Supporting Information. Dynamic Hosts for High Performance Li-S Batteries. Studied by Cryogenic Transmission Electron

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1 Supporting Information Dynamic Hosts for High Performance Li-S Batteries Studied by Cryogenic Transmission Electron Microscopy and In Situ X-ray Diffraction Xiao-Chen Liu,,# Yao Yang,,# Jingjie Wu,, Miao Liu, Sophia Zhou, Barnaby D. A. Levin, Xiao-Dong Zhou, Hengjiang Cong, David A. Muller,, Pulickel M. Ajayan, Héctor D. Abruña, * and Fu-Sheng Ke, * College of Chemistry and Molecular Sciences, Wuhan University, Wuhan , China Department of Chemistry and Chemical Biology, Baker Laboratory, Cornell University, Ithaca, NY 14853, USA Department of Materials Science and Nano Engineering, Rice University, Houston, TX, 77005, USA Energy Technologies Area, Lawrence Berkeley National Laboratory, Berkeley, CA School of Applied and Engineering Physics, Cornell University, Ithaca, NY 14853, USA Department of Chemical Engineering, Institute of Materials Research and Innovations, University of Louisiana at Lafayette, Lafayette, LA 70504, USA Kavli Institute for Nanoscale Science, Cornell University, Ithaca, New York 14853, USA # X.-C. L and Y. Y. contributed equally to this work. S-1

2 *corresponding address: Table of Contents Section S1 Experimental Methods S3-S5 Section S2 Structural and compositional characterization of TiS 2 - S composites S5-S7 Section S3 Electrochemical characterization of TiS 2 -S composites S8-S11 Section S4 Reference In situ XRD study of the dynamic changes of TiS 2 and TiS 2 -S composite electrodes S12 S13 S-2

3 Section S1. Experimental Methods Materials: Titanium powder (99.99%, 300 mesh) was purchased from Aladdin Reagents Co., Ltd. Sulfur (99.5%) was purchased from Sinopharm Chemical Reagent Co. Ltd. Bis(trifluoromethane) sulfonimide lithium salt (LiTFSI, 99.95%), Poly(acrylic acid) (average Mv ~450,000), N-methyl-2- pyrrolidinone (NMP, 99%), polyvinylidene fluoride (PVDF, average Mw ~534,000), 1, 3-dioxolane (DOL, 99.8%) and 1, 2-dimethoxyethane (DME, 99.5%) were purchased from Sigma-Aldrich. All materials were used without further purification. Synthesis of TiS 2 : Bulk TiS 2 powders were synthesized by the solid-state reaction of Ti powder and S (4 at.% excess). The powdered mixture was ground in a mortar for 30 min, and then put into a quartz tube in an Ar filled glove-box (H 2 O< 1 ppm, O 2 < 1 ppm). After moving the tube out of the glove-box, it was evacuated to 0.1 torr and then sealed. The samples were heated slowly to 850 o C for 7 days, and then cooled to room temperature. Synthesis of TiS 2 -S and BP2000-S: TiS 2 and BP2000 were degassed at 120 o C for 12 h, then transferred into a glove-box, and mixed with sulfur. The mixture was grounded for 30 min to obtain a uniform mixture, and then sealed in a pyrex tube and heated at 155 o C for 20 h at a heating rate of 0.5 o C min -1. Characterization: X-ray diffraction (XRD) analyses were carried out on a Smartlab (Rigaku) with filtered Cu Kα radiation (Rigaku D/max-2500, λ = Å). The morphology of the samples was characterized with a FESEM (Zeiss Ltd.) operated at 20 kv. For cryo-stem and XEDS experiments, TiS 2 -sulfur composite particles were dispersed in ethanol and transferred onto TEM grids. The TEM grids, containing the sample, were loaded into a Gatan model 914 cryo-holder under nitrogen gas, near liquid nitrogen temperature. The holder kept the sample at a stable temperature of about -180 o C. STEM images were acquired using an FEI Tecnai F-20 microscope operated at 200 kev. XEDS elemental mapping was performed using an Oxford X-Max detector. Thermo gravimetric analyses (TGA) were carried out under a N 2 atmosphere on a TG-209F1 instrument at a heating rate of 10 o C min -1 from room temperature to 500 o C, to determine the sulfur loading in the composites. The nitrogen adsorption isotherms were measured at 77 K in the Quantachrome iq automatic volumetric instrument. S-3

