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_x VSe₂ 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 V₅ S_8- _x and V₂ O_5- _x -based heterojunctions, marking a conceptual breakthrough in advanced electrode design for constructing high-performance sodium-ion batteries.