Carbon Disulfide Cosolvent Electrolytes for High-Performance Lithium

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Carbon Disulfide Cosolvent Electrolytes for High-Performance Lithium Sulfur Batteries Sui Gu, Zhaoyin Wen, Rong Qian, Jun Jin, Qingsong Wang, Meifen Wu, and Shangjun Zhuo ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b11619 • Publication Date (Web): 22 Nov 2016 Downloaded from http://pubs.acs.org on November 28, 2016

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ACS Applied Materials & Interfaces

Carbon Disulfide Cosolvent Electrolytes for HighPerformance Lithium Sulfur Batteries Sui Gu,a,b,c Zhaoyin Wen,a,* Rong Qian, b,* Jun Jin, a Qingsong Wang, a,cMeifen Wu,a Shangjun Zhuo b a

CAS Key Laboratory of Materials for Energy Conversion, Shanghai Institute of Ceramics,

Chinese Academy of Sciences, Shanghai 200050, P. R. China. b

National Center for Inorganic Mass Spectrometry in Shanghai, Shanghai Institute of Ceramics,

Chinese Academy of Sciences, Shanghai 200050, P. R. China. c

University of Chinese Academy of Sciences, Chinese Academy of Sciences, Shanghai 200050,

P. R. China.

ABSTRACT: Development of lithium sulfur (Li-S) batteries with high coulombic efficiency and long cycle stability remains challenging due to the dissolution and shuttle of polysulfides in electrolyte. Here, a novel additive, carbon disulfide (CS2), to the organic electrolyte is reported to improve the cycling performance of Li-S batteries. The cells with the CS2-additive electrolyte exhibit high coulombic efficiency and long cycle stability, showing average coulombic efficiency ＞99% and a capacity retention of 88% over entire 300 cycles. The function of the CS2 additive is two-fold: 1) It inhibits the migration of long-chain polysulfides to anode by

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forming complexes with polysulfides, and 2) It passivates electrode surfaces by inducing the protective coatings on both anode and cathode.

KEYWORDS: carbon disulfide, electrolyte, additive, SEI, lithium sulfur battery

INTRODUCTION Rechargeable lithium batteries with long cycle life and high energy density are in great market demand for energy storage applications. The lithium sulfur (Li-S) battery is attracting intense interest because of its high theoretical capacity (1675 mAh g-1) and large energy density (2600 Wh kg-1).1 Combing the other attractive features, such as large abundance, low cost and nontoxic nature, Li-S batteries turn out to be one of the most promising next-generation energy storage systems, especially for large scale applications.2-3 Despite above considerable advantages, several technical bottlenecks related to sulfur chemistry are to be overcome: 1) The intermediate products of sulfur reduction (polysulfides) dissolve in ether-based electrolytes and shuttle between electrodes, leading to the loss of active sulfur and low coulombic efficiency.4 The polysulfides shuttle also causes serious corrosion reactions on lithium surface.5 2) The intrinsic electrical insulating nature of sulfur and the discharge products Li2S/Li2S2 can result in large polarization of cells during operation.6 Moreover, the inhomogeneous aggregation of sulfur species together with their insulating nature also brings about "dead" sulfur. The rapid capacity decay in Li-S batteries is closely associated with the shuttle of polysulfides. Various strategies have been employed to avoid or suppress the shuttle, such as confinement of sulfur within porous carbons7-11, employment of polymer templates12-15 or metal oxides16-18 through physical or chemical absorption, physical barriers19-20 to prevent the contact between


