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Polyphenylene Tetrasulfide as an Inherently Flexible Cathode Material for Rechargeable Lithium Batteries Amruth Bhargav, Michaela Bell, Yi Cui, and Yongzhu Fu ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b01350 • Publication Date (Web): 25 Oct 2018 Downloaded from http://pubs.acs.org on October 26, 2018

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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.

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ACS Applied Energy Materials

Polyphenylene Tetrasulfide as an Inherently Flexible Cathode Material for Rechargeable Lithium Batteries

Amruth Bhargava, Michaela Elaine Bella, Yi Cuiac, and Yongzhu Fuab* a

Department of Mechanical Engineering, Indiana University-Purdue University Indianapolis, Indianapolis, IN 46202, United States

b

College of Chemistry and Molecular Engineering, Zhengzhou University, Zhengzhou 450001, China *Corresponding author: [email protected] (Y. Fu)

c

School of Mechanical Engineering, Purdue University, West Lafayette, IN 47907, United States

KEYWORDS – organopolysulfide, polyphenylene tetrasulfide, flexible cathode, polymer cathode, flexible lithium battery

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Abstract Li-ion batteries have transformed personal electronics from chunky devices to lightweight, flexible, and even wearable ones. Higher capacity and energy density along with flexibility is offered by sulfur-based cathode materials. In this work, the condensation reaction of 1,4-benzenedithiol with elemental sulfur has been utilized to synthesize polyphenylene tetrasulfide (PPTS) possessing a theoretical specific capacity of 788 mAh g-1. This elastic material can accommodate a strain of up to 334% while the carbon nanotube-based cathode can handle strains of 107%. This good flexibility also comes with favorable cycling performance in a lithium battery with a high capacity (633 mAh g-1 at 1C), good rate performance, high Coulombic efficiency (~99.4%), and a low capacity decay (under 0.07% per cycle). PPTS and its analogues thus offer promising cathodes for flexible lithium batteries.

Batteries have become the dominant power supplies in portable electronics. However, they are also the hurdle for more advanced devices as their energy density cannot increase significantly.1 In addition, as portable electronics become exponentially more present in daily life, it is necessary to adapt battery storage methods to appease the demands of cutting edge technologies. An emerging condition for electronics is the ability to bend, stretch, and incorporate curved infrastructure. This flexibility requirement has challenged the electronics industry and within the past decade, there has been enormous progress taken in the materials, design, and manufacturing of flexible and stretchable electronic subcomponents.2-3 An essential element in conceiving fully functional bendable electronics is an adaptable source of energy storage and delivery.

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Since the 1990s, lithium-ion (Li-ion) batteries have been the frontline contender of power supply for portable electronic devices due to their high energy density and long cycle life.1 The challenge in achieving flexible batteries is to design and fabricate materials that are well functioning with high capacity, cycling stability, good conductivity, sufficient safety and finally robust flexibility. In contrast to traditional batteries, flexible batteries must have all flexible interior components, such as the anode, cathode, electrolyte and separator, which can withstand frequent multi-directional mechanical strains.4 In recent years there have been many efforts to develop flexible Li-ion batteries including conductive textile electrodes,5 gel polymer electrolytes,6 structure changes, i.e., wire-like,7 and origami assemblies.8 Over the past 27 years, typical Li-ion batteries based on electrochemical intercalation techniques have played a crucial role in enabling widespread portable electronics; however, Liion batteries are quickly reaching their limits in specific capacities between 100 - 250 mAh g-1, and specific energies between 150 - 250 Wh kg-1.9-10 Lithium-sulfur (Li-S) batteries are a promising alternative to Li-ion batteries due to the high theoretical capacity and specific energy of 1,675 mAh g-1 and 2,600 Wh kg-1, respectively, of sulfur cathode.11 However, upon discharge in a Li-S battery, elemental sulfur is converted to lithium polysulfides which are soluble in commonly used organic electrolytes, leading to loss of active material and a significant decline in battery performance. In an effort to exploit the high capacity of sulfur as well as introduce a flexible framework for electrode materials, researchers are looking towards sulfur containing polymers as a cathode material. Among the organosulfur polymers studied, the specific capacities range between 360 - 580 mAh g-1 and the specific energies range between 720 - 1240 Wh kg-1.12-13 According to the results, polymer cathode materials containing sulfur show great promise to replace elemental sulfur while improving the performance of lithium-based batteries.

