High-voltage lithium cathode materials

Single cell lithium battery systems can operate over 4.5 V vs. Li/Li+ on the basis of reversible extraction of lithium from several cathodes based on complex spinel oxides Li2MM3'O-8 and LiMM'O-4: MM3' = CrMn3, FeMn3, CoMn3, NiMn3, CuMn3; MM' = CrMn, CoMn, NiV. Except for LiNiVO4, Li+ fully occupies tetrahedral sites and can, therefore, move through the well-known tetrahedral site-empty octahedral site pathway; Mn occupies octahedral sites, and exhibits mixed-valence states, so that fast electronic transport is expected to occur between neighboring Mn3+/Mn4+. In all cases, the electrochemical process over 4.5 V is attributable to the redox couples of M situated in octahedral sites: Cr3+/4+, Fe3+/4+, Co3+/4+, Ni2+/4+, Cu2+/3+. Of a potentially large family of spinel-based cathodes in the systems Li-M-M'-O, the largest capacity over 4.5 V achievable so far occurs when tetrahedral sites are fully occupied by lithium which is reversibly extracted by means of the redox couples of M located in octahedral sites. For example, a cell with LiCoMnO4 exhibits a discharge capacity of ca. 95 mA h g(-1) at a plateau centered on 5.0 V and, therefore, has superior energy density and operating voltage to LiMn2O4, the favored cathode in next-generation lithium batteries. (C) 1999 Elsevier Science S.A. All rights reserved.