Anion-regulated interfacial stability in Prussian blue analog cathodes for quasi-solid-state aqueous sodium-ion batteries

Quasi-solid-state gel polymer electrolytes (GPEs) offer a promising strategy to enhance the safety, mechanical robustness, and interfacial stability of aqueous sodium-ion batteries (ASIBs). In this work, carboxymethyl cellulose (CMC)-based GPEs incorporating four different sodium salts (NaClO 4 , NaNO 3 , Na 2 SO 4 , and NaOAc) were systematically investigated to elucidate the role of anion chemistry. Optimization studies revealed that salt concentrations ≥2 M were necessary to retain gel integrity, with electrolyte absorption and ionic conductivity trends strongly influenced by Hofmeister-type kosmotropic/chaotropic effects. The compatibility of the different GPEs with Prussian blue analog (PBA) cathode was systematically investigated using three-electrode and full-cell electrochemical measurements. Although all electrolyte systems exhibited similar Fe(III)/Fe(II) redox behavior in cyclic voltammetry, significant differences emerged during long-term cycling. Among the investigated systems, the acetate-based electrolyte (GPE-A) demonstrated the highest electrochemical stability, maintaining an average coulombic efficiency of ∼95% over 350 cycles along with the lowest interfacial resistance growth. In contrast, sulfate-, nitrate-, and perchlorate-based systems showed rapid degradation, unstable cycling behavior, and pronounced cathode dissolution, as confirmed by post-cycling structural and morphological analyses. The enhanced stability of GPE-A is likely associated with the kosmotropic nature of acetate ions aided by the mildly basic pH of the electrolyte, which together help stabilize the Fe–CN framework and suppress interfacial degradation. Overall, this work highlights the critical role of electrolyte anion engineering in improving cathode stability in quasi-solid-state aqueous sodium-ion batteries.