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In this work, we have developed a highly flexible sulfur-containing polymer as a cathode material, polyphenylene tetrasulfide (PPTS). The fabricated polymer consists of a repeating unit of a phenyl ring with four sulfur atoms attached linearly. The four S–S bonds in the repeating unit can accept 6 Li leading to a high theoretical specific capacity of 788 mAh g-1 corresponding to a theoretical specific energy of 1,586 Wh kg-1. We believe that the polymer developed in this study has the potential to be a major contributor in the progress of flexible battery technology.

Many methods to synthesize sulfur-based polymers have been recently reported.14-15 The reaction of organohalides with sodium polysulfides and the process of inverse vulcanization being the most common techniques for sulfur polymer synthesis.16-17 Such polymers have been shown to work as excellent cathode materials in Li-batteries.18-21 Another effective synthesis route for the formation of a high sulfur content polymer relies on the reaction between sulfur and dithiols.22

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Figure 1. (a) Visual representation of the polyphenylene tetrasulfide (PPTS) synthesis with the equation of the reaction occurring. Color change in lead acetate-based test strip used to confirm the H2S evolution is also depicted. (b) Optical images of the cast PPTS membrane showing its flexibility and transparent property (c) Optical images of the PPTS-CNT cathode membrane showing its flexible nature (d) SEM image of the PPTS-CNT cathode with EDS image in the inset detailing the microstructure and material distribution.

Here we exploit the deprotonation of 1,4-benzenedithiol’s (BDT) characteristic thiol groups to incorporate a polysulfide chain. The high reactivity of the thiol groups facilitated by the conjugated nature of the benzyl ring favor the elimination of the protons on a condensation reaction with elemental sulfur.23 Excess sulfur abets the reaction by seizing the removed protons and evolving hydrogen sulfide gas as shown in Figure 1a. This reaction proceeds without the 5 ACS Paragon Plus Environment

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need for additional heating or a catalyst in CS2/toluene (1:1 v/v) solvent upon dissolution and stirring of the precursors for 12 hours. The resulting PPTS polymer stays dissolved in solution as the H2S is eliminated (see supporting information for detailed synthesis procedure). Figure 1a also shows the evidence of H2S gas evolution via the color change of the lead acetate infused gas test strip when exposed to the reaction. A membrane of the PPTS polymer can be cast upon heating the solution to 140°C to remove the solvent for 10 hours in a polytetrafluoroethylene dish. As displayed in Figure 1b, the membrane is highly flexible for bendable electronic applications and sharply transparent, which has potential to be applied in other areas.17 Battery cathodes were prepared by infusion of the PPTS solution into commercial carbon nanotube (CNT) paper followed by drying to eliminate the solvent. The CNT paper acts as a high strength supporting matrix for the polymer.24 The PPTS-CNT composite cathode is displayed in Figure 1c which demonstrates the flexibility of the cathode proving its ability to easily fold. Scanning electron microscopy (SEM) was used to observe the microstructural details of the cathode. SEM image of the cathode’s surface is shown in Figure 1d. The SEM reveals a network of porous CNT paper with a blanket of PPTS interwoven in the depths of the material. In order to characterize the spatial homogeneity of the material, energy dispersive spectroscopy (EDS) elemental mapping was employed, as shown in the insert image. EDS reveals an even distribution of sulfur signal from PPTS within the cathode. SEM and EDS prove that the polymer is uniformly distributed in the CNT system, ensuring consistent electrical contact between the CNT matrix and PPTS while also imparting channels for electrolyte penetration.

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Figure 2. (a) XRD spectrum of the precursors – sulfur and 1,4-benzenedithiol and the products – PPTS polymer and PPTS-CNT cathode membranes. (b) FTIR spectra of 1,4-benzenedithiol and PPTS polymer with the highlighted region showing change occurring at the thiol group. (c) 1HNMR spectra proving the deprotonation of thiol to yield the polymer. The residual peak indicates the polymer is capped by thiol groups.

