Aluminum Doping Enhances Structural Integrity and Electrochemical Performance in NASICON-Type Cathodes for Sodium-Ion Batteries

In this work, we report the synthesis of aluminum-doped Na4–xVMn1–xAlx(PO4)3/C cathodes via a solid-state reaction and evaluate their structural and electrochemical performances for sodium-ion batteries. Structural analyses reveal that Al incorporation modifies both the long-range lattice and the local coordination of V and Mn sites. Electrochemical results show that although the specific capacity varies nonlinearly with Al content, moderate Al substitution yields smoother voltage profiles, reduced polarization, and markedly enhanced cycling stability, with doped electrodes retaining over 92% of their initial capacity after 500 cycles at 2C. Density functional theory (DFT) calculations further clarify that Al substitution distorts the MnO6 and VO6 octahedra and alters V–O and Mn–O bonding through modified cation–oxygen hybridization. The projected density of states (PDOS) and Bader charge analyses reveal pronounced charge redistribution around V atoms, accounting for the merged redox potentials and improved structural stability. Overall, electrochemically inactive Al3+ substitution effectively stabilizes the NASICON framework and mitigates phase transitions, offering atomic-level insights for the design of durable, high-performance sodium-ion batteries.