LAUSR

Layered niobium phosphates have been considered very promising energy storage materials because of their high theoretical operating voltage window and the rich oxidation states of niobium. However, their development has been stymied by the phase‐controlled synthesis due to the insolubility of niobium sources except in concentrated hydrofluoric (HF) acid systems. Herein, a new avenue is opened for layered acid niobium phosphate (2NbOPO4·H3PO4·H2O) synthesis in a mild oxalic acid system. Taking advantage of this strategy, in situ growth of sub‐5 nm 2NbOPO4·H3PO4·H2O nanosheet (NPene) arrays on conductive carbon fiber cloth (CFC) substrates is achieved as self‐standing electrodes for solid‐state supercapacitors. Interestingly, the NPene@CFC electrode exhibits a typical cation (H+ or Li+)‐intercalation kinetics with a wide potential window of 0–1.0 V in aqueous electrolytes. Given the wide potential window and highly exposed active surface, the solid‐state asymmetric supercapacitors constructed from such a NPenes@CFC electrode display a high working potential of 2.0 V, energy density of 122.2 W h kg−1 at a power density of 589.7 W kg−1, cycle stability with a capacitance retention of 94.2% after 10 000 cycles, and also outstanding flexible and wearable characteristics.