Electronic origin of structural degradation in Li-rich transition metal oxides: The case of Li2MnO3 and Li2RuO3

Abstract

Li2MnO3 and Li2RuO3 represent two prototype Li-rich transition metal (TM) oxides as high-capacity cathodes for Li-ion batteries, which have similar crystal structures but show quite different cycling performances. Here, based on the first-principles calculations, we systematically studied the electronic structures and defect properties of these two Li-rich cathodes, in order to get more understanding on the structural degradation mechanism in Li-rich TM oxides. Our calculations indicated that the structural and cycling stability of Li2MnO3 and Li2RuO3 depend closely on their electronic structures, especially the energy of their highest occupied electronic states (HOS), as it largely determines the defect properties of these cathodes. For Li2MnO3 with low-energy HOS, we found that, due to the defect charge transfer mechanism, various defects can form spontaneously in its host structure as Li ions are extracted upon delithiation, which seriously deteriorates its structural and cycling stability. While for Li2RuO3, on the other hand, we identified that the high-energy HOS prevents it from the defect formation upon delithiation and thus preserve its cycling reversibility. Our studies thus illustrated an electronic origin of the structural degradation in Li-rich TM oxides and implied that it is possible to improve their cycling performances by carefully adjusting their TM components.