Interfacial Dipole Reorganization Enables Ultrafast Sodium Storage in Na ₄ Fe ₃ (PO ₄ ) ₂ P

Abstract

As a promising cathode material for large‐scale sodium‐ion energy storage, Na 4 Fe 3 (PO 4 ) 2 P 2 O 7 (NFPP) still suffers from sluggish Na + kinetics and insufficient electronic conductivity. Herein, we propose a bulk‐to‐interface coupling framework in which substitution‐induced electronic regulation propagates from the crystal lattice to the cathode surface, thereby reorganizing the adjacent electrolyte environment. This strategy is implemented by introducing Ni 2 + with high crystal‐field stabilization energy into NFPP. Ni 2+ substitution modulates the local electronic structure, strengthens Fe─O d‐p hybridization, and promotes medium‐range structural ordering. The resulting surface charge redistribution reshapes the interfacial electrostatic potential, suppresses solvent polarization and Na‐ClO 4 association, and facilitates interfacial Na + exchange. Consequently, the optimized cathode delivers 118 mAh g −1 at 0.1 C and retains 74% of its capacity after 10 000 cycles at 20 C. A pouch cell with a cathode areal capacity of 0.5 mAh cm −2 further demonstrates its feasibility in a full‐cell configuration. This work extends substitution chemistry beyond bulk transport optimization toward the deliberate regulation of electrode‐electrolyte interactions.