Solid-state processed novel germanium/reduced graphene oxide composite: Its detailed characterization, formation mechanism and excellent Li storage ability

Abstract

By using a simple solid-state reduction process, germanium oxide (GeO2) and graphene oxide (GO) are reduced to germanium (Ge) and reduced graphene oxide (rGO), respectively, to form Ge/rGO composite. Transmission electron microscopy showed that clusters of Ge nanoparticles are decorated on the graphene layers of rGO. Detailed x-ray diffraction, Raman scattering, and x-ray photoelectron spectroscopic analyses confirmed Ge/rGO composite formation. The thermal stability of Ge/rGO composite and the reduction mechanism through which the composite is formed are experimentally elucidated. Further, Brunauer–Emmett–Teller (BET) specific surface area, average pore volume, and average pore size of Ge/rGO composite are measured to be ∼115 m2/g, ∼0.4 cm3/g, and ∼14 nm, respectively. The thermal analysis quantified the amount of rGO in the Ge/rGO composite to be ∼66%. As-synthesized Ge/rGO composite is tested as anode material in Li-ion batteries. When cycled at 160 mA/g current density and in the 0.005–3.0 V voltage range, the composite showed an excellent reversible capacity of ∼539 mAh/g that corresponds to 98% capacity retention at the end of the 100th cycle. The Ge/rGO composite also showed high reversible capacity retention, good rate capability, and excellent cycle life when tested at 80 and 320 mA/g in the same voltage range. The redox peaks in the cyclic voltammetry (CV) curves revealed that alloying-dealloying and electrochemical adsorption-desorption mechanisms are the primary reasons for the lithium-ions storage by Ge and rGO, respectively. A constant area under the CV curves throughout the test indicated the stable capacity. Also, the CV results align with the galvanostatic cycling results.