Engineering the structural and electronic properties of VO2 through Fe substitution for high-performance aqueous zinc-ion batteries

Abstract

Rechargeable aqueous zinc-ion batteries (AZIBs) are gaining attention for large-scale energy storage owing to safety and low cost. Among the numerous cathodes for AZIBs, VO 2 is a promising candidate due to its tunnel-like structure and high theoretical capacity; however, its practical application is limited by its low electronic conductivity. To address this issue, doping with several transition metals has been attempted; however, exploration of Fe doping is rare despite its cost-effectiveness and abundance. Herein, a series of Fe-substituted VO 2 cathodes (Fe x V 1-x O 2 : x = 0.15, 0.25, and 0.35) have been synthesized via a simple one-step hydrothermal method and systematically evaluated for AZIB applications. PXRD studies indicate a noticeable lattice expansion upon Fe substitution, providing relatively wider pathways for Zn 2+ transport within the cathode framework. Among the investigated compositions, Fe 0.25 V 0.75 O 2 exhibited the best electrochemical performance, delivering a high specific capacity of 380.11 mAh g -1 at 0.1 A g -1 , retaining 56% capacity at 5 A g -1 , and maintaining 82.75% capacity retention after 500 cycles at 2 A g -1 , significantly outperforming the pristine VO 2 cathode. Post-cycling characterization further revealed improved tolerance toward cycling-induced structural and morphological changes, while ICP-OES analysis showed approximately 92.3% lower vanadium dissolution for FeVO 2 -25% than pristine VO 2 . The Fe substitution effect is also rationalized by DFT calculations, which showed a modified electronic structure of VO 2 , including band-gap narrowing through Fe 3d–V 3d/O 2p hybridization, consistent with the experimentally observed reduction in optical band gap. This work demonstrates that Fe, an earth-abundant and cost-effective dopant, provides an effective strategy for simultaneously tuning the structural and electronic properties of VO 2 and improving its Zn 2+ storage performance in aqueous zinc-ion batteries.