Supporting Information

Transcription

1 Supporting Information A Class of Organopolysulfides as Liquid Cathode Materials for High Energy Density Lithium Batteries Amruth Bhargav, Michaela Elaine Bell, Jonathan Karty, Yi Cui, and Yongzhu Fu # * Department of Mechanical Engineering, Indiana University-Purdue University Indianapolis, Indianapolis, IN 46202, United States Department of Chemistry, Indiana University, Bloomington, IN 47405, United States School of Mechanical Engineering, Purdue University, West Lafayette, IN 47907, United States # College of Chemistry and Molecular Engineering, Zhengzhou University, Zhengzhou , China *Corresponding author: yfu@zzu.edu.cn (Y. Fu) S-1

2 Experimental Materials: Benzenethiol (PhSH, C 6 H 5 SH, 99%, Acros Organics), sulfur (S 8, 99.5+%, Acros Organics), carbon nanotubes (CNT, 95+%, OD: 8-15nm, L: 50µm, Nanostructure and Amorphous Materials, Inc.), CNT buckypaper (20 GSM, NanoTechLabs, Inc), ethanol (C 2 H 5 OH, Fisher Chemical), phenyl disulfide (PDS, C 12 H 10 S 2, 99%, Sigma Aldrich), lithium sulfide (Li 2 S, 99.98%, Sigma Aldrich), lithium bis(trifluoromethanesulfonimide) (LiTFSI, LiN(CF 3 SO 2 ) 2, 99%, Acros Organics), lithium nitrate (LiNO 3, %, Acros Organics), 1,2- dimethoxyethane (DME, 99.5%, Sigma Aldrich), 1,3-dioxolane (DOL, 99.8%, Sigma Aldrich), chloroform (CHCl 3, ACS spectrophotometric grade, 99.8%, Sigma Aldrich), hydrogen sulfide test strips (Sigma Aldrich), and potassium bromide (KBr, FTIR Grade, Alfa Aesar) were purchased and used as received. Phenyl polysulfide synthesis: Appropriate amount of sulfur (3, 4 and 5 equivalents for phenyl tetrasulfide (PTS), phenyl pentasulfide (PPS), and phenyl hexasulfide (PHS), respectively) was added slowly into a vial containing 2 equivalents of benzenethiol (typically g was used) under constant stirring. After the initial vigorous reaction, the mixture was heated to 40 C for 6 hours. This yielded a clear polysulfide liquid without any visible sulfur particles which was used in the following experiments. Evolution of H 2 S gas was detected using standard test strips containing lead acetate. The test strips were held over the vial for approximately a minute before noting the color change. S-2

3 Typical cell fabrication: Commercial binder-free multi-walled carbon nanotube paper called buckypaper (BP) was used as the conventional current collector in this study except for high-loading cells. The BP was cut into 0.97 cm 2 discs (7/16 inch diameter, weighing about 1.8 mg each) and dried at 100 C for 24 h in a vacuum oven before use. CR2032 type coin cells were fabricated inside the glove box with 1.0 M LiTFSI and 0.2 M LiNO 3 in mixture solvent of DME and DOL (1:1 v/v) as the electrolyte. First, 1 µl of the phenyl polysulfide was added into the BP current collector followed by 20 µl of electrolyte. Then a Celgard 2400 separator was placed on the top of the BP electrode followed by adding 10 µl electrolyte on the top of the separator. This was topped off with a lithium foil and nickel foam spacer before the coin cell was crimped. The active-material loading was calculated based on the density of the polysulfide. High-loading cell fabrication: 225 mg of CNT was ultrasonicated using a vibracell VC505 sonicator for 1 hour causing the CNT to interweave. This was vacuum filtered on to a 7 cm filter paper and washed repeatedly with copious amount of D.I. water. A free-standing, binder-free CNT paper resulted. The CNT paper was cut into 0.97 cm 2 discs (7/16-inch diameter, weighing about 5.5 mg each) and dried at 100 C for 24 h in a vacuum oven before use. To prepare the cell, 9 µl of phenyl hexasulfide was added into the current collector followed by 20 µl of electrolyte. Then a Celgard 2400 separator was placed on the top of the BP electrode followed by adding 15 µl electrolyte on the top of the separator. Following this, lithium foil anode was placed and the cell was crimped. The phenyl hexasulfide loading was 11.9 mg and composed 68.4% of the cathode mass. Total volume of the electrolyte in the cell yielded an electrolyte to active material ratio of 3 µl mg -1. S-3