4 Electrochemical measurements: Electrochemical experiments were carried out in CR2016-type coin cells. The various TiS 2 -S (BP2000-S) composites were mixed with BP2000 carbon black and poly(acrylic acid) binder in N- methyl-2-pyrrolidinone (NMP) with mass ratios of 70:20:10 to form a slurry. The slurry was coated onto aluminum foil current collectors and dried at 60 o C for 12 h under vacuum. The sulfur mass loading of the composite electrodes ranged from 0.8 to 1.2 mg cm -2. The microporous interlayers were fabricated by mixing BP2000 and PVDF binder in N-methyl-2-pyrrolidinone (NMP) with mass ratios of 60:40 to form a slurry, and coated onto waterleaf carbon paper, and dried at 80 o C for 12 h under vacuum. The areal loading of the BP2000 is about 1.0 mg cm -2. CR 2016-type coin cells were assembled in an argon-filled glove-box using Li foil as the counter electrode, microporous carbon interlayers and Celgard 2400 as separator. The electrolyte was 1.0 M lithium bis(trifluoromethanesulfonyl) imide (LiTFSI) in a mixture of 1, 3-dioxolane (DOL) and 1, 2- dimethoxyethane (DME) 1:1 (Vol%). The electrolyte-to-sulfur ratio is 30 μl per milligram sulfur. The cells were galvanostatically discharged and charged on a battery test system (SLAN BT100, Wuhan) between 1.5 and 3.0 V at room temperature. Cyclic voltammetry (CV) was carried out on a VMP3 potentiostat/galvanostat station (Bio-logic Science Instruments). In situ XRD experiments were conducted in a home-made in situ XRD cell. The TiS 2 slurry was coated onto stainless steel nets, while the TiS 2 -S composite slurry was coated onto mircroporous interlayers. The cell was assembled in a glove-box and beryllium (Be) was used as an X-ray transmissive window. In situ XRD was performed on a Smartlab (Rigaku) with filtered Cu Kα radiation (Rigaku D/max-2500, λ = Å) and a high power of 9 kw. In situ XRD was done by sequential scans, with each scan collected between 10 o and 50 o at a scanning rate of 20 o min -1. Computational details: We employed density functional theory (DFT) calculations to investigate the formation energy and reaction dynamics in Ti-Li-S system. The Vienna ab initio software package (VASP) was employed. The projector augmented-wave (PAW) method was used to describe the wave functions near the core and the generalized gradient approximation (GGA) within the Perdew Burke Ernzerhof (PBE) parameterization was employed as the electron exchange correlation functional. The surface slab consisted of a 3 3 unit cell in the TiS 2 in-plane direction to have enough area for Li-S molecules to diffuse, rotate and relax, so that a clear view of the interactions between the TiS 2 -surface and lithium polysulfides could be extracted. There are four layers of TiS 2 in the vertical direction to prevent the interaction of two TiS 2 surfaces. For formation energy calculations, the Brillouin zone sampling of was adopted to ensure good accuracy. We used the ab initio molecular dynamics (AIMD) to mimic the formation of the Ti-Li-S bindings. The Li-S and TiS 2 are initially placed apart at a distance of 3.5 Å; and we use room temperature and allow the system to relax for 500 steps. An energy minimization under 0 K was carried out thereafter to obtain accurate ground S-4

