Polysulfides Capture-Copper Additive for Long Cycle Life Lithium

Oct 18, 2016 - Copper powder was introduced into the lithium sulfur battery system to capture intermediate polysulfides and CuxS (x = 1 or 2) species ...
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Polysulfides Capture-Copper Additive for Long Cycle Life Lithium Sulfur Batteries Lei Jia,† Tianpin Wu,‡ Jun Lu,§ Lu Ma,§ Wentao Zhu,† and Xinping Qiu*,† †

Key Laboratory of Organic Optoelectronics and Molecular Engineering, Department of Chemistry, Tsinghua University, Beijing 100084, China ‡ X-ray Science Division and §Chemical Sciences and Engineering Division, Argonne National Laboratory, Argonne, Illinois 60439, United States S Supporting Information *

ABSTRACT: Copper powder was introduced into the lithium sulfur battery system to capture intermediate polysulfides and CuxS (x = 1 or 2) species was generated depending on the chain length of polysulfides. This phenomenon was verified by X-ray absorption near edge structure technique. The results indicated that copper can be oxidized to CuS by Li2Sx (x ≥ 6), and a mixture of Cu2S and CuS was obtained when x ranges from 3 to 6. While Cu2S is eventually formed in the presence of Li2S3. After several cycles activation, the polysulfide-shuttle effect and self-discharge phenomenon which hinder the application of lithium sulfur batteries are found nearly eliminated Further experiments demonstrated that in the case of Cu2S generation, a high specific sulfur capacity of 1300 mAh g−1 could be delivered, corresponding to 77.6% sulfur utilization, while the Coulombic efficiency approximates around 100%. Self-discharge experiment further demonstrated that polysulfides almost disappear in the electrolyte, which verified the polysulfide-capture capability of copper. KEYWORDS: sulfur utilization, polysulfides capture, shuttle effect free, in situ generation, Coulombic efficiency, lithium sulfur battery

1. INTRODUCTION Sulfur, as a new energy storage material, possesses a high specific capacity of 1675 mAh g−1, which is 4−5 times greater than that of conventional Li-ion battery materials based on insertion/ extraction reactions.1,2 Moreover, sulfur is cheap, environmental friendly, and abundant, which make the lithium sulfur battery the most promising next-generation energy storage system.3−6 However, the electrochemical reaction between sulfur and metal lithium suffers from two serious problems that hinder its commercial application.7,8 It is the intrinsic insulating characteristic of sulfur and the shuttle phenomenon that decrease the utilization of sulfur and seriously impact the battery Coulombic efficiency. To commercialize the lithium sulfur (Li−S) battery, extensive approaches have been developed, such as design novel sulfur host materials,9,10 introducing electrolyte additives11 and electrode surface modification.12−14 It is well-known that the redox reaction in the Li−S battery is complicated and its mechanism is still not sufficiently understood. Element sulfur (S8), undergoes a multiple-electron reduction process to the final product Li2S. Sulfur will electrochemically be reduced gradually and generate polysulfides with different length chains, depending on the degree of discharge.15−17 Those intermediate polysulfides are soluble and could move back and forth between anode and cathode electrodes, causing degradation in cycle performance and Coulombic efficiency. The sulfur utilization and the battery © 2016 American Chemical Society

Coulombic efficiency could be improved simultaneously if polysulfide dissolution and diffusion could be restrained. In the discharge−charge process, polysulfides would be generated through coexisted direct electrochemical and pure chemical reaction. In order to investigate the discharge mechanism in detail, fundamental research has been carried out by using in situ or ex situ analytical methods such as Raman,18 UV/vis,19 HPLC ESI/MS,20 and CV.21 Based on abundant research results, it was a widely accepted concept that S82− and S62− were the major species generated in the first reduction process. However, Qu’s report recently gave a much different conclusion that S42− and S52− polysulfides were mainly formed at the first reduction wave by using in situ derivation with HPLC.22 For the first time, derivatization reagent methyl triflate was used to immobilize the newly generated polysulfides for analysis. Moreover, it was demonstrated that element sulfur could react with S2− and various polysulfides (Sn2−, n = 2−9) could be produced.20 Otherwise, chemical and disproportionation reactions also exist among various polysulfides, such as23 Received: August 18, 2016 Accepted: October 18, 2016 Published: October 18, 2016 30248

