Highly Reversible Diphenyl Trisulfide Catholyte for Rechargeable

Nov 14, 2016 - Organotrisulfide (RSSSR) is a new class of high-capacity cathode materials for rechargeable lithium batteries. The organic R group can ...
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Highly Reversible Diphenyl Trisulfide Catholyte for Rechargeable Lithium Batteries Min Wu,† Amruth Bhargav,† Yi Cui,† Amanda Siegel,‡,§ Mangilal Agarwal,†,‡ Ying Ma,*,∥ and Yongzhu Fu*,†,‡ †

Department of Mechanical Engineering, ‡Integrated Nanosystems Development Institute (INDI), and §Department of Chemistry and Chemical Biology, Indiana University−Purdue University Indianapolis, Indianapolis, Indiana 46202, United States ∥ Materials Science and Engineering Center, University of WisconsinEau Claire, Eau Claire, Wisconsin 54702, United States S Supporting Information *

ABSTRACT: Organotrisulfide (RSSSR) is a new class of high-capacity cathode materials for rechargeable lithium batteries. The organic R group can tune the structure and property of organotrisulfide as well as electrochemical performance in batteries. Herein, a nominal diphenyl trisulfide (DPTS, C6H5SSSC6H5) catholyte is reported for rechargeable lithium batteries. Three sulfur atoms allow 4e− storage per molecule, affording DPTS with a theoretical capacity of 428 mAh g−1. The DPTS catholyte is synthesized from a coupling reaction of diphenyl disulfide (DPDS) and elemental sulfur in liquid electrolyte at 70 °C. It is found that the DPTS catholyte is a mixture of DPTS, DPDS, and elemental sulfur in the electrolyte. The lithium cell with the DPTS catholyte delivers an initial specific capacity of 330 mAh g−1DPTS and retains 79% of the initial capacity over 100 cycles at the C/2 rate. The cell delivers an initial discharge specific energy of 751 Wh kg−1DPTS with a high energy efficiency of over 95% at the C/5 rate. The achievable energy density of the DPTS catholyte (1.0 M) is 158 Wh L−1. This study shows that DPTS is a promising high-capacity cathode material for highly reversible lithium batteries. cathode material for rechargeable lithium batteries.15 Dimethyl trisulfide (DMTS, CH3SSSCH3) was used as a model compound for the initial study. It is found that DMTS can take almost 4e− per molecule, and a very low electrolyte/ DMTS mass ratio (e.g., 3:1 μL mg−1) can be used without compromising battery performance. Therefore, a relatively high specific energy can be achieved. DMTS is a volatile compound, which results in instability in battery operation. Alternative organotrisulfides that are less volatile than DMTS are worth pursuing. The organic group R provides an opportunity to tune the structure and property of organotrisulfides in order to achieve desirable electrochemical performance in batteries. In this contribution, we report a nominal diphenyl trisulfide (DPTS, C6H5SSSC6H5) catholyte for rechargeable lithium batteries. Compared to the methyl group in DMTS, the phenyl electron-withdrawing group can enable higher discharge voltage and better cycling stability in batteries. In DPTS, the three sulfur atoms can also allow 4e− of storage per molecule to form two lithium thiophenoxides (C6H5SLi) and one lithium sulfide (Li2S), making DPTS a novel cathode with a theoretical

