Supporting Information Powering Lithium Sulfur Battery Performance by Propelling Polysulfide Redox at Sulfiphilic Hosts Zhe Yuan,, Hong-Jie Peng,, Ting-Zheng Hou,, Jia-Qi Huang, Cheng-Meng Chen, Dai-Wei Wang, Xin-Bing Cheng, Fei Wei, Qiang Zhang *, Beijing Key Laboratory of Green Chemical Reaction Engineering and Technology, Department of Chemical Engineering, Tsinghua University, Beijing 100084, China Key Laboratory of Carbon Materials, Institute of Coal Chemistry, Chinese Academy of Sciences, 27 Taoyuan South Road, Taiyuan 030001, China *E-mail: zhang-qiang@mails.tsinghua.edu.cn (Q. Zhang) Z. Y., H.-J. P., and T.-Z. H. contributed equally to the work. SI1
1. Supplementary Text Experimental section 1.1. Material characterization The morphologies of the samples were characterized by a JSM 7401F (JEOL Ltd., Tokyo, Japan) SEM operating at 3.0 kv and a JEM 2010 (JEOL Ltd, Tokyo, Japan) TEM operating at 120.0 kv. The structure of cobalt disulfide was determined by an X-ray powder diffractometer (D8-Advance, Bruker, Germany). The thermogravimetric analysis was performed using TGA/DSC1 STARe system under N 2 atmosphere. BET specific surface area of the CoS 2 and graphene samples was calculated from N 2 adsorption/desorption isotherm collected with Autosorb-IQ2-MP-C system. 1.2. Synthesis of graphene Graphene was synthesized according to a previously reported route. 1 Graphene oxide (GO) was synthesized in advance via a modified Hummers method while graphene was obtained by thermally annealing GO at 250 C under vacuum. To clarify the influence of oxygen functional groups on graphene, graphene with less oxygen content was prepared by thermally annealing GO at higher temperature of 1000 o C under vacuum and was denoted as HT-G. 1.3. Synthesis of CoS 2 and CoS 2 + graphene (CoS 2 + G) mixture CoS 2 was synthesized by a hydrothermal process as reported previously. 2 CoCl 2 6H 2 O (5 mmol) and Na 2 S 2 O 3 (10 mmol) were dissolved in 25 ml of deionized water. The pink solution was transferred into a 40 ml Teflon-lined autoclave and was maintained at 140 C SI2
for 12 h. CoS 2 powder was obtained by vacuum filtration and drying at 60 C. CoS 2 + G mixture was fabricated by milling graphene and CoS 2 powder in a mortar at designated weight ratios (70:30 and 85:15). 1.4. Visualized adsorption test Li 2 S 4 solution was prepared by adding Li 2 S and sulfur at a molar ratio of 1:3 in DME followed by vigorous magnetic stirring. Solutions with Li 2 S 4 concentration of 10 mmol L -1 (1.4 mg ml -1 ) were used for adsorption test. CoS 2 and graphene with same surface area of 0.3 m 2 were added to 3.0 ml of Li 2 S 4 /DME solutions separately, and the mixtures were vigorously stirred to realize thorough adsorption. 3.0 ml Li 2 S 4 /DME solution was used as a comparison. The suspensions were centrifuged before photographs were taken, because no difference in polysulfide adsorption could be observed as both suspension were black before centrifugation. CoS 2 and CoS 2 (30 %) + G with the same weight (20.0 mg) were also used for static adsorption with the same procedures. 1.5. Li S cell assembly and measurements Sulfur/host composites were fabricated via a melt-diffusion method. 60 mg of host materials (CoS 2 + G mixture or graphene) were separately mixed with 180 mg sulfur by milling in a mortar. The mixtures were co-heated at 155 C in a sealed flask for 3.0 h. The as-obtained composites with Ketjen black and poly(vinylidene fluoride) (PVDF) binder at a weight ratio of 80:10:10 were dispersed in N-methyl pyrrolidone (NMP) to form slurry by magnetic stirring for 16.0 h. Positive electrodes were fabricated by coating the slurry on aluminum foils, drying at 60 C for 6.0 h and punching the foil into small disks with a SI3
