We investigated the electrochemical behaviors of nonstoichiometric silicon suboxides (SiOx, for x = 0.17-1.34) as the anode material for lithium rechargeable batteries. The amorphous SiOx (a-SiOx) films were synthesized by reactive evaporation of Si with oxygen gas. The initial charge (lithiation) and discharge (delithiation) capacities were strongly related to the value of x in SiOx. We proposed an electrochemical reaction model of SiOx with lithium (Li) to describe the relationships quantitatively by assuming SiOx was composed of a reversible Si and an irreversible SiO2 component. Lithiated products were identified by XPS as a fully lithiated Si, Li2O, Li silicates (Li4SiO4, Li2SiO3), and SiO2. After discharge, a portion of the Li in Li-Si was found to remain undischargeable as LizSi (z; the function of x). This was an additional factor of the irreversible capacity. Peak positions assigned to LizSi in XPS Si2p spectra suggest that the value of z depends on the value of x. The a-SiOx (for x = 1.02, 1.34) films demonstrated excellent cyclability at a range of 0.005-1.5 V vs. Li/Li+. Absence of peaks at 0.45 V in differential capacity vs. voltage (dQ/dV) profiles indicates the suppression of crystallization of lithiated a-Si, even under deep cycle conditions. (C) 2013 Elsevier B.V. All rights reserved.