Method for producing composition for forming positive electrode material mixture layer and method for producing lithium ion secondary battery

A method for producing a composition for forming a positive electrode material mixture layer of the present invention either includes the steps of; (1) mixing a positive electrode active material and phosphoric acid or a phosphate compound to form a mixture; and mixing the mixture and the binder to form a composition, or includes the steps of: (2) forming a mixture containing a positive electrode active material and a binder, and mixing the mixture and phosphoric acid or a phosphate compound to form a composition, wherein the formed composition for forming a positive electrode material mixture layer contains the solvent and has a viscosity adjusted to 15000 mPa·s or less.