An electrode element (1) for an energy storage unit (200), such as a capacitor, has an electrode body (100) made of an active electrode material (E), wherein the electrode body (100) includes one or more of: at least one cavity (110) on its surface or in its interior; at least one partial volume (120) of lower density; and/or a surface coating (D) covering at least a portion of the surface of the electrode body (100), such that the surface area covered by the surface coating (D) remains unwetted when in contact with an electrolyte. Energy storage units (200) incorporating the electrode element (1) are particularly suitable for use in implantable electrotherapeutic devices.