In order to validate the above-stated synthesis process, the PPTS polymer and the cathode composite were subjected to a variety of material characterization techniques. We begin by utilizing X-ray diffraction (XRD) as shown in Figure 2a. The XRD pattern for sulfur shows multiple peaks characteristic of its fddd orthorhombic structure. XRD peaks of the other precursor highlight the high crystallinity of BDT. Upon conversion to PPTS, a broad feature is 7 ACS Paragon Plus Environment

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observed in the 20°-35° region due to the non-crystalline nature of the polymer material. Additionally, there is no XRD evidence of elemental sulfur or BDT in the formed PPTS. The XRD analysis of the electrode material, composed of polymer embedded in a CNT matrix, reveals a polymer-like spectra with a significant peak at 2 = 26.3 which is associated with graphitic (002) peak of the CNT matrix.23 XRD sufficiently characterized the material as a polymer, however, further insight into the mechanism of the reaction was examined via Fourier transform infrared spectroscopy (FTIR). We monitor this through the transformation of the thiol vibration (ν(SH)) represented in Figure 2b. This characteristic absorption peak appears at 2,550 cm-1 corresponding to the presence of thiol groups in BDT.25 However, upon PPTS formation, this peak is absent owing to its deprotonation and successful polymer formation. This transformation is also observed for the ν(CSH) vibration at 901 cm-1 (Figure S1, supporting information). Furthermore, an enhancement of C-C and the C-H aromatic stretching at 15001650 cm-1 region and 2900-3100 cm-1 region, respectively, points to localization of conjugation within the phenyl ring as a consequence of the polymerization.26Further understanding of the degree of polymerization can be obtained through nuclear magnetic resonance (NMR) analysis. Figure 2c shows the 1H-NMR spectrum of BDT wherein the peak corresponding to a chemical shift of 3.35 ppm (shown by purple dot) belongs to the proton of the thiol group and the peak at 7.1 ppm (shown by green dot) is that of protons on the phenyl ring.27 On examination of the spectrum for PPTS, a slight shift in the phenyl protons (7 ppm) without the loss of intensity is observed. Whereas, a 20-fold reduction in the thiol intensity is observed indicating that the polymer strands are capped by the thiol groups. Additionally, NMR analysis (Figure S2, supporting information) was used to determine the average molecular weight. It was estimated to

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be about 1200 Da. Hence, this combination of spectroscopic techniques verifies the successful synthesis of the PPTS polymer chains while following the stoichiometry presented in Figure 1a.

Figure 3. (a) Cyclic voltammetry (CV) of the PPTS cathode during 2nd scan performed at 50 µV s-1 showing the polymer redox potentials. (b) Voltage profile of the first cycle of the cathode cycled at C/20 and 1C. The different redox potentials are distinguishable at C/20 (c) long term cycling performance at 1C rate. (d) Capacities delivered by the cell at different C-rates. The cycling rate was based on active material mass in the cathode with 1C = 788 mA g-1.

The electrochemical performance of the synthesized PPTS-CNT cathode was evaluated using standard CR-2032 type coin cells. The electrolyte solution used in this process was 1.0 M of lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) in 1,2-dimethoxyethane (DME) and 1,39 ACS Paragon Plus Environment

