| Summary: | <p>The Prussian blue analogs (PBAs) are a family of electrochemically reactive compounds with great promise as electrode materials in next-generation sodium- and potassium-ion batteries. They are also an excellent platform for the computational study of interesting phenomena within electronic-structure theory, including mixed valency, redox activity, photo-induced spin transition, and charge disproportionation. However, modeling the PBAs with density-functional theory (DFT) is difficult because the electrons in these compounds are strongly correlated. In this work, I demonstrate how important electrochemical properties of the PBAs are calculable from first principles, but advanced, DFT-based methods like non-local, hybrid functionals or DFT+U must be used to overcome the self-interaction error in these strongly correlated systems.</p>
<p>With these advanced, DFT-based methods, I am able to show with a hybrid functional that three metal hexacyanoferrates (NaxM[Fe(CN)6]), which are important cathode candidates, have very heavy effective masses for charge-carrying electrons and holes, and, thus, small-polaron hopping is an important mechanism for PBA electronic conductivity. Also with the use of a hybrid functional, I provide strong theoretical support to the hypothesis that the high specific capacity of sodium manganese hexacyanomanganate arises from the insertion of a third sodium ion per formula unit to form Na_3Mn{II}[MnI(CN)6], answering a long-standing question from experiment. Finally, I use DFT+U to predict materials properties like reduction potential, ion-hopping activation energy, direct magnetic exchange-coupling coefficient, and infrared-spectroscopy absorption frequencies in four defect-free alkali-metal chromium hexacyanochromates, AxCr[Cr(CN)6].</p>
<p>This work provides critical, new insight into the electronic structure and electrochemical properties of the PBAs. Technically, it contributes another example system to the list of strongly correlated systems successfully simulated with hybrid functionals and DFT+U. And it is an important work in the ongoing, urgent, and global effort to make next-generation batteries better.</p>
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