Integrating Nanoreactor with O–Nb–C Heterointerface Design and Defects Engineering Toward High‐Efficiency and Longevous Sodium Ion Battery

Abstract Sodium‐ion batteries (SIBs) have attracted much attention for their advantages of high operating voltage, environmental friendliness and cost‐effectiveness. However, the intrinsic defects of anode materials (such as poor electrical conductivity, sluggish kinetics, and large volume changes) hinder them from meeting the requirements for practical applications. Herein, a Nb₂O₅@carbon nanoreactor containing both a O–Nb–C heterointerface and oxygen vacancies (Nb₂O_5‐x@MEC) as an anode material is designed to drive SIBs toward extraordinary capacity and ultra‐long cycle life. The heterostructured nanoreactor both effectively immobilizes defective Nb₂O₅ by forming O‐Nb‐C heterointerface and offers homogeneous dispersion of Nb₂O₅ with desirable content to prevent their agglomeration. In addition, vast active interfaces, favored electrolyte infiltration, and a well‐structured ion–electron transportation channel are enabled by the framework, improving sodium ion storage and enhancing redox reaction kinetics. The enhancement brought by spatial confinement, defect implantation and heterointerface design give the composites a highly reversible sodiation–desodiation process and remarkable structural stability. By virtue of these superiorities, Nb₂O_5‐x@MEC delivers excellent performance, i.e., high areal capacity over 1.1 mAh cm^‐2, admirable rate capability up to 20 A g^‐1, and ultra‐long cycling performance over 5000 cycles, holding great promise for utilization in practically viable SIBs.