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ithium−sulfur (Li−S) batteries have been considered as a promising high-energy-density storage system due to the high theoretical capacity of sulfur, that is, 1672 mAh g−1, which is an order of magnitude higher than those of cathodes used in Li-ion batteries.1−3 However, there are several technical issues facing sulfur cathodes such as low sulfur utilization, severe self-discharge, poor cycle life, shuttle effect, and low specific energy due to excess electrolyte use.4,5 At room temperature, elemental sulfur is a solid in the form of an eightatom crown (S8). When lithiation starts, the ring opens and lithium polysulfides (Li2Sx, 2 < x ≤ 8) are formed. These polysulfides are soluble in ether-based electrolyte and could shuttle between the cathode and anode during cycling, leading to low Coulombic efficiency and rapid capacity fading.6 In addition, excess electrolyte has to be used in Li−S cells to enable complete reduction of S8 to lithium sulfide (Li2S); for example, the electrolyte/sulfur ratio has to be ≥8:1 in order to maintain good battery performance.5,7 Several strategies have been developed to overcome these issues. The efforts are primarily focused on holding these intermediate lithium polysulfides within the cathode side by using mesoporous carbon, conductive polymers, carbon interlayers, and metal oxide hosts.8−14 Recently, our group demonstrated that organotrisulfide (RSSSR, R is an organic group) is a promising high-capacity © XXXX American Chemical Society

Received: October 14, 2016 Accepted: November 14, 2016 Published: November 14, 2016 1221

DOI: 10.1021/acsenergylett.6b00533 ACS Energy Lett. 2016, 1, 1221−1226

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ACS Energy Letters

Figure 1. (a) Schematic illustration of the synthesis of DPTS in liquid electrolyte at elevated temperature (i.e., 70 °C); (b) photograph of 1.0 M DPTS catholyte in a vial; and (c) schematic representation of the addition of the DPTS catholyte into a multiwalled carbon nanotube (MWCNT) paper current collector.

Figure 2. (a) XRD patterns of DPTS, DPDS, and sulfur; (b) DSC spectra of DPTS, DPDS, and a mixture of DPDS/sulfur; (c) GC spectrum of DPTS in DME/DOL solution; and (d) mass spectrum of the DPTS compound at 23.6 min.

capacity of 428 mAh g−1. The nominal DPTS catholyte was synthesized by a coupling reaction of diphenyl disulfide (DPDS) and elemental sulfur with a molar ratio of 1:1 at 70 °C in an ether-based electrolyte, as shown in Figure 1. The S−S bonds in DPDS and elemental sulfur can break, and a sulfur atom can be bonded in between two C6H5S• radicals to form a DPTS molecule. A homogeneous solution with yellow color is formed, which can be evaluated as a catholyte in lithium batteries using a carbon nanotube current collector.15,16 In contrast, the mixture of DPDS and sulfur in the electrolyte without heating results in phase separation (Figure S1 in the Supporting Information). The solubility of nominal DPTS can easily reach up to 2.0 M in the electrolyte, corresponding to 8 Faradays of charge storage per liter within the 4e− reduction reaction. Given the average operating voltage of 2.2 V (shown later), the theoretical energy density for the 2.0 M DPTS catholyte is up to 472 Wh L−1.

To characterize the synthesized DPTS, X-ray diffraction (XRD), differential scanning calorimetry (DSC), gas chromatography−mass spectrometry (GC-MS), and 13C-nuclear magnetic resonance (13C NMR) were performed. The XRD and DSC samples were prepared by forming DPTS in a mixture of 1,2-dimethoxyethane (DME)/1,3-dioxolane (DOL) without lithium salt. Once the solvent was removed by evaporation, a gel-like sample was formed in a carbon nanotube paper substrate. For comparison, DPDS and sulfur samples were also obtained from the solution-drying process in DME/DOL mixture solvent and carbon disulfide, respectively. The XRD patterns in Figure 2a show that both sulfur and DPDS samples exhibit multiple crystal peaks besides the carbon peak at 26°.17 In contrast, the DPTS sample almost shows no crystal peaks except the carbon peak at 26°, indicating its amorphous structure. The DSC spectra in Figure 2b show DPDS, and the mixture of DPDS/sulfur exhibits a strong endothermic peak at 61−62 °C, which is due to the melting of DPDS. In contrast, 1222

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Figure 3. (a) CV of a Li/DPTS cell: the arrows indicate the potential scan directions; (b) voltage−capacity profile of a Li/DPTS cell in the first cycle at the C/10 rate; (c) equilibrium structure of DPTS; (d) electrostatic potential of DPTS; note that colors toward red correspond to a negative potential, while colors toward blue correspond to a positive potential; (e) relaxed structure of DPTS + Li; and (f) LUMO plot of DPTS, where the blue color indicates a positive phase whereas the red color refers to a negative phase.

