A Rationally Designed High Sulfur Content Polymeric Cathode

Subscriber access provided by Columbia University Libraries

Energy, Environmental, and Catalysis Applications

A Rationally Designed High Sulfur Content Polymeric Cathode Material for Lithium-Sulfur Batteries Amruth Bhargav, Chi-Hao Chang, Yongzhu Fu, and Arumugam Manthiram ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b21395 • Publication Date (Web): 16 Jan 2019 Downloaded from http://pubs.acs.org on January 17, 2019

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 25 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Applied Materials & Interfaces

A Rationally Designed High Sulfur Content Polymeric Cathode Material for LithiumSulfur Batteries

Amruth Bhargav, † Chi-Hao Chang, † Yongzhu Fu, ‡,* and Arumugam Manthiram†, 

† Materials Science & Engineering Program and Texas Materials Institute, The University of Texas at Austin, Austin, TX78712, United States

‡ College of Chemistry and Molecular Engineering, Zhengzhou University, Zhengzhou 450001, China

Keywords: lithium-sulfur batteries, organopolysulfide, polyethylene hexasulfide, high-sulfurcontent polymer, electrochemistry *

Corresponding authors: [email protected] (A. Manthiram) [email protected] (Y. Fu) 1 ACS Paragon Plus Environment

ACS Applied Materials & Interfaces 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Abstract Polyethylene hexasulfide (PEHS) is investigated as a cathode material in lithium batteries. By utilizing a condensation reaction, ethylene groups are inserted between six linear sulfurs in a chain to obtain PEHS while simultaneously exercising control over the polysulfur chain length. Additionally, by selecting a low molecular weight organic group, PEHS contains 87 wt.% sulfur, thus maximizing the material-level specific capacity to 1,217 m Ah g-1. Furthermore, this synthesis method is validated using a host of materials characterization techniques. In a battery, PEHS prevents the formation of soluble long-chain intermediates that plague traditional sulfur cathodes. This enables a high material-level capacity of 774 mA h g-1 at 1C rate alongside a stable performance over 350 cycles with a capacity fade rate of only 0.083% per cycle. We also elucidate the unique reaction pathway of our short-chain polysulfur material and provide a useful foundation for further development of sulfur-containing polymer-based cathode materials.

1. Introduction It is estimated that ~ 70 billion kilograms of elemental sulfur is obtained as a by-product of the hydrodesulfurization process during petroleum refining.1-2 This provides an abundant and inexpensive feedstock for the synthesis of sulfur-bearing polymers that have a host of interesting properties such as unique electrochemical redox activities, non-linear refractive indices, and selfhealing behaviors.1, 3-4 Consequently, the use of electroactive sulfur and sulfur-bearing polymers in lithium batteries has garnered significant research attention over the past decade owing to their high theoretical specific capacity.5-10 However, the practical application of this system is hindered by several challenges. The most prominent among them is the “shuttle effect”, which involves the migration of the electrolyte-soluble electroactive species viz. lithium polysulfides

2 ACS Paragon Plus Environment

Page 2 of 25

Page 3 of 25 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(Li2Sx, 4 ≤ x ≤ 8) from the cathode to the lithium-metal anode through the porous polymeric separator. Apart from using engineered inorganic materials to confine the polysulfides within the cathode, a popular approach is to use polymers to encapsulate sulfur and its reduced species.11-13 In these polymers such as poly(sulfur‐r-1,3‐diisopropenylbenzene), active N-based, O-based, or S-based organic groups facilitate favorable interaction and bind to the polysulfide species, thus mitigating the shuttle effect. Additionally, in polymers bearing linear polysulfur chains, such as sulfurized polyacrylonitrile, the shuttle effect is significantly mitigated by limiting the order of the polysulfides formed during cycling to electrolyte-insoluble short chain polysulfides.14-15 This strategy has proven very effective as evidenced by the good performance exhibited even at high sulfur loading in a previous work.16 However, owing to the nature of the synthesis process of such polymers, it is difficult to control the length of the polysulfur chain. This leaves it prone to the formation of higher-order polysulfides, leading to faster capacity decay. To overcome this limitation, we present here a rational design of an inherently high sulfur content polymer while exercising control over the polysulfide chain length to maximize the material-level specific capacity. In this work, polyethylene hexasulfide (PEHS) is synthesized by a simple condensation reaction route while using sulfur as a feedstock. This polymer is imbibed onto an intertwined network of carbon nanotubes that forms the composite cathode. The synthesis mechanism and the nature of the polymer and the electrode have been characterized with various spectroscopic techniques. Electrochemical and materials characterization techniques have been used to understand the behavior and performance of this novel sulfur-containing polymer in a lithium battery.