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polysulfides and anodes, and modification of SEI (solid electrolyte interphase) films21-22 to protect lithium from corrosion reaction. However, any one of the known absorption approaches in encapsulating polysulfides provides limited success only. Once the polysufides start dissolving in electrolyte, the shuttle between electrodes is inevitable. Besides, such approaches including physical barriers generally decrease the energy density and increase the complexity of cells fabrication. In-situ generation of protective coatings on anodes can be an effective method in eliminating the polysulfides shuttle by preventing the contact of sulfur species from anode, consequently avoiding the shuttle and corrosion reactions. For instance, LiNO3 is widely used as an additive or co-salt in electrolytes to form a protective film on anodes,23 and P2S5 can passivate the lithium surface with a Li3PS4-based SEI.24 Improved cycle performance and coulombic efficiency are confirmed in Li-S cells with these additives. However, such approaches, in general singe additive in Li-S batteries, do not yet provide satisfactory improvement due to insufficient toughness of the formed passivate films on anodes. As a result, long cycle stability still needs significant improvement. Herein, we report the successful use of CS2 added into electrolytes to induce formation of insitu protective coatings not only on anode but also on cathodes with high coating uniformity. CS2 has been regarded as a surface passivation agent on the graphite for lithium ion batteries25. In LiS cells, CS2 is observed to enable the formation of complexes with polysulfides, which is also confirmed by Gewirth's work.26 These complexes are insoluble in ether-based solvents and alter the sulfur reduction mechanism. We propose that CS2 serves two major functions in the Li-S batteries: it inhibits the migration of long-chain polysulfides to anode by forming complexes and also passivates electrode surfaces by inducing the protective coatings. In addition, CS2 can


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partially dissolve S8 on cathode, and activate the insulated "dead" sulfur, therefore finally facilitate better battery performance.

EXPERIMENTAL SECTION Preparation of electrolytes: The lithium salts were a mixture of Lithium bis (trifluoromethanesulfonyl) imide (LiTFSI) (99.99%, Sigma Aldrich) and LiNO3 (99%, Sigma Aldrich) with desired concentration in solvent. The solvent was a mixture of 1, 2dimethoxyethane (DME) and 1, 3-dioxolane (DOL) (v/v = 1:1) (99.99%, Sigma Aldrich). Two types of electrolytes were used in this work, including 1 M LiTFSI in DOL/DME with 2 wt% LiNO3 as reference and 1 M LiTFSI in DOL/DME with 2 wt% LiNO3 and 40 wt% CS2 (99%, Sigma Aldrich) for study. The amount of liquid electrolyte used in the cells was 40 ul. Synthesis of polysulfides：：Sulfur (99.5%, Sigma Aldrich) and lithium sulfide (Li2S) (99.9%, Sigma Aldrich) were mixed together in proper stoichiometric ratio in DME and then magnetically stirred for 24 h at 60 °C. The calculated concentration of polysulfides in DME was 0.1 mol L-1. All processes were operated in a glove box (content of oxygen and water less than 1 ppm). Electrochemical measurements：： CR2025-type coin cells were assembled in a glove box with oxygen and water contents less than 1 ppm. For cathode, slurry was prepared by mixing 80 wt% S/KB (Ketjen Black) (w/w=6:4), 10 wt% acetylene black, 5 wt% styrene-butadiene rubber and 5 wt% carboxymethyl cellulose and ball milling (with a proper amount of deionized water). Then the uniform slurry was cast onto an aluminum foil substrate and further dried at 60 ºC under vacuum. The typical mass loading of sulfur was about 1 mg cm-2. The cells contained Celgard 2320 as the separator and lithium foils as both the counter and reference electrodes. The


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cells were equilibrated for 12 h before operation, and cycled between 1.8 and 2.6 V versus Li/Li+ in galvanostatic mode on LAND CT2001A battery test system (Wuhan, China). Cyclic voltammetry (CV) was performed at scan rate of 0.2 mV s-1 in the range of 1.8 to 2.6 V using the Autolab Electrochemical Workstation (Wuhan, China). AC impedance (EIS) of the cell was measured with a frequency response analyzer (FRA) on an Autolab Electrochemical Workstation over the 1 MHz to 100 mHz range with amplitude of 10 mV. Both CV and EIS measurements were conducted by a coin cell-based three-electrode configuration with lithium as both the counter electrode and the reference electrode. All performances were carried out at 25 °C. Characterization：：The ionic conductivity was measured by a conductivity meter equipped with a two-electrode epoxy conductivity probe (REX, China) in a glove box with oxygen and water contents less than 1 ppm. The electrode surface morphology before and after cycling was characterized by Scanning Electron Microscope (SEM, HITACHI S-3400 N). X-ray Photoelectron Spectroscopy (XPS, Thermo Fisher Scientific) was employed to detect the chemical composition of both cathode and anode surfaces after 50 cycles. All XPS spectra were fitted with Gaussian–Lorentzian functions and a Shirley-type background. S 2p peaks were fitted using two equal full-width half maximum (FWHM) doublets with about 2:1 area ratios. Other peaks (1s) were fitted with approximatively equal FWHM. The binding energy values were all calibrated using the C 1s peak (248.8 eV). Samples for the study of the reaction between CS2 and polysulfides were prepared by adding 5 wt% CS2 in Li2Sn solution and retaining for 1 h in a glove box. Samples for SEM and XPS characterization were prepared by disassembling cells and rinsed with DME in a glove box. All samples were sealed in a vial before being quickly transferred to the chamber of testing system.


