NOVELTY - An energy storage device (200) comprises: cathode (210) and anode (230) with separator (220) between them; where at least one of cathode or anode comprises a rigid polymer matrix (260); an active material within the matrix; and elongated electrically conducting particles comprising ion conducting moieties bonded to it, where the particles are located within the matrix and bonded to the matrix to provide mechanical reinforcement. The rigid polymer matrix comprises a solid electrolyte. The separator comprises elongated electrically insulated particles comprising ion conducting moieties.    USE - As energy storage device, in particular a reinforced structural composite energy storage device (claimed). The energy storage device includes batteries and super-capacitors, used in automotive (vehicle accessory power, hybrid vehicles, etc.), military (micro unmanned air vehicles, unmanned air ship, soldier power, etc.), aerospace products (accessory power for sensors, structural sensors for composites, etc.), and in consumer items (portable devices, clothing, fabric, etc.).    ADVANTAGE - The device is inexpensive, mechanically robust, and it does not need additional structural elements, and provides a means to monitor the health status of the structure. It enables the fabrication of multi-functional power structures, such as batteries or super-capacitors, which can also function as structural members, allow structural deformation, and/or provide sensor capabilities. By varying the matrix type of the storage device, the structural properties of the device can be changed.    DETAILED DESCRIPTION - An energy storage device (200), comprises: a cathode (210) and an anode (230) with a separator (220) between them; and at least one of the cathode or anode comprises: a rigid polymer matrix (260); an active material within the rigid polymer matrix; and elongated electrically conducting particles comprising ion conducting moieties bonded to it, where the elongated electrically conducting particles are located within the rigid polymer matrix and bonded to the polymer matrix so as to provide mechanical reinforcement; where the rigid polymer matrix comprises a solid electrolyte; and the separator comprises elongated electrically insulated particles comprising ion conducting moieties bonded to it, where the elongated electrically insulated particles are mixed throughout the separator. An INDEPENDENT CLAIM is included for a method for forming an energy storage device, involving: forming solid polymer matrix comprising a cathode, an anode, and a separator; forming a solid electrolyte material comprising bonding a chemical moiety to a conductive particle of at least one of: a cathode active material (213); or an anode active material (233); and locating the solid electrolyte material within the solid polymer matrix so as to be capable of providing mechanical reinforcement; and mixing elongated electrically insulated particles comprising ion conducting moieties bonded to it throughout the separator.    DESCRIPTION OF DRAWING(S) - The figure shows a cut away side view of a battery.    Battery (200)    Cathode (210)    Cathode active material (213)    Separator (220)    Anode (230)    Anode active material (233)    Polymer matrix (260)