(Digital Presentation) Prussian Blue Analogs – a Wide Variety of Promising Cathode Materials with Peculiar Electrochemical Properties
Authors: Morozova Polina A. , Fedotov Stanislav S. , Abakumov Artem M. 
Published: July 2022

Today, post-lithium energy storage technologies are now rapidly progressing due to the high price of a net Li-ion battery, which also depends on the desired capacity and power. Among sodium, potassium, calcium, magnesium, and even aluminum-based alternatives, young potassium-ion batteries demonstrate high capacity and energy density, notable ionic transport in electrolytes, the possibility to employ graphite anodes, and a wide variety of possible electrode materials: layered oxides, polyanionic, organic compounds, Prussian Blue analogs. However, the latter ones are generally considered as the most promising and practically viable.

Prussian Blue analogs form a big family of electrode materials with the general formula KxM1[M2(CN)6]∙nH2O, where x=0...2, and Mi are any possible 3d transition metals. The most well-known and commercially available is based on a hexacyanoferrate anion [Fe(CN)6]n-, while other transition metals can also form hexacyanometallate complexes, but are poorly studied or not known at all. The most part of published works shed light on the morphology and low content of water defects inside crystallites counting the lack of hexacyanoferrate, and their influence on realized capacity and capacity fades, while the fundamental principles and real water position presence which guide the electrochemical activity of high- and low-spin cations in these materials are totally missed.

In our work, we also started with the intrinsic water defects and their impact on crystal and physicochemical properties in K2Mn[Fe(CN)6]∙nH2O but with sensitive to light atoms neutron diffraction technique. We observed that water content does not effect the whole crystal symmetry but slightly amend unit cell parameters. Besides the fact of decreasing a decomposition temperature in “watered” Prussian Blue analog, electrochemical properties were found close. Therefore, we concluded that intrinsic water does not notably influence material properties. Continuing with potassium manganese hexacyanoferrate and electronic structure impact to the compound properties, to reveal the best synergetic stabilizing agent during cycling, we synthesized and studied the system K2-γMn1-xCox[Fe(CN)6]∙nH2O with x=0, 0.05,...1. In addition to symmetry and composition transformations, magnetic and electrochemical properties also significantly differ, while higher cobalt content increases the redox potential of iron, but drastically decreases total capacity due to the inability of reaching iron oxidation. The fact of notable changing of redox potentials in K2-γMn1-xCox[Fe(CN)6]∙nH2O is inspiring, and we have been extending with other hexacyanometallates. Hexacyanomanganate-ion [Mn(CN)6]n- is one of the promising pathways to investigate such systems with a more fundamental point of view, and the obtained experimental results confirm the hypothesis. We discovered that cation position exchange totally alters the electrochemistry of the Prussian Blue cathode materials, and the interpretation still raises a lot of questions and assumptions, and together with computational chemistry, we will try to answer the fundamental questions about electronic, crystal structures and electrochemical properties. In this report we will present the new correlations of redox potential in Prussian Blue analogs depending on transition metal position and electronic structure, evaluate diffusion of potassium in these materials, and try to answer the possibility to use these compounds in potassium-ion batteries.