Reversible Multi-Electron Redox Chemistry in NASICON-Type Cathode Toward High-Energy-Density And Long-Life Sodium-Ion Full Batteries.

Na-superionic-conductor (NASICON)-type cathodes (e.g., Na 3 V 2 (PO 4 ) 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 (PO 4 ) 3 (NVFP) is designed by Fe substitution strategy, which not only reduces the exorbitant cost of vanadium, but also realize the high-voltage multi-electron 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 (Fe 2+ /Fe 3+ , V 3+ /V 4+ and V 4+ /V 5+ ) 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 50C), 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. This article is protected by copyright. All rights reserved.