An energy storage device is provided. The energy storage device includes a cathode material and a separator in electrical communication with the cathode material. The cathode material includes zinc. The separator has a first surface that defines at least a portion of a first chamber, and a second surface that defines a second chamber. The first chamber is in ionic communication with the second chamber through the separator. The separator includes an alkali-metal-ion conducting material and a toughening material. A method for operating the energy storage device is also provided. Furthermore, an energy storage system including the energy storage device is provided.