Supporting Information for

Transcription

1 Supporting Information for Hollow Carbon Nanofiber-Encapsulated Sulfur Cathodes for High Specific Capacity Rechargeable Lithium Batteries Guangyuan Zheng, Yuan Yang, Judy J. Cha, Seung Sae Hong and Yi Cui * Department of Chemical Engineering, Department of Materials Science and Engineering and Department of Applied Physics, Stanford University, Stanford, California 94305, United States. *Corresponding Author: Y. C., yicui@stanford.edu These authors contributed equally to this work. Materials and Synthesis Synthesis of hollow carbon nanofiber-encapsulated sulfur cathode Anodic aluminum oxide (AAO) membranes (Whatman, pore size ~200 nm, thickness ~60 µm) were used as the template for making carbon nanofibers. Typically, 120 mg of AAO was placed in an alumina boat and 2 ml polystyrene (PS) suspended in dimethylformamide (DMF, 0.1 g/ml) was dropped onto the template as the carbon precursor. The carbonization was done by heating the AAO/PS/DMF composite at 750 C for 4 hours under a slow flow of N 2 gas. After cooling down, 15 mg of carbon-coated AAO template was loaded into a small glass vial and 300 µl of 1% sulfur solution in toluene was dropped onto the template. After drying, the mixture was heated up to 155 C and kept for 12 hours to facilitate sulfur diffusion into the hollow nanofibers. The AAO template provided a barrier to prevent sulfur from

2 coating onto the outer surface of the carbon nanofibers. Sulfur residue sticking on the surface of the template was washed away using methanol. Total sulfur loading (~1 mg) in the final electrode sample was calculated by weighing the sample before and after sulfur infusion (using a Sartorius SE2 Ultra Micro Balance, mg readability and ± mg repeatability). The AAO template was removed by immersing the as-prepared sample in a solution of 2 M H 3 PO 4 for 10 hours. Characterizations FEI XL30 Sirion SEM with FEG source was used for SEM characterizations. FEI Tecnai G2 F20 X-TWIN Transmission Electron Microscope was used for TEM characterizations. A Renishaw RM1000 Raman microscope at the Extreme Environments Laboratory at Stanford University was used for the Raman spectroscopy. Electrochemical Measurement To evaluate the electrochemical performance of the hollow carbon nanofiber/sulfur composite, 2032-type coin cells (MTI Corporation) were fabricated. The prepared samples were pressed onto aluminum substrate as the working electrode without any binder or conductive additives. Lithium foil (Alfa Aesar) was used as the counter electrode. The electrolyte is 1 M lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) in 1,3-dioxolane and 1,2-dimethoxyethane (volume ratio 1:1). For electrolyte with LiNO 3 additive, LiNO 3 (Sigma Aldrich) was first dried at 100 under vacuum over

3 night, before being added to the electrolyte to reach a concentration of 0.1 mol/l. The mass loading of sulfur in the working electrode was about 1.0 mg/cm 2. Batteries testings were performed using a 96-channel battery tester (Arbin Instrument). The voltage range was V vs. Li/Li +. SEM characterization of AAO template. Figure S1. SEM images showing different pore size distributions for the two sides of AAO template. (a) The side with pore openings of about 250 nm and (b) the other side with pore openings of about 200 nm. Raman spectroscopy measurement

4 Figure S2. Raman spectra of the four samples: pure sulfur (brown), carbon coated AAO template (red), hollow carbon nanofiber-encapsulated sulfur (black) and pure AAO template (blue). No sulfur signal was detected in the hollow carbon nanofiber-encapsulated sulfur. X-ray Diffraction characterizations Figure S3. Comparison of the XRD pattern for pristine sulfur and hollow carbon

5 nanofiber-encapsulated sulfur. Inset is the zoom-in image of the encapsulated sulfur XRD pattern between 22 and 24. A series of peaks are observed in XRD pattern for AAO template heated to 780 C, while AAO template heated to 750 C shows no diffraction peaks. This indicates a phase transition of AAO between 750 C and 780 C. Figure S4. XRD pattern of AAO template of after carbon coating at 750 C (red) and 780 C (black).

6 Figure S5. Charge/discharge voltage profiles of hollow carbon nanofiber-encapsulated sulfur at C/10 and C/5.