Fe-rich NASICON cathode with enhanced energy density and structural stability enabled by Fe/Mo co-doping-induced electronic coupling

Abstract

Designing cost-effective and environmentally benign cathode materials is crucial for advancing sodium-ion battery technology. In this regard, partial substitution of vanadium with iron in the NASICON-type Na<inf>3</inf>V<inf>2</inf>(PO<inf>4</inf>)<inf>3</inf> (NVP) framework not only offers economic advantages but also improves charge storage capacity through activation of the V4+/V5+ redox couple. However, excessive substitution of V3+ with larger Fe2+ often leads to framework instability and performance deterioration. This work introduces a co-doping strategy combining Fe2+ and trace Mo6+ to obtain a Fe-rich NASICON composition, Na<inf>4</inf>VFe<inf>0.9</inf>Mo<inf>0.1</inf>(PO<inf>4</inf>)<inf>3</inf> (NVFMP) (⁓50% Fe substitution), without compromising structural stability while significantly improving charge storage performance. The NVFMP cathode delivers a high reversible capacity of 161.3 mAh g−1 at 0.1C, excellent rate capability up to 20C, and 84% capacity retention after 500 cycles at 5C. The full cell exhibits an energy density of 334.8 Wh kg−1 with excellent rate capability and cyclic performance, even at high discharge rates. Density functional theory calculations reveal enhanced V-Fe-Mo-O electronic communication facilitated by additional Fe<inf>3d</inf>-Mo<inf>4d</inf> hybridized states near the Fermi level, leading to significant band-gap narrowing and enhanced charge delocalization. This study provides a rational Fe/Mo co-doping strategy to achieve Fe-rich NVP cathodes with high energy density and structural stability. © 2026 Elsevier B.V.