the presence of DPTS in the synthesized catholyte is confirmed. However, both results may not reflect the true compositions in the as-synthesized DPTS solution because the measurement conditions could affect the reaction equilibrium between DPDS and DPTS in the samples, which has been observed with other organic polysulfides due to their instability.20 For example, DPTS may transform back to DPDS in the injection period in the GC measurement due to the high temperature (180 °C). The dilution of DPTS in NMR sample preparation could result in the decomposition of DPTS into DPDS. Additionally, the reduced bond strength in the case of trisulfides may favor their conversion to disulfides under these circumstances.19 In any case, the stoichiometric formula of the active material in the catholyte is still C6H5SSSC6H5 with a nominal capacity of 428 mAh g−1. The electrochemical performance of the DPTS catholyte was evaluated in lithium half-cells. A certain amount of 0.5 M DPTS catholyte was injected into a binder-free carbon nanotube paper current collector, which served as a reservoir for holding discharged and charged products. LiNO3 additive was used in the cell to improve stability of the lithium metal anode by forming a passivation layer that reduced the reactions between lithium metal and DPTS, therefore increasing the cycling stability.15 Figure 3a shows the cyclic voltammogram (CV)

the DPTS sample shows a much lower melting point at around 49 °C, which is consistent with its amorphous characteristic. Figure 2c presents the GC spectrum of DPTS. Two strong peaks at retention times of 14.9 and 23.6 min and a small peak at 21.6 min are seen. The peak at 23.6 min is attributed to DPTS as it shows three strong mass/charge ratio (m/z) peaks at 250 (C6H5SSSC6H5), 141 (C6H5SS), and 109 (C6H5S) in Figure 2d. The peaks at 14.9 and 21.6 min can be assigned to DPDS and elemental sulfur, respectively, based on the m/z ratios in Figure S2. No more peaks can be seen after 25 min in the GC spectrum, indicating that longer organosulfides may not form such as diphenyl tetrasulfide or pentasulfide. The peak area ratio of DPDS/DPTS/sulfur in GC is 46.7:50.6:2.7, as shown in Table S1, indicating that the DPTS catholyte is a mixture solution rather than a pure DPTS solution. 13C NMR analysis was also conducted to verify the presence of DPTS upon synthesis. The 13C NMR spectrum for DPDS (Figure S3a) shows the chemical shift for the quaternary carbon (marked in red) at 135.74 ppm, in agreement with literature values for DMSO-d6 solvent.18 The spectrum for the DPTS (Figure S3b) shows a clear chemical shift of the quaternary carbon (marked in blue) to 135.08 ppm. This lowered chemical shift corresponds to the increase in chain length, meaning that DPTS is formed.19 On the basis of GC-MS and NMR analysis, 1223

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Figure 4. (a) Cycling performance and Coulombic efficiency of a Li/DPTS cell at the C/2 rate; (b) rate performance of a Li/DPTS cell; (c) cyclability of Li/DPTS cells with different DPTS loadings (3.9 and 7.7 mg cm−2) at the C/5 rate; the specific energy is based on the mass of DPTS in the catholyte; and (d) energy density of the DPTS catholyte with different concentrations (0.5 and 1.0 M) in the 20th cycle in (c).

