Supporting Information

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1 Supporting Information Highly Reversible Diphenyl Trisulfide Catholyte for Rechargeable Lithium Batteries Min Wu, a Amruth Bhargav, a Yi Cui, a Amanda Siegel, b,c Mangilal Agarwal, a,b Ying Ma,*,d and Yongzhu Fu*,a,b a Department of Mechanical Engineering, Indiana University-Purdue University Indianapolis, Indianapolis, IN 46202, United States b Integrated Nanosystems Development Institute (INDI), Indiana University-Purdue University Indianapolis, Indianapolis, IN 46202, United States c Department of Chemistry and Chemical Biology, Indiana University-Purdue University Indianapolis, Indianapolis, IN 46202, United States d Materials Science and Engineering Center, University of Wisconsin-Eau Claire, Eau Claire, WI 54702, United States 1

2 Experimental Section: Materials: Diphenyl disulfide (C12H10S2, 99%, Sigma Aldrich), sulfur (99.5%, Alfa Aesar), lithium bis(trifluoromethanesulfonimide) (LiTFSI, LiN(CF3SO2)2, 99%, Acros Organics), lithium nitrate (LiNO3, %, Acros Organics), 1,2-dimethoxyethane (DME, 99.5%, Sigma Aldrich), and 1,3-dioxolane (DOL, 99.8%, Sigma Aldrich) were purchased and used as received. Electrolyte and diphenyl trisulfide (DPTS) catholyte preparation: The ether electrolyte is composed of 1.0 M LiTFSI and 0.1 M LiNO3 in a mixture solvent of DME and DOL (1:1 v/v). To prepare DPTS catholyte, a stoichiometric ratio of diphenyl disulfide and sulfur were added into the electrolyte to render 0.5 or 1.0 M DPTS in the solution. The mixture solution was heated at 70 C in the glovebox for overnight to produce a yellow solution. Li/DPTS cell fabrication and electrochemical evaluation: Commercial binder-free carbon nanotube paper called buckypaper (NanoTechLabs, Inc) was used as the current collector in this study. The carbon paper was cut into 0.97 cm 2 discs (D = 11 mm, about 4.4 mg each) and dried at 100 C for 24 h in a vacuum oven before use. CR2032 coin cells were used and the cells were made in an Ar-filled glove box. First, 30 μl DPTS catholyte (0.5 or 1.0 M) was added into the current collector, the DPTS loadings are 3.8 mg (3.9 mg cm -2 ) and 7.5 mg (7.7 mg cm -2 ) corresponding to a DPTS weight percent of 46% and 63% in the electrode, respectively. Then, a Celgard 2400 separator was placed on the top of the electrode followed by adding 20 μl blank electrolyte on the separator. Finally, a piece of lithium foil and nickel foam as a spacer was placed on the separator. The cell was crimped and taken out of the glove box for the electrochemical evaluation. 2

3 Cyclic voltammetry (CV) was performed on a BioLogic VSP potentiostat. The potential was swept from open circuit voltage to 1.7 V and then cycled between 1.7 and 2.7 V at a scanning rate of 0.05 mv s -1. Cells were galvanostatically cycled on an Arbin BT2000 battery cycler at different C rates (1C = 428 ma g -1 ) and voltage ranges (C/10 and C/5: V, C/2: V, 1C: V, 2C: V). Characterizations: The X-ray diffraction (XRD) data of the DPTS, DPDS, and elemental sulfur electrodes were collected on a Bruker D8 Discover XRD Instrument equipped with Cu Kα radiation. The DPTS sample was prepared by forming DPTS in a mixture of DME/DOL without lithium salt. Once the solvent was removed by evaporation, DPTS gel was formed in a carbon paper substrate. For comparison, DPDS and sulfur samples were also prepared from the solution-drying process in DME/DOL and carbon disulfide (CS2), respectively. The scanning rate was 2 min 1, and 2θ was set between 15 and 65. The discharged electrode of DPTS was protected by a Kapton tape and characterized with the same procedure. The morphological characterization of the cycled DPTS electrodes was conducted with a JEOL JSM-7800F field emission scanning electron microscopy (SEM). The elemental mapping was performed with energy-dispersive X-ray spectroscopy (EDS) attached to the SEM with 15 kv. Differential scanning calorimetry (DSC) was performed on TA 2000 Thermogravimetric Analysis. The DPTS sample was prepared from the solution-drying process in DME/DOL. The DPDS/sulfur sample was prepared by grinding a mixture of DPDS and elemental sulfur with a 3

