Abstract
Several series of 
samples prepared at different temperatures and in different oxygen environments were studied in order to further understand the dependence of the electrochemical performance on the sample heating temperature, on the amount of excess lithium x, and on the oxygen stoichiometry. We show that a peak in the differential capacity of 
electrochemical cells at 3.3 V on discharge always appears with the 4.5 V peak previously reported by Tarascon et al. Powder x‐ray diffraction and Rietveld refinement show that the mixing of Li and Mn cations in 8a sites is not the origin of these capacity peaks. Samples heated to 900°C and samples that are more oxygen deficient exhibit larger 3.3 and 4.5 V peaks. Samples made at 750°C in air or oxygen with x = 0.1 show almost no evidence for the 3.3 and 4.5 V peaks. Furthermore, these samples do not degrade even after cells are charged to 4.9 V, suggesting these samples should be excellent for Li‐ion battery applications. A simple way of determining x with thermal gravimetric analysis is also demonstrated.