An energy storage device comprises a pair of electrodes in contact with an electrolyte and a dielectric separator therebetween. The separator comprises a chelating material capable of binding parasitic to ions with the electrolyte, so to prevent the free circulation of the parasitic ions within the electrolyte. In one embodiment, the chelating material comprises chitosan, which may be provided in fibrous form and which may be included within a non-woven fabric of cellulosic and/or olefinic, or which may be instead coated on the separator or otherwise comprised in the separator. In another embodiment, the chelating material comprises chitosan, and the ion-binding capability of the chitosan is enhanced by chemically binding a chelating agent to the chitosan so to create a multi-dentate ligand that binds with the parasitic ions, particularly multi-valent metal ions, to create coordination compounds. One such chelating agent is ethylendiamine tetraacetic acid (EDTA).