Development of Lithium Iron Oxide Cathode Materials for Lithium Secondary Batteries

抄録

Two lithium iron oxides have been synthesized at low temperatures by the conventional solid-state method. Orthorhombic LiFeO₂ has been synthesized using LiOH and γ-FeOOH at 150°C was composed of orthorhombic LiFeO₂ and small amount of spinel LiFe₅O₈ phases. A Li/LiFeO₂ cell showed not only a fairly high initial discharge capacity of over 150 mAh g⁻¹ but also a good cycle retention rate at room temperature. It was found that the orthorhombic phase of the LiFeO₂ underwent a structural change to the LiFe₅O₈ spinel phase in the first cycle. Especially, it showed a severe structural change during the first discharge process, which might be the main reason for the capacity loss of the Li/LiFeO₂ system. An amorphous-like Li_xFe_yO_z has been synthesized using LiOH and α-FeOOH at 200°C consisted of three kinds of structures, LiFe₅O₈, Li₅FeO₄, and a trace of cubic α-LiFeO₂ A Li/Li_xFe_yO_z cell showed a very high initial discharge capacity (215 mAh g^−l) as well as an excellent cycle retention rate (95%) from the 11th to the 50th cycle. It was found that Li_xFe_yO_z material was transformed into the spinel LiFe₅O₈ and tetragonal β-LiFeO₂ phases after long-term cycling.