4 molar ratio of 1:1. All the samples are about 5 mg and tested in a nitrogen atmosphere with a temperature ramping rate of 5 C. GC-MS data were obtained with an Agilent 7890A gas chromatograph coupled to an Agilent 5975C mass spectrometer. The column is an Agilent J&W HP-5ms Ultra Inert GC Column, with the following specifications: 30 m column length, 0.25 mm internal diameter, 0.25 µm film thickness. 0.5 M DPTS in DME/DOL was placed inside a 3 ml testing vial, then 1 µl solution was injected into the GC machine with microneedle with a split ratio of 100:1. The oven temperature program utilized an initial temperature of 60 C followed by a ramp of 8 C min -1 to 180 C, and held at 180 C for 35 min. The MS transfer line temperature was 180 C. The mass spectrum in the National Institute of Standards and Technology (NIST) version 14 mass spectral database (DPDS) was used. The spectrum for DPTS was consistent with expected fragmentation patterns based on the mass spectra of DPDS. 13 C-Nuclear Magnetic Resonance ( 13 C NMR) analysis of DPDS, the synthesized DPTS, and the recharged cathode was performed on a Varian Inova 500 MHz NMR spectrometer. The material of interest was dissolved in dimethyl sulfoxide-d6 (99.9 atom % D, Sigma Aldrich) to yield an approximately 0.05 M solution. It was then transferred to an NMR tube before subjecting it to the NMR test. Chemical shifts (δ) are referenced downfield from tetramethylsilane ((CH3)4Si) using the residual solvent peak as an internal standard ((CD3)2SO, ppm for 13 C NMR). The cathode for NMR study was cycled with only LiNO3 as the lithium salt in the electrolyte to exclude other carbon species in the 13 C NMR spectra. 4

5 Computational Section: Geometry optimization and total energy calculations based on the density functional theory (DFT) were performed using SPARTAN software package [1]. The Becke-Lee-Yang-Parr (B3LYP) hybrid exchange-correlation functional [2, 3] and the G** basis set. To simulate the effect of the solvent, a polarizable continuum model (PCM) was used, and the dielectric constant was set to that of DME that was used as electrolyte in this study (the dielectric constant of DOL is similar to that of DME). [1] Y. Shao, L.F. Molnar, Y. Jung, J. Kussmann, C. Ochsenfeld, S.T. Brown, A.T.B. Gilbert, L.V. Slipchenko, S.V. Levchenko, D.P. O Neill, R.A. DiStasio Jr., R.C. Lochan, T. Wang, G.J.O. Beran, N.A. Besley, J.M. Herbert, C.Y. Lin, T. Van Voorhis, S.H. Chien, A. Sodt, R.P. Steele, V.A. Rassolov, P.E. Maslen, P.P. Korambath, R.D. Adamson, B. Austin, J. Baker, E.F.C. Byrd, H. Dachsel, R.J. Doerksen, A. Dreuw, B.D. Dunietz, A.D. Dutoi, T.R. Furlani, S.R. Gwaltney, A. Heyden, S. Hirata, C-P. Hsu, G. Kedziora, R.Z. Khalliulin, P. Klunzinger, A.M. Lee, M.S. Lee, W.Z. Liang, I. Lotan, N. Nair, B. Peters, E.I. Proynov, P.A. Pieniazek, Y.M. Rhee, J. Ritchie, E. Rosta, C.D. Sherrill, A.C. Simmonett, J.E. Subotnik, H.L. Woodcock III, W. Zhang, A.T. Bell, A.K. Chakraborty, D.M. Chipman, F.J. Keil, A.Warshel, W.J. Hehre, H.F. Schaefer, J. Kong, A.I. Krylov, P.M.W. Gill and M. Head-Gordon, Phys. Chem. Chem. Phys., 8, 3172 (2006). [2] Becke, A. D., J. Chem. Phys. 1988, 88, [3] Lee, C, Yang, W., Parr, R. G. Phys. Rev. B 1988, 37,

6 a) b) Figure S1. a) A photograph of 2.0 M DPTS catholyte after the reaction at 70 C; b) a photograph of equivalent moles of DPDS and sulfur powder in the liquid electrolyte without heating, DPDS dissolves but sulfur remains unreacted in the electrolyte. 6

7 a) b) Figure S2. The mass spectra of a) diphenyl disulfide (DPDS) compound at the retention time of 14.9 min and b) elemental sulfur (S8) at the retention time of 21.6 min in the GC in Figure 2c. Table S1. The integration areas of different compounds in the GC spectrum in Figure 2c. Integration area/area percent (%) DPDS DPTS Elemental sulfur 0.5M DPTS 1.2E+09/ E+09/ E+07/2.7 7

8 a) b) c) Figure S3. The 13 C NMR spectra of a) pure DPDS; b) as-synthesized DPTS; c) the recharged catholyte of DPTS with LiNO3 as the lithium salt. 8

9 Figure S4. XRD pattern of the washed, discharged electrode of DPTS. 9

10 Figure S5. a) SEM image of the cycled electrode of DPTS. EDS mapping of (b) carbon, (c) sulfur, and (d) overlap. 10

11 a) b) S Figure S6. a) SEM image of the cycled lithium anode surface; b) the corresponding sulfur mapping on the lithium metal. 11

12 Figure S7. The representative voltage-capacity profiles of the DPTS catholyte at different rates. 12