A
lithium ion secondary battery comprising: a positive
electrode comprising a positive
electrode active material; a negative
electrode comprised mainly of a material capable of storing and releasing
lithium ions; and an electrolytic liquid,the positive electrode active material being a
lithium-iron-
manganese complex oxide having a layered rock salt structure and represented by a
chemical formula:LixFesM1(z-s)M2yO2-δ wherein 1.05≦x≦1.32, 0.06≦s≦0.50, 0.06≦z≦0.50, 0.33≦y≦0.63, and 0≦δ≦0.80; M1 represents a
metal selected from the group consisting of Co, Ni, Mn and a mixture thereof; and M2 represents a
metal selected from the group consisting of Mn, Ti, Zr and a mixture thereof,the electrolytic liquid comprising 1,1,2,3,3,3-hexafluoropropyl difluoromethyl
ether represented by the following formula (1):