Lithium Ion Insertion Properties of Solution-Exfoliated Germanane Andrew C. Serino, Jesse S. Ko, Michael T. Yeung, Jeffrey J. Schwartz, Chris B. Kang, Sarah H. Tolbert,,, Richard B. Kaner,,, Bruce S. Dunn,*,, and Paul S. Weiss*,,, Department of Materials Science and Engineering, University of California, Los Angeles, Los Angeles, California 90095, United States Department of Chemistry and Biochemistry, University of California, Los Angeles, Los Angeles, California 90095, United States Department of Physics and Astronomy, University of California, Los Angeles, Los Angeles, California 90095, United States California NanoSystems Institute, University of California, Los Angeles, Los Angeles, California 90095, United States *Corresponding authors: psw@cnsi.ucla.edu (PSW), bdunn@ucla.edu (BSD) Supporting Information: Intensity (a.u.) 2 Theta ( ) Figure S1. Powder X-ray diffraction of a crushed CaGe 2 ingot. The peaks index to CaGe 2 with some impurity peaks present (CaO and Ge). 1
Atomic Force Microscopy of Solution-Exfoliated Single Sheets of GeH Atomic force micrographs (Figure S2) of exfoliated GeH sheets, drop-cast and dried in air on silicon substrates, were collected using a Bruker Dimension Icon atomic force microscope in tapping mode. Three-point plane-fitting was performed using Gwyddion S1 to flatten the data prior to subsequent analysis. Histograms depicting the distribution of pixel heights exhibit two distinct peaks, indicative of two characteristic feature heights visible in the images, which correspond to the exposed substrate surface and the tops of nanosheets containing an identical number of layers. An automated routine using a combination of constant-threshold and morphological opening and closing operations was used to identify all nanosheets (and their borders) within the field of view of each image, in addition to the exposed substrate surface. The heights of individual nanosheets were measured as the difference between the mean height of all pixels internal to an identified nanosheet and the mean substrate height. Border regions, ~10 20 pixels wide, straddling the edges of the nanosheets and the exposed substrate, were excluded from the computation of the mean feature height. All nanosheets were found to be ~0.81 nm thick, corresponding to ~1.4 GeH sheets assuming a single-sheet thickness of 0.57 nm. We attribute the discrepancy in apparent heights to the effects of non-uniform solvent adsorption across the hydrophilic sheets and relatively hydrophobic substrate, even after nominally drying in air. S2,S3 As such, we hypothesize that the sheets shown here are single layers of GeH, with residual solvent accounting for the ~0.24 nm difference in apparent and expected heights. In these data, nanosheets containing multiple identifiable layers are not observed, precluding analysis of interlayer step heights within a single particle. We note that these are regions highlighting single-sheet structures and are not representative of the entire dispersion. 2
Figure S2. (A and B) Atomic force micrographs of solution-exfoliated germanane, drop-cast on a Si wafer surface. The scale bars in both images represent 100 nm. Histograms depict number of pixels at a given height. 3
Figure S3. Powder X-ray diffraction of germanane after cycling against lithium. Germanane was drop-cast out of isopropanol onto a stainless steel electrode. The electrode was cycled using cyclic voltammetry from 2.5 to 0.1 V at 0.1 mv/s. Following completion, the sample was immediately rinsed with propylene carbonate, dried under an Ar stream, and sealed in a polyethylene terephthalate (PET) envelope to prevent oxidation. A stainless steel background was subtracted from this spectrum for clarity. Figure S4. Galvanostatic charge/discharge curves at C/10 of germanane for the first and second cycles, in the voltage window of 2.5 and 0.1 V vs Li/Li +, show solid-electrolyte-interphase (SEI) layer formation. 4
The First Cycle The results discussed thus far focus on the performance of the germanane anode after the second electrochemical cycle; however, it would be more accurate to describe them as amorphous germanium nanosheets due to the structural change that occurs during the Ge-Li alloying/dealloying reaction after the first cycle. A cyclic voltammogram of germanane during the first cycle is shown in Figure S5A, with distinctive reduction and oxidation peaks at 0.53 and 0.93 V, respectively, and the Coulombic efficiency between these two peaks is ~95%. To investigate the source of these peaks, a pristine GeH anode was swept from the open circuit voltage to 0.45 V vs Li/Li + (below the previously tested voltage of the first peak) and its Raman spectrum was immediately measured (Figure S5B). Ex situ analysis of this sample showed that the germanane had become amorphous by this potential. From these data, it appears that the reduction reaction is reversible within the first cycle and that the sample undergoes oxidation after the structure becomes amorphous and hydrogens are removed. However, the reduction reaction no longer occurs after the lithium-induced structural change. It is unclear what the source of this extra redox reaction is, whether it is structural or hydrogen related. Additional studies are needed in order to elucidate the exact mechanism for these redox peaks. Figure S5. (A) Cyclic voltammogram of cycle 1 and 2 of a germanane thin-film, cycled between 2.5 and 0.1 V vs Li/Li + at 0.1 mv/s. Red circle highlights germanane specific redox peaks. (B) Raman spectra of pristine GeH (top, blue) and partially cycled GeH, where a thin film of GeH was swept to 0.45 mv vs Li/Li +. 5
Kinetics The kinetics of the germanane anode were investigated further with cyclic voltammetry, as shown in Figure S6A, with cycling at rates from 0.2 to 50 mv/s. As the rate increases, the anodic and cathodic peaks broaden and the cathodic peak shifts to the right. Similar effects can be seen with bulk germanium (Figure S6B). This shift in potentials limits our ability to conduct b-value analysis. Nonetheless, we can infer the charge-storage mechanism at different charge/discharge rates by assessing the shape of the cyclic voltammogram. As the rate increases, the redox peaks of the germanane (GeH) anode (Figure S6A) begin to match those of the carbon matrix (Figure S6C), suggesting that the rate is too high for Ge-Li alloying to occur, and carbon dominates any charge storage. The bulk Ge CVs broaden and have a peak shift as well; however, much of the original peak shape is maintained at higher sweep rates (Figure S6B). An advantage of the nanostructures is better depicted in Figure S6D, where it is evident that a higher capacity is achievable at higher charge/discharge rates. These results support the hypothesis that the amorphous nanosheet structure of GeH maintains a higher reaction rate by reducing diffusionbased limitations; however, definitive conclusions cannot be made due to the instability of both bulk germanium and germanane anodes. 6
Figure S6. Cyclic voltammograms (second cycles) for (A) germanane (GeH), (B) bulk Ge, and (C) carbon matrix cycled at 0.2, 2, 5, 10, 20, and 50 mv/s between 2.5 and 0.1 V vs Li/Li +. Reductive capacity at different sweep rates (D) is plotted against sweep rate for GeH and bulk Ge. 7
Figure S7. Capacities for each galvanostatic charge/discharge curve for (A) germanane and (B) bulk Ge cycled at rates of C/10, C/5, C/2, C, 2C, C/10, and finally cycled to 100 cycles at a rate of 1C within a voltage window of 2.5 and 0.1 V vs Li/Li +. Plots also contain the Coulombic efficiencies for each complete cycle. Figure S8. Capacities for each galvanostatic charge/discharge capacities (black and red, respectively) of GeH plotted against cycled number. The sample was cycled at 1 C within a voltage window of 2.6 and 0.1 V vs Li/Li +. Plot also contains the Coulombic efficiencies for each complete cycle (blue). 8
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