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RESULTS AND DISCUSSION

Figure 1. Electrochemical performance of Li-S cells with basic and 40 wt% CS2 electrolytes. a. The initial charge-discharge voltage profiles at 0.5 C rate. b. The 100th charge-discharge profiles at 0.5 C rate. c. Cycle performance of Li-S cells with different electrolytes at 0.5 C. The ionic conductivity of the electrolytes with CS2 is shown in Table S1. Basic electrolyte offers the highest ionic conductivity of 9.5 mS cm-1 (in good agreement with literature27), whereas CS2-40 wt% exhibits the lowest value of 2.6 mS cm-1. This is due to the low solubility of lithium salt in CS2 (Figure S1) and less solvent is available to act as 'lubricant' for Li+. To investigate the effect of CS2-containg electrolyte on the electrochemical performance of LiS batteries, we select KB with commercial sulfur as the cathode materials. The mechanical ball-


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milling method is used to prepare the cathode material. The basic electrolyte employed is 1M LiTFSI in DOL/DME with 2 wt% LiNO3. Figure 1a and 1b show the selected charge-discharge curves of the Li-S batteries with/without CS2. The S/C cathode in basic electrolyte shows an initial capacity of 962 mAh g-1 with two typical voltage plateaus at 2.3 V and 2.0 V, corresponding to the reduction of S8 and high order polysulfides (Sn, 8 ≤ n ≤ 4 ), and the further reduction of low order polysulfides (Sn, 4 ≤ n ≤ 2 ), respectively.28 The capacity ratio of the high plateau to the low plateau is about 1:3. It is consistent with the electron transfer of sulfur at each discharge step: 1/4 electrons at high plateau and the remaining 3/4 at low plateau.24 In comparison, using the same amount of 40 wt% CS2 electrolyte, the first charge-discharge curve exhibits significant differences: 1) the discharge capacity is 632 mAh g-1, far lower than the value of basic electrolyte, and the second plateau almost doesn't appear, while the first high plateau contributes main capacity of 550 mAh g-1, even more than the theoretical value of 1/4 electrons contributions (419 mAh g-1); 2) the charge plateau remarkably increases from 2.2-2.3 V to 2.3-2.4 V and the main discharge plateau also increases from 2.1 V to 2.3V, resulting the potential difference (△E) between the charge and the discharge plateau much smaller than that of basic electrolyte, suggesting a kinetically efficient reaction process with smaller barriers.29 However, the 100th voltage profile with CS2 shows typical Li-S electrochemistry with two discharge plateaus, demonstrating a consumptive nature of CS2 to sulfur reduction. The △E is smaller than that of basic electrolyte. The discharge capacity maintaining at 835 mAh g-1 is higher than that of the basic electrolyte at 701 mAh g-1. Obviously, the cell with CS2-containing electrolyte experienced complex electrochemical processes.


