Electronic Structure and Electron-Transport Properties of Three Metal Hexacyanoferrates

Metal hexacyanometallates, or Prussian blue analogs (PBAs), are active materials in important electrochemical technologies, including next-generation sodium- and potassium-ion batteries. They have tunable properties, including reduction potential, ionic conductivity, and color. However, little is known about their electronic conductivities. In this work, we use density-functional theory to model the electronic structure and to explore the likely electron-conduction mechanism in three promising cathodes (manganese, iron, and cobalt hexacyanoferrate) in each of three oxidation states. First, we demonstrate that hybrid functionals reliably reproduce experimentally observed spin configurations and geometric phase changes. We confirm these materials are semiconductors or insulators with band gaps ranging from 1.90 eV up to 4.94 eV. We further identify that for most of the compounds, the electronic band edges originate from carbon-coordinated iron orbitals, suggesting that doping at the carbon-coordinated site may strongly affect carrier conductivity. Finally, we calculate charge-carrier effective masses, which we find are very heavy. This study is an important foundation for making electronic conductivity a tunable PBA material property.