NOVELTY - Battery (10) comprises a positive electrode (22), a negative electrode (23) and an electrolyte. The positive electrode includes a positive electrode active material layer on at least one surface of a positive electrode current collector. The positive electrode active material layer includes a binder and a positive electrode active material of a deoxidized lithium transition metal composite oxide.    USE - The battery is useful: in an electric vehicle, a battery pack, a power system and a power storage device; and for supplying power to an electronic device (all claimed).    ADVANTAGE - The battery: suppresses oxygen generation from the positive electrode at the time when the battery temperature is increased by short circuit; suppresses the reaction between oxygen and the electrolyte and a process of a burning reaction; suppresses heat generation and a decrease in battery characteristics; and exhibits excellent cycling characteristics.    DETAILED DESCRIPTION - Battery (10) comprises a positive electrode (22), a negative electrode (23) and an electrolyte. The positive electrode includes a positive electrode active material layer on at least one surface of a positive electrode current collector. The positive electrode active material layer includes a binder and a positive electrode active material of a deoxidized lithium transition metal composite oxide. The positive electrode active material layer shows a first peak and a second peak of oxygen amounts generated from a type of the positive electrode active material in the positive electrode active material layer when the positive electrode active material layer is heated in a charge state of 4.2-4.5 V in a lithium antipode potential. The second peak appears in a temperature region higher than a temperature region of the first peak, in which at least the second peak appears in a temperature region higher than 220 degrees C. INDEPENDENT CLAIMS are also included for:    (1) a positive electrode active material comprising at least one of: a deoxidized lithium transition metal composite oxide having a layered rock salt structure in which a peak intensity ratio (I003/I104) which is a ratio of diffracted peak intensity (I003) of a peak attributed to a (003) plane to diffracted peak intensity (I104) of a peak attributed to a (104) plane, is 0.65-0.8; and a deoxidized lithium transition metal composite oxide having a spinel structure in which a peak intensity ratio (I311/I111), which is a ratio of diffracted peak intensity (I311) of a peak attributed to a (311) plane to diffracted peak intensity (I111) of a peak attributed to a (111) plane, is 0.3-0.4, where the diffracted peak intensity is measured in an X-ray diffraction measurement using a copper-potassium alpha-ray for an X-ray source;    (2) the positive electrode;    (3) manufacturing the positive electrode active material comprising mixing the lithium transition metal composite oxide and a reducing agent, and baking the reducing agent and the lithium transition metal composite oxide under a non-oxygen atmosphere at a temperature higher than or equal to a temperature at which the reducing agent starts reducing;    (4) a battery pack comprising the battery, a controller configured to control the battery, and a package including the battery;    (5) an electric vehicle comprising the battery, a converting device configured to convert power supplied from the battery into driving power of a vehicle, and a control device configured to perform information processing about vehicle control based on information about the battery;    (6) a power storage device comprising the battery, where the power storage device supplies power to an electronic device connected to the battery; and    (7) a power system configured to enable power supply from the battery or to enable power supply to the battery from a power generation device or a power network.    DESCRIPTION OF DRAWING(S) - The figure shows a cross-sectional view of a cylindrical battery.    Battery (10)    Safety valve (14)    Positive electrode (21)    Negative electrode (22)    Separator (23)