Supporting Information

Transcription

1 Supporting Information In Situ-formed Li 2 S in Lithiated Graphite Electrodes for Lithium-Sulfur Batteries Yongzhu Fu, Chenxi Zu, Arumugam Manthiram Electrochemical Energy Laboratory & Materials Science and Engineering Program The University of Texas at Austin, Austin, Texas 78712, United States Materials and Methods Materials Toray carbon paper (Fuel Cell Earth, thickness: 370 µm, 0% Teflon coating), liquid carbonate electrolyte (Novolyte, 1 M LiPF 6 in ethylene carbonate/diethyl carbonate (1:1 v/v)), lithium trifluoromethanesulfonate (LiCF 3 SO 3, 98%, Acros Organics), lithium nitrate (LiNO 3, 99+%, Acros Organics), dimethoxy ethane (DME, 99+%, Acros Organics), 1,3-dioxolane (DOL, 99.5%, Acros Organics), sublimed sulfur powder (99.5%, Acros Organics), lithium sulfide (Li 2 S, 99.9%, Acros Organics), tetraglyme ( 99%, Sigma-Aldrich), and sulfolane (99%, Sigma- Aldrich) were purchased and used as received. Methods The dissolved polysulfide in liquid electrolyte was prepared in an argon-filled glove box (H 2 O content: < 1.2 ppm, O 2 content: < 1.6 ppm). LiCF 3 SO 3 and LiNO 3 in a DME/DOL (1:1 v/v) mixture solvent to render a 1 M LiCF 3 SO 3 and 0.1 M LiNO 3 blank ether electrolyte was prepared first. Then sulfur powder and an appropriate amount of Li 2 S were added to the proper amount of blank ether electrolyte to render 1.5 M sulfur (i.e., 0.25 M Li 2 S 6 ) in the form of Li 2 S 6 in the solution. The mixture solution was heated at 45 C inside the glove box over night to produce a dark yellow solution with a moderate viscosity. Alternative catholytes were prepared * Corresponding author. Tel: ; fax: address: manth@austin.utexas.edu (A. Manthiram) S1

2 for a comparison; they are 0.5 M Li 2 S 6 in 1 M LiCF 3 SO 3 /0.1 M LiNO 3 in DME/DOL (1:1 v/v), 0.25 M Li 2 S 6 in 1 M LiCF 3 SO 3 in tetraglyme, and 0.25 M Li 2 S 6 in 1 M LiCF 3 SO 3 in sulfolane. Lithiation of graphitic carbon paper and graphite MCMB electrode The carbon paper was cut into circular discs with a diameter of 1.2 cm and mass of 17.6 ± 0.1 mg, and then assembled in CR2032 coin cells inside the glove box. For assembling the half cells for lithiation, 40 µl of liquid carbonate electrolyte was added into a carbon paper electrode, followed by a Celgard 2400 separator, 20 µl of additional electrolyte, and lithium metal anode. Finally, the cells were crimped for the electrochemical discharge outside the glove box. The graphite MCMB electrode containing 80 wt.% of graphite (MCMB), 10 wt.% of Super P carbon, and 10 wt.% of polyvinylidene fluoride (PVdF) binder was prepared on a copper foil by a slurrycasting method. For assembling the half cells for lithiation, 20 µl of liquid carbonate electrolyte was added onto the graphite electrode. The cells were galvanostatically discharged to 0.01 V at C/40 rate (1C = 167 ma g -1 ) for the carbon paper and C/20 rate (1C = 372 ma g -1 ) for the graphite MCMB electrode and then opened inside the glove box. A cell containing the carbon paper electrode was cycled between V for 50 cycles to demonstrate the electrochemical behavior of the carbon paper as a graphite anode. The lithiated graphite electrodes were washed with DME/DOL (1:1 v/v) mixture solvent thoroughly to remove all soluble species, e.g., lithium salt LiPF 6 and carbonate solvent, for characterization and use in the Li/polysulfide cells. Li/polysulfide cell assembly and electrochemical characterization Li/polysulfide cells containing lithiated carbon paper electrodes were assembled inside the glove box. First, 40 µl of polysulfide Li 2 S 6 electrolyte was added into the lithiated carbon S2