DOI: 10.1021/acsami.6b10366 ACS Appl. Mater. Interfaces 2016, 8, 30248−30255

Research Article

ACS Applied Materials & Interfaces

Figure 1. TEM images of template Al2O3 (a), CaCO3 (b), obtained porous carbon (c, d), and PC-S composites (e, f); SEM images of PC-S composites (g) and the corresponding elemental mapping images of sulfur (h). drying process, the pristine porous carbon product was produced. This carbon powder was refluxed in concentrated nitric acid at 50 °C for 8 h, then centrifuged and washed again, and finally soaked in stronger ammonia−water for another 12 h, and the obtained final porous carbon was denoted as PC. 2.2. Sulfur Impregnation. The mixture powder of PC and sulfur with a mass ratio of 1:2 was heated to 155 °C for 30 min in the tube furnace under the H2/Ar (5:95) atmosphere. After maintaining these conditions for 5 h, the temperature was increased to 180 °C in 30 min and maintained for another 1 h to remove the nonadsorbed sulfur; meanwhile, the atmosphere was changed to flow at the rate of 50 mL min−1. The sulfur content in the composite was calculated according to the mass loss of the mixture. The sulfur impregnated PC were marked as PC-S. In this work, the sulfur mass percent was about 60%. 2.3. Battery Assembly. A total of 2025 coin cells were assembled with lithium foil as the counter electrode in an Ar-filled glovebox with moisture and oxygen contents below 1 ppm. The mass ratio of sulfur contained composite material, copper powder (morphology can be seen in Figure S1 in Supporting Information, and the size of the copper particle is about 1 μm), PVDF, and conducting carbon black (Super P) in the slurry was 26:64:5:5 (the mole ratio of Cu and S slightly exceeded 2:1), which can make sure the sulfur would be totally transferred to Cu2S. Slight copper as additives added in the composite was proved to have beneficial effect on the battery performance; however, the polarization was severe and the energy density was low.24 In this case, the mass loading of sulfur in the working electrode was about 1.3 mg cm−2. The electrolyte was composed of 1.0 mol l−1 lithium bistrifluoromethanesulfonylimide (LiTFSI) in a mixed solvent of 1, 3dioxolane and 1, 2-dimethoxyethane (DOL/DME, 1:1, v/v). The galvanostatic tests were performed with a relaxation period of 1 min at the end of each charge/discharge process. 2.4. Structure Characterization. Morphologies were observed by transmission electron microscopy (TEM, Hitachi H-7650B) and scanning electron microscopy (SEM, HITACHI S-5500) operated at 80 and 15 kV, respectively. XRD spectra were collected using an X-ray diffractometer (Rigaku D8 Advance) with Cu Kα radiation source (0.15417 nm), scanned at a rate of 8° min−1, with a step of 0.02°, in the range of 10° and 80°. Raman spectra were recorded on a HORIBA Jobin Yvon HR Evolution spectrometer with He−Ne 532 nm laser excitation in the range of 200−2000 cm−1. ICP measurements were carried out using the ESCALAB250 version made in company THERMO in U.S.A. IR measurement was carried out on German Horiba Bruker Infrared Spectrometer in the wavenumber range of 500−4000 cm−1. Sulfur Kedge XANES data were collected at the Advanced Photon Source (APS) on the beamline 9-BM-B with electron energy of 7 GeV and average current of 100 mA. The radiation was monochromatized by a Si (111)

