A bifunctional Na-deficient strategy induced pure phase Na4−xFe3(PO4)2P2O7 cathode with high capacity for sodium-ion batteries

Abstract

Iron-based mixed phosphate-pyrophosphate Na₄Fe₃(PO₄)₂(P₂O₇) (NFPP) as a cathode material for sodium-ion batteries (SIBs) exhibits promising commercial prospects due to its excellent structural stability, high theoretical capacity, and cost advantages. However, the easy generation of inactive NaFePO₄ impurities during synthesis is a major factor causing lower specific capacity. This study proposes a Na-deficient strategy for structural modulation of NFPP, successfully synthesizing pure-phase Na_4−xFe₃(PO₄)₂(P₂O₇). The abundant Fe³⁺/Fe²⁺ redox pairs and improved crystal structure allow more Na⁺ to migrate, achieving a near-theoretical specific capacity of 127.2 mA h g⁻¹ at 0.1 C and 114.6 mA h g⁻¹ at 1 C, with a capacity retention rate of 100% after 450 cycles at 1 C. Enhanced Na⁺ dynamics are confirmed by electrochemical impedance spectroscopy (EIS), the galvanostatic intermittent titration technique (GITT), and cyclic voltammetry (CV). Additionally, the coin-type full cell based on the NFPP-ND-1 cathode and hard carbon anode exhibits a discharge capacity of 108 mA h g⁻¹ at 0.1 C. This study validates the significant improvement in the electrochemical performance of NFPP achieved through the Na-deficient strategy, and proposes a low-cost, highly stable, and industrially feasible SIB cathode material.