Heterogeneous Dual Hollow Spindles of Amorphous Fe₂(MoO₄)₃@TiO₂ as Anode Materials for Superior Lithium Storage

In the field of lithium-ion batteries, Fe₂(MoO₄)₃ has attracted considerable interest because of its distinctive three-dimensional open framework structure, high oxidation states of two metal elements, and high theoretical capacity. However, practical applications of Fe₂(MoO₄)₃ are hindered by its substantial volume changes. Herein, a metal–organic-framework-engaged strategy is proposed to construct heterogeneous dual hollow spindles of amorphous Fe₂(MoO₄)₃@TiO₂. The synthesis approach relies on the etching reaction of MIL-88A in the Na₂MoO₄ solution and Kirkendall effect, as well as the controlled hydrolysis of titanium isopropoxide. Material characterizations based on x-ray diffraction, Raman spectra, transmission electron microscopy, scanning electron microscopy, and x-ray photoelectron spectroscopy disclose successful preparation of the dual hollow spindles. Its surface area is 11 m² g⁻¹. The hollow spindle mitigates the volume changes of Fe₂(MoO₄)₃, while the TiO₂ coating further enhances the structural robustness of Fe₂(MoO₄)₃. Moreover, the hollow spindle also facilitates the storage of electrolyte and Na⁺ transport. These structural advantages result in high reversible capacities, stable cycling performance, and outstanding rate capability. Specifically, Fe₂(MoO₄)₃@TiO₂ maintains a high reversible capacity of 1204 mAh g⁻¹ at 1.0 A g⁻¹ after 450 cycles and 462 mAh g⁻¹ at 5.0 A g⁻¹ after 540 cycles. Furthermore, the high surface capacitive contribution and rapid Li⁺ diffusion induce fast reaction kinetics. This study demonstrates that the heterogeneous dual hollow spindle is an advanced composite structure for Fe₂(MoO₄)₃.