2. Experimental 2.1 Materials and chemicals 1,2-ethanedithiol (EDT, HSC2H4SH, ≥98.0%, Sigma Aldrich), sulfur (S8, 99.5+%, Acros organics), carbon disulfide (CS2, extra pure, 99.9%, Acros organics), toluene (C6H5CH3, anhydrous, 99.85%, Acros organics), diethylamine ((CH3CH2)2NH, 99+%, Acros Organics), carbon nanotubes (CNT) “buckypaper” (20 GSM, NanoTechLabs, Inc), lithium bis(trifluoromethanesulfonimide) (LiTFSI, LiN(CF3SO2)2, 99%, Acros Organics), lithium nitrate (LiNO3, 99+%, Acros Organics), 1,2-dimethoxyethane (DME, 99+%, Acros Organics), 1,3dioxolane (DOL, 99.5%, Acros Organics), chloroform-d (CDCl3, 99.8 atom % D, Acros organics), and potassium bromide (KBr, FTIR Grade, Alfa Aesar) were purchased and used as received.

2.2 Polyethylene hexasulfide (PEHS) polymer synthesis The polymer stock-solution was synthesized inside an argon-filed glovebox. In a typical synthesis, 5 equivalents of elemental sulfur powder (usually 480 mg or 15 mmol) was added to 4 mL of a 1:1 v/v mixture of toluene/CS2 solvents, followed by vigorous stirring until all the sulfur dissolved. Next, one equivalent of EDT (typically 283 mg or 3 mmol) was added dropwise to the above solution and stirred for half an hour. On injecting catalytic amount (2 µL) of diethyl amine, strong effervescence of H2S gas was observed, which subsided within 15 minutes. This solution was either stirred overnight to obtain the PEHS polymer as a precipitate or used immediately to prepare the cathodes.


Page 4 of 25

Page 5 of 25 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


2.3 PEHS-CNT cathode fabrication First, CNT paper was cut into 7/16th inch discs weighing roughly 1.9 mg (2 mg cm-2) and dried in a vacuum oven overnight. To prepare cathodes, 20 µL of the 0.75 M PEHS solution synthesized as described above was slowly injected in to the CNT paper discs and allowed to sit in the glovebox over-night. This led to the completion of the reaction in-situ followed by slow precipitation of the polymer as the low boiling point CS2 evaporated. The cathodes were then removed outside the glovebox and dried at 80°C under air flow in an oven for 12 h to remove the toluene solvent. The PEHS-CNT cathodes thus obtained had a polymer loading of 3.3 mg (3.4 mg cm-2) corresponding to a loading of ~ 64% by weight of cathode mass. These cathodes were used for further electrochemical and materials characterization tests.

2.4 S-CNT control cathode fabrication Elemental sulfur was dissolved in CS2 at a concentration of 0.12 mg µL-1, and 20 µL of this solution was deposited on CNT paper electrodes as used above, followed by slow drying to give S-CNT cathodes with a loading of 2.4 mg, corresponding to the same areal capacity as the PEHS-CNT cathodes.

2.5 Coin-cell fabrication Electrochemical tests were performed with CR-2032 type coin cells fabricated inside an argon-filled glove box with 1.0 M LiTFSI and 0.2 M LiNO3 dissolved in a mixture of DME and DOL (1:1 v/v) as the electrolyte. PEHS-CNT electrode was the cathode and Li metal foil was the anode. 20 µL of the electrolyte was evenly added on both electrodes giving a total electrolyte amount of 40 µL within the cell. This resulted in an electrolyte to active material ratio of about



12 µL mg-1. Higher loading PEHS cells were fabricated with two cathode discs and 45 µL of electrolyte within the cell. This affords a PEHS loading of 6.6 mg (6.8 mg cm-2) and an electrolyte to active material ratio of 6.8 µL mg-1. The control sulfur cells were prepared the same way as regular cells except with S-CNT as cathodes.

2.6 Electrochemical cell testing BioLogic VSP potentiostat was used for cyclic voltammetry (CV) tests with the potential being swept from open-circuit voltage (OCV) to 1.8 V and then back to 3.0 V at a scanning rate of 50 µV s-1. Electrochemical impedance spectroscopy (EIS) was performed in the frequency range of 1 MHz to 100 mHz. The coins cells were galvanostatically cycled on an Arbin battery cycler at various C rates with a rate of 1C corresponding to 1,217 mA g-1 of PEHS present by mass in the cathode of the cell. The cells were discharged to 1.8 V at C/20 and C/10 rates, 1.7 V at C/5 and C/2 rates, and 1.6 V at 1C rate to ensure material utilization while also preventing LiNO3 decomposition. All cells were recharged to 3 V.

2.7 Materials characterization Materials characterization were performed on cells that were cycled at C/20 rate to the appropriate cutoff. The cathodes from these cells were washed with pure DME to remove traces of LiTFSI salt followed by drying before mounting onto instrument sample holders. The samples were transferred to the instruments with care to avoid exposure to air in the case of air-sensitive samples.


