Influence of Dual Mo/Co Substitution on the Structure and Electrochemical Behaviour of Prussian Blue Analogue Cathodes for Sodium-Ion Batteries

Prussian Blue Analogues (PBAs) are among the most promising cathode materials for sodium-ion batteries; however, their electrochemical behaviour strongly depends on the composition and local coordination environment of transition-metal sites. In this work, a dual Mo/Co-substituted Prussian Blue Analogue (MoCo-PBA) was synthesized via a controlled co-precipitation route and systematically investigated to elucidate the influence of simultaneous transition-metal substitution on the structural, physicochemical, and electrochemical properties of the PBA framework. The material was comprehensively characterized using ICP-OES, CHN/O elemental analysis, XRD, XPS, FT-IR, Raman spectroscopy, SEM-EDS, BET surface area analysis, thermogravimetric analysis, and electrochemical techniques including cyclic voltammetry and galvanostatic charge–discharge measurements. Structural and spectroscopic analyses indicated the formation of a face-centred cubic PBA framework with both Mo and Co detected by elemental analysis, while nitrogen adsorption measurements revealed a mesoporous architecture with a specific surface area of 117 m2 g−1. Electrochemical measurements demonstrated reversible Na+ insertion/extraction within the dual-substituted framework under practically active mass loading conditions (~3.1 mg cm−2). The combined physicochemical and electrochemical results indicate that simultaneous Mo/Co substitution modifies the framework chemistry and influences the electrochemical response of the PBA cathode. At 10 mA g−1, the MoCo-PBA cathode delivered first-cycle charge and discharge capacities of approximately 145 and 46 mAh g−1, respectively, indicating substantial initial irreversibility. The subsequent discharge capacity was approximately 39 mAh g−1 at 10 mA g−1 and decreased to approximately 24 mAh g−1 at 50 mA g−1. Although the reversible capacity remains below that of state-of-the-art PBA cathodes, the present study provides insight into the relationship between dual-metal substitution, framework characteristics, and sodium-storage behaviour, contributing to the rational design of chemically engineered Prussian Blue Analogues for sodium-ion batteries.