To provide a positive active material comprising a lithium manganese oxide, keeping a high charge discharge capacity and suppressing capacity deterioration in charge discharge cycles, and to provide a positive electrode containing the active material and a secondary battery.

In the positive active material of the secondary battery comprising a lithium-containing metal oxide capable of inserting and releasing lithium. The lithium-containing metal oxide is represented by a general formula, LiMn2O4, a lattice constant (a) before the undoping of lithium is in a range of 8.220 Å≤a≤8.235 Å, and in a reversible reaction represented by a formula; LiMn2O4←→Li1-nMn2O4++nLi++ne-, a rate of change attendant at inserting and releasing lithium in the charge discharge in a range of 0≤n≤1 is 1.3×10-3Å or less per 1 mAh/g, a value of a lattice constant at a charging end (4.35 V vs Li) is 8.07 Å or more, and a value of a lattice constant at a discharge end (3.2 V vs Li) is 8.235 Å or less, and the positive electrode contains the active material, and the secondary battery contains the positive electrode.

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