Pre-Modulation of NH₄⁺ to Synthesize Composite N-Doped FeVO₄/Fe₂V₄O₁₃/H_<i>x</i>V₂O₅ Nanosheets as Film Electrodes: With Dual Fe/V Redox Sites

By modulating the quantity of complexed NH₄⁺ in the deposition solution, three distinct morphologies of multiphase composite films composed of iron vanadate and hydrogen vanadate bronze were grown on an indium tin oxide (ITO) conductive substrate using the liquid-phase deposition technique. Among them, N-doped FeVO₄/Fe₂V₄O₁₃/H_<i>x</i>V₂O₅ triphase composite nanosheets exhibit enhanced discharge capacities at a high current density and retain a capacity of 115.1 mAh m^-2 after 300 cycles. Cyclic voltammetry tests and galvanostatic charge-discharge tests revealed that the reduction/oxidation potential corresponding to the Fe³⁺/Fe²⁺ redox couple during cycling was 0.14/0.40 V (vs Ag/AgCl in 3 M KCl). X-ray photoelectron spectroscopy further elucidated the contribution of the Fe³⁺/Fe²⁺ valence transition to the capacity of the film electrode, as well as the presence of vanadium in both V⁵⁺ and V⁴⁺ oxidation states in FF₂H-S. During the first cycle, the molar ratios of V⁵⁺/V⁴⁺ were 2.91, 2.08, and 2.52, respectively. Then, X-ray diffraction tests elucidated the insertion/extraction mechanism of Na⁺, and FeVO₄ had an oversized storage space for Na⁺. It helped H_<i>x</i>V₂O₅ and Fe₂V₄O₁₃ to further increase the Na⁺ transport channels and provided more active sites to facilitate diffusion kinetics (the average <i>D</i>_Na⁺ during the discharge phase was 5.49 × 10^-13 cm² s^-1). Moreover, the formation of a solid electrolyte interphase (SEI) film on the surface of the triphase nanosheets resulted in the lowest overdischarge and highest Coulombic efficiency under the same conditions. The results demonstrated that multiphase composite films exhibit excellent electrical properties, offering a new perspective for the design of sodium-ion battery electrodes.