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., Na₃ V₂ (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, Na_3.5 V_1.5 Fe_0.5 (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.