Influences of Dopants on the Properties of LiMnO2 and Li2MnO3 in Olo Cathode for Li Ion Batteries

Abstract

The over-lithiated-oxides (OLOs), a composite of layered structures of Li2MnO3 and LiMO2 (M = Mn, Fe, Co, Ni), have shown much higher storage capacity than the traditional layered oxides for Li ion battery cathode because of the Li2MnO3 phase. However, Li2MnO3 is not stable after the 1st charge-discharge cycle and will partly transform into layered LiMnO2, which indicates that the practically used phase is a mixture of both Li2MnO3 and LiMnO2. During the subsequent cycles, the OLO voltage decreases due to the phase transition of layered LiMnO2 into spinel. Experimentally, the effective dopants satisfying multiple cathode materials requirements of thermodynamic stability, optimized voltage and improved kinetics based on ionic and electronic conductivities are investigated to overcome the voltage degradation and to improve the power capacity. In this work, redox potential, lithium ion diffusion and charge carrier transportation of both phases are examined in details using the ab initio density-functional theory (DFT) simulations. The calculations find, due to the Jahn-Teller effect of Mn3+ atoms, Li vacancy migration in LiMnO2 has special behaviors and hole polaron and electron polaron will form in LiMnO2 and Li2MnO3 phases, respectively. Based on the understanding of the pure phase properties, the effects of 10 cationic (Mg, Ti, V, Nb, Fe, Ru, Co, Ni, Cu, Al) and 2 anionic (N, F) dopants on the redox potential, ionic and electronic conductivity are investigated. These DFT findings could provide conceptual guidance in the experimental search for the effective dopants enabling the practical application of OLO cathodes.