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In addition, marked improvement in capacity retention of 40 wt% CS2 electrolyte is observed in Figure 1c. After cycling the 40 wt% CS2 cell over 300 cycles at 0.5 C, the capacity remains at 747 mAh g-1, corresponding to a capacity retention of 88% of its highest value (848 mAh g-1, 120th) and a very small capacity decay of 0.561 mAh g-1 per cycle. The cell without CS2 shows relatively poor electrochemical performance. The capacity remains at 561 mAh g-1 after 300 cycles and the capacity retention is only 58.3%. The coulombic efficiency with CS2 is always ＞ 99.0% over the entire cycling range, much better than that with the basic electrolyte accompanying continuous decline, which mainly results from the consumption of LiNO3 along with the repeated break and reconstruction of SEI films.28 It is noteworthy that the capacity of CS2-containing cell increases gradually during the first 50 cycles, as is evident by the extended low discharge plateau in Figure 1b. This result is likely to contribute to the specific functions of CS2 for electrochemical reaction, which will be discussed.


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Figure 2. Electrochemical properties of Li-S cells with 40 wt% CS2 electrolyte. a. Rate performance. b. EIS spectra at different cycles after fully charged at 0.5 C. Figure 2a shows the rate capability of the Li-S batteries with 40 wt% CS2. The cell was cycled at various current densities after the first 30 cycles (0.2 C), where capacity has been found to be stable. The cell shows high reversible capacities of 852, 795, 711 and 650 mAh g-1 at 0.5, 1, 2 and 3 C, respectively. Even when the current density reaches 4 C, a capacity of 557 mAh g-1 is achieved. After rate testing, the cell recovers to 0.2 C tests with the capacity returning to 855 mAh g-1. Both excellent rate capability and stability are evident and attributed to the good reaction kinetics of the cathode with CS2 electrolyte. Discharge-charge profiles (Figure S2) of the S/C cathode with 40 wt% CS2 electrolytes further exhibit the low polarization of the cell at large current density. The electrochemical performance of cells with different CS2 contents are


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also evaluated (Figure S3 and S4). Higher content of CS2 results better capacity stability. It is worth mentioning that CS2 becomes non-intersoluble in DOL/DME when the content increased to 50 wt% (Figure S5). Because of the intrinsic insulation nature of sulfur and discharge product Li2S/Li2S2, the inhomogeneous aggregation of these insulating substances during cycling will cause severe polarization and affect battery performance.6 We propose that CS2 added into the electrolyte will release sulfur from "dead" aggregation on cathode by partly dissolving it, and increase contact between these insulating substances and conductive carbon. To get further insight into the electrochemical reaction process, electrochemical impedance spectra (EIS) of the cell with 40 wt% CS2 electrolyte at different cycles are measured and presented in Figure 2b. The impedance plots for the cell before cycling are composed of a semicircle in the high-frequency region and a sloping straight line in the low-frequency domain. The semicircle and the straight line correspond to the charge-transfer process (Rct) and a semiinfinite Warburg diffusion process, respectively. The intercept of the semicircle in the highfrequency region on the Z' axis represents the electrolyte resistance Re.30-32 After cycling, the second semi-circle in the high-frequency region corresponding to the interfacial resistance (Ri) has emerged, implying the formation of a SEI layer.32 The Rct decreases dramatically once cycled and remain stable after 80 cycles, revealing that the surface electrochemical activity of the cathode is initiated by the discharge/charge reaction, and the deposition and aggregation of insulating substances on the surface of electrode are not serious. With respect to the Re, the value increases slightly in 80 cycles and then remains stable. The dissolved polysulfides, their reaction products with CS2, and complex decomposition products of lithium salts and solvents would contribute to the change of Re. For the first 10 cycles, the Ri is less than Rct, indicating a chargetransfer controlled process and a good lithium-ion conducting SEI layer. After 10 cycles, Ri is


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slightly increased and keeps stable after 80 cycles, which can be attributed to the deposition and aggregation of insulating products on electrodes with the battery cycling.

