Regulating Na/Fe Antisite Defects and Suppressing Elemental Segregation Toward a Phase-Pure Na4Fe2.91(PO4)2(P2O7) Cathode with Fast Intercalation Kinetics

Abstract

Na₄Fe_2.91(PO₄)₂(P₂O₇) (NFPP) is an attractive cathode material for commercial sodium-ion batteries. However, local phosphorus aggregation and Na/Fe antisite defects lead to the formation of a low-capacity Na₂FeP₂O₇ (N2FP) impurity and slow Na⁺ diffusion, which diminish its electrochemical performance. To address these challenges, we introduce a dual-site tungsten (W) doping strategy. The doping of W⁶⁺ at the Fe site suppresses Na/Fe antisite defects, redistributes electrons around Fe atoms, and creates Na vacancies through a charge compensation mechanism. This process reduces the band gap (from 2.82 to 0 eV) and lowers the Na⁺ migration energy barriers (from 3.30 to 1.76 eV). Furthermore, W forms [WO₄] polyanion groups that partially replace [PO₄], reducing local phosphorus concentration and the formation energy of NFPP phase, thereby limiting the generation of N2FP impurity. Notably, decreased Na/Fe antisite defect levels prevent unfavorable structural rearrangements during early cycles, reducing capacity decay in the first 40 cycles from 1.8% to 0%. As a result, W-doped NFPP exhibits a high discharge specific capacity (113.8 mAh g^-1 at 0.1C between 2-4 V), long cycle life (stable for 2000 cycles at 20C), and excellent temperature stability (from -20°C to 50°C). This study provides a novel approach for designing high-performance NASICON-type cathode materials and advances the commercialization of sodium-ion batteries.