Method for producing positive electrode active material

A method for producing a positive electrode active material, the method comprising the following steps of:

• (1) mixing an activation agent containing one type or two or more types of alkali metal compounds with an electrode material mixture which contains a positive electrode active material, a binder, and an electrolyte and from which at least a part of the electrolyte has been removed by contact with an electrolyte cleaning solvent to obtain a mixture;
• (2) heating the mixture at a retention temperature equal to or higher than a melting start temperature of the activation agent;
• (3) bringing the mixture after step (2) into contact with a water-containing liquid to obtain a slurry containing a solid component and a liquid component, and then separating the slurry into a solid component and a liquid component;
• (4) removing water from the obtained solid component by subjecting the solid component to heating and/or a reduced-pressure environment, wherein an ambient pressure at which the solid component exists and/or a temperature of the solid component is changed so that a ratio of saturated water vapor pressure at the temperature of the solid component to the ambient pressure becomes 80% or more within 24 hours after the slurry is separated into the solid component and the liquid component;
• and
• (5) heat-treating the solid component after step (4) at lower than 900° C.