Page 6 of 25

Page 7 of 25 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


X-ray diffraction (XRD) patterns were collected on a Rigaku Miniflex 6000 X-ray diffractometer with Cu Kα radiation source. The samples were protected in the sample holder with kapton film. The scanning rate was 1.5° min−1 from 20° to 60°. Fourier transform infrared (FTIR) absorption spectra were recorded on a Thermo Scientific-Nicolet iS5 FTIR spectrometer. 32 scans in the 400 to 4000 cm−1 range were recorded per sample that was impregnated in a KBr pellet. 1

H-Nuclear magnetic resonance (NMR) spectroscopy was performed on a Bruker Avance

III 500 MHz NMR spectrometer. The samples were stirred with 1 mL of CDCl3 : CS2 (1 : 1 v/v) solution overnight. 700 µL of the solution was then subject to NMR analysis and the chemical shifts (δ) were calibrated using the residual solvent peak as an internal standard. To determine the molecular weight, PEHS was dissolved with PEO (polyethylene oxide, average Mw = 8000 Da) as a reference in 4 : 3 weight ratio. The integrals of the polymer signals were used to calculate the approximate molar masses. Scanning electron microscopy (SEM) was performed with a FEI Quanta 650 microscope at 10 kV. Sulfur and carbon elemental mapping were carried out to understand the material distribution using energy-dispersive X-ray spectroscopy (EDX) detector attached to the SEM. X-ray photoelectron spectroscopy (XPS) was performed with a Kratos AXIS Ultra DLD spectrometer with monochromatic Al Kα radiation. All regions were scanned with a step size of 0.1 eV and a dwell time of 2,000 ms per step without the charge-neutralizer. The acquired spectrum was processed with the CASA XPS software. The calibration was performed by setting the adventitious carbon peak at 284.8 eV. Shirley background was used and the sulfur 2p spectrum was deconvoluted using Gaussian/Lorentzian peaks.



3. Results and Discussion Most commonly utilized synthesis routes of vulcanization or inverse-vulcanization for obtaining sulfur-bearing polymers involve temperatures greater than 180°C.1, 4, 13 At these high temperatures, sulfur undergoes a radical polymerization and becomes long polymeric sulfur chains, which are added onto the organic precursor to yield the sulfur-containing polymer. However, sulfur is able to maintain its orthorhombic octa-sulfur crystalline configuration up to 95°C.17 Therefore, operating below this temperature affords well-defined polymer structures with well-controlled sulfur chain length. To facilitate this, we begin with an organic precursor containing the thiol functional group (-SH). The high acidity of thiol group facilitates a condensation reaction with elemental sulfur while offering selectivity in polysulfide order through simple stoichiometry, thus making it a suitable choice.18 When utilizing dithiols (HS-RSH), it has been shown that this reaction yields oligomers and polymers at room temperatures under the catalytic action of bases such as secondary amines.19-21 Therefore, in this work, we utilize 1,2-ethanedithiol (EDT) as the organic precursor wherein the low molecular mass of the ethylene group helps maximize the sulfur content in the polymer. In a typical synthesis, 5 equivalents of sulfur are first dissolved in a 1:1 v/v mixture carbon disulfide (CS2) and toluene (C6H5CH3, PhMe) under stirring, followed by the addition of one equivalent of EDT. Upon the addition of a catalytic amount of diethylamine ((CH3CH2)2NH, (Et)2NH), the reaction proceeds as shown by the equation in Figure 1a, leading to a vigorous evolution of H2S gas. In 15 minutes, the H2S evolution subsides, giving a clear yellow solution as shown in Figure 1a. After an hour, the reaction reaches completion and the transparent, yellow-colored solid polyethylene hexasulfide (PEHS) polymer precipitates out, which is insoluble in typical organic solvents. In order to circumvent this issue and prepare the cathodes,


