SUPPORTING INFORMATION. Graduate School of EEWS (WCU), Korea Advanced Institute of Science and Technology, 373-1

Transcription

1 SUPPORTING INFORMATION Electrospun Core-Shell Fibers for Robust Silicon Nanoparticle Based Lithium Ion Battery Anodes Tae Hoon Hwang, Yong Min Lee, Byung Seon Kong, Jin-Seok Seo, and Jang Wook Choi,,* Graduate School of EEWS (WCU), Korea Advanced Institute of Science and Technology, Guseong Dong, Yuseong Gu, Daejon , Korea Department of Applied Chemistry, Hanbat National University, 125 Deokmyeong Dong, Yuseong Gu, Daejeon, , Korea KCC Central Research Institute, 83, Mabook Dong, Giheung Gu, Yongin-Si, Gyunggi-Do, , Korea KAIST Institute Nano Century, Korea Advanced Institute of Science and Technology, Guseong Dong, Yuseong Gu, Daejon , Korea * Corresponding author jangwookchoi@kaist.ac.kr S1

2 Experimental procedures In order to synthesize samples containing 50 wt.% Si, the shell polymer solution was prepared by dissolving 11.5 wt.% polyacrylonitrile (PAN, M w =150,000, Aldrich) in N,N-dimethylformamide (DMF, Junsei). On the other hand, the core polymer solution was prepared by dissolving 11.5 wt.% of poly(methyl methacrylate) (PMMA, M w =996,000, Aldrich) and 4.5 wt.% of Si NPs (KCC) in the cosolvent of acetone (Aldrich) and DMF (1:1 = w:w). These solutions were dispersed by stirring at 80 for at least 6 hours. The core polymer solution was loaded into the syringe connected to the inner channel of a dual nozzle (NNC-DN-1723, NanoNC, Korea), which has a 23-gauge inner needle (OD: 0.63 mm, ID: 0.33 mm) and a 17-gauge outer needle (OD: 1.47 mm, ID: 1.07 mm). The shell polymer solution in the other syringe was connected to the outer channel of the same dual-nozzle through a Teflon tube. The flow rates of the core and shell solutions were 1.5 and 2.0 ml/h, respectively. A high voltage of 10.5 kv was applied to the dual-nozzle, and the distance between the needle and the rotating drum collector was ~9 cm. The as-electrospun fibers were peeled off from the collector and then the sample was transferred into an alumina tube furnace for stabilization and carbonization. The stabilization was performed at 280 at atmosphere for 1 hour to stabilize the electrospun fibers. The carbonization was performed at 1000 with a 250 sccm Ar-flow for 5 hours. Each heating step was performed at a heating rate of 5 /min. To synthesize samples with 26 and 37 wt.% of Si, the concentration of PAN for the shell polymer solution was increased to 12 wt.% and the flow rates of the core polymer solution were also changed to 1.0 and 2.0 ml/h, respectively. The carbonization was performed at the same conditions as above. Characterization The morphology and cross-sectional images of carbonized fibers were investigated using a field emission scanning electron microscope (FE-SEM, Sirion). The core section of fibers was investigated using field emission transmission electron microscopy (FE-TEM, TECNAI) and selected area electron S2

3 diffraction (SAED). The elemental analysis of the samples was performed using an energy-dispersive X- ray spectroscopy (EDXS) system. The crystal structure of bare silicon nanoparticles was characterized by X-ray diffraction (micro XRD, Rigaku). The Si weight portion was calculated based on the thermogravimetric analysis (TGA, Netzsch) data. Electrochemical Characterization The core-shell fibers was mixed with Super p and poly(acrylic acid) (PAA, M w =3,000,000, Aldrich) in the weight ratio of 70 (active) : 15 (binder) : 15 (Super P) and was then added in 1-Methyl-2- pyrrolidinone (NMP, Aldrich) to form a homogeneous slurry. The slurry was pasted onto the copper current collector (18 μ m thick Cu foil, Hohsen, Japan) using the doctor blade method. The pasted electrode was dried in the vacuum oven at 70 for 6 hours and punched into circular discs. The electrodes of SiNP alone and C@SiNP for the control samples were prepared under the same conditions. CR2032 coin cells were assembled in an Ar-filled glove box. 1M LiPF 6 solution in a mixture of ethylene carbonate (EC) and diethyl carbonate (DEC) (1:1 = v/v, PANAX E-TEC, Korea) with vinylene carbonate (3 wt%, PANAX E-TEC, Korea) was used as the electrolyte. Celgard 2400 film was used as a separator. The galvanostatic charge and discharge method (same charge/discharge current) were applied with voltage cutoffs of 0.005~1.5 V vs. Li/Li + using a WBCS 3000 battery cycler (Wonatech, Korea) and electrochemical impedance spectroscopy (EIS) was conducted using VMP3 (Bio Logic, France). S3

4 Figure S1. SEM images in (A) low and (B) high magnifications of dual nozzle electrospun SiNP carbon fibers without the non-solvent effect (no acetone in the core polymer solution). It is clearly observed that the diameters of the fibers are irregular and Si NPs are on the surfaces of carbon fibers. Figure S2. An X-ray diffraction pattern of bare Si NPs. The diffraction peaks are indexed based on the JCPDS file no S4

5 Figure S3. Thermogravimetric Analysis (TGA) results of the samples with various Si/C ratios prepared by controlling experimental conditions. Figure S4. SEM images of (A) bare Si NPs (SiNP alone) and (B) Si NPs attached onto carbon fibers prepared using a single nozzle electrospinning for control studies. S5

6 Figure S5. Cycling performance of at a higher rate of A/g. About 80.9 % of the initial discharge capacity was retained after 1500 cycles (361 mah/g) as compared to that at the 20 th cycle (446 mah/g). The sample was cycled at 0.15 A/g for the first cycle but at a higher rate of 6.89 A/g for subsequent cycles. Figure S6. A TEM image of SiNP@C after 500 cycles. Consistently with the SEM image, the TEM image shows that Si NPs remain encapsulated within the core region. The particles on the surfaces of the fibers are not Si NPs, but Super-P. S6