Microwave-Assisted Synthesis of Reduced Graphene Oxide with Hollow Nanostructure for Application to Lithium-Ion Batteries

Abstract

In this study, reduced graphene oxide (RGO) with a hollow nanostructure was successfully synthesized by layer-by-layer self-assembly using electrostatic interactions and van der Waals forces between building blocks, and its lithium storage characteristics were investigated. After 800 cycles at a current density of 1 A/g, the microwave-irradiated RGO hollow spheres (MRGO-HS) maintained a capacity of 626 mA h/g. In addition, when the charge/discharge capacity was measured stepwise in the current density range of 0.1-2 A/g, the discharge capacity of the RGO rapidly decreased to 156 mA h/g even at the current density of 2 A/g, whereas MRGO-HS provided a capacity of 252 mA h/g. Even after the current density was restored at a current density of 0.1 A/g, the MRGO-HS capacity was maintained to be 827 mA h/g at the 100th cycle, which is close to the original reversible capacity. Thus, MRGO-HS provides a higher capacity and better rate capability than those of traditionally synthesized RGO.

Keywords: Exfoliation; hollow spheres; layer-by-layer self-assembly; lithium-ion batteries; reduced graphene oxide.

Figure 1

Schematic illustration of the MRGO-HS synthesis.

Figure 2

XRD patterns of (a) graphene oxide (GO), (b) reduced GO (RGO), (c) MRGO-HS, (d) PS beads, and (e) PS/GO core–shell.

Figure 3

SEM images of the (a,b) PS beads, (c,d) PS/GO core–shell, and (e,f) MRGO-HS.

Figure 4

TEM images of (a–c) MRGO-HS. (d) HRTEM image of MRGO-HS with the multi-layer structure indicated by the red line.

Figure 5

Brightness profile of the red lines in Figure 4d.

Figure 6

FTIR spectra of (a) GO, (b) RGO, (c) MRGO-HS, (d) PS beads, and (e) PS/GO core–shell.

Figure 7

The Raman spectra of GO, RGO, and MRGO-HS.

Figure 8

C 1s and O 1s core level X-ray photoemission spectra of the (a,b) GO, (c,d) RGO, and (e,f) MRGO-HS samples. The table below the spectra shows the atomic percentages of C, O, and N in GO, RGO, and MRGO-HS in the XPS analysis.

Figure 9

N₂ adsorption/desorption isotherms of GO and MRGO-HS. The inset image is the pore size distribution curves of MRGO-HS.

Figure 10

TGA curves of MRGO-HS and GO.

Figure 11

(a) Cycling performance and coulombic efficiency of RGO and MRGO-HS (at the current density of 0.1 A/g), (b) at the current density of 1 A/g, (c) at various current densities. Galvanostatic charge/discharge curves of (d) RGO, and (e) MRGO-HS between 0.01 and 3.0 V at various current densities.

Figure 12

(a) Nyquist plots for a fresh cell and (b) Nyquist plots after 70 cycles. Warburg plots for a fresh cell (c) and after 70 cycles (d).

Figure 13

Area covered under the CV curves of (a) MRGO-HS and (b) RGO.

Figure 14

CV curves of (a) MRGO-HS and (b) RGO at different scan rates. (c) The log(i) versus log(v) plots of MRGO-HS and RGO. Pseudocapacitive (blue area) and diffusion (red area) contribution to the charge storage of (d) MRGO-HS and (e) RGO at 0.8 mV/s. (f) Proportion of pseudocapacitive contribution at different scan rates of MRGO-HS and RGO.