spinels (M 5 Cr, Ni, Cu;x ’ 0.2) prepared at 500°C was analyzed by using accurate analytical spectroscopic techniques ~mass spectroscopy, nuclearmagnetic resonance! to examine the electrolyte behavior. The spectra revealed organic solvents to be stable as no decompositionproducts were detected, thus excluding the electrolyte oxidation as a side reaction accounting for the cell overcharge. However,these spinels contain excess oxygen in an amount that was quantified from thermogravimetric data. The excess oxygen plays aprominent role in the electrochemical response of the spinel. The cyclic voltammetry and galvanostatic results support theassumption that the excess oxygen can be released above 4.5 V. The additional capacity obtained and that required to release theoxygen were quite consistent. This must be the origin of both the overcharge and the poor performance of the cells compared withspinels of similar composition but synthesized at higher temperatures ~800°C!, the excess of oxygen in which was smaller.© 2004 The Electrochemical Society. @DOI: 10.1149/1.1824037# All rights reserved.Manuscript submitted March 30, 2004; revised manuscript received May 20, 2004. Available electronically November 17, 2004.