A Multielectron-Reaction and Low-Strain Na_3.5Fe_0.5VCr_0.5(PO₄)₃ Cathode for Na-Ion Batteries

Abstract

Natrium superionic conductor (NASICON)-type phosphates have attracted widespread attention as cathodes for sodium-ion batteries (SIBs) due to their 3D open frameworks facilitating Na + diffusion, but they are characterized by mediocre energy density or rapid capacity decay. Herein, we delicately design a multielectron-reaction and low-strain Na 3.5 Fe 0.5 VCr 0.5 (PO 4 ) 3 /C cathode material featuring a high working voltage (∼3.43 V), high reversible capacity (148.5 mAh g –1 ), and high cycling stability (95.1% capacity retention over 2000 cycles). The deviation in the reaction potential of each redox couple (Fe 2+ /Fe 3+, V 3+ /V 4+ /V 5+, and Cr 3+ /Cr 4+ ) efficaciously alleviates the lattice strain accumulation, ensuring a small cell volume variation of 3.87% during the highly reversible charge–discharge processes, as confirmed by systematic in situ / ex situ analyses. Moreover, the fast reaction kinetics and the unexpected reversible Na1-ion (6b site) release/uptake are elucidated via multiple electrochemical characterizations and theoretical computations. This rational design strategy of incorporating versatile redox couples with different roles will broaden the horizons of high-performance NASICON-type cathodes.