Hierarchical Porous Carbon by Ultrasonic Spray. Pyrolysis Yields Stable Cycling in Lithium-Sulfur. Battery

Transcription

1 SUPPORTING INFORMATION Hierarchical Porous Carbon by Ultrasonic Spray Pyrolysis Yields Stable Cycling in Lithium-Sulfur Battery Dae Soo Jung, Tae Hoon Hwang, Ji Hoon Lee, Hye Young Koo, Rana A. Shakoor, Ramazan Kahraman, Yong Nam Jo, Min-Sik Park, and Jang Wook Choi,* Graduate School of Energy, Environment, Water, and Sustainability (EEWS) and Center for Natureinspired Technology (CNiT), KAIST Institute NanoCentury, Korea Advanced Institute of Science and Technology (KAIST), 291 Daehakro, Yuseong-gu, Daejeon , Republic of Korea Powder Technology Department, Korea Institute of Materials Science (KIMS), 797 Changwondaero, Seongsan-gu, Changwon, Gyeongnam, , Republic of Korea Department of Chemical Engineering, Qatar University, P. O. Box 2713, Doha, Qatar Advanced Batteries Research Center, Korea Electonics Technology Institute (KETI), 25, Saenari-ro, Bundang-gu, Seongnam-si, Gyeonggi-do, , Republic of Korea *Corresponding author. Tel: ; Fax: address: S1

2 Experimental procedures Synthesis of the hierarchical porous carbon (HPC) particles using spray pyrolysis Sucrose (C 12 H 22 O 11, Aldrich) and sodium carbonate (Na 2 CO 3, Aldrich) were used as starting materials for synthesis of HPC particles. The spray precursor solution was prepared by introducing sucrose (0.5 M) and Na 2 CO 3 (0.5 M) into distilled water and then continuously stirring the solution for 3 h. The prepared precursor solution was atomized by an ultrasonic nebulizer with six droplet outlets operating at 1.7 MHz. The generated droplets were flown into an electronic furnace via an Ar flow, and the furnace was maintained at 800 o C. The flow rate was 5 L/min, so the residence time required for each droplet to pass from the ultrasonic generator to the end of the furnace was 5 sec. A schematic diagram of the ultrasonic spray pyrolysis system is shown in Figure S1. A quartz reactor with 1200 mm in length and 50 mm in diameter was used. The particles collected by a Teflon bag filter were washed several times with distilled water to remove salt byproducts. Synthesis of the HPC-S, AC1600-S, and HPC-S 400 The HPC particles were homogeneously mixed and ground with sulfur powder (Aldrich) in a mass ratio of 50:50. The mixture was then sealed in a glass container and transferred into a tube furnace, and the mixture was annealed at 155 o C for 20 h under Ar flow at 50 sccm to obtain the HPC-S composite. AC1600-S used as a control sample was prepared by the same procedure. HPC-S 400 was prepared by heating HPC-S in a sealed glass container at 400 o C for 5 h under Ar flow at 50 sccm. Characterization The morphologies of the HPC, AC1600, HPC-S, and AC1600-S were characterized by field-emission scanning electron microscopy (FESEM, HITACHI, S-4800/UHR-SEM, FEI, Magellan 400) and high resolution transmission electron microscopy (HRTEM, FEI, Tecnai G2 F30). An energy dispersive S2

3 spectrometer (EDS) attached to the TEM apparatus was used for microscopic elemental analyses. The sulfur weight portions in the given composites were determined based on thermogravimetric analysis (TGA, NETZSCH, TG 209 F3) measurements. The nitrogen adsorption and desorption isotherms were attained to characterize the porosities of the samples (Micrometrics, ASAP2010) after degassing the samples at 383 K for 5 h. More detailed structural information on HPC-S 400 was obtained by X-ray photoelectron spectroscopy (ESCA 2000, MultiLab). To verify the robust structural nature and sulfur loading of the HPC-S after cycling, the coin cells were opened inside a glovebox, and the composite particles from the electrodes were then transferred to an SEM holder. Electrochemical Characterization The HPC-S, HPC-S 400, and AC1600-S were mixed with super-p and poly(acrylic acid) (PAA, M w =3,000,000, Aldrich) in a weight ratio of 70 (composite) : 15 (binder) : 15 (super P), and 1-methyl-2- pyrrolidinone (NMP, Aldrich) was added to form homogeneous slurries. After rigorous stirring, the slurries were cast onto the aluminum current collectors (20 µm thick Al foil, Hohsen, Japan) using the doctor blade method. The cast electrodes were dried and punched into 12 pi circular discs. The mass loadings of the whole electrode components for all the samples were ~1 mg cm -2. Another control sample, the bare sulfur electrode, was prepared by the same procedure but based on sulfur powder, AC1600, binder, and super P in a mass ratio of 35:35:15:15. CR2032 coin-type half-cells were assembled in an Ar-filled glove box. 1 M lithium bis(trifluoromethane sulfonyl)imide in 1,3-dioxolane (DOL) and 1,2-dimethoxyethane (DME) (1:1 = v:v) containing 1 wt% LiNO 3 was used as the electrolyte. 1M LiPF 6 in a mixture of ethylene carbonate (EC) and diethyl carbonate (DEC) in a volume ratio of 1:1 was also used at the commercial carbonate electrolyte. The cells were assembled by sandwiching separators (polypropylene, Celgard 2400) with the working electrodes and lithium metal (Reference/counter electrode, Hohsen, Japan). The cells were electrochemically cycled between 1.7 and 2.6 V versus Li + /Li at 0.1 A/g (1C=1675mA/g) under constant current mode for both charge and discharge in the first cycle and cycled at different current S3

