Self-Supporting Quasi-1D TaS₃ Nanofiber Films with Dual Cationic/Anionic Redox for High-Performance Mg-Li Hybrid Ion...

Magnesium-lithium hybrid ion batteries (MLIBs) offer a promising energy storage technology that combines the safety and dendrite-free plating/stripping of Mg anodes with the rapid Li⁺-dominated diffusion in cathode materials. However, for electrodes that undergo significant volume/structural changes during cycling, conventional slurry-cast fabrication often leads to microstructural degradation, active material detachment, and consequently, poor cycling stability and rapid capacity fading. Here, we report a self-standing, carbon- and binder-free tantalum trisulfide (TaS₃) nano fibrous (NF) film, synthesized via a facile one-step physical vapor transport reaction that addresses these challenges through mechanistic innovations. Mechanistic investigations reveal that the TaS₃ NF electrode undergoes dual cationic (Ta⁵⁺/Ta³⁺) and anionic (S₂^2-/S^2-) redox reactions, accompanied by electrochemically induced phase transitions and in situ exfoliation. The dual redox couples provide a large number of Li⁺ ion storage sites, while the structural changes lead to fiber-level nanosizing, which in turn promotes fast (near) surface ion storage and pseudo capacitive behavior. Despite these significant transformations, the robust fibrous architecture retains structural integrity throughout prolonged cycling, as confirmed by in operando and ex situ characterization. This dual-redox, in situ exfoliation, and architecture-driven mechanism underpins the electrode's exceptional cycling stability and high rate capability. As a result, the TaS₃ NF electrode achieves a high reversible capacity of 178.5 mA h g^-1 at 50 mA g^-1, maintains 91.6% of reversible capacity after 100 cycles, and delivers 144.4 and 119.0 mA h g^-1 at 500 and 1000 mA g^-1, respectively, surpassing those of slurry-cast bulk TaS₃ controls. Furthermore, the maintenance of a flexible film structure after extended cycling suggests potential applicability in next-generation wearable and structurally adaptive energy storage systems. These findings highlight the potential of self-standing, carbon- and binder-free film electrodes in advancing the cycling stability, energy density, and design versatility of MLIB systems and beyond.