High-performance Na3.5Fe0.5V1.5(PO4)3 cathodes enabled by phase control and CQD-containing carbon coating

Abstract

Fast-charging sodium-ion batteries (SIBs) demand cathode materials with rapid redox kinetics and robust structural integrity. Herein, a series of Na_3+xV_2−xFe_x(PO₄)₃/C (0 ≤ x ≤ 1) cathodes are synthesized via a citric acid-assisted spray-drying strategy. Citric acid acts as both a chelating agent and a carbon source, effectively suppressing electrochemically inert NaFePO₄ impurity formation and enabling a phase-pure NASICON framework with a carbon coating containing carbon quantum dots (CQDs). The optimized composition, Na_3.5Fe_0.5V_1.5(PO₄)₃/C, delivers a high reversible capacity of 124 mA h g⁻¹ at 0.5C and 95 mA h g⁻¹ at 20C, while retaining 93.1% of its initial capacity over 1000 cycles at 20C. Detailed kinetic analysis reveals that the elimination of NaFePO₄ and the composite carbon layer with CQDs greatly enhance the Fe²⁺/Fe³⁺ and V⁴⁺/V⁵⁺ redox activity, especially under high-rate conditions. This study underscores the importance of phase and interface engineering in optimizing NASICON-type cathodes and provides a scalable route toward high-power, long-life SIBs.