DEVELOPMENTAL RESEARCH FOR INTERCALATION ELECTRODE MATERIALS FOR HIGH ENERGY DENSITY BATTERIES

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The results of our research projects are summarized as follows1. Layred-rocksalt oxides LiMO_2 Oxides LiMO_2 with M=Co and/or Ni are promising cathode materials. However, the ability to extract and reinsert Li depends critically on the stoichiometry and ordering of the Li and M atoms into alternate planes of the structure. The withdrawal of Li creates Li_<1-X>MO_2 compositions metastable relative to atomic displacements, and displacement of M atoms to the Li layrs on Li extraction lowers sharply the Li+-ion mobility in the Li layrs. We focused on the influence of synthesis and the deintercalation process on the degree of cation stoichiometry, order, and stability of the ordered phase first for the system Li_<1-y>NiO_2. We clarified (a) the phase relationships in the Li-Ni-O ternary system in order to optimize the performance of the Li_<1-X>NiO_2 cathode system, (b) the synthetic conditions that yield precise control of stoichiometry and cation order in the starting lithiated compounds, (c) the decomposition or disordering processes operative at higher temperatures versus the Li concentration, (d) the relationship between stoichiometry, order, and physical properties, (e) the lattice changes induced by deintercalation, and (f) the influence of cation disorder on the deintercalation process.2. New oxide hosts. We attempted to synthesize new oxide hosts by unconventional routes such as ion exchange, high pressure, and high oxygen pressure. LiFeO_2 does not crystallize in the layred-rocksalt structure, but NaFeO_2 does ; it was possible to prepare metastable LiFeO_2 with the layred-rocksalt structure by ion exchange of Li+ for Na+ in molten LiCl/KCl. Furthermore, we synthesized new cathode, LiFeO_2 with the zig-zag layred structure. We clarifed the structure and electrochemical properties of these LiFeO_2 cathodes with the layred structure and zig-zag layred structures.