3S2 2 − ↔ S4 2 − + 2S2 −, 2S6 2 − ↔ S52 − + S7 2 −, 2S4 2 − ↔ S52 − + S32 −

It can be speculated that a complicated connection existed between the various pure chemical reactions and direct electrochemical reactions. And obviously, any factors affecting the chemical reactions would also influence the electrochemical process and the battery performance. Then strategies changing the chemical routes would be effective alternative choices to affect and propel the Li−S battery cycle performance. Therefore, just like the derivatization reagent, some additives with the capability of reacting with various polysulfides could be added into the cathode electrode. Under ideal conditions, the shuttle effect due to the diffusion and migration of the polysulfides would be eliminated and electrochemical performance would be improved obviously. In this study, copper powder acting as polysulfides capture was introduced into the Li−S battery system to eliminate the shuttle phenomenon and propel the Coulombic efficiency. The reaction mechanism between copper and different polysulfides was studied by XANES (X-ray absorption near edge structure) technique. The results indicated that copper can be oxidized to Cu2S by Li2S3 and to CuS by Li2Sx(x ≥ 6), respectively. When the x in Li2Sx ranges from 3 to 6, the product is a mixture of Cu2S and CuS. It was estimated that with discharge proceeds, the length of the polysulfide anions became shorter18 and the Cu2S became the final stable product. After the Cu2S becomes the final active material, the battery exhibits high electrochemical performance with a sulfur utilization of 77.6% and maintains approximately 100% Coulombic efficiency.

2. EXPERIMENTAL SECTION 2.1. Synthesis of Porous Carbon. Nanostructure aluminum oxide, calcium carbonate, and sucrose were added in water in the weight ratio of 1:2:3; after the sucrose was completely dissolved, the above turbid solution was dried at 80 °C under magnetic stirring. The obtained product was baked in an oven for 12 h at 180 °C. Finally, the brown sample was carbonized in a tube furnace for 8 h at 900 °C under a flow atmosphere (H2/Ar, 5:95 in V/V); the flow rate was 50 mL min−1. The thermal decomposition product calcium oxide from the template calcium carbonate was removed by stirring in 4.5 mol l−1 HCl solution for 12 h; successively aluminum oxide was removed in 10 mol L−1 NaOH solution at 85 °C for 24 h; and after centrifuging, washing, and 30249

DOI: 10.1021/acsami.6b10366 ACS Appl. Mater. Interfaces 2016, 8, 30248−30255

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ACS Applied Materials & Interfaces

Figure 2. XRD pattern (a, b) and Raman spectra (c, d) of pristine PC, PC, PC-S, and pure sulfur; IR spectroscopy (e) of pristine PC and PC. double-crystal monochromator. Harmomic rejection was accomplished with an Rh-coated mirror. The samples were measured in a He-purged sample chamber and data were collected in fluorescence mode using a four-element Vortex Si-Drift detector. Sodium thiosulfate was used for energy calibration by setting the Gaussian fitted first peak position at 2469.2 eV.

with different chain lengths. During the carbonization process, the carbon dioxide generated from the thermal decomposition of CaCO3 would break the pore wall and make the pore structure more complex. It can be seen from Figure 1c that the morphology of the PC seemed like foam with a pore size of about 100 nm, and because of the gas release effect, the pore was fractured. Moreover, as shown in Figure 1d, inside the carbon wall there exist several nanometers pores supposed to come from the template Al2O3. Figure 1e,f shows the TEM images of PC-S composite; it can be seen from the visual field that sulfur was distributed homogeneously within the porous carbon, indicating that sulfur had sufficient contact with the carbon matrix surface whether inside the pore or adhere to the outer wall. The

3. RESULTS AND DISCUSSION To understand the morphology of the obtained materials, TEM and SEM techniques were operated. As shown in Figure 1a,b, it can be indicated that the particle size of the Al2O3 and CaCO3 were about 100−200 nm and several nanometers, respectively. The mixture of templates would endow the porous carbon sufficient pore sizes, which is beneficial for absorbing polysulfides 30250

DOI: 10.1021/acsami.6b10366 ACS Appl. Mater. Interfaces 2016, 8, 30248−30255

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ACS Applied Materials & Interfaces

Figure 3. Charge−discharge curves and cycle performances of PC-S (a, b).

