Graphene oxide (GO) presents a unique chemical complexity due to the number of defect sites and chemical groups introduced by the harsh oxidative treatment. In this work, we elucidate the chemical nature of GO and thermally-reduced GO at different temperatures. These materials were characterized by a variety of techniques such as FT-IR, Raman spectroscopy, TGA, SEM, XRD, XPS, TEM, surface area, and elemental analysis. Furthermore, galvanostatic experiments demonstrate that the electrochemical performance of reduced-GOs for Li intake is optimal when GO is reduced at a relatively mild temperature of 250 C regardless of the chemical environment. Mildly reduced-GOs show a high first cycle specific capacity of over 2000 mAh/g (charge) and 1000 mAh/g (discharge), at a large current density of 500 mA/g. After 100 cycles, the reversible capacity remains stabilized at 500 mAh/g. Our characterization results combined with density functional theory calculations suggest that the presence of specific ketone groups at the edge sites rather than their gross morphology is responsible for the enhanced performance of the material as anode electrodes.