4 Lithium polysulfide cathode and cell fabrication: Lithium polysulfide solution in ethanol was prepared by adding stoichiometric amounts of sulfur powder and Li 2 S and stirring overnight to give a 0.75 M Li 2 (4.5 M sulfur) solution. Then, 20 µl Li 2 solution was added into a CNT paper current collector and dried in the glove box. Afterwards, another 18 µl of Li 2 was added, and the Li 2 /CNT was dried again. The corresponding sulfur mass in the electrode is 5.47 mg (5.64 mg cm -2 ) which equates to an areal capacity of ~9.45 mah cm -2 which closely matches that of high-loading PHS cathode. The cells were fabricated similar to that of PHS cathode while adjusting the total volume of electrolyte in the cell to obtain 3, 5 and 10 µl mg -1 electrolyte to active material ratios. Electrochemical cell testing: Cyclic voltammetry (CV) was performed on a BioLogic VSP potentiostat. The potential was swept from open circuit voltage (OCV) to 1.8 V and then swept back to 3.0 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 = 570 ma g -1 for PTS, 683 ma g -1 for PPS and 775mA g -1 for PHS and based on the mass of active material in the cells). The cells were discharged to 1.8 V at C/10, C/2 and 1C, 1.7 V at 2C, 1.6 V at 3C and 4C, 1.55 V at 6C and 1.5 V at 10C to enable maximum material utilization while avoiding LiNO 3 decomposition. All cells were recharged to 3 V. S-4

5 Materials characterization: Cells used for materials characterization was cycled using a 0.5 M LiNO 3 in DME/DOL (1:1 v/v) electrolyte (to avoid presence of sulfur from it) at C/10 till the appropriate cut-off voltage. The cell was then opened, the cathode was extracted, washed using DME and dried in glove box atmosphere before mounting on instrument sample holders. The samples were transferred to the instrument in an argon-filled airtight container. X-ray photoelectron spectroscopy (XPS) experiments were performed using PHI Versa Probe II instrument equipped with focused monochromatic Al Kα source. The X-ray power of 50 W at 15 kv was used for 200 µm beam size. The PHI dual charge compensation system was used on all samples. XPS spectra with the energy step of 0.1 ev were recorded using software SmartSoft XPS v2.0 and processed using Casa software. The spectrum was calibrated using HOPG strips attached alongside the samples as standard and setting its C 1s binding energy (BE) to ev and verified using adventitious (aliphatic) carbon BE. The XPS spectra were fitted using a combination of Gaussians and Lorentzians with 0-50% of Lorentzian contents. Shirley background was used for curve-fitting. The S 2p 3/2 and S 2p 1/2 doublets were constrained using peak areas of 2:1 with a splitting of 1.18 ev. X-ray diffraction (XRD) data were collected on a Bruker D8 Discover XRD Instrument equipped with Cu Kα radiation. The samples were protected in the sample holder with kapton film. The scanning rate was 2 min 1, for 2θ between 20 and 60. Fourier transform infrared (FTIR) absorption spectra were recorded on a Thermo Scientific-Nicolet is10 FTIR spectrometer. 64 scans between 400 cm 1 to 4000 cm 1 were recorded per sample. Samples were prepared by grinding the cathodes with KBr within the glove box and pelletizing it using an FTIR die set. S-5

6 Ultraviolet-visible (UV-Vis) spectroscopy was performed on Thermo Scientific Genesys 10 UV-Vis spectrophotometer. UV-Vis absorption spectra were collected using a 1 cm quartz cuvette over the wavelength range of nm. All spectra were collected using 2.5 ml of 0.1 M solution of the sample in chloroform. Blank chloroform was used as a background for these measurements. Electron ionization mass spectrometry (EI-MS) was performed using a Thermo MAT- 95XL magnetic sector instrument. Samples were diluted to approximately 1 mg ml -1 in CHCl 3 ; 2 µl of this solution was placed in an aluminum crucible for MS. The crucible was placed in the source ( mbar pressure) and heated a temperature sufficient to volatilize the sample and generate ions (manually ramped between 75 C and 225 C). The mass scale was internally calibrated with ions from perfluorokerosene vapor that was leaked into the source throughout the experiment. Scanning electron microscopy (SEM) of the electrodes was performed using a JEOL JSM-7800F microscope at 10 kv. The elemental mapping was performed with energy-dispersive X-ray spectroscopy (EDX) attached to the SEM to identify carbon and sulfur species. Volume change calculation Table S1. Physical properties of the different cell materials Ph 2 PhSLi Li 2 S S 8 Molar mass (g mol -1 ) Density (g cc -1 ) Volume per mol (cc mol -1 ) S-6

7 Volume change in PHS cathode: 10 Li 1 Ph 2 2 PhSLi + 4 Li 2 S = cc yields 2 (124.31) cc + 4 (27.68)cc = cc yields cc cc V V = cc yields = cc = % expansion = Similarly, volume expansion in PPS cathode was calculated to be 37.32% and in PTS cathode it was 36.9%. Volume change in Sulfur cathode: 16 Li 1 S 8 8 Li 2 S = cc yields 8 (27.68) cc V V = cc yields = cc = 79.06% expansion = S-7

