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

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 NbS₃ 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 Nb⁴⁺/Nb³⁺ and S₂ ^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 NbS₃ 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 NbS₃ 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.