3 paper electrode, corresponding to 1.9 mg (1.7 mg cm -2 ) of sulfur. The theoretical capacity of sulfur in the polysulfide electrolyte is: 1.9 mg mah mg mah. Then a Celgard 2400 separator was placed on top of the electrode. 20 µl of blank ether electrolyte was added onto the separator followed by the lithium metal anode. The cells were crimped for electrochemical evaluation outside the glove box. Cells were stabilized over a specific period of time (days). Some were then opened inside the glove box for analysis. The electrodes with in situ reduced sulfur compounds were taken out of cells for ex situ characterization. A control Li/polysulfide cell was prepared for a comparison. The cell preparation procedure is same as that of the Li/polysulfide cells described above. Instead of the lithiated carbon paper electrode, pristine carbon paper was used as the electrode in the control cell. For the Li/polysulfide cell containing the lithiated graphite MCMB electrode, 10 µl of 0.5 M Li 2 S 6 catholyte was added onto the lithiated graphite electrode. For the cells with tetraglyme or sulfolane solvent, 40 µl of 0.25 M Li 2 S 6 in tetraglyme or sulfolane catholyte was added into the lithiated carbon paper electrodes. Cyclic voltammetry data were collected on a VoltaLab PGZ402 with Li/polysulfide cells (after 6-day stabilizing) between 1.8 and 3.0 V at a scan rate of mv s -1. The potential was swept from the open-circuit voltage to either 3.0 or 1.8 V. Electrochemical performances of the Li/polysulfides cells were galvanostatically cycled with an Arbin battery test station between 1.8 and 3.0 V at various C rates (1C = 1,672 ma g -1 of sulfur). The capacity values shown in this paper are based on the mass (1.9 mg) of sulfur in the polysulfide electrolyte used. S3

4 Characterization Morphological characterization was carried out with a FEI Quanta 650 scanning electron microscope (SEM). The elemental mapping results were examined with an energy dispersive spectrometer (EDS) attached to the FEI Quanta 650 SEM. The X-ray diffraction (XRD) data were collected on a Philips X-ray diffractometer equipped with CuKα radiation in steps of XRD samples were covered by Kapton films in the glove box. X-ray photoelectron spectroscopy (XPS) data were collected at room temperature with a Kratos Analytical spectrometer and monochromatic Al Kα ( ev) X-ray source. The XPS samples were transferred to the spectrometer via a custom interface (built at the Surface Analysis Laboratory of the Texas Materials Institute at the University of Texas at Austin) to avoid any exposure to atmosphere. All spectra were fitted with Gaussian-Lorentzian (30% Gaussian) functions and a Shirley-type background by the de-convolution software (CasaXPS, Casa Software). Three constraints on the component peaks were applied: the position (1.18 ev between S 2p 3/2 and S 2p 1/2 ), the peak area ratio (S 2p 3/2 : S 2p 1/2 = 2 : 1), and equal full width at half maximum. All reported binding energy values are calibrated to the graphitic C 1s peak with a value of ev. The lithiated carbon paper (Li-CP) was characterized by SEM and XRD. Some Li-CP electrodes that were electrochemically de-lithiated after various charge times (5 h, 10 h, and full charge) in lithium half cells were characterized by ex situ XRD. The electrodes in the Li/polysulfide cells after various stabilizing time (1 6 days) were characterized by XRD. The Li 2 S-CP electrode before and after the 1 st cycle (charge and discharge) were characterized by XPS, SEM, and EDS. These electrodes after washed with DME/DOL (1:1 v/v) mixture solvent thoroughly were also characterized by XPS. S4

5 Table S1. Diffraction angle 2θ and interplanar d 002 spacing of the lithiated carbon paper as a function of stabilizing time t, obtained from the XRD patterns shown in Fig. S3 t (day) 2θ ( ) d 002 (Å) Table S2. Compositions of reduced sulfur compounds in the Li 2 S-CP electrode before and after the 1 st cycle; the electrodes were washed with DME/DOL (1:1 v/v) mixture solvent before the measurement and the data were obtained from the XPS analysis shown in Fig. S8 Sample Li 2 S (%) (~160.1 or ev) Li 2 S 2 (%) (~161.7 or ev) Li 2 S x (%) (~163.2 ev) Li 2 S-CP (washed) cycled Li 2 S-CP (washed) S5

6 Fig. S1. SEM images of the surface of a pristine carbon paper, showing the woven carbon fibers with large voids between these fibers. S6

7 Time (hour) (a) Capacity (mah) Coulombic efficiency (%) Cycle number (b) Fig. S2. (a) A representative voltage profile of the first two cycles and (b) cycle life and Coulombic efficiency of a Toray carbon paper electrode at a rate of C/40 (1C = 167 ma g -1 ). S7

8 de-li-cp Intensity (a.u.) 6 d 5 d 4 d 3 d 2 d Intensity (a.u.) de-li-cp-10h de-li-cp-5h 1 d Li-CP θ ( ) θ ( ) (a) (b) Fig. S3. XRD patterns of (a) electrodes in Li/polysulfide cells after various stabilizing times (d = day) and (b) lithiated carbon paper (Li-CP) electrodes after 5 h (de-li-cp-5h), 10 h (de-li-cp- 10h), and full charge (de-li-cp) in lithium half cells, showing the shifts of the (002) reflection to high angles as the de-lithiation occurs. Only the (002) reflection regions are shown for clarity. S8

