019) 6365. [29] L.Y. Du, Q. Wu, L.J. Yang, X. Wang, R.C. Che, Z.Y. Lyu, W. Chen, X.Z. Wang, Z. Hu, Nano Energy 57 (2019) 34e40. [30] J.Q. Zhou, H.Q. Ji, J. Liu, T. Qian, C.L. Yan, Energy Storage Mater 22 (2019) 256e264.
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Feather duster liked CeO2 as efficient adsorber host material for advanced lithiumesulfur batteries Baoyun Ye a, *, Chenhe Feng a, Guohao Zhu a, Shuang Wang a, Ali Fakhri b a b
School of Environment and Safety Engineering, North University of China, Taiyuan, Shanxi, 030051, China Science and Research Branch, Islamic Azad University, Tehran, Iran
a r t i c l e i n f o
a b s t r a c t
Article history: Received 12 December 2019 Received in revised form 3 January 2020 Accepted 7 January 2020 Available online 8 January 2020
Many kinds of host materials have been investigated to deal with the issues of the LithiumeSulfur batteries, such as poor electronic conductivity and severe capacity fade during the electrochemical cycles. Among various host materials, metal oxides have become a promising host material due to their strong adsorption ability for the soluble polysulfide. Herein, feather duster liked CeO2 ceramics (FDCO) are synthesized via hydrothermal reaction with high surface area (198 m2 g1). While the sulfur is immersed into the feather duster liked CeO2, the as-prepared FDCO/S composites show superior capacity of 1436 mAh g1 at 0.1 C and a low capacity decay of 0.041% per cycle after 500 cycles at 2 C. The superior electrochemical performance is attributed to the strong chemical bond of polysulfide to CeO2, as well as the mesopores in the CeO2 matrix. © 2020 Elsevier B.V. All rights reserved.
Keywords: Lithium-sulfur batteries Polysulfide Shuttle effect Cycle performance
1. Introduction High energy density and long cycle life rechargeable batteries are greatly important for the electronic vehicles [1e3]. Lithiumsulfur batteries have drawn much attention because of their high specific capacity (1675 mAh g1) and energy density (2600 Wh Kg1), which are according to the redox reaction of sulfur to Li2S via various lithium polysulfides (Li2Sx, x ¼ 2e8) [4e8]. Besides, the sulfur is abundant, low cost and environment friendly [9]. However, the practical use for the lithium-sulfur batteries is hindered by the low sulfur utilization and poor cycle performance even at low current rates [10,11]. This is mainly caused by the low electronic conductivity and migration of soluble polysulfide, which is the socalled “shuttle effect” [12e15]. To deal with these problems, various host materials have been designed to limit the migration of the polysulfide. Especially, carbonaceous materials are mainly used to improve the electrochemical performance via enhancing the electronic conductivity. Whereas, the adsorption between the polar polysulfide and nonpolar carbonaceous materials is physical trapping and/or weak chemical bonding [16,17]. This will limit the adsorption ability between polysulfide and the host materials. Therefore, the
* Corresponding author. E-mail address: [email protected] (B. Ye). https://doi.org/10.1016/j.jallcom.2020.153743 0925-8388/© 2020 Elsevier B.V. All rights reserved.
carbonaceous materials are not likely to meet the requirements for the practical employment of the lithium-sulfur batteries [18]. Besides, great amounts of metal oxides are also attractive host materials due to their strong chemical bonding with polysulfide, such as TiO2 [19], Co3O4 [20], Al2O3 [21]. These metal oxides have been studied to improve the cycle stability via inhibiting the shuttle effect of the soluble polysulfide. Gao et al. firstly used yolk-shell S@TiO2 composites as cathode materials [22]. The as-prepared S@TiO2 yolk-shell structure could keep the structure stability. Moreover, the meatl oxide TiO2 could inhibit the migration of soluble polysulfide. As a result, the as-prepared S@TiO2 yolk-shell composites exhibited superior cycle stability and high specific capacities. Polar CeO2 materials have been widely used for in LieO2 batteries owing to its moderate band gap, multiple valence states [23]. It can still maintain its face cubic structure and the oxygen vacancy could also act as catalytic centers, which could improve the redox kinetics of polysulfide. In this work, feather duster liked CeO2 ceramics (FDCO) are synthesized via hydrothermal reaction with high surface area (198 m2 g1). While the sulfur is immersed into the feather duster liked CeO2, the as-prepared FDCO/S composites show superior capacity and excellent cycle stability. The superior electrochemical performance is attributed to the strong chemical bond of polysulfide to CeO2, as well as the mesopores in the CeO2 matrix.
