Superior Additive of Exfoliated RuO 2 Nanosheet. Oxide over Graphene

Transcription

1 Supporting Information Superior Additive of Exfoliated RuO 2 Nanosheet for Optimizing the Electrode Performance of Metal Oxide over Graphene Seul Lee, a, Xiaoyan Jin a, In Young Kim, a Tae-Ha Gu, a Ji-Won Choi, b Sahn Nahm, c and Seong-Ju Hwang*, a a Department of Chemistry and Nanoscience, College of Natural Sciences, Ewha Womans University, Seoul 03760, Korea b Electronic Materials Research Center, Korea Institute of Science and Technology, Seoul 02792, Korea c Departments of Materials Science and Engineering, Korea University, Seoul 02841, Korea

2 Figure S1. Photoimage of the coated electrode. The slurry of active electrode material was pressed on a stainless steel substrate with the area of 1 1 cm 2. For the measurement of the electrode performance, the electrode was masked with an insulating tape. S1

3 Figure S2. (Left) X-ray photoelectron spectroscopy (XPS) survey spectra and (right) Ru 3p XPS spectra of (a) LMR0, (b) LMR1, (c) LMR2.5, and (d) LMR4. As shown in Figure S2, all the RuO 2 -incorporated nanocomposites exhibit notable spectral weight in the Ru 3p XPS region, confirming the incorporation of RuO 2 nanosheet into the present nanocomposite. S2

4 Figure S3. Transmission electron microscopy (TEM) images of (a) exfoliated MnO 2 nanosheet and (b) exfoliated RuO 2 nanosheet. As can be seen clearly from Figure S3, both the exfoliated MnO 2 and RuO 2 nanosheets commonly show 2D sheet-like morphology. S3

5 Figure S4. Energy dispersive X-ray spectrometry (EDS) elemental maps of the Li MnO 2 RuO 2 nanocomposites of (a) LMR1, (b) LMR2.5, and (c) LMR4. The elemental distributions of the LMR nanocomposites are probed with EDS elemental mapping analysis. As illustrated in Figure S4, all of the Mn, Ru, and O elements are uniformly distributed in the entire part of the LMR nanocomposites. This finding provides strong evidence for the homogeneous mixing of RuO 2 nanosheets with MnO 2 ones. S4

6 Figure S5. Galvanostatic charge discharge (CD) curves of initial few cycles at a constant current density of 1 A g 1 of the Li MnO 2 RuO 2 nanocomposites of (a) LMR0, (b) LMR1, (c) LMR2.5, and (d) LMR4. The capacitive behaviors of the LMR nanocomposites were examined by the galvanostatic CD cycling measurements at a constant current density of 1 A g 1. As plotted in Figure S5, all the materials commonly show a linear variation of potential with time, confirming the capacitive behavior of these materials. S5

7 Figure S6. (a) Cyclic voltammetry (CV) curves and (b) capacitance retention plots of the Li RuO 2 nanocomposite. As shown in Figure S6, the Li RuO 2 nanocomposite prepared by the restacking of RuO 2 nanosheets with Li + ions delivers a specific capacitances of 213 F g 1 for the maximum point and 176 F g 1 for the 5000th cycle, and is smaller than those of the RuO 2 -incorporated LMR nanocomposites with a smaller RuO 2 content. This finding strongly suggests that a small amount of RuO 2 nanosheet incorporated in the present Li MnO 2 RuO 2 nanocomposites induces synergistic effect on their electrode activity as a conductive additive. S6

8 Figure S7. (a) Powder X-ray diffraction (XRD) patterns, (b) field emission-scanning electron microscopy (FE-SEM) images, and (c) transmission electron microscopy (TEM) images of the Li MnO 2 rg-o nanocomposites of (i) LMG2.5, (ii) LMG5, and (iii) LMG10. The crystal structures and morphologies of rg-o-incorporated LMG nanocomposites were examined by powder XRD, FE-SEM, and TEM analyses. As illustrated in Figure S7a, the LMG nanocomposites display intense (00l) XRD peaks, indicating the formation of the layerby-layer-stacked structure of exfoliated MnO 2 and rg-o nanosheets. From the XRD result, the LMG10 with a high graphene content shows the impurity peak, because of the selfaggregating tendency of graphene nanosheets. The FE-SEM and TEM images in Figures S7b and S7c clearly demonstrate the mesoporous morphologies of the LMG nanocomposites formed by the house-of-cards-type stacking of MnO 2 and rg-o nanosheets. S7