9 Time (hour) Fig. S4. Voltage profile of the 1 st two cycles of a control Li/polysulfide cell with a pristine carbon paper electrode at a rate of C/10 (1C = 1,672 ma g -1 of sulfur); the polysulfide used in the cathode is same as that used in the cell with the lithiated carbon paper electrode (40 µl of polysulfide Li 2 S 6 electrolyte, corresponding to 1.9 mg of sulfur). S9

10 1.0 Current density (ma cm -2 ) Initial (OCV -> 3.0 V) 1 st cycle 2 nd cycle (a) 1.0 Current density (ma cm -2 ) (b) Initial (OCV -> 1.8 V) 1 st cycle 2 nd cycle Fig. S5. Cyclic voltammograms of two Li/polysulfide cells with Li 2 S-CP electrodes; the potential was swept either (a) from OCV to 3.0 V initially or (b) from OCV to 1.8 V initially and then between 1.8 and 3.0 V at a rate of mv s -1. S10

11 Charge first Discharge first Time (hour) Fig. S6. Voltage profile of two Li/polysulfide cells with Li 2 S-CP electrodes; one cell was charged to 3.0 V first (black) and the other cell was discharged to 1.8 V first (red) and then cycled between 1.8 and 3.0 V at a rate of C/10. S11

12 V 2.12 V V V V V V Time (day) (a) end of 1 st charge Time (hour) (b) Fig. S7. (a) OCVs of a Li/polysulfide cell with a Li-CP electrode as a function of stabilizing time (day) and (b) voltage profile of the 1 st cycle (charge and discharge) of a cell with the Li 2 S-CP electrode after 4-day stabilizing; the polysulfide electrolyte contained no LiNO 3 additive, and the 1 st charge step time was capped at 15 h which equals to a capacity of 4.8 mah. S12

13 V 2.13 V M Li 2 S 6 Tetraglyme Sulfolane 0.93 V 2.12 V V 0.43 V 0.08 V Time (day) (a) M Li 2 S 6 Tetraglyme Sulfolane Time (h) (b) Fig. S8. (a) OCVs and (b) voltage profiles of the 1 st cycle (charge and discharge) of the Li/polysulfide cells with Li-CP electrodes and different catholytes as a function of stabilizing time (day); the catholytes used are 40 µl of 0.5 M Li 2 S 6 in 1 M LiCF 3 SO 3 /0.1 M LiNO 3 in DME/DOL (labeled as 0.5 M Li 2 S 6 ), 40 µl of 0.25 M Li 2 S 6 in 1 M LiCF 3 SO 3 in tetraglyme (labeled as Tetraglyme), and 40 µl of 0.25 M Li 2 S 6 in 1 M LiCF 3 SO 3 in sulfolane (labeled as Sulfolane). S13

14 V 2.05 V 2.08 V 1.56 V V V V Time (day) (a) Time (h) (b) Fig. S9. (a) OCVs of a Li/polysulfide cell with a lithiated graphite MCMB electrode as a function of stabilizing time and (b) voltage profile of the cell after stabilizing. The catholyte used was 10 µl of 0.5 M Li 2 S 6 in 1 M LiCF 3 SO 3 /0.1 M LiNO 3 in DME/DOL. S14

15 Intensity (a.u.) Li 2 S x (163.2) Li 2 S 2 (161.7) Li 2 S (160.1) Binding energy (ev) (a) Li 2 S 2 (161.9) Li 2 S (160.3) Intensity (a.u.) Binding energy (ev) (b) Fig. S10. S 2p XPS spectra of the Li 2 S-CP electrodes (a) before and (b) after the 1 st cycle (charge and discharge); the electrodes were washed with DME/DOL (1:1 v/v) mixture solvent thoroughly before the measurement. S15

16 Intensity (a.u.) cycled Li 2 S-CP θ ( ) Li 2 S-CP Fig. S11. XRD pattern of the Li 2 S-CP electrode before (Li 2 S-CP) and after the 1 st cycle (cycled Li 2 S-CP); only the (002) reflection region is shown for clarity. S16

17 Fig. S12. (a) Surface and (d) cross-section SEM images of a Li 2 S-CP electrode, carbon and sulfur elemental mapping of (b, c) the surface and the (e, f) cross-section of the electrode. S17

18 1000 Capacity (mah g -1 ) C/5 C/2 1C Cycle number Fig. S13. Discharge capacities of half cells with Li 2 S-CP electrodes at various rates; the capacities are based on the mass (1.9 mg) of sulfur in the polysulfide Li 2 S 6 used in the half cells. S18