Figure 4. Self-discharge experiment results at the rate of 0.1C: a 48 h rest during the charge (a) and discharge (b) process; a 6 day rest after fully discharged (c); and 15 day rest after fully charged (d); the tests were carried out at room temperature.

about 1580 and 1710 cm−1 could be attributed to the relatively higher content of carboxylic ammonium salt on PC compared to pristine PC. Galvanostatic tests were carried out at a rate of 0.1 C based on the weight of sulfur. Figure 3 shows the charge−discharge plateaus and cycle stability of the PC-S. From Figure 3a it can be seen that the discharge profiles exist two voltage plateaus at about 2.1 and 1.7 V. While the charge voltages lie at about 1.85 and 2.3 V, respectively, much different from the typical lithium sulfur battery. For lithium sulfur batteries, it is well know that two discharge plateaus at about 2.3 and 2.1 V were usually exhibited, and the charge voltage lies at about 2.3 V.25 However, when the copper powder was added into the electrode slurry, the voltage plateau changed a lot, denoting different electrochemical reactions. With cycle proceeds, the discharge plateau at 2.1 V and charge plateau at 2.3 V disappeared gradually, as shown in Figure 3a. After about 30 cycles there nearly just existed about discharge plateau at 1.7 V and a charge plateau at 1.85 V, and the plateau was very flat and stable. The PC-S composites exhibited high initial specific capacity about 1500 mAh g−1. Figure 3b

morphology of PC-S and its corresponding sulfur elemental mapping are shown in Figure 1g,h; the TEM and element mapping images confirmed that the sulfur distribution was uniform and homogeneous. XRD and Raman measurements were operated and the results are shown in Figure 2a−d. Comparing Figure 2a and b, the XRD characteristic peaks of the pure S phase could not be detected in the PC-S composite, and after treated by nitric acid and ammonium water in succession, the carbon XRD changed a little. The Raman results were in good agreement with the XRD pattern that the sulfur Raman shift peak in the PC-S composite also disappeared. According to the XRD and Raman results, a uniform sulfur distribution was further confirmed. Fourier Transform Infrared Spectroscopy (FTIR) was carried out to detect the nitric acid−ammonia−water treatment effect on the porous carbon. As shown in Figure 2e, the wavenumber at 3298.38 cm−1 of PC or 3305.01 cm−1 of pristine PC denotes the existence of carboxyl groups, and the weak transmission peak 3733.10 cm−1 indicated that hydroxyl groups were generated on the PC carbon surface. The relative stronger peak intensity at 30251

DOI: 10.1021/acsami.6b10366 ACS Appl. Mater. Interfaces 2016, 8, 30248−30255

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ACS Applied Materials & Interfaces

Figure 5. XANES spectra of S K-edge for Cu−Li2Sx (a) and Li2Sx solution (b).

Figure 6. Identification of the products from the reactions between Cu and different Li2Sx solutions (x = 3, 4, 5, 6, 8, 9) corresponding to (a)−(f).

Self-discharge experiments were accomplished to demonstrate the complete transformation from sulfur to Cu2Sx. It is well acknowledged that, in lithium sulfur batteries, there exist

shows the cycle stability of PC-S composite. The PC-S exhibited a stable specific capacity of 1300 mAh g−1 and the Coulombic efficiencies approached 100%. 30252

DOI: 10.1021/acsami.6b10366 ACS Appl. Mater. Interfaces 2016, 8, 30248−30255

Research Article

ACS Applied Materials & Interfaces

Figure 7. Schematic illustration for the transformation from sulfur to Cu2S: (a) discharge intermediate polysulfides can react with copper; (b) CuxS (x = 1 or 2) species is generated depending on the chain length of polysulfides; (c) The length of the polysulfide anions is shortened and only Cu2S forms during further discharge.

