In a lithium-ion secondary battery using a positive-electrode active material that includes a lithium-manganese-based oxide which includes a lithium (Li) element and a tetravalent manganese (Mn) element and whose crystal structure belongs to a layered rock-salt structure, adding a compound being selected from the group consisting of Compounds (a) through (i) into the electrolytic solution leads to the following: degradations due to oxidation-reduction decompositions of the electrolytic solution, and so on, are inhibited; and not only the shelf or storage capacity and recovered capacity upgrade in the case of being stored at high temperatures, but also the rise of internal resistance is inhibited.