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

Abstract

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.

Keywords: cycling stability; magnesium−lithium hybrid ion batteries; mixed anionic and cationic redox; nanofiber; self-standing electrodes; tantalum trisulfide (TaS3).

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(a) Fitted PXRD diffraction profile for TaS₃ NFs (transmission geometry, capillary mode). Crystal structure of TaS₃ NF showing projections down the (b) b-axis, (c) a-axis, and (d) c-axis, with unit cell shown, as visualized by Vesta software. (e) Raman spectra of as-made TaS₃ NFs and bulk TaS₃. High-resolution XPS spectra of the: (f) Ta 4f and (g) S 2p regions for TaS₃ NFs.

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Results of electron microscopy characterization on the as-prepared TaS₃ NFs displaying: (a) low-magnification (×20k) and (b) high-magnification (×120k) SEM images; (c) SEM-EDS elemental maps for Ta, S, and O, respectively; (d) low-magnification, (e) high-magnification TEM, and (f) HRTEM images.

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Galvanostatic cycling performance of TaS₃ NF and bulk TaS₃ electrodes: (a) Initial six CV curves of the self-standing TaS₃ NF electrodes at a scan rate of 0.1 mV s^–1 between 0.4 and 2.2 V and (b) the initial four (dis)­charge curves of the same electrode at a current density of 50 mA g^–1 in an MLIB. (c) Cycling performance at a current density of 50 mA g^–1 (TaS₃ NF in LiAPC and APC, TaS₃ bulk in LiAPC), (d) variable current densities over 70 cycles (TaS₃ NF and TaS₃ bulk in LiAPC), and (e) at a high current density of 500 mA g^–1 to evaluate long-term cycling performance (TaS₃ NF and TaS₃ bulk in LiAPC).

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SEM images of the TaS₃ NF electrode materials in the (a) uncharged, pristine state, and after the first (b) discharge (1D0.4 V) and (c) charge (1C2.2 V), (d) the second discharge (2D0.4 V) and (e) charge (2C2.2 V), and (f) the 50th charge (50C2.2 V) cycles. The yellow dashed circles highlight the regions where exfoliations occur.

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High-resolution XPS spectra of (a) Li 1s and Mg 2p, (b) Ta 4f, and (c) S 2p regions, and (d) the S K-edge XAS spectra of the TaS₃ NF electrode at different (dis)­charge states.

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(a) Initial three (dis)­charge curves (left) of the TaS₃ NF electrode obtained at a current density of 50 mA g^–1 and their contour plots (right) of the corresponding in operando PXRD patterns. Note that the red marks indicate the presence of an inactive unknown minor impurity in the electrode material in operando cell, and blue marks indicate the diffraction of stainless steel from the cell casing. (b) Zoomed in operando PXRD plot taken from (a), in which red, pink, and blue arrows represent the peaks for pristine TaS₃ NF, phase M, and pseudo-TaS₃, respectively. (c) Raman spectra of the uncycled, the first discharge (1D0.4V), first charge (1C2.2 V), second discharge (2D0.4 V), and second charge (2C2.2 V) states of the TaS₃ NF electrode.

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Charge storage mechanism, Li⁺ ion diffusion kinetics, and interfacial resistance properties of the TaS₃ NF electrode: (a) CV curves obtained at different scan rates of 0.1 mV s^–1 (green), 0.2 mV s^–1 (orange), 0.3 mV s^–1 (violet), 0.4 mV s^–1 (pink), and 0.5 mV s^–1 (light green), respectively. (b) Plots of the measured and fitted log reductive (green) and log oxidative (orange) peak currents against the log of scan rates v, in which the solid lines represent the linear fits (adjusted R-squared values, R_R ² = 0.9999, R_O ² = 0.9995). (c) Histogram of the separate capacitance and diffusion contributions to the charge storage at various scan rates. (d) Discharge and charge GITT curves at a current density of 25 mA g^–1. (e) Plots of discharge and charge diffusivities against Li⁺ cation level applying the data derived from the GITT curves. (f) Measured (thin lines with open circles) and fitted (thick lines) EIS spectra for the (−)­Mg|LiAPC|TaS₃ NFs­(+) cells after the first charge and the 20th charge cycles. The equivalent circuit is shown as an inset in the graph.