The present invention provides an active material for a
lithium secondary battery with a high
discharge capacity, particularly for a
lithium secondary battery that can increase the
discharge capacity in a potential region of 4.3 V or lower, a method for producing the same, a
lithium secondary battery having a high
discharge capacity, and a method for producing the same. The active material for a lithium secondary battery includes a
solid solution of a lithium
transition metal composite oxide having an α-NaFeO2 type
crystal structure, in which the composition ratio of Li, Co, Ni, and Mn contained in the
solid solution satisfies Li1+(1 / 3)xCo1−x−yNi(1 / 2)yMn(2 / 3)x+(1 / 2)y (x+y≦1, 0≦y and 1−x−y=z); in an Li[Li1 / 3Mn2 / 3]O2(x)-LiNi1 / 2Mn1 / 2O2(y)-LiCoO2(z) type
ternary phase diagram, (x, y, z) is represented by values in a range present on or within a line of a heptagon (ABCDEFG) defined by the vertexes; point A(0.45, 0.55, 0), point B(0.63, 0.37, 0), point C(0.7, 0.25, 0.05), point D(0.67, 0.18, 0.15), point E(0.75, 0, 0.25), point F(0.55, 0, 0.45), and point G(0.45, 0.2, 0.35); and the
intensity ratio between the
diffraction peaks on (003) plane and (104) plane measured by X-
ray diffractometry before charge-discharge is I(003) / I(104)≧1.56 and at the end of discharge is I(003) / I(104)>1. The invention provides a method for producing the active material for a lithium secondary battery using a
coprecipitation method, a lithium secondary battery including a positive
electrode containing the active material and a method for producing the lithium secondary battery.