Activating reversible multi-electron reaction of Na₃(VO)₂(PO₄)₂F cathode via Fe/F dual-doping for high-energy and stable sodium storage

Na₃(VO)₂(PO₄)₂F cathode has garnered extensive interest for its stable structure, abundant Na⁺ migration channels, and high working voltage, though higher energy densities are sought for commercial applications. This study enhances energy density by activating multi-electron reactions through the partial substitution of V⁴⁺ and dangling O²⁻ with Fe³⁺ and F⁻, respectively, using a straightforward hydrothermal method. This substitution successfully activates the V³⁺/V⁴⁺ redox couple, facilitating multi-electron reactions. The modified cathode, Na₃(VO)_1.8Fe_0.2(PO₄)₂F_1.2 (N(VO)_1.8Fe_0.2PF_1.2), exhibits a reversible specific capacity of 213.3 mAh g⁻¹ at 50 mA g⁻¹. Characterization techniques, including in situ X-ray diffraction and ex-situ X-ray photoelectron spectroscopy, confirm that the activated V³⁺/V⁴⁺ redox reaction proceeds via a solid-solution mechanism. Density functional theory analysis suggests that Na₃(VO)_1.8Fe_0.2(PO₄)₂F_1.2 offers improved electronic conductivity and structural stability, elucidating the origins of low Na⁺ migration energy barriers and ideal diffusion kinetics. When paired with a hard carbon (HC) anode, the full cell (HC//N(VO)_1.8Fe_0.2PF_1.2) achieves a reversible capacity of 196.6 mAh g⁻¹ and an energy density of 287.0 Wh kg⁻¹ at 50 mA g⁻¹, demonstrating exceptional long-term cyclic stability with a capacity retention of 94.7% after 200 cycles at 500 mA g⁻¹. This study opens new avenues for the commercial application of sodium-ion batteries (SIBs) cathodes.