Supporting Information Towards High-Safety Potassium-Sulfur Battery Using Potassium Polysulfide Catholyte and Metal-Free Anode Jang-Yeon Hwang, Hee Min Kim, Chong S. Yoon, Yang-Kook Sun* Department of Energy Engineering, and Department of Materials Science and Engineering, Hanyang University, Seoul, 04763, Republic of Korea * To whom correspondence should be addressed. E-mail: yksun@hanyang.ac.kr 1
Experimental Section Preparation of 3D-FCN film: The 3D-FCN film was prepared via a two-step vacuum filtration process. In brief, 0.26 g of MWCNT was first dispersed in a mixture of deionized (DI) water (200 ml) and isopropyl alcohol (IPA; 20 ml) (DUKSAN) with sonication for 10 min. The solution was then filtered by vacuum filtration through a membrane (Advantec No. 2). Then, the resulting filtered MWCNT film was dried in an oven at 60 C for 12 h and the membrane was peeled off to obtain the 3D-FCN film. Preparation of series of potassium-polysulfide catholyte K 2 S x (1 x 6): The standard K 2 S x -free electrolyte was prepared in an argon-filled glove box by dissolving 0.5 M KTFSI (Sigma Aldrich, USA) in DEGDME (Sigma Aldrich, USA). The 0.05 M K 2 S x -containing electrolyte (1 x 6) was prepared by adding small pieces of potassium metal (Sigma Aldrich, USA) and elemental sulfur (Sigma Aldrich, USA) with the appropriate molar ratio (K: S) of 2:1, 2:2, 2:3, 2:4, and 2:5 in K 2 S x - free electrolyte (0.5 M KTFSI in DEGDME). The mixture was continually stirred for 24 h at room temperature to produce a dark-red colored solution in which no residual sulfur or potassium metal could be detected. Material Characterization: The surface morphology and thickness of the prepared electrodes were visualized by scanning electron microscopy (NOVA NANO SEM 450, FEI). The crystalline phases were characterized by powder X-ray diffraction (XRD; Rint-2000, Rigaku) in the 2θ range between 10 and 80 with a step size of 0.03 under Cu Kα radiation. 0.05 M K 2 S x (5 x 6) potassium polysulfide was characterized by UV/vis spectroscopy (V-670, JASCO, Japan). Transmission electron microscopy (TEM, JEM 2010, JEOL) and energy dispersive X-ray spectroscopy (EDX, JEM 2100F, JEOL) analyses were also performed. The charge/discharge product was confirmed via Raman spectroscopy (InVia TM Raman Microscope, Renishaw, United Kingdom). 2
Electrochemical Test: Electrochemical characterization was conducted using a 2032 coin-type cell. The fabricated cathode and a potassium metal anode were separated using both a glass fiber (Advantec) and polypropylene (Celgard 2400) to prevent short circuiting. The cells were typically cycled in the constant current mode at a 0.1 C-rate within the voltage range of 1.2 2.4 V versus K/K + at room temperature, where 1 C=558 ma g -1. All cells used 50 μl of 0.05 M K 2 S x (5 x 6) potassium polysulfide for the electrochemical test. We calculated the gravimetric capacity of cathode for both the half-cell and full cell based on weight of sulfur (0.44 mg) in 0.05 M K 2 S x (5 x 6) catholyte. The anode was fabricated by blending the active materials (80 wt. %) and polyvinylidene fluoride (20 wt. %). The resulting slurry was applied onto the copper foil and dried at 110 C for 12 h in a vacuum oven. The full cell balance was achieved by adjusting the capacity ratio of the anode to the cathode (N/P ratio) to be 1.2:1. 3
A B Solid phase K:S=2:1 K:S=2:2 K:S=2:3 K:S=2:4 C Liquid Phase K:S=2:5 Figure S1. (A) Schematic illustration of synthesis method for potassium polysulfides. Digital photographs of a series of potassium polysulfides: (B) solid phase potassium polysulfides (K:S = 2:1, 2:2, 2:3, and 2:4, molar ratio) (C) liquid phase potassium polysulfides ((K:S = 2:5, molar ratio). 4
Absorbance 3 2 1 290 nm 226 nm 0 200 250 300 350 400 450 wavenumber / nm Figure S2. (A) UV/vis spectra of K 2 S x (5 x 6) polysulfide catholyte. 5
3D-FCN Film 200 μm Figure S3. Cross-sectional SEM image of the 3D-FCN film 6
Voltage / V 3.0 Shuttle Reaction (> 2.5 V) 2.5 2.0 1.5 1.0 0 200 400 600 800 1000 Specific capacity / mah g -1 Figure S4. Initial charge-discharge curves in the voltage range of 1.2-2.5 V at 0.1 C-rate. 7
dq dv -1 / mah V -1 1.0 0.5 2.05 V 2.23 V 0.0-0.5 1.8 V 2.1 V -1.0 1.0 1.5 2.0 2.5 Voltage / V Figure S5. The dq dv -1 curves at initial charge-discharge of K K 2 S x (5 x 6) polysulfide catholyte 3D-FCN half-cell. 8
Intensity / A.U. ( 0 4 1 ) ( 0 2 3 ) ( 1 1 2 ) 1 3 0 ) *( * ( 2 0 2 ) ( 2 2 1 ) ( 1 3 1 ) * * * * * K 2 S 3 Peak K 2 S 3 - JCPDS #71-234 1 st charge Ⅱ 1 st discharge Ⅰ 10 30 40 50 60 Cu K θ Figure S6. Ex situ XRD patterns after the 1 st discharge (brown line) and 1 st charge (black line). 9
(a) 1 st Discharge CNT (b) Carbon K 2 S 3 250 μm 250 μm (c) sulfur (d) Potassium 250 μm 250 μm Figure S7. TEM image (a) and corresponding EDX elemental mapping (b d) after the 1 st discharge. 10
Voltage / V A 0.05M K 2 S x (5 x 6) in DEGDME 3.0 2.5 2.0 1.5 1.0 1.95 V 2.23 V 2.05 V 2.1 V 1.9 V 1.2-2.4 V, 0.1 C, 30 o C 1.8 V 1.65 V 5 th cycle dq dv -1 / mah V -1 B 1.0 0.5 0.0-0.5 5 th cycle 2.23 V 2.05 V 1.95 V 1.65 V 1.8 V 1.9 V 2.1 V 0.5 0 100 200 300 400 Specific capacity / mah g -1 (s) -1.0 1.0 1.5 2.0 2.5 Voltage / V Figure S8. (A) Charge-discharge curves at 5 th cycle in K K 2 S x (5 x 6) polysulfide catholyte 3D-FCN cell and (B) corresponding dq dv -1 curves. 11
Voltage / V 0.05M K 2 S x (5 x 6) in DEGDME 2.0 1.5 Hard Carbon 0.01-1.5 V, 0.1 C, 30 o C 1.0 0.5 0.0 0 100 200 300 Specific capacity / mah g -1 (c) Figure S9. Initial charge-discharge curves of K K 2 S x (5 x 6) polysulfide catholyte Hard Carbon half-cell. 12
A DME:DOL (1:1, v/v) solvent (Flammable) B DEGDME solvent (Non-Flammable) C 0.05M K 2 S x (5 x 6) in DEGDME solution (Non-Flammable) Figure S10. Flammability test for different solvent: (A) DME:DOL (1:1, v/v) solvent, (B) DEGDME solvent, and (C) 0.05 M K 2 S x (5 x 6) in DEGDME solution. 13