Origin of the high voltage (> 4.5 V) capacity of spinel lithium manganese oxides

With an objective to develop a better understanding of the origin of high voltage (>4.5 V) capacities, the electrochemical behaviors of a number of spinel lithium manganese oxides with and without other transition metal ions have been studied and compared. The oxides investigated are LiMn2-yMyO4 (M=Co, Ni, and Cu), LiMn2-y-zMyLizO4, LiMn2-yLiyO4, Li2Mn4O9-delta, and Li4Mn5O12. The LiMn2-yLiyO4 (0.05less than or equal toyless than or equal to0.12) oxides are found to exhibit capacity above 4.5 V although they do not contain other transition metal ions. In the case of Li2Mn4O9-delta and Li4Mn5O12, charging above 4.5 V is found to increase the discharge capacity below 4.5 V. While both LiMn2-yCoyO4 and LiMn2-yCoyLizO4 show a similar behavior at >4.5 V, LiMn2-yNiyO4 and LiMn2-y-zNiyLizO4 differ significantly. Also, while LiMn1.9Li0.1O4 and LiMn1.8Co0.2O4 show a rapid decrease in their discharge capacity above 4.5 V with rest time allowed in the fully charged state (self-discharge), LiMn2-yNiyO4 does not suffer from such a difficulty. Additionally, long-range periodicity with good crystallinity is found to be essential to observe discharge capacity above 4.5 V. The results are explained on the basis of the participation of the O2-:2p band in the redox process and the relative positions of the other transition metal ion redox energies with respect to the top of the O2-:2p band. (C) 2003 Elsevier Ltd. All rights reserved.