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.