To improve both discharge capacity and a charging-discharging cycle characteristic by composing positive electrode active material of a spinel type Li-Mn based compound oxide of a specific composition as a main component.
The composition of this compound oxide is represented by the formula, where 0.05≤A≤2.3, X+Y+Z=2, Y+Z=0.02-0.04, and the atom number ratio of Y to Z is 1:9-9:1, desirably A=1, Y+Z=0.05-0.15, and the atom number ratio Y:Z is 8.2-3:7. If Y+Z being larger than 0.4, an impurity layer other than a spinel phase is apt to be generated. The compound oxide expressed by the formula is manufactured by heat-sintering a specified ratio of mixture of an oxide, a hydroxide, carbonate or nitrate composed of elements other than oxygen, at 500-1,000°C for 1-50 hours in the atmosphere. Positive electrode active material may also be formed by using one or more kinds of compound oxide expressed by the formula, in combination with a small quantity of an Li-Mn compound oxide or an Li-Co compound oxide excluding the compound oxide expressed by the formula, if necessary.
SUKETANI SHIGENORI
TATEISHI KAZUYUKI
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