First principles molecular orbital calculations for LiMO2 and MO2 have been carried out using the discrete variational Xα cluster method. Li in LiMO2 is nearly completely ionized. Strong covalent interaction between M and O is noted. The removal of Li significantly increases the interaction between M and O. This results in the “oxidation” associated with the de-intercalation not of M but of O. The formal redox notation for the de-intercalation, i.e., M(IV)/M(III), is thus far from reality. The difference in the de-intercalation capability among four LiMO2 compounds is ascribed to the difference in the magnitude of electrostatic repulsion between the oxygen layers in MO2.

A systematic study of the electronic structures and chemical bonding of the titanium dichalcogenide TiX2 (X=S, Se, and Te) layered structures is performed by a first-principles molecular orbital calculation using the discrete-variational (DV)-Xα cluster method. The intra- and interlayer chemical bonding properties are also investigated using the bond overlap population. Valence band structures obtained by the calculation are in good agreement with experimental results obtained by X-ray photoemission spectroscopy. Each peak in the density of states (DOS) is identified from the viewpoint of chemical bonding. There is a considerably strong covalent bonding between Ti and chalcogen atoms in TiX2. The covalency of chemical bonding is shown to increase and the ionicity to decrease in the series of TiS2–TiSe2–TiTe2. Both in intra- and interlayer chalcogen-chalcogen bonding and intralayer Ti–Ti bonding are much weaker than Ti-chalcogen intralayer bonding. However, interlayer interaction is found to be not purely of the van der Waals type.

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A systematic study of the electronic structure and chemical bonding of the Li-intercalated titanium dichalcogenides, LiTiX2 (X=S, Se, and Te), is performed by a first-principles molecular-orbital method using a model cluster composed of 75 atoms. The discrete-variational (DV)-Xα method was employed and Mulliken's population analyses were thoroughly conducted. The net charge of Li is found to be approximately 0.1 independently of X-species. Net charges of Ti and X are not significantly affected by the Li intercalation. Strong covalent bonding is formed between Li and X with a bond-overlap population (BOP) of 0.173–0.176. The BOP of Ti–X bonding decreases by about 10% due to the presence of Li. These results should be important for determining battery properties when the dichalcogenide are used for positive electrodes.

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