Page 8 of 25

Page 9 of 25 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


the reaction solution is drop-casted onto commercial carbon nanotube (CNT) paper discs. The high boiling point and wettability of PhMe allows for its infiltration into the pore spaces within the CNT network followed by in-situ completion of the reaction until the polymer precipitates. The prepared cathode is further dried at 80°C to remove the excess solvent while also consuming any unreacted sulfur forming a PEHS-CNT composite cathode. The SEM image of the PEHSCNT cathode in Figure 1b shows a uniform precipitation of the PEHS polymer on the CNT fibrils resulting in a conformal coating of the polymer within the porous CNT matrix. This homogeneous distribution of the polymer is validated with the elemental maps of sulfur (yellow dots) and carbon (red dots) by EDX spectroscopy, shown in inset images in Figure 1b. It is interesting to note that the porous, intertwined nature of the CNT paper is maintained allowing for electrolyte penetration and rapid electron transfer, which are pre-requisites for good electrode kinetics. Upon synthesis, various spectroscopic techniques were applied to confirm the synthesis protocol and understand the chemical nature of the polymer and its composite cathode. The XRD investigation shown in Figure 2a reveals that PEHS does not exhibit any of the characteristic peaks of sulfur indicating its compete consumption. Furthermore, XRD reveals its polymeric nature from the broad peaks at low angles of 2-theta up to 35°, which is due to scattering by the amorphous material. XRD pattern of the composite cathode also exhibits this broad polymeric feature along with the suppressed peaks for the (002), (100), and (004) peaks of CNT at 26°, 43.5° and 54°.22 This is followed by an investigation into the transformation of the EDT precursor through 1H-NMR spectroscopy (Figure 2b). The spectrum of EDT displays peaks centered at 1.67 ppm arising from the proton on the thiol group and peaks at 2.75 ppm arising from the ethylene group hydrogens as indicated in the figure.23 The peak (thiol groups) at 1.67



ppm, however, completely disappears after polymerization due to condensation reaction whereas the ethylene group within the polymer retains its peaks albeit with a small shift of 0.15 ppm owing to polymerization. Further NMR analysis (Figure S1, supporting information) revealed that the molecular weight of PEHS was about 1990 Da. We then turned to FTIR spectroscopy for a comprehensive understanding of the bond changes as EDT is converted to PEHS (Figure 2c). Most importantly, as we convert EDT to PEHS, the thiol character signified by the S-H vibration at 2560 cm-1 disappears and concurrently, the S-S vibration bands at 490 cm-1 appears.18, 24-25 Additionally, it is interesting to note that there is a loss in the C-S vibrations and increase in the C-H vibrations occurring in the 650 – 1150 cm-1 range, resulting from the change in the degrees of freedom from EDT to PEHS.24-25 XPS of the cathode was then used to validate the order of the polysulfides. The S 2p spectrum in Figure 2d shows the presence of two forms of sulfur namely, the linear polysulfide (S-S) chains having their 2p3/2 peaks centered around 163.9 eV and the C-S bonds originating from the EDT precursor having the 2p3/2 peak at 163.5 eV, which are in agreement with literature values.15, 26-27 As there are 4 S-S bonds and 2 C-S bonds in PEHS, the ratio of peak areas should be 2 : 1. The observed ratio of 1.96 : 1 establishes the hexasulfide nature of PEHS. When put together, these characterization results point to the successful formation of PEHS polymer in accordance with the scheme in Figure 1a leading to a polymer containing 87% sulfur. After ascertaining its chemical composition, the behavior of the PEHS-CNT cathode in a lithium half-cell was studied. PEHS-CNT electrode discs were used as freestanding and binderfree cathodes. In a lithium battery, PEHS can theoretically undergo a 10-electron redox reaction, giving it a specific capacity of 1217 mA h g-1 and a high specific energy of 2600 W h kg-1. The electrochemical response of the cell under cyclic voltammetry (CV) at a scan rate of 50 µV s-1 is


Page 10 of 25

Page 11 of 25 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


shown in Figure 3a and the discharge/charge behavior at C/20 rate is captured by the voltage profile in Figure 3b. The cathodic scan results in 2 small peaks I and II at 2.35 V and 2.15 V, respectively, followed by a strong reduction peak (III) at 2.0 V. This is in good agreement with corresponding voltage plateaus seen during discharge of the cell as indicated in the voltage profile. Plateaus I and II correspond to the scission of the bond between sulfur attached to the ethylene group (indicated in blue in Figure 3b) and the sulfur of the polysulfide chain (indicated in red in Figure 3b) due to its low bond energy, leading to the formation of lithium 1,2ethanedithiolate (Li-SC2H4S-Li) and lithium tetrasulfide (Li2S4).18, 28-29 This is followed by a further reduction of Li2S4 to lithium sulfide (Li2S) at 2.0 V (corresponding to plateau II) as also observed in conventional Li-S cells.5 The overall discharge reaction is indicated in Figure 3b. During charge, the organic thiolate mediates the formation of Li2S4 as indicted by strong oxidation peak IV in the anodic scan of CV and the corresponding plateau in the voltage profile, followed by a reversal to the polymeric state at the peak/plateau labeled as V. XPS was also utilized to validate the reaction pathway as shown in Figure 3c. The S 2p spectrum of the cathode discharged to 1.8 V at C/20 rate indicates the presence of the primary discharge products of Li2S (2p3/2 at 160 eV) and Li-SC2H4S-Li (2p3/2 at 161.6 eV) along with partially reduced lithium polysulfide (Li2Sx, 2 ≤ x ≤ 4 with 2p3/2 at 163.3 eV) owing to an incomplete discharge.18, 30-32 Furthermore, upon analyzing the peak areas, it can be seen that the partially reduced polysulfides account for 17% of the species present on discharge, which in close agreement with an average material utilization 86% achievable at C/20 rate as is common for such materials.32 The reformation of the polymer on recharge can be seen through the XPS spectrum of the cathode at the end of first cycle (Figure S2, supporting information). The morphological transformations occurring during battery operation were investigated with SEM



