Fe-substituted Na_3.5V_1.5Mn_0.5(PO₄)₃ NaSICON cathode with multi-electron reactions and improved energy output

Sodium-ion batteries have emerged as a promising alternative to conventional lithium-ion batteries due to the abundance and low cost of sodium resources. NaSICON-based materials represent a highly competitive class of cathodes for sodium-ion batteries. Engineering NaSICON-type cathodes through rational design is a compelling approach to enhancing the overall energy density of sodium storage systems. In this study, the known Na_3.5V_1.5Mn_0.5(PO₄)₃ phase was strategically modified by partially substituting Mn with Fe using a Pechini route, followed by an ex situ carbon coating, leading to the formation of a novel ternary NaSICON-type Na_3.5V_1.5Mn_0.25Fe_0.25(PO₄)₃/C (NVMFP/C) cathode with enhanced properties. Structural and morphological analyses confirmed the high crystallinity and phase purity of the designed material (NVMFP/C). Electrochemical evaluation versus Na⁺/Na demonstrated a reversible capacity of 167 mA h g^-1 (∼0.30 mA h cm^-2) at 0.1C, outstanding rate capability, and a high energy density of 412 Wh kg^-1 at 1C. Furthermore, the material exhibited remarkable cycling stability, with 74% capacity retention after 1400 cycles at 10C within the 1.5-4.3 V potential window. This promising electrochemical performance is attributed to the fast Na⁺ diffusion kinetics, as confirmed by the galvanostatic intermittent titration technique (GITT), and the structural stabilization, resulting from this compositional engineering. Moreover, the synergy between the Pechini route, ex situ carbon coating, and Fe incorporation activates multiple redox reactions and enables the designed cathode to operate within an expanded potential range with significant capacitive contribution, providing a robust pathway toward high-energy-density sodium-ion batteries.