profile of the cell; two cathodic peaks at 2.2 and 2.0 V and an anodic peak at 2.4 V are seen, which are distinct from only one major cathodic peak observed with the DMTS catholyte in our previous report.15 The initial voltage profile of the charge/ discharge of the cell at the C/10 rate (1 C = 428 mA g−1) is shown in Figure 3b; the discharge specific capacity is 398 mAh g−1, which is 93% of the theoretical capacity of the nominal DPTS catholyte. The high discharge voltage plateau consists most of the total capacity (72.5%), which is consistent with the CV curve. Also, we can see that the voltage difference between the charge and discharge curve is fairly small, which is good for achieving high energy efficiency. The washed, discharged electrode shows very broad (220) and (311) plane peaks of Li2S in the XRD pattern (Figure S4), indicating that the formed Li2S crystals are rather small, which is good for the recharge of Li2S and maintaining a low overpotential.21 The morphology of the discharged electrode is shown in Figure S5, where the nanopores in the carbon paper are filled with the discharge products. EDS mapping confirms that the sulfur-containing species are uniformly distributed within the carbon paper. 13C NMR spectra for the recharged electrode (Figure S3c) in comparison with that of the synthesized DPTS show that upon battery recharge the catholyte composition reverts back to the DPTS state. To identify the reaction mechanisms, first-principles calculations based on density functional theory (DFT) were performed (for more detailed information, please refer to the Computational Section in the Supporting Information). Figure 3c plots the equilibrium geometry of DPTS, while the electrostatic potential, which is the interaction energy of a positive charge with the molecule, is depicted in Figure 3d. It can be seen that there are three attractive positions corresponding to the three sulfur atoms along the chain. For simplicity, we label the three sulfur positions from left to right as S1, S2, and S3, respectively. Thus, the first step during the discharge process corresponds to the formation of a Li−S bond

on one of those positions due to electrostatic attraction. To reveal the structural changes of the DPTS molecule after this initial lithiation step, DFT geometry relaxation was performed and a significant increase in the S−S bond length was observed. In fact, if the Li+ is bonded to S1, the S1−S2 bond length increases from 2.1 to 4.3 Å, as shown in Figure 3e. Furthermore, no charge overlap exists between S1 and S2, which essentially means that the S1−S2 bond breaks and the DPTS molecule splits up to form C6H5SLi and C6H5SS• radical, the latter of which will further obtain a Li+ and one electron to form C6H5SSLi. A similar increased bond length and reduced charge overlap are also observed in the case that Li+ is bonded to S2, and the changes involving Li+−S3 bonding is the same as that of Li+−S1 due to symmetry. Thus, the overall initial reaction is a two-electron process and can be described by C6H5SSSC6H5 + 2Li ↔ C6H5SLi + C6H5SSLi, corresponding to a first discharge plateau. The second lithiation step will continue to involve the S−S bond in C6H5SSLi to form C6H5SLi and Li2S, as given by C6H5SSLi + 2Li ↔ C6H5SLi + Li2S. It is known that for molecules with similar structures, the lowest unoccupied molecular orbital (LUMO) level correlates with the reduction potential, and a lower LUMO energy corresponds to a higher reduction potential.22,23 The LUMO energy levels of DPTS (Figure 3f) and C6H5SSLi are −2.15 and −0.99 eV, respectively, which is consistent with the lower voltage of the second plateau. Interestingly, the LUMO energy level of DPTS is lower than that of DMTS (−1.62 eV). In other words, the two phenyl electronwithdrawing groups decrease the energy level of the LUMO orbital, leading to a higher reaction voltage of DPTS (2.4 V) than that of DMTS (2.1 V). In the charge, only a single voltage is seen, indicating a single reaction. Overall, the four-electron reaction can be described by C6H5SSSC6H5 + 4Li ↔ 2C6H5SLi + Li2S. Figure 4a presents the cycling performance and Coulombic efficiency of the cell at the C/2 rate. The initial discharge 1224