Figure 3. a. Selected voltage profiles of Li-S cells with 40 wt% CS2 electrolyte at 0.5 C rate. b. Cyclic voltammograms of cells with S/C cathode and carbon paper (CP, sulfur-free cathode) at scan rate of 0.2 mV s-1 with 40 wt% CS2 electrolyte. c. The initial discharge voltage curves of LiS cells with CS2 electrolytes (0 wt%, 25 wt% and 40 wt%). d. XPS spectra of S 2p in S/C cathodes after initial discharge. Figure 3a shows the charge and discharge curves of cell with 40 wt% CS2 electrolyte. The multiple changes of capacity and plateau length are further demonstrated by the cyclic voltammetry (CV) results in Figure 3b (CV for basic electrolyte is shown in Figure S6). As the


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cycle number increased, the reduction peak at 2.0 V grows higher in intensity and the one at 2.3 V becomes less significant. Sharper peak during cycling indicates better kinetics of Li ions diffusion in CS2-containing cells, which may be a result of better SEI properties. The reduction peaks shift to higher potential and oxidation peaks to lower potential with increase in cycle number, indicating an improvement of reversibility of the cell with cycling. Based on the results above, the use of CS2-containing electrolytes can significantly boost capacity of the first discharge plateau for first dozens of cycles. Interestingly, no redox peak is observed in the same CS2-containing electrolytes with carbon paper as sulfur-free cathode between 1.8 and 2.6 V (Figure 3b, blue line), which proves no extra capacity contributed by CS2-containing electrolyte. This indicates a different discharge-charge mechanism of Li-S cells with CS2 electrolyte. After dozens of cycles, the regained typical discharge curve shows the consumption of CS2. According to the conjecture of reported paper26, CS2 added into the electrolyte will react with S82- and form stable compounds. We would like to clarify that: 1) because of the solubility of sulfur in CS2, the utilization of active sulfur reduced for the first few cycles, which results the decreased discharge capacity; 2) CS2 reacts with long chain polysulfides and inhibits the further reduction of long chain polysulfides, not only causing the lowered capacity of the second voltage plateau, but also increased capacity of the first plateau. The reaction products of CS2 and polysulfides contribute extra capacity for the first reduction process. After the initial discharge (Figure 3c), the cathodes of Li-S cell with different content of CS2 were extracted for X-ray photoelectron spectroscopy (XPS) study. As shown in Figure 3d, in the range of 164-158 eV, the peaks in the S 2p spectra can be attributed to Li2S2 (161.8 eV) and Li2S (160.3 eV),33-34 produced by the reduction of loworder polysulfides and possible reactions between high-order polysulfides and CS2. The three curves (basic, 25%, 40%) match well with each other. These results suggest that the discharge


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products of Li-S cells with CS2 are consistent with that of typical sulfur reduction. CS2 only has influence on the reduction process of high-order polysulfides. The nature of the interaction of CS2 with polysulfides is determined from S 2p XPS analysis. The results are summarized in Figure 4 and 5. The photo in Figure 4 visually presents the change of Li2S8, Li2S4, and Li2S with CS2 in DME. The dark-yellow vials show that Li2S8 and Li2S4 are soluble in DME, and the colorless one shows that Li2S is insoluble. With CS2 added in the three vials, the dark-yellow grows lighter, and new unidentified substances appear, which is phaseseparated from solvents. The polysulfides are consumed and insoluble sulfocompounds are formed. The vial of Li2S has almost no visual change. XPS analysis was conducted on both Li2S8 and Li2S4 with/without CS2. High-order polysulfides contain two types of sulfur atom, the bridge sulfur (S0) and the terminal sulfur (S-1). The spectrum of Li2S8 (Figure 5a) shows two S 2p2/3 contributions at 160.9 eV (S-1) and 163.0 eV (S0),35 which dominates the peak area because of the higher atom ratio of bridge sulfur than the terminal in Li2S8. Figure 5b exhibits the S 2p2/3 spectrum of Li2S8/CS2. There are two main changes as compared with pure Li2S8: 1) peaks shift to the higher binding energy range, from 160.9 eV, 163.0 eV to 161.3 ev, 163.4 eV, respectively. 2) The intensity ratio of S0 to S-1 becomes lower, which further confirms the structure-chain change of polysulfides after the introduction of CS2. The S 2p2/3 peaks of pure Li2S4 at 161.1 eV and 163.0 eV can be assigned to S-1 and S0. The 1:1 ratio of the two contributions is consistent with the chain-structure of Li2S4. The spectra of S 2p2/3 of Li2S4/CS2 display the same trend in peak shift and intensity ratio compared with the Li2S8/CS2. These findings suggest the structure-chain change of polysulfides and the formation of new sulfocompounds, which likely accounts for the increased capacity of the high-order polysulfides reduction.