diameter of 13.0 mm. Electrochemical measurements were conducted in standard 2025 coin-type cells. In Li S battery assembly, lithium foils were used as counter electrode, and Celgard 2400 polypropylene membranes worked as separators. The electrolyte was 1.0 mol L -1 lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) dissolved in mixed solvent of 1,3-dioxolane (DOL) and DME (v/v = 1/1) with 1 wt% of LiNO 3 as additive. Typical sulfur loading was 0.4 mg cm -2 with the consideration of avoiding diffusion resistance and full demonstration of composite cathode materials for Li S batteries. A practical electrode with an areal loading amount of 2.9 mg cm -2 was also prepared for Li S cells with high energy density. 12 μl electrolyte was added to each Li S cell. The cells were galvanostatically cycled within a voltage range of 1.7-2.8 V on a Neware multichannel battery tester under different C rates. Cyclic voltammetric (CV) measurements were conducted on a Solartron 1470E electrochemical workstation at a scanning rate of 0.1 mv s -1. Certain cells cycled at 0.2 C were disassembled at charged, discharged and half-discharged (beginning of the lower plateau) states, whose cathode were used for SEM and TEM characterizations before washed by DOL/DME and dried under Ar atmosphere. 1.6. Symmetrical cell assembly and measurements. The electrodes for symmetrical cells were fabricated without the presence of elemental sulfur. Each host material (CoS 2 + G mixture and graphene) and PVDF binder at a weight ratio of 3:1 were dispersed in NMP. Slurry was produced under stirring and was coated on aluminum foils. Electrode disks with a diameter of 13.0 mm were punched out of slurry-coated aluminum foils after thorough drying. Those disks were used as identical SI4
working and counter electrodes, while 40 μl electrolyte containing 0.5 mol L -1 Li 2 S 6 and 1 mol L -1 LiTFSI dissolved in DOL/DME (v/v = 1/1) was added. Li 2 S 6 was prepared by adding Li 2 S and S into corresponding solvent at a molar ratio of 1:5. Same electrolyte without Li 2 S 6 was used as a control. Electrodes with sulfur and Li 2 S were also prepared for symmetrical cell test. Sulfur electrodes were fabricated via the above-mentioned route, and Li 2 S electrodes were prepared by discharging sulfur cathodes to 1.0 V at 0.2 C. CV measurements of the symmetrical cells were performed at scan rate of 50 mv s -1. Electrochemical impedance spectroscopy (EIS) tests were conducted on a Solartron 1470E electrochemical workstation, using potentiostatic mode at open circuit potential. A sinusoidal voltage with an amplitude of 10 mv and a scanning frequency from 200 khz to 10 MHz were applied. The slight difference in electrode weight is uniformized for comparability. 1.7. Calculations The first principle calculations were conducted, using spin-polarized Perdew Burke Ernzerhof (PBE) exchange-correlation functional 3 in the framework of CASTEP 4 in Materials Studio of Accelrys Inc. A convergence criterion of 5.0 10-2 ev Å -1 for the maximum final force was used for geometry optimization, and a convergence criteria of 2.0 10-3 Å for the maximum final displacement and 2.0 10-5 ev atom -1 for the total energy of the system was utilized for all computations. Vanderbilt-type ultrasoft pseudopotentials 5 were generated in slab calculations, and the free surfaces of CoS 2 slab was separated by a 15 Å vacuum layer. We used a 3 3 1 Monkhorst Pack mesh 6 for the sampling of the SI5
Brillouin zone, whereas valence electrons were expanded within plane wave basis set with a cut-off energy of 300 ev. The threshold for self-consistent-field (SCF) density convergence was set to 2.0 10-6 ev atom -1. Adsorption was allowed on only one side of the exposed surfaces, with the dipole moment corrected accordingly in the z direction. For quantitatively measuring the interaction between the substrates and Li 2 S 4, we defined the binding energy E b as follows: E b = E(Sub) + E(Li 2 S 4 ) E(Total) where E(Sub), E(Li 2 S 4 ), and E(Total) represent the total energies of the substrate, the Li 2 S 4 cluster, and the adsorption pair of the substrate and Li 2 S 4, respectively. SI6
2. Supplementary Figures Figure S1. Morphology of CoS 2 and CoS 2 + G. SEM images of (a) CoS 2 and (b, c) CoS 2 + G. The CoS 2 primary particles are indicated by white dotted circles in c. Scale bar, (a, b) 500 nm and (c) 10 μm. SI7
Figure S2. Material characterizations of CoS 2. (a) High-resolution TEM image with corresponding FFT pattern (inset) of CoS 2 secondary particles. Scale bars, 10 nm. (b) XRD pattern of as-synthesized CoS 2. SI8
Figure S3. N 2 sorption isotherm of (a) CoS 2 and (b) graphene. SI9
Figure S4. Visualized adsorption of Li 2 S 4 on graphene and pristine CoS 2 (30 %) + G with the same weight (20.0 mg). SI10
Figure S5. Top view of the optimized CoS 2 (111)/Li 2 S 4 adsorption geometry. Colored spheres of violet, yellow, and blue represent lithium, sulfur, and cobalt, respectively. SI11
Figure S6. Equivalent circuit used for fitting the EIS spectra of symmetrical cells. SI12
