Triggering Atomic Reconstruction in Dopant‐Free Na _3.12 Fe _2.44 (P ₂ O ₇ ) ₂ Cathode via Sodium Deficiency Chemistry for Practical Sodium‐Ion Batteries

ABSTRACT Pyrophosphate Na 3.12 Fe 2.44 (P 2 O 7 ) 2 (NFP) is noted for its stability and safety, yet kinetic and interfacial constraints limit performance. This work proposes a dopant‐free NFP cathode based on sodium (Na) deficiency chemistry. By precisely adjusting Na content, we synchronized local structure and charge compensation to develop a superior NFP‐3.05 cathode. Structural analysis reveals that moderate Na deficiency reduces oxygen vacancies and introduces Fe 3+ , thereby optimizing charge compensation while restoring Fe‐O‐P framework continuity. This enhances both Na + transport and electronic conductivity. Consequently, NFP‐3.05 delivers 104 mAh g − 1 at 0.1 C and maintains 91.2% capacity after 1500 cycles at 1 C, outperforming NFP‐3.12 and NFP‐2.98. In situ tests confirm high reversibility with a minimal 3.6% volume change. Notably, in situ DEMS reveals significantly suppressed gas evolution O 2 , CO 2 , CO/C 2 H 4 , proving that Na deficiency inhibits high‐voltage interfacial side reactions. To assess adaptability, we built additive‐free coin cells and additive‐based pouch cells. The coin cell achieved 92.2 mAh g −1 with 89.5% retention after 200 cycles at 1 C, verifying intrinsic compatibility. The pouch cell maintained 93.2% capacity after 150 cycles at 80 mA, demonstrating practical potential. This Na‐deficient strategy provides a pathway for optimizing NFP cathodes and a reference for synergistic structure‐interface regulation in polyanionic systems.