High energy density redox flow device

The present invention provides an energy storage device comprising: at least one flowable electrode comprising an electroactive zone between a current collector and an ion permeable membrane, the electroactive zone including an ion-storing redox composition; wherein the ion-storing redox composition is a semi-solid ion-storing redox composition comprising a solid phase ion-storing redox material in an electrolyte, the solid phase ion-storing redox material is capable of taking up or releasing ions without dissolving in the electrolyte, and the volume percentage of the solid phase ion-storing redox material in the composition is between 5% and 70 %; and/or wherein the ion-storing redox composition is a condensed liquid ion-storing redox composition comprising a condensed liquid ion-storing redox material in an electrolyte, the condensed liquid ion-storing redox material is capable of taking up or releasing ions, and the weight percentage of the condensed liquid ion-storing redox material in the composition is at least 10%. [0002] Also provided are a method of operating the energy storage device, a vehicle comprising the energy storage device and uses of the energy storage device.