Supporting Information for: High-Performance Silicon Battery Anodes Enabled by Engineering Graphene Assemblies Min Zhou,, Xianglong Li, *, Bin Wang, Yunbo Zhang, Jing Ning, Zhichang Xiao, Xinghao Zhang, Yanhong Chang, and Linjie Zhi CAS Key Laboratory of Nanosystem and Hierarchical Fabrication, National Center for Nanoscience and Technology, Beijing 100190, China Department of Environmental Engineering, University of Science and Technology of Beijing, Beijing 100083, China * E-mail: lixl@nanoctr.cn Methods Fabrication of TCG-Si. Graphene oxide sheets (GO) were prepared according to the literature reported elsewhere; commercial silicon nanoparticles (~100 nm in diameter) were used as received. The fabrication of TCG-Si assemblies is realized by electrostatic self-assembly of as-prepared bovine serum albumin (BSA)-coated silicon nanoparticles and GO sheets during vacuum filtration followed by thermal annealing. Typically, silicon nanoparticles (~100 nm in diameter) were dispersed into a certain amount of BSA solution (0.5 wt %), stirred for 2 h at room temperature, and then collected by centrifugation. Afterwards, the thus-obtained BSA-coated silicon nanoparticles were re-dispersed in 100 ml deionized water, and then mixed with a desired amount of GO solution (1 mg ml -1 ) under continuous stirring for 2 h to obtain
a homogeneous dispersion. Upon adjusting the ph value to <5 by addition of dilute hydrogen chloride (HCl) solution, the dispersion was vacuum-filtered, during which BSA-coated silicon nanoparticles and GO sheets were assembled via electrostatic interactions. The obtained filtrate cake was washed, dried, and then thermally annealed at 700 ºC for 2 h under argon/hydrogen (Ar/H 2 ; 90: 10) atmosphere (with a ramp rate of 5 ºC min -1 ), thus completing the fabrication of silicon nanoparticle-impregnated assemblies of templated carbon bridged oriented graphene (denoted as TCG-Si). Hereof, the thermal annealing process was applied aiming at thermally reducing GO sheets into graphene sheets (G), as well as making BSA coatings on silicon nanoparticles carbonized into templated carbon (TC) hinges. In some cases, the TCG-Si assemblies were further immersed into 5% sodium hydroxide (NaOH) aqueous solution under ~80 ºC for 1 h, washed, and then dried, thus obtaining templated carbon bridged oriented graphene assemblies (denoted as TCG). The G-Si control samples with the similar silicon content (~64 wt %) were prepared using the same procedure used to fabricate the TCG-Si, except for the use of pristine silicon nanoparticles. Characterization. The morphology and structure of the samples were investigated by FE-SEM (Hitachi S4800) and FE-TEM (FEI Tecnai G2 20 STWIN and Tecnai G2 F20 U-TWIN). The X-ray diffraction (XRD) instrument type was D/MAX-TTRⅢ (CBO). X-ray photoelectron spectroscopy (XPS) measurements were carried out on an ESCALAB250Xi apparatus with an Al Kα X-ray source. Raman spectra were collected using a Renishaw invia Raman microscope with a laser wavelength of
514.5 nm. All electrochemical measurements were conducted with coin-type half-cells (2032). Except for the SiNP control electrode which was prepared by mixing silicon nanoparticles, super P carbon black and PVDF binder at a set weight ratio of 65:25:10 and then casting on current collectors, all the self-supporting assemblies (TCG-Si, G-Si, and TCG) were directly used as the working electrode, and assembled into coin-type half cells in an argon-filled glove box (<1 ppm of oxygen and water) with lithium foil as the counter electrode, porous polypropylene film as the separator, and 1M LiPF 6 in 1:1 (v/v) ethylene carbonate/diethyl carbonate (EC/DEC) with 5 wt% fluoroethylene carbonate as the electrolyte. The mass loading of all the tested TCG-Si electrodes is 0.8~3.7 mg cm -2. The cycle-life and rate capability tests were performed using a CT2001A battery program controlling test system within the voltage range of 0.02-1.0 V. Cyclic voltammetry was carried out in the potential range of 0.02-1.0 V at a rate of 0.1 mv s -1 with a CHI660D electrochemical station. Electrochemical impedance spectroscopy (EIS) measurements were conducted in the frequency range from 100 KHz to 0.01 Hz with an AC perturbation of 5 mv. The specific capacities reported are on the basis of the total electrode weight; the volumetric capacity is calculated as product of the packing density and gravimetric capacity of the electrode. Figure S1-S10
