A Molten Salt Strategy to Convert Bulk Nb ₂ O ₅ Into 2D Nonlayered Niobium Nitride Nanosheets for Superior Pseudocapacitive Storage

ABSTRACT Two‐dimensional (2D) nonlayered niobium nitrides have attracted considerable interest for electrochemical energy storage. However, the synthesis of these materials is hampered by their nonlayered crystal structures, which are characterized by strong three‐dimensional chemical bonding. Herein, we demonstrate the direct synthesis of nonlayered 2D NbN 0.8 /Nb 5 N 6 heterojunction nanosheets from commercial bulk Nb 2 O 5 through a scalable molten‐salt‐assisted route. The in situ formed molten KNbO 3 intermediate, confined within the K 2 CO 3 /KCl flux, guides the 2D growth of niobium nitride under NH 3 , yielding nanosheets with 4.8 nm thickness and lateral sizes of 3–5 μm. The as‐synthesized 2D NbN 0.8 /Nb 5 N 6 nanosheets deliver a large capacitance of 918.2 F cm −3 (324.1 F g −1 ) at 1 A cm −3 , superior rate performance of 649.8 F cm −3 (229.4 F g −1 ) at 20 A cm −3 , and exceptional long‐term cycling stability with 95.4% retention over 10,000 cycles, surpassing most reported niobium‐based nitrides. Furthermore, the solid‐state symmetric supercapacitor assembled with the two electrodes of 2D NbN 0.8 /Nb 5 N 6 nanosheets delivers a high volumetric energy density of 13 mWh cm −3 . This work reports a novel high‐performance 2D niobium nitride electrode material and offers a generalizable synthesis strategy for 2D nonlayered transition metal nitrides.