Monocrystalline copper((II)) oxide nanoparticles were made by scalable flame spray pyrolysis (FSP) and analyzed by X-ray diffraction (XRD), nitrogen adsorption (BET), transmission electron microscopy (TEM) and X-ray absorption near edge structure (XANES). Their primary particle diameter was closely controlled from 6 to 50 nm by varying the FSP conditions. Their electrochemical performance as Li-ion battery materials was tested in composite electrodes vs. Li-metal. Near theoretical specific charges were obtained for intermediate CuO sizes of 20 and 50 nm (d(BET)). In contrast, larger, commercially available CuO (d(BET) = 670 nm) exhibited significantly lower practical specific charge due to incomplete oxidation in the delithiation cycle as indicated by the remaining Cu and Cu2O by XRD and XANES analysis. (C) 2013 Elsevier B.V. All rights reserved.