Synergistic Fe/Mo Co-substitution Enables High-Energy and Ultra-stable NASICON Cathode for Sodium-Ion Batteries

Sodium-ion batteries have emerged as promising alternatives to lithium-ion systems due to the abundance and low cost of sodium resources. Among various cathode candidates, NASICON-type Na3V2(PO4)3 (NVP) has garnered substantial attention because of its robust framework and rapid Na+ diffusion. However, its poor electronic conductivity hinders rate performance and cycling stability. Elemental substitution is widely regarded as a powerful strategy to enhance electrochemical behavior while preserving the inherent structural advantages of the NASICON framework. In this work, a ternary NASICON composite cathode, Na3V1.4Fe0.5Mo0.1(PO4)3/C (NFVMP), is rationally constructed via Fe/Mo co-substitution. Fe acts as an electrochemically active dopant, introducing Fe2+/Fe3+ redox centers that contribute to the enhanced capacity together with the V3+/V4+/V5+ redox reactions. In contrast, Mo primarily serves as a structural stabilizer by reinforcing the NASICON framework through strengthened Mo–O bonding and lattice contraction, thereby suppressing structural distortion during cycling. Consequently, NFVMP delivers a high specific capacity of 126 mAh g–1 at 0.1 C and retains 84.5 mAh g–1 even at 20 C, while maintaining 86.6% capacity retention after 5000 cycles. In situ X-ray diffraction analysis confirms highly reversible structural evolution with negligible volume variation during cycling, indicating superior structural stability. Furthermore, the NFVMP||HC full cell exhibits a discharge capacity of 107.9 mAh g–1 and retains 83.9% of its capacity after 100 cycles, corresponding to a remarkable energy density of 247 Wh kg–1. These findings demonstrate that multi-metal ion substitution is an effective strategy for simultaneously enhancing the electrochemical performance and reducing the cost of NVP-based cathodes for sodium-ion batteries.