and EDX. The SEM and EDX result of a discharged cathode in Figure 3d shows the uniform deposition of the various discharge products within the CNT. The image also indicates good compatibility between the organic and inorganic discharge products without any visual segregation.29 This potentially promotes the reformation of the polymer on recharge while demoting the growth of long chain polysulfides.33 Additionally, the CNT network seems to successfully accommodate the larger volume fraction of the discharge products present in comparison to the pristine polymer in Figure 1b, thus preserving the physical integrity of the cathode during cycling.34 This would allow for good rechargeability, promote high Coulombic efficiency, and maintain a long cycle life. Inspection of the cathode upon recharge (Figure S3, supporting information) indicates the preservation of the conformal coating nature of the polymer on CNT, which is comparable to the pristine electrode. The practical application of this material in a battery necessitates insight into its behavior under different C rates and extended cycling. The performance of the PEHS-CNT cathode when tested under C rates ranging from C/20 to 1C is shown in Figure 4a. The cathode can deliver a high capacity of 1108 mA h g-1, which corresponds to 1274 mA h g-1 with respect to sulfur in the polymer and accounts for an electrochemical utilization of 89%. A capacity of 970, 895, and 815 mA h g-1 (1115, 1030 and 937 mA h g-1 with respect to sulfur present in the polymer) can be extracted, respectively, at C/10, C/5 and C/2 rates. Even at high current densities of 1C, the cell can deliver a high capacity of 750 mA h g-1 or 862 mA h g-1 based on sulfur mass, highlighting the favorable kinetics of PEHS and the electrode. Moreover, as the cycling rate returns to C/5, the capacity recovers to 895 mA h g-1, establishing the robustness of the material to withstand the electrochemical stress. On looking at the discharge/charge curves at these rates (Figure S4, supporting information), only a 100 mV overpotential increase is observed despite a 20-fold


Page 12 of 25

Page 13 of 25 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


increase in current density, revealing that PEHS has favorable power capability. As PEHS is capable of high capacities at practical current densities of 1C, this C-rate was used to examine the long-term cycling stability of the cathode. Figure 4b shows that an initial specific capacity of 774 mA h g-1 (889 mA h g-1 with respect to sulfur) is obtained along with a Coulombic efficiency (CE) of 98%. The CE increases to 99.6% in the following cycle and maintains an average value of 99.3% through 350 cycles. The high CE value signifies an effective suppression of the polysulfide shuttle effect resulting in a high retention rate (71%) of its initial capacity after 350 cycles and a low capacity decay rate of barely 0.083% per cycle.35 Furthermore, the cell maintains its discharge/charge pathway over 350 cycles as seen through the consistency of the voltage curves in Figure S5 (supporting information), thus validating the excellent reversibility of PEHS. The performance of PEHS under high-loading (6.8 mg cm-2) and low electrolyte (6.8 µL mg-1) condition was also investigated as shown in Figure S6 (supporting information). At a C/5 cycling rate, the cell can deliver 885 mA h g-1 (1017 mA h g-1 with respect to sulfur) after an activation period of 6 cycles, which is comparable to that of lower loading cells. Furthermore, nearly 85% of this capacity can be retained after 150 cycles, thus proving its suitability to operate under these conditions. When PEHS is compared against polymers that are processed with the common vulcanization or inverse-vulcanization techniques (see Table S1 in supporting information), it can be seen that PEHS offers the highest material level specific capacity and thus can offer the highest areal capacity normalized to the active material in the cathode. While this work seeks to show the first-order benefits of using this material, significant improvements can be expected with better cathode design. To highlight the minimized formation of higher-order polysulfides in PEHS and further examine its benefits, it was compared against a S-CNT cathode with a matching areal capacity.



