nergy usage is experiencing a large andfast shift toward electricity as the mainpower source. Reversible storage and releaseof electricity is an essential technology,driven by the needs of portableconsumer electronics and medical devices,electric vehicles, and electric grids, as well asthe emerging Internet of Things and wearabletechnologies. These applications and the needto store energy harvested by triboelectric andpiezoelectric generators (e.g., frommusclemovements),as well as solar panels, wind powergenerators, heat sources, and moving machinery,call for considerable improvement anddiversification of energy storage technology.In this context, materials with nanometersized structural features and a large electrochemically active surface can change the paradigm for energy storage from within theelectrode bulk to surface redox processes thatoccur orders of magnitude faster and allow agreatly improved power and cycle life (1–3).High electronic and ionic conductivities combinedwith intrinsic strength and flexibility oflow-dimensional materials allow ultrathin, flexible,and structural energy storage solutions.