NOVELTY - Secondary battery comprises: a cathode (21) comprising a lithium transition metal complex compound including lithium, one, two or more transition metals, magnesium and oxygen as constituent elements; an anode (22); and an electrolytic solution.    USE - The secondary battery is useful in a battery pack, electric vehicle, electric power storage system, electric power tool and electronic device, where the secondary battery is lithium ion secondary battery (all claimed), which is useful in mobile electronic devices such as video camcorder, digital still camera, mobile phone, notebook personal computer, cordless phone, headphone stereo, portable radio, portable television and personal digital assistant, and the battery pack is useful as an electric power source of a notebook personal computer and a medical electronic device such as a pacemaker and a hearing aid.    ADVANTAGE - The secondary battery has improved performance, battery capacity and cycle characteristics and long life.    DETAILED DESCRIPTION - Secondary battery comprises: a cathode (21) comprising a lithium transition metal complex compound including lithium, one, two or more transition metals, magnesium and oxygen as constituent elements; an anode (22); and an electrolytic solution, where the battery has: a standardized X-ray absorption spectrum, having a horizontal axis indicating X-ray energy and a vertical axis indicating X-ray absorption intensity, of the lithium transition metal complex compound measured by an X-ray absorption spectroscopic method; a first absorption edge having absorption edge energy (E1) in X-ray absorption intensity of 0.5, a X-ray energy that is 1303-1313 eV, in a discharged state in which a discharge voltage is 3 V; and a second absorption edge, having absorption edge energy (E2) in X-ray absorption intensity of 0.5, in a charged state in which a charge voltage V is 4.3-4.5 V, and the absorption edge energies E1 and E2 and the charge voltage (V) satisfy a relation of E2-E1 greater than or equal to (V-4.25)x 4. INDEPENDENT CLAIMS are included for:    (1) a battery pack comprising the secondary battery, a control section for controlling a usage state of the secondary battery and a switch section for switching the usage state of the secondary battery to an instruction of the control section;    (2) an electric vehicle comprising the secondary battery, a conversion section for converting electric power supplied from the secondary battery to drive power, a drive section operating to drive power and the control section;    (3) an electric power storage system comprising the secondary battery, one, two or more electric devices and a control section for controlling supply of electric power from the secondary battery to the electric device;    (4) an electric power tool comprising the secondary battery and a movable section, which is supplied with electric power from the secondary battery.    DESCRIPTION OF DRAWING(S) - The figure shows a schematic view of the secondary battery.    Battery can (11)    Battery cover (14)    Cathode (21)    Anode (22)    Separator. (23)