First Principles Study of Li-site Doping Effect on the Properties of LiMnO_2 and Li_2MnO_3 Cathode Materials

摘要

Due to a high voltage of 4.6 V and large practical capacity of ～~260 mAh/g, the over-lithiated-oxides (OLOs) which are often represented as Li_2MnO_3-LiMO_2 (M = Mn, Co, Ni), have been intensely investigated as a promising candidate of the next generation cathode materials for Li-ion battery. The Li_2MnO_3 composite structure plays an important role in providing high capacity and phase stability to the system. However, during cathode charge-discharge operation, Li_2MnO_3 is unstable and partly transforms into LiMnO_2, indicating that the phase used in practice has mixed both Li_2MnO_3 and LiMnO_2. In this work, the effects of Li-site doping from 10 cationic dopants (Mg, Ti, V, Nb, Fe, Ru, Co, Ni, Cu, Al) on the electrochemical properties of both oxides are studied using density functional theory. The calculations show that, comparing with the Mn-site doped phases, Li-site doping is thermodynamically unstable for the ground states, but the small transition barriers can be easily overcome under high thermal fluctuations during the realistic cathode synthesis process. The redox potentials of both oxides can be lowered by most of the Li-site dopants. For example, Nb strongly lowers the redox potential of the LiMnO_2 phase, and Ru shows an unexpected effect on the Li_2MnO, phase: it activates the Li atoms in the Li-layer and, at the same time, it immobilizes the Li atoms in the Li-Mn mixed-layer by increasing the redox potential. These results support the experimental observations about Li-site doping and provide an explanation about the effects of Li-site doping on the electrochemical properties.