Systematic research on insertion materials based on superlattice models in a phase triangle of LiCoO2-LiNiO2-LiMnO2. I. First-principles calculation on electronic and crystal structures, phase stability and new LiNi1/2Mn1/2O2 material

The first-principles calculations on the lithium insertion materials have been done to give the rationale for materials research on advanced lithium-ion batteries. To apply the computational methods, the model crystals are constructed by assuming in-plane two-dimensional superlattices based on alpha-NaFeO2-type structure for three sides of a phase triangle of LiCoO2-LiNiO2-LiMnO2. Model crystals consisting of a [2 x 2] superlattice gives two transition metal elements in the ratio of 3 to 1 and a [root3 x root3] R30degrees superlattice model gives 2 to 1. For the ratio of 1 to 1, two model crystals are assumed, i.e., so-called straight'' and zigzag'' models. The first-principles calculations for these model crystals indicate that (i) LiCo1-xNixO2 is a solid solution between LiCoO2 and LiNiO2, (ii) LiCo1-xMnxO2 is not expected to be a solid solution between LiCoO2 and LiMnO2 (unstable, phase separation is expected), and (iii) LiNi1-xMnxO2 is not a solid solution between LiNiO2 and LiMnO2 (LiNi1/2Mn1/2O2 is expected to be a stable phase). Among these compounds calculated, LiNi1/2Mn1/2O2 having a layered structure may be a new compound consisting of Ni2+ and Mn4+ in their formal charges, which is neither a solid solution between LiNiO2 and LiMnO2 nor between NiO and Li2MnO3. From these theoretical results, materials design toward the implementation of novel lithium insertion materials for advanced lithium-ion batteries is discussed. (C) 2004 The Electrochemical Society.