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2. Experimental 2.1. Preparation of feather duster liked CeO2 nanofibers Typically, 0.2 g of methenamine and 0.8 ml of 0.1 M Ce(NO3)3$6H2O solutions were added into 30 ml ethanol with vigorously stirring for 30 min. Then, the mixture was transferred to a 50 ml sealed container for hydrothermal reaction at 180 C for 12 h. After that, the samples was collected and washed by deionized water for 3 times, and dried at 100 C for 24 h. Then, the obtained products were inserted into the tube furnace and heated at 800 C for 1 h under argon atmosphere with a heating rate of 5 C min1 to obtain the feather duster liked CeO2 materials. 2.2. Preparation of FDCO/S composites FDCO/S composites were obtained by a modified melt-diffusion method. The FDCO was mixed with sulfur with a weight ratio of 1:3. The mixture was heated at 155 C for 12 h in a sealed vial under Ar atmosphere.
a Bruker diffractometer at 40 kV and 40 mA with Cu Ka radiation. The thermogravimetric (TG) analyses were performed on a NETZSCH STA409PC instrument in a nitrogen atmosphere with a temperature ramp of 10 C min1. Scanning electron micrographs were acquired using a JSM-7001F field emission scanning electron microscope (FESEM) with pressure of 105-109. Transmission electron microscopy (TEM) analyses were carried out on a JEM2100F microscope operated at 200 kV coupled with an energy dispersive X-ray spectrometer (EDS). 2.4. Polysulfide adsorption test Li2S6 solution was prepared by mixing sublimed sulfur and lithium sulfidein a molar ratio of 5:1 in 1,3-dioxolane/1,2-dimethoxyethane (DOL/DME, 1:1 by volume and stirred overnight at 60 C. 20 mg FDCO and acetylene black were separately added to 10 ml of Li2S6 solution, respectively, and stirred for 24 h to facilitate adsorption. After that, 2 mL of each of the supernatant solutions was transferred to UVeVis analysis. 2.5. Electrochemical measurements
2.3. Materials characterization X-ray diffraction (XRD) measurements were carried out by using
Electrochemical performance was measured by using coin 2032 half batteries. The as-prepared FDCO/S composite was used as
Fig. 1. (a) and (b) SEM images of the feather duster liked CeO2 nanofibers. (c) SEM image of FDCO/S composites. (d), (e) and (f) Corresponding elemental mapping of O, Ce and S for the FDCO/S composites.
