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

Abstract

Iron-based mixed phosphate Na₄Fe₃(PO₄)₂P₂O₇ (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 (NH₄F) 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 NH₄F 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.