Plasma‐Tailored Bulk‐Interface‐Surface Trinity Engineering of Iron‐Based Mixed Phosphate Cathodes for Advanced Sodium Ion Batteries

ABSTRACT Iron‐based mixed phosphate Na 4 Fe 3 (PO 4 ) 2 P 2 O 7 (NFPP) is one of the most promising cathodes for sodium‐ion batteries due to its good rate capability and long lifespan, while its practical application is hindered by sluggish ionic/electronic kinetics and interfacial instability. Herein, we report a novel solid‐source ammonium fluoride (NH 4 F) plasma‐driven synergistic “Trinity” engineering strategy to realize simultaneous reconstruction of NFPP cathodes in bulk, interface, and surface architectures. Mechanistic investigations reveal that the coupling reactions between the NH 4 F plasma and NFPP lattice/surface trigger simultaneous bulk F‐substitution and F/N interface doping as well as surface reconstruction. Specifically, the bulk F − substitution strengthens Fe─O bonding and widens Na + channels. Concurrently, plasma‐generated radicals promote the formation of F/N co‐doped carbon network and NaF at the interface, while also promoting the development of a NaF‐rich cathode electrolyte interphase at the surface via modulating the NFPP/electrolyte status. This trinity engineering establishes fast transport pathways and a stable cathode electrolyte interface, effectively minimizing charge transfer impedance while suppressing deleterious side reactions. Consequently, the optimized cell exhibits high capacity and superior high‐rate cycling life with 95.5% retention after 6000 cycles at 30 C. The developed plasma‐driven approach offers mechanistic insights for the synergistic optimization of polyanionic cathodes for advanced sodium ion storage.