8 Additional Data Figure S1. Photograph of the reaction minutes after mixing benzenethiol and sulfur. S-8

9 Intensity (a.u.) Normalized absorbance (a.u.) PHS S-S bond stretching out-of-plane phenyl ring deformation PPS PTS Wavenumber (cm -1 ) Figure S2. FTIR spectrum of the synthesized phenyl polysulfides in the cm -1 range showing the characteristic polysulfide peaks PTS Figure S3. EI-MS spectrum confirming the presence of phenyl tetrasulfide (PTS) at m/z of 282. m/z S-9

10 Intensity (a.u.) Intensity (a.u.) PPS m/z Figure S4. EI-MS spectrum confirming the synthesis of phenyl pentasulfide (PPS) at m/z of 314 along with a trace amount of phenyl hexasulfide (PHS) at m/z of PHS m/z Figure S5. EI-MS spectrum confirming the synthesis of phenyl hexasulfide (PHS) at m/z of 346 along with a trace amount of phenyl heptasulfide at m/z of 378. S-10

11 Intensity (a.u.) CNT (002) Intensity (a.u.) CNT (002) Li 2 S (220) Li 2 S (311) PHS PPS PTS (degrees) Figure S6. XRD spectra of the washed, discharged cathode of PTS, PPS and PHS. Broad peaks of Li 2 S are discernable indicating its presence in amorphous state. PHS PPS PTS (degrees) Figure S7. XRD spectra of the washed, recharged cathode of PTS, PPS and PHS. Only CNT peaks are visible indicating the absence of active materials. S-11

12 Figure S8. SEM image of discharged PPS cathode. The inset image is the EDX map of the cathode. Figure S9. SEM image of discharged PTS cathode. The inset image is the EDX map of the cathode. S-12

13 Voltage vs Li/Li + (V) Figure S10. SEM image of washed, recharged cathode. The inset image is the EDX map of the cathode. The absence of particles indicates the reformation of polysulfides on recharge that were removed upon washing Cycle # Specific capacity (mah g -1 ) Figure S11. Voltage profile for PHS cathode during long term cycling performance at 1C. The cycling rate was based on active material mass in the cathode with 1C PHS = 775 ma g -1. S-13

14 Specific energy (Wh kg -1 ) Voltage vs Li/Li + (V) C/2 4C 1C 6C 2C 10C 3C Specific capacity (mah g -1 ) Figure S12. Voltage profile for PHS cathode at various C-rates. The cycling rate was based on active material mass in the cathode with 1C PHS = 775 ma g -1. The discharge cutoff was set so as to enable maximum material utilization Specific power (W kg -1 ) Figure S13. Gravimetric Ragone plot of PHS cathode based on performance at various C-rates. S-14

15 Specific energy (Wh L -1 ) Specific power (W L -1 ) Figure S14. Volumetric Ragone plot of PHS cathode based on performance at various C-rates. Figure S15. Photograph of the lithium metal anode (left) and the separator (right) extracted out of the high PHS loading cell operated at 3 µl mg -1. Lithium dendrite formation and subsequent penetration of the separator membrane leading to shorting of the cell is visibly evident. S-15

16 Areal capacity (mah cm -2 ) Voltage vs Li/Li + (V) Theoretical electrode capacity L mg -1 Li 5 L mg -1 Li 3 L mg -1 Li 10 L mg Areal capacity (mah cm -2 ) Figure S16. Comparison of voltage profiles of high loading PHS cathode with sulfur cathodes at different electrolyte to sulfur ratios. At 3 µl mg -1 the sulfur cell shows instability during the charge process. The cells were cycled at the same current density L mg -1 Li 5 L mg -1 Li 3 L mg -1 Li 10 L mg Cycle number Figure S17. Cycling stability of high loading PHS cathode withrespect to those of sulfur cathodes at different electrolyte to sulfur ratios. The cells were cycled at the same current density. S-16

17 Cell specific energy (Wh kg -1 ) Coulombic efficiency (%) L mg -1 Li 5 L mg -1 Li 3 L mg -1 Li 10 L mg Cycle number Figure S18. Coulombic efficiency of high loading PHS cathode along with those of sulfur cathodes at different electrolyte to sulfur ratios. The 3 µl mg -1 based sulfur cell shows high Coulombic efficiency due to cell failure. The cells were cycled at the same current density L mg -1 Li 3 L mg -1 Li 5 L mg -1 Li 10 L mg -1 Figure S19. Cell level specific energy of high loading PHS cathode compared with sulfur cathodes at different electrolyte to sulfur ratios. Cell level specific energy was determined by accounting for mass of the cathode, electrolyte, separator and stoichiometric amount of lithium. S-17