Unlocking the Design Paradigm of In‐Plane Heterojunction with Built‐in Bifunctional Anion Vacancy for Unexpectedly Fast Sodium Storage

Abstract Transition metal chalcogenide (TMD) electrodes in sodium‐ion batteries exhibit intrinsic shortcomings such as sluggish reaction kinetics, unstable conversion thermodynamics, and substantial volumetric strain effects, which lead to electrochemical failure. This report unlocks a design paradigm of VSe_2−x/C in‐plane heterojunction with built‐in anion vacancy, achieved through an in situ functionalization and self‐limited growth approach. Theoretical and experimental investigations reveal the bifunctional role of the Se vacancy in enhancing the ion diffusion kinetics and the structural thermodynamics of Na_xVSe₂ active phases. Moreover, this in‐plane heterostructure facilitates complete face contact between the two components and tight interfacial conductive contact between the conversion phases, resulting in enhanced reaction reversibility. The VSe_2−x/C heterojunction electrode exhibits remarkable sodium‐ion storage performance, retaining specific capacities of 448.7 and 424.9 mAh g⁻¹ after 1000 cycles at current densities of 5 and 10 A g⁻¹, respectively. Moreover, it exhibits a high specific capacity of 353.1 mAh g⁻¹ even under the demanding condition of 100 A g⁻¹, surpassing most previous achievements. The proposed strategy can be extended to other V₅S_8−x and V₂O_5−x‐based heterojunctions, marking a conceptual breakthrough in advanced electrode design for constructing high‐performance sodium‐ion batteries.