Figure S7. Sulfur content in composite cathodes. Thermogravimetric (TG) curves of sulfur cathodes with host materials of graphene, CoS 2 (15 %) + G, and CoS 2 (30 %) + G. SI13
Figure S8. Onset potential for Li S redox reactions. Differential CV curves of (a) G, (c) CoS 2 (15 %) + G, (e) CoS 2 (30 %) + G, and (g) HT-G. The baseline voltage and current density are defined as the value before the redox peak, where the variation on current density is the smallest, namely di/dv = 0. Baseline voltages are denoted in gray for cathodic peak I, II and in black for anodic peak III, respectively. CV curves and corresponding onset SI14
potentials of redox peak I, II, and III (inset): (b) G, (d) CoS 2 (15 %) + G, (f) CoS 2 (30 %) + G, and (h) HT-G. Following a common definition employed in electrocatalysis, the onset potential is determined when the current density goes 10 μa cm -2 beyond the corresponding baseline current density (more specifically, 10 μa cm -2 more negative than baseline current density for cathodic peaks or 10 μa cm -2 positive than baseline current density for anodic peaks). As shown in the inset of b, d, and f, the baseline voltages are the same as in a, c, and e while the colored region indicates the difference in current density (10 μa cm -2 ). SI15
Figure S9. Influence of annealing temperature on graphene electrodes. (a) CV curves of G and HT-G. (b) Onset potential of G, HT-G, CoS 2 (15 %) + G, and CoS 2 (30 %) + G. SI16
Figure S10. Stability of composite cathodes. Capacity retention of sulfur cathodes with host materials of graphene, CoS 2 (15 %) + G, and CoS 2 (30 %) + G at 0.5 C. SI17
Figure S11. Cycling performance of sulfur cathodes with host materials of G, HT-G, and CoS 2 (15 %) + G at 0.5 C. SI18
Figure S12. Reduced hysteresis by CoS 2 addition. Galvanostatic charge-discharge profiles of (a) S/G, (b) S/CoS 2 (15 %) + G, and (c) S/CoS 2 (30 %) + G cathodes at different current densities. (d) Polarization voltage and (e) rate performance of S/G, S/CoS 2 (15 %) + G, and S/CoS 2 (30 %) + G cathodes at different current densities. The polarization voltage was obtained at 50 % depth of discharge of each profile. SI19
Figure S13. Performance of high-sulfur-loading electrodes based on CoS 2 hosts. Discharge capacity of S/CoS 2 (15 %) + G electrodes with a high sulfur loading amount of 2.9 mg cm 2. The current density was set as 0.2 C after three cycles of activation under 0.1 C. SI20
Figure S14. Cyclic performance of CoS 2 (15 %) + G. A current density of 0.33 ma cm -2, equivalent to 0.5 C based on sulfur, was applied within a voltage window of 1.7-2.8 V, using LiTFSI-DOL/DME as electrolyte. SI21
Figure S15. Sluggish kinetics of liquid-liquid conversion. EIS spectra of symmetrical cells (S S, Li 2 S 6 Li 2 S 6 and Li 2 S Li 2 S). SI22
Figure S16. Li 2 S nuclei. High resolution TEM image at intermediate state of S/CoS 2 (15 %) + G cathodes. Scale bar, 5 nm. SI23
Figure S17. Cyclic performance of S/CoS 2 (30 %) + G and S/G cathodes at 0.2 C. SI24
Figure S18. Correlations between surface areas of the electrode materials and specific capacities: CoS 2 /graphene hybrids (red) and pure graphene (blue). SI25
3. Supplementary Table Table S1. The fitting results of EIS spectra of symmetrical Li 2 S 6 -Li 2 S 6 cells. R s (Ω) R ct (Ω) Y dl (Ω -1 s n ) n dl CoS 2 (30 %) + G 4.06 116 1.14 10-4 0.63 CoS 2 (15 %) + G 4.59 436 2.59 10-5 0.71 G 5.33 1202 3.21 10-5 0.71 The constant phase element (CPE) impedance is expressed as: Z CPE =Y -1 (jω) -n. SI26
Table S2. The onset potential of sulfur cathodes with different host materials. Onset potential (V) Host materials I: S 8 Li 2 S x (x = II: Li 2 S x (x = 4-8) III: Li 2 S Li 2 S x (x 4-8) Li 2 S = 4-8) S 8 G 2.371 2.016 2.146 HT-G 2.417 2.098 2.183 CoS 2 (15 %) + G 2.385 2.100 2.144 CoS 2 (30 %) + G 2.396 2.102 2.113 SI27
4. Supplementary References (1) Chen, C. M.; Zhang, Q.; Yang, M. G.; Huang, C. H.; Yang, Y. G.; Wang, M. Z. Carbon 2012, 50, 3572 3584. (2) Chen, X. H.; Fan, R. Chem. Mater. 2001, 13, 802 805. (3) Perdew, J. P.; Burke, K.; Ernzerhof, M. Phys. Rev. Lett. 1996, 77, 3865 3868. (4) Clark, S. J.; Segall, M. D.; Pickard, C. J.; Hasnip, P. J.; Probert, M. I. J.; Refson, K.; Payne, M. C. Z. Kristallogr. 2005, 220, 567 570. (5) Vanderbilt, D. Phys. Rev. B 1990, 41, 7892 7895. (6) Monkhorst, H. J.; Pack, J. D. Phys. Rev. B 1976, 13, 5188 5192. SI28