Ionic Polarization‐Driven Defect Engineering in Na ₄ Fe ₃ (PO ₄ ) ₂ (P ₂ O ₇ ) Cathode: Fast Charging and Ultra‐Long Cycle Life of Sodium‐Ion Batteries

The Na₄Fe₃(PO₄)₂(P₂O₇) (NFPP) cathode material faces the challenge of coordinating the improvement of high-rate performance and long-cycle stability for sodium-ion batteries (SIBs). This study proposes an ionic polarization-driven defect engineering strategy, which regulates the electronic structure and Na⁺ transmission dynamics of NFPP through Bi³⁺ doping. Experimental results and theoretical calculations show that Bi³⁺ with (18 + 2) electron configuration significantly enhances the crystal structure stability of NFPP by strengthening the covalency of Bi─O bonds. Meanwhile, the heterovalent Bi³⁺ doping optimizes the bandgap of the material (from 3.29 to 0.16 eV) and promotes Na⁺ diffusion, while introducing lattice defects to provide additional sodium storage sites. The optimized 0.02Bi-NFPP cathode exhibits excellent electrochemical performance as the half-cell only takes 31.6 min to charge to 80% at a rate of 1 C, and the capacity decay is only 0.000495 mA h g^-1 per cycle (86.9% capacity retention) over 20,000 cycles at 20 C. The full battery based on hard carbon anode maintains 95.5% capacity retention after 200 cycles at 1 C. This study reveals the synergistic mechanism between ion polarization effect and lattice defects, and provides a new strategy for designing SIBs cathode materials with both fast charging/discharging capabilities and ultra-long life.