9 Figure S8. N 2 adsorption desorption isotherms of the Li MnO 2 rg-o nanocomposites of (a) LMG2.5, (b) LMG5, and (c) LMG10. The open and close symbols represent the adsorption and desorption data, respectively. Figure S8 represents the N 2 adsorption desorption isotherms of the LMG nanocomposites. According to the fitting analysis based on the Brunauer Emmett Teller (BET) equation, the surface areas of the LMG nanocomposites are estimated to be 136, 118, and 111 m 2 g 1 for LMG2.5, LMG5, and LMG10, respectively, which are notably smaller than that of the LMR2.5 nanocomposite (156 m 2 g 1 ). This result strongly suggests that the incorporation of RuO 2 nanosheet is more effective in increasing the porosity of restacked Li MnO 2 nanocomposite than that of rg-o nanosheet. This result is ascribable to the weak selfaggregating tendency of exfoliated RuO 2 nanosheet. S8

10 Figure S9. CV curves of the Li MnO 2 rg-o nanocomposites of (a) LMG2.5, (b) LMG5, and (c) LMG10, and (d) capacitance retention plots of these nanocomposites. As plotted in Figure S9, the rg-o-incorporated nanocomposites of LMG2.5, LMG5, and LMG10 deliver the specific capacitances of 240, 258, and 231 F g 1 for the maximum point and 210, 251, and 211 F g 1 for the 5000th cycle, respectively, which are smaller than that of RuO 2 -incorporated LMR2.5 nanocomposite with a smaller RuO 2 content. This result provides strong evidence for a better role of RuO 2 nanosheet in improving the electrode performance of the Li MnO 2 nanocomposite compared to the rg-o nanosheet. S9

11 Figure S10. Contact angles of the exfoliated nanosheets of (a) MnO 2, (b) RuO 2, and (c) rg-o. The surface natures of the exfoliated MnO 2, RuO 2, and rg-o nanosheets are examined by measuring the contact angles of restacked membranes of these nanosheets. As illustrated in Figure S10, the contact angles of these nanosheets are estimated to be 19.9, 26.5, and 83.3 for the MnO 2 nanosheet, RuO 2 nanosheet, and rg-o nanosheet, respectively. Both the freestanding membranes of RuO 2 and MnO 2 nanosheets show much smaller contact angles than does the rg-o nanosheet. This result clearly demonstrates the hydrophilic surface characteristics of the RuO 2 and MnO 2 nanosheets and the hydrophobic surface nature of the rg-o nanosheet. The present finding provides strong evidence for more efficient chemical interaction between the RuO 2 and MnO 2 nanosheets than between rg-o and MnO 2 nanosheets. The resulting efficient improvement of electron conduction upon the RuO 2 incorporation contributes to the better electrode performances of the LMR nanocomposites than those of the LMG nanocomposites. S10

12 Table S1. Electrical resistivities of the present electrode materials. Sample LMR0 LMR1 LMR2.5 LMR4 LMG2.5 LMG5 LMG10 Li rg-o Li RuO 2 Resistivity (Ω cm 1 ) To verify the superior role of RuO 2 nanosheet over the rg-o nanosheet, the electrical conductivities of these materials were measured using a four-point probe method. As listed in Table S1, the resistivity of exfoliated RuO 2 nanosheet is smaller than that of rg-o nanosheet, confirming a promising role of the exfoliated RuO 2 nanosheet as efficient conductive additive in enhancing the electrical conductivity of MnO 2 material. S11

13 Figure S11. Plots of the real part of impedance as a function of the inverse square root of angular frequency in Warburg region for the Li MnO 2 RuO 2 nanocomposites of LMR0 (circles), LMR1 (triangles), LMR2.5 (squares), and LMR4 (diamonds), and the Li MnO 2 rg-o nanocomposites of LMG2.5 (inverse triangles), LMG5 (hexagons), and LMG10 (close circles). As shown in Figure S11, the slope of Z re vs. ω 0.5 plot in the Warburg region provides the Warburg coefficient (σ w ). Among the present RuO 2 -incorporated nanocomposites, the LMR2.5 nanocomposite shows the highest Na + ion diffusivity with the smallest value of σ w (LMR0: σ w = Ω s 0.5 ; LMR1: σ w = Ω s 0.5 ; LMR2.5: σ w = Ω s 0.5 ; LMR4: σ w = Ω s 0.5 ). The lowering of Warburg coefficient in the LMR nanocomposites indicates that the polarizability of these electrode materials is improved by the incorporation of the RuO 2 nanosheet. The Warburg coefficients of all the present LMG nanocomposites are 26.39, 25.22, and Ωs 0.5 for LMG2.5, LMG5, and LMG10, S12

14 respectively. This result demonstrates that the incorporation of the rg-o nanosheet is also effective in improving the Na + ion diffusivity of the Li MnO 2 nanocomposite but the beneficial effect of rg-o incorporation is weaker than that of RuO 2 addition. S13