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Figure 4. Photographs of Li2S8, Li2S4, Li2S and Li2S8/CS2, Li2S4/CS2, Li2S/CS2 in DME. The concentration of Li2Sn in DME in each vial is 0.1 M, and the volume ratio of CS2 to DME is 1:5.

Figure 5. XPS spectra of S 2p: a) pure Li2S8, b) Li2S8/CS2, c) pure Li2S4, d) Li2S4/CS2. The proposed discharge mechanism for sulfur cathodes with CS2 electrolyte is summarized in Figure 6. In the basic electrolyte system (Figure 6a), the two discharge plateau is corresponded to the reduction (①) of S8 to high order polysulfides (Li2S8) and medium order polysulfides (Li2S4), and the further reduction (②) of Li2S4 to Li2S2/Li2S, respectively.1, 5 After the introduction of CS2 in electrolyte (Figure 6b), the reductions (①) of S8 to Li2S8 and Li2S4 still occur, while the reduction of low order polysulfides (Li2S4) to Li2S2/Li2S is prevented. Polysulfides (Li2Sn,

8 ≤ n ≤ 4 ) react with CS2, resulting the formation of new sulfocompounds with -S-Sn-S- (②).


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The discharge/charge nature of Li-S battery is the electrochemical reactions of polysulfides (-SSn-S-). As discussed above, the reaction products of Li2Sn and CS2 contain the structure of -S-SnS-, which also would take part in the discharge(③④). The final discharge products include Li2S2/Li2S, which is consistent with that of other organic sulfocompounds (dimethyl disulfide, RSn-R).36 We propose that the increased capacity in the first voltage plateau is attributed to the conversion of chemical energy to electrical energy. The suppressed process of second plateau is possibly due to the low solubility of LiTFSI and byproducts in CS2-containing solvents. As shown in Figure 3c, the increase of CS2 content causes larger polarization of cells. Therefore the solid-state conversion reaction of Li2S2/Li2S is likely prevented in the CS2-rich electrolyte. Besides, CS2 is known to dissolve sulfur. After the introduction of CS2 into electrolyte, the elemental sulfur in S/C cathodes has been partially dissolved in electrolyte and lost its electrochemical activity. With cycling of the cell, CS2 reacted with polysulfides and has been gradually consumed. After the exhaustion of CS2, the discharge behavior returns to the typical two-step reduction. The unusual voltage behavior in the first several discharge of CS2 cells is attributed to the reaction of Li2Sn and CS2. In these cells, the initial discharge products are not only the products of Li2Sn-CS2 reaction, but also the discharge products of reaction products of Li2Sn-CS2. The discharge/charge nature of Li-S battery is the electrochemical reactions of polysulfides (-S-Sn-S-). As discussed above, the reaction products of Li2Sn and CS2 contain the structure of -S-Sn-, which also would take part in the discharge. These reactions change mechanism of Li-S cell and affect the cell’s voltage profile.


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Figure 6. Schematic diagram of discharge process for sulfur cathode. a. Typical discharge process of sulfur cathode in basic electrolyte. b. Discharge process of sulfur cathode in CS2containing electrolyte. Scanning electron microscopy (SEM) images of cycled Li anode foils are presented in Figure 7. The surface morphology of pristine Li foil is shown in Figure 7a. After cycling in basic electrolyte, the surface become relatively rough and loose (Figure 7b). Possible explanations are the consumption of LiNO3 and corrosion reaction of polysulfide shuttle.28 In contrast, a uniform and dense surface can be observed with the 40 wt% CS2 electrolyte (Figure 7c). Even after 300 cycles, the surface of Li anode keeps dense and has no obvious dendrites or rough layer of deposited sulfides (Figure 7d and Figure S7). The passivation surface films (SEI) formed on the Li anode after cycling may protect the lithium from damage of shuttle and corrosion. To determine the chemical composition of the SEI, S 2p spectra of the Li anode are studied. Peaks in the S 2p spectra (Figure 7e) for basic electrolyte can be assigned to Li2S (160.1 eV) and Li2S2 (161.9 eV).34 For the CS2-containing electrolyte (Figure 7f), the new significant contribution of S


