Lithium-iron-manganese-based composite oxide and lithium-ion secondary cell using same

To provide a lithium-iron-manganese-based composite oxide capable of providing a lithium-ion secondary cell in which the capacity retention rate in charge/discharge cycles is increased and generation of gas by the charge/discharge cycles is suppressed. A lithium-iron-manganese-based composite oxide having a layered rock-salt structure, at least a part of the surface of the lithium-iron-manganese-based composite oxide being represented by the following formula (1): Li x M 1 (y-p) Mn p M 2 (z-q) Fe q O (2-δ) (1) (where: 1.05 ≤ x ≤ 1.32, 0.33 ≤ y ≤ 0.63, 0.06 ≤ z ≤ 0.50, 0 < p ≤ 0.63, 0.06 ≤ q ≤ 0.50, 0 ≤ δ ≤ 0.80, y ≥ p, and z ≥ q; M 1 is an element which is Ti and/or Zr, and M 2 is at least one element selected from the group consisting of Co, Ni, and Mn), is coated by an inorganic material.