B. Ye et al. / Journal of Alloys and Compounds 823 (2020) 153743
cathode. The mass loading of sulfur in the electrode was about at 3.5 mg cm2. Lithium was used as anode. Celgard 2300 was as the separator, the electrolyte was composed of 1.0 M (LiTFSI) salt in DOL and DME (1:1 vol) with 1 wt% LiNO3 as an additive. Galvanostatic charge-discharge tests were performed at different scan rates in the voltage range from 1.7 to 2.8 V. EIS was conducted in the frequency range of 105e102 Hz with an amplitude of 5 mV characterized by using a CHI 660 E instrument. 3. Results and discussion The morphology of CeO2 nanofibers and FDCO/S composites were observed by using scanning electron microscopy (SEM) and corresponding elemental mapping. Fig. 1a shows the morphology of the CeO2 materials. It can be seen that the as-prepared CeO2 exhibits feather duster liked nanofiber structure. The dimeter of the FDCO is located at 80e90 nm. To clear observe the morphology, large magnification image was made for the FDCO. As shown in Fig. 1b, it can be observed that the surface of the FDCO consists of
3
many feather liked fibers. Fig. 1c displays the SEM image of the FDCO/S composites. The as-prepared FDCO/S composites show similar morphology with that of the FDCO materials. This confirms that the element sulfur was immersed into the FDCO. Fig. 1def shows the corresponding elemental mapping of O, Ce and S for the FDCO/S composites. The elements O, Ce and S are uniformly distributed in the whole FDCO/S composites. To prove the crystal structure, XRD was conducted for the FDCO. As shown in Fig. 2a, the as-prepared FDCO exhibits a typical cubic fluorite structure with Fm3m space group [24]. All of these diffraction peaks could be attributed to related hkl planes. There are no impurity peaks, demonstrating superior crystallinity. To obtain the sulfur content of the FDCO/S composites, TG analysis was conducted for the FDCO/S composites. As shown in Fig. 2b, it can be calculated that the sulfur content in the FDCO/S composites ia about 74.8%. This is significant important for the calculation of the specific capacity of the lithium-sulfur battery. Fig. 3a shows the schematic illustration for the adsorption of polysulfide. The feather duster liked polar CeO2 nanofiber could
Fig. 2. (a) XRD pattern of FDCO. (b) TG curve of the FDCO/S composites.
Fig. 3. (a) Schematic illustration for the adsorption of polysulfide. (b) and (c) Photographs and UVevis spectra of Li2S6 solution with FDCO and AB. (d) N2 adsorption-desorption isotherm of FDCO.
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effectively adsorb the soluble polysulfide, and inhibit the shuttle effect. Besides, the mesoporous CeO2 also could provide weak physical adsorption for the polysulfide. To confirm this effect, FDCO and AB are added into Li2S6 solution to observe the color change with the increase of the time (Fig. 3b). It can be seen that the color of FDCO/Li2S6 mixture becomes colorless after 24 h, showing superior adsorption ability. While for the AB/Li2S6 mixture, it still remains weak yellow. After that, the UVevis spectra of Li2S6 solution with FDCO and AB were conducted. As shown in Fig. 3c, strong absorbance at about 400e500 cm1 could be observed for pure Li2S6 solution. Li2S6 solutions with the FDCO and AB show lower absorbance in 400e500 cm1. This peak is related to the presence of the Li2S6. What is more, the absorbance for the FDCO/Li2S6 mixture is much lower than AB/Li2S6 mixture, demonstrating stronger chemical absorption ability of FDCO. This is beneficial for improving the cycle stability and specific capacity [25]. Therefore, it can be concluded that the as-prepared FDCO has superior adsorption ability for the soluble polysulfide. N2 adsorptionedesorption analysis of FDCO was displayed in Fig. 3d. It indicates that the specific surface area of FDCO is 198 m2 g1. Therefore, the FDCO with high specific surface area could provide high active site and
fast electron transfer when used as cathode for LieS batteries. To further demonstrate the strong adsorption ability of polysulfide by using FDCO, S2p XPS spectra for the pristine Li2S6 and FDCO/Li2S6 mixture was conducted. As shown in Fig. 4a, for the pristine Li2S6, two S 2p3/2 cores at 160.8 and 162.3 eV can be observed, which are related to terminal sulfur (ST1) and bridging sulfur (S0B) respectively. After adding FDCO into the Li2S6 for 12 h, considerable shifting can be observed for the FDCO/Li2S6 mixture, which is corresponding to the reduction of electron cloud density in sulfur atoms as a result of strong chemical interactions between Li2S6 and FDCO [26]. This result is consistent with the previous report [27,28]. Fig. 5a exhibits the theory discharge transition, demonstrating chemical reduction and various products during the discharging process. Initially, the ring S8 molecules are transformed into Li2S8 and Li2S6 at 2.3 V. After that, long chain polysulfides were transformed into the insoluble Li2S2 and Li2S at 2.1 V [29]. Finally, all electrochemical reactions are completed and the theoretical specific capacity of 1675 mAh g1 could be achieved. Fig. 5b shows the initial discharge and charge profiles of the FDCO/S composites and pristine S electrode. It can be clearly seen that the initial capacity of
Fig. 4. High resolution S 2p XPS spectra of (a) pristine Li2S6 and (b) FDCO/Li2S6 mixture.