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capacity is up to 330 mAh g−1. After 100 cycles, the discharge capacity is 260 mAh g−1, retaining 79% of the initial capacity. Capacity fading mainly resulted from the loss of DPTS as it can diffuse to the anode side and react with the lithium metal. The SEM image and sulfur mapping in Figure S6 clearly show that the cycled lithium metal anode is coated with a layer of compounds containing sulfur species. The Coulombic efficiency is above 99.5% for all cycles except the first one. Compared to DMTS, DPTS shows better cycling stability, which is due to the bulky phenyl groups and low volatility of DPTS. Less DPTS would diffuse to the anode side than DMTS under the same operating conditions. The rate performance of a cell with the DPTS catholyte is shown in Figure 4b. The cell shows discharge capacities of 376, 345, 312, 258, and 199 mAh g−1 at C/10, C/5, C/2, 1 C, and 2 C, respectively. At each rate, the discharge capacity slowly decreases with cycles. When the rate returned to C/10, the discharge capacity recovered to 322 mAh g−1. The corresponding voltage profiles are shown in Figure S7; at higher C rates, the charge/discharge voltage profiles remain the same except that the ohmic overpotential increases. The high discharge voltage region and low discharge voltage region remain relatively the same ratios. Figure 4c shows the cycling performance of cells with DPTS loadings of 3.9 and 7.7 mg cm−2 at the C/5 rate; the high loading of DPTS is obtained by using a 1.0 M DPTS catholyte. At low loading, the initial discharge specific energy is 751 Wh kg−1DPTS. After 50 cycles, the cell discharge specific energy is still as high as 673 Wh kg−1DPTS, corresponding to a specific energy fading rate of 0.2%. The energy efficiency is above 95% for all cycles except the first cycle. When the loading is increased to 7.7 mg cm−2, the initial discharge specific energy is 686 Wh kg−1DPTS, and the end discharge specific energy is 551 Wh kg−1DPTS after 50 cycles. The energy efficiency is still maintained over 91% for all cycles except the first cycle. The high energy efficiency is not only good for energy saving but can also improve cell safety as little heat is produced upon cycling.24 Figure 4d shows the energy density of the DPTS catholyte in the 20th cycle; the high DPTS loading cell shows an energy density of 158 Wh L−1, which is comparable to or better than recent reports about liquid batteries or semiliquid batteries.25−28 In summary, we synthesized a nominal DPTS catholyte for rechargeable lithium batteries. The synthesis method is facile, and the obtained catholyte consists of DPDS, DPTS, and elemental sulfur. DFT calculation clarifies the lithiation process of DPTS and indicates the 4e− reduction reaction. The initial discharge specific capacity at the C/10 rate is 398 mAh g−1DPTS (93% of the theoretical capacity of DPTS). At the C/2 rate, the cell shows an initial capacity of 330 mAh g−1DPTS and retains 79% of the initial capacity over 100 cycles with DPTS loading of 3.9 mg cm−2. At the C/5 rate, the initial specific energy is up to 751 Wh kg−1DPTS, with an energy retention of 91% over 50 cycles. The energy efficiency is >95% for all cycles except the first cycle. With a higher loading of 7.7 mg cm−2, the cell delivers an initial discharge specific energy of 686 Wh kg−1DPTS and a discharge specific energy of 551 Wh kg−1DPTS after 50 cycles. This study reveals that DPTS, as another organotrisulfide compound, can be a promising cathode material for highly reversible lithium batteries with high energy efficiency.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsenergylett.6b00533. Experimental methods, mass spectra, 13C NMR, additional XRD, SEM, and EDS mapping (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (Y.M.). *E-mail: [email protected] (Y.F.). ORCID

Yongzhu Fu: 0000-0003-3746-9884 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by a startup grant from the Purdue School of Engineering and Technology and the Department of Mechanical Engineering at Indiana University−Purdue University Indianapolis (IUPUI). We would like to acknowledge the Integrated Nanosystems Development Institute (INDI) for use of their Bruker D8 Discover XRD instrument, which was awarded through NSF Grant MRI-1429241 and for use of their JEOL7800F field emission scanning electron microscope, which was awarded through NSF Grant MRI-1229514. The authors are grateful to Dr. Bruce D. Ray of the Department of Physics, IUPUI for help in collecting the NMR data. Y.M. would like to acknowledge support from the Materials Science and Engineering Center at the University of WisconsinEau Claire.

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