An electrochemical energy storage device disclosed by the present invention is provided with a positive electrode that contains a positive electrode active material; a negative electrode; and a nonaqueous electrolyte solution that is in contact with the positive electrode and the negative electrode. The positive electrode active material contains, in a discharged state, at least one substance that is selected from the group consisting of alkali metal chlorides, alkaline earth metal chlorides and quaternary alkyl ammonium chlorides. The nonaqueous electrolyte solution contains, as a solvent, an ionic liquid that contains a cation having an alkoxy alkyl group as a constituent.