On the structural integrity and electrochemical activity of a 0.5Li2MnO3·0.5LiCoO2 cathode material for lithium-ion batteries

Abstract

Structural changes in a 0.5Li₂MnO₃·0.5LiCoO₂ cathode material were investigated by X-ray absorption spectroscopy. It is observed that both Li₂MnO₃ and LiCoO₂ components of the material exist as separate domains, however, with some exchange of transition metal (TM) ions in their slab layers. A large irreversible capacity observed during activation of the material in the 1^st cycle can be attributed to an irreversible oxygen release from Li₂MnO₃ domains during lithium extraction. The average valence state of manganese ions remains unchanged at 4+ during charge and discharge. In the absence of conventional redox processes, lithium extraction/reinsertion from/into Li₂MnO₃ domains occurs with the participation of oxygen anions in redox reactions and most likely involves the ion-exchange process. In contrast, lithium deintercalation/intercalation from/into LiCoO₂ domains occurs topotactically, involving a conventional Co³⁺/Co⁴⁺ redox reaction. The presence of Li₂MnO₃ domains and their unusual participation in electrochemical processes enable LiCoO₂ domains of the material to sustain a higher cut-off voltage without undergoing irreversible structural changes.