Crystal structure Control and Application to electrode materials of Low-expanding Composite Oxides

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Lithium secondary batteries using lithium-cobalt oxide, LiCoO_2 as the cathode, and carbon materials as the anode (so called "lithium ion batteries") have been commercialized, and the production is repidly expanding. However, the natural resources for cobalt is so limited (less than 9 million tons) that new cathode materials which replace cobalt are strongly expected. Manganese is one of the abundant elements, and lithium-manganese spinel oxide, LiMn_2O_4 can also successfully work as the reversible cathode of lithium secondary batteries, however, the stability of this compound in a long term charge-discarge cycling is not satisfactory when it is compared with LiCoO_2. In the present project, we have focused our efforts on the stabilization of the LiMn_2O_4 crystal lattice by substituting other metal elements for a part of manganese ion in LiMn_2O_4, i.e., LiM_yMn_<2-y>O_4. As the M elements, such metals that have stronger affinities to oxide ions compared to that of Mn-O have been chosen, e.g., Cr, Ni, Co.Notable improvement in the charge-discharge cycling properties have been observed for LiM_yMn_<2-y>O_4. Especially, when y=1/6 and M=Co, more than 90% of the initial discharge capcity of 115 mAh/g was maintained even after 200 cycle, while in the case of LiMn_2O_4, the discharge capacity after the same cycle number was almost 60% of initial capacity. When M=Ni, although the initial capacity was not large (95mAh/g), reduction in the cycling capacity was also small. Variation of the diffusion coefficient of lithium (D_<Li>) with the lithium content x in LiM_yMn_<2-y>O_4 were also measured. It was also found by using CPR method that the D_<Li> value at x=0.5 was higher by two orders of magnitude (10^<-7.5>cm^2s^<-1>) compared with those in other x region, where lithium ions were expected to be arranged in a ordered form.