Unexpected Chain of Redox Events in Co-Based Prussian Blue Analogues

The electronic structure of electrode materials for metal-ion batteries has a great impact on their charge compensation mechanism and, consequently, electrochemical behavior. In this paper, we report on the cobalt doping in the potassium manganese hexacyanoferrate positive electrode material for potassium-ion batteries, resulting in the formation of a system of K2−δCoxMn1–x[Fe(CN)6] compounds with x = 0...1 and provide their comprehensive characterization including crystal structure evolution and charge compensation mechanisms upon K de/intercalation. Synthesized by a coprecipitation method, K2−δCoxMn1–x[Fe(CN)6] forms two series of solid solutions with monoclinic (Co-poor) and cubic (Co-rich) structures. According to energy-dispersive X-ray analysis, the K content diminishes with increasing x value. Electrochemical properties of electrode materials based on K2−δCoxMn1–x[Fe(CN)6] in K-metal half cells are also strongly dependent on Co doping regarding both specific capacity and redox potential. Attempts to interpret the results led to an unexpected conclusion that cobalt has influence on iron and manganese redox potentials, forming the following oxidation sequence: Co2+/3+, Mn2+/3+, and Fe2+/3+ in K2−δCoxMn1–x[Fe(CN)6], which is inverse to that of Co-free K2−δMn[Fe(CN)6] (Fe2+/3+, Mn2+/3+), as validated by ex situ, operando X-ray absorption spectroscopy, and 57Fe Mössbauer spectroscopy.