NOVELTY - The lithium ion secondary battery comprises a positive electrode (21), a negative electrode (22), and an electrolytic solution. The positive electrode includes a first lithium composite oxide and a second lithium composite oxide expressed by an equation (1), as a positive electrode active material. A charge capacity (vs lithium metal) per unit volume during a charge and discharge of a first cycle is larger in the second lithium composite oxide compared to the first lithium composite oxide.    USE - The lithium ion secondary battery is useful: in an electric tool; in an electric vehicle; in a power storage system (all claimed) such as a household battery system for an emergency; in a portable electronic apparatus such as a video camera, a digital still camera, a mobile telephone, a notebook, a personal computer, a wireless telephone, a headphone stereo, a portable radio, a portable television and a portable personal digital assistant; in a household electric apparatus such as an electric shaver, a storage device such as a backup power source and a memory card; and in a medical electronic apparatus such as a pacemaker or a hearing aid, where the electric tool includes an electric drill and an electric sheer and the electric vehicle includes a hybrid vehicle.    ADVANTAGE - The compact and effective lithium ion secondary battery exhibits long operational lifespan, light weight, high battery capacity enhanced performance, and excellent cycle characteristics and storage characteristics.    DETAILED DESCRIPTION - The lithium ion secondary battery comprises a positive electrode (21), a negative electrode (22), and an electrolytic solution. The positive electrode includes a first lithium composite oxide and a second lithium composite oxide expressed by an equation (1), as a positive electrode active material. A charge capacity (vs lithium metal) per unit volume during a charge and discharge of a first cycle is larger in the second lithium composite oxide compared to the first lithium composite oxide. A discharge voltage (vs lithium metal) during the charge and discharge of the first cycle is lower in the second lithium composite oxide compared to the first lithium composite oxide. The first lithium composite oxide includes compounds expressed by equations (2) to (4). A charge capacity ratio (a charge capacity (vs lithium metal) per unit volume during the charge and discharge at a second cycle/a charge capacity (vs lithium metal) per unit volume during the charge and discharge of the first cycle) is larger in the first lithium composite oxide compared to the second lithium composite oxide. The negative electrode includes a negative electrode active material. A charge and discharge efficiency (a discharge capacity (vs lithium metal) per unit volume during the charge and discharge of the first cycle/a charge capacity (vs lithium metal) per unit volume during the charge and discharge of the first cycle) is higher in the first lithium composite oxide compared to the negative electrode active material. INDEPENDENT CLAIMS are included for:    (1) a positive electrode active material;    (2) an electric tool;    (3) an electric vehicle; and    (4) a power storage system.    DESCRIPTION OF DRAWING(S) - The diagram shows a schematic cross-sectional view of a lithium ion secondary battery.    Battery casing (11)    Lid (14)    Safety valve mechanism (15)    Positive electrode (21)    Negative electrode. (22)