Abstract: The disclosure provides a novel positive electrode active material for an aqueous potassium ion battery, and a novel potassium ion secondary battery. The positive electrode active material for an aqueous potassium ion battery is represented by the general formula LixMn2O4, where 0<x<2. The aqueous potassium ion secondary battery comprises a positive electrode active material represented by the general formula LixMn2O4 where 0<x<2, and an aqueous electrolyte solution, wherein the aqueous electrolyte solution has a pH of 7.0 to 13.0 and comprises an aqueous solvent and a potassium salt dissolved in the aqueous solvent. The potassium salt is preferably potassium pyrophosphate.

Abstract: A power storage device includes a positive electrode, a negative electrode, and a non-aqueous electrolyte. The negative electrode comprises a plurality of types of negative electrode active materials, wherein each of the plurality of types has different lithium-ion absorption potentials.

Abstract: An electric double layer capacitor electrode includes a positive electrode having a positive electrode active material layer including a positive electrode side porous body and a negative electrode having a negative electrode active material layer including a negative electrode side porous body. An oxidation-reduction substance causing an oxidation-reduction reaction during charging and discharging is adsorbed onto at least one of the positive electrode side porous body of the positive electrode active material layer and the negative electrode side porous body of the negative electrode active material layer.