Electrolyte-regulated CEI evolution and interfacial kinetics of Na4Fe3(PO4)2(P2O7) cathodes for sodium-ion batteries

The electrochemical behavior of cathodes in sodium-ion batteries is strongly influenced by electrolyte-induced interfacial processes, yet the underlying origins remain unclear. Here, Na 4 Fe 3 (PO 4 ) 2 (P 2 O 7 ) (NFPP) is selected as a model cathode to examine how electrolyte chemistry affects cathode electrolyte interphase (CEI) evolution and interfacial kinetics. Four carbonate-based electrolyte systems with different sodium salts and FEC additive contents were systematically compared. Among them, NaClO 4 @EC/PC/5% FEC shows the most favorable electrochemical behavior, featuring an oxidation limit of 4.7 V vs. Na/Na + and a relatively low apparent interfacial activation energy of 0.510 eV, as determined from Na||Na symmetric cells. Mechanistic analyses indicate that this electrolyte favors the development of a well-organized CEI featuring NaF-enriched inorganic domains and sufficiently compliant organic components, thereby enabling a more stable cathode/electrolyte interface and smoother Na + transport during repeated cycling. In contrast, the other electrolyte systems lead to either excessive accumulation of organic species or mechanically fragile interphases, causing interfacial deterioration and surface corrosion. This work highlights the decisive role of electrolyte chemistry in governing CEI evolution and interfacial stability, and provides broadly applicable guidance for electrolyte engineering in advanced sodium-ion battery cathodes.