EIS spectra in Figure S7 (supporting information) reveal that the charge-transfer resistance of sulfur electrode is 103 Ω, which is nearly 5 times that of the PEHS electrode (23 Ω). This points to improved electron transfer through the polymeric PEHS as compared to the insulating nature of elemental sulfur. Additionally, long-term cycling at the same current density (Figure S8, supporting information) shows that in 350 cycles, the sulfur cathode has a lower initial capacity of 791 mA h g-1 compared to 889 mA h g-1 in the case of PEHS and also can retain only 54% of its capacity. Additionally, an average CE of only 95% is seen with a drop to just 92.7% by the 350th cycle. The negative impact of the shuttle effect can be seen at the Li-anode side after 350 cycles. As seen in Figure S9 (supporting information), the shuttle effect promotes dendritic Li growth whereas the suppressed polysulfide formation in the case of PEHS leads to denser and smoother Li anode. These tests clearly highlight the benefits of polysulfide shuttle prevention by utilizing a rationally designed polymer such as PEHS. Although the shuttle effect is minimized, capacity fade is still observed in PEHS. To uncover the cause for this, post-mortem XRD analysis of the cathode was performed after the 150th charge (Figure S10, supporting information). On comparing with the pristine electrode, only an increase in the intensity of the CNT peaks is observed, while no obvious peaks of sulfur are seen; This is counter to what would be expected in the case of detachment of sulfur from the polymer chain. Thus, we believe that active material delamination or inaccessibility upon prolonged cycling is the prevailing capacity fade mechanism. Confining the polymer within an alternative matrix such as doped graphene could provide a potential solution for this problem.8, 16 Thus, we hope that PEHS could provide the research community with an alternative pathway for truly superior Li-S cathode development.


Page 14 of 25

Page 15 of 25 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


4. Conclusion In summary, the condensation reaction of ethanedithiol with elemental sulfur has been used to synthesize PEHS which has a high sulfur content of 87%. Despite its high sulfur content, the synthesis procedure allows for a precise control over the polysulfide chain length, which has been verified through a host of materials characterization techniques. Electrochemical testing reveals the unique cycling behavior of this polymer material. Importantly, it is observed that limiting the polysulfur linkages in a polymer greatly minimizes the formation of high order polysulfides during cycling, leading to extended cycle life. This work thus introduces a new material and a design rationale for the family of sulfur-based cathode materials. We hope this inspires exploration of similar materials and sulfur encapsulation solutions to facilitate the continued improvement of Li-S batteries.

Supporting Information Additional materials characterization and electrochemical data are available in the supporting information. This material is available free of charge via the Internet at http://pubs.acs.org

Acknowledgements This work was supported by the Department of Energy, Office of Energy Efficiency and Renewable Energy (EERE) grant number DE-EE0007218. YF acknowledges the support from Thousand Youth Talents Program of China.

Competing interests The authors declare no competing financial interests.



Corresponding Authors [email protected] (A. Manthiram) [email protected] (Y. Fu) ORCID A. Manthiram: 0000-0003-0237-9563 Y. Fu: 0000-0003-3746-9884

References 1.

Griebel, J. J.; Glass, R. S.; Char, K.; Pyun, J., Polymerizations with Elemental Sulfur: A Novel Route to High Sulfur Content Polymers for Sustainability, Energy and Defense. Prog. Polym. Sci. 2016, 58, 90-125.

2.

Boyd, D. A., Sulfur and Its Role In Modern Materials Science. Angew. Chem. Int. Ed. 2016, 55, 15486-15502.

3.

Lim, J.; Pyun, J.; Char, K., Recent Approaches for the Direct Use of Elemental Sulfur in the Synthesis and Processing of Advanced Materials. Angew. Chem. Int. Ed. 2015, 54, 3249-3258.

4.

Chung, W. J.; Griebel, J. J.; Kim, E. T.; Yoon, H.; Simmonds, A. G.; Ji, H. J.; Dirlam, P. T.; Glass, R. S.; Wie, J. J.; Nguyen, N. A.; Guralnick, B. W.; Park, J.; Somogyi, Á.; Theato, P.; Mackay, M. E.; Sung, Y.-E.; Char, K.; Pyun, J., The Use of Elemental Sulfur as an Alternative Feedstock for Polymeric Materials. Nat. Chem. 2013, 5, 518.

5.

Manthiram, A.; Fu, Y.; Chung, S.-H.; Zu, C.; Su, Y.-S., Rechargeable Lithium–Sulfur Batteries. Chem. Rev. 2014, 114, 11751-11787.


Page 16 of 25

Page 17 of 25 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


6.

Manthiram, A.; Chung, S. H.; Zu, C., Lithium–Sulfur Batteries: Progress and Prospects. Adv. Mater. 2015, 27, 1980-2006.

7.

Peng, H. J.; Huang, J. Q.; Cheng, X. B.; Zhang, Q., Review on High‐Loading and High‐ Energy Lithium–Sulfur Batteries. Adv. Energy Mater. 2017, 7, 1700260.

8.

Pang, Q.; Liang, X.; Kwok, C. Y.; Nazar, L. F., Advances in Lithium–Sulfur Batteries Based on Multifunctional Cathodes and Electrolytes. Nat. Energy. 2016, 1, 16132.

9.

Liang, Y.; Tao, Z.; Chen, J., Organic Electrode Materials for Rechargeable Lithium Batteries. Adv. Energy Mater. 2012, 2, 742-769.

10.

