NOVELTY - The device e.g. high-energy ultracapacitor (200), has an electroactive separator (230) i.e. electronic insulator, positioned between a positively charged electrode (210) i.e. cathode, and a negatively charged electrode (220) i.e. anode. Two pairs of electric double layers (260, 270, 280, 285) are formed on respective electrode side surfaces of the separator. The separator is made of ferroelectric material, anti-ferroelectric material, composite of a particulate material in a polymer matrix or composite of a particulate material in a glass matrix.    USE - Electric storage device e.g. high-energy ultracapacitor, pseudocapacitor, battery, ultracapacitor-battery hybrid or pseudocapacitor-battery hybrid, for a hybrid electric vehicle (all claimed). Can also be used for a large device e.g. electric vehicle, industrial equipment, electrical grid or microgrid load-leveling system, power backup for telecommunication center, blade pitch control for a wind turbine, rubber-tire gantry, electrical utility black-start system, and cordless power tool, and small device e.g. implantable defibrillator, cell phone, camera, rechargeable torch and remote door locking device.    ADVANTAGE - The device exhibits higher capacitance, operating voltages, higher power density, faster charge-discharge cycles and longer cycle life. The separator supports double-layer formation at the electrolyte/separator interfaces, thus improving energy capacity of the device. The separator is densified, so as to decrease voids and porosity in the separator, thus improving physical properties such as strength, thermal conductivity, electrical or ionic conductivity, of the separator.    DETAILED DESCRIPTION - An INDEPENDENT CLAIM is also included for a method for storing electrical energy.    DESCRIPTION OF DRAWING(S) - The drawing shows a schematic sectional view of an ultracapacitor including an electroactive separator.    High-energy ultracapacitor (200)    Positively charged electrode (210)    Negatively charged electrode (220)    Electroactive separator (230)    Cathodic compartment (240)    Anodic compartment (250)    Electric double layers (260, 270, 280, 285)