Fig. 5. (a) The theoretical specific capacity obtained by the discharge curve. (b) The constant discharge and charge profiles of the pristine sulfur and FDCO/S composites electrode at 0.1 C. (c) Rate capability of the pristine sulfur and FDCO/S composite electrode at various current densities from 0.1 C to 2 C. (d) EIS of the pristine sulfur and FDCO/S composite electrode. (e) Long cycle performance of the FDCO/S electrode at 2 C for 500 cycles.
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the FDCO/S composites is as high as 1436 mAh g1 at 0.1 C, which is much higher than the pristine S electrode. Fig. S1 shows the CV curves of the FDCO/S composites. It can be seen that there are two reduction peaks and one oxidation peak at 2.3 V, 2.1 V, 2.4 V, respectively. The two reduction peaks are corresponding to the chemical reaction from sulfur to polysulfide and Li2S. The oxidation peak is related to the inverse reaction from Li2S to pristine sulfur. Fig. 5c displays the rate performance of the FDCO/S and pristine S electrode. The FDCO/S composites show excellent rate capability at various current densities. Even at the high current densities of 2 C, the specific capacity could achieve 965mAh g1, demonstrating superior rate performance. To investigate the electrochemical impedance, EIS was tested for the FDCO/S composite and pristine S electrode [30]. It can be seen that the FDCO/S composites exhibit smaller resistance than the pristine S electrode. Finally, long cycle performance for the FDCO/S composites was tested at 2 C for 500 cycles. The FDCO/S composites exhibits a high initial capacity of 1050 mA h g1 at 2 C and retains about 836 mA h g1 at 2 C after 500 cycles corresponding with a capacity decay of 0.041% per cycle. 4. Conclusions In summary, feather duster liked CeO2 nanofiber ceramics are successfully prepared via hydrothermal reaction with high surface area (198 m2 g1). While the sulfur is immersed into the feather duster liked CeO2, the as-prepared FDCO/S composites show superior capacity of 1436 mAh g1 at 0.1 C and a low capacity decay of 0.041% per cycle after 500 cycles at 2 C. The superior electrochemical performance is attributed to the strong chemical bond of polysulfide to CeO2, as well as the mesopores in the CeO2 matrix. CRediT authorship contribution statement Baoyun Ye: Writing - original draft, Writing - review & editing. Chenhe Feng: Data curation. Guohao Zhu: Formal analysis. Ali Fakhri: Writing - review & editing. Acknowledgement We thank the financial support from the North University of China. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.jallcom.2020.153743.
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Declaration of competing InterestCOI We declare that we have no interest conflict.