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2p spectra between 167.1 eV and 169.2 eV can be fit with two sulfur species. The peak at 167.1 eV is in accord with the thiosulfate37, which is regarded as an internal mediator to anchor highorder polysulfides and trigger conversion to lower polysulfides. Another peak at 169.2 eV is assigned to polythionate complex, products of the polysulfides catenation to the thiosulfate. As described by Eq (1) 16:

According to the XPS data (Figure S8) and reported studies34, the other components of the SEI are similar, including decomposition products of lithium salt and solvents. Though the generation mechanism of thiosulfate is not entirely clear in cells with CS2 electrolyte, these insoluble thiosulfate species help suppress the shuttle and stabilize the Li anode surface through the reactions with polysulfides, especially after the damage of LiNO3-induced SEI. To investigate the interfacial stability of the CS2-containing electrolyte in the Li electrodes, AC impedance measurement for Li/electrolyte/Li batteries was also performed (Figure S9). In the battery assembled with CS2-containing electrolyte, the value of Ri displays the same trend compared with that of basic electrolyte, indicating few undesired side reactions between CS2-containing electrolyte and Li anode.


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Figure 7. SEM images of fresh Li foil (a), Li anodes after 50 cycles with (b) basic electrolyte and (c) 40 wt% CS2 electrolyte, and Li anodes after 300 cycles with (d) 40 wt% CS2 electrolyte. S 2p XPS spectra of Li anodes after 50 cycles with (e) basic electrolyte and (f) 40 wt% CS2 electrolyte. The morphology of S/C cathodes with 40 wt% CS2 electrolyte has no obvious change over the entire cycling (Figure 8a), and the complex decomposition of solvents and lithium salts may account for the molten-like surface after 50 cycles. SEM images for fresh S/C cathode and cycled S/C cathode with basic electrolyte are shown in Figure S10. The chemical compositions on S/C cathodes surface are determined by XPS analysis after 50 cycles. Figure 8b shows the S 2p spectra of S/C cathode. When the cell is fully charged, the elemental sulfur is evidenced by S 2p contributions at 164.2 eV.38 Li2S2 and Li2S are also detected, the cause of incomplete electrochemical reactions or the dispropornation of metastable polysulifdes. As with the Li anode (Figure 7f), strong S 2p3/2 contributions at 167.3 and 169.1 eV corresponded to thiosulfate and polythionate are also detected. It is proved that the generation of CS2-induced SEI happens not


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only on Li anode, but also cathode. The spectra of other elements are presented in Figure S11. The components mainly comprise of LiF, LiNOx, LiOR decomposed by lithium salts/solvents.

Figure 8. a. SEM images of S/C cathodes after 50 cycles. b. S 2p spectra on S/C cathode after 50 cycles (fully charged). Based on the collected evidences, we propose that both the chemical and electrochemical reactions of polysulfides with CS2 are responsible for the formation of the long-term protection layer on electrodes (anode and cathode). After the damage of LiNO3-induced films, the thiosulfate-hosted protective layer not only blocks the shuttle by physical barrier, but also chemically reacts with long-chain polysulfides resulting in regeneration of thiosulfate-containing SEI.


