To improve the electrochemical performance of Co3O4/graphene composites towards high-performance lithium ion batteries, various facile chemical methods have been developed to form hybrid Co3O4/graphene composites, but the compositions of these composites were considerably different. In this study, a two-step solvothermal synthesis method was employed to accurately control the concentration of Co3O4 nanoparticles (NPs) with particle sizes less than 10 nm in the composite. The Co3O4/reduced graphene oxide (RGO) composites exhibited the highest reversible capacity of 1600 mAh g(-1) at 43 wt% Co3O4 NPs. The synergic effects of the sub-10-nm sized Co3O4 and the RGO sheets resulted in improved reversible capacity, cycling stability, and rate capability, which are caused by a large number of active sites and the short diffusion pathways provided by the nano-sized Co3O4, as well as the elastic buffer space and conductive pathway provided by the RGO sheets. (C) 2016 Elsevier B.V. All rights reserved.