The copper foil samples were then sealed in Al bags to be kept in an Ar atmosphere before being transferred out of glovebox. The samples were put into the He-purged sample chambers immediately after being taken out of the Al bag for XANES measurement. As shown in Figure 5a, with the x in Li2Sx increases, the peak intensity located about 2469 eV becomes stronger. Compared with pure Li2Sx solutions, whose XANES measurement results were shown in Figure 5b, the 2468 eV peak and the 2470 eV peak were just found. Those obvious differences from the spectra of Cu−Li2Sx products and the corresponding Li2Sx solution indicated that the 2469 eV peak was not originated from the Li2Sx solutions, but from the new species due to the existence of copper. In order to distinguish the products, the comparison with standard CuS and Cu2S spectra was carried out, as shown in Figure 6. The blue and pink curves underneath every image belong to the standard Cu2S and CuS, respectively. It can be seen that the sharp 2469 eV peak belonged to CuS, and the broad 2472 eV peak with a shoulder belonged to Cu2S. And it can be found from Figure 6a that when reacted with Li2S3 solution, Cu was oxidized to Cu2S. When the solution was changed to Li2Sx (x = 6, 8, 9) the product became CuS and the results are shown in Figure 6d−f. When the x in Li2Sx was 4 or 5 as shown in Figure 6b,c, a mixture product of Cu2S and CuS were detected. In conclusion, copper could be oxidized to Cu2S by Li2S3, when x in Li2Sx is greater than 6, all copper was oxidized to CuS. As x ranged from 3 to 6, a mixture of Cu2S and CuS were produced and the amount of CuS also got increased with the x in Li2Sx. This schematic of transformation process from sulfur to Cu2S is illustrated in Figure 7. ICP (Inductively Coupled Plasma) tests were also carried out to detect the interaction between copper and the electrolyte (1 M LiTFSI in DME/DOL) currently used in lithium sulfur battery; the results indicated that electrode can corrode Cu and release copper ions or cuprous ions with a concentration of 611.5 ng/100 μL. Thus, the element copper in the final Cu2S species could come from the oxidation and the corrosion by polysulfides and electrolyte, respectively, which will accelerate the transformation from sulfur to Cu2S simultaneously.

discharge intermediate product polysulfides that will dissolute into the electrolyte and diffuse to the metal lithium.26,27 In the shelf life of a lithium sulfur battery, a certain amount of polysulfides will stay in the electrolyte, which will move back and forth between the positive and negative electrode and cause a low Coulombic efficiency. Moreover, the polysulfides will react with metal lithium to produce electrochemical inert substances (Li2S and Li2S2), causing poor cycle stability and serious self-discharge phenomenon. However, in our case, element sulfur all existed in the Cu2S form, and the polysulfides will not be generated. As a result, the shuttle and the self-discharge phenomenon will be eliminated at the same time. All the self-discharge tests were carried out on the same coin cell with copper foil as the current collector, while the slurry just contains sulfur-contained composite, Super P, and PVDF with a ratio of 8:1:1. Copper foil had been demonstrated owning a similar effect to copper powder on the transformation from sulfur to Cu2S.28 As shown in Figure 4a, in the 27th cycle, the cell was charged part-filled, the cell was allowed to rest for 48 h without any operation, and after that, the cell was continued to cycle. The two separated charge values were 586.13 and 288.54 mAh g−1, respectively. The subsequent discharge specific capacity was about 867.83 mAh g−1; therefore, the corresponding Coulombic efficiency of this cycle was about 99.22%. Similarly, in the discharge process, as shown in Figure 4b, among the 37th cycle after a specific capacity of 585.97 mAh g−1 was let out of the cell, the same shelving time of 48 h was introduced; subsequently, the cell could still have a discharge specific capacity of 273.71 mAh g−1, and the prior charge capacity was 868.86 mAh g−1, exhibiting a high Coulombic efficiency of about 98.94%. As exhibited in Figure 4c,d, when the rest time was extended to 6 or 15 days, the battery cycle ability remained satisfying, and the Coulombic efficiency still approached 100%. The results demonstrated that no polysulfides would be generated in the cell when sulfur was transferred to Cu2S. The XANES technique was carried out to identify products generated from the reaction of copper and different lithium polysulfide solutions. The energy positions of the sulfur K-edge are characteristic of each compound, such as CuS and Cu2S, and can be ascribed to the transitions of the sulfur 1s core electrons to the unoccupied p-type molecular orbitals, which can be used to identify element sulfur contained compounds.29 For the preparation of Li2Sx solution, S and Li2S powder were dissolved in DME with various specific ratios in the glovebox. A specific polysulfide anion solution is impossible to prepare because of the disproportionation in equilibrium,30,31 so in our work, Li2Sx solution stands for an average value in atom lithium and sulfur by mixing S and Li2S powder by the particle stoichiometric proportions in the solvent DME. After soaking in Li2Sx solution for about 30 min, the copper foils were taken out from the solutions, dried, and sealed by Mylar film on the sample holder.

