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

Abstract The Na 4 Fe 3 (PO 4 ) 2 (P 2 O 7 ) (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 Bi 3+ doping. Experimental results and theoretical calculations show that Bi 3+ with (18 + 2) electron configuration significantly enhances the crystal structure stability of NFPP by strengthening the covalency of Bi─O bonds. Meanwhile, the heterovalent Bi 3+ 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.