Abstract: The present invention relates to an energy storage device comprising a positive electrode, a negative electrode, and an aqueous polymer electrolyte disposed between the positive electrode and the negative electrode. At least one of the electrodes is an organic electrode. The aqueous polymer electrolyte comprises a metal ion component comprising a metal cation being Na+ or K+; a polymer or copolymer comprising at least one monomer unit being a carboxylic acid. At least 20 mol-% of a total amount of monomers in the polymer is monomers comprising carboxylic acid.