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dioxolane (DOL) (1:1 v/v) with a 0.2 M lithium nitrate (LiNO3) additive to passivate the Li metal anode. In theory, the 4 sulfur linkages in the polymer can undergo 6-electron redox reactions to provide a specific capacity of 788 mAh g-1. Figure 3a shows the cyclic voltammetry (CV) response of the cell during the 2nd cycle at a scan rate of 50 µV s-1. In the cathodic scan, the onset of reduction is indicated by the four peaks at 2.24 V, 2.2 V, 2.06 V and 1.97 V. These peaks correspond to the stepwise reduction of the four S-S linkages in the polymer. On the reverse, anodic scan, two oxidation steps at 2.27 V and 2.38 V seems to occur. However, this might be the result of overlapping multi-step oxidation processes with very small free energy changes.28 This multi-step redox of the polymer is further evident during galvanostatic cycling as shown in Figure 3b. The distinctive 4-step reduction process and the combinatorial oxidation process is clearly observed when cycling at low rates such as C/20 as shown. However, during fast rate cycling such as 1C, the multi-step reduction occurring near 2.2 V overlap and seem like one plateau. On a capacity basis, it is interesting to note that nearly two-thirds of the capacity is delivered over 2 V thus indicating a higher average operating voltage thus differentiating it from the behavior of elemental sulfur. In addition to electrochemical response, the chemical changes occurring at the cathode were also determined through XRD. Concurrent with the voltage profile, there is a step-wise scission of the four S-S bonds along with a 6e- and 6Li+ transfer yielding lithium 1,4-benzenedithiolate and lithium sulfide in a 1:2 mole ratio as shown in Figure 3b. This is evidenced through the presence of their characteristic peaks in XRD of the discharged cathode (Figure S3, supporting information). On charge, the bonds reform to yield the polymer as no sulfur or lithium 1,4-benzenedithiolate peaks appears in the XRD spectrum of the recharged cathode (Figure S4, supporting information). It is interesting to note that the broad feature reappears denoting the reformation of the polymer. In addition to chemical changes,

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morphological transitions during cycling were observed using SEM. SEM image along with the EDS mapping of the discharged cathode as in Figure S5 (supporting information) shows a conformal and uniform deposition of the discharge products in the CNT matrix. Moreover, these deposits appear porous which would assist in electrolyte penetration thus enabling good material utilization and enhancing performance.29 The image of the recharged cathode (Figure S6, supporting information) re-affirms the homogeneous reformation of the polymer. These characterizations suggest good reversibility which is further verified through longterm cycling at a rate of 1C. As can be seen in Figure 3c, the cell delivers a capacity of 633 mAh g-1 at 1C and can retain 77% of its initial capacity after 350 cycles. Also, the cell can maintain an average Coulombic efficiency (CE) of 99.2% over the 350 cycles suggesting good reversibility. Another support for this comes from the sustenance of the various voltage plateaus over the 350 cycles as shown in Figure S7 (supporting information). The origin of cycling stability and high CE could be attributed to the low order of the polysulfides formed due to the limiting nature of the tetrasulfide polymer, good confinement of the polymer as well as the discharge products within the CNT matrix and the lithium 1,4-benzenethiolate mediated recharge.30-31 Despite this, the eventual fade observed could be the result of breakdown of the length of the polymer chain over cycles which could lead to its dissolution in the electrolyte leading to shuttle and active material losses. However, a capacity fade rate of only 0.065% per cycle ensures longevity in a personal electronic device like operating scenario. Another imperative metric for the use of such materials is its rate-performance. Therefore, the cathode was tested under various C-rates up to 4C as in Figure 3d. At 1C, 636 mAh g-1 can be obtained on an average which is only a 11% reduction in capacity from 714 mAh g-1 at C/20 despite a 20-fold increase in current density. This performance extends to 2C, 3C and 4C rate wherein 551, 512 and 418 mAh g-1 can be

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delivered by the cell, respectively. Furthermore, upon returning to 1C, the capacity can be recovered to 590 mAh g-1 showcasing the stability of this material. The representative voltage profiles under these rates as shown in Figure S8 (supporting information). There is a 280 mV overpotential increase during discharge which suggests a tolerable power penalty for this polymer at high current density cycling.

