Rice husks as a sustainable source of nanostructured silicon for high performance Li-ion battery anodes

Transcription

1 Supplementary Information Rice husks as a sustainable source of nanostructured silicon for high performance Li-ion battery anodes Nian Liu 1, Kaifu Huo 2,3, Matthew T. McDowell 2, Jie Zhao 2 & Yi Cui 2,4 * 1 Department of Chemistry, Stanford University, Stanford, California 94305, USA. 2 Department of Materials Science and Engineering, Stanford University, Stanford, California 94305, USA. 3 Wuhan National Laboratory for Optoelectronics, Huazhong University of Science and Technology (HUST), Wuhan, , China. 4 Stanford Institute for Materials and Energy Sciences, SLAC National Accelerator Laboratory, 2575 Sand Hill Road, Menlo Park, CA 94025, USA. Correspondence and requests for materials should be addressed to Y.C. (* yicui@stanford.edu). Note: this Supplementary Information contains Supplementary Figure S1-13 and Supplementary Table S1. SUPPLEMENTARY FIGURES Figure S1. Pictures of (a) rice grown on a farm (photographed by Huo, K.) and (b) rice husks (photographed by Liu, N.). (c) Schematic showing the microstructure of rice husks, which is a natural reservoir of silica nanoparticles. S1

2 Figure S2. (a) Optical microscopy image and (b) SEM image of a typical piece of heat treated rice husk (rice husk ash). The macroscopic morphology is maintained after the heat treatment, while the removal of organics makes the remnant silica brittle and loose. (c) X-ray diffraction pattern of rice husk ash, showing its amorphous nature. S2

3 Figure S3. Images of the homemade reactor used to carry out magnesiothermic reduction in this report. The reactor is made by assembling one 5/8 in. Swagelok brass union with two 5/8 in. Swagelok stainless steel plugs. Swagelok was chosen because its tight sealing prevents Mg vapor from escaping. Also, it is commercially available and easy to handle. The size and shape of the reactor was chosen so that it can fit into a 1 in. tube furnace in the laboratory. The different component materials (brass and stainless steel) prevent them from fusing at elevated temperature. The total internal volume is measured to be 2 ml and the reactant powders fill one third of the volume. The reactor can be used repeatedly as long as it is washed with dilute hydrochloric acid to remove residual material after each reaction. S3

4 Figure S4. Low-magnification SEM image of macroporous Si recovered from RHs (Si-RH-40) with ramp rate of 40 o C min -1 from 400 o C to 650 o C during magnesiothermic reduction. S4

5 Figure S5. TEM selected area electron diffraction (SAED) pattern of the nano-si recovered from RHs (Si-RH-5). Large numbers of SiNPs were in the selected diffraction area. The diffraction rings are indexed to cubic Si. Figure S6. XRD patterns of the mixture after magnesiothermic reduction. The reaction vessel was heated from room temperature to 400 o C over 10 min, then to 650 o C with 40, 5, and 0.5 o C min -1 ramp rate for different scans, respectively. S5

6 Figure S7. (a) Cross section SEM image of a pristine nano-si pellet pressed on Cu foil for I-V curve measurement. The pellet was made from drop-casting of a ethanol dispersion of nano-si onto Cu foil followed by drying and roll-pressing. (b) I-V curve at room temperature averaged over 11 devices. Indium tin oxide (ITO, 2 mm 2 mm) was deposited by sputtering on top of nano-si pellet as an electrical contact. Inset, schematic of the device. Figure S8. Secondary ion mass spectrometry (SIMS) analysis of the purity of nano-si recovered from RHs. The depth profiles of the Si and trace metal atomic concentration were measured using SIMS with an approximate sputter rate of 0.2 nm min -1. The nano-si pellet as shown in Fig. S7a was used as the sample. S6

7 Figure S9. Voltage profiles of a nano-si half cell cycled between 0.01 V and 1 V versus Li/Li +. The first, 100th, 200th, and 300th cycles are plotted. Figure S10. Impedance measurements (Nyquist plot) of a nano-si electrode after different numbers of CV scans over the potential window of 0.01 to 1 V versus Li/Li +. For example, 1 denotes the impedance after one CV scan cycle. All the measurements were performed in the delithiated state. The inset is a plot of the charge transfer resistance versus scan number extracted from the Nyquist plot. S7

8 Figure S11. SEM images of nano-si electrodes after 60 galvanostatic cycles. The nano-si electrodes were delithiated to 1 V before being taken out of the cells. For (a), electrodes were washed with only acetonitrile to remove the excess electrolyte. For (b), electrodes were washed first with acetonitrile and then with dilute hydrochloric acid to remove the solid-electrolyte interphase (SEI). (c) is a magnified image of (b) to show the porous nature of Si. Yellow arrows in (c) indicate Super P conducting carbon additives. Figure S12. (a) Delithiation capacity and (b) voltage profiles of the nano-si anode cycled at various rates from C/50 to 2C in the potential window of 0.01 to 1 V versus Li/Li +. 1C = 4.2 A g -1 Si. S8

9 Figure S13. Gram-scale fabrication of (a) nano-sio 2 and (b) nano-si from rice husks. The net weight of the products were shown on the screen of the balance. To scale up the magnesiothermic reduction step, larger reactors and a tube furnace with inner diameter of 100 mm were utilized. To diminish heat accumulation during the exothermic reaction, the whole heating process was slowed down. The reactors were heated to 350 o C over 30 min, then to 650 o C with a ramp rate of 1 o C min -1, and held at 650 o C for 1 h. S9

10 SUPPLEMENTARY TABLE Table S1. Summary of BET specific surface area and pore volume of the SiO 2 and Si recovered from rice husks. SiO 2 Si-RH-40 Si-RH-5 BET surface area (m 2 g -1 ) Pore volume (cm 3 g -1 ) S10