Ab initio study of doping effects on LiMnO2 and Li2MnO3 cathode materials for Li-ion batteries

Abstract

For the over-lithiated-oxides (OLOs), a composite of layered Li₂MnO₃ and LiMO₂ (M = Mn, Co, Ni), the Li₂MnO₃ part is not stable after the 1st charge–discharge cycle and partly transforms into layered LiMnO₂, which in practice indicates that the phase used is actually a mixture of both Li₂MnO₃ and LiMnO₂. In the present work, the influence of 10 cationic (Mg, Ti, V, Nb, Fe, Ru, Co, Ni, Cu, and Al) and 2 anionic (N and F) dopants on the phase stability, redox potential, ionic and electronic conductivity of both Li₂MnO₃ and LiMnO₂ is investigated in detail using density functional theory. The calculations show that all the cationic dopants and F can be thermodynamically stable in the layered structures. The redox potential of both oxides is quite sensitive to some of the dopants, like V, Nb, and Ru, due to the appearance of gap states introduced by those dopants. The Jahn–Teller effect has a strong influence on the Li vacancy diffusion behavior in both LiMnO₂ and its doped phases. Li vacancy diffusion behavior in Li₂MnO₃, including both interlayer and intralayer pathways, is relatively more complex and some dopants like Mg, Ti, Nb, and Ru can decrease the barriers of the diffusion paths. The calculations also show the evidence of hole polaron formation in LiMnO₂ and electron polaron formation in Li₂MnO₃ which should be the reason why these phases have low electronic conductivities. Based on these findings, possible ways to improve the electronic conductivity through the doping process are discussed.