Figure 4. Optical images comparing specimens without and at maximum strain of (a) PPTS and (b) PPTS-CNT membranes demonstrating their elastic property. The membrane is outlined with a dashed line as a guide to the eye. The brown bracket seen is the specimen clamp. (c) Voltage profile of the cathodes cycled at 1C on subjecting to different types of mechanical strains (d) 12 ACS Paragon Plus Environment

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Cycling performance of cathodes subjected to mechanical tests carried out at 1C. The cycling rate was based on active material mass in the cathode with 1C = 788 mA g-1

It has been shown that the polymer electrode is electrochemically capable, but, its behavior under compression and tension needs to be examined to suggest implementation into flexible electronics.4, 29 This was done by subjecting the PPTS polymer and the composite cathode to a series of mechanical tests. Figure 4a and 4b show the maximum range of tensile strain (just before failure) for the PPTS membrane and the PPTS-CNT composite as determined by a tensile testing machine. Figure 4a shows a 0.6 mm thick PPTS membrane could stretch to 334% of its initial length revealing the hyperelastic property of the material. In comparison when PPTS is embedded in a CNT matrix, with a combined thickness of 0.45 mm, the electrode can stretch to 107% before failure as shown in Figure 4b whereas, CNT alone can only afford a 12% tensile strain. Thus, the addition of PPTS material to a CNT network affords an 89% increase in elasticity. Besides tensile strain, the electrode could sustain repeated bending over a 5 mm diameter tube as shown in Figure S9 (supporting information) without developing any visible cracks or creases. In addition to physically testing the electrodes elasticity, the electrochemical performance of the electrode was evaluated in a coin cell after 50 cycles of stretching to 30% strain to simulate elasticity of human skin or 50 cycles of bending.32 Figure 4c reveals voltage profiles for comparison of the effect mechanical strain has on the electrode. The average discharge voltage was 2 V, 2.05 V, and 1.95 V for stain-free, bent, and stretched electrodes respectively. As shown, the discharge voltage increase in the bent electrode could be due to the relaxation of bonds occurring due to combination of tensile and compressive strains during bending. Meanwhile, 50 mV lower voltage of the stretched cathode could be due to the storage

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of strain energy in the S-S bonds of the polymer. Nevertheless, this small difference in working voltage has little difference in the energy output of the cell. Figure 4c compares the cycling characteristics under the three types of mechanical strains in question. It shows that all three electrodes offer approximately the same initial discharge capacities around 625 mAh g-1, signifying that the electrochemical performance has not been compromised. Over 200 cycles, the capacity retention is 86%, 79%, 86% for the cathodes undergoing no strain, bending and, stretching respectively which shows favorable capacity retention under duress. This is further complimented by CEs of 99.5%, 99.8% and 99.3% respectively. This encouraging performance of the polymer asserts its potential as a suitable cathode material for flexible batteries.

To recapitulate, the PPTS polymer can be synthesized through the condensation reaction of BDT with elemental sulfur in an appropriate solvent. This polymer can be easily molded to a membrane or a cathode through infusion into a flexible, porous, and conductive scaffold such as CNT paper. Both membranes exhibit excellent flexibility and stretchablity. Furthermore, this cathode material shows favorable electrochemical performance such as reversibility, high capacity, rate capability, and high Coulombic efficiency along with acceptable calendar life even under strained conditions. These characteristics cover the requisites for a high-capacity and energy cathode material for flexible lithium-based batteries.

Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Experimental section, additional electrochemical and materials characterization data are listed in the Supporting Information. 14 ACS Paragon Plus Environment

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Corresponding Author [email protected] (Y. Fu)

ORCID Yongzhu Fu: 0000-0003-3746-9884 Amruth Bhargav: 0000-0002-1793-8340 Author contributions A.B. proposed the concept of the battery. A.B and M.E.B. conducted measurements and analysis under the Y.F.’s supervision. Y.C. performed the SEM and assisted in material synthesis. A.B. and Y.F. prepared the manuscript. All the authors discussed the results and commented on the manuscript. Acknowledgments YF acknowledges the Release Time for Research grant from Office of the Vice Chancellor for Research at Indiana University-Purdue University Indianapolis (IUPUI) and the support from Thousand Youth Talents Program of China. This work was supported by the US National Science Foundation under Grant No. 1335850. We would like to acknowledge the Integrated Nanosystems Development Institute (INDI) for use of their Bruker D8 Discover XRD Instrument, which was awarded through the NSF grant MRI-1429241 and for use of their JEOL7800F Field Emission SEM, which was awarded through NSF grant MRI-1229514 at IUPUI. We also thank the Department of Chemistry and Chemical Biology at IUPUI for the use of their FTIR and NMR facility.

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