Spray Drying Method for Large-Scale and High. Performance Silicon Negative Electrodes in Li-ion. Batteries

Transcription

1 SUPPORTING INFORMATION Spray Drying Method for Large-Scale and High Performance Silicon Negative Electrodes in Li-ion Batteries Dae Soo Jung, Tae Hoon Hwang, Seung Bin Park, and Jang Wook Choi,,* Graduate School of EEWS (WCU) and Department of Chemical and Biomolecular Engineering, Korea Advanced Institute of Science and Technology, 291 Daehak-ro, Yuseong-gu, Daejon , Republic of Korea * S1

2 Experimental procedures 2.25g Silicon nanoparticles (Si NPs, average diameter=70 nm, KCC Korea), 1.2g silica NPs (average diameter=10 nm, Degussa, Germany), and 13.7g sucrose (Aldrich) were used as starting materials in the precursor solution for synthesis of Silica NPs were used as a template for pore formation in the composite particles, and sucrose was used as a carbon source. The precursor solution was prepared by first introducing the starting materials into distilled water, and the solution was then agitated for dispersion using a high energy tip sonicator (750 W, VCX 750; Sonics & Materials Inc., Newtown, CT). Next, droplets in the sizes of 10~20 µm were generated through an atomization process of the precursor solution using an ultrasonic spray drying process (USD). For this, an ultrasonic atomizer with six droplet outlets operating at 1.7 MHz was used. Once the droplets were generated, they were flown into an electronic furnace via an air flow in which the droplets were dried at 500 o C. The flow rate was 10 L/min, so the residence time required for droplet flow from the ultrasonic generator to the end of the furnace was only 2.1 sec. After the drying step, typical sphere dimensions were 1~2 µm. The prepared Si/silica/sucrose composite spheres were carbonized at 700 o C for 30 min under nitrogen atmosphere. After the carbonization, the composite spheres were chemically etched with hydrofluoric acid (HF) to remove silica NPs, thus generating nano-dimensional pores in the final spheres. The bare porous carbon spheres (po-c) were prepared by the same procedure, but from the precursor solution that did not contain Si NPs. Characterization The morphologies of the Si@po-C and po-c 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 spectrometer (EDS) attached to the SEM and TEM apparatus was used for local elemental analyses. Particle size S2

3 distribution and mean particle size of were measured uisng a particle size analyzer (Cilas, 1064). The Si weight portion in Si@po-C was obtained based on thermogravimetric analysis (TGA, NETZSCH, TG 209 F3) measurements. The nitrogen adsorption and desorption isotherms of the Si@po-C and po-c were attained using the Brunauer-Emmett-Teller (BET, Micrometrics, ASAP2010) method after degassing the samples at 383 K for 5 h. To verify the robust structural nature of the Si@po-C after cycling, the coin cells were opened and the electrodes were thoroughly washed with acetonitrile inside a glove box. Then, the electrodes were transferred to an SEM holder and the holder was completely sealed to prevent exposure to air. The samples were exposed to air for < 30 s during transfer to the vacuum chamber of SEM. Electrochemical Characterization The Si@po-C was mixed with super-p and poly(acrylic acid) (PAA, M w =3,000,000, Aldrich) in the weight ratio of 60 (active) : 20 (binder) : 20 (super P) and was then added into 1-Methyl-2-pyrrolidinone (NMP, Aldrich) to form a homogeneous slurry. The slurry was cast onto the copper current collector (18 µm thick Cu foil, Hohsen, Japan) using the doctor blade method. The loading of the active materials was ~1 mg cm -2. The cast electrodes were dried in a vacuum oven at 70 o C for 10 h and punched into circular discs for coin-cell fabrication. The electrodes of Si NPs alone and po-c were also prepared under the same conditions. CR2032 coin cells were assembled in an Ar-filled glove box. 1M lithium hexafluorophosphate (LiPF 6 ) solution in a mixture of ethylene carbonate (EC) and diethyl carbonate (DEC) (1:1 = v:v) containing 5 wt% FEC (PANAX E-TEC, Korea) was used as the electrolyte. The cells were assembled by sandwiching separators (polypropylene, Celgard 2400) with the Si@po-C electrodes (working electrodes) and lithium metal (Reference/counter electrode, Hohsen, Japan). The cells were electrochemically cycled between 0.01 and 1.2 V versus Li + /Li at 0.1 A/g under constant current mode for both charge and discharge in the first cycle and cycled at different current rates S3

4 thereafter using a cycle tester (Wonatech, Korea). In the second and subsequent cycles, the charge and discharge rates were same. Figure S1. (A) A schematic procedure for synthesis of porous carbon spheres (po-c). (B) A series of photographs showing the initial, intermediate, and final states during the synthesis of po-c. From the left, colloidal precursor solution containing silica NPs and sucrose, silica spheres coated by decomposed sucrose after the spray drying process, and the final po-c obtained after carbonization (700 o C for 30 min under nitrogen atmosphere) and an HF treatment. S4

5 Figure S2. SEM images during the synthesis of po-c. (A) Silica NPs used as templates for pores, (B) sucrose coated silica spheres after the spray drying process, the same spheres but after (C) carbonization and (D) HF treatment. Higher resolution images for the sample in D were also obtained using TEM and STEM, and are presented in the next Supporting Figure. S5

6 Figure S3. Morphological characteristics of po-c. (A) A TEM image and (right) its magnified image showing the porous structure. (B) An STEM image and (C) an energy-dispersive X-ray spectrum from the red box in (B). (D-E) A nitrogen adsorption and desorption isotherm and a pore size distribution. S6

7 Figure S4. SEM images of sucrose-si composite prepared based on the same residential time as for the case of in the main text, but without silica NPs. The sucrose-si composite does not form spheres rather generates random collapsed morphologies. Figure S5. An SEM image of the as-synthesized particles in a low magnification. S7

8 Figure S6. The particle size distribution of measured by dynamic light scattering. Figure S7. An SEM image of after a thermal treatment at 650 o C for 1h to remove the porous carbon matrix and thus trace the internal distribution of Si NPs within each of the composite particle. S8

9 Figure S8. A TGA curve of Si@po-C indicating that the final Si@po-C contains 41 wt% of Si. Figure S9. XPS spectra of Si@po-C and bare Si. S9

10 Figure S10. Electrochemical performance of po-c. (A) The first potential profiles of po-c when measured at a current rate of 0.05C for both charge and discharge in the potential range of 0.01~ 1.2 V S10

11 vs Li + /Li. (B) Discharge capacities at various discharge rates from 0.15C to 12C (1C= 0.23 A/g). The charge rate was fixed at 1C. (C) Charge-discharge potential profiles and (D) Charge-discharge capacities and CEs over cycling when measured at a current rate of 12C in the potential range of 0.01~1.2 V vs. Li + /Li. Figure S11. A typical SEM-EDS image with elemental mapping of Si and C for Si@po-C. The shown sphere was within the electrode film, and the image was taken after the rate performance test. This image indicates that Si NPs remain encapsulated within the carbon matrix. Color code: Si=yellow, C=red. S11