Reversible Multielectron Redox Chemistry in a NASICON-Type Cathode toward High-Energy-Density and Long-Life Sodium-Ion Full Batteries

Na-superionic-conductor (NASICON)-type cathodes (e.g., Na3V2(PO4)3) have attracted extensive attention due to their open and robust framework, fast Na+ mobility, and superior thermal stability. To commercialize sodium-ion batteries (SIBs), higher energy density and lower cost requirements are urgently needed for NASICON-type cathodes. Herein, Na3.5V1.5Fe0.5(PO4)3 (NVFP) is designed by an Fe-substitution strategy, which not only reduces the exorbitant cost of vanadium, but also realizes high-voltage multielectron reactions. The NVFP cathode can deliver extraordinary capacity (148.2 mAh g-1), and decent cycling durability up to 84% after 10 000 cycles at 100 C. In situ X-ray diffraction and ex situ X-ray photoelectron spectroscopy characterizations reveal reversible structural evolution and redox processes (Fe2+/Fe3+, V3+/V4+, and V4+/V5+) during electrochemical reactions. The low ionic-migration energy barrier and ideal Na+-diffusion kinetics are elucidated by density functional theory calculations. Combined with electron paramagnetic resonance spectroscopy, Fe with unpaired electrons in the 3d orbital is inseparable from the higher-valence redox activation. More competitively, coupling with a hard carbon (HC) anode, HC//NVFP full cells demonstrate high-rate capability and long-duration cycling lifespan (3000 stable cycles at 50 C), along with material-level energy density up to 304 Wh kg-1. The present work can provide new perspectives to accelerate the commercialization of SIBs. A novel Fe-substituted Na3.5V1.5Fe0.5(PO4)3 cathode with Na-superionic-conductor (NASICON)-type structure is proposed, which realizes reversible structural evolution and multielectron redox reactions (Fe2+/Fe3+, V3+/V4+, and V4+/V5+) by incorporating iron, whose unpaired electrons in the 3d orbital are inextricably related to the activation of higher-voltage redox couple.image

Automated summary

In this study, a novel Fe-substituted Na3.5V1.5Fe0.5(PO4)3 (NVFP) cathode with Na-superionic-conductor (NASICON) structure is proposed, which realizes reversible structural evolution and multi-electron redox reactions (Fe2+/Fe3+, V3+/V4+, and V4+/V5+) by incorporating iron. The NVFP cathode delivers excellent capacity and cycling stability, and the low ionic migration energy barrier and ideal Na+ diffusion kinetics are elucidated. Coupling with a hard carbon anode, HC//NVFP full cells demonstrate high-rate capability and long cycling lifespan, with material-level energy density up to 304 Wh kg-1.