Figure S1. (a-d) SEM images at different magnifications of G-Si. The dashed lines in (d) are superimposed to highlight continuous interlayers of graphene sheets sandwiching silicon nanoparticles. Figure S2. Representative XRD patterns of (a) TCG-Si and (b) G-Si. In both cases, five sharp peaks appeared at 2θ=28, 47, 56, 69, and 76 can be assigned to the (111), (220), (311), (400), and (331) reflections of crystalline silicon (JCPDS card no. 27-1402), respectively, indicating the structural preservation of crystal silicon nanoparticles in both cases after experiencing our designed processing. Furthermore, a relatively broad peak observed at 26 may be related to the presence of TC and graphene for the TCG-Si, and graphene for the G-Si, respectively.
Figure S3. XPS spectra of TCG-Si and G-Si. Figure S4. TGA curves of (a) TCG-Si and (b) G-Si. It can be deduced that the silicon content is ca. 62 and 64 wt% in the TCG-Si and the G-Si, respectively. Figure S5. Gravimetric capacities versus cycle number of the TCG electrode obtained under the same testing conditions as those for the TCG-Si electrodes.
Figure S6. Voltage profiles for the TCG-Si electrode plotted for the 5th, 50th, 100th, 150th, and 200th cycles, exhibiting typical electrochemical features of silicon. Note: the current rate is 2 A g -1 for all cycles. Figure S7. Typical cyclic voltammetry (CV) curve of TCG-Si, in which one peak at 0.18 V in the cathodic branch and two peaks at 0.37 V and 0.52 V in the anodic branch correspond to lithiation and delithiation of amorphous Si, respectively, consistent with other reported silicon-based electrodes.
Figure S8. SEM images of a TCG-Si electrode (a) before and (b-d) after cycling. The TCG-Si electrode maintains its original morphology, reflecting the structural stability; the electrode thickness remains almost the same as the initial one (~8 μm), implying better accommodation of the volume change of silicon nanoparticles by the TCG in the TCG-Si electrode. Figure S9. Voltage profiles obtained at different current rates for (a) 6 μm TCG-Si, (b)
5 μm G-Si, (c) 14 μm TCG-Si, and (d) 28 μm TCG-Si. Figure S10. Nyquist plots of the TCG-Si electrodes of different thicknesses obtained from electrochemical impedance spectroscopy (EIS) measurements, after annotated charge/discharge cycles. The typical Nyquist plot of the G-Si electrode is also superimposed for comparison. All the plots obtained are composed of an inclined line in the low-frequency region and a depressed semicircle in the high-frequency region, a behavior that is typical for silicon anodes. The inclined line corresponds to the lithium-ion diffusion impedance, and the depressed semicircle mainly consists of the interfacial charge transfer impedance at the medium-to-high frequency range although high-frequency SEI film impedance contributes to it. The comparison of the semicircle diameter indicates that the charge transfer resistance of G-Si is relatively larger than that for TCG-Si, reflecting relatively poor electrical conductivity in the G-Si than in the TCG-Si. The comparable semicircle diameter in the plots for all tested TCG-Si electrodes after 10 cycles indicates that the charge transfer resistance of the TCG-Si electrodes is less dependent of the electrode thickness, providing an explanation for the achieved almost constant rate capability for all the TCG-Si electrodes of different thicknesses. Furthermore, the slight increase in charge transfer resistances of all the tested electrodes after 100 cycles in comparison with those after 10 cycles are observed, indicating the stability of the TCG-Si electrodes during cycling.