4. CONCLUSION In summary, copper powder as the polysulfides capture was introduced in the electrode slurry to transform sulfur to Cu2S species. The transformation mechanism is studied by using the XANES and ICP technique. In the discharge process, Cu2S and CuS were generated depending on the length of the polysulfides. In addition, the electrolyte will also corrode the copper to release Cu2+/Cu+ ions coupled with the oxide reaction, the Cu2+/Cu+ ions would get increased as the cycle undergoes. Finally, Cu2S as the active material exhibits good electrochemical performance, high sulfur utilization of 77.6% and approximately 100% Coulombic efficiency are obtained. Self-discharge experiment 30253

DOI: 10.1021/acsami.6b10366 ACS Appl. Mater. Interfaces 2016, 8, 30248−30255

Research Article

ACS Applied Materials & Interfaces

(11) Zhou, G.; Li, L.; Wang, D.-W.; Shan, X.-Y.; Pei, S.; Li, F.; Cheng, H.-M. A Flexible Sulfur-Graphene-Polypropylene Separator Integrated Electrode for Advanced Li-S Batteries. Adv. Mater. 2015, 27, 641−647. (12) Zhou, W. D.; Chen, H.; Yu, Y. C.; Wang, D. L.; Cui, Z. M.; DiSalvo, F. J.; Abruna, H. D. Amylopectin Wrapped Graphene Oxide/ Sulfur for Improved Cyclability of Lithium-Sulfur Battery. ACS Nano 2013, 7, 8801−8808. (13) Yu, M.; Ma, J.; Song, H.; Wang, A.; Tian, F.; Wang, Y.; Qiu, H.; Wang, R. Atomic Layer Deposited TiO2 on a Nitrogen-Doped Graphene/Sulfur Electrode for High Performance Lithium-Sulfur Batteries. Energy Environ. Sci. 2016, 9, 1495−1503. (14) Yu, M.; Wang, A. J.; Tian, F. Y.; Song, H. Q.; Wang, Y. S.; Li, C.; Hong, J. D.; Shi, G. Dual-Protection of a Graphene-Sulfur Composite by a Compact Graphene Skin and an Atomic Layer Deposited Oxide Coating for a Lithium-Sulfur. Nanoscale 2015, 7, 5292−5298. (15) Wang, D.-W.; Zeng, Q.; Zhou, G.; Yin, L.; Li, F.; Cheng, H.-M.; Gentle, L. R.; Lu, G. Q. M. Carbon-Sulfur Composites for Li-S Batteries: Status and Prospects. J. Mater. Chem. A 2013, 1, 9382−9394. (16) Bresser, D.; Passerini, S.; Scrosati, B. Recent Progress and Remaining Challenges in Sulfur-Based Lithium Secondary Batteries - a Review. Chem. Commun. 2013, 49, 10545−10562. (17) Manthiram, A.; Fu, Y.; Chung, S.-H.; Zu, C.; Su, Y.-S. Rechargeable Lithium-Sulfur Batteries. Chem. Rev. 2014, 114, 11751− 11787. (18) Chen, J.-J.; Yuan, R.-M.; Feng, J.-M.; Zhang, Q.; Huang, J.-X.; Fu, G.; Zheng, M.-S.; Ren, B.; Dong, Q.-F. Conductive Lewis Base Matrix to Recover the Missing Link of Li2S8 during the Sulfur Redox Cycle in Li-S Battery. Chem. Mater. 2015, 27, 2048−2055. (19) Patel, M. U. M.; Cakan, R. D.; Morcrette, M.; Tarascon, J. M.; Gaberscel, M.; Dominko, R. Li-S Battery Analyzed by UV/Vis in Operando Mode. ChemSusChem 2013, 6, 1177−1181. (20) Zheng, D.; Qu, D.; Yang, X. Q.; Yu, X.; Lee, H. S.; Qu, D. Quantitative and Qualitative Determination of Polysulfide Species in the Electrolyte of a Lithium-Sulfur Battery using HPLC ESI/MS with OneStep Derivatization. Adv. Energy Mater. 