References [1] Z.Q. Ye, Y. Jiang, J. Qian, W.L. Li, T. Feng, L. Li, F. Wu, R.J. Chen, Nano Energy 64 (2019) 103965. [2] N. Li, K.H. Chen, S.Y. Chen, F. Wang, D.D. Wang, F.Y. Gan, X. He, Y.C. Huang, Carbon 149 (2019) 564e571. [3] R. Xu, J. Lu, K. Amine, Adv. Energy Mater. 5 (2015) 150048. [4] Y.P. Li, T.Y. Jiang, H. Yang, D. Lei, X.Y. Deng, C. Hao, F.X. Zhang, J.L. Guo, Electrochim. Acta 330 (2020) 135311. [5] J. Li, L. Zhang, F.R. Qin, B. Hong, Q. Xiang, K. Zhang, J. Fang, Y.Q. Lai, J. Power Sources 442 (2019) 227232. [6] M.D. Walle, M.Y. Zhang, L. Zeng, Y.J. Li, Y.N. Liu, Appl. Surf. Sci. 497 (2019) 143773. [7] J. Fang, W.J. Shen, S.H. Cheng, S. Ghashghaie, H.K. Shahzad, C.Y. Chung, J. Power Sources 441 (2019) 227202. [8] L.L. Fan, M. Li, X.F. Li, W. Xiao, Z.W. Chen, J. Lu, Joule 3 (2019) 361e386. [9] H. Cheng, N. Cai, M. Wang, Solid State Ion. 337 (2019) 12e18. [10] L. Wu, Z.F. Wang, C.H. An, G. He, J. Alloy. Comp. 806 (2019) 881e888. [11] B.E. Li, Z.H. Sun, Y. Zhao, Z.S. Zhang, Mater. Lett. 255 (2019) 126529. [12] J.L. Cheong, A.A. Hamid, J.Y. Ying, Nano Energy 66 (2019) 104114. [13] C.L. Wang, L.S. Sun, X.X. Wang, L.M. Wang, Carbon 153 (2019) 691e698. [14] C.Y. Zha, D.H. Wu, T.K. Zhang, J.H. Wu, H.Y. Chen, Energy Storage Mater 17 (2019) 118e125. [15] W. Zhang, J.F. Zhang, Y. Zhao, X. Wang, Mater. Lett. 255 (2019) 126595. [16] B.Y. Hao, H. Li, W. Lv, Y.B. Zhang, S.Z. Niu, Q. Qi, S.J. Xiao, J. Li, F.Y. Kang, Q.H. Yang, Nano Energy 60 (2019) 305e311. [17] J.M. Chabu, K. Zeng, W.S. Chen, A. Mustapha, Y.J. Li, Y.N. Liu, Appl. Surf. Sci. 493 (2019) 533e540. [18] S.P. Li, X. Chen, F. Hu, R. Zeng, Y.H. Huang, L.X. Yuan, J. Xie, Electrochim. Acta 304 (2019) 11e19. [19] H.F. Xu, Y.Z. Shi, S.B. Yang, B. Li, J. Power Sources 430 (2019) 210e217. [20] S. Lin, Y.R. Cai, F.X. Ruan, J. Wu, B.D. Sireesh, X. Yao, J.K. Gao, J.M. Yao, J. Alloy. Comp. 779 (2019) 412e419. [21] J.H. Kim, D.J. Byun, H.S. Kim, W.C. Choi, S.O. Kim, J. Power Sources 427 (2019) 165e173. [22] X.N. Gao, X.Y. Wang, Z. Yang, Y.H. Shen, A.J. Xie, Appl. Surf. Sci. 475 (2019) 135. [23] W.T. Qi, W. Jiang, F. Xu, J.B. Jia, C. Yang, B.Q. Cao, Chem. Eng. J. 382 (2020) 122852. [24] Z.S. Han, F.J. Kong, X.L. He, S. Tao, X.F. Jiang, B. Qian, J. Phys. Chem. Solids 134 (2019) 187e192. [25] X.L. Lu, Q.F. Zhang, J. Wang, S.H. Chen, J.M. Ge, Z.M. Liu, L.L. Wang, H.B. Ding, D.C. Gong, H.G. Yang, X.Z. Yu, J. Zhu, B.G. Lu, Chem. Eng. J. 358 (2019) 955e961. [26] Z.K. Kong, Y. Li, Y.L. Wang, Y.Z. Zhang, K.L. Shen, X. Chu, H.C. Wang, J.Y. Wang, L. Zhan, Chem. Eng. J. 4 (2019) 123697. [27] Y. Zheng, S.S. Zheng, H.G. Xue, H. Pang, J. Mater. Chem. A. 7 (2019) 3469. [28] L. Y. Du, X. Y. Cheng, F. J. Gao, Y. B. Li, Y. F. Bu, Z. Q. Zhang, Q. Wu, L. J. Yang, X. Z. Wang and Z. Hu, 55 (2019) 6365. [29] L.Y. Du, Q. Wu, L.J. Yang, X. Wang, R.C. Che, Z.Y. Lyu, W. Chen, X.Z. Wang, Z. Hu, Nano Energy 57 (2019) 34e40. [30] J.Q. Zhou, H.Q. Ji, J. Liu, T. Qian, C.L. Yan, Energy Storage Mater 22 (2019) 256e264.