In Situ Electrochemical Activation of Pseudo‐Layered NbS ₃ via Interlayer Expansion and Dual Redox for High Mg‐Ion Storage

ABSTRACT Magnesium‐ion batteries (MIBs) are a promising alternative to lithium‐ion technologies due to their inherent safety and potential for sustainable, large‐scale energy storage, yet their development remains hindered by the scarcity of high‐capacity cathode materials. In this study, we reveal a significant leap in performance in a structurally unique quasi‐1D pseudo‐layered NbS ₃ cathode for MIBs, achieved through electrochemical interlayer engineering. In operando and ex situ PXRD, SEM‐EDS, and XPS confirm the intercalation of 1‐butyl‐1‐methylpyrrolidinium (BMPyrr ⁺ ) cations, which results in substantial expansion of the interlayer spacing. This expansion not only enhances magnesium ion diffusion but also activates the dual Nb ⁴⁺ /Nb ³⁺ and S ₂ ²⁻ /S ²⁻ redox processes for access to abundant ion storage sites, as elucidated by ex situ XAS analysis. In addition, multiple coupled factors, including BMPyrr ⁺ ‐enabled channel opening and electrochemically induced morphological reconfiguration (i.e., nanosizing/fragmentation) further promote pseudo capacitive behavior. Consequently, the expanded NbS ₃ electrode delivers a high reversible capacity of up to 200 mA h g ⁻¹ at 50 mA g ⁻¹ , plus excellent cycling stability, significantly outperforming its unmodified counterpart. This work highlights NbS ₃ as a novel dual‐redox trichalcogenide cathode for MIBs and demonstrates the power of interlayer expansion in unlocking inherent redox reactions for improving performance in multivalent‐based energy storage systems.