Three-dimensional K₃V₃(PO₄)₄@Carbon core-shell coaxial nanowire networks for High-rate sodium-ion batteries

Potassium vanadium phosphate is an emerging sodium superionic conductor (NASICON) material with great potential as a cathode for sodium-ion batteries, owing to its high operating voltage, structural stability, and low cost. However, its poor electronic conductivity, along with significant volume expansion and stress effects during Na⁺ insertion and extraction, severely limits its electrochemical performance, hindering its practical application. In this study, a three-dimensional (3D) carbon-coated K₃V₃(PO₄)₄ core-shell coaxial nanowire network (KVP@C) was synthesized using a simple electrospinning method combined with high-temperature annealing. The outer carbon layer helps mitigate the volume expansion of KVP during charge and discharge cycles. Simultaneously, the 3D carbon framework enhances the interaction between the electrode material and the electrolyte, facilitating the transport and diffusion of ions and electrons. They lead to an improved specific capacity and outstanding rate performance. The KVP@C half-cell delivers an initial capacity of 103.9 mAhg^-1 at a current density of 10mAg^-1, a rate capability of 66.5 mAhg^-1 at 200 mAg^-1, and a capacity retention of 83.69 % after 500 cycles. Electrochemical kinetics analysis reveals that the 3D core-shell network with a carbon coating improves both electrical conductivity and ion diffusion rates, leading to superior rate performance. Ex-situ characterization further elucidates the sodium storage mechanism of KVP@C: the concentration gradient of Na⁺ between the electrolyte and the electrode induces spontaneous Na⁺ diffusion into the electrode during the resting state. During the initial charge, both Na⁺ and K⁺ are extracted from the KVP@C electrode, with the remaining ions acting as structural supports between the KVP layer. In the subsequent charge-discharge cycles, electrochemical reactions are dominated by the reversible insertion and extraction of Na⁺, while K⁺ no longer participates in the reactions. Density functional theory calculations confirm the chemical interaction between the carbon layer and KVP, which alters the energy band structure of the material and enhances its electrical conductivity. These findings demonstrate that the as-synthesized 3D carbon-coated KVP core-shell coaxial nanowire network offers exceptional sodium storage properties, laying a solid foundation for the practical development of high-performance sodium-ion batteries.