Chung, S.-H.; Chang, C.-H.; Manthiram, A., Progress on the Critical Parameters for Lithium–Sulfur Batteries to be Practically Viable. Adv. Funct. Mater. 2018, 28, 1801188.

11.

Dirlam, P. T.; Glass, R. S.; Char, K.; Pyun, J., The Use of Polymers in Li-S Batteries: A Review. J. Polym. Sci., Part A: Polym. Chem. 2017, 55, 1635-1668.

12.

Lifen, X.; Yuliang, C.; Jie, X.; Birgit, S.; H., E. M.; V., S. L.; Zimin, N.; J., E. G.; Jun, L., A Soft Approach to Encapsulate Sulfur: Polyaniline Nanotubes for Lithium-Sulfur Batteries with Long Cycle Life. Adv. Mater. 2012, 24, 1176-1181.

13.

Kim, H.; Lee, J.; Ahn, H.; Kim, O.; Park, M. J., Synthesis of Three-Dimensionally Interconnected Sulfur-Rich Polymers for Cathode Materials of High-Rate Lithium–Sulfur Batteries. Nat. Commun. 2015, 6, 7278.

14.

Simmonds, A. G.; Griebel, J. J.; Park, J.; Kim, K. R.; Chung, W. J.; Oleshko, V. P.; Kim, J.; Kim, E. T.; Glass, R. S.; Soles, C. L.; Sung, Y.-E.; Char, K.; Pyun, J., Inverse Vulcanization of Elemental Sulfur to Prepare Polymeric Electrode Materials for Li–S Batteries. ACS Macro Lett. 2014, 3, 229-232.



15.

Wei, S.; Ma, L.; Hendrickson, K. E.; Tu, Z.; Archer, L. A., Metal–Sulfur Battery Cathodes Based on PAN–Sulfur Composites. J. Am. Chem. Soc. 2015, 137, 12143-12152.

16.

Chang, C.-H.; Manthiram, A., Covalently Grafted Polysulfur–Graphene Nanocomposites for Ultrahigh Sulfur-Loading Lithium–Polysulfur Batteries. ACS Energy Lett. 2018, 3, 7277.

17.

Currell, B. R.; Williams, A. J., Thermal Analysis of Elemental Sulphur. Thermochim. Acta 1974, 9, 255-259.

18.

Bhargav, A.; Bell, M. E.; Karty, J.; Cui, Y.; Fu, Y., A Class of Organopolysulfides As Liquid Cathode Materials for High-Energy-Density Lithium Batteries. ACS Appl. Mater. Interfaces 2018, 10, 21084-21090.

19.

Bordoloi, B. K.; Pearce, E. M., Kinetics of the Base-Catalyzed Reactions of Cyclooctameric and Catenapolymeric Sulfur with Dithiol. J. Appl. Polym. Sci. 1979, 23, 27572761.

20.

Bordoloi, B. K.; Pearce, E. M., Oligomeric Alkenyl Polysulfide: Synthesis and Characterization by NMR Analysis. J. Polym. Sci. Polym. Chem. Ed. 1978, 16, 3293-3300.

21.

Bhargav, A.; Bell, M. E.; Cui, Y.; Fu, Y., Polyphenylene Tetrasulfide as an Inherently Flexible Cathode Material for Rechargeable Lithium Batteries. ACS Appl. Energy Mater. 2018, 1, 5859-5864.

22.

Cao, A.; Xu, C.; Liang, J.; Wu, D.; Wei, B., X-ray Diffraction Characterization on the Alignment Degree of Carbon Nanotubes. Chem. Phys. Lett. 2001, 344, 13-17.

23.

Brändle, A.; Khan, A., Thiol–Epoxy ‘Click’ Polymerization: Efficient Construction of Reactive and Functional Polymers. Polym. Chem. 2012, 3, 3224-3227.


Page 18 of 25

Page 19 of 25 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


24.

Joo, S. W.; Han, S. W.; Kim, K., Multilayer Formation of 1,2-Ethanedithiol on Gold: Surface-Enhanced Raman Scattering and Ellipsometry Study. Langmuir 2000, 16, 53915396.

25.

Li, Y.-S.; Li, S., FTIR Spectra of HSCH2CH2SH, CH3SCH2SH, and CH3SSCH3 in Argon and Nitrogen Matrices. Spectrochim. Acta A 1994, 50, 509-519.

26.

Preefer, M. B.; Oschmann, B.; Hawker, C. J.; Seshadri, R.; Wudl, F., High Sulfur Content Material with Stable Cycling in Lithium‐Sulfur Batteries. Angew. Chem. Int. Ed. 2017, 56, 15118-15122.

27.

Xue, L. J.; Li, J. X.; Hu, S. Q.; Zhang, M. X.; Zhou, Y. H.; Zhan, C. M., Anthracene Based Organodisulfide Positive Active Materials for Lithium Secondary Battery. Electrochem. Commun. 2003, 5, 903-906.

