NOVELTY - An electrode active material (1) is formed by coating the surface of particles (2) containing lithium complex with coating layer (3) containing lithium oxide complex. The electrode active material has secondary region in which discharge potential is fall in discharge curve, after primary region in which discharge potential is substantially fixed, and tertiary region in which discharge potential fluctuation rate is less than the average discharge potential fluctuation rate of the secondary region.    USE - Electrode active material is used for lithium-ion battery (claimed) for power supply device. Uses include but are not limited to portable telephone apparatus, notebook-type personal computer, electric vehicle, hybrid vehicle and electric vehicle.    ADVANTAGE - The electrode active material has excellent stability and durability, and is safe to use. The compact lithium-ion battery comprising the electrode active material has excellent load characteristics, cycling characteristics and energy density. The final state of discharge can be detected easily.    DETAILED DESCRIPTION - An electrode active material is formed by coating the surface of particles containing lithium complex (LiwAxDO4, where A is manganese or cobalt, D is phosphorus, silicon or sulfur, w is more than 0 to 4, and x is more than 0 to 1.5) with coating layer containing lithium oxide complex (LiyEzGO4, where E is iron, or iron and nickel, G is phosphorus, silicon or sulfur, y is more than 0 to 2, and z is more than 0 to 1.5). The electrode active material has secondary region in which discharge potential is fall in discharge curve, after primary region in which discharge potential is substantially fixed, and tertiary region in which discharge potential fluctuation rate is less than the average discharge potential fluctuation rate of the secondary region. INDEPENDENT CLAIMS are included for the following:    (1) lithium-ion battery;    (2) detection method of discharge state of electrode active material; and    (3) manufacture of electrode active material.    DESCRIPTION OF DRAWING(S) - The drawing shows a sectional view of the electrode active material.    Electrode active material (1)    Particles (2)    Coating layer (3)