To suppress deterioration of a battery in order to put a lithium-ion secondary battery using lithium manganate for a positive electrode and having high capacity to practical use.

Sodium phosphate is attached to lithium manganate particles by mixing lithium manganate and sodium phosphate together. By using such active material for the positive electrode, negative electrode deterioration is suppressed by reducing manganese dissolution from the positive electrode, and violent deterioration of this lithium ion secondary battery is suppressed. In lithium manganate, a part of Mn may be replaced with foreign metal ions.