2015, 5, 1401888. (21) Guillanton, G. L.; Do, Q. T.; Elothmani, D. Determination of Mixtures of Polysulfides by Cyclic Voltammetry. J. Electrochem. Soc. 1996, 143, L223−L225. (22) Zheng, D.; Zhang, X.; Wang, J.; Qu, D.; Yang, X.; Qu, D. Reduction Mechanism of Sulfur in Lithium-Sulfur Battery: From Elemental Sulfur to Polysulfide. J. Power Sources 2016, 301, 312−316. (23) Wild, M.; O’Neill, L.; Zhang, T.; Purkayastha, R.; Minton, G.; Marinescu, M.; Offer, G. J. Lithium Sulfur Batteries, a Mechanistic Review. Energy Environ. Sci. 2015, 8, 3477−3494. (24) Zheng, S.; Yi, F.; Li, Z.; Zhu, Y.; Xu, Y.; Luo, C.; Yang, J.; Wang, C. Copper-Stabilized Sulfur-Microporous Carbon Cathodes for Li-S Batteries. Adv. Funct. Mater. 2014, 24, 4156−4163. (25) Xiao, Z.; Yang, Z.; Wang, L.; Nie, H.; Zhong, M. E.; Lai, Q.; Xu, X.; Zhang, L.; Huang, S. A Lightweight TiO2/Graphene Interlayer, Applied as a Highly Effective Polysulfide Absorbent for Fast, Long-Life LithiumSulfur Batteries. Adv. Mater. 2015, 27, 2891−2898. (26) Thieme, S.; Bruckner, J.; Meier, A.; Bauer, I.; Gruber, K.; Kaspar, J.; Helmer, A.; Althues, H.; Schmuck, M.; Kaskel, S. A Lithium-Sulfur Full Cell with Ultralong Cycle Life: Influence of Cathode Structure and Polysulfide Additive. J. Mater. Chem. A 2015, 3, 3808−3820. (27) Song, J. X.; Gordin, M. L.; Xu, T.; Chen, S. R.; Yu, Z. X.; Sohn, H.; Lu, J.; Ren, Y.; Duan, Y. H.; Wang, D. H. Strong Lithium Polysulfide Chemisorption on Electroactive Sites of Nitrogen-Doped Carbon Composites For High-Performance Lithium-Sulfur Battery Cathodes. Angew. Chem., Int. Ed. 2015, 54, 4325−4329. (28) Fei, H.; Wen-Cui, L.; Duo, L.; An-Hui, L. In Situ Electrochemical Generation of Mesostructured Cu2S/C Composite for Enhanced Lithium Storage: Mechanism and Material Properties. ChemElectroChem 2014, 1, 733−740. (29) Patel, M. U. M.; Arcon, I.; Aquilanti, G.; Stievano, L.; Mali, G.; Dominko, R. X-ray Absorption Near-Edge Structure and Nuclear Magnetic Resonance Study of the Lithium-Sulfur Battery and its Components. ChemPhysChem 2014, 15, 894−904.

demonstrates that polysulfides never exist in the battery after the fully transformation. After the above transformation the discharge−charge voltage lies at the stable and flat plateau about 1.7−1.85 V, which is a little lower than the lithium sulfur battery working voltage; however, the sulfur utilization and cycle stability are greatly enhanced. Thus, this simple strategy is a promising avenue to utilize the cheap, environment friendly, and abundant sulfur.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b10366. SEM images of the copper powder (PDF).

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Fax: +86-10-62794234. Tel.: +86-10-62794234. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors appreciate the support from National Key Project on Basic Research (2015CB251104), China-US Electric Vehicle Project (S2016G9004), National Natural Science Foundation of China (51361130151), and Beijing Science Foundation (2120001). Use of the Advanced Photon Source, an Office of Science User Facility operated for the U.S. Department of Energy (DOE) Office of Science by Argonne National Laboratory, was supported by the U.S. DOE under Contract No. DE-AC0206CH11357.

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REFERENCES

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