28.

Yang, Y.-M.; Yu, H.-Z.; Sun, X.-H.; Dang, Z.-M., Density Functional Theory Calculations on S―S Bond Dissociation Energies of Disulfides. J. Phys. Org. Chem. 2016, 29, 6-13.

29.

Guo, W.; Wawrzyniakowski, Z.; Cerda, M.; Bhargav, A.; Pluth, M.; Ma, Y.; Fu, Y., Bis(aryl) Tetrasulfides as Cathode Materials for Rechargeable Lithium Batteries. Chem. Eur. J. 2017, 23, 16941-16947.

30.

Klein, M. J.; Goossens, K.; Bielawski, C. W.; Manthiram, A., Elucidating the Electrochemical Activity of Electrolyte-Insoluble Polysulfide Species in Lithium-Sulfur Batteries. J. Electrochem. Soc. 2016, 163, A2109-A2116.

31.

Fu, Y.; Zu, C.; Manthiram, A., In Situ-Formed Li2S in Lithiated Graphite Electrodes for Lithium–Sulfur Batteries. J. Am. Chem. Soc. 2013, 135, 18044-18047.



32.

Wu, M.; Cui, Y.; Bhargav, A.; Losovyj, Y.; Siegel, A.; Agarwal, M.; Ma, Y.; Fu, Y., Organotrisulfide: A High Capacity Cathode Material for Rechargeable Lithium Batteries. Angew. Chem. Int. Ed. 2016, 55, 10027-10031.

33.

Hua, W.; Yang, Z.; Nie, H.; Li, Z.; Yang, J.; Guo, Z.; Ruan, C.; Chen, X. a.; Huang, S., Polysulfide-Scission Reagents for the Suppression of the Shuttle Effect in Lithium–Sulfur Batteries. ACS Nano 2017, 11, 2209-2218.

34.

Peng, H.-J.; Xu, W.-T.; Zhu, L.; Wang, D.-W.; Huang, J.-Q.; Cheng, X.-B.; Yuan, Z.; Wei, F.; Zhang, Q., 3D Carbonaceous Current Collectors: The Origin of Enhanced Cycling Stability for High-Sulfur-Loading Lithium–Sulfur Batteries. Adv. Funct. Mater. 2016, 26, 6351-6358.

35.

Peng, H.-J.; Huang, J.-Q.; Liu, X.-Y.; Cheng, X.-B.; Xu, W.-T.; Zhao, C.-Z.; Wei, F.; Zhang, Q., Healing High-Loading Sulfur Electrodes with Unprecedented Long Cycling Life: Spatial Heterogeneity Control. J. Am. Chem. Soc. 2017, 139, 8458-8466.


Page 20 of 25

Page 21 of 25 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 1. (a) Optical image of a vial containing a 0.5 M solution of the polyethylene hexasulfide (PEHS) polymer in 1:1 v/v mixture of CS2/toluene on the left. Also presented within is the synthesis scheme with different colors as a guide to the eye. The resultant PEHS-CNT cathode is shown on the right. (b) SEM image of the PEHS-CNT cathode along with EDX elemental mapping of sulfur (yellow) and carbon (red) in the inset.



Figure 2. (a) XRD pattern of the sulfur, PEHS polymer, and PEHS-CNT cathode. (b) 1H-NMR spectra showing the loss of the hydrogen associated with thiol upon polymer synthesis. (c) FTIR spectra with the grey regions indicating the loss of thiol character and the formation of polysulfide linkages on polymer synthesis. (d) Sulfur 2p XPS spectrum indicating the two types of sulfur present in the polymer.


Page 22 of 25

Page 23 of 25 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 3. (a) CV scan of the PEHS-CNT cathode performed at 50 µV s-1 with labels for the various redox potentials observed. (b) Voltage profile of the cathode cycled at C/20 rate with labels for the different voltages corresponding to the potentials seen in the CV. The discharge/charge reaction is also indicated. (c) Sulfur 2p XPS spectrum of the cathode at the end of first discharge at C/20 rate. Color of the peaks correspond to the colors in the reaction scheme in (b). (d) SEM image of the discharged cathode along with EDX maps for sulfur (yellow) and carbon (red) in the inset. The C rate was calculated based on PEHS loading in the cathode with 1C = 1217 mA g-1.



Figure 4. (a) Performance of the PEHS-CNT cathode at different C rates. (b) Long-term cycling performance of the cathode when cycled at 1C rate. The C-rate was calculated based on PEHS loading in the cathode with 1C = 1217 mA g-1. Capacities are shown with respect to both the polymer and sulfur mass in the cathode.


Page 24 of 25

Page 25 of 25 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


TOC Figure


A Rationally Designed High Sulfur Content Polymeric Cathode

Recommend Documents