4 rates thereafter using a WBCS 3000 battery cycler (Wonatech, Korea). In the second and subsequent cycles, the charge and discharge rates were maintained to be fixed. Figure S1. Schematic illustration showing the formation of HPC during spray pyrolysis as well as the subsequent sulfur infiltration to generate the final HPC-S and HPC-S 400. S4

5 Figure S2. (A) XRD patterns of the HPC particles before and after washing. (B-C) SEM images of the HPC particles prepared by the spray pyrolysis (B) before and (C) after washing. Figure S3. (A) An SEM image of the HPC-S particles after the sulfur infiltration at 155 o C. This image confirms the absence of the residual bulk elemental sulfur after the heat treatment. (B) An EDX pattern of the HPC-S composite obtained for the given STEM image (inset). S5

6 Figure S4. (A) Nitrogen adsorption-desorption isotherms of HPC-S. (B) The pore size distribution obtained from the desorption branch using the density functional theory (DFT) method. S6

7 Figure S5. Galvanostatic profiles of HPC-S when PAA (left) and PVDF (right) were used as binders. Figure S6. Characterization of the conventional activated carbon (AC1600). (A) SEM and (B) TEM images of AC1600 with open micro-, and meso-pores. (C) Nitrogen adsorption-desorption isotherms. The surface areas and pore volumes are denoted inside. (D) The pore size distribution obtained from the desorption branch using the density functional theory (DFT) method. S7

8 Figure S7. Characterization of AC1600-S. Its SEM images in (A) low and (B) high magnifications. TEM images of AC1600-S in (C) low and (D) high magnifications. (E) An XRD pattern showing the infiltrated sulfur is amorphous. S8

9 Figure S8. The discharge capacities originating from the upper plateaus in the voltage profiles in Figures 4C and D in the main text. (A) HPC-S and (B) AC1600-S. S9

10 Figure S9. The sulfur infiltration process into HPC at low and high temperatures. Normally, when heated above 118 o C, sulfur first turns into a mobile, amber molten form containing S 8 rings, and the viscosity of the molten sulfur becomes lowest around 155 o C. When the molten sulfur is cooled, the mobile S 8 rings or S 8 chains are crystallized into more stable alpha-sulfur form under the orthorhombic symmetry, where the sulfur consists of puckered large S 8 rings in the shape of crowns. Thus, when the molten sulfur infiltrated into HPC by capillary forces at 155 o C is cooled and solidified, most of the sulfur in HPC is accommodated in the relatively large meso- and macro pores to produce the thermodynamically stable phase. However, after an additional heat-treatment of HPC-S at 400 o C, the sulfur can be stabilized in the micropores in the form of small sulfur (S 2-4 ) for the reason described in the main text. S10

11 Figure S10. (A) The sulfur line-scan across the HPC-S 400. (B) XRD patterns and (C) TGA curves of HPC-S and HPC-S 400. S11

12 Counts (/s) S1: ev (2p3/2) S2: ev (2p3/2) S3: ev (2p3/2) S4: ev (2p3/2) S4 S3 S2 S1 R aw S 2p3/2-1 S 2p3/2-2 S 2p3/2-3 S 2p3/2-4 S um Binding Energy (ev) Figure S11. S 2p 3/2 signal obtained from XPS analysis of HPC-S 400. The sub-peaks (S4 & S3) at and 168 ev arise from short sulfur chains, S1 and the lower peak locations (S2 & S1) at and ev compared to those of bare sulfur indicate C-S bond and S=C=S bond, S2 respectively. Figure S12. (A) Nitrogen adsorption-desorption isotherms of HPC-S 400. (B) The pore size distribution obtained from the desorption branch using the density functional theory (DFT) method. S12

13 Figure S13. Electrochemical performance of HPC-S 400 in the carbonate-based electrolyte without LiNO 3 additive. (A) The 1 st and 2 nd discharge/charge voltage profiles at a 0.05C rate (1C=1675 mah/g). (B) Voltage profiles during 100 cycles at 0.2C. (C) Long term cycling performance when measured at 0.5C and (D) the corresponding discharge/charge voltage profiles. Reference S1. Li, Z.; Yuan, L.; Yi, Z.; Sun, Y.; Liu, Y.; Jiang, Y.; Shen, Y.; Xin, Y.; Zhang, Z.; Huang, Y. Adv. Energy Mater. 2014, 4, S2. Park, M.-S.; Yu, J.-S.; Kim, K. J.; Jeong, G.; Kim, J.-H.; Jo, Y.-N.; Hwang, U.; Kang, S.; Woo, T.; Kim, Y.-J. Phys. Chem. Chem. Phys. 2012, 14, S13