Engineering fibrous-interconnected potassium bis(dioxovanadium) phosphate frameworks for fast-charging and high-rate sodium-ion supercapacitors

Flexible and high-performance sodium-ion storage systems are essential for next-generation energy technologies. Here, orthorhombic K(VO₂)₂(PO₄) nanostructures were synthesized on carbon cloth through a controlled phosphorization process for 4 h (4KVOP-C). The 4KVOP-C electrode exhibited a fibrous network morphology, providing abundant active sites, short Na⁺ diffusion pathways, and strong contact with the conductive substrate. Moreover, its robust P–O bonds and open ion-diffusion channels enhanced its structural stability and charge transport. The 4KVOP-C electrode delivered outstanding electrochemical performance, with a high areal capacitance and excellent rate capability in a three-electrode configuration. The phosphate-stabilized vanadyl framework of KVOP enables delocalized charge redistribution across the V–O–P networks during Na adsorption, resulting in a higher quantum capacitance and density of states at the Fermi level. This electronic preconditioning underlies its superior areal capacitance, fast charge–discharge, and enhanced Na-ion accommodation compared with those of potassium-intercalated vanadium oxide. Moreover, a symmetric 4KVOP-C//4KVOP-C supercapacitor was assembled, which operated over a wide voltage window of 2.0 V, achieving an energy density of 50 µW h cm⁻² at a power density of 1980 µW cm⁻², along with excellent cycling stability. These results demonstrate that the fibrous K(VO₂)₂(PO₄) nanostructures synthesized via optimized phosphorization exhibit excellent intrinsic electrochemical properties, making them potential electrode materials for flexible, high-energy-density and durable sodium-ion supercapacitors.