Electrochemical Interlayer Expansion and Dual Redox Activation for Fast Mg-Ion Transport and High Capacity in Quasi-1D TiS _<b>3</b>

Magnesium ion batteries (MIBs) offer promising solutions for next-generation sustainable energy storage systems owing to their intrinsic safety and cost-effectiveness, yet their development is hindered by the scarcity of high-capacity cathode materials, primarily due to poor magnesium ion transport and a limited number of electrochemically active sites. Here, we report a significant performance breakthrough in a structurally and electrochemically distinct, underexplored quasi-1D pseudolayered titanium trisulfide (TiS₃) cathode through interlayer engineering and exploitation of dual cationic/anionic redox chemistry. <i>In operando</i> and <i>ex situ</i> characterization reveal that interlayer expansion, induced by the intercalation of 1-butyl-1-methylpyrrolidinium (BMPyrr⁺), weakens electrostatic interactions within the sulfide sublattice, enhances magnesium ion diffusion kinetics, and increases accessible redox sites. These modifications activate reversible Ti⁴⁺/Ti³⁺ and S₂ ^2-/S^2- redox couples, complemented by nanosizing-induced pseudocapacitance, synergistically underpinning the exceptional electrochemical performance. As a result, the expanded TiS₃ cathode delivers outstanding reversible capacities (up to 300 mA h g^-1 at 100 mA g^-1), excellent rate performance (181 mA h g^-1 at 1000 mA g^-1), and long-term cycling stability, surpassing its pristine counterpart and many state-of-the-art MIB cathodes. This work underscores the combined role of interlayer engineering and dual-ion redox chemistry in advancing multivalent energy storage and introduces pseudolayered TiS₃ as a new structural platform beyond conventional layered sulfides.