5 state energies for all the systems. We used a DFT-D2 method in all the calculations to include the intra-molecular van der Waals forces as they play important roles in S-anchoring materials. Section S2. Structural and compositional characterization of TiS 2 -S composites (a) (b) Figure S1. SEM images of as-synthesized TiS 2 (a) and its corresponding high-magnification (b). It displays a layered structure with sizes of 20-30um and thicknesses of several micrometers for the as-synthesized TiS 2. Figure S2. (a) Powder XRD patterns and (b) Nitrogen adsorption-desorption isotherms of assynthesized TiS 2. The PXRD pattern is demonstrated as-synthesized TiS 2 with hexagonal structure (space group, P-3ml), and without any other phase. The nitrogen adsorption-desorption isotherm show almost no nitrogen adsorption, indicating that the TiS 2 is nonporous. S-5

6 Figure S3. (a) PXRD pattern of BP S composite. (b) Nitrogen adsorption-desorption isotherms and (c) pore size distribution of BP2000 and BP S. Figure S4. (a) TGA curves under a N 2 atmosphere of TiS 2 with different sulfur loadings, (b) XRD patterns of TiS 2, S, and TiS 2 -S with different sulfur loadings. S-6

7 Figure S5. (a) Cryo-HAADF-STEM image of a TiS 2-60S composite. (b,c,d) XEDS elemental mapping of S, Ti and color overlay of S vs. Ti. (e) XEDS spectra of reference TiS 2 (black), sum spectrum from the whole XEDS map (blue), TiS 2 region (red, from upper white dashed box in (d)) and S region (green, from lower white dashed box in (d)). The intensities of the X-ray peaks are proportional to elemental concentration, and changes in the ratio of peak intensities in an XEDS [1, 2] dataset can be used to assess changes in the relative concentrations of the specific elements. Based on peak integration of S Kα and Ti Kα, S/Ti ratio in TiS 2 region (box on the right side in Fig. S5d) is 1.69, comparable to the S/Ti ratio of 1.79 in pure TiS 2, which indicates sulfur cannot infiltrate into the layered structure of TiS 2. S-7

8 Section S3. Electrochemical characterization of TiS 2 -S composites Figure S6. (a) Plots of specific capacity versus cycle number of a TiS 2-60S electrode during 480 cycles. (b) Selected voltage profiles of TiS 2-60S and (c) BP S composites electrodes at a rate of 0.3 C. S-8

9 Figure S7. Voltage profiles and cyclic capacity performance of TiS 2 electrodes at current densities of (a, b) 50 ma g -1, (c, d) 50 to 6000mA g -1, and (e, f) BP2000 electrode in 1 M LiTFSI DME/DOL (1:1, vol.) electrolyte. S-9

10 Figure S8. Voltage profiles and plots of specific capacity versus cycle number of (a, b) TiS 2-70S and (c, d) TiS 2-80S electrodes at a rate of 0.1 C. Figure S9. Differential capacity plots of TiS 2 and BP2000 electrodes for the stabilized cycles in Figure S7. S-10

11 Figure S10. Cyclic voltammograms of a TiS 2 electrode with scanning rate (a) 0.1 mv s -1 and (b) mv s -1. Figure S11. Stable CV profiles of TiS 2-60S and BP S electrodes at a scanning rate of 0.1 mv s -1 in coin cells. Details of coin cell assembly can be found in previous experimental section. Section S4. In situ XRD study of the dynamic changes of TiS 2 and TiS 2 -S composite electrodes S-11

12 Figure S12. Selected in situ XRD patterns of TiS 2 electrode during cycling at (a) 2θ= o, (b) 2θ= o, and (c) 2θ= o. Figure S13. Selected in situ XRD patterns of TiS 2-60S electrode during cycling at (a) 2θ= o, (b) 2θ= o, and (c) 2θ= o. References S-12

13 (1) Cliff, G.; Lorimer, G. The Quantitative Analysis of Thin Specimens. J. Microscopy 1975, 103, (2) Levin, B.; Zachman, M.; Werner, J.; Sahore, R.; Nguyen, K.; Han, Y.; Xie, B.; Ma, L.; Archer, L.; Giannelis, E.; Wiesner, U.; Kourkoutis L.; Muller. D. Characterization of Sulfur and Nanostructured Sulfur Battery Cathodes in Electron Microscopy without Sublimation Artifacts. Microsc. Microanal. 2017, 23, S-13