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CONCLUSIONS In summary, we have reported that the introduction of the CS2-containing electrolyte in Li-S batteries results in remarkable performance improvement. The cells with the 40 wt% CS2 electrolyte have exhibited high coulombic efficiency and long cycle stability, showing average coulombic efficiency ＞99% and a capacity retention of 88% over entire 300 cycles. We have proposed that CS2 in electrolyte serves two functions: it changes the S reduction by reacting with polysulfides, and it passivates the surface of both anode and cathode by inducing the formation of thiosulfate-containing protective layers. The passivation layer not only suppresses the polysulfides shuttle, thus stabilizing the lithium surface, but also improves the electrochemical performance. Nevertheless, the mechanisms, reaction with polysulfides and formation of thiosulfate, are still not clear, and deeper understanding is needed.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge via the Internet at http://pubs.acs.org. This includes ionic conductivity voltage profiles of CS2-containing cells at different current density, cyclic voltammograms of basic cells, and voltage profiles on different cycles and cycle performance of CS2-containing cells with different content, SEM images of Li foils with CS2additive after different cycles, and XPS spectra of Li anode and S/C cathode after 50 cycles. (PDF) AUTHOR INFORMATION Corresponding Authors


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*E-mail: [email protected] *E-mail: [email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of China (No.51402330, No.51372262, and No.51472261). REFERENCES (1) Manthiram, A.; Fu, Y. Z.; Chung, S. H.; Zu, C. X.; Su, Y. S., Rechargeable LithiumSulfur Batteries. Chem. Rev. 2014, 114, 11751-11787. (2) Ji, X. L.; Nazar, L. F., Advances in Li-S Batteries. J. Mater. Chem. 2010, 20, 9821-9826. (3) Yin, Y. X.; Xin, S.; Guo, Y. G.; Wan, L. J., Lithium-Sulfur Batteries: Electrochemistry, Materials, and Prospects. Angew. Chem.-Int. Edit. 2013, 52, 13186-13200. (4) Mikhaylik, Y. V.; Akridge, J. R., Polysulfide Shuttle Study in the Li/S Battery System. J. Electrochem. Soc. 2004, 151, A1969-A1976. (5) Barghamadi, M.; Best, A. S.; Bhatt, A. I.; Hollenkamp, A. F.; Musameh, M.; Rees, R. J.; Ruther, T., Lithium-Sulfur Batteries-The Solution is in the Electrolyte, but is the Electrolyte a Solution? Energy Environ. Sci. 2014, 7, 3902-3920. (6) Li, G.-C.; Li, G.-R.; Ye, S.-H.; Gao, X.-P., A Polyaniline-Coated Sulfur/Carbon Composite with an Enhanced High-Rate Capability as a Cathode Material for Lithium/Sulfur Batteries. Adv. Energy Mater. 2012, 2, 1238-1245. (7) Ji, X. L.; Lee, K. T.; Nazar, L. F., A Highly Ordered Nanostructured Carbon-Sulphur Cathode for Lithium-Sulphur Batteries. Nat. Mater. 2009, 8, 500-506. (8) Shi, J. L.; Tang, C.; Peng, H. J.; Zhu, L.; Cheng, X. B.; Huang, J. Q.; Zhu, W.; Zhang, Q., 3D Mesoporous Graphene: CVD Self-Assembly on Porous Oxide Templates and Applications in High-Stable Li-S Batteries. Small 2015, 11, 5243-5252. (9) Zhang, B.; Qin, X.; Li, G. R.; Gao, X. P., Enhancement of Long Stability of Sulfur Cathode by Encapsulating Sulfur into Micropores of Carbon Spheres. Energy Environ. Sci. 2010, 3, 1531-1537. (10) Jayaprakash, N.; Shen, J.; Moganty, S. S.; Corona, A.; Archer, L. A., Porous Hollow Carbon@Sulfur Composites for High-Power Lithium-Sulfur Batteries. Angew. Chem.-Int. Edit. 2011, 50, 5904-5908. (11) Xin, S.; Gu, L.; Zhao, N. H.; Yin, Y. X.; Zhou, L. J.; Guo, Y. G.; Wan, L. J., Smaller Sulfur Molecules Promise Better Lithium-Sulfur Batteries. J. Am. Chem. Soc. 2012, 134, 1851018513.


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Table of Contents graphic

CS2 (Carbon disulfide) as cosolvent in ether-based electrolyte significantly enhances the longterm cycling stability of lithium sulfur batteries.


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Figure 1

Figure 2


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Figure 3

Figure 4



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Figure 7

Figure 8


Carbon Disulfide Cosolvent Electrolytes for High-Performance Lithium

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