High-Performance All-Solid-State Lithium-Sulfur. Battery Enabled by a Mixed-Conductive Li 2 S

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1 Supporting information High-Performance All-Solid-State Lithium-Sulfur Battery Enabled by a Mixed-Conductive Li 2 S Nanocomposite Fudong Han, Jie Yue, Xiulin Fan, Tao Gao, Chao Luo, Zhaohui Ma, Liumin Suo, Chunsheng Wang* Department of Chemical and Biomolecular Engineering, University of Maryland, College Park, Maryland, United States, *Corresponding Author: cswang@umd.edu Experimental Methods Synthesis: Li 6 PS 5 Cl solid electrolyte was synthesized using high-energy mechanical milling. 1 Li 2 S (Sigma-Aldrich, 99.98%), P 2 S 5 (Sigma-Aldrich, 99%) and LiCl (Sigma-Aldrich, 99%) were used as starting materials. These materials were weighed in the molar ratio of Li 2 S/P 2 S 5 /LiCl = 5/1/2 in an argon-filled glovebox, subjected to a zirconia ceramic, and ball-milled (PM 100, Retsch) at 610 rpm for 50 hours. Polycrystalline Li 10 GeP 2 S 12 powder was prepared with the same method reported elsewhere. 2 80Li 2 S 20P 2 S 5 glass-ceramic solid electrolyte was also prepared by high-energy ball milling process (510 rpm for 50 hours) followed by a heating process (243 C for 4 hours) under argon atmosphere. To prepare the Li 2 S-Li 6 PS 5 Cl-C nanocomposite, Li 2 S

2 powder, polyvinylpyrrolidone (PVP, Sigma-Aldrich, average Mw ) powder and Li 6 PS 5 Cl powder were separately dissolved in anhydrous ethanol inside the glovebox to obtain 0.5 M Li 2 S in ethanol, 10 wt. % PVP in ethanol, and 10 wt. % Li 6 PS 5 Cl in ethanol solutions. These three solutions were then mixed together with the weight ratio of Li 2 S/PVP/Li 6 PS 5 Cl = 2/2/1. The mixed solution was stirred for 24 hours to get a dark brown solution, and then dried at 100 C for 24 hours under vacuum to completely evaporate the solvent. The obtained powder was then was heated at 550 C for 5 hours under Ar to enable the carbonization of the PVP. After that, the Li 2 S-Li 6 PS 5 Cl-C nanocomposite was obtained. A Li 2 S-C nanocomposite was also prepared using the same procedure except without the addition of Li 6 PS 5 Cl in the mixed solution. Characterization: Powder X-ray diffraction patterns were obtained with a D8 Advance with LynxEye and SolX (Bruker AXS, WI, USA) using Cu Kα radiation. The morphologies of the sample were examined using a Hitachi a SU-70 field-emission scanning electron microscope and JEOL 2100F field emission transmission electron microscope. The electronic conductivity of the nanocomposite was measured by a four-probe method (Signatone SP4) using a pressed pellet of the nanocomposites. Raman spectra were measured on a Horiba Jobin Yvon Labram Aramis using a 532 nm diode-pumped solid-state laser. Electrochemistry: The ionic conductivity of the nanocomposite was measured by electrochemical impedance spectra of an electron-blocking Pt/Li 10 GeP 2 S 12 /nanocomposite/li 10 GeP 2 S 12 /Pt cell, which was prepared by sandwiching 20 mg Li 10 GeP 2 S 12 powder, 120 mg nanocomposite powder, and 20 mg Li 10 GeP 2 S 12 powder by cold-pressing, followed by the Pt sputtering. The obtained Li 2 S-C and Li 2 S-Li 6 PS 5 Cl-C were used as the active materials for all-solid-state batteries. 80Li 2 S 20P 2 S 5 glass-ceramic was used as the solid electrolyte layers for the all-solid-state battery because of its high ionic conductivity. Composite materials of Li 2 S-Li 6 PS 5 Cl-C, carbon black,

3 and 80Li 2 S 20P 2 S 5 glass-ceramic with a weight ratio of 60 : 10 : 30 were mechanically milled (360 rpm for 5 hours) to form the Li 2 S-Li 6 PS 5 Cl-C working electrode. Similar method was used to prepare the Li 2 S-C working electrode by mechanically milling the mixtures of Li 2 S-C, carbon black, and 80Li 2 S 20P 2 S 5 glass-ceramic but with a different weight ratio of 42 : 10 : 48. The varied weight ratio is to ensure the same amounts of the carbon and solid electrolyte as in the Li 2 S-Li 6 PS 5 Cl-C working electrode. For the assembly of the all-solid-state cells, the Li 2 S- Li 6 PS 5 Cl-C (or the Li 2 S-C) working electrode (10 mg) was put on the top of the 80Li 2 S 20P 2 S 5 glass-ceramic powder (150 mg) and cold pressed together under 360 Mpa in a PTFE tank with a diameter of 13 mm. After that, a 100 µm thick indium metal was attached on the other side of the 80Li 2 S 20P 2 S 5 layer as a counter and reference electrode. The formed three-layered pellet was then cold-pressed under 120 Mpa between two stainless steel rods which function as current collectors. All the electrode preparation and cell assembly processes were performed in the glovebox. The equilibrium (open-circuit) potentials of the cells were obtained by galvanostatic intermittent titration technique (GITT), which consists of a series of current pulses at 50 ma/g for 0.5 hour, followed by a 10 hours relaxation process. The open-circuit-voltage at the end of relaxation is considered as the thermodynamically equilibrium potential. The charge/discharge behavior was tested at room temperature using an Arbin BT2000 workstation (Arbin Instruments, TX, USA). The electrochemical impedance spectrum was measured on an electrochemistry workstation (Solartron 1287/1260).

4 Figure S1. XRD pattern of the as-synthesized Li 6 PS 5 Cl solid electrolyte. Figure S2. EDS results of the Li 2 S-Li 6 PS 5 Cl-C nanocomposite from four different spots of the sample. The top right corner indicates the carbon content in the Li 2 S-Li 6 PS 5 Cl-C nanocomposite.

5 Figure S3. EIS plot of the Pt/Li 10 GeP 2 S 12 /Li 2 S-Li 6 PS 5 Cl-C/Li 10 GeP 2 S 12 /Pt cell. The ionic conductivity of the nanocomposite electrode was measured by the EIS using an electron-blocking Pt/Li 10 GeP 2 S 12 /nanocomposite/li 10 GeP 2 S 12 /Pt cell, which was prepared by sandwiching 20 mg Li 10 GeP 2 S 12 powder, 120 mg nanocomposite powder, and 20 mg Li 10 GeP 2 S 12 powder by cold-pressing, followed by the Pt sputtering. The high frequency semicircle represents the grain boundary resistance. We used the intercept to real axis (R Li = 300 Ω) at the high frequency to determine the total resistance (including bulk and grain boundary resistances) of both Li 10 GeP 2 S 12 and the nanocomposite electrode. Since Li 10 GeP 2 S 12 has a very high ionic conductivity (7*10-3 S/cm) and the total thickness of the two Li 10 GeP 2 S 12 layers is only 12 µm, the ion resistance contributed from LGPS is only ~0.034 Ω (out of 300 Ω) and therefore could be ignored. The ionic conductivity of the Li 2 S-Li 6 PS 5 Cl-C nanocomposite electrode is then determined by the equation: σ = (1/R Li )(l/a), where l (144 µm) and a (5 cm 2 ) represent the thickness and area of the nanocomposite, respectively.

6 Figure S4. Charge/discharge curves of the Li 2 S-C and Li 2 S-Li 6 PS 5 Cl-C composite electrodes with liquid electrolyte at 100 ma/g. 1M LiTFSI dissolved in DOL/DME with 1 wt. % LiNO 3 was used as the liquid electrolyte. Figure S5. EIS plot of the Li-In/80Li 2 S 20P 2 S 5 /Li 2 S cells for the first few cycles

7 REFERENCES: (1) Yubuchi, S.; Teragawa, S.; Aso, K.; Tadanaga, K.; Hayashi, A.; Tatsumisago, M. J. Power Sources 2015, 293, (2) Han, F.; Gao, T.; Zhu, Y.; Gaskell, K. J.; Wang, C. Adv. Mater. 2015, 27,