Fe Substitution-Enabled Performance Optimization of Na₃V₂(PO₄)₃@C Cathodes for Quasi-Solid-State Sodium Metal Batteries

Solid-state sodium metal batteries (SSBs) are considered promising candidates for next-generation energy storage owing to their intrinsic safety and cost advantages. However, their development is limited by the poor stability and sluggish kinetics of commonly used polyanionic cathodes. To overcome these challenges, a dual-modification strategy is proposed by simultaneously introducing Fe³⁺ substitution into Na₃V₂(PO₄)₃ and applying a carbon coating, where Fe³⁺ partially substitutes V³⁺ to enhance structural stability (denoted as Na₃Fe_xV_2-x(PO₄)₃@C, x = 0, 0.8, 1.0, 1.2), and the conductive carbon layer improves electronic conductivity. Structural characterization confirms that Fe substitution retains the rhombohedral framework while inducing lattice contraction resulting from Fe²⁺ incorporation, Na⁺ vacancies, and simultaneously improves the graphitization of the carbon coating. Electrochemical evaluations show that optimized Na₃Fe_0.8V_1.2(PO₄)₃@C enables multi-redox activity, enhances pseudocapacitive behavior, and improves rate capability and cycling stability. The Na₃Fe_0.8V_1.2(PO₄)₃@C achieves a capacity retention of 92.2% after 400 cycles at 1 C in quasi-solid-state sodium metal batteries. Moderate Fe substitution enables uniform Na deposition and stabilizes electrode–electrolyte interfaces, as evidenced by structural and surface analyses. These results confirm the